lee myungjae 201106 phd thesis

Adsorption of alkaline copper quat components in wood –

mechanisms and influencing factors

by

Myung Jae Lee

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Graduate Department of Forestry

University of Toronto

© Copyright by Myung Jae Lee 2011

ii

Adsorption of alkaline copper quat components in wood –

mechanisms and influencing factors

Myung Jae Lee

Doctor of Philosophy

Graduate Department of Forestry

University of Toronto

2011

Abstract

Mechanisms of adsorption of alkaline copper quat (ACQ) components in wood were investigated

with emphasis on: copper chemisorption, copper physisorption, and quat adsorption. Various

factors were investigated that could affect the adsorption of individual ACQ components in red

pine wood. Copper chemisorption in wood was affected by ligand types coordinating with Cu

and the stability of the Cu-ligand complexes in solution. For Cu-monoethanolamine (Cu-Mea)

system, the prevailing active solvent species at the solution pH, [Cu(Mea)2-H]+ complexes with

wood acid sites and loses one Mea molecule through a ligand exchange reaction. The amount of

adsorbed Cu was closely related to the cation exchange capacity of wood. An increase in Mea/Cu

ratio increased the proportion of the uncharged Cu-Mea complex and resulted in decreased Cu

chemisorption in wood. Copper precipitation is also an important Cu fixation mechanisms of Cu-

amine treated wood. X-ray diffraction analysis revealed that in vitro precipitated Cu was a

mixture of copper carbonates (azurite and malachite) and possibly Cu2O. Higher concentration

Cu-amine solutions retarded the Cu precipitation to a lower pH because of higher free amine in

the preservative-wood system. The changes in zeta potential of wood in relationship to the

quaternary ammonium (alkyldimethylbenzylammonium chloride: ADBAC) adsorption isotherm

showed two different adsorption mechanisms for quat in wood: ion exchange reaction at low

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concentration and additional aggregation form of adsorption by hydrophobic interaction at high

concentration. Because of the aggregation effect, when wood was treated with ACQ, high

amounts of ADBAC were concentrated near the surface creating a steep gradient with depth.

This aggregated ADBAC was easily leached out while the ion exchanged ADBAC had high

leaching resistance. Free Mea and Cu of ACQ components appeared to compete with ADBAC

for the same bonding sites in wood.

.

iv

Acknowledgments

It was a pleasant and precious journey to study in Faculty of Forestry in University of Toronto

and the author is indebted to the many that have contributed to the completion of this thesis.

Foremost, I would like to thank my supervisor, Professor Paul Cooper for all support,

encouragement and guidance throughout my Ph.D. study. Looking back, meeting with him to

discuss and share ideas was the most awaited and valuable time during all the years we worked

together. It is a truly blessing to have supervisor like him. I also appreciate the advice and

assistance given by the members of my advisory and examination committees, including

Professors M. Sain, N. Yan, D. Heyd, S. Krigstin, and J. Ruddick.

My gratitude is extended to all my colleagues and friends for their help and support, particularly

Tony for his help, comments and technical support.

Finally, I wishes to express my thankfulness to my parents, my wife Seung-Hee and son, Paul

Junsu for their unconditional love and understanding ever since.

v

Table of Contents

Acknowledgments.......................................................................................................................... iv

Table of Contents............................................................................................................................ v

List of Tables .............................................................................................................................. ix

List of Figures ............................................................................................................................... x

List of Appendices ....................................................................................................................... xiv

List of Abbreviations .................................................................................................................... xv

Chapter 1 Introduction and background................................................................................. 1

1.1 Introduction......................................................................................................................... 1

1.2 Literature review................................................................................................................. 4

1.2.1 Copper amine fixation in wood............................................................................... 4

1.2.2 Factors affecting copper stabilization in wood ..................................................... 12

1.2.3 Quaternary ammonium adsorption in wood ......................................................... 14

1.3 Objectives, Experimental approaches and Hypotheses .................................................... 19

Chapter 2 Copper monoethanolamine adsorption in wood and its relation to cation

exchange capacity (CEC)...................................................................................... 21

2.1 Abstract ............................................................................................................................. 21

2.2 Introduction....................................................................................................................... 21

2.3 Materials and methods ...................................................................................................... 22

2.3.1 Sample preparation ............................................................................................... 22

2.3.2 Cation exchange capacity of red pine ................................................................... 22

2.3.3 Effect of pH on Cu and Mea adsorption capacity................................................. 23

2.3.4 Fourier transformed infrared (FTIR) spectroscopy............................................... 23

2.4 Results and discussion ...................................................................................................... 23

vi

2.4.1 Cation exchange capacity (CEC) of red pine dust as a function of pH ................ 23

2.4.2 Mea adsorption capacity of wood ......................................................................... 25

2.4.3 Cu-amine adsorption capacity of wood ................................................................ 25

2.4.4 FTIR spectral analysis........................................................................................... 29

2.5 Conclusions....................................................................................................................... 31

Chapter 3 Effect of amine ligand, copper/amine ratio and pH on copper adsorption into

wood...................................................................................................................... 32

3.1 Abstract ............................................................................................................................. 32

3.2 Introduction....................................................................................................................... 32



3.3.2 Effect of Cu/Mea ratio on copper amine solubility .............................................. 33

3.3.3 Effect of amine type, Cu/amine ratio and pH on Cu-amine adsorption capacity . 34


3.4.1 Effect of Cu/Mea ratio on Cu-amine solubility .................................................... 35

3.4.2 Effect of Cu/ethanolamine ratio and pH on Cu adsorption capacity .................... 35

3.4.3 Cu-amine adsorption capacity of wood in Cu-En and Cu-Am systems ............... 45

3.5 Conclusions....................................................................................................................... 48

Chapter 4 Copper precipitation in Cu-monoethanolamine preservative treated wood ........ 50

4.1 Abstract ............................................................................................................................. 50

4.2 Introduction....................................................................................................................... 50


4.3.1 Effect of Cu-Mea concentration on in vitro copper precipitation......................... 52

4.3.2 XRD and XRF analysis of precipitated copper..................................................... 52

vii

4.3.3 Relationship between Cu adsorption and pH in wood during conditioning ......... 53

4.3.4 Evaluation of CEC changes and Cu(I) formation ................................................. 53


4.4.1 In vitro copper precipitation of Cu-Mea ............................................................... 54

4.4.2 Relationship between Cu stabilization and pH during the conditioning process.. 58

4.4.3 Copper stability after drying and influence of oxidation of wood components ... 62

4.5 Conclusions....................................................................................................................... 66

Chapter 5 Alkyldimethylbenzylammonium chloride (ADBAC) adsorption mechanism on

wood...................................................................................................................... 68

5.1 Abstract ............................................................................................................................. 68

5.2 Introduction....................................................................................................................... 68



5.3.2 CMC measurement of ADBAC............................................................................ 70

5.3.3 Zeta potential measurement .................................................................................. 71

5.3.4 Adsorption isotherm of ADBAC in wood ............................................................ 71

5.3.5 Fitting to adsorption isotherm models .................................................................. 72


5.4.1 CMC of ADBAC .................................................................................................. 72

5.4.2 Relationship between zeta potential of wood and ADBAC adsorption at

different quat concentrations................................................................................. 73

5.4.3 Effect of concentration on adsorption of ADBAC on wood................................. 75

5.4.4 Application of adsorption isotherm models to ADBAC adsorption in wood....... 78

5.5 Conclusions and implications ........................................................................................... 79

viii

Chapter 6 Effects of ionic strength, Mea, Cu, and pH on alkyldimethylbenzylammonium

chloride (ADBAC) adsorption in wood................................................................ 81

6.1 Abstract ............................................................................................................................. 81

6.2 Introduction....................................................................................................................... 81



6.3.2 Effect of ionic strength on ADBAC adsorption.................................................... 83

6.3.3 Effect of Mea increase on ADBAC adsorption .................................................... 83

6.3.4 Effect of copper and solution pH on ADBAC adsorption .................................... 83

6.3.5 ACQ treatment of solid wood – concentration gradient and leachability of

components ........................................................................................................... 84

6.3.6 Analysis of Cu, Mea, ADBAC and Cl.................................................................. 84


6.4.1 Effect of ionic strength on ADBAC adsorption.................................................... 85

6.4.2 Effect of Mea increase on ADBAC adsorption .................................................... 86

6.4.3 Effect of Cu on ADBAC adsorption..................................................................... 87

6.4.4 pH effect on ADBAC adsorption.......................................................................... 88

6.4.5 Gradient adsorption of ADBAC and its leachability in solid wood samples ....... 90

6.5 Conclusions and implications ........................................................................................... 93

Chapter 7 Summary, conclusions and proposed future work............................................... 94

7.1 Summary and Conclusions ............................................................................................... 94

7.2 Proposed future work........................................................................................................ 97

Bibliography ........................................................................................................................... 100

Appendices ........................................................................................................................... 112

Peer reviewed journal papers ...................................................................................................... 117

ix

List of Tables

Table 3.1 Distribution of Cu-Mea species in aqueous solution with Cu concentration of 42 mmol

l-1

(equivalent to 0.33% CuO in solution) (Pankras et al. 2009)................................................... 36

Table 3.2 Distribution of Cu-Dea and Cu-Tea species in aqueous solution as a function of pH

(Tauler and Casassas 1986, Casassas et al. 1989). ....................................................................... 42

Table 5.1 The effect of additives on CMC of ADBAC measured at 22 C................................... 73

Table 5.2 Adsorption constants estimated from simulations with Langmuir, BET, and Freundlich

models for isotherms of ADBAC in different media using wood dust as an absorbent............... 78

x

List of Figures

Figure 1.1 Different charge states of copper amine complexes in solution (Zhang and Kamdem

2000b). ............................................................................................................................................ 5

Figure 1.2 (a) Copper-ethylene diamine (Cu-En) complex and (b) copper-ammonium (Cu-Am)

complex........................................................................................................................................... 6

Figure 1.3 Chemical structure of alkyldimethylbenzylammonium chloride (ADBAC) used as

wood preservative. ........................................................................................................................ 17

Figure 2.1 Cation exchange capacity of red pine dust for sodium (Na+) from NaOH/HNO3

solutions (0.05 mol l-1

and 0.1 mol l-1

) as a function of pH (a) and its comparison with Mea

adsorption on red pine dust at different pH (b)............................................................................. 24

Figure 2.2 Chemisorption of Cu (a) and Mea (b) of Cu-Mea in red pine dust and comparison with

CEC of red pine ............................................................................................................................ 26

Figure 2.3 Chemisorption of copper from Cu(II) acetate and Cu(II) sulfate solutions in red pine

dust ................................................................................................................................................ 28

Figure 2.4 FTIR spectra of Na+-treated wood dust at different pH and control wood dust (a) and

Na+-, Cu-Mea-, and Mea-treated wood at pH 9 and control wood dust (b).................................. 30

Figure 3.1 Percent Cu precipitation as affected by Cu to Mea ratios for Cu-Mea solution.......... 35

Figure 3.2 Chemisorption of Cu (a) and Mea (b) from different Mea ratios of Cu-Mea in red pine

dust and comparison with CEC of red pine .................................................................................. 37

Figure 3.3 Chemical adsorption of Cu and Mea from high amine ratio Cu-Mea (1:70 molar ratio)

in red pine dust in comparison with CEC of red pine................................................................... 38

Figure 3.4 Proposed mechanism for copper-amine-wood interactions at carboxylic acids (above)

and phenolic hydroxyl groups (below) ......................................................................................... 39

Figure 3.5 Comparison of Cu adsorption for wood treated with Cu-Mea (1:70 molar ratio) at

different pHs and then retreated with Cu-Mea at pH 6.8.............................................................. 40

xi

Figure 3.6 Cu+ content of Cu-Mea (1:70 molar ratio) treated wood dust at different pHs........... 41

Figure 3.7 Chemisorption of Cu (a) and Dea (b) from different ratios of Cu-Dea in red pine dust

....................................................................................................................................................... 43

Figure 3.8 Chemisorption of Cu (a) and Tea (b) from different ratios of Cu-Tea in red pine dust

....................................................................................................................................................... 44

Figure 3.9 Chemisorption of Cu and En in red pine dust from Cu-En solution ........................... 46

Figure 3.10 Chemisorption of Cu and NH3 in red pine dust from Cu-Am solution ..................... 48

Figure 4.1 Monitoring Cu stabilization by expressing solution from Cu-Mea treated wood ...... 53

Figure 4.2 In vitro Cu precipitation of commercial Cu-Mea at different pH (22 ºC): (a) effect of

concentration on precipitation pH; (b) HNO3 consumption for the Cu precipitation; (c) ionic

concentration effect on 0.67% Cu-Mea precipitation. .................................................................. 55

Figure 4.3 (a) X-ray diffraction pattern of precipitated copper in Cu-mea solution at pH 6.5: (b)

standard stick pattern for malachite (CuCO3·Cu (OH)2) ............................................................. 56

Figure 4.4 Effect of concentration on in vitro Cu precipitation of formulated Cu(acetate)-Mea at

different pH (22 ºC) ...................................................................................................................... 57

Figure 4.5 (a) X-ray diffraction pattern of precipitated copper in formulated Cu-mea solution; (b)

standard stick XRD pattern for Cu(OH)2 ..................................................................................... 58

Figure 4.6 Change in pH of Cu-Mea solution expressed (a,d), and its relevance to Cu fixation

amount (b,e) and percent fixation (c,f) in wood blocks conditioned at room temperature (a,b,c)

and at 50 ºC (d,e,f) without drying................................................................................................ 59

Figure 4.7 (a) X-ray diffraction pattern of precipitated copper in dried Cu-Mea solution; (b)

standard stick pattern for Cu2O .................................................................................................... 62

Figure 4.8 Cation (Na+)

2) exchange capacity of Cu-Mea treated wood and control wood at pH 9

after 6 week conditioning.............................................................................................................. 63

xii

Figure 4.9 (a) Copper retention (Cu(I) and Cu(II)) of Cu-Mea treated wood and (b) total copper

before and after accelerated leaching............................................................................................ 66

Figure 5.1 Variation of zeta-potential of red pine sawdust with the concentration of ADBAC in

distilled water at 22°C (pH 5.0-7.3) (a) and Equilibrium adsorption of ADBAC on red pine

sawdust solution controlled to pH 9.5 by Mea (b)........................................................................ 74

Figure 5.2 Schematic representation of ADBAC adsorption into wood. (a) monomer (b) two-

dimensional aggregation forms and (c) three-dimensional aggregation with the counter-ion (ion

pair) forms of adsorption depending on the concentration and (d) after leaching........................ 75

Figure 5.3 Equilibrium adsorption of ADBAC into red pine sawdust in pH 8.0 phosphate buffer

solution.......................................................................................................................................... 77

Figure 5.4 Langmuir, BET, and Freundlich model fitting of ADBAC adsorption in wood before

(a) and after leaching (b) (with logarithm Ce scale) ..................................................................... 79

Figure 6.1 Effect of ionic strength on ADBAC adsorption on wood dust.................................... 85

Figure 6.2 Effect of Mea/HCl on ADBAC after-leaching adsorption on wood dust ................... 86

Figure 6.3 Effect of competition between Quat (ADBAC) and Cu on equilibrium adsorption of

quat at pH 9.5 (a) before leaching (b) after leaching. ................................................................... 87

Figure 6.4 Quat (ADBAC) and Mea adsorption on red pine dust from ADBAC-Mea solution (a)

ADBAC adsorption before leaching and (b) ADBAC and ADBAC + Mea adsorption after

leaching compared to the CEC of red pine ................................................................................... 89

Figure 6.5 Retention gradients of Cu, ADBAC, and Cl of ACQ (2%) in red pine block (30 60

100 mm) which were adsorbed through longitudinal direction (a, c, e) and radial direction (b,

d, f) ................................................................................................................................................ 91

Figure 7.1 Conceptual changes in the proportions of precipitated Cu and soluble Cu in ACQ

treated wood by reducing free Mea .............................................................................................. 97

xiii

Figure 7.2 Proposed scheme of wood-1,3-diaminopropane copper complex structure (Klemm et

al. 1998) ........................................................................................................................................ 98

xiv

List of Appendices

Appendix 2.1 Cation exchange capacity of red pine dust for sodium (Na+) from NaOH/HNO3

solutions (0.05 M) depending on time. ....................................................................................... 112

Appendix 3.1 Change in lignin content (%) after treating with different Cu-amine solutions... 113

Appendix 3.2 Changes in wood components after treating with Cu-En and Cu-Am................. 113

Appendix 4.1 Copper precipitation on 0.2% and 0.67% Cu-Mea treated wood: (a) transverse

surfaces and (b) radial surfaces................................................................................................... 114

Appendix 5.1 Langmuir, BET, and Freundlich model fitting of ADBAC adsorption in wood

before (a) and after leaching (b) (with normal Ce scale) ............................................................ 115

Appendix 6.1 Langmuir, BET, and Freundlich model fitting of ADBAC adsorption isotherm in

Cu-Mea media before (a) and after leaching (b)......................................................................... 116

Appendix 6.2 Adsorption constants estimated from simulations with Langmuir, BET, and

Freundlich models for isotherms of ADBAC in Cu-Mea media using wood dust as an absorbent

..................................................................................................................................................... 116

xv

List of Abbreviations

AAS atomic absorption spectroscopy

ACQ alkaline copper quat

ADBAC alkyldimethylbenzylammonium chloride

Am ammonia

AWPA American wood protection association

CA copper azole

CCA chromate copper arsenate

CDDC copper dimethyldithiocarbamate

CEC cation exchange capacity

CMC critical micelle concentration

CSA Canada standard association

Cu-AEEA copper aminoethylethanolamine

Cu-Am copper ammonia

Cu-Dea copper diethanolamine

Cu-En copper ethylenediamine

Cu-HDO bis-(N-cyclohexyldiazeniumdioxy)-copper

Cu-Mea copper monoethanolamine

Cu-Polyim copper polyethyleneimine

Cu-Tea copper triethanolamine

CX copper HDO

DDAC didecyldimethylammonium chloride

Dea diethanolamine

EN ethylenediamine

EPR electron paramagnetic resonance spectroscopy

ESR electron spin resonance spectroscopy

FSP fiber saturation point

FTIR fourier transform infrared spectroscopy

HDTMA hexadecyltrimethylammonium

ICP-AES inductively coupled plasma atomic emission spectroscopy

xvi

KBr potassium bromide

Mea monoethanolamine

Mea-H deprotonated (hydroxyl group) monoethanolamine

MeaH+ protonated monoethanolamine

Quat quaternary ammonium

Tea triethanolamine

UV VIS ultraviolet visible photo spectroscopy

XPS X-ray photoelectronic spectroscopy

XRD X-ray diffraction

XRF X-ray fluorescence spectroscopy

1

Chapter 1 Introduction and background

1.1 Introduction

Wood has long been one of the most important materials for various purposes but it is prone to

biological attack by fungi, harmful insects, or marine borers. Therefore, suitable treatment or

maintenance is necessary to protect wood, especially when it is used in environments where it is

vulnerable to these organisms. There are records of wood treatment with olive oil for bridge

wood in ancient Greece and with tar for wooden ship hulls by Romans, and treatment of wood

has been carried out for almost as long as the use of wood itself (Richardson 1978). Commercial

wood pressure treatment commenced in the latter half of the 1800s with the protection of railroad

crossties using creosote, and many different types of wood preservatives were introduced,

including chromated copper arsenate (CCA) which was the most widely used wood preservative

since the 1970s (Richardson 1978; U.S. EPA 2008). By January 2004, when CCA was

voluntarily withdrawn for residential applications, more than 2 million cubic meters of wood

were treated with CCA annually in Canada of which more than 80% was for residential products

(Stephens 1999). After voluntary withdrawal of CCA for residential applications in 2004 in USA

and Canada because of environmental and human health concerns, copper amine based

preservatives have been considered potential replacements for CCA (AWPA 2006b; Humar et al.

2007a). These include alkaline copper quat (ACQ), copper dimethyldithiocarbamate (CDDC),

copper HDO (CX), and copper azole (CA). Alkaline copper systems were initially based on

ammonia as solvent for copper (e.g. ammonical copper arsenate and early ACQ formulations

such as ACQ types A and B) (AWPA 2006b). Replacement of the ammonia solvent by

monoethanolamine in later formulations (ACQ-C, ACQ-D, CA and CX) allowed easier treating

practices, easier handling, lower costs, and uniform color of treatment (Jiang 2000). All copper

amine based systems were developed as safer and more environmentally friendly alternatives to

CCA and have played their part as mainstream wood preservatives in different countries.

However, particulate (micronized or dispersed) copper preservatives have recently been

introduced and their market share is increasing in the USA and Europe (Freeman and Mclntyre

2008).

2

The development of wood preservatives has been achieved with many research studies

investigating how different inorganic metals and co-biocides become effective as wood

preservatives and the bonding or fixation that occurs between the preservative components and

wood (Thomason and Pasek 1997). Most preservative formulations which have been developed

since the latter part of the 1970s use copper as the primary biocide component because of its

excellent fungicidal properties (Jiang 2000; Freeman and Mclntyre 2008). Important factors for

an effective wood preservative include its biological efficacy and its stability in wood to prevent

leaching. Other factors such as solubility and stability in solution, toxicity and corrosiveness

should also be considered. The major concern with copper amine preservatives is their relatively

higher copper leaching during service. For example, Edlund and Jermer (2002) reported that

copper depletion from stakes after 5 years in field test was 19% for ACQ, 15% for CBA, and 14-

30% for CX while it was 0-10% for CCA. Board samples exposed to natural rain for 2 years

showed significant copper loss about 32% and 29% for ACQ and CX treated boards. Hingston

(2002) showed 10 times higher copper leaching from Cu-Mea treated wood than CCA in an

aquatic environment, and suggested Cu-Mea was not suitable for aquatic uses and that the

industry had to find a way to fix copper better in these systems. Indeed, Cu-Mea based

preservatives are not used extensively in aquatic environments, and this limits their application

as a result of leaching issues. Therefore, the chemical leaching, especially from copper amine

treated wood in service is a major concern in the wood preservation industry and it should be

resolved to achieve environmentally friendly and long-lasting wood material.

The leachability issues of preservative components are related to their fixation in wood and

therefore, extensive studies were focused on CCA and its fixation and leachability of its

components (Pizzi 1982; Ruddick et al. 1993; Cooper and Kamdem 1997). Copper fixation of

CCA in wood can be summarized as physical or chemical adsorption in wood by forming water

insoluble compounds such as Cu-chromates or Cu-arsenates and by generating new ligands or

new sites in wood due to the chromium oxidation effect (Craciun et al. 2009). Significant effort

was also devoted toward addressing the copper fixation issue of copper amine preservatives

(Thomason and Pasek 1997; Zhang and Kamdem 2000b,c; Ruddick et al. 2001; Craciun 2009).

The copper fixation mechanism of copper amine systems is very different compared to that of

CCA, resulting in the following differences in stabilization and leaching characteristics

compared to CCA: i) CCA components stabilize to a very high degree regardless of the

3

preservative retention; copper amine systems stabilize to a lower degree, especially at high

preservative retentions ii) Drying during stabilization slows CCA fixation significantly, while

drying accelerates copper stabilization in copper amine systems. iii) Long term leaching

properties are quite different; CCA releases small amounts of copper over many years while

copper amine systems have high initial rates of copper emission, which level off after 1-2 years

of exposure (Cooper and Ung 1993; Waldron et al. 2005; Tascioglu et al. 2005). Despite its good

performance in general, the reported poor stabilization of copper when wood is treated to high

retentions, limits its application for high performance uses such as treated poles and pilings

where high retentions are needed. Also related to the stabilization of preservative components is

the tendency of the treatments to corrode metal fasteners due to free unreacted copper, amine and

chloride and the propensity of treated wood to develop mould, possibly due to the nitrogen in the

monoethanolamine co-solvent. Understanding copper amine fixation mechanisms in wood will

provide solutions to overcome its disadvantages and optimize its systems for better efficacy and

less impact on the environment. However, the fixation or stabilization mechanisms of copper

amine preservatives are still not well understood due to the difficulty in studying in situ, the

complex reactions between the preservative ingredients and wood components. Moreover, most

studies have focused on copper, and there has been little study of co-biocide and co-solvent

effects. According to Craciun et al. (2009) “There are still many uncertainties and grey areas for

the newer systems based on copper.”

The objective of this study was to elucidate the fixation mechanism of copper amine based wood

preservative with focus on alkaline copper quat (ACQ). One of the difficulties with the study of

in situ copper fixation comes from the complicated reactions in wood. For example, when

categorizing copper stabilization in wood into chemisorption and physical precipitation (Rennie

et al. 1987; Cooper 1991; Zhang and Kamdem 2000a,b,c; Ruddick et al. 2001; Jiang and

Ruddick 2004), the reactions occur concurrently and are interrelated, which makes it more

difficult to interpret the results. Therefore, an attempt was made to divide the involved reactions

into copper chemisorption, copper physisorption, and quaternary ammonium (quat) adsorption to

ensure a clear understanding of their fixation mechanisms and factors influencing their

adsorption.

Most chapters of the thesis are based on already published peer reviewed journal papers. In

Chapter 2, copper monoethanolamine (Mea) chemisorption in wood is investigated compared to

4

the cation exchange capacity (CEC) of wood. Amine ligand effects on copper chemisorption are

explored in Chapter 3. The 4th

chapter covers copper physisorption in wood. Chapter 5 and 6

investigate the adsorption mechanisms of the co-biocide, quaternary ammonium compound

(quat) in wood and several important factors limiting its adsorption. Finally, comprehensive

results and implications about them are discussed in Chapter 7.

1.2 Literature review

1.2.1 Copper amine fixation in wood

1.2.1.1 Copper amine complex in solution

Like other chemical reactions, copper(II) tends to be stabilized by reacting with other chemicals.

In water, copper(II) ions can be stabilized by forming the hexaaquacopper(II) ions, [Cu(H2O)6]2+

through coordinate bonding with six water molecules. When amine or ammonia solution is added

in this system, the amine/ammonia replaces four water molecules to form more stable copper

complexes; this is known as a ligand exchange reaction (Eq. 1.1).

[Cu(H2O)6]2+

+ 4NH3 [Cu(NH3)4(H2O)2]2+

+ 4H2O (Eq. 1.1)

Likewise, [Cu(NH3)4(H2O)2]2+

can be further reacted to form a more stable complex with other

materials like wood through acid-base reaction or the ligand exchange reaction. The stability of a

copper complex formed in water is different depending on the ligand and is also affected by

other factors such as pH and ionic strength (Housecroft and Sharpe 2005). Copper coordination

features in amine/ammonia solution appear to be strongly related to its fixation chemistry in

wood. Several amines such as monoethanolamine (Mea), ethylenediamine (En), and ammonia

(Am) have been considered as co-solvents for better copper solubility and fixation. Sometimes,

copper fixation with different amine or ammonia ligands in wood is considered to be by the same

mechanism. However, different copper coordination characteristics in different solvents must

result in different fixation reactions and different copper stabilization in wood.

Copper ions form various types of complexes with ethanolamines in solution, depending on the

solution pH (Hancock 1981; Tauler and Casassas 1986; Casassas et al. 1989). Cupric ions react

with Mea to form stable five-membered chelate rings in solution (Fisher and Hall 1962; Davies

5

and Patel 1968; Tauler and Casassas 1986; Casassas et al. 1989). This chelation is formed by the

amine action as a bidentate ligand via the N and O atoms. The hydroxyl groups of the amines,

when coordinated to copper, become acid-like and dissociate to partially or fully neutralize the

copper charge resulting in divalent (Figure 1.1a), monovalent (Figure 1.1b) or neutral complexes

(Figure 1.1c) depending on the pH (Davies and Patel 1968; Tauler and Casassas 1986).

Considering that the pH of ACQ solution is about 9.5, neutral and monovalent Cu-amine

complexes are prevalent in ACQ solution and divalent complexes can also exist as the solution

pH decreases with solution penetration into wood. Therefore, the questions for understanding

copper chemisorption mechanism of Cu-Mea system can be summarized as follows: what are the

active copper species among the different Cu-Mea complexes in solution that react with wood,

where does the active copper react in wood and how does the copper react with wood?

OH

Cu

H2N

NH2

HO

2+

O

Cu

H2N

NH2

HO

+

O

Cu

H2N

NH2

O0

(a) (b) (c)

Figure 1.1 Different charge states of copper amine complexes in solution (Zhang and Kamdem

2000b). Note: Divalent copper ion stay as an octahedral structure with six coordination number,

but two water molecules perpendicularly ligated to Cu is not shown here for simplicity.

Ethylenediamine (En) also acts as a bidentate ligand when it forms Cu-En complexes in solution

via the two N atoms, forming stable five-membered chelate rings. Ethylenediamine is in the

neutral form when complexing copper, even at acidic pH values unlike ethanolamine (Aksu and

Doyle 2002). Thereby, although copper forms Cu-En complexes with different ratios of En such

as CuEn2+

, Cu(En)22+

and Cu(En)32+

, the overall valency states of the complexes are always +2.

Considering the solution pH and Cu/En ratio of Cu-amine preservatives, Cu(En)22+

formed at pH

5 to 13 is regarded as a main active species (Figure 1.2a) (Klemm et al. 1998; Aksu and Doyle

2002).

6

Ammonia is also an excellent Lewis base and combines with copper cation to form copper

ammonium complex with four ammonia molecules surrounding each Cu2+

ion (Figure 1.2b)

(Kotz et al. 2009). Unlike the Cu-ethanolamine and Cu-En, the complex ions are formed from

monodentate ligand. Because monodentate complexes are less stable than bidentate ligand

complexes in solution (Klemm et. al 1998), Cu-Am is expected to adsorb on wood more readily

than bidentate copper complexes. The copper can also coordinate with different amounts of

ammonia e.g., as Cu(NH3)2+

, Cu(NH3)22+

, Cu(NH3)32+

, and Cu(NH3)42+

(Stumm and Morgan

1995; Wang et al. 2006).

Figure 1.2 (a) Copper-ethylene diamine (Cu-En) complex and (b) copper-ammonium (Cu-Am)

complex. Note: two water molecules perpendicularly ligated to Cu is not shown for simplicity.

1.2.1.2 Fixation chemistry of copper amine in wood

Since copper combines with different amines to form different copper amine complexes in

solution, the fixation characteristics of copper amine complexes in wood are affected by the

nature of amine ligands. Nevertheless, many studies had some common results about copper

bonding sites in wood but had somewhat different results and interpretations for the different

copper ligand systems, which are discussed below.

Copper chemisorption in wood occurs by means of ion exchange or other chemical complex

reactions (Rennie et al. 1987; Cooper 1991; Thomason and Pasek 1997; Kamdem et al. 2001;

Ruddick et al. 2001). The bonding sites are weak acidic groups of wood components, e.g.,

carboxylic and phenolic hydroxyl groups, which dissociate according to the ambient pH. At pH

above 5, most of the carboxylic acid components of hemicelluloses and pectic substances are

7

dissociated and capable of adsorbing or complexing with copper. At pH over 8, the lignin

phenolic groups are significantly dissociated and are essentially completely dissociated at pH

10.5 and higher (Rennie et al. 1987; Cooper 1991; Kamdem et al. 2001). According to Pizzi

(1982), copper forms stable complexes with orthodihydroxy phenols and orthodihydroxylmethyl

phenols at pH > 7. Cooper (1991) showed that maximum ion exchange capacity is about 6.5 mg

copper per gram of wood (about 0.2 milliequivalents) at pH 10.5. Assuming the specific gravity

of wood is 0.4, the corresponding copper retention would be about 2.6kg/m3. At higher pH, the

cation exchange capacity increases further according to the dissociation of other weak acid

groups. Considering the recommended copper retention 2.1-5.1 kg/m3 (Copper basis) (AWPA

2006b), it is clear that copper fixation still relies on physical precipitation for high retention

applications. Lebow and Morrell (1995) showed that phenolic extractives in Douglas-fir

heartwood also provide reactive sites for copper in wood.

For copper amine based formulations, studies performed by Hughes et al. (1994) using electron

paramagnetic resonance spectroscopy (EPR) showed that copper was complexed with 3 or 4

nitrogen atoms, even after leaching. For nitrogen free systems, copper was complexed to four

oxygen atoms of wood and two oxygen atoms of water molecules in a distorted octahedral

arrangement. However, no conclusions were made about wood bonding sites involved. A model

for the fixation of copper from copper amine preservative was proposed by Ruddick et al. (2001).

They employed X-ray crystallography, FTIR and electron spin resonance (ESR) to resolve the

structure of vanillin, a lignin model compound, reacted with copper in an ethanolamine solution

and suggested that reaction between the lignin guaiacyl groups and copper ethanolamine solution

may take place to form a lignin-copper-ethanolamine complex. This finding is somewhat

different from that of Hughes et al. (1994) in that copper bonded with two oxygen atoms and one

nitrogen atom from the ethanolamine ligands, and one oxygen atom from the hydroxyl group in

vanillin to form square planer structure. Although this model provides insight into the copper

fixation mechanism, the model compound may not represent heterogeneous and more complex

wood substrates. Kamdem and Zhang (2000) showed that isolated holocellulose, lignin, and

xylan adsorbed significant copper while cellulose adsorbed very little. When treating with 0.5%

copper amine solution, copper uptake was less than 0.1% in cellulose, but oxidized cellulose

with high contents of carboxylic groups had 0.63% uptake.

8

Zhang and Kamdem (2000b) proposed a ligand exchange reaction and a complexation reaction

as copper fixation mechanisms. In a ligand exchange reaction, copper amine complexes

exchange deprotonated ligands with wood and release one or two amine molecules (Eqs. 1.2 and

1.3), which was suggested by Kamdem et al. (1996), Craciun et al. (1997) and Thomason and

Pasek (1997).

COO - + [Cu(Mea-H)(Mea-H)]

0 [COO

-(CuMea-H)

+]

0 + [Mea-H]

- (Eq. 1.2)

Ph-O - + [Cu(Mea-H)(Mea-H)]

0 ! [Ph-O

-(CuMea-H)

+] 0 + [Mea-H]

-

(Eq. 1.3)

Where, Mea-H indicates of deprotonation from OH group in Mea.

However, according to Klemm (1998), ethanolamine as a ligand to copper has very strong

binding of the deprotonated ligand to the central copper atom, which impedes subsequent ligand

exchange with acidic groups.

In another possible complexation reaction, wood acidic groups such as carboxylic and phenolic

groups may complex with the charged copper amine species to form a stable wood-copper-amine

complex (Eq. 1.4-1.7).

COO - + [Cu(Mea)(Mea-H)]

+ [COO

-(CuMeaMea-H)

+]

0 (Eq. 1.4)

Ph-O - + [Cu(Mea)(Mea-H)]

+ [Phenol-O

-(CuMeaMea-H)

+] 0

(Eq. 1.5)

2 COO - + [Cu(Mea)(Mea)]

2+ [(COO

-)2(CuMea2)

2+]

0 (Eq. 1.6)

2 Ph-O - + [Cu(Mea)(Mea)]

2+ [(Ph-O

-)2(CuMea2)

2+] 0

(Eq. 1.7)

The concept of this complexation reaction is likely that of cation exchange reaction. It is

noticeable that simply charged copper complex binds to one wood acidic site (Eq. 1.4 and 1.5)

while the 2+ charged complex requires two acidic sites for bonding reaction (Eq. 1.6 and 1.7). As

another feature for this proposed complexation reaction, copper to amine molar ratio reacting

with wood acidic sites should be 1:2 like the copper amine complex ratio in solution. This was

supported by the studies of Zhang and Kamdem (2000a) and Mazela et al. (2005) while different

ratios (1:3 or 1:4) were reported by Hughes et al. (1994). The structure of copper complexes in

9

copper amine treated wood samples was investigated by the application of EPR (Zhang and

Kamdem 2000a). The copper complexes in both treating solution and treated wood were in the

form CuN2O2, where copper was ligated with 2 nitrogens and 2 oxygens. This is in agreement

with the complexation reaction in that amine/Cu ratio of Cu complex fixed in wood is 2. The

stereo-structure of copper complexes in copper amine treated wood was either tetragonal-based

octahedral or square-based pyramidal along with two weak ligands (water) perpendicular to the

plane. Another EPR study by Mazela et al. (2005) indicated Cu(Mea)2O2 and Cu(Mea)2O

complexes with oxygen atoms from wood functional groups were formed.

In situ copper to amine molar ratios were determined after leaching copper amine treated wood

blocks (Jiang 2000; Lucas and Ruddick 2002). The fixed Mea to copper mole ratio in wood was

about 1.4:1 (Lucas and Ruddick 2002) and the amine to copper mole ratio in the treated wood

was about 1.2:1 when different types of amine were applied (Jiang 2000). These results were

different from the EPR studies discussed above. Accordingly, Lucas and Ruddick (2002) also

proposed that while copper amine compounds are formed in wood, simple copper complexes

without amine ligand are also present.

Thomason and Pasek (1997) proposed a reaction only between copper and carboxylic groups

without copper-lignin complexation, which contradicts other studies (Kamdem and Zhang 2000;

Ruddick et al. 2001). Southern yellow pine thermally modified at 200 ºC to degrade carboxylic

groups did not retain any adsorbed copper after leaching. The point that lignin does not totally

degrade at the high temperature condition but could not adsorb any copper, led them to the

conclusion that copper is bound only to the carboxylic groups contained within the hemicellulose

structure. However, thermally modified wood can cause other chemical changes besides the

degradation of carboxylic acid groups, such as increasing soluble lignin and extract content

which could react with Cu but are easily leachable (Windeisen et al. 2007). Thermal treatment

also can alter the permeability of wood.

Hoffmann et al. (2003) proposed that copper amine complexes are attached to cellulose

molecules of wood. From the results obtained by EPR study, Cu(Mea)2(H2O)2 was present in Cu-

Mea solution and the addition of alkyldimethylbenzylammonium chloride (ADBAC) resulted in

an exchange of water for nitrogen in the ADBAC molecules forming Cu(Mea)2(ADBAC)2. In

10

the treated wood, Cu(Mea)2 from Cu(Mea)2(H2O)2 was thought to be attached to the OH groups

of cellulose and Cu(Mea)2(ADBAC)2 complex could also be stabilized in wood by ion exchange.

For the copper ammonia system, it has been proposed that water insoluble copper salts are

precipitated due to evaporation of ammonia (Hartford 1972). The loss of ammonia from the

tetra-ammonia-copper present in wood is rapid and this initiates the copper fixation. With the

ammonia evaporation, there is a stepwise water substitution described by the reaction sequence

shown below (Jiang 2000).

Xie et al. (1995) proposed a fixation model of ammoniacal copper in wood and suggested that

ammoniacal copper can form stable copper-nitrogen-lignin complexes through reaction with the

guaiacyl group in lignin. Vanillin, a lignin model compound was reacted with copper ammonia

and the precipitate was characterized as [Cu(vanillin)2(NH3)2] by spectroscopic study.

Ethylenediamine (Cu-En) as a ligand was also explored using X-ray photoelectron spectroscopy

(XPS) and Fourier Transformed Infrared Spectroscopy (FTIR) by Jiang and Ruddick (1999).

FTIR results showed Cu-En complexes react with the carboxylic acid and phenolic groups in

wood to form stable complexes. XPS results supported the reaction of copper with guaiacyl

groups in lignin similar to that suggested for copper ammonia reactions.

Copper stabilization in wood through precipitation was studied, with particular focus on its

leachability (Jiang and Ruddick 1997, 2004). The leachability of copper carbonate artificially

precipitated in wood was compared with that of copper-diammine complexes formed in wood.

After several steps of severe water leaching, 64% of copper carbonate was resistant to leaching

and over 90% of copper-diammine complex resisted leaching whereas only 17% of copper

remained after leaching of copper sulphate treated wood. Jiang and Ruddick (2004) also

proposed that the fixed copper of Cu-Mea treated Scots pine blocks consisted of 35% in

precipitated form and 50% in chemisorbed form as Cu-Mea-wood complex. This was based on

-NH3[Cu(H2O)2(NH3)4]

2+ [Cu(H2O)3(NH3)3]2+ [Cu(H2O)4(NH3)2]

2+

[Cu(H2O)5(NH3)]2+ [Cu(H2O)6]

2+

-NH3

-NH3

-NH3

11

the observation that 85% of the copper remained after water leaching and 50% of the copper

remained after citrate buffer leaching (0.6g NaOH and 1.6g citric acid per liter of solution).

However, it is possible that the buffer solution dissolves both the precipitated copper, and the

chemically adsorbed copper. Copper chemisorption or ion-exchange processes are equilibrium

processes (Craciun et al. 2009) and therefore, copper chemisorption can be re-equilibrated to that

at buffer solution pH (estimation of pH 5) and the buffer leaching process can also leach

chemically adsorbed copper.

Cuprous (Cu(I)) and cupric (Cu(II)) types of copper as well as a copper-amine complexes are

catalysts that are widely used in a broad range of applications to form phenoxyl radicals, and

oxidize alcohols or hydroquinone under certain solvent environments with atmospheric oxygen

(Chaudhuri et al. 1999; Kumbhar and Kishore 2003; Li and Trush 1993; Marko et al. 1996;

Velusamy et al. 2006). Copper also has a catalytic effect in the reaction of isolated cellulose with

oxygen at low alkalinities. The rate of alkaline autoxidation of cellulose, resulting in a decrease

of the weight-average degrees of polymerization, was 4-10 times higher in the presence of

copper even at low alkalinity conditions. The rate of autoxidation was dependent on the

availability of oxygen, concentration of NaOH and time, but independent of the concentration of

copper as a catalyst (Kimura and Kubo 1951). However, these reactions occurred in vitro

between wood components and copper/copper amine and it does not seem to be easy for wood

components in situ to be oxidized because of the entangled intra-bonding structure of wood

components. Cellulose contains alcoholic hydroxyl groups, which are ionized only in very strong

base and so, the amount of copper absorbed from ACQ solution by cellulose was almost

negligible (Kamdem and Zhang 2000).

Humar et al. (2007b) proposed that at high pH and concentration, Mea of Cu-Mea system could

depolymerize lignin and increase copper leaching. It was also reported that rusting iron can cause

decomposition of all wood constituents (Emery and Schroeder 1974). In that study, cellulose was

oxidized in the presence of rusting iron to form an oxycellulose and direct depolymerization of

cellulose and xylan also occurred as the isolated cellulose was oxidized. This opens the

possibility that copper can also catalytically oxidize the wood components in situ in alkaline

conditions. If cellulose could be oxidized in situ by copper amine, hemicelluloses and lignin

would be degraded in advance at the same condition because of their more vulnerable nature in

alkaline conditions (Fengel and Wegener 1984).

12

If the same oxidation reactions occur during copper amine treatment with wood and the reactions

increase with time as the studies of Emery and Schroeder (1974) and Michie and Neale (1959)

indicate, this process would not be limited to the preservative treatment process, but would

continue during the period of fixation and even in service under the appropriate humidity and

temperature environment. Tascioglu et al. (2008) explored the possibility that copper amine

could induce oxidation of wood to create additional cation exchange sites with copper reduction

from Cu(II) to Cu(I) during elevated temperature post-conditioning of ACQ treated wood. If

these reactions occur, they could affect copper fixation in three ways; i) by creating more binding

sites for copper, resulting in higher copper fixation; ii) if divalent copper needs two wood sites

for bonding, monovalent copper might need only one site, resulting in higher copper fixation in

wood; iii) if un-reacted free copper (Cu(II)) converts to less soluble cuprous form (Cu(I)),

potentially leachable copper could be reduced, resulting in less copper leaching. Any of these

would enhance copper fixation either through chemisorption or physisorption in wood. However,

Tascioglu et al. (2008) could not see any significant difference in the amount of copper leaching

between wood conditioned at high temperature and at ambient temperature.

1.2.2 Factors affecting copper stabilization in wood

1.2.2.1 Formulation factors

The rate and extent of copper and Mea adsorption were highly dependent on the solution

concentration; copper stabilized much faster at lower ACQ concentrations (Tascioglu et al. 2005;

Ung and Cooper 2005). Different copper sources and different amine ligands were compared for

leaching characteristics by Zhang and Kamdem (2000b). Different copper sources affected the

leachability of copper and the amount of copper leaching decreased as the molecular weight of

amine ligands increased. Copper concentration and initial amine to copper ratio in the presence

of monoethanolamine also affected the copper fixation (Lucas and Ruddick 2002). High initial

copper concentration and high initial amine (Mea) to copper ratio decreased copper fixation. A

large excess of amine interfered with copper fixation in wood. Thomason and Pasek (1997) also

proposed that protonated amine competes with copper for the same anionic sites in the wood and

copper adsorption could decrease as copper amine concentrations increase. However, for

ethylenediamine, the initial amine to copper mole ratio did not affect copper fixation (Lucas and

Ruddick 2002) and the addition of up to 1 percent ammonia in Cu-Mea treated solution did not

13

significantly influence the copper retention or the amount of copper leached (Cui et al. 2005).

Jiang and Ruddick (2004) explored the leaching resistance of copper as affected by amine

species. Copper monoethanolamine (Cu-Mea), copper ethylenediamine (Cu-En), copper

aminoethylethanolamine (Cu-AEEA) and copper polyethylenimine (Cu-Polyim: mixed with 20%

of monoethanolamine) were compared and among them, Cu-Mea and Cu-Polyim showed the

greatest water leaching resistance with approximately 91% and 83% of the copper remaining,

respectively.

1.2.2.2 Factors of post-treatment conditioning

To enhance the copper stabilization in wood, various conditions have been investigated as post-

treatment conditioning. Many studies on CCA fixation showed that copper fixation rates

increased with temperature (Peek and Willeitner 1988; Alexander and Cooper 1993; Boone et al.

1995). Especially, non-drying condition at high temperature can increase the fixation rate and

reduce copper losses (Avramidis and Ruddick 1989; Chen et al. 1994). Copper fixation rate of

ACQ treated wood is also affected by temperature (Alma and Kara 2008).

However, the application of high temperature treatment can result in problems such as reduction

of mechanical properties and bleeding of resin (Yu et al. 2008). One of the major issues with

application of high temperature to ACQ treated wood is copper reduction. The cuprous ion

(Cu(I)) disproportionates rapidly in aqueous solution to form copper(II) and copper(0). This

change is favored by the high hydration energy of Cu2+

compared with Cu+. Cupric ions are the

most common form of copper in water and are mildly hydrolyzed in near-neutral solution,

forming the dimer Cu2(OH)22+

(Georgopoulos et al. 2001). Nevertheless, some cupric copper of

copper amine treated wood may convert to cuprous form during high temperature conditioning.

Ruddick (2003) reported that significant Cu(I) was observed in alkaline copper treated wood at

temperatures greater than 50 ºC. Zhang and Kamdem (2000d) reported that about 50% of Cu(II)

in copper naphthenate (Cu-N) treated wood was reduced to Cu(I) after post treatment steaming at

115 ºC, whereas no cuprous oxide was formed in non-steamed Cu-N treated wood. Because

Cu(I) has low efficacy against decay organisms (Cui 1999; Barnes et al. 2000), it was

recommended that copper amine treated wood not be exposed to temperatures above 50 ºC

during post treatment conditioning (Ruddick 2003).

14

Tascioglu et al. (2008) also investigated the effect of elevated temperature, ACQ retention and

wet and dry condition on the conversion of divalent copper (Cu(II)) to monovalent copper

(Cu(I)). They showed that the copper conversion ratio increased with increasing temperature

with relatively higher conversion at lower ACQ retentions. The conversion at moderate

temperature (50 ºC and 75 ºC) was higher in dry conditions while the conversion at high

temperature (105 ºC) was higher in wet conditions. Exposure of treated wood to a leaching

procedure which mimicked natural exposure, resulted in a significant component of the Cu(I) in

treated wood being oxidized back to Cu(II).

Various post treatment conditions for accelerating copper fixation were investigated by Yu et al.

(2008). They found that hot air treatment (70 ºC) combining high relative humidity (80 %) and

air circulation accelerated copper fixation with less than 10% copper conversion whereas

steaming and microwave treatments did not enhance copper fixation. However, there are

conflicting results that steaming treatment accelerated copper fixation within 30 min. with

negligible copper reduction (Kang et al. 2008). Tascioglu et al. (2005) and Ung et al. (2005)

observed that copper stabilized much faster when conditioned at 50 ºC than 22 ºC. Delayed

drying and CO2 application was investigated by Tascioglu et al. (2009). Delayed drying resulted

in a higher degree of copper fixation only at higher temperature, 50 ºC, while this effect was not

observed at 22 ºC. Pressurizing with CO2 after ACQ treatment reduced pH of the solution in

wood and resulted in rapid fixation of the copper. However, this effect was temporary as CO2

evaporated, indicating pressurized CO2 did not form insoluble copper carbonate.

1.2.3 Quaternary ammonium adsorption in wood

Quaternary ammonium compounds (quats) are surfactants with many applications in the textile,

agriculture, dye, cosmetics, chemical and mining industries (Ersoy and Celik 2003; Schaeufele

1985). The study of quat as a wood preservative was initiated by Thompson (1965) and Oertel

(1965) and it was commercialized as a wood preservative in New Zealand in 1978 (Jin and

Preston 1991). Specific quats, such as alkyldimethylbenzylammonium chloride (ADBAC), are

used as co-biocides in alkaline copper quat (ACQ) wood preservatives to provide supplemental

biological resistance against copper tolerant fungi and insects. ACQ Type C contains 67% CuO

and 33 % ADBAC with Mea and water co-solvents (CSA 2008).

15

1.2.3.1 Quat adsorption mechanism in wood

Quat cation exchange on carboxylic and phenolic hydroxyl groups may be the dominant

mechanism, while electrostatic and hydrophobic effects may also play roles in quat adsorption

onto cellulose (Preston et al. 1987; Jin and Preston 1991; Loubinoux and Malek 1992; Doyle and

Ruddick 1994; Zabielska-Matejuk 2005). Jin and Preston (1991) found that the quat,

didecyldimethylammonium chloride (DDAC) adsorbed onto softwood lignin with much lower

amounts adsorbed onto isolated cellulose. The adsorption was greater as pH increased, which

was more apparent with lignin than with cellulose. The amount of DDAC adsorbed on wood

meal at equilibrium was 0.38, 0.37, and 0.26 mmol/g wood, respectively when treating with 0.75,

0.50, and 0.25 % DDAC at pH 11.5. If the DDAC adsorption mechanism is only defined as a

cation exchange reaction, the amount of DDAC adsorbed on wood meal should not increase by

the amounts observed (from 0.26 to 0.38) at the same pH, because cation exchange sites are

limited in wood. This implies other important reactions are involved. Preston et al. (1987)

proposed that in addition to ion exchange reaction, ion pair and hydrophobic effects contribute to

quat adsorption in wood. According to their results, 25-40% of quat was leachable when treated

with 1% quat solution. Loubinoux et al. (1992) reported that the quat retention in wood increased

with the lipophilicity of quat and water leaching of the quat increased with increased adsorption.

According to Loubinoux and Malek (1992), the nature of the anion was not important in the

adsorption of quat and a higher proportion of ammonium cations compared to halide anions were

bound to wood. This anion adsorption can be explained by ion pair absorption, i.e., cationic

surfactant adsorption followed by counter-anion adsorption near the critical micelle

concentration (CMC) of the surfactant (Sexsmith and White 1959). Loubinoux et al. (1992)

showed that maximum quat adsorption was reached after 48h, regardless of wood species. Wood

extractives also play an important role in its adsorption. Maximum adsorption on beech sawdust

was about 0.48 mmol g-1

wood for ADBAC equilibrium concentration near 1%.

Because the potential reaction sites for copper and quat are the same, the two components might

be in competition for the same sites in wood. Indeed, Tascioglu et al. (2005) observed higher

copper adsorption from Cu-Mea solution than from corresponding ACQ (Cu-Mea + quat)

solution and proposed that quat competes with copper for the same reaction sites. They also

observed that quat adsorbed quickly and to a higher degree than copper. Cooper and Ung (2009)

reported that preferential adsorption of quat near the wood surface resulted in a steep

16

concentration gradient and higher quat rate of leaching than copper after 330 days of exposure to

natural weathering.

1.2.3.2 Characteristics of cationic surfactants related to their adsorption

ADBAC used in wood preservatives is a cationic substance that contains a hydrophobic (non-

polar) hydrocarbon chain (C12 to C18) (Figure 1.3) and a hydrophilic or polar group associated

with the positively charged nitrogen group. This hydrophilic group makes the surfactant soluble

in polar solvents such as water as a result of hydrogen bonding (Farn 2006). Due to the positive

charge of the nitrogen head group in quat, at low concentration, this cationic surfactant strongly

adsorbs onto negatively charged surfaces such as wood acidic groups.

When a cationic surfactant is added to water, surfactant molecules move towards the interface

(water-air) and the hydrophobic tail of the molecule either lies flat on the surface or aligns itself

to air while the hydrophilic head orientates itself towards the bulk water. Surfactants at low

concentration in water exist as monomers. These monomers pack together at the interface, form a

monolayer and contribute to lowering surface and interfacial tension. As the surfactant

concentration increases, the available area at the surface for surfactant molecules diminishes and

surfactant monomers start accumulating in the solution. However, the hydrophobic tails of the

surfactant molecules have extremely low solubility in water. Hence, the hydrophobic effect will

drive surfactant monomers to form closed, self-assembled aggregates (micelles) in which the

hydrophobic tails are shielded from water while the hydrophilic heads face water. This occurs

above a certain aggregate concentration (critical micelle concentration, CMC) (Shaw 1994; Farn

2006). CMC indicates the concentration at which the surface or interface is saturated with

surfactant monomers. Accordingly, below the CMC, the surface or interfacial tension decreases

dramatically with an increase of surfactant concentration in the water, while there are no

significant changes in the surfactant properties, including surface tension of the solution, above

the CMC. Therefore, the CMC can be determined by measuring the changes in physical

properties such as electrical conductivity, surface tension, and interfacial tension of the surfactant

solutions (Fuguet et al. 2005; Farn 2006; Jiang et al. 2003; Rahman 1983; Shaw 1992).

According to Farn (2006), the CMC decreases with temperature to a minimum which appears to

be at about 25ºC and then increases with further increase in temperature. This behavior is related

17

to two competitive effects. The initial decrease of the CMC with temperature is caused by

reduced hydration of the hydrophilic group, due to the lower probability of hydrogen bond

formation at higher temperatures. The increase in temperature also causes increased breakdown

of the structured water surrounding the hydrophobic group, and this retards micelle formation

(Castedo et al. 1997; Chen et al. 1998). The CMC of surfactant can be decreased or increased by

various factors (Farn 2006):

Decreasing factors include: an increase in the number of carbon atoms (hydrophobic

hydrocarbon chain length), the existence of polyoxypropylene group, fluorocarbon structure,

an increased degree of binding of the counter-ions, the addition of electrolyte, the existence

of polar organic compounds (such as alcohols and amides), the addition of xylose and

fructose.

Increasing factors include: branched hydrophobic structure, double bonds between carbon

atoms, polar groups (O or OH) in hydrophobic tail, strongly ionized polar groups (sulphates

and quaternaries), hydrophilic groups placed in the surfactant molecule center, trifluoro-

methyl group, an increase in the effective size of the hydrophilic head, an increase in the pH

of weak acids, addition of urea, formamide, guanidinium salts, dioxane, ethylene glycol and

water soluble esters.

RR=CnH2n+1

n=12, 14, 16, 18

Cl_

N+ R

R=CnH2n+1

n=12, 14, 16, 18

Cl_

N+

Figure 1.3 Chemical structure of alkyldimethylbenzylammonium chloride (ADBAC) used as

wood preservative.

For the adsorption of cationic surfactants at the solid-liquid interface, it is generally accepted that

surfactant ions are adsorbed as individual ions. Once the adsorbed ions reach a certain critical

concentration, they begin to associate into two-dimensional patches (hemimicelle or admicelle)

or three-dimensional patches (micelle or admicelle) of ions in much the same way as they

associate into three-dimensional aggregates to form micelles in bulk solution (Garcia-Prieto et al.

18

2007; Gu and Rupprecht 1990; Harwell et al. 1985; Somasundaran et al. 1964). Therefore, the

adsorption between surface-active species and the solid surface at a low concentration can be

derived by electrostatic interaction, van der Waals interaction, or ion exchange reaction and the

surfactants are also adsorbed at higher concentration through the hydrophobic interaction

between the hydrophobic tails of the surfactant adsorbed on the hydrophilic solid surface and the

hydrophobic tails of the surfactant from the liquid phase (Biggs et al. 2007; Gu and Rupprecht

1990; Huang et al. 2006; Paria and Khilar 2004; Somasundaran et al. 1964; Sexsmith and White

1959). Measurement of zeta potential is an excellent way to observe and describe surface

reactions at a hydrophilic solid–water interface. Therefore zeta potential data have been used to

explain the adsorption mechanisms of ionic surfactants (Ersoy and Celik 2003; Paria and Khilar

2004; Somasundaran et al. 1964).

Many factors can affect the adsorption of quat into wood. The concentration of quat is the most

important factor for the size (or aggregation number) and shape of micelles (Chandar et al. 1987;

Gu and Rupprecht 1990; Paria and Khilar 2004; Tummino and Gafni 1993) and therefore, will

also affect quat adsorption in wood. pH determines both the cation exchange capacity of wood

(Cooper 1991), and the CMC of quat because hydrogen ions may decrease the electrostatic

repulsion of the hydrophilic charge heads, by decreasing the charge density on the surface of the

micelles, thereby changing their stability (Oyanedel-Craver and Smith 2006; Rahman 1983).

Various additives such as amine and buffer components also decrease the CMC of quat (Fuguet

et al. 2005; Jiang et al. 2003). In addition, increasing temperature decreases the adsorption of

cationic surfactants on solid surfaces because the solubilities of ionic surfactants increase with

increased temperature (Farn 2006).

In summary, many studies have reached the same conclusion, that copper of copper amine

systems complexes with lignin, hemicellulose, and extractives and also forms insoluble copper

precipitates in wood. This fixation can be accelerated by different formulations and different

post-treatments. These different formulations and post-treatment conditions must be related to

copper fixation chemistry in wood, but no clear explanations have been provided. Examining the

reactions of copper with model compounds of wood components gave an indication of copper

fixation mechanisms, but it does not always give a true picture of the reactions between the

preservative and the heterogeneous wood substrate (Thomason and Pasek 1997; Jiang 2000).

Compared to copper, quat has received less attention as a research topic, since it is used as a co-

19

biocide. However, it is also important to understand how its aggregation feature affects ACQ

fixation and its distribution in wood. Basic studies about ACQ fixation chemistry and factors

influencing its fixation will provide a better understanding of the ACQ system and suggest

solutions to optimize fixation.

1.3 Objectives, Experimental approaches and Hypotheses

The objective of this research was to elucidate the fixation mechanism of copper amine wood

preservatives with particular focus on the ACQ system. To achieve this goal, adsorption studies

were performed for each ACQ component, copper, quat and Mea, by appropriate approaches to

investigate a number of research questions.

For copper chemisorption study, an excess amount of amine was used for formulating copper

amine solution to prevent copper precipitation and the relationship between cation exchange

capacity (CEC) of wood and copper chemisorption was explored. Copper is divalent in solution,

and also forms copper complexes with amine/ammonia with different valency states. If divalent

copper complexes bind with two wood sites, copper chemisorption might be half of monovalent

chemicals like Na+. The hypothesis in this chapter, therefore, is that “Copper complexes having

+2 charges will show only half of copper chemisorption in wood compared to monovalent

chemicals.” If this hypothesis is supported, the monovalent state of copper complexes formed in

amine solution will promote higher copper chemisorption in wood. In the case of triethanolamine

(Tea) as a ligand, the dimeric singly charged complex species (Cu2Tea2)+ present in Cu-Tea

solutions at pH 5.8-10 (Tauler and Casassas 1985), might increase copper chemisorption by

binding two equivalents of copper while consuming only one reaction site in wood. Through this

study, the following questions should be answered;

a) What is the CEC of wood as a function of pH?

b) How much divalent copper can be chemically adsorbed in wood?

c) What is the molar ratio of copper to amine involved in copper amine fixation?

d) How do different ligands affect copper chemisorption in wood?

20

For the study of copper physisorption, in vitro copper precipitation conditions were compared to

in situ conditions to investigate the potential for additional copper stabilization over wood’s

chemisorption capacity. Because copper can precipitate in wood only either by amine

evaporation or pH drop of the system to neutral pH, both possibilities were investigated.

Although additional copper adsorption over chemisorption can be achieved by insoluble copper

precipitation, it could also result from additional copper chemisorption, if copper amines can

oxidize some alcohol groups in wood as mentioned in the literature review. Therefore, copper

stabilization was investigated based on the further hypothesis that “Copper amine oxidization of

wood components results in two modes of copper stabilization: deposition of low solubility

copper and stabilization of additional copper to the new oxidized sites.” Through this study, the

following questions should be answered:

a) Under which conditions does insoluble copper form in wood?

b) What is the chemical form of insoluble copper formed in wood?

c) Is the CEC of wood increased after copper amine treatment?

Although quat has unique features as a surfactant that could affect its adsorption and distribution

in wood, ion exchange reaction has been the main focus as its adsorption mechanism. Quat

adsorption characteristics were studied with consideration of its hydrophobic effect by exploring

its adsorption isotherm. The study was on the basis of a hypothesis that “The aggregation nature

of quat leads to the poor leaching resistance of ACQ treated wood.” Also, as described, quat is

known to bind to the same sites in wood as copper; thus it is of interest to investigate how they

compete for the sites. Through this study, the following questions should be answered;

a) How does aggregation of quat affect its leachability?

b) How do other components of ACQ affect aggregation of quat?

c) How do different cations of ACQ compete for the same sites?

21

Chapter 2 Copper monoethanolamine adsorption in wood and its relation to cation exchange capacity (CEC)

2.1 Abstract

To investigate the chemical adsorption capacity of copper-monoethanolamine (Cu-Mea)

components on wood, the Na+

cation exchange capacity (CEC) of red pine (Pinus resinosa Ait.)

was determined and compared to the adsorption capacity of free Mea, Cu2+

and Cu-Mea

complexes. Red pine showed higher CEC as pH increased. Free Mea adsorption as a function of

pH followed the Na adsorption curve except at pH over 9, when it exceeded the CEC. Below pH

5, where Cu-Mea complexes do not form, divalent Cu2+

was adsorbed as if it were monovalent.

Cu-Mea adsorbed up to the CEC at pH 9.0-9.5 apparently as [CuMea-H]+, whereas the complex

in solution is predominantly of the form [Cu(Mea)2-H]+. FTIR analysis showed that the same sites

(carboxylic acid groups and phenolic hydroxyl groups) of wood are related to Mea, Na, and Cu

adsorption.

2.2 Introduction

Copper ions form various types of complexes with monoethanolamines (Mea) in solution

depending on the solution pH (Hancock 1981; Tauler and Casassas 1986; Casassas et al. 1989).

These complexes can be divalent, monovalent or neutral depending on dissociation number of

hydroxyl groups in amine ligand, which is solely determined by pH. The different charge states

of copper complexes are expected to react with wood in a different way or with a different

number of wood acid groups.

Wood also has a pH dependant number of sites for reaction with copper (Cooper 1991). These

bonding sites are weak acidic groups of wood components, e.g., carboxylic and phenolic

hydroxyl groups, which dissociate according to the ambient pH. Chemisorption occurs by means

of ion exchange or other chemical complex reactions (Rennie et al. 1987; Cooper 1991;

Thomason and Pasek 1997; Kamdem et al. 2001; Ruddick et al. 2001). Understanding the

22

relationship between both pH-dependant Cu-amine species and pH dependant CEC of wood

might provide insight into Cu-amine adsorption mechanisms and identify the best pH and

formulations for high copper fixation.

In this chapter, copper, Mea and copper-Mea adsorption capacities in wood and their relationship

with CEC of wood are investigated at different pH values.

2.3 Materials and methods

2.3.1 Sample preparation

Red pine (Pinus resinosa Ait.) (35-60 mesh) was washed with 0.5 mol l-1

HCl to remove all

metal ions initially bound to the wood and rinsed repeatedly with distilled water until the pH of

washings was constant. The sample was then dried at 60°C for 48 h.

2.3.2 Cation exchange capacity of red pine

Sodium in salts always ionizes to one positive small-dimension ion, and it is an appropriate

chemical to achieve equilibrium adsorption in an anion exchange material like wood. Therefore,

NaOH was used to measure the total cation exchange capacity of red pine wood. The pH’s of 50

ml of 0.1 mol l-1

and 0.2 mol l-1

NaOH solutions were adjusted to the range from 13 to 2 with

HNO3 with continuous stirring, then solutions were diluted to 100 ml with distilled water for

final concentrations of 0.05 mol l-1

and 0.1 mol l-1

respectively. Three grams of the prepared

wood dust were added to 100 ml of pH-adjusted NaOH solution. The suspensions were

continuously stirred for 48 h at 22 ; although ion exchange is a quick reaction, pH of the

treating solution decreased after mixing with wood dust and Na retention also decreased with

decreasing pH and reached equilibrium after 20 h, based on our preliminary test (Appendix 2.1).

After recording the final pH of the suspension, the wood dust was vacuum filtered and washed

repeatedly with distilled water. The ion exchanged sodium content of wood dust was determined

by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 7300 DV,

Perkin Elmer, Waltham, MA, USA) analysis of sodium in wood samples, digested according to

AWPA standard A7-04 (AWPA 2006a).

23

2.3.3 Effect of pH on Cu and Mea adsorption capacity

Mea adsorption studies were conducted with 0.5 mol l-1

and 2.0 mol l-1

Mea solutions.

Independent copper adsorption studies were conducted with 0.2% and 0.6% Cu (II) acetate and

0.5 to 2.0% Cu (II) sulfate without pH adjustment. For Cu-Mea adsorption, Cu (II) acetate

solutions were formulated with Mea at Cu:Mea molar ratio = 1:3.7. This proportion is usual in

commercial alkaline copper quat (ACQ) solutions (AWPA 2006b). The pH’s of 50 ml of each

Mea and Cu-Mea solution were adjusted with HNO3, and diluted to 100 ml with distilled water.

For Cu-Mea, the target concentrations were 0.2 – 1.0% Cu. These concentrations would

correspond to concentrations of ACQ preservative solution of 0.38% to 1.9% (AWPA 2006b).

Three grams of wood dust were added to each solution and continuously stirred for 48 h at 22 C.

The wood dust was vacuum-filtered and washed repeatedly with distilled water. The adsorbed

Cu content of wood was analyzed by digestion followed by atomic absorption spectroscopy

(AAS, AAnalyst 100-Perkin Elmer, Waltham, MA, USA). Total nitrogen content was measured

for Mea determination by an elemental combustion system (ECS 4010, Costech, Valencia, CA,

USA) and corrected for N content in untreated wood.

2.3.4 Fourier transformed infrared (FTIR) spectroscopy

Infrared analysis of untreated wood and the Na+-, Mea- and Cu-Mea-treated wood at different

pHs was conducted using a Bruker Tensor 27 (Billerica, MA, USA) FTIR spectrometer. Wood

dust was mixed with KBr powder, ground, and pelletized for analysis. FTIR spectra were

acquired at a resolution of 4 cm-1

, 60 scans, on a 400-4000 cm-1

wave number range. All spectra

were displayed in absorbance and presented as the narrower range of 1000-2000 cm-1

region for

clarity after vector normalization.

2.4 Results and discussion

2.4.1 Cation exchange capacity (CEC) of red pine dust as a function of pH

When wood dust was added to the standardized NaOH solutions, the pH of the suspensions

initially decreased as Na+ occupied the acidic sites of wood releasing H

+ into the suspension

(Appendix 2.1). Figure 2.1 (a) shows the CEC of red pine as a function of suspension

24

equilibrium pH. There are somewhat steeper increases between pH 3 and 5 and between pH 7

and 11 at both Na concentrations. The increase at lower pH results from sodium ions complexing

with the dissociated carboxylic acid groups of hemicelluloses and pectins, and the steep increase

at high pH results from their complexing with dissociated phenolic acid groups of lignin (Rennie

et al. 1987; Cooper 1991; Kamdem et al. 2001). The CEC of red pine was approximately 0.09

mmol g-1

wood at pH 7 and 0.20 mmol g-1

wood at pH 11; these values are similar to those

estimated for copper (Cooper 1991). The 0.09 mmol g-1

wood at pH 7 is also comparable with

reported 0.091 mmol g-1

carboxylic acid content of holocellulose (Kamdem and Zhang 2000).

This pH-dependent CEC of red pine was the reference to compare the adsorption of copper and

Mea.

0.00

0.05

0.10

0.15

0.20

0.25

2 3 4 5 6 7 8 9 10 11 12 13 14

Na

+ a

dso

rptio

n

(mm

ol g

-1 w

oo

d)

0.05 N

0.1 N

(a)

0.00

0.05

0.10

0.15

0.20

0.25

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Me

a a

dso

rptio

n

(mm

ol g

-1 w

oo

d)

CEC of red pine

0.5 M Mea

2 M Mea(b)

Figure 2.1 Cation exchange capacity of red pine dust for sodium (Na+) from NaOH/HNO3

solutions (0.05 mol l-1

and 0.1 mol l-1

) as a function of pH (a) and its comparison with Mea

adsorption on red pine dust at different pH (b).

25

2.4.2 Mea adsorption capacity of wood

Amine plays an important role in ACQ solutions as a co-solvent to improve copper solubility and

in the binding of Cu to wood acidic groups by formation of Cu-amine complexes (Zhang and

Kamdem 2000b; Ruddick et al. 2001; Humar et al. 2007a). To understand the mechanisms of

Cu-amine fixation, the interaction of amine only with wood is relevant. Figure 2.1 (b) shows the

Mea adsorption in wood that was treated with Mea only compared with the CEC of red pine at

different pH values. As the pH increased, the amine adsorption followed the same trend, and the

adsorbed amount was almost the same as the CEC below pH 8 for both 0.5 and 2.0 mol l-1

Mea.

However, Mea data for the two concentrations deviated somewhat from the CEC curve and from

each other above pH 9. Humar et al. (2003) found by FTIR analysis that ethanolamine may react

with COOH groups of hemicelluloses and C1, C3, and C4 positions of benzene rings in lignin.

Ion exchange and hydrogen bonding are two possible reactions between Mea and wood. As Mea

adsorption was similar to the CEC at low and moderate pH values, ion exchange reaction is

considered as the most likely bonding mechanism between Mea and wood. MeaH+ ions dominate

in aqueous Mea below pH 9.5 (Eq. 2.1), and these may bond with dissociated acidic groups of

wood (e.g. carboxylic acid and phenolic hydroxyl groups).

NH3+CH2CH2OH " NH2CH2CH2OH + H

+ (pKa # 9.5) (Eq. 2.1)

As pH increases above 9.5, only a decreasing amount of MeaH+ can react through ion exchange

with acidic functional groups on wood because of increasing neutral Mea0 formation. The latter

may interact with lignin through the formation of hydrogen bonds between the hydroxyl groups

and alcohols or carbonyls present in the lignin (Spencer et al. 1985; Rodrigues et al. 2001).

Therefore, peculiarities above pH 9 may reflect adsorption of Mea0 by hydrogen bonding, and

high adsorption at higher concentration may be the result of combined adsorptions by ion

exchange (of MeaH+) and by hydrogen bonding (of Mea

0).

2.4.3 Cu-amine adsorption capacity of wood

Figure 2.2 (a) compares the Cu adsorption in wood for different concentrations of Cu with the

normal commercial ratio of Cu:Mea =1:3.7. The pH range 5 to 9 is not included in the figure

since significant Cu precipitation occurred in this range. At pH 9-9.5, approximately 0.17 mmol

26

g-1

of copper is adsorbed; this corresponds to a Cu uptake of about 4.2 kg m-3

in red pine. In

general, even though wood is treated with commercial wood preservative solutions containing

Cu-Mea at pH 9-9.5, the equilibrium pH of treated wood decreases to 6-8 due to the buffering

capacity of wood and other pH-affecting reactions, which can result in higher Cu fixation in high

retention treatments by precipitation of Cu(OH)n or other low solubility compounds in wood.

0.00

0.05

0.10

0.15

0.20

0.25

2 3 4 5 6 7 8 9 10 11 12 13

Cu a

dsorp

tion (

mm

ol g

-1 w

ood)

0.2% 0.4%

0.6% 1.0%

CEC

(a)

0.00

0.05

0.10

0.15

0.20

0.25

2 3 4 5 6 7 8 9 10 11 12 13

pH

Mea a

dsorp

tion (

mm

ol g

-1 w

ood)

0.2% 0.4%

0.6% 1.0%

CEC

(b)

Figure 2.2 Chemisorption of Cu (a) and Mea (b) of Cu-Mea in red pine dust and comparison

with CEC of red pine.

However, use of highly alkaline solutions or high concentration of ACQ may lead to higher

equilibrium pH of 9 - 9.5 without precipitation. For the pH values evaluated, the Cu adsorption

was similar to the CEC value, regardless of the Cu-Mea concentration (Figure 2a). This indicates

that cation sorption sites in wood are limited and are saturated with copper, even at low

concentrations. Also, it shows that both carboxylic groups and phenolic hydroxyl groups are

27

responsible for Cu-Mea fixation as proposed earlier (Cooper 1991; Kamdem and Zhang 2000;

Ruddick et al. 2001). It does not appear that hypothesized oxidation of wood to create additional

sites occurs, because copper adsorption does not exceed the CEC.

Mea alone and Mea in the presence of Cu react differently (Figure 2.2 (b) vs. Figure 2.1 (b))

because of the diverse interactions between Cu and Mea. At pH below 4, virtually all of the Mea

is protonated and will not complex with copper (Tauler and Casassas 1986; Pankras et al. 2009)

and free Cu2+

can react directly with wood in low quantities reflecting the dissociation of –

COOH. Since the protonated MeaH+ was not adsorbed at pH below 5 (Figure 2.2 (b)), it is

evident that Cu2+

is selectively adsorbed over MeaH+ or displaces adsorbed MeaH

+. Both Cu and

Mea adsorption followed the CEC curve at pH 9 to 9.5, indicating that the Cu to Mea ratio was

1:1 which is an indication that the complex fixed in wood was [CuMea-H]+. This interpretation is

consistent with findings of the literature. Ruddick et al. (2001) showed that Cu-Mea (with a 1:1

ratio of Cu to Mea) reacted with a lignin model compound. Jiang and Ruddick (2004) performed

experiments with different amines and the ratio of Cu to the amines in wood after leaching was

about 1.2:1. Lucas and Ruddick (2002) reported a Cu:Mea ratio in wood after leaching of 1.4:1.

However, the Cu-Mea complexes vary depending on pH and in a Cu-Mea treating solution near

pH 9, [Cu(Mea)2-H]+ species is dominant (Casassas et al. 1989; Pankras et al. 2009). This

indicates that the [Cu(Mea)2-H]+ complex loses one Mea molecule during fixation in wood.

The 1:1 ratio of Cu to Mea may be explained by a ligand exchange reaction proposed by Craciun

et al. (1997) and Zhang and Kamdem (2000b), i.e. non-conducting Cu-amine complexes – such

as [Cu(Mea)2-2H]0 – exchange Mea ligand with wood and release one or two amine molecules.

On the other hand, it was shown in chapter 3 that with high Mea ratio Cu-Mea formulations, Cu

adsorption was considerably decreased at pH 11 where the most abundant copper Mea complex

is in the form [Cu(Mea)2-2H]0 indicating that ligand exchange is not occurring under these

conditions. Moreover, significant amounts of [Cu(Mea)2-H]0

will not be formed at pH 9-9.5

studied in the present study. Therefore, we consider that [Cu(Mea)2-H]+ must react with wood and

eliminate one Mea molecule in the process of fixation, drying or washing with neutral water.

Unexpectedly, even though Cu2+

and many of its complexes with Mea are divalent at lower

pH’s, copper adsorption followed the CEC curve which suggests that the divalent Cu reacted

with only one reaction site in wood below pH 5. The absorbed uncomplexed Cu must be divalent

28

because less than 3% of Cu2+

converts to Cu+ even after Cu-Mea treatment with high Mea ratios

(Figure 3.6). To confirm if this is related to the presence of Mea in the solution, wood was

treated with Cu (II) acetate and Cu (II) sulfate without Mea at low pH, and the results were the

same as with the Cu-Mea system (Figure 2.3). A possible explanation is that the sparse anionic

sites in wood permit divalent complexes to behave as monovalent ones, as illustrated in Eq. 2.2.

In nitrogen free formulations, electron paramagnetic resonance (EPR) investigations of Hughes

et al. (1994) demonstrated that Cu is complexed to 4 oxygen atoms to form square planer in a

distorted octahedral environment in wood and the four oxygen atoms come from water or

functional groups of wood. Similar monovalent-like Cu adsorption was reported by Staccioli et

al. (2000).

(Eq. 2.2)

a) O provided by either water or other functional groups but not by other carboxylic group.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

pH

Cu a

dsorp

tion (

mm

ol g

-1 w

ood) 0.2% Cu ( ) acetate

0.6% Cu ( ) acetate

0.5% Cu ( ) sulfate

1% Cu ( ) sulfate

2% Cu ( ) sulfate

CEC

Figure 2.3 Chemisorption of copper from Cu(II) acetate and Cu(II) sulfate solutions in red pine

dust.

29

2.4.4 FTIR spectral analysis

Figure 2.4 (a) shows the FTIR spectra of control wood and Na-treated wood at different pH

values. The band at 1738 cm-1

, assigned to carboxyl stretching vibration in carboxylic acid

(Hortling et al. 1997; Tolvaj and Faix 1995), diminished as pH increased and disappeared at high

pH while the intensity of the band at 1600 cm-1

assigned to C=O stretching in carboxylate,

increased and overlapped the peak at 1655 cm-1

as pH increased. Many previous studies showed

that the reduction of the band at 1738 cm-1

in treated wood is from the interaction of cation (i.e.

copper complex) with carboxyl group of wood, leading to a shift of this peak to 1600 cm-1

attributing to carbonyl stretching vibration in carboxylate salt (Craciun et al. 1997; Jiang and

Ruddick 1999; Zhang and Kamdem 2000c). The peak intensity at 1220 cm-1

, assigned to

phenolic O-H deformation (Zhang and Kamdem 2000c; Michell 1993), and the peak intensity at

1260 cm-1

, assigned to C-O stretching in guaiacyl group (Michell 1993; Pandey and Pitman

2003) decreased as pH increased. This implies the reaction of phenolic hydroxide and guaiacyl

groups in lignin with Na.

Figure 2.4 (b) is a comparison of the spectra of Na-, Cu-Mea-, and Mea-treated wood at pH 9

and control for references. Although the change of the bands in Mea treated wood were weaker

than in Na and Cu-Mea treated wood, the band at 1738 cm-1

decreased while that at 1600 cm-1

increased in all three spectra. Because most carboxylic acid groups must be dissociated at pH 9

(Cooper 1991), the 1738 cm-1

peak was expected to disappear at this pH, but this was not the

case. Jiang and Ruddick (1999) showed that a small amount of free carboxylic acid

corresponding to this peak was reformed during leaching.

The intensity of the band at 1220 cm-1

and 1260 cm-1

decreased in Na, Cu-Mea, and Mea treated

wood although the change in Mea treated wood was weaker than for the others. Considering the

similar spectral changes of Na, Cu-Mea, and Mea treated woods, it can be concluded that the

same reactive sites of wood are involved in the bonding of Na, Mea, and Cu-Mea.

30

Figure 2.4 FTIR spectra of Na+-treated wood dust at different pH and control wood dust (a) and

Na+-, Cu-Mea-, and Mea-treated wood at pH 9 and control wood dust (b).

2000 1800 1600 1400 1200 1000

a

Absorb

ance

Control

pH 4.0

pH 7.5

pH 10

pH 13

1738

1655

1600

1220

1260

2000 1800 1600 1400 1200 1000

b

Absorb

ance

Wavenumber (cm-1)

Control

Mea

Na

Cu-Mea

1738

1655

1600

1220

1260

31

2.5 Conclusions

Wood has a limited but variable CEC as a function of pH, and generally, CEC increased as pH

increased. Free Mea adsorbed into wood by ion exchange at pH below 9 and possibly by

hydrogen bonding and ion exchange at pH over 9. Accordingly, free Mea can affect Cu

adsorption. However, at Cu to Mea ratios and pH usually found in commercial solutions (pH#

9~9.5), Cu adsorbed into wood up to the CEC level in [CuMea-H]+ form, which indicates that

[CuMea-H]+

is selectively adsorbed into carboxylic and phenolic hydroxyl groups of wood over

MeaH+. Below pH 4.5, Cu does not form complexes with Mea and is directly adsorbed into

wood like a monovalent ion and was adsorbed preferentially over protonated Mea.

32

Chapter 3 Effect of amine ligand, copper/amine ratio and pH on copper adsorption into wood

3.1 Abstract

To measure the chemical adsorption capacity of wood for copper (Cu) and amine in Cu-amine

solution, Cu was formulated with different ratios of mono- (Mea), di- (Dea), tri- (Tea)

ethanolamine, ethylenediamine (En) and ammonia (Am), and the Cu adsorption was compared

with the cation exchange capacity (CEC) of red pine (Pinus resinosa Ait.). The chemisorption

capacity of the wood for Cu was highly pH dependant and varied with ligand types used in this

study. Although wood chemisorption capacity increased with pH, high amine ratio Cu-

ethanolamine complexes showed very limited adsorption at high pH due to competition with free

ethanolamine in combination with formation of uncharged ion complexes, [Cu(Mea)2-2H] that are

very stable in solution. During Cu-Mea treatment, negligible Cu2+

was converted to Cu+

and no

significant delignification was detected even at very high Mea ratios. Cu appeared to be adsorbed

as if it were singly charged, even though most of the complexes present are +2 charged. The

three ethanolamine ligands generally showed similar adsorption tendencies, although their pH

dependencies differed. Cu in En was much less adsorbed at intermediate pH compared to the

CEC, but had higher adsorption at high pH. Cu in Am also showed higher adsorption at high pH

compared with Mea and [Cu(NH3)(H2O)5]2+

form of Cu might be fixed in wood.

3.2 Introduction

In chapter 2, it was reviewed that wood has a limited but increasing cation exchange capacity

(CEC) as pH increases, and Cu adsorption capacity of Cu-Mea (molar ratio 1:3.7 in solution) is

similar to the CEC value at pH 9-9.5. However, Cu chemisorption capacity in wood near neutral

pH could not be determined because of immediate Cu precipitation. Since Cu is fixed and

stabilized in wood near neutral pH in Cu-amine preservative treatments, study of reactions near

neutral pH has practical application; this can be achieved without Cu precipitation by increasing

33

the amine ratio. Therefore, Cu-Mea formulations with high amine ratios were evaluated to

extend the copper soluble region to neutral pH conditions.

Copper ions form various complexes not only with monoethanolamine, but also with di-, tri-

ethanolamine, ethylenediamine, and ammonia in solution depending on type of amine and pH

(Hancock 1981; Tauler and Casassas 1985, 1986; Casassas et al. 1989). This opens the

possibility that different Cu-amine complexes can react differently with wood, resulting in

different amounts of Cu adsorption and bonding strengths. It is also of interest to determine if the

dimeric singly charged complex species [Cu2Tea2]+

present in Cu-Tea solutions at pH 5.8-10

(Tauler and Casassas 1985), might increase Cu chemisorption by binding two equivalents of Cu

while consuming only one reaction site in wood.

In this study, Cu adsorption characteristics were investigated at different pH values, and

compared with the CEC of wood to provide insight into the selective Cu ion adsorption in wood

and whether there are chemical reactions such as oxidation of wood that could result in higher

Cu adsorption. Copper adsorption characteristics of copper monoethanolamine (Cu-Mea) were

compared with those of Cu-diethanolamine (Cu-Dea) and Cu-triethanolamine (Cu-Tea) to see

how secondary and tertiary ethanolamines affect Cu adsorption. Ethylenediamine (En) and

ammonia were also compared with ethanolamine to evaluate the effect of solvent (or ligand) on

Cu adsorption.



Particulate red pine (Pinus resinosa Ait.) (35-60 mesh) was used in the study. The sample was

washed with 0.5 M HCl to remove all metal ions initially bound to the wood and rinsed

repeatedly with distilled water until the pH of washings was constant (Staccioli et al. 2000). The

sample was then dried at 60°C for 48 h.

3.3.2 Effect of Cu/Mea ratio on copper amine solubility

30 ml of Cu solution (as 50 mmol l-1

copper acetate), in which Cu/Mea molar ratios were

between 1:3.7 and 1:70, were adjusted to different pH values with HNO3 (11 N and 0.5 N). After

34

48 h, the amount of precipitate formed in the solutions at different pH levels was determined

gravimetrically via vacuum filtering followed by drying the precipitate at 105°C.

3.3.3 Effect of amine type, Cu/amine ratio and pH on Cu-amine adsorption capacity

Copper acetate solutions (0.2-1.2%, Cu based) were formulated at different Cu/ligand molar

ratios with Mea (1:3.7, 1:5, 1:10, 1:30 and 1:70), Dea (1:3.7, 1:15 and 1:70), Tea (1:3.7 and

1:70), En (1:3.7 and 1:6.6), and NH4OH (1:5.6, 1:11.2, 1:16.7 and 1:33.5). The commercial

cellulose solvent cupriethylendiamine (Lab Chem. Inc., Pittsburgh, PA, USA. Cu:En = 1:2 molar

ratio) was also employed for investigating Cu-En systems. The pH of 50 ml of each Cu-amine

solution was adjusted with HNO3, and diluted to 100 ml with distilled water to achieve the target

concentration. 3 g wood dust were added to the solution and continuously stirred for 48 h at

22 C. Then, the solid material was vacuum filtered and washed repeatedly with distilled water.

Wood samples were digested according to AWPA A7-04 (2006a) and the adsorbed Cu content

was determined by atomic absorption spectroscopy (AAS, AAnalyst 100-Perkin Elmer,

Waltham, MA, USA). Amine content was determined from total nitrogen content as measured by

elemental combustion system (ECS 4010, Costech, Valencia, CA, USA) and corrected for N

content in untreated wood.

To test if excess Mea or different pH’s could oxidize wood resulting in reduction of cupric to

cuprous ions, cuprous copper content of Cu-Mea treated wood dust was measured

spectrophotometrically. Wood (0.05 g) was extracted by ultrasonic extraction for 20 min at room

temperature in 10 ml of 0.1% 2, 2’-biquinoline in glacial acetic acid (Aldrich chemicals Ltd., St.

Louis, MO, USA) as solvent. Each extraction solution was filtered before UV-VIS analysis at

540 nm (Cui et al. 2005; Tascioglu et al. 2008). Instrument: Shimadzu UV-160

spectrophotometer (Columbia, MA, USA) equipped with a 10-mm light path silica cuvette

35


3.4.1 Effect of Cu/Mea ratio on Cu-amine solubility

At moderate ratios of Cu to Mea, significant Cu was precipitated when the solution approached

neutral pH (Figure 3.1). However, above a molar ratio of 1:70, there was no copper precipitation.

Copper precipitation is prevented when free amine prevails over hydroxyl ion, so only negligible

amounts of hydroxo compounds are formed (Bjerrum 1957; Fisher and Hall 1962; Davies and

Patel 1968; Casassas et al. 1989).

0

20

40

60

80

100

1 2 3 4 5 6 7 8 9 10 11 12

pH

Cu

pre

cip

ita

tio

n (

%)

Cu:Mea=1:3.6

Cu:Mea=1:20

Cu:Mea=1:30

Cu:Mea=1:50

Cu:Mea=1:70

Figure 3.1 Percent Cu precipitation as affected by Cu to Mea ratios for Cu-Mea solution.

3.4.2 Effect of Cu/ethanolamine ratio and pH on Cu adsorption capacity

3.4.2.1 Cu-Mea

Cu-Mea complexes have diverse ion species and different Cu/Mea ratios and their relative

abundances depend on pH and on Cu/Mea ratios in solution as exemplified in Table 3.1 (Pankras

et al. 2009). Figure 3.2 (a) compares Cu adsorption in wood with the cation exchange capacity

(CEC) as determined by Na+ adsorption (Figure 2.1 (a)). A number of high Mea molar ratio Cu-

Mea formulations (Cu:Mea = 1:5, 1:10 and 1:30) are compared with that of Cu:Mea = 1:3.7

(usual ratio in most commercial wood preservative solutions, AWPA 2006). There are no data

available for pH 5 to 8.5 for lower Mea ratios, due to significant Cu precipitation. At the Cu/Mea

ratio of 1:3.7, Cu adsorption was similar to the CEC level at pH < 5 and pH 9 and pH 9.5, but

36

addition of Mea to increase pH reduced the Cu adsorption to below the CEC values; as more

Mea was added, less Cu was adsorbed. Increasing the Mea/Cu ratio of the formulation extends

the uncharged complex, [Cu(Mea)2-2H]0 formation to the lower pH and increases the proportion

of [Cu(Mea)2-2H]0. For the 1:30 molar ratio (D in Figure 3.2), Cu adsorption at pH 9.5, where

charged Cu-Mea complexes are present, was similar to that at pH 11.5, where the uncharged

complex predominates (Table 3.1). This indicates that increasing Mea ratio is not favorable to

achieve high copper retention and high leaching resistance because the accompanying high pH

both prevents Cu precipitation and reduces copper adsorption and thus leads to increasing

amounts of unfixed soluble Cu in wood.

Table 3.1 Distribution of Cu-Mea species in aqueous solution with Cu concentration of 42 mmol

l-1

(equivalent to 0.33% CuO in solution) (Pankras et al. 2009).

Cu complexes pH range of distribution pH a)

% b)

% c)

Cu2+

< 7.0 < 4.0 100 100

[CuMea]2+

3.0-7.5 5.5 24 38

[Cu(Mea)2]2+

5.0-9.0 6.8 24 48

[Cu(Mea)3]2+

6.0-11.0 8.2 1 14

[Cu(Mea)4]2+

8.0-11.0 9.5 0 5

[Cu(Mea)2-H]+ d)

5.5-12.0 8.8 84 74

[Cu(Mea)2-2H] 0 > 8.0 > 11.0 95 95

a) Optimum pH for maximum formation of each complex.

b) Maximum concentration at optimum

pH with 1:3 Cu to Mea molar ratio. c)

Maximum concentration at optimum pH with 1:10 Cu to

Mea molar ratio. d)

(Mea)2-H indicates of deprotonation from one of the Mea coordinating with

Cu.

In the previous chapter, it was observed that wood exposed only to a Mea solution at different

pH conditions adsorbed Mea at levels (mmol g-1

) similar to its CEC. However, Mea in the Cu-

Mea system (Figure 3.2 (b)) was not adsorbed at low pH, even though it is in the protonated form

in acidic solutions. Charged MeaH+ does not complex with Cu

2+ at these pHs and it is evident

that Cu2+

is selectively adsorbed over MeaH+. In chapter 2, it was also found that near pH 9.5,

the adsorbed Cu complex was likely of the form (CuMea-H)+, not (Cu(Mea)2-H)

+, which is the

complex in solution at this pH. Thus, the adsorbed complex loses one Mea molecule and

(CuMea-H)+ is selectively adsorbed over MeaH

+ at this pH. Ruddick et al. (2001) showed that

37

precipitates formed by reaction of CuMea with lignin model compounds formed a complex with

1:1 Cu/Mea ratio. However, at higher pH and higher Mea ratios of Cu-Mea, Mea adsorption

exceeded the CEC curve, while Cu adsorption was low. Low Cu adsorption may be explained by

formation of an uncharged complex, i.e. [Cu(Mea)2-2H]0, the amount of which is maximized

above pH 11 (Table 3.1). The uncharged complex may also be abundant enough to be a factor

even at relatively low pH 9.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

Cu a

dsorp

tion (

mm

ol g

-1 w

ood)

0.2% 0.4%0.6% 1.0%

0.2% 0.4%0.6% 1.0%

0.2% 0.4%0.6% 1.0%0.2% 0.4%

0.6% 1.0%CEC

A

B

C

D

(a)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

pH

Mea a

dsorp

tion (

mm

ol g

-1 w

ood)

0.2% 0.4%

0.6% 1.0%

0.2% 0.4%

0.6% 1.0%

0.2% 0.4%

0.6% 1.0%

0.2% 0.4%

0.6% 1.0%

CEC

A

B

C

D

(b)

Figure 3.2 Chemisorption of Cu (a) and Mea (b) from different Mea ratios of Cu-Mea in red

pine dust and comparison with CEC of red pine. Note: In the figure legend A is 1:3.7, B is 1:5, C

is 1:10, and D is 1:30 Cu:Mea molar ratio. The CEC values of red pine and the adsorption values

of 1:3.7 ratios were obtained from Figure 2.1 (a).

However, the high Mea retention at high pH – where unprotonated Mea0 is formed along with

low Cu adsorption at high pH – is difficult to explain. Speciation analysis of Cu-Mea in solution

(Table 3.1) indicates that the proportions of [Cu(Mea)3]2+

and [Cu(Mea)4]2+

increase with

38

increasing Mea ratio, which could explain some of the excess Mea adsorbed in wood. An

alternative explanation is that at high pH and/or high Mea ratio, when most of the Mea is

unprotonated and [Cu(Mea)2-2H]0 prevails over [Cu(Mea)2-H]

+ in solution, unprotonated Mea

interferes with the Cu adsorption or pre-occupies the reaction sites through hydrogen bonding.

For MeaH+ " Mea

0 + H

+ (pKa # 9.5), theoretically 50% unprotonated Mea is present at pH 9.5,

90% at pH 10.5, and 99% at pH 11.5..

Figure 3.3 presents the Cu and Mea adsorptions in wood treated with an excess of amine

(Cu:Mea = 1:70 molar ratio) for the entire pH range without precipitation. Varying solution

concentrations had little effect on their adsorption over the pH range, although Mea adsorption

showed deviation at high pH. In this excess amine formulation, [Cu(Mea)2-2H]0 formation can be

further extended to lower pH and prevail over [Cu(Mea)2-H]+ near pH 9-10. However, Cu

adsorption decreased dramatically at pH > 6.8, which is a much lower pH than observed for

lower Mea systems. This finding supports the view that [Cu(Mea)2-2H]0 cannot react with wood;

alternatively, free unprotonated Mea competes with (or interferes with) Cu complex adsorption

on wood because in the high Mea system, there is a greater absolute amount of unprotonated

Mea which competes with Cu-Mea even at lower pH. Theoretically 50% unprotonated Mea is

present at pH 9.5, 10% at pH 8.5, and 1% at pH 7.5 and there may be enough umprotonated Mea

even at lower pH if the Mea concentration is high.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

pH

Cu a

nd M

EA

adsorp

tion

(mm

ol g

-1 w

ood)

0.2% Cu Mea

0.4% Cu Mea

0.6% Cu Mea

CEC

Figure 3.3 Chemical adsorption of Cu and Mea from high amine ratio Cu-Mea (1:70 molar ratio)

in red pine dust in comparison with CEC of red pine

39

A cation exchange fixation mechanism between Cu-amine and wood, is very reasonable based

on the results but it is difficult to explain the discrepancy in the amount of amine coordinated

with Cu found in wood Cu(Mea) and solution Cu(Mea)2. Therefore, we consider coordinate

bonding of Cu-amine with wood accompanying the elimination of one amine by ligand exchange

(Gadd 1982; Klemm et al. 1998). Similarly, several researchers proposed ligand exchange

reaction between wood and non-charged species of Cu-amine i.e. [Cu(Mea)2-2H]0

as one of the

possible mechanism (Kamdem et al. 1996; Thomason and Pasek 1997; Zhang and Kamdem

2000b).

Figure 3.4 Proposed mechanism for copper-amine-wood interactions at carboxylic acids (above)

and phenolic hydroxyl groups (below).

However, as shown in the results, [Cu(Mea)2-2H]0

is not helpful to increase Cu fixation in wood

and cannot be an active species for strong bonding that contributes to long term protection in

treated wood. This is because of the very strong binding of the deprotonated ligand to the central

Cu atom, which impedes subsequent ligand exchange with the wood materials (Gadd 1982;

Klemm et al. 1998). The higher the charge on them, the greater the electrostatic attraction and

the more stable the resultant complex (Nicholls 1975). Accordingly, the coordination bonding

between wood and [Cu(Mea)2-2H]0 which is very stable in solution (log K#19.9) (Casassas et al.

40

1989), is not likely to happen and elimination of amine seems a key reaction for Cu

chemisorption. Therefore, [Cu(Mea)2-H]+ (log K#14.9) is thought to be an active species and its

less stable protonated part may be exchanged by positively charged wood acidic groups.

Accordingly, stable complexes are formed by interaction of [Cu(Mea)-H]+ with the acidic groups

such as carboxylic groups and phenolic hydroxyl groups, with coordination sites of the copper

atom being occupied by the deprotonated O atoms at wood acidic groups and the other two sites

binding Mea molecules (Figure 3.4). This coordinate bond seems only to happen at dissociated

acid groups, which may be the driving force to attract the cationic [Cu(Mea)2-H]+ and therefore,

shows a very close relationship with CEC of wood.

Another hypothesis to explain the low Cu adsorption at high pH and high Mea concentration

treatments is that the Mea has depolymerized accessible lignin, resulting in fewer adsorption

sites in wood. It was previously reported that at high pH and concentration, Mea could

depolymerize lignin which increased the Cu leaching and caused low Cu retention (Humar et al.

2007b).

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

2 3 4 5 6 7 8 9 10 11 12

pH

Cu a

dsorp

tion (

mm

ol g

-1 w

ood)

Treatment at different pH

Retreatment at pH 6.8

Figure 3.5 Comparison of Cu adsorption for wood treated with Cu-Mea (1:70 molar ratio) at

different pHs and then retreated with Cu-Mea at pH 6.8

The decrease in Cu adsorption observed here started to occur at relatively low pH if the Mea

concentration was high. To investigate whether wood exposed to high concentration Mea

solutions had reduced Cu adsorption capacity, samples treated at different pH were re-treated at

41

pH 6.8. Cu adsorption in all samples was about 0.1 mmol g-1

wood, similar to that of fresh

treated samples at pH 6.7 (Figure 3.5). This demonstrates that appreciable lignin-

depolymerization with loss of reaction sites did not occur to reduce Cu adsorption even at high

Mea concentrations.

The finding that copper amine complex adsorption is strongly related to the CEC of the wood

can be interpreted that no oxidation of wood (accompanied by reduction of Cu) occurred, which

would produce more reaction sites. In the Cu conversion test of Cu-Mea treated wood dust

illustrated in Figure 3.3, only about 2-3 % of total adsorbed Cu was present as Cu+ (Figure 3.6).

The amount of reduced copper adsorbed was only dependent on total adsorbed Cu.

0.000

0.001

0.002

0.003

0.004

2 3 4 5 6 7 8 9 10 11

pH

Cu I

(m

mol g-1

wood)

Figure 3.6 Cu+ content of Cu-Mea (1:70 molar ratio) treated wood dust at different pHs.

3.4.2.2 Cu-Dea and Cu-Tea

The Cu-Dea and Cu-Tea complexes formed in aqueous solution (Table 3.2) are similar to those

of Cu-Mea, although the protonation constants (pKa) of the ligands are different (e.g. Mea # 9.5,

Dea # 9.0, Tea # 8.0) (Tauler and Casassas 1985, 1986). For Cu-Dea, the general Cu adsorption

capacity was also similar to that of Cu-Mea, but the pH’s after Cu-Dea treatment were lower

than for Cu-Mea due to the lower alkalinity of Dea. For a 1:3.7 molar ratio (A in Figure 3.7 (a)),

Cu was adsorbed up to the CEC at pH 8.4 but the adsorption decreased a little at pH 8.5-8.7,

indicating effects of unprotonated Dea and [Cu(Dea)2-2H]

0 above this pH. At Cu:Dea = 1:15 (B),

42

the Cu adsorptions were higher at pH 8.5 than at pH 10 although their amine contents were the

same, which may result from the more uncharged complex, [Cu(Dea)2-2H]0 and unprotonated Dea

intervention at pH 10 rather than formation of different complexes. At Cu/Dea = 1:70 (C), a

decrease in Cu adsorption was detected at all pHs except at very low pH where Cu does not

complex with Dea. At pH 5, there is approximately 0.01 % of unprotonated Dea in solution

which corresponds to 0.35 mmol l-1

at 1:70 Cu:Dea solution, indicating again there might be

enough unprotonated Dea and [Cu(Dea)2-2H]0 present to affect Cu adsorption with the high Dea

system. At the Cu to Dea ratio of 1:3.7, the adsorption ratios of amine to Cu were less than 1 at

pH near 8 which is implausible stochiometrically (Figure 3.7 (b)). This suggests that both

[Cu(Dea)2-H]+ and [Cu(Dea)2]

2+ ions are regarded as active species to bond with wood; [Cu(Dea)2-

H]+

ions lose one protonated Dea molecule and [Cu(Dea)2]2+

ions lose two protonated Dea

molecules for ligand exchange with wood sites as shown in Figure 3.4.

Table 3.2 Distribution of Cu-Dea and Cu-Tea species in aqueous solution as a function of pH

(Tauler and Casassas 1986, Casassas et al. 1989).

Cu complexes pH range of distribution Optimum pH for max.

formation

Cu2+

< 6.0 < 3.0

[CuDea]2+

3.0-7.4 5.0

[Cu(Dea)2]2+

4.6-8.6 6.2

[Cu(Dea)2-H]+ 6.0-10.0 7.7

Dea

[Cu(Dea)2-2H]0 > 7.0 11-12

Cu2+

< 5.4 < 2.0

[CuTea] 2+

2.0-7.2 4.7

[CuTea-H] +

4.6-8.8 7.0

[CuTea-2H]0 > 6.8 10-11

[Cu2(Tea)2-2H]2+

4.8-8.4 6.6

[Cu2(Tea)2-3H]+ 5.8-10 7.8

[Cu2Tea2-4H]0 > 7.0 9.7-10

[Cu2(Tea)2-5H]- - 12.0

Tea

[Cu(Tea)2-2H]0 - 11.0

43

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

Cu a

dsorp

tion (

mm

ol g

-1 w

ood)

0.2% Cu0.6% Cu

3.0% Cu0.2% Cu0.6% Cu

0.2% CuCEC

A:

B:

C:

(a)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

pH

De

a a

ds

orp

tio

n (

mm

ol

g-1

wo

od

) 0.2% Dea

0.6% Dea

3.0% Dea0.2% Dea0.6% Dea

0.2% DeaCEC

A:

B:

C:

(b)

Figure 3.7 Chemisorption of Cu (a) and Dea (b) from different ratios of Cu-Dea in red pine dust.

Note: In the figure legend A is 1:3.7, B is 1:15, and C is 1:70 Cu:Dea molar ratio.

In the Cu-Tea system, Cu was only precipitated in a narrow range of pH, so Cu chemisorption

could be observed even at neutral pH with the Cu:Tea ratio, 1:3.7 (Figure 3.8). Over the

complete pH range, Cu or Cu-Tea complexes behaved like singly charged ions, although Tea

makes more variable Cu complex ions compared with Mea and Dea and even forms dimeric

species in solution depending on pH (Table 3.2). The expected high copper uptake from

adsorption of [Cu2(Tea)2-3H]+ at pH near 8 was not realized. The Tea adsorption was about half of

the Cu adsorption regardless of different Cu-Tea species in solution at different pH values.

44

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

Cu a

dsorp

tion (

mm

ol g

-1 w

ood)

0.2% Cu

0.6% Cu

3% Cu

0.2% Cu

CEC

A:

B:

(a)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

2 3 4 5 6 7 8 9 10 11 12 13

pH

Tea a

dsorp

tion (

mm

ol g-1

wood)

0.2% Tea

0.6% Tea

3% Tea

0.2% Tea

CEC

A:

B:

(b)

Figure 3.8 Chemisorption of Cu (a) and Tea (b) from different ratios of Cu-Tea in red pine dust.

Note: In the figure legend A is 1:3.7 and B is 1:70 of Cu:Tea molar ratio

Probably, all Cu-amines investigated (Cu-Mea, Cu-Dea, and Cu-Tea) lose one or two amine

molecules during their fixation and elimination of amine molecule is thought to be an essential

process for Cu complexation. At the Cu:Tea ratio 1:70, (B in Figure 3.8 (a)), lower Cu and high

Tea was adsorbed similar to the Cu-Mea and Cu-Dea systems. Thus, in general, the three

ethanolamine formulations adsorbed to wood in similar ways. However, Mea is a more effective

45

ligand in a copper-amine preservative system compared to Dea and Tea. Cu-Mea has a broader

pH range of precipitation and higher natural pH to cause more acidic groups in wood to

dissociate. It also has a higher pKa value to shift the formation of uncharged free amine and

uncharged Cu-amine complexes to higher pH conditions compared to Cu-Dea and Cu-Tea.

3.4.3 Cu-amine adsorption capacity of wood in Cu-En and Cu-Am systems

Ethylenediamine (En) also acts as a bidentate ligand via the two N atoms, forming stable five-

membered chelate rings; however, it forms more stable ion complexes than ethanolamine and it

is in the neutral form when complexing copper, even in an acidic pH range (Aksu and Doyle

2002). Neat En can exist in solution in the three forms shown below:

It forms different complexes with copper such as CuEn2+

near pH 5 and Cu(En)22+

at pH 5 to 13.

CuOHEn2+

and Cu(En)32+

complexes are formed only at very high En/Cu ratios; CuO also can be

formed in strongly alkaline condition (Aksu and Doyle 2002). Figure 3.9 presents the Cu and En

adsorption into wood after treating with different concentrations and different ratios of Cu-En.

Unlike with the Cu-ethanolamine systems, Cu/En ratio has little effect on Cu adsorption. This

may be because there is no uncharged Cu complex formed in solution. At pH below 4.5, Cu

adsorption follows the CEC curve and it is expected that it is adsorbed without complexation

with En. However, there also appears to be some En adsorption in wood. At pH between 5 and

10, the Cu adsorption was less than half of the CEC. This indicates that the reactions between

Cu-En ions and wood are different from those of Cu-ethanolamine. This might be from its higher

stability in solution (log K = 19.6 for Cu(En)22+

) than [Cu(Mea)2-H]+ (log K = 14.9) (Casassas et

al. 1989; Aksu and Doyle 2002). When amines form chelating complexes with copper, the ligand

arrangement around the Cu has a distorted octahedral structure (Inada et al. 1993; Hughes et al.

1994). For Cu(En)22+

, Cu bonds with four N for equatorial interaction, but bonds with two N and

two O for Cu-ethanolamine. Stronger Cu-N bonding might be more stable in solution and less

exchangeable (e.g. by de-amination) to be adsorbed in wood.

En2+

+H3N-CH2-CH2-NH3

+ pKa1=6.8En

+

+H3N-CH2-CH2-NH2

pKa2=9.9En

0

H2N-CH2-CH2-NH2

46

The decrease in Cu adsorption at pH > 5 suggests that some Cu-En complex formation occurs

and that the adsorption of Cu and En results from a combination of Cu2+

and Cu(En)22+

/ CuEn2+

adsorption. At pH between 10 and 12, the Cu adsorption was approximately half of the CEC.

Unlike with Cu-ethanolamine, Cu was not adsorbed as if it were singly charged and there was no

decrease in Cu adsorption at high pH. Ethylenediamine adsorption followed the CEC at pH < 10

indicating that free En occupies the rest of the reaction sites after Cu-En adsorption. Higher En

adsorption over CEC at pH > 10 might be an indication of En0 formation. The higher Cu

adsorption than CEC at pH over 13 may be the result of lignin depolymerization and Cu

adsorption in cellulose. Cupriethylendiamine and cuprammonium are well-known cellulose

solvents and can complex with cellulose glycol groups (Nevell and Zeronian 1985) (Appendix

3.1 and 3.2). CuO formation in wood is also considerable at high pH and copper precipitation

could result in the observed high copper content.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Cu a

dsorp

tion (

mm

ol g

-1 w

ood)

0.3% Cu 0.6% Cu

1.2% Cu En

0.6% Cu En

0.6% Cu En

CEC

A:

B:

C:

Figure 3.9 Chemisorption of Cu and En in red pine dust from Cu-En solution. Note: In the figure

legend A is commercial Cu-En (Cu:En=1:2 molar ratio), B is 1:3.7 Cu:En molar ratio (1:2.75

CuO:En weight ratio), and C is 1:6.6 Cu:En molar ratio (1:5 CuO:En weight ratio).

Free Cu ions can react with free ammonia to form Cu-Am complexes in an ammonia solution.

However, their complex ions are formed from monodentate ligand while the other ligands

47

considered in this study form bidentate chelates. The overall formation constants (logK(NH3)) for

Cu(NH3)2+

, Cu(NH3)22+

, Cu(NH3)32+

, and Cu(NH3)42+

are, respectively, 4.0, 7.5, 10.3, and 11.8

(Stumm and Morgan 1995; Wang et al. 2006). The complexes are always 2+ charged even

though they have different numbers of ammonia ligands. The Cu-Am fixation in wood is driven

by loss of ammonia leading to breakdown of tetramminocopper ion shown in the following

(Ruddick 2003).

[Cu(NH3)4(H2O)2]2+

! [Cu(NH3)4-n(H2O)2+n]2+

+ nNH3 "

Xie et al. (1995) found that precipitates formed by reaction of ammoniacal copper with lignin

model compounds formed [Cu(lignin)2(NH3)2], indicating 1:2 Cu:NH3 ratio reacting with 2 wood

sites.

Figure 3.10 shows the Cu and NH3 adsorption into wood at different pH values, controlled by

different Cu/NH3 weight ratios ranging from 1:1.5 to 1:9.0 (from 1:5.6 to 1:33.5 molar ratios).

Even though Cu-Am treated wood dust was washed repeatedly with distilled water, Cu

adsorption was far higher than the CEC (unfilled squares in Figure 3.9). Copper hydroxide

species can be formed in copper ammonia solutions, with decreasing amounts with increasing

pHs (Stumm and Morgan 1995; Johnson et al. 2005; Wang et al. 2006). Thus, the possibility that

some copper hydroxide was precipitated in the wood was evaluated by re-measuring the Cu

contents after leaching with 3% NH4OH for 48 h followed by washing with distilled water (filled

squares in Figure 3.10). The Cu adsorption after leaching with NH4OH was similar to the CEC of

red pine, indicating that 2+ charged Cu-Am ions also occupy only one anionic site in wood,

assuming that bonding was not affected by delignification or ammonia reaction with cellulose.

Staccioli et al. (2000) also observed that copper was adsorbed on wood as if it were singly

charged. The 1:1 ratio of Cu:NH3 adsorption in wood demonstrates that the fixed Cu species

might be (CuNH3)2+

as described in the following equation:

[Cu(NH3)4(H2O2)2]2+

! [Cu(NH3)(H2O)5]2+

+ 3NH3 "

48

As noted above, Xie et al. (1995) observed a ratio of copper to substrate reaction sites of 1:2 and

ratio of copper to ammonia of 1:2; this confirms that reactions with wood may differ from those

with model compounds.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

2 3 4 5 6 7 8 9 10 11 12 13

pH

Cu

an

d N

H3 a

dso

rptio

n (

mm

ol g

-1 w

oo

d)

0.4% Cu before leaching

0.4% Cu after leaching

NH3 after leaching

CEC

Figure 3.10 Chemisorption of Cu and NH3 in red pine dust from Cu-Am solution. Note: filled

square shows the results after leaching with 3% NH4OH solution.

3.5 Conclusions

While the maximum copper adsorption is defined by the cation exchange capacity of wood, it

also depends on ligand type, copper/ligand ratio and pH. Copper in ethanolamine solution can be

adsorbed into wood up to its cation exchange capacity (CEC) regardless of the types of Cu-

amine species. It is irrelevant, whether the Cu complex is 1+ or 2+ charged in treating solution.

However, copper adsorption was considerably decreased as uncharged Mea and [Cu(Mea)2-2H]0

formed. As a Cu-Mea fixation mechanism in wood, [Cu(Mea)2-H]+ seems an active Cu species

and it exchange one Mea molecule with wood acidic sites to form stable [Cu(Mea)-H]-wood

complexes. Mea is superior to Dea and Tea as a ligand in that it induces Cu precipitation in a

broader range of pH. Its higher pH capacitates more acidic sites of wood for dissociation, and its

higher pKa value is able to retard formation of uncharged free amine and uncharged Cu-amine

49

complexes. Unlike the adsorption pattern with copper-ethanolamines, copper-ethylenediamine

showed less than half the amount of Cu adsorption at moderate pH and highest Cu adsorption at

high pH, possibly due to adsorption onto cellulose. In the cuprammonium system, copper

hydroxide precipitation increased the apparent copper adsorption at high pH. After removal of

the precipitate with ammonium hydroxide, 2+ charged Cu species appeared to be adsorbed on

wood as if they were singly charged.

50

Chapter 4 Copper precipitation in Cu-monoethanolamine preservative treated wood

4.1 Abstract

Copper precipitation is one of the important Cu fixation mechanisms of Cu-amine treated wood.

Possible pathways of Cu precipitation, effects of pH and temperature, and Cu species formed in

Cu-Mea treated wood were investigated through comparing in vitro and in situ studies. Higher

concentration Cu-monoethanolamine (Cu-Mea) solutions needed higher amounts of acid for Cu

precipitation and also required a lower pH to initiate Cu precipitation because of higher free Mea

in the preservative-wood system. For this reason, Cu fixation during conditioning of wood

treated to high ACQ retention (2.0% solution) resulted in only a slight pH decrease and low Cu

fixation. For wood treated with lower concentration solutions (0.67% and 0.2%), the pH

decreased enough for Cu precipitation and there was a much higher Cu fixation rate driven by

both chemisorption and precipitation. However, evaluation of leaching after drying and

conditioning showed that additional Cu precipitation could occur during drying. Conditioning of

Cu-Mea treated wood at high temperature (50 ºC) showed outwardly faster and higher Cu

fixation, but showed ultimately higher Cu leaching. X-ray diffraction analysis revealed that the

in vitro precipitated Cu was a mixture of azurite and malachite and possibly Cu2O formed as a

result of Cu-Mea decomposition.

4.2 Introduction

Copper amine preservative systems have become widely used wood preservatives since

withdrawal of chromated copper arsenate (CCA) for residential applications and great efforts

have been made to understand their fixation mechanisms in wood. Understanding Cu-amine

preservative fixation mechanisms in wood will identify the best conditions to minimize leaching

to ensure maximum lifetime of the treated wood and reduced its impact on the environment.

The fixation mechanism of Cu from Cu-amine preservatives in treated wood is thought to be

complexation or ion exchange of Cu complex cations at the sites of proton dissociation from

51

weak acid sites of wood (Rennie et al. 1987; Cooper 1991; Zhang and Kamdem 2000b,c;

Ruddick et al. 2001), followed by precipitation of low solubility Cu compounds (Jiang and

Ruddick 2004) as the pH in the treated wood system decreases. Therefore, fixation can be

characterized as chemisorption accompanied by physical precipitation; however there has been

less focus on Cu precipitation because it is not easy to evaluate as an isolated process, since the

fixation processes are complicated, concurrent, and interrelated. Because Cu chemisorption

capacity through ion exchange (or complexation) is limited, treating wood with low

concentration of Cu-amine can lead to higher and quicker Cu fixation by chemisorption with low

Cu leaching. However, Cu contents above the cation exchange capacity of wood should be

stabilized through insoluble Cu precipitation; otherwise, the still remaining soluble Cu ends up

leaching during service (Tascioglu et al. 2005; Ung and Cooper 2005). In other words, higher Cu

retention of treated wood can be stabilized only through higher Cu precipitation due to the

limited chemisorption sites of wood.

Except Cu-HDO (N-cyclohexyl-diazenium-dioxy) and CDDC (Copper dimethyl-dithio-

carbamate) systems which incorporate strong complexing agents to generate insoluble Cu

complexes (Craciun et al. 1997; Jiang 2000), the Cu precipitation mechanism of Cu-amine relies

on pH drop of the system. In Cu-ammonium systems (e.g., ACQ-types A and B), the pH drop

can be easily driven by ammonia evaporation (Jiang 2000) but it is not certain how Cu

precipitation can be initiated in Cu-Mea system (ACQ-types C and D) because of the high

boiling point of Mea (# 170 ºC). Since Mea cannot evaporate like ammonia, it should be

investigated whether H+ released through ion exchange is enough to lower the solution pH and if

other factors affecting Cu precipitation can be identified.

Like iron catalyzed oxidation of wood components (Emery 1974), transition metal Cu (I and II)

salts and complexes are also known as catalysts to oxidize various organic compounds

(Muralidharan and Freiser 1989; Li and Trush 1993; Markó et al 1996; Chaudhuri et al. 1999;

Tsai et al. 2005; Velusamy et al. 2006). Therefore, it is of interest whether Cu-Mea can induce

oxidation of ROH in wood to create additional reaction sites.

In the present study, Cu-monoethanoamine (Cu-Mea) fixation processes in wood are investigated

focusing on its physisorption through precipitation. Several important factors affecting Cu

52

precipitation and precipitated Cu species are discussed through comparing in vitro and in situ

studies.


4.3.1 Effect of Cu-Mea concentration on in vitro copper precipitation

To measure the pH at which Cu-Mea starts to precipitate, samples of commercial Cu-Mea (31%

Mea, 9% Cu: Osmose Inc. NY) and formulated Cu-Mea (Cu-acetate mixed with Mea at 2.75

times the equivalent CuO weight, AWPA 2006) were used. Thirty ml of 0.2%, 0.67% and 2.0%

Cu-Mea solutions were prepared in 50 ml test tubes and the solution pH was sequentially

reduced by adding different amounts of 0.1N – 1N HNO3. After 5 days, the supernatant was

carefully collected for soluble Cu analysis by X-ray fluorescence spectroscopy (Oxford

Instruments LAB X 3000 Abingdon, Oxfordshire, UK). The % of Cu precipitated was calculated

based on the initial concentration. To check the influence of temperature on Cu precipitation, an

additional batch was prepared with pH adjusted to slightly higher than that of precipitation

(0.2%: pH 9.5, 0.67%: pH 9.0, 2.0%: pH 8.5) and conditioned with and without lids at 50 ºC.

Because ionic strength can affect Cu ion activity and the Cu solubility, the ionic concentration

effect was also explored at the same Cu-Mea concentration (0.67 %) with different ionic

concentrations (0, 0.5, and 1.0 M KCl).

4.3.2 XRD and XRF analysis of precipitated copper

To determine the species of precipitated Cu, the precipitates were analyzed by X-ray diffraction

(PW 1830 HT generator, PW 1050 goniometer, and PW 3710 control electronics, Philips

Analytical, Natick, MA, USA) and X-ray fluorescence spectroscopy (Philips PW 2404, Philips

Analytical, Natick, MA, USA) after washing with distilled water and drying at 50 ºC. XRD

analysis was carried out using Cu-K$ radiation at 40 kV and 40 mA. The diffraction angle (2%)

was measured from 5 º to 65 º (copper carbonate and copper hydroxide) or 25 º to 75 º (copper

(I) oxide) in steps of 0.02 º 2 %, with measuring time per step of 2 seconds. Reference x-ray

patterns for the Cu compounds, were obtained from X-pert highscore software and the searchable

ICDD powder diffraction file (2005) database were used.

53

4.3.3 Relationship between Cu adsorption and pH in wood during conditioning

Twenty mm cubes of red pine (Pinus resinosa

Ait.) sapwood were cut from untreated air-dry

red pine pole sections and conditioned to 10 %

moisture content. The samples were treated

(vacuum at 11 kPa absolute pressure for 10

min followed by a pressure at 1020 kPa for 20

min) with 0.2%, 0.67%, and 2.0% Cu-Mea

solution (Osmose Inc. NY). After treatment,

samples were conditioned in a plastic bag

either at 22 ºC (room temperature) for 34 days

or at 50 ºC for 10 days. At different times

after treatment, three randomly selected

replicate samples were squeezed in a press

(2,500lb cm-2

) to express treating solution (Figure 4.1) to measure pH and analyze CuO content

(by XRF) of the expressed solution. The difference in concentration of the expressed solution

relative to the initial treating solution along with solution uptake were used to calculate fixed Cu

content and fixation ratio. The initial solution concentration was corrected for the effect of cell

well hydration assuming the fibre saturation point moisture content of 35% (Cooper 1998).

4.3.4 Evaluation of CEC changes and Cu(I) formation

To explore if Cu-Mea can oxidize wood components during the conditioning process, CEC and

Cu conversion rate from Cu(II) to Cu(I) were measured with Cu-Mea treated blocks after the

conditioning described above. Blocks conditioned at 22 ºC and at 50 ºC were removed from the

bags and dried at the same conditions for 2 weeks (< 8% MC) then ground to pass a 40 mesh

screen. The Cu(I) content in prepared wood dust was measured colorimetrically described in

3.3.3 (Cui 1999; Basem et al 2006). The CEC of the wood dust was determined following

extraction of adsorbed Cu with 0.5 M HCl. The remaining Cu after extracting with HCl was

negligible (< 0.002 mmol g-1

wood). Differences among groups were determined using ANOVA

and Tukey multiple comparison intervals ($ = 0.05). To accelerate leaching, 2g of the wood dust

Figure 4.1 Monitoring Cu stabilization by

expressing solution from Cu-Mea treated

wood

54

were placed on a shaker and leached with 100 ml distilled water for 10 days (Waldron et al.

2005). The water was changed every second day. The Cu contents before and after leaching were

determined by X-ray fluorescence spectroscopy (Oxford Instruments, LAB X 3000).


4.4.1 In vitro copper precipitation of Cu-Mea

4.4.1.1 Commercial Cu-Mea

In general, even though wood is treated with pH 9-9.5 Cu-Mea solutions, the equilibrium pH of

treated wood decreases to 6-8 due to the buffering capacity of wood and other pH-affecting

reactions. This can result in higher Cu fixation in high retention treatments by precipitation of

low solubility Cu compounds in wood (Jiang and Ruddick 2004). Therefore, Cu-Mea injected in

wood exceeding the cation exchange capacity (CEC) may still be immobilized in wood. Cu-Mea

concentration had a significant effect on Cu precipitation (Figure 4.2). The Cu of 0.2% Cu-Mea

started to precipitate near pH 9 while precipitation did not occur until pH 8 was reached for 2.0%

Cu-Mea, indicating it is difficult to induce the Cu precipitation for higher concentrations of Cu-

Mea. The higher Cu-Mea concentrations required more acid to drop the pH to the Cu

precipitation point (Figure 4.2b) and moreover, the precipitation pH was shifted to a lower pH

(Figure 4.2a). The retarded precipitation pH may be related to the absolute amount of free amine

in the Cu-Mea system; considering Mea to Cu molar ratio in ACQ system is 3.3-3.9 (AWPA

2006b) which is higher than the Cu-Mea complex ratio (1:2) at pH around 9 (Tauler and

Casassas 1986), higher concentration ACQ has higher absolute amount of free Mea that

functions as a pH buffer. Furthermore there is a higher concentration of unprotonated Mea which

can form complexes with Cu to keep it in solution. This may be one of the reasons why low

concentration Cu-Mea stabilizes better and faster in wood than high concentrations (Ung and

Cooper 2005). Because ionic strength can affect Cu ion activity and the Cu solubility, the ionic

concentration effect was also explored (Figure 4.2c). Higher ionic strengths increased the

proportion of Cu precipitation. Generally, Cu solubility increases as KCl is added to Cu solution

by forming soluble chloride-cupric complex (Jin et al. 2010), but in the Cu-Mea system, chloride

could also react with MeaH+ especially in the presence of high chloride concentration which

55

resulted in lower Cu solubility. Nevertheless, the higher ionic strengths had little effect on the pH

which initiated Cu precipitation.

0

20

40

60

80

100

6.577.588.599.510

Cu

pre

cip

ita

tio

n (

%) 0.2 %

0.67 %

2.0 %

(a)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

6.577.588.599.510

HN

O3 (

mm

ol l-1

)

(b)

0

5

10

15

20

25

8.08.59.09.510.0

pH

Cu

pre

cip

ita

tio

n (

%)

0 M

0.5 M

1.0 M

(c)

Figure 4.2 In vitro Cu precipitation of commercial Cu-Mea at different pH (22 ºC): (a) effect of

concentration on precipitation pH; (b) HNO3 consumption for the Cu precipitation; (c) ionic

concentration effect on 0.67% Cu-Mea precipitation.

Cu precipitated at pH 7.5 had a distinct blue tinge and was amorphous (no XRD pattern); it

showed the best match with azurite ((CuCO3)2·Cu(OH)2) based on CuO content (69.75% CuO

compared to 69.77% CuO based on formula) by XRF analysis. The XRD pattern of the

precipitate collected at pH 6.5 showed that it was poorly crystallized, but contained malachite

(CuCO3·Cu(OH)2) illustrated in Figure 4.3. Therefore, the precipitated Cu formed near neutral

pH is likely a mixture of azurite with a small amount of malachite both of which are common

56

basic copper carbonates. Even though most of the precipitate was the azurite form, azurite is

unstable in open air compared to malachite; the weathering process of treated wood containing

azurite would result in the replacement of some CO2 with H2O and conversion to malachite as

shown in Eq. 4.1 (Mansour 1994).

2((CuCO3)2 ·Cu(OH)2) + H2O ! 3(CuCO3·Cu(OH)2) + CO2 (Eq. 4.1)

2-Theta scale

10 20 30 40 50 60

Inte

nsity,

arb

itr.

units

nn

(a)

(b)

Figure 4.3 (a) X-ray diffraction pattern of precipitated copper in Cu-mea solution at pH 6.5: (b)

standard stick pattern for malachite (CuCO3·Cu (OH)2) (ref. # 00-001-0959: PDF-2, 1998).

4.4.1.2 Cu-acetate + Monoethanolamine

To investigate the effect of carbonate which is contained in commercial ACQ (AWPA 2006b),

comparison precipitation tests were performed with formulated Cu-Mea which does not contain

carbonate anions (Figure 4.4). Like commercial Cu-Mea, the pH where Cu precipitation was

initiated was lowered as the concentration increased. However, compared to that of commercial

Cu-Mea, overall Cu precipitation pHs were also lowered, indicating that the absence of

carbonate anion in the formulation inhibited precipitation. For 0.2% Cu, the precipitation pH was

around 8.0, compared to 9.0 for the commercial formulation; at 0.67% Cu it was about pH 7.4

compared to 8.5 and at 2.0% Cu the precipitation pH was 8.0 for commercial Cu-Mea and 6.5 for

copper acetate-Mea. The XRD analysis showed that the precipitate was a crystallized Cu(OH)2

57

regardless of the pH (Figure 4.5). Thus, the species of Cu precipitated depends on whether there

is a CO2 source; the CO2 might come from different sources (e.g., CO2 added to the concentrate

to adjust its pH, dissolved basic copper carbonate as the copper source, or carbonate/bicarbonate

counterion in quat). The Cu(OH)2 formed in a narrower pH range (initiated at lower pH) than

basic copper carbonate formed in commercial Cu-Mea or CO2 facilitated Cu precipitation when

the same amount of Mea was used for the two formulations. For formulated Cu-Mea, 3.6 times

(mol) higher Mea than Cu was used according to AWPA (2006b); however if Cu-Mea could be

formulated at a lower Mea ratio, the precipitation would initiate at a higher pH because of lower

free Mea. This suggests a possible formulation change to lower Mea/Cu ratio to promote Cu

precipitation in wood.

0

20

40

60

80

100

5.56.57.58.59.510.5

pH

Cu

pre

cip

ita

tio

n (

%)

0.2%

0.67%

2.0%

Figure 4.4 Effect of concentration on in vitro Cu precipitation of formulated Cu(acetate)-Mea at

different pH (22 ºC)

58

2-Theta-scale

10 20 30 40 50 60

Inte

nsity,

arb

itr.

units

(a)

(b)

Figure 4.5 (a) X-ray diffraction pattern of precipitated copper in formulated Cu-mea solution;

(b) standard stick XRD pattern for Cu(OH)2 (ref. # 00-003-0307: PDF-2, 1998).

4.4.2 Relationship between Cu stabilization and pH during the conditioning process

Figure 4.6 shows the relationship between pH change and Cu fixation based on the expressed

solution from Cu-Mea treated blocks. This expressed solution method demonstrates the Cu

stabilization during conditioning, just before the drying step, because wood block samples should

always be in wet condition for this method. When conditioned at 22 ºC (Figure 4.6a, b, c), the pH

of 2 % Cu-Mea treated blocks was about 8.9 even after 34 days (816 h) and did not reach pH

about 8 required for precipitation (Figure 4.6a). Therefore, Cu fixation should be driven only by

chemisorption, with little or no precipitation. Indeed, the fixed Cu after 34 days was about 0.18

mmol g-1

wood (Figure 4.6b), which is just slightly higher than the CEC of 0.16 mmol g-1

wood

at pH around 9 (Figure 2.1a). As a fixation ratio, it corresponds to only 32 % for the copper

present in the wood (Figure 4.6c). Copper chemisorption or ion-exchange processes are

equilibrium processes (Craciun et al. 2009), so it is not easy for Cu chemisorption to attain the

maximum CEC values when low concentration Cu-Mea is applied. The fixed Cu amount for

0.67% Cu-Mea treated blocks stabilized at about 0.11 mmol g-1

wood after 2 weeks (336 h), but

it increased to about 0.15 mmol g-1

after the pH reached about 8.7, close to the value where

59

precipitation is expected to become significant. This was close to the fixed Cu value of 2.0%

treated blocks. Total fixed Cu ratio was about 79% (Figure 4.6c). 0.2% Cu-Mea treated blocks

showed very quick and high Cu fixation ratio (90%) which was driven by both chemisorption

and precipitation (Appendix 4.1) following fast pH decrease (Figure 4.6a), but there was a low

amount of fixed Cu due to low Cu content in the treating solution.

5

6

7

8

9

10

0 200 400 600 800

pH

0.20% 0.67% 2.00%

(a)

5

6

7

8

9

10

0 30 60 90 120 150 180 210 240

0.20% 0.67% 2.00%

(d)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 200 400 600 800

Fix

atio

n (

mm

ol g

-1 w

oo

d)

(b)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 30 60 90 120 150 180 210 240(e)

0

20

40

60

80

100

0 200 400 600 800

Time (h)

Fix

atio

n (

%)

(c)

0

20

40

60

80

100

0 30 60 90 120 150 180 210 240

Time (h)(f)

Figure 4.6 Change in pH of Cu-Mea solution expressed (a,d), and its relevance to Cu fixation

amount (b,e) and percent fixation (c,f) in wood blocks conditioned at room temperature (a,b,c)

and at 50 ºC (d,e,f) without drying.

60

The Cu stabilized faster with higher percent fixation accompanied by greater pH drop when

conditioned at 50 ºC (Figure 4.6d, e, f), and the fixation mechanisms involved seem more

complex than at 22 ºC. For example, 2.0 % treated blocks had fixed Cu amounts of about 0.3

mmol g-1

wood after 2 days, which is far above the CEC while the pH did not reach a low

enough pH to induce precipitation. Temperature plays a significant role in the solubility of Cu in

water. The higher temperature can increase the Cu precipitation pH and increase the precipitation

ratio as well (Dortwegt 2001). Parekh and Andrew (1968) showed malachite transformation time

from Na2CO3-CuSO4 was reduced from 850 min at 20 ºC to 27 min at 50 ºC. The average

malachite particle size formed was also affected by temperature: 3.7 microns at 20 ºC and 10.8

microns at 50 ºC. Larger particles should be advantageous in terms of reducing Cu leaching.

Therefore, higher Cu fixation than the CEC might result from the accelerated Cu precipitation at

high temperature. To determine if accelerated Cu precipitation occurred at high temperature,

2.0%, 0.67%, and 0.2% commercial Cu-Mea solutions with pH adjustment to 8.5, 9.0, and 9.5

respectively were sealed in jars and conditioned at 50 ºC for 5 days. However, no precipitate was

observed, which indicates temperature is not the only factor affecting precipitation.

Because of high pH drops at 50 ºC from 9.5 to 8.4 in 2.0% Cu-Mea treated blocks regardless of

high concentration, Mea evaporation during conditioning was considered, and the same in vitro

test was conducted at the same condition (50 ºC) without a lid allowing Mea evaporation.

However, no precipitate was formed and pH actually increased as only water evaporated because

of the high boiling point of Mea (170 ºC). Even after the water totally dried up, the pH did not

decrease, indicating little Mea evaporation. If this phenomenon occurs during drying of Cu-Mea

treated wood, Cu would never be precipitated in wood because of concentration of Mea and the

resulting high pH. Therefore there might be other reactions involving Mea0 loss in wood, such as

Mea0 adsorption by wood and Mea

0 reaction with CO2 by an acid-base neutralization reaction to

form a soluble carbonate salt.

The alkalinity of Cu-Mea comes from free Mea0. Therefore, a high amount of Mea adsorption by

wood will decrease the free Mea in absorbed Cu-Mea solution. This will promote Cu

precipitation in a manner similar to the system at low Cu-Mea solutions (precipitation at high

pH). Indeed, as shown in Chapter 2, higher Mea adsorption than cation exchange capacity of

wood above pH 9 occurred, possibly through hydrogen bonding. Tascioglu et al. (2005) also

reported that changes in the Mea concentration of expressed solutions indicated that 2.6-4.9

61

moles of Mea were adsorbed per mole of Cu with the ratio increasing with retention. Since the

Cu to Mea chemisorption ratio is about 1:1 (Jiang and Ruddick 2004), this indicates that a

significant amount of free amine is adsorbed in the wood cell wall, which could result in more

Cu precipitation. The hydrogen bonding between wood and free Mea would be accelerated as the

temperature increases and the water evaporates.

On the other hand, Mea is a well known CO2 sorbent forming a soluble carbonate salt (Eq. 4.2)

(Strazisar et al. 2001, 2003; Maceiras et al. 2008).

2RNH2 + CO2 + H2O ! (RNH3)2CO3 (Eq. 4.2)

Consumption of Mea by reacting with CO2 from the air, from the CO2 formulated in commercial

ACQ and/or from the quat anion (carboquat) might also induce Cu precipitation in commercial

ACQ treatment. The reaction of CO2 with Mea solution increases with temperature and with Mea

concentration (Maceiras et al. 2008). Considering the relatively low Mea concentration as CO2

adsorbent in commercial Cu-Mea solutions, CO2 adsorption might be increased only by

concentrated Mea in wood with water evaporation. Tascioglu et al. (2009) reported that CO2

pressurization immediately after ACQ treatment could not increase Cu fixation over the long

term because the initial pH drop following the introduction of CO2 was recovered in time as CO2

dissipated from the solution.

Another possible reaction is insoluble Cu (I) precipitation through decomposition of Cu-amine.

In the in-vitro study described above, long time exposure (7 days) of uncovered Cu-Mea solution

resulted in evaporation of water from the solution, leaving a reddish brown material. After

adding water up to the initial Cu-Mea volume and sonicating it in order to dissolve soluble Cu,

the solution pH was measured and the insoluble reddish brown material was filtered for XRD

analysis. There was little pH change even after the solution dried up, indicating little Mea

evaporation. XRD results showed that the insoluble material was Cu2O (Figure 4.7). Because no

precipitate was observed before the solution dried up, the Cu2O may be decomposed from Cu-

Mea complex rather than from basic copper carbonate. Eqs. 3 and 4 show the possible

decomposition and reduction processes (Wendlandt 1963; Hoffmann et al. 2003; Pike et al.

2006). The decomposition processes occur at greatly elevated temperature, but were also

observed when Cu-Mea was conditioned at relatively low temperature (50 ºC) for a long enough

time for water to be evaporated. Because in the current study, all treated wood samples were

62

conditioned under wet conditions, this decomposition process likely did not occur. However,

these processes are to be expected on the partially dried wood parts especially on wood surfaces

during high temperature drying of Cu-Mea treated wood.

Cu(Mea)2(H2O)2 ! CuO + 2Mea + H2O + H2 (Eq. 4.3)

4CuO ! 2Cu2O + O2 (Eq. 4.4)

Despite all of these possible Cu precipitation processes, Cu fixation ratio of 2.0 % treated blocks

was only about 64 % even after 9 days (220h) while those of 0.67% and 0.2% treated blocks

were about 87 % and 92 % respectively (Figure 4.6f).

2-Theta-scale

30 40 50 60 70

Inte

nsity,

arb

itr.

units

(a)

(b)

Figure 4.7 (a) X-ray diffraction pattern of precipitated copper in dried Cu-Mea solution; (b)

standard stick pattern for Cu2O (ref. # 01-071-4310: PDF-2, 1998).

4.4.3 Copper stability after drying and influence of oxidation of wood components

Although the observed higher Cu stabilization over CEC can be caused by insoluble Cu

precipitation, it could also result from additional Cu chemisorption if Cu amines can oxidize

some alcohol groups in wood during the conditioning process. For reference, there was no

evidence of oxidation or delignification of wood during Cu-Mea treatment without conditioning

63

(Chapter 3). The wood blocks conditioned at 22 ºC and 50 ºC were dried and ground for CEC

determination with 0.05M NaOH followed by extracting with 0.5 M HCl. Figure 4.8 shows the

CEC changes after conditioning of Cu-Mea treated wood. The CEC of 0.2% Cu-Mea treated

wood was about 0.15 mmol g-1

wood, similar to control wood at both temperatures while 0.67%

and 2.0% treated wood had increased CEC values. The wood conditioned at higher temperature

had noticeably higher CEC. The CEC of 2.0% Cu-Mea treated wood conditioned at 50 ºC

increased 47% compared to control wood. This indicates that the Cu-Mea system can oxidize

wood components and the oxidation process can be accelerated by high temperature. Copper

complexes and copper salts have been widely used as catalysts for the oxidation of organic

compounds even in the aerobic condition although it commonly used in high temperature

(Muralidharan and Freiser 1989; Li and Trush 1993; Markó et al. 1996; Chaudhuri et al. 1999;

Tsai et al. 2005; Velusamy et al. 2006). The increase in the CEC at higher Cu-Mea concentration

may result from the high alkalinity of Mea while Cu acts as a catalyst (Kimura and Kubo 1959).

0.00

0.05

0.10

0.15

0.20

0.25

0.2% 0.67% 2.0% 0.2% 0.67% 2.0%

Control 22ºC 50ºC

To

tal C

EC

(m

mo

l g

-1 w

oo

d)

a1) ab

b

c

ab

cd

Figure 4.8 Cation (Na+)

2) exchange capacity of Cu-Mea treated wood and control wood at pH 9

after 6 week conditioning. * Note:

1) Different letters indicate statistical differences (ANOVA,

Tukey intervals, $ = 0.05).

Studies of Cu complexes as redox catalysts showed that oxidation can be driven by Cu(I) (Markó

et al 1996; Tsai et al. 2005) and sometimes, it can also be driven by Cu(II) accompanying Cu

reduction to monovalent Cu(I) (Li and Trush 1993). Therefore, we expected that there might be

some correlation between the proportion of Cu(I) and increased CEC. Figure 4.9a compares the

64

proportion of Cu(I) conversion between samples treated and conditioned at different

concentrations and temperatures. As generally known from other studies (e.g., Tascioglu et al.

2008), treated wood conditioned at higher temperature has higher conversion of copper from

Cu(II) to Cu(I). However, the results show that the wood conditioned at lower temperature had

more Cu(I), indicating that there might be more factors affecting the Cu conversion. The wood

conditioned at 22 ºC was never exposed to high temperature even during the drying process, but

was in a wet condition for a longer time than the sample conditioned at 50 ºC. This prolonged

wet condition might increase the Cu reduction to Cu(I). Unlike our expectation, there was no

correlation between the proportion of Cu(I) (Figure 4.9a) and CEC increase (Figure 4.8); the

wood conditioned at 50 ºC showed less Cu(I) content but had a greater increase in CEC.

The stability of the Cu in wood was evaluated by a leaching test. After drying followed by

conditioning, the treated wood blocks were ground for accelerated leaching (Waldron et al.

2005) to estimate the total potential for Cu leaching. Figure 4.9b shows the Cu content in wood

before and after leaching and the latter represents stabilized Cu in wood after drying. According

to Figure 4.6 and Figure 4.8, the wood conditioned at 50 ºC was expected to have higher

leaching resistance. However, higher leaching resistance was observed in the wood conditioned

at 22 ºC regardless of concentration, whereas the 2.0% Cu-Mea treated wood conditioned at 50

ºC showed very high leaching ratio (36%) even though it had the highest Cu stabilization during

conditioning up to 0.32 mmol g-1

wood and highest CEC (0.22 mmol g-1

wood). It is difficult to

explain the discrepancy and more specific investigation is needed. Nevertheless, comparing the

results in Figure 4.6 and Figure 4.9b provides some insights into possible reactions of Cu. First,

there may be more Cu stabilization by precipitation during the drying process. For example, the

Cu stabilization test in Figure 4.6 was terminated while the wood blocks were still wet and the

blocks were dried for the leaching test. The stabilized Cu of 2.0% Cu-Mea treated blocks was

0.18 mmol g-1

wood in Figure 4.6b with little pH decrease, but after accelerated leaching

followed by air drying for 2 weeks, it increased significantly to 0.26 mmol g-1

wood (Figure

4.9b). Considering there was little decrease in pH of expressed solution during conditioning

(Figure 4.6a) and CEC of 0.18 mmol g-1

wood (Figure 4.8), the greater amount of stabilized Cu

might come from its precipitation during the drying process. Second, higher Cu stabilization of

wood conditioned at 22 ºC (Figure 4.9b) might simply result from higher insoluble Cu(I)

formation (Figure 4.9a). Although treated wood conditioned at high temperature had greater

65

CEC (Figure 4.8), if more soluble/leachable Cu in wood conditioned at low temperature was

converted to insoluble Cu(I), higher Cu stabilization could be realized. Last and most probably,

the Cu-Mea system could depolymerize wood components. Depolymerization of lignin during

Cu-ethanolamine treatment was also proposed by Humar et al. (2007b). In the current study, Cu

adsorbed in depolymerized lignin could be interpreted as a fixed Cu (0.32 mmol g-1

wood) by the

expressate method, while it potentially could be leachable during the leaching process. Mea is a

well known solvent for high temperature alkaline pulping (Claus et al. 2004), but it can also

depolymerize wood lignin even at low temperature. Wise et al. (1939) reported that different

species of wood sections treated with Mea for 10 days at 15 ºC and 28 ºC showed evidence of

delignification. Kimura and Kubo (1951) observed 4-10 times higher alkaline autoxidation of

cellulose in the presence of Cu(II) even at low alkali concentration. Thus, Cu catalyzed alkaline

depolymerization of wood components could be possible during conditioning, which could result

in higher Cu leaching of 2.0% treated wood conditioned at 50 ºC. The difference in color

between wood conditioned at 22 ºC (green) and wood conditioned at 50 ºC (dark brown) might

be an indication of some depolymerization in wood. This premise also matches with the result in

Figure 4.8. Higher oxidation degree may result from higher depolymerization, leading to higher

Cu leaching. Although it needs to be confirmed, the long conditioning period in the current study

may have increased the oxidation rate and possible depolymerization. Therefore, delayed drying

and a long conditioning time is not recommended for higher Cu fixation of Cu-Mea treated

wood. The expressed solution method for determining Cu-Mea fixation rate in wood (Tascioglu

et al. 2005; Ung and Cooper 2005) should be interpreted carefully because any additional Cu

stabilization during the drying process is not observed; similarly any depolymerization of wood

components resulting in higher Cu leaching would also not be seen.

66

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.2% 0.67% 2.0% 0.2% 0.67% 2.0%

22ºC 50ºC

Cu

re

ten

tio

n (

mm

ol g

-1 w

oo

d)

Cu (II) Cu (I)

(a)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.2% 0.67% 2.0% 0.2% 0.67% 2.0%

22ºC 50ºC

Cu

re

ten

tio

n (

mm

ol g

-1 w

oo

d)

Before leaching After leaching

(b)

Figure 4.9 (a) Copper retention (Cu(I) and Cu(II)) of Cu-Mea treated wood and (b) total copper

before and after accelerated leaching.

4.5 Conclusions

In vitro studies showed that at higher Cu-Mea solution concentration, Cu was precipitated at

lower pH indicating that it is harder to form Cu precipitate in higher concentration Cu-Mea

treated wood. This is because of higher amount of free Mea in higher concentration solution,

which suggests that Cu precipitation fixation reactions may be enhanced by reformulating with

67

lower Mea content. The copper carbonate precipitates formed were amorphous azurite and semi-

crystalline malachite. The Cu-Mea formulated in the laboratory without CO2 formed crystalline

copper hydroxide; it formed at lower pH than copper carbonate. Insoluble Cu2O precipitation

through Cu-Mea decomposition is also possible. Copper fixation rate in Cu-Mea treated wood at

high concentration (2%) during conditioning, was dominated by chemisorption because the pH

was too high for precipitation. Cu fixation was driven by both chemisorption and precipitation at

intermediate (0.67%) and low concentration (0.2%) copper solutions. However, the drying

process applied after the initial conditioning could induce more Cu precipitation of high

concentration (2%) Cu-Mea treated wood. Higher temperature conditioning accelerated Cu

stabilization but caused higher Cu leaching, probably because of accelerating depolymerization

of wood components at higher temperature. Additional study is needed to clarify the reactions

during fixation at high moisture content followed by drying.

68

Chapter 5 Alkyldimethylbenzylammonium chloride (ADBAC) adsorption mechanism on wood

5.1 Abstract

The adsorption of alkyldimethylbenzylammonium chloride (ADBAC) on wood was investigated.

The changes in zeta potential curves of wood and ADBAC adsorption with increasing ADBAC

concentration were highly correlated and showed two different mechanisms for ADBAC

adsorption on wood: ion exchange reaction at low concentration and additional aggregation form

of adsorption by hydrophobic interaction at high concentration. ADBAC adsorbed at a low

solution concentration had high leaching resistance while ADBAC adsorbed into wood above the

critical micelle concentration (CMC) had low leaching resistance. The CMC decreased with

addition of Mea, Cu-Mea, and buffer chemicals. The anion, Cl- of ADBAC was only adsorbed at

solution concentrations above the CMC and was easily leached out. The adsorption isotherm of

ADBAC on wood before and after leaching was fit to the Langmuir, BET, and Freundlich

isotherm models; the BET and Freundlich models fit the adsorption isotherm well before

leaching and the Langmuir and the Freundlich models showed better fits to the adsorption

isotherm after leaching. The adsorption capacity of ADBAC into wood by cation exchange did

not achieve the cation exchange capacity (CEC) of wood.

5.2 Introduction

ACQ Type C contains 67% copper compound (expressed as CuO) and 33 % ADBAC with Mea

and water co-solvents (CSA 2008). ADBAC as a cation surfactant has a feature forming

aggregation (micelle) above a CMC. As described in chapter 1, many factors can affect the

CMC. At concentrations above the CMC, the physical properties of micelle solutions change,

and the CMC can be determined by measuring the conductivity of the surfactant solutions

(Rahman 1983; Shaw 1992; Jiang et al. 2003; Fuguet et al. 2005).

When ADBAC molecules contact with a solid interface like wood, they also begin to associate

into patches (hemimicelle or admicelle) even below a CMC whereas they are adsorbed as

69

individual ions at low concentration (Somasundaran et al. 1964; Fan et al. 1997; Paria and Khilar

2004). If different interactions resulting from aggregation formation are involved in adsorption

of quat into wood, they could affect both the quat adsorption process and quat leaching from

wood, because different interactions result in different bonding strengths. For example, relatively

high quat adsorption in wood (Loubinoux and Malek 1992; Loubinoux et al. 1992; Zabielska-

Matejuk 2005) and relatively high leaching compared with the copper component of

preservatives (Cooper and Ung 2009) have been reported. Because measurement of zeta

potential has been used to explain the adsorption mechanisms of ionic surfactants

(Somasundaran et al. 1964; Ersoy and Celik 2003; Paria and Khilar 2004), it should provide

insight into the adsorption of quat at the wood/water interface.

Adsorption isotherms are commonly used to predict adsorption characteristics of liquid or

dissolved solid to a wood surface. The most commonly employed models are the Freundlich,

Brunauer-Emmett-Teller (BET) and Langmuir isotherms. The Freundlich model is purely

empirical and based on the assumption of adsorption on heterogeneous surfaces and possibly by

mutilayer adsorption. The BET isotherm has a theoretical basis and has parameters that can

determine multilayer adsorption behavior, monolayer adsorption capacity and heat of adsorption

at various adsorption layers. The Langmuir model is also a theoretical expression obtained by

assuming a uniform monolayer adsorption of the adsorbate molecules (Bueno et al. 2008; Li and

Bai 2005, Liu et al. 2006). The mathematical forms of the three isotherms for adsorption from

solution are given for the Langmuir model and its linear form in Eq. 5.1 and 5.2, for the

Freundlich model and its linear form in Eq. 5.3 and 5.4 and for the BET model in Eq. 5.5.

e

eme

Cb

Cqbq

!"

!!#

1, (5.1)

e

mme

e Cqqbq

C 11"

!

# , (5.2)

For adsorption of ADBAC on wood, eC (mmol l-1

) is the equilibrium ADBAC concentration,

eq (mmol g-1

) is the amount of ADBAC on wood at adsorption equilibrium, b is the Langmuir

isotherm constant and mq is the maximum adsorption capacity.

n

efe CKq /1!# , (5.3)

70

efe Cn

Kq ln1

lnln "# , (5.4)

where fK (mmol 1-1/n

L 1/n

/ g) is the Freundlich constant and 1/n is a constant.

e

eebbeb

bbme C

CCKCCC

CKqq

))(( $!"$

!!# , (5.5)

Where mq is the mass uptake (mmol g-1

) to give complete monolayer coverage of the wood

surface, bC is the maximum (or saturation) concentration of the ADBAC at which upward

curvature of the BET isotherm is observed, and bK is the energy constant.

In the present study, the adsorption capacity of quat in wood and its leaching resistance and their

relevance to aggregation formation are studied by comparing adsorption on wood under different

conditions. The above three isotherms were applied to the experimental results to check their

applicability and to get the best fit that can represent the adsorption behavior of quat in wood.



Air-dried red pine (Pinus resinosa Ait.) was ground to pass a 35 mesh screen and be retained on

a 60-mesh screen, washed with 0.5 M HCl to remove all metal ions initially bound to the wood

and rinsed repeatedly with distilled water until the pH of washings was constant. The sample was

then dried at 60 °C for 48 h. For treatment solutions, 50% alkyl (C12 67%, C14 25%, C16 7%,

C18<2%) dimethyl benzyl ammonium chloride (ADBAC, Lonza Inc., Allendale, NJ, USA) was

diluted to the target concentrations for use.

5.3.2 CMC measurement of ADBAC

To determine the CMCs of ADBAC, ADBAC in 1.4% or 2.8% Mea, and ADBAC in Cu (0.4%)-

Mea (1.4%) solutions, the conductivities of unbuffered solutions of different concentrations were

measured at 22 °C. The CMCs were determined as the break point of two linear portions with

71

different slopes in plots of V (conductivity) vs. C (concentration of ADBAC). Three replicate

samples were evaluated.

5.3.3 Zeta potential measurement

Zeta potential measurements were made with ZetaPlus (Brookhaven Instruments Corp.,

Holtsville, NY, USA). The ZetaPlus automatically calculates the electrophoretic mobility of the

particles and converts it to the zeta potential using the Smoluchowski equation (Zeta potential

analyzer instruction manual 2003). Wood particles were prepared by grinding with a ball mill for

48 h followed by passing through a 400 mesh screen. Ten mg of the particles were added to 50

ml of different initial concentrations of ADBAC ranging from 0.001 to 100 mmol l-1

(3.5 x 10-5

to 3.5 %). The tubes were closed with screw caps and conditioned on a shaker for 5 days. The

final pH’s of the samples were from 5.0 to 7.3 depending on the concentration. The zeta-

potential for each sample was determined ten times at 22 °C and the values averaged.

5.3.4 Adsorption isotherm of ADBAC in wood

The adsorption experiments were conducted with 2 L of ADBAC solution at different initial

concentrations ranging from 0.005 to 100 mmol l-1

(1.75×10-4

% to 3.5 %). The pH of treatment

solution was controlled with Mea to 9.5 ± 0.2, the approximate pH of commercial ACQ

solutions. Another batch was treated in 50 mmol l-1

phosphate buffer solutions (pH 8.0). Two g

of prepared red pine sawdust were added to each solution and conditioned at 22 °C for 5 days

(Loubinoux and Malek 1992) with shaking and adjusting pH every day to reach equilibrium at

the target pH. The mixtures were vacuum-filtered and the sawdust dried at 60 °C for 48 h. A 1 g

subsample of the treated sawdust was placed on a shaker and leached with 150 mL distilled

water at 200 rpm for 10 days. The water was changed every second day. 0.5 g of leached and

unleached samples (oven-dry basis) were extracted with denatured ethanol for cationic ADBAC

and 0.3 g of samples were extracted with distilled water for chloride by ultrasonic extraction

according to AWPA A16-93 (AWPA 2006c). Extracts were analyzed for ADBAC and chloride

contents by ion chromatography (Dionex DX 600, Sunnyvale, CA, USA) suppressed

conductivity using a CS14A analytical column for ADBAC and AS11-HC analytical column for

chloride, run isocratically at 1.0 ml/min flow rate. The eluents used for the ADBAC analysis

were 20% 20 mmol l-1

methanesulfonic acid and 80% acetonitrile, and for the chloride analysis

72

were 12 mmol l-1

NaOH as the mobile phase. Adsorption values before leaching were corrected

for excess ADBAC and chloride in lumen with moisture content, assuming the fiber saturation

point is 35% (Cooper 1998).

5.3.5 Fitting to adsorption isotherm models

The Langmuir, BET, and Freundlich models were applied to the data for ADBAC adsorption on

wood from ADBAC/Mea and ADBAC/buffer solutions, both before and after leaching. The

Langmuir and Freundlich equations have two adjustable parameters and their linearized forms

(Eq. 2 and 4) are frequently used for data analysis. However, when we applied the linearized

model to the experimental data, even though the correlation coefficients were high (R2 # 0.999),

the fitting with the non-linear model using the parameter values which were determined by

linearized models showed poor fits, especially at low concentrations. The use of linearized forms

of non-linear models to obtain estimates of parameters can have a problem in that the distribution

of measurement errors may be invalid after the transformation of the data (Edgehill and Lu 1998;

Robinson 1985). The BET model contains three parameters and the value of one of them, Cb

(Eq. 5) cannot be determined by merely studying adsorption behavior (Ebadi et al. 2009).

Therefore, we used non-linear regression models and all model parameters were estimated by

using an optimization algorithm that minimizes the mean square errors between the measured

and fitted ADBAC cation uptake (Al-Futaisi et al. 2007). The MATLAB program was used to

program the algorithm.


5.4.1 CMC of ADBAC

Considering the application of ADBAC in wood preservatives, no buffer or electrolytes for ionic

strength were used in this study; only the effects of additional ACQ components, Mea and Cu

were evaluated. The addition of Mea and Cu reduced the CMC values (Table 5.1). This may

result from the incorporation of Mea or Cu-Mea into the hydrophilic head groups of ADBAC,

increasing the distances between ionic head groups and decreasing the charge density of the

micellar surface (Fuguet et al. 2005; Jiang et al. 2003). Considering the CMC, of 0.027% in

Cu0.4% -Mea solution, micelles are expected to form in ACQ solution (AWPA 2006b) above

73

0.08% solution concentration. This suggests that micelles always form in the practical

concentration levels of ACQ solution; ACQ working solution (0.5-2%) typically contains 0.17-

0.7% quat.

Table 5.1 The effect of additives on CMC of ADBAC measured at 22 C

5.4.2 Relationship between zeta potential of wood and ADBAC adsorption

at different quat concentrations.

Figure 5.1 shows the zeta potential changes of wood particles as a function of the initial

concentration of ADBAC (a) and its relationship to ADBAC adsorption on wood (b). The zeta

potential curve exhibited three regions with two break points indicated as A and B (Figure 5.1

(a)) and similarly, ADBAC adsorption before leaching also showed three regions with two

transition points (Figure 5.1 (b)). Although small amounts of Mea were added to the solutions for

the ADBAC adsorption study, while DI water was used for zeta potential, considering the similar

CMC values for water and Mea1.4% (Table 5.1), the two different studies should be comparable.

Indeed, two transition points were observed at very similar concentrations at the break points A

and B. The initial negative zeta potential values reflect the negatively charged wood surface. At

the very low concentration below A, there was an increase in ADBAC adsorption and in this low

concentration range, the zeta potential remained almost constant. This indicates that cationic

ADBAC adsorbed onto wood released an equivalent amount of cations (or protons) by ion

exchange. However, in the region between A and B, a sharp increase in zeta potential values and

transition from negative to positive zeta potential (isoelectric point # 0.6 mmol l-1

) is observed

with only moderate ADBAC adsorption. The isoelectric point indicates the most stable state of

the system, at which point quat may have maximum chemisorption and high leaching resistance.

The sharp increase in zeta potential value with small ADBAC adsorption may reflect an increase

in polar (cationic) groups of ADBAC in the diffuse layer by formation of two-dimensional

CMCADBAC Component

(mmol l-1

) (%) pH range

ADBAC in Distilled Water 4.57 0.160 5.6-7.2

ADBAC in Mea1.4% 3.91 0.137 11.2-11.7

ADBAC in Mea2.8% 2.74 0.096 11.7-11.9

ADBAC in Mea1.4% + Cu0.4% 0.77 0.027 8.9-9.2

74

aggregations. In this region, adsorption occurs by increasing the number of aggregates on the

negative sites of the particle (Paria and Khilar 2004).

1E-3 0.01 0.1 1 10 100

-30

-20

-10

0

10

20

(a)

B

Zeta

pote

ntial (m

V)

A

0.0

0.1

0.2

0.3

0.4

0.001 0.01 0.1 1 10 100

Initial concentration of ADBAC (mmol l-1)

Ad

so

rptio

n (

mm

ol g

-1w

oo

d) ADBAC (Before leaching)

ADBAC (Af ter leaching)

Cl (Before leaching)

Cl (Af ter leaching)

CEC at pH 9.5

(b)

Figure 5.1 Variation of zeta-potential of red pine sawdust with the concentration of ADBAC in

distilled water at 22°C (pH 5.0-7.3) (a) and Equilibrium adsorption of ADBAC on red pine

sawdust solution controlled to pH 9.5 by Mea (b). Note: The CEC values of red pine are obtained

from Figure 2.1 (a). For estimation of ACQ concentration, 10 mmol l-1

= 0.35 % ADBAC,

corresponding to 1.05 % ACQ solution and 0.1 mmol g-1

wood # 35 mg g-1

wood

When the negative charge on the particle is neutralized, however, the energetic situation favors

the growth of existing aggregates rather than the formation of new aggregates, resulting in

significantly increased size of aggregates with adsorption density (Paria and Khilar 2004). The

break point B shows a transition point in the zeta potential value, at a concentration similar to the

CMC of ADBAC in distilled water (Table 5.1). Above this point, a significant amount of

75

ADBAC was adsorbed on wood as the solution concentration increased, which may be an

indication of micelle formation. Because of the significant increase in cationic ADBAC on the

wood surface, a sharp increase in positive zeta value was expected; however, it remained almost

constant as the concentration increased. This can be explained by adsorption of anionic chloride

above the CMC (Figure 5.1 (b)). Considering the sharp increase in ADBAC adsorption in wood

and almost constant zeta value, similar amounts of ADBAC cation and anion (Cl-) must be

absorbed, as confirmed in Figure 5.1 (b). Comparing the maximum adsorption value of about

0.37 mmol g-1

at 100 mmol l-1

with the adsorption of about 0.1 mmol g-1

near a concentration of

0.03 mmol l-1

(point A in Figure 5.1 (b)), by ion exchange, the aggregation number of micelles,

i.e., the number of monomers per aggregation seems to be low in wood. We estimate that about

3-4 molecules are aggregated at pH 9.5 from 100 mmol l-1

quat in Mea solution; this aggregation

number may vary depending on the pH and solution concentration (ionic strength). The

described adsorption mechanisms are shown schematically in Figure 5.2. Although micelles

could have different geometry such as sphere, cylinder, or bilayer shapes (Farn 2006), for

illustration purposes, it was assumed that ADBAC micelle has spherical shape.

(a) (b) (c) (d)

Figure 5.2 Schematic representation of ADBAC adsorption into wood. (a) monomer (b) two-

dimensional aggregation forms and (c) three-dimensional aggregation with the counter-ion (ion

pair) forms of adsorption depending on the concentration and (d) after leaching.

5.4.3 Effect of concentration on adsorption of ADBAC on wood

Figure 5.1 (b) shows the adsorption of ADBAC on wood before and after leaching, with

concentrations presented on a logarithmic scale. There was a steady increase in adsorption up to

near 0.1 mmol l-1

, where the isotherm flattened with only a slight increase in adsorption until 3

ClQuat

Wood--O- Wood--O

- Wood--O-

Wood--O-

76

mmol l-1

. The two break points on the adsorption curve are very similar to the aggregation

forming concentration and CMC described in Figure 5.1 (a). This shows that below the

aggregation forming concentration, ADBAC adsorption into wood is mainly determined by ion

exchange and above the concentration, a relatively small amount of aggregation may affect the

ADBAC adsorption as discussed above. Above 3.0 mmol l-1

concentration, the adsorption

increased sharply due to three-dimensional aggregation and saturation was not reached in the

tested concentration range.

The samples that were leached for 10 days showed much lower ADBAC concentrations in wood

compared to the unleached samples, especially for the higher concentration treatments. It

appeared that the ADBAC adsorbed by ion exchange was resistant to leaching, while almost all

of the ADBAC adsorbed by aggregation was leached out. Cooper and Ung (2009) reported that

38 mm boards had a steep quat (DDAC) concentration gradient and that there was relatively high

leaching of DDAC on the wood surface relative to the center after 330 days of natural

weathering. This suggests that the high concentration of DDAC adsorbed near the surface might

have been easily depleted as a result of high aggregation form of adsorption on the surface.

The cation exchange capacity (CEC) of red pine at pH 9.5 was about 0.17 mmol g-1

(Figure 2.1

(a)) and the maximum ADBAC adsorption after leaching in this study was about 0.14 mmol g-1

,

which indicates that ADBAC adsorption does not reach the CEC even at high concentration.

Sullivan et al. (1997) reported that the diameter of the polar (-(CH3)3N+) group of HDTMA

(hexadecyltrimethylammonium) is 0.694 nm with a cross-sectional area of 0.38 nm2. The

limiting pore diameter for penetration of molecules into wood cell walls is controversial, but has

generally been estimated to be in the range of 1 to 8 nm (Hill and Papadopoulos 2001). The

ADBAC monomer appears to have no difficulty in penetrating into wood cell walls, but the size

of ADBAC molecule is still big compared with the size of atomic copper, 0.066 nm2 (Clementi

and Raimondi 1963), and the lower adsorption of ADBAC relative to the CEC may be because

of steric hindrance from its greater molecular size. It is unclear how ADBAC aggregates

penetrate or form in the cell wall micro-capillaries. The dimensions of micelles are proportional

to the length of the hydrocarbon chain. The micelle radius for sodium dodecyl sulfate (C12) is

approximately 1.7 nm in water but about 30 nm in 0.8M NaCl solutions (Hayashi and Ikeda

1980; Tanford 1980). Therefore, very large micelles would be difficult to penetrate into the cell

wall or deform to penetrate into the cell wall in case of external pressure. The chloride anion that

77

was adsorbed above the CMC was not stable to leaching and none remained after the leaching

treatment.

The adsorption isotherm of ADBAC from phosphate buffered solution (pH 8.0) (Figure 5.3) was

very similar to that in Mea solution at pH 9.5 (Figure 5.1 (b)). However, the aggregation forming

concentration was decreased to 0.03 mmol g-1

and the concentration of the second increase in

adsorption also decreased to 0.06 mmol g-1

due to the chemicals used for the buffer solution. The

maximum adsorption after leaching also decreased to 0.08 mmol g-1

compared to about 0.14

mmol g-1

in Figure 5.1 (b). The lower adsorption results from the lower pH and possibly from

competition with cationic Na+ used in the buffer solution.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.001 0.01 0.1 1 10 100


Ad

so

rptio

n (

mm

ol g

-1w

oo

d)

ADBAC (Before leaching)

ADBAC (Af ter leaching)

Cl (Before leaching)

Cl (Af ter leaching)

CEC at pH 8.0

Figure 5.3 Equilibrium adsorption of ADBAC into red pine sawdust in pH 8.0 phosphate buffer

solution

Sexsmith and White (1959) proposed an ion-pair adsorption mechanism for quaternary

ammonium compounds whereby the surfactant counter-ion (anion) is adsorbed along with the

surfactant into cellulosic materials in the vicinity of the CMC of the surfactant. Adsorption of Cl-

followed this pattern in all samples used in this study (Figures 5.1 (b) and 5.3). At the

concentration of 3-7 mmol l-1

, Cl- was detected and the amount adsorbed increased as the

solution concentration increased. The amount of adsorbed Cl- was dependant on total adsorbed

ADBAC and was similar to that observed by Loubinoux and Malek (1992). However, Cl- was

not detected in any of the samples after leaching. This indicates that the anions were adsorbed to

78

the polar groups of micelles and were washed out with the micelles after leaching as illustrated in

Figure 5.2.

5.4.4 Application of adsorption isotherm models to ADBAC adsorption in wood

The fits of the three isotherm models to the experimental adsorption isotherm data before and

after leaching are shown in Figure 5.4 as a logarithm Ce scale (Appendix 5.1: normal Ce scale)

and the corresponding isotherm parameters determined from the fitting calculation are given in

Table 5.2. Regardless of different formulations, the Freundlich and the BET isotherm models fit

the isotherm before leaching better than the Langmuir model, suggesting a rather heterogeneous

surface of the adsorbent and multilayer adsorption behavior of ADBAC due to the aggregation

formation of ADBAC (Bueno et al. 2008; Liu et al. 2006). In the BET model, the monolayer

adsorption capacities, qm, were estimated to be about 0.25 and 0.19 mmol g-1

wood in different

media (Table 5.2) which are far higher than our estimates of 0.14 and 0.08 mmol g-1

wood based

on direct adsorption measurements.

Table 5.2 Adsorption constants estimated from simulations with Langmuir, BET, and Freundlich

models for isotherms of ADBAC in different media using wood dust as an absorbent

(Before leaching)

Langmuir BET Freundlich Medium pH

qm b R2 qm Kb Cb R

2 Kf n R

2

Mea 9.5 0.33 1.14 0.83 0.25 950 275 0.91 0.17 5.6 0.99

Buffer 8.0 0.22 1.63 0.89 0.19 985 337 0.93 0.12 5.8 0.98

(After leaching)

Mea 9.5 0.12 50.0 0.82 0.11 1000 284 0.46 0.09 9.6 0.94

Buffer 8.0 0.07 25.3 0.83 0.07 1000 300 0.68 0.05 8.5 0.95

The ADBAC adsorption isotherm after leaching showed apparently a typical Langmuir model

form because of the constant ADBAC concentration in wood at higher concentrations with the

increase in equilibrium ADBAC concentration. However, the Freundlich model showed a better

fit than the Langmuir model, indicating adsorbed ADBAC in wood after leaching is somewhat

heterogeneous compared to the monolayer model. This may be because of an inflection in the

curvature at high ADBAC concentration, which is reflected in the Langmuir model as a little

79

higher adsorption. The maximum adsorption capacity, qm, was about 0.12 and 0.07 mmol g-1

wood which values are very close to our estimates for adsorption after leaching. The better fitting

with Langmuir than the BET model suggests that ADBAC adsorption in wood after leaching is

rather homogeneous and ADBAC was adsorbed into wood in a monolayer manner like Cu

adsorption into wood (Cooper 1991).

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

qe

(m

mo

l g-1

wo

od

)

Experim ental data

Langm uir

BET

Freundlich

ADBAC in Mea

(a) 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

ADBAC in Mea

(b)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

1E-04 0.001 0.01 0.1 1 10 100

Ce (mmol l-1)

qe

(m

mo

l g-1

wo

od

)

ADBAC in Buf fer

(a)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

1E-04 0.001 0.01 0.1 1 10 100

Ce (mmol l-1)

ADBAC in Buf fer

(b)

Figure 5.4 Langmuir, BET, and Freundlich model fitting of ADBAC adsorption in wood before

(a) and after leaching (b) (with logarithm Ce scale)

5.5 Conclusions and implications

The surfactant ADBAC showed two different adsorption mechanisms on wood: ion exchange

reaction at low concentration and an additional aggregation form of adsorption by hydrophobic

interaction at higher concentration, especially above the critical micelle concentration (CMC).

The adsorption by ion exchange reaction is homogeneous with a monolayer type reaction

between wood and ADBAC ions and ion-exchanged ADBAC has high leaching resistance. The

adsorption of ADBAC molecules by hydrophobic aggregation is heterogeneous and multilayer

and has low leaching resistance. The CMC was decreased by adding Mea and Cu and was about

80

0.03% in 0.5% Cu-Mea solution, which indicates that micelle formation and aggregation of quat

always occur in practical application of ACQ. Anionic chloride is also always adsorbed in

practical ACQ treated wood because working solutions are above the CMC.

81

Chapter 6 Effects of ionic strength, Mea, Cu, and pH on alkyldimethylbenzylammonium chloride (ADBAC) adsorption in wood

6.1 Abstract

Various factors were investigated that could affect the adsorption of alkyldimethylbenzyl

ammonium chloride (ADBAC) on red pine wood. An increase in ionic strength of ADBAC

solution had little effect on ion exchange but allowed higher hydrophobic uptake of ADBAC in

wood. ADBAC solution containing high amounts of monoethanolamine (Mea) and Cu decreased

the chemisorption of ADBAC; free Mea and Cu appear to compete with ADBAC cations for the

same bonding sites in wood. When ADBAC in Mea solution was adsorbed on wood under

different pH conditions, ADBAC adsorption increased with increasing pH, but was considerably

lower than the cation exchange capacity (CEC) of red pine. Red pine blocks were treated radially

and longitudinally with alkaline copper quat (ACQ) solution to verify how the micelle form of

ADBAC penetrates into wood. Copper penetrated evenly into 50 mm thick wood samples with

little gradient with depth; however, high amounts of ADBAC were concentrated on the surface

creating a steep gradient with depth. After accelerated leaching, considerable amounts of

physically adsorbed ADBAC leached out, especially from the surface.

6.2 Introduction

Since commercialized as a wood preservative in 1978 (Jin and Preston 1991), alkylammonium

compounds (AACs) have received considerable attention as low toxicity wood preservatives

(Hedley et al. 1982; Preston et al. 1987). However, after problems arose in the early 1980’s

regarding its poor performance in ground contact (Ruddick 1983; Tillott and Coggins 1981; Jin

and Preston 1991), there was more focus on its use as a co-biocide.

In terms of its adsorption in wood, an important consideration is that quat is formulated with

other chemicals. Indeed, ACQ solution components are primarily cationic and include charged

82

copper amine complexes, free protonated amine, and quaternary ammonium compound (quat)

which could compete with or limit other components for the anionic bonding sites of wood.

Copper chemisorption capacity for wood is pH dependant and controlled by the cation exchange

capacity (CEC) of wood. Moreover, Cu chemisorption in wood can be affected by amine species

and Cu is also known to compete with quat for the same bonding sites of wood (Tascioglu et al.

2005). This indicates that all ACQ components are interrelated in their adsorption in wood and

can affect adsorption of other components. Therefore, it is of interest to investigate how CEC,

pH, and other components affecting Cu chemisorption are related to quat adsorption in wood.

Increasing ionic strength or adding other chemicals generally induces agglomeration of the quat

(Garidel et al. 2000; Jiang et al. 2003; Fuguet et al. 2005) and could affect its penetration and

adsorption although ionic strength of the solution has little or no effect on copper amine

speciation (Pankras et al. 2009) and Cu adsorption (Hayes et al. 1987). pH of solution could

affect both chemisorption and physisorption of quat because it affects the CMC and aggregation

numbers of quat (Rahman 1983).

Unlike for Cu, the amount of quat adsorption in wood dust was lower than the CEC value,

raising the question of whether micelles formed in solution can penetrate readily into the wood

cell wall.

In this study, the adsorption capacity of quat in wood was investigated at different levels of pH,

ionic strength, and Mea and Cu concentration to evaluate the factors influencing adsorption and

the potential for competition among the different cationic species such as Mea and Cu for wood

adsorption sites. The effects of penetration of ACQ components into solid wood were also

studied in terms of the adsorption and solubilities of ACQ components penetrating into wood

blocks.



Six boards of air-dried red pine (Pinus resinosa Ait.) from three different pole sections were

passed through a thickness planer. Mixed shavings were ground and the fraction that passed a 35

83

mesh screen and was retained on a 60-mesh screen was used. Samples were washed with 0.5 M

HCl to remove all metal ions initially bound to the wood and rinsed repeatedly with distilled

water until the pH of washings was constant at about pH 5.5. The sample was then dried at 60°C

for 48 h. For treatment solutions, 50% alkyl (C12 67%, C14 25%, C16 7%, C18<2%) dimethyl

benzyl ammonium chloride (ADBAC, Lonza Inc., Allendale, NJ, USA) was diluted to the target

concentrations for use.

6.3.2 Effect of ionic strength on ADBAC adsorption

The influence of ionic strength on ADBAC adsorption by wood dust was examined. Different

amounts of KCl were added to ADBAC solutions to produce different ionic strengths (0 M, 0.5

M and 1 M) and the pH was adjusted to 9.5 ± 0.2 with Mea, followed by diluting the solution to

100 ml with distilled water to a target ADBAC concentration of 0.33% (# 9.5mmol l-1

). Three

replicate samples of red pine sawdust (2 g) were added to each solution and conditioned at 22°C

for 5 days (Loubinoux and Malek 1992) with shaking and adjusting pH every day to reach

equilibrium at the target pH=9.5. The mixtures were vacuum-filtered and the sawdust dried at

60°C for 24 h. A subsample of the treated sawdust (1 g) was placed on a shaker and leached with

100 ml distilled water at 200 rpm for 10 days. The water was changed every second day.

6.3.3 Effect of Mea increase on ADBAC adsorption

Solution pH was adjusted by two different methods throughout this study; for fixed pH 9.5,

several drops of Mea were added to ADBAC solution to reach the target pH (low Mea

concentration). To investigate the effects of pH, ADBAC was first formulated with 5.5 times

higher concentration (m/m basis) of Mea, which is the proportion for commercial alkaline copper

quat (ACQ) solution (AWPA 2006b) but without copper, and pH’s were adjusted with HCl (high

Mea concentration). Chemisorption (after leaching) of ADBAC by 2 g of wood dust in 100 ml of

5, 20, 100 mmol l-1

ADBAC was compared for low and high concentrations of Mea at pH 9.5.

6.3.4 Effect of copper and solution pH on ADBAC adsorption

Copper acetate was added to ADBAC solutions to produce ADBAC concentrations ranging from

0.005 to 100 mmol l-1

(1.75×10-4

% to 3.5%) with a constant 0.4% Cu concentration when

84

diluted to a final volume of 2l. Treatment solution pH was controlled with Mea to 9.5 ± 0.2 and

solutions were treated with 2 g of red pine dust as described above.

The effect of pH on adsorption capacity of ADBAC was determined on red pine dust before and

after leaching. ADBAC solutions (10, 40, and 200 mmol l-1

) were formulated with 5.5 times

higher concentration of Mea. The pH of 50 ml of each quat-Mea solution was adjusted with HCl,

and diluted to 100 ml with distilled water for target concentrations of 5, 20, and 100 mmol l-1

over pH range of 3 to 12. These quat concentrations correspond to ACQ preservative solution

concentrations of 0.53, 2.1, and 10.5% (AWPA 2006b). Three grams of red pine dust were

treated with these solutions as described above.

6.3.5 ACQ treatment of solid wood – concentration gradient and leachability of components

For solid wood treatment, defect-free quarter-sawn blocks (30 X 60 X 100 mm3) of red pine

were cut from three different pole sections. Three replicate samples for both longitudinal (L) and

radial (R) direction penetration were sealed with silicone adhesive except for one cross-sectional

surface (for longitudinal penetration) or one tangential surface (for radial penetration). The

prepared samples were treated with 2% ACQ solution (1.33% CuO and 0.67% ADBAC: Lonza

Inc., Allendale, NJ, USA). The treating procedure included a vacuum at -90 kPa relative to gauge

(or absolute pressure of 11 kPa) for 30 min, followed by a pressure level of 1020 kPa for 5 h.

The treated samples were conditioned in a plastic bag at a temperature of 60°C for 7 days,

followed by drying at the same temperature for 7 days. Sections were cut every 4 mm (including

the saw blade kerf of 0.7 mm) from the treated surface with a bandsaw and ground to pass a 30

mesh screen for analysis of copper, quat and chloride. A subsample of the sawdust (3 g) was

leached on a shaker with 100 ml distilled water at 200 rpm for 10 days with water changes every

second day.

6.3.6 Analysis of Cu, Mea, ADBAC and Cl

Instrument for Cu analysis: X-ray fluorescence spectroscopy (Oxford Instruments, LAB X 3000,

Abingdon, Oxfordshire, UK). Mea determination: elemental combustion system (ECS 4010,

85

Costech, Valencia, CA, USA); results were corrected for the natural N content in the wood and

the N contained in the ADBAC molecule.

One half gram of the leached and unleached samples (oven-dry basis) was extracted with

analytical grade ethanol for cationic ADBAC and a 0.3 g sample was extracted with distilled

water for chloride analysis by ultrasonic extraction according to AWPA standard A16-93

(AWPA 2006c). Extracts were analyzed for ADBAC and chloride contents by ion

chromatography (Dionex DX 600, Sunnyvale, CA, USA) with suppressed conductivity on a

CS14A analytical column for ADBAC and an AS11-HC analytical column for chloride; isocratic

run at 1.0 ml min-1

flow rate. The eluents for the ADBAC analysis: 20% 20 mmol l-1

methanesulfonic acid and 80% acetonitrile; for the chloride analysis: 12 mmol l-1

NaOH.

Adsorption values before leaching of sawdust were corrected for excess ADBAC and chloride in

retained moisture above the fiber saturation point (FSP), assuming the FSP was 35% (Cooper

1998). Adsorption of Cu and ADBAC were compared with the CEC of the wood.


6.4.1 Effect of ionic strength on ADBAC adsorption

The effects of ionic strength (0, 0.5, and 1.0 mmol l-1

) on ADBAC adsorption on wood at pH 9.5

are shown in Figure 6.1.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 0.5 1

Ionic s trength (m m ol l-1)

AD

BA

C (

mm

ol g

-1 w

oo

d)

before leaching after leaching

a1)

b

c c c

b

Figure 6.1 Effect of ionic strength on ADBAC adsorption on wood dust. Note: 1)

Different

letters indicate statistical differences (ANOVA, Tukey intervals, $ = 0.05).

86

Before leaching, slightly more ADBAC was adsorbed on wood as ionic strength increased.

Increasing amount of inorganic salts may increase the binding of counter ions on the micelle

surface. This reduces the electrostatic repulsion between ionic head groups of surfactants

facilitating micelle formation and increasing the aggregation number (Jiang et al. 2003).

However, no significant differences were observed in adsorption after leaching in the tested

concentration range.

6.4.2 Effect of Mea increase on ADBAC adsorption

Figure 6.2 compares the ADBAC chemisorption on wood dust at pH 9.5 with two different Mea

concentrations. Overall, ADBAC containing more Mea showed far less chemisorption in wood.

Increased Mea concentration reduces the CMC (Table 5.1) and increases the aggregation

number. The larger micelles could hinder bonding with anionic wood sites due to steric

hindrance.

0.00

0.04

0.08

0.12

0.16

5 20 100

ADBAC conc. (mmol l-1)

Ad

so

rptio

n (

mm

ol g

-1 w

oo

d)

Low Mea conc. High Mea conc.

Figure 6.2 Effect of Mea/HCl on ADBAC after-leaching adsorption on wood dust. Note: For

estimation of ACQ concentration, 10 mmol l-1

= 0.35 % ADBAC, corresponding to 1.05 % ACQ

solution and 0.1 mmol = 35 mg quat g-1

wood.

However, in the low Mea concentration system, increased ADBAC concentration resulted in

higher adsorption even though increasing ADBAC concentrations also causes the aggregation

number to increase; this finding makes the hypothesized mechanism unlikely. Alternatively, the

higher MeaH+ content in higher concentration ADBAC solutions may lead to greater competition

87

between ADBAC and Mea for anionic bonding sites in wood resulting in lower ADBAC

adsorption.

6.4.3 Effect of Cu on ADBAC adsorption

Figure 6.3 shows the adsorption isotherm of ADBAC in the presence of 0.4% Cu solution.

Before leaching (Figure 6.3 (a)), ADBAC adsorption increased starting at a concentration of

about 0.05 mmol l-1

and increased more sharply above 1 mmol l-1

. Accordingly, the aggregation

concentration may be near 0.05 mmol l-1

and the lowest concentration to saturate all available

sites may be near 1 mmol l-1

in this medium. This can also be inferred from the after-leaching

curve (Figure 6.3 (b)).

0.00

0.10

0.20

0.30

0.40

0.50

Adsorp

tion (

mm

ol g

-1w

ood)

Quat Cu

Cu+Quat Cl

CEC at pH 9.5

(a)

0.00

0.05

0.10

0.15

0.20

0.001 0.01 0.1 1 10 100


Adsorp

tion (

mm

ol g

-1w

ood)

(b)

Figure 6.3 Effect of competition between Quat (ADBAC) and Cu on equilibrium adsorption of

quat at pH 9.5 (a) before leaching (b) after leaching. Note: For estimation of ACQ concentration,

10 mmol l-1

= 0.35 % ADBAC, corresponding to 1.05 % ACQ solution and 0.1 mmol g-1

wood #

35 mg g-1

wood for ADBAC and # 8 mg g-1

wood for CuO.

88

When the ADBAC concentration was increased in solution with constant Cu concentration, the

amount of ADBAC adsorbed increased and the amount of Cu adsorbed decreased accordingly.

After leaching (Figure 6.3 (b)), the sum of adsorbed ADBAC and Cu (ADBAC + Cu) at each

concentration was almost constant, about 0.16 mmol g-1

wood (standard deviation 0.01), similar

to the CEC of 0.17 mmol g-1

for red pine at pH 9.5 (Figure 2.1 (a)). This confirms Cu

competition with ADBAC for the same reaction sites in wood, as suggested by Tascioglu et al.

(2005). Therefore, the application of higher concentrations of ACQ with more free Mea and Cu-

Mea complexes to compete with quat (ADBAC) will decrease chemisorption but increase the

physisorption of quat resulting in an increased amount of quat leaching.

The maximum ADBAC adsorption from the Cu-Mea-ADBAC solution after leaching was 0.09

mmol g-1

which was lower than its maximum adsorption of 0.14 mmol g-1

in Mea solution

(Figure 5.1 (b)), but the total Cu and ADBAC adsorption, 0.16 mmol g-1

was higher than that of

ADBAC alone in Mea solution. Thus, while Cu competed with ADBAC for reaction sites, it also

occupied some reaction sites that were inaccessible to ADBAC. A Cu concentration of 0.4 %

corresponds to a 0.75 % solution of ACQ-C with 0.25 % (7.1 mmol l-1

) of ADBAC (CSA 2008).

At this concentration (Figure 6.3 (b)), similar mole ratios of Cu (Mw 63.5) and ADBAC (Mw

350) were adsorbed but on a mass basis, a much higher amount of ADBAC was adsorbed

compared to Cu. Moreover, before leaching (Figure 6.3 (a)) at this concentration, a higher

amount of ADBAC was adsorbed, demonstrating that wood can adsorb more ADBAC (mass

basis) than Cu. However, during actual wood treatments, the availability of ADBAC in the cell

lumens is greatly restricted (mole ratio Cu:ADBAC = 8.9:1 in ACQ-C solution); therefore,

competition only becomes important at very high retentions (Tascioglu et al 2005). On the

surface of wood, where the availability of ADBAC is not restricted, a high adsorption of quat is

expected. Chloride was adsorbed at ADBAC concentrations above 1 mmol l-1

; the leaching

procedure removed all of the Cl- from the wood. The ADBAC adsorption isotherm and different

adsorption models are compared in Appendice 6.1 with adsorption constants (Appendix 6.2).

6.4.4 pH effect on ADBAC adsorption

The effect of pH on ADBAC adsorption onto wood dust before leaching is presented in Figure

6.4 (a). ADBAC adsorption was less affected by pH below 9, but its adsorption increased sharply

above this value. Considering the pKa value of Mea (# 9.5), uncharged Mea seems to relate to

89

this sharp increase above pH around 9.5. The alcohol-like nature of uncharged Mea bound to

micelles results in replacement of the water molecules at the interface, which weakens the

electrostatic repulsion between polar head-groups and increases the aggregation number of the

micelles (Jiang et al. 2003).

0.0

0.1

0.2

0.3

0.4

0.5

2 3 4 5 6 7 8 9 10 11 12 13

Ad

so

rptio

n (

mm

o g

-1 w

oo

d)

5mM 20mM

100mM

(a)

0.00

0.05

0.10

0.15

0.20

0.25

3 4 5 6 7 8 9 10 11 12

pH

Mea a

nd Q

uat

(mm

ol g

-1 w

ood)

(5mM) Quat Quat+Mea

(20mM) Quat Quat+Mea

(100mM) Quat Quat+Mea

CEC of red pine

(b)

Figure 6.4 Quat (ADBAC) and Mea adsorption on red pine dust from ADBAC-Mea solution (a)

ADBAC adsorption before leaching and (b) ADBAC and ADBAC + Mea adsorption after

leaching compared to the CEC of red pine. Note: The CEC values of red pine are obtained from

Figure 2.1 (a). (a) and (b) are the results of unmatched samples. For estimation of ACQ

90

concentration, 10 mmol l-1

= 0.35 % ADBAC, corresponding to 1.05 % ACQ solution and 0.05

mmol g-1

wood # 17.5 mg g-1

wood.

Figure 6.4 (b) shows the remaining ADBAC in wood dust treated at different pHs after 10 days

of leaching. The results illustrate that ADBAC chemisorption increased as pH increased in all

concentrations. This is reasonable because wood has increasing chemisorption sites such as

dissociated carboxylic groups and phenolic hydroxyl groups as pH increases. However, ADBAC

adsorption could not reach the CEC level. It is also noticeable that chemisorption in wood was

lower at higher ADBAC concentration. For all solutions with pH greater than 5, sufficient Mea

was adsorbed to bring the total adsorbed values (ADBAC + Mea) close to the CEC curve. This

supports the premise that Mea+ competes with ADBAC for the same sites because ADBAC

adsorption decreased at the high ADBAC concentration which also had higher Mea+ content.

The higher adsorption values (ADBAC + Mea) than the CEC below pH 5 may result from re-

equilibration of the wood dust suspension to a pH close to that of the distilled water (#5.5) used

to leach the samples. The higher pH distilled water could dissociate more carboxylic groups

allowing Mea+ in the leachate to adsorb to these new sites, resulting in uptake similar to the

value at pH 5.

6.4.5 Gradient adsorption of ADBAC and its leachability in solid wood samples

The adsorption of ACQ components at different depths in solid red pine is illustrated in Figure

6.5. Copper retention in the longitudinal direction (Ld) (Figure 6.5 (a)) was uniform with depth

except for slightly higher adsorption near the surface. This trend could still be seen in the

samples after leaching to remove unreacted copper. In the radial direction (Rd) (Figure 6.5 (b))

the Cu variation was larger, probably due to more tortuous passage through rays and multiple

bordered pit pairs in both early wood and late wood (Zahora 2010). Nevertheless, there was

relatively uniform Cu adsorption for both treating directions ranging from 0.18 to 0.24 mmol g-1

wood. Expected average component absorption values based on solution uptake of 170% (st.d.

16.9) for longitudinal penetration (L) and 151% (st.d. 15.0) for radial penetration (R) were 0.288

and 0.253 mmol copper g-1

wood respectively, which were similar to the average analyzed

retentions of 0.283 (L) and 0.250 (R) mmol copper g-1

wood. Copper retentions after leaching in

both longitudinally and radially penetrated samples were similar and slightly higher than the

91

CEC (# 0.16 mmol g-1

wood) at pH 9. The higher Cu retention could reflect precipitation of

Cu(OH)n or other low solubility compounds in wood (Jiang and Ruddick 2004). After 10 days of

leaching, average Cu losses were similar: for the two directions, 17.2% (L) and 18.8% (R).

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 4 8 12 16 20 24 28 32 36 40 44 48 52

Cu

(m

mo

l g

-1 w

oo

d)

Before leaching

After leaching

Expected absorption

(a)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 8 16 24 32 40 48 56 64 72 80

Before leaching

After leaching

Expected absorption

(b)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 4 8 12 16 20 24 28 32 36 40 44 48 52

Qu

at (

mm

ol g

-1 w

oo

d)

(c)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 8 16 24 32 40 48 56 64 72 80(d)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 4 8 12 16 20 24 28 32 36 40 44 48 52

Depth from a surface (m m )

Cl (m

mo

l g

-1 w

oo

d)

(e)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0 8 16 24 32 40 48 56 64 72 80

Depth from a surface (m m )(f)

Figure 6.5 Retention gradients of Cu, ADBAC, and Cl of ACQ (2%) in red pine block

(30 60 100 mm) which were adsorbed through longitudinal direction (a, c, e) and radial

direction (b, d, f). Note: For estimation of ACQ concentration, 0.1 mmol g-1

wood = 8 mg CuO g-

1 wood, 0.01 mmol = 3.5 mg quat g

-1 wood and 0.01 mmol = 0.36 mg chloride g

-1 wood.

92

Unlike the Cu adsorption, ADBAC had steep retention gradients through the Ld (Figure 6.5 (c))

and Rd (Figure 6.5 (d)). These different adsorption profiles indicate that quat penetrates and

reacts on wood independently of Cu. For the Ld, the ADBAC retention gradually decreased

through the full sapwood depth from about 0.1 to 0.02 mmol g-1

wood. However, for the Rd, the

penetration was inhibited and retention sharply decreased from 0.12 mmol g-1

to 0.02 mmol g-1

at

28 mm and concentrations at greater depths were below 0.01 mmol g-1

. Considering the molar

ratio of 1:8.9 ADBAC:Cu in ACQ-C solution, the proportion of adsorbed ADBAC relative to Cu

was much higher at the surface as discussed in the context of Figure 6.3; the ratio was 1:4 at the

surface and decreased to a ratio 1:8.8 at 38 mm for the L penetration. This confirms that the

micelles of ADBAC could penetrate at least up to 38 mm in depth. However, this ratio was

reached at 26 mm in the R direction, indicating relatively difficult micelle penetration.

Based on solution uptake, the expected absorption values through the sapwood were 0.032 (L)

and 0.029 (R) mmol g-1

wood. These values were reached at 38 mm in depth for L and 26 mm in

depth for R, indicating significantly higher than expected adsorption of ADBAC near the surface.

The high adsorption near the surface resulted in higher total average adsorption (0.045 (L) and

0.035 (R) mmol g-1

) compared to the expected values. This discrepancy may be due to stripping

of ADBAC from the solution into the surface layers of the wood. The average adsorption for L

penetration was higher than for R penetration, but the former showed a lower leaching amount,

71% compared to 83%. Accordingly, L penetration allows ADBAC to react with more bonding

sites in wood (chemisorption) than through the R penetration by allowing deeper penetration. We

assume this steep penetration profile was caused by a chromatographic effect and/or filtration

effect related to micelle formation. The adsorbed ADBAC molecules near the surface form larger

adsorbed micelles and allow only excess ADBAC molecules to penetrate deeper

(chromatographic effect). Alternatively, smaller micelles may penetrate more while larger

micelles are filtered as they penetrate the wood structure.

The ADBAC concentration in wood after leaching was almost constant and very low,

independent of the initial adsorption for L adsorption. For R adsorption, the concentration in

wood after leaching was lower than that for Ld and only a small amount (< 0.01 mmol g-1

or <

3.5mg g-1

) remained at depths greater than 20 mm. Considering that the copper retention after

leaching was above the CEC, it is clear that in the wood environment, copper was preferentially

93

adsorbed on the ion exchange sites compared to ADBAC and only a small proportion of the

anionic sites had adsorbed quat (<0.02 mmol g-1

).

Figures 6.5 (e) and 6.5 (f) confirm that Cl- can penetrate very deep into red pine. Although Cl

-

was detected at all depths for both L and R penetration, their profiles are different. Chloride

retention gradually decreased for L penetration, similar to ADBAC and adsorbed ADBAC cation

to anionic Cl- molar ratio was about 1:1 through the whole depth. On the other hand, similar

amounts of Cl- were detected at all depths in the R direction, which indicates lower anion

adsorption to charged micelles near the surface and greater penetration compared to that of

cationic ADBAC. The inconsistency seems to show independent penetration and adsorption of

cations and anions from ADBAC molecules for R penetration.

6.5 Conclusions and implications

Quat (ADBAC) is adsorbed onto wood by chemisorption and additional physisorption. The

former is due to ion exchange of individual ions on anionic sites in wood and resists leaching.

The latter is achieved by hydrophobic interaction of quat molecules on the same sites and the

formed micelle structures can be easily washed out. An increase in ionic strength of the ADBAC

solution facilitated ADBAC uptake but had little effect on the chemisorption. Thus increasing

ionic strength in treating formulations will likely increase ADBAC leaching. Higher uptake of

ADBAC compared to Cu on the wood surface is expected based on the equilibrium distribution

of the two components when exposed to sawdust; this was confirmed by the solid wood

treatments. In the same way, ADBAC and free Mea compete for the same sites in wood.

Therefore, the application of higher concentration of ACQ will decrease the ADBAC

chemisorption, and increase the amount of leaching. ADBAC chemisorption was slightly

elevated at higher pH, but was still considerably lower than the CEC of red pine. The aggregated

ADBAC can penetrate wood, but chromatographic/filtration effects cause a steep retention

gradient; this was more apparent through radial penetration. The high surface concentrations are

highly susceptible to leaching.

94

Chapter 7 Summary, conclusions and proposed future work

7.1 Summary and Conclusions

This dissertation describes a comprehensive study of ACQ adsorption mechanisms and factors

influencing fixation of ACQ components in wood. Each individual study on copper

chemisorption, copper physisorption, and quat adsorption in wood provides information about

Cu chemisorption capacity, required Cu/Mea ratio and importance of ligand selection for Cu

fixation. It also provides explanations for Cu and quat adsorption mechanism. The consolidated

study shows how they interact and contribute to their adsorption in an integrated ACQ system.

The observations and experimental results about adsorption of ACQ components in wood can be

summarized as follow:

1. Wood has a limited but increasing CEC as pH increases because wood acid sites such as

carboxylic groups and phenolic hydroxyl groups dissociate almost proportionally to pH

increase. The adsorption of individual free copper, free Mea, and Cu-Mea complex

cations present in Cu-Mea solution follows the CEC curve as a function of pH.,

indicating their adsorptions are relevant to ion exchange properties and divalent copper

occupies only one wood acid site. When Cu-Mea complexes coexist with free Mea

(protonated or unprotonated) in solution, Cu-Mea complexes are selectively adsorbed

into wood over free amine. While [Cu(Mea)2-H]+ complex is predominant in solution at

pH 9-9.5, adsorbed copper in wood was apparently as [CuMea-H]+ which suggests copper

complexation with wood acidic sites through ligand exchange. Copper chemisorption

capacity of wood was about 0.15 mmol g-1

wood at pH 9 which corresponds to 4.8 kg

CuO m-3

wood (specific gravity #0.4).

2. Besides pH, ligand type and Cu/ligand ratio also affect Cu adsorption in wood. Copper in

ethanolamine solution adsorbed in wood up to its CEC regardless of mono-, di-, and tri-

ethanolamine at normal Cu/amine molar ratio (1:3.7). However, increased amine ratio in

Cu-amine solution decreased the Cu adsorption. This is because as amine ratio increased,

95

the proportion of uncharged Cu-amine species (which are more stable in solution than in

wood) increased, preventing ligand exchange reactions with wood. In terms of high Cu

adsorption, Mea is superior to Dea and Tea as a ligand because it induces Cu

precipitation in a broader pH range, its higher pH dissociates more wood acid sites, and

its higher pKa value retards formation of uncharged Cu-amine complex in solution. When

En is used as a ligand, Cu adsorption was less than half of the CEC at intermediate pH

and exceeded the CEC at over pH 13 possibly due to adsorption of Cu(En)22+

in cellulose.

When [Cu(NH3)4(H2O)2]2+

(copper ammonia) treated with wood, apparently

[Cu(NH3)(H2O)5]2+

was adsorbed up to the CEC as if it were a singly charged complex.

3. In addition to the Cu chemisorption, Cu precipitation is also an important mechanism of

Cu fixation in wood, especially for high retention treatments. As Cu-Mea solution

concentration increased, it was harder to initiate Cu precipitation in wood because of the

higher amount of free Mea in the solution; higher free amine retarded Cu precipitation to

a lower pH while requiring more acid to reduce the pH. This suggests that Cu

precipitation fixation reactions can be enhanced by reformulating with lower Mea

content. For this reason, high concentration Cu-Mea treated wood has a relatively low Cu

fixation rate driven only by chemisorption, while low concentration Cu-Mea treated

wood has a higher Cu fixation rate driven by both chemisorption and precipitation during

conditioning without drying. During the drying process, additional Cu precipitation can

occur even in high concentration treated wood. Wet conditioning at high temperature (50

ºC) showed faster and higher Cu fixation based on solution expressate, but showed

ultimately higher Cu leaching, possibly due to the depolymerization of wood

components. The Cu leaching rate should be checked in a dry condition to confirm if wet-

condition or high temperature caused high Cu leaching through depolymerization of

wood. In vitro, Cu precipitated in wood as amorphous azurite, semi-crystalline malachite

and possibly Cu2O formed as a result of Cu-Mea decomposition.

4. ADBAC showed two different adsorption mechanisms on wood; ion exchange reaction at

low concentration and an additional aggregation form of adsorption by hydrophobic

interaction at higher concentration, especially above the CMC. The adsorption by ion

exchange reaction is homogeneous with a monolayer type reaction between wood and

ADBAC ions and has high leaching resistance. The adsorption of ADBAC molecules by

96

hydrophobic aggregation is heterogeneous and multilayer and has low leaching

resistance. The CMC decreased with addition of Mea, Cu-Mea, and buffer chemicals.

5. Increasing ionic strength in ADBAC solution will increase ADBAC leaching because it

facilitates ADBAC uptake through greater aggregation but has little effect on its

chemisorption. ADBAC compete with Cu and free Mea for the same sites in wood. Thus,

the application of higher concentration of ACQ will decrease the ADBAC chemisorption,

and increase the amount of leaching. The aggregated ADBAC can penetrate wood, but

chromatographic/filtration effects cause a steep retention gradient; this was more

apparent through radial penetration. Therefore, more ADBAC is adsorbed on the wood

surface, but this concentrated surface ADBAC is highly susceptible to leaching.

From these findings regarding ACQ adsorption mechanism, there are several factors which

should improve the ACQ formulation: use a ligand that forms less stable complexes with copper,

re-formulate with lower Mea content and lower quat content and increase the CMC of quat:

1. Cu chemisorption in wood is decided by the stability of Cu-ligand complexes in solution.

If the Cu-ligand complexes are too stable in solution such as [Cu-ethanolamine]0 and

[Cu(En)2]2+

, the complexes are less reactive with wood regardless of their charge. This

results in less Cu chemisorption in wood. [Cu(Mea)2-H]+ can exchange the ligand with

wood sites at intermediate pH but forms too stable and uncharged [Cu(Mea)2-2H]0 in

solution at high pH which prevents Cu chemisorption where wood exhibits a higher CEC.

For higher copper fixation, therefore, some other ligand that is less stable than

[Cu(Mea)2-H]+ (log K#14.9) even at high solution pH is desirable.

2. Lower Mea content of the ACQ formulation has advantages in two ways. Low amount of

Mea retards and reduces uncharged Cu-Mea complex which restricts Cu chemisorption. It

also reduces free Mea content in ACQ solution and thus, promotes Cu precipitation in

wood, which reduces the leachable Cu as shown in Figure 7.1.

97

Figure 7.1 Conceptual changes in the proportions of precipitated Cu and soluble Cu in ACQ

treated wood by reducing free Mea.

3. Higher concentration of quat forms higher aggregation causing its steep adsorption

gradient and high leaching. Therefore, re-formulating with lower amount of quat will

reduce undesirable (leachable) quat concentration on wood surface. Because Cu and Mea

concentration also affect the aggregation size of quat, applying a low concentration of

ACQ is desirable in terms of reducing quat leaching. Replacing with other quat with high

CMC will also reduce the aggregation and the resulting high leaching and uneven

adsorption in wood.

7.2 Proposed future work

1. As an example of a less stable ligand for Cu complex, 1,3-diaminopropane complexes with

copper by two bidentate diaminopropane units and the resulting six-membered Cu complex is

less stable than the five-membered ones in the case of Cu-En (Klemm et al. 1998). Therefore, it

is expected that a ligand exchange with deprotonated wood acidic sites can take place rather

easily, as indicated in Figure 7.2.

98

Figure 7.2 Proposed scheme of wood-1,3-diaminopropane copper complex structure (Klemm et

al. 1998)

2. As described in Chapter 4, additional study is needed to clarify the reactions during fixation at

high moisture content followed by drying. It is also of interest to investigate the source of Cu (I).

Cu (I) formation may not be detrimental and could be a practical way to speed up the Cu

stabilization reaction if low solubility Cu (I) is converted from un-reacted soluble Cu.

Higher ionic strengths increased the Cu precipitation ratio with little effect on the pH which

initiated Cu precipitation (Figure 4.1c). It raises the question of whether this has practical

significance, e.g., can some electrolytes be formulated in the ACQ solution to increase ionic

strength so that precipitation can occur, and more of the soluble Cu is precipitated?

3. The current study showed that low Mea content of ACQ formulation reduces free Mea which

retards/reduces uncharged Cu-Mea complex formation and also accelerates Cu precipitation in

wood. Therefore, low Mea content for ACQ formulation is desirable for high Cu fixation in

wood. However, ACQ solution is generally recovered after wood treatment and re-used for

another treatment. Considering high free amine adsorption in wood, Mea/Cu formulation ratio in

re-used ACQ solution may be reduced every time, which could cause Cu precipitation in treating

99

solution when too low Mea/Cu ratio is applied. Therefore, in the practical point of view, finding

the optimum Mea/Cu ratio is necessary to balance high Cu fixation in wood and good working

solution stability.

4. As shown in the literature review (Chapter1), the CMC of quat can be controlled by

temperature and other chemicals. It is necessary to study if these factors are practically

applicable to increase CMC of quat and thus, to achieve even distribution and better fixation of

quat in wood.

5. Copper present in Cu-amine treated wood after the fixation reactions can be divided into three

types: chemically reacted Cu with wood, physically precipitated Cu, and soluble Cu. Because the

biocidal efficacy of Cu based wood preservatives depends on the affinity of the cupric ion Cu2+

(Eaton and Hale 1993), it raises the question whether well fixed Cu (chemically adsorbed Cu and

precipitated Cu) in wood have similar biocidal efficacies to that of soluble Cu. It would be an

essential study in investigating Cu fixation to make sure that appropriate recommendations are

made. The Cu-Mea treatment method can be modified to promote chemically reacted Cu without

precipitation or to induce Cu precipitation in wood (e.g., treating with high Mea ratio Cu-Mea to

prevent Cu precipitation used in Chapter 3: treating with sodium carbonate followed by treating

with Cu to induce Cu precipitation by Jiang and Ruddick (1997)).

100

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Appendices

Appendix 2.1 Cation exchange capacity of red pine dust for sodium (Na+) from NaOH/HNO3

solutions (0.05 M) depending on time.

9.0

9.5

10.0

10.5

11.0

11.5

12.0

0.0 0.5 2.0 4.0 17.0 28.0 56.0

Time (h)

pH

0.0

0.1

0.2

0.3

0.4

exch

an

ge

d N

a

(m

mo

l g

-1 w

oo

d)

pH

exchanged Na

113

Appendix 3.1 Change in lignin content (%) after treating with different Cu-amine solutions

Treating Sol. pH Lignin

(%)

Std.

(%)

CuO

(%)

Amine

(%)

Lignin in

pure

wood (%)

Std.

Control 28.06 0.05 28.06 0.05

Cu-Mea 9.5 27.30 0.27 1.33 0.99 27.95 0.28

Cu-Dea 8.5 27.79 0.41 1.05 0.89 28.34 0.42

Cu-Tea 7.7 27.57 0.13 0.76 0.69 27.98 0.13

Cu-En 12.4 26.61 0.39 1.08 1.1 27.20 0.40

Cu-amine

treated

wood

Cu-Am 11.8 26.30 0.19 1.49 0.38 26.80 0.19

There is no change in lignin content after Cu-ethanolamine treatment while Cu-En and Cu-Am

treated wood showed slightly lower lignin contents. Accordingly, further analysis for Cu-En and

Cu-Am treated wood dust was conducted and the results are summarized as follows:

Appendix 3.2 Changes in wood components after treating with Cu-En and Cu-Am

Symbol Treating sol.

(ml/3g wood)

Initial

pH

Final

pH CuO(%)

Amine

(%)

Weight loss

(%)

Control Con

En1 6 12.0 7.3 0.46 1.45 3.27

En2 6 10.0 6.8 0.26 1.10 2.82

En3 100 12.0 11 0.71 1.67 6.03 Cu-En

En4 100 10.0 9.7 0.42 1.38 4.96

Am1 6 11.1 8.2 1.02 0.35 1.93

Am2 6 10.6 7.8 1.07 0.49 2.46

Am3 100 11.1 10.8 2.47 0.31 3.02 Cu-Am

Am4 100 10.6 10.3 2.43 0.36 3.66

Chemical content based on weight loss

Lignin (%) Holocellulose (%) Cellulose (%)

ave. Std. ave. Std. ave. Std. Total (%)

Con 25.9 a* 0.3 75.8 a 0.7 35.5 a 0.4 101.7

En1 26.7 b 0.3 72.2 b 0.9 34.7 a 0.4 99.0

En2 25.9 a 0.3 73.2 b 1.2 34.3 a 1.2 99.1

En3 26.9 b 0.4 70.0 c 1.1 32.1 b 1.2 96.9

En4 25.7 a 0.3 73.1 b 0.3 34.9 a 0.2 98.8

Am1 24.8 b 0.4 72.1 b 0.5 35.0 a 0.6 97.0

Am2 25.6 a 0.1 71.2 bc 0.6 33.6 ab 0.6 96.8

Am3 25.6 a 0.3 70.0 c 1.1 33.1 b 1.2 95.6

Am4 25.4 ab 0.2 69.9 c 1.1 33.8 ab 0.9 95.3

* ANOVA: individual 95% Cls for mean based on Pooled StDev.

114

Appendix 4.1 Copper precipitation on 0.2% and 0.67% Cu-Mea treated wood: (a) transverse

surfaces and (b) radial surfaces.

115

Appendix 5.1 Langmuir, BET, and Freundlich model fitting of ADBAC adsorption in wood

before (a) and after leaching (b) (with normal Ce scale)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

qe

(m

mo

l g-1

wo

od

)

Experim ental data

Langm uir

BET

Freundlich

ADBAC in Mea

(a) 0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

ADBAC in Mea

(b)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 10 20 30 40 50 60 70 80 90 100

Ce (mmol l-1)

qe

(m

mo

l g-1

wo

od

)

ADBAC in Buf fer

(a)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 10 20 30 40 50 60 70 80 90 100

Ce (mmol l-1)

ADBAC in Buf fer

(b)

116

Appendix 6.1 Langmuir, BET, and Freundlich model fitting of ADBAC adsorption isotherm in

Cu-Mea media before (a) and after leaching (b)

(Logarithm Ce scale)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0001 0.01 1 100

qe

(m

mo

l g-1

wo

od

) Experimental data

Langmuir

BET

Freundlich

(a)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

1E-04 0.001 0.01 0.1 1 10 100

(b)

(Normal Ce scale)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40 50 60 70 80 90 100

Ce (mmol l-1)

qe

(m

mo

l g-1

wo

od

)

Experimental data

Langmuir

BET

Freundlich

(a)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 10 20 30 40 50 60 70 80 90 100

Ce (mmol l-1)

(b)

Appendix 6.2 Adsorption constants estimated from simulations with Langmuir, BET, and

Freundlich models for isotherms of ADBAC in Cu-Mea media using wood dust as an absorbent

(Before leaching)

Langmuir BET Freundlich Treating pH

qm b R2 qm Kb Cb R

2 Kf n R

2

9.5 0.443 0.743 0.983 0.3 490 235 0.962 0.17 4.07 0.922

(After leaching)

9.5 0.091 3.5 0.991 0.085 1000 412.2 0.947 0.05 5.82 0.86

117

Peer reviewed journal papers

1. Myung Jae Lee and Paul Cooper (2010) Copper monoethanolamine adsorption in wood

and its relation with cation exchange capacity (CEC). Holzforschung 64:653-658

2. Myung Jae Lee and Paul Cooper (2010) Effect of amine ligand, copper/amine ratio, and

pH on copper adsorption into wood. Holzforschung 64:659-665

3. Myung Jae Lee and Paul Cooper (2010) Alkyl dimethyl benzyl ammonium chloride

adsorption mechanism on wood. Cellulose 17:1127-1135

4. Myung Jae Lee and Paul Cooper (2011) Effect of ionic strength, monoethanolamine,

copper, and pH on alkyl dimethyl benzyl ammonium chloride adsorption in wood.

Holzforschung : DOI: 10.1515/HF.2011.023

5. Paper in preparation: Myung Jae Lee and Paul Cooper. Copper precipitation in Cu-

monoethanolamine preservative treated wood. For submission to Wood Science and

Technology.

Conference paper

1. Myung Jae Lee and Paul Cooper (2010) Adsorption of ACQ components in wood.

IRG/WP 10-30522. Int. Res. Group on Wood Protection. Stockholm. Sweden

lee myungjae 201106 phd thesis

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