ACTIVATED CARBON CHARACTERIZATIONS: EXTENT OF BOEHM TITRATION

USAGE

By

UPASNA BHAGWANTH RAI

A THESIS PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2015

© 2015 Upasna Bhagwanth Rai

To Mummy and Papa

4

ACKNOWLEDGMENTS

I would like to thank my Advisor, and Committee Chair, Dr. David Mazyck, who has

guided me through the toughest of times and granted me this opportunity to conduct research in

the field of Air Pollution. I thank him for seeing the potential in me and working alongside to

ensure I complete my term successfully here at UF. I would also like to acknowledge Dr.

Treavor Boyer and Dr. Jason Weaver for being on my committee and helping me despite their

busy schedule through my Thesis.

A special thank you goes out to Regina Rodriguez, for her constant help and supervision

with the research. She was a pillar of support who helped me put it together during my

difficulties.

I am grateful to have been given a supervisory role to my student workers, Kristen Croft

and Naim Vilabrera. They made working in the laboratory a lot easier alongside an interactive

learning experience. I also thank Kokil Bansal, for being my encouragement in the laboratory

and otherwise as a friend and close companion at all times

A whole lot of thanks to Roxie for being patient with editing my thesis and ensuring I

cleared it in the first attempt.

Last, but not the least, I would like to thank my family, back in India and my best friend,

Swagnik Guhathakurta, for inspiring, reassuring and being patient with all my decisions. Despite

being thousands of miles away from home, their love and prayers have always by my side and

has never let me break down even at the toughest of times.

5

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...............................................................................................................4

LIST OF TABLES ...........................................................................................................................7

LIST OF FIGURES .........................................................................................................................8

ABSTRACT .....................................................................................................................................9

CHAPTER

1 INTRODUCTION ..................................................................................................................11

Opening Statement ..................................................................................................................11 Hypothesis ..............................................................................................................................13

Objectives ...............................................................................................................................13

2 LITERATURE REVIEW .......................................................................................................14

Activated Carbon ....................................................................................................................14 Surface Chemistry of Activated Carbon .................................................................................14 Acidic Functional Groups .......................................................................................................15

Basic Functional Groups .........................................................................................................16 Boehm Titrations ....................................................................................................................17

Standardizations in Boehm Titration Methodology ...............................................................18

Fourier Transform InfraRed (FTIR) Spectroscopy .................................................................19

Activated Carbon Oxidation Treatment ..................................................................................20 Mercury Removal ...................................................................................................................21

3 MATERIALS AND METHODS ...........................................................................................24

Chemicals and Materials .........................................................................................................24

Material Synthesis: Nitric Acid Treatment .............................................................................24 Activated Carbon Physical Characterization Methods ...........................................................24

Surface Area, Pore Volume and Average Pore Size .......................................................24 Tap Density .....................................................................................................................26

Activated Carbon Chemical Characterization Methods .........................................................26

Contact pH .......................................................................................................................26

Boehm Titrations Methodology ......................................................................................26

Unfiltered Aliquots (Method 1) ................................................................................27 Filtered Aliquots (Method 2) ....................................................................................27 Acidic Functional Group Concentrations .................................................................28 Basic Functional Group Concentrations...................................................................28

FTIR Methodology ..........................................................................................................28 Gas Phase Mercury Adsorption ..............................................................................................29

6

4 RESULTS AND DISCUSSION .............................................................................................30

Activated Carbon Oxidation ...................................................................................................30 Boehm Titrations Comparison for Method 1 and Method 2 ..................................................30 Unfiltered Aliquots (Method 1) Boehm Titration Comparison for the Acid Treated

Carbon .................................................................................................................................32 FTIR Results ...........................................................................................................................33 Comparison between Boehm and FTIR Results .....................................................................34 Mercury Adsorption Performance Analysis ...........................................................................34

5 CONCLUSIONS AND RECOMMENDATIONS .................................................................48

APPENDIX: BOEHM TITRATION CALCULATIONS .............................................................49

LIST OF REFERENCES ...............................................................................................................50

BIOGRAPHICAL SKETCH .........................................................................................................53

7

LIST OF TABLES

Table page

4-1 Physical and chemical characteristics of F820, F820 1 M, F820 5 M and F820 10 M .....36

4-2 IR absorptions ....................................................................................................................36

8

LIST OF FIGURES

Figure page

2-1 Possible functional groups on carbon surfaces ..................................................................23

2-2 Illustration of the approach used to calculate concentrations of reactive functional

groups in discrete pKa ranges using Boehm titrations.......................................................23

4-1 Boehm titrations methodology comparison of F820 .........................................................37

4-2 Boehm titrations of F820, F820 1 M, F820 5 M and F820 10 M following Method 1

(µmol/g AC) .......................................................................................................................38

4-3 DRIFT spectrum corrected for interference and baseline ..................................................39

4-4 Common scale transmission spectrum, corrected for water vapor, CO2, baseline and

inverted ..............................................................................................................................40

4-5 Zoomed from 3500–800 cm-1- common scale transmission spectrum, corrected for

water vapor, CO2, baseline and inverted ............................................................................41

4-6 Boehm titrations results for the sum of carboxyl and lactone groups of F820, F820 1

M, F820 5 M and F820 10 M following Method 1 (µmol/g AC) ......................................42

4-7 Mercury removal performance measure on F820, F820 1 M, F820 5 M and F820 10

M ........................................................................................................................................43

4-8 Compare μmol of carboxyl group/g AC of F820 samples against mercury removed

(μg Hg/g AC) .....................................................................................................................44

4-9 Compare μmol of lactone group/g AC of F820 samples against mercury removed (μg

Hg/g AC) ............................................................................................................................45

4-10 Compare μmol of phenol group/g AC of F820 samples against mercury removed (μg

Hg/g AC) ............................................................................................................................46

4-11 Compare μmol of total acidity/g AC of F820 samples against mercury removed (μg

Hg/g AC) ............................................................................................................................47

9

Abstract of Thesis Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Master of Science

ACTIVATED CARBON CHARACTERIZATIONS: EXTENT OF BOEHM TITRATION

USAGE

By

Upasna Bhagwanth Rai

December 2015

Chair: David Mazyck

Major: Environmental Engineering Sciences

Characterization of activated carbon is essential to understand its surface chemistry. This

work characterizes a virgin carbon and acid treated carbons both physically and chemically. The

physical characterizations included surface area, pore volume, average pore size, and density.

The chemical characterization techniques adopted to study the amphoteric nature of carbon were

Boehm titrations and Fourier Transform Infrared (FTIR) Spectroscopy.

Traditionally Boehm titration has been used to determine three acidic functional groups

(carboxyl, lactone, and phenolic) on activated carbon based on the concept of acid-base

neutralization. This qualitative and quantitative technique has been extensively used on activated

carbons of different origins to analyze for surface oxygen functional groups.

Two titration methods were compared. One such method was to titrate the filtrate after

mixing carbon with deionized water. An alternative to this, was titration on solutions in the

presence of carbon. Both methods were compared and confirmed that activated carbon’s

presence was essential to produce consistent results using Boehm titration. To an extent, Boehm

and FTIR were also comparable.

10

Virgin and acid treated activated carbons were compared for Hg adsorption and all acid

treated carbons showed greater removal than the virgin carbon. This suggests that oxygen

functional groups, enhanced with the acid treatment, were beneficial for Hg adsorption.

11

CHAPTER 1

INTRODUCTION

Opening Statement

Activated carbon is one of the most widely used adsorbents and catalysts in the water and

air treatment industry for the removal of organic and inorganic compounds. While the porous

structure of activated carbon is more regularly considered when choosing one carbon over

another for a given application, its surface chemical nature may play a more critical role in

adsorption and catalytic applications. The sheer magnificence of activated carbon lies in its

complexities that allow researchers to work towards enhancing its performance in the field of

study.

Activated carbon’s physical characteristics (e.g., surface area, average pore size, and pore

size distribution) are easy to measure, albeit there is some controversy regarding the best

techniques. Activated carbon’s surface chemistry is far more difficult to characterize whereby

the most popular approaches (Boehm titrations and FTIR) have limitations and may not correlate

with each other. There are numerous papers that suggest the surface chemistry of activated

carbon is more important than its physical properties (Coughlin et al., 1968; Vidic and Siler,

2001; Ramon et al., 2002; Quinlivan et al., 2005), (i.e., optimizing physical attributes without

concern for also optimizing surface properties will not result in the ideal activated carbon for a

given application). Some of these papers focus on the role of oxygen-containing functional

groups (i.e., oxygen heteroatoms). These functional groups exercise a profound influence on

surface acidity, cation exchange capacity, selective adsorption of metal ion/metallic species from

solutions, direct sites for catalytic reactions (Li et al., 2011) and adsorption of polar and nonpolar

gases vapors (Pradhan and Sandle, 1999).

12

In full-scale activated carbon manufacturing, surface oxygen complexes are formed when

the carbon cools exiting the furnace and is exposed to air at temperatures less that its ignition

temperature (i.e., atmospheric oxygen chemisorbs to the carbon surface). Therefore, and beyond

the scope of this research, oxygen functional groups are not presently engineered for activated

carbons. Nonetheless, the identification of these functional groups is essential from an

application point of view. The two methods investigated herein are the Boehm titration method

and the Fourier Transform InfraRed (FTIR) Spectroscopy method.

The Boehm titration method was first developed by HP Boehm (H.P. Boehm et al., 1964;

H.P. Boehm, 2008) in the 1960s. Since then, it has been used as a chemical method to identify

specific oxygen surface functional groups on carbon materials. Boehm titration works on the

principle that oxygen groups on carbon surfaces have different acidities and can be neutralized

by bases of different strengths (Boehm et al., 1964; Boehm, 2008; Goertzen et al., 2010; Oickle

et al., 2010). Sodium hydroxide (NaOH) is the strongest base generally used, and is assumed to

neutralize all Brønsted acids (including phenols, lactonic groups and carboxylic acids), while

sodium carbonate (Na2CO3) neutralizes carboxylic and lactonic groups (e.g. lactone and lactol

rings) and sodium bicarbonate (NaHCO3) neutralizes carboxylic acids (Boehm, 2008).

Fourier Transform InfraRed Spectroscopy (FTIR) is a “dry” analytical method that can be

used parallel to the Boehm method, to qualitatively determine the functional groups on activated

carbon. Correlation of different analytical methods has been used to understand the surface

structure of carbons (Barkauskas and Dervinyte, 2004) and in part is a significant goal for this

work, for FTIR is an easier approach than performing titrations.

Here in, a coal-based activated carbon was treated to increase its acidic oxygen functional

groups to assess how Boehm titrations and FTIR would characterize activated carbon with

13

varying degrees of acidity. Moreover, one must consider the amphoteric nature of activated

carbon during titrations. Therefore, two titrations methods were completed (an “Unfiltered

Aliquots” (Method 1) titration of the carbon and a “Filtered Aliquots” (Method 2) titration of the

filtrate produced from the varying carbons-both described in more detail in Chapter 3). The

author is not aware of any article that performs such a comparison.

Characterization is an important aspect for improving sorbents and therefore the

performance of the acid treated activated carbons for the removal of elemental mercury (Hg)

from air is compared. Furthermore, it will be determined whether Boehm titrations and/or FTIR

measurements correlate to Hg capture

Hypothesis

Oxygen functional groups are chemisorbed to the carbon surface, and therefore, to best

measure these functional groups the activated carbon will need to be present during titration.

These oxygen functional groups can accept electrons, and therefore, adsorption of Hg will

increase for the acid treated activated carbons.

Objectives

1. Treat activated carbon with different acid concentrations to create varying concentrations

of oxygen functional groups.

2. Compare two different Boehm titration techniques to qualitatively and quantitatively

determine the carbon surface functional groups.

3. Analyze the correlation between “Unfiltered Aliquots” (Method 1) of the Boehm titration

methodology and FTIR.

4. Compare a virgin and three acid treated activated carbons for elemental Hg adsorption.

14

CHAPTER 2

LITERATURE REVIEW

Activated Carbon

Activated carbons can be prepared from a variety of raw materials such as wood, coal,

peat, and coconut shells. Depending on the manufacturing method, partial oxidation takes place

and the carbon surface becomes rich in a variety of hetero atoms with a broad range of

concentrations depending on the method of activation, chemicals impregnated before and after

activation, and temperature of preparation (Salame and Bandosz, 2001).

Surface Chemistry of Activated Carbon

The surface chemistry of carbon allows carbon to form unusually strong C-C, C=C and

C≡C bonds. Along with this, carbon also forms many strong double and triple covalent bonds

with a number of nonmetals, known as heteroatoms such as N, O, P, H, Cl and S.

The surface of carbons consists of basal planes and edge sites. The edge sites have been

found to be much more reactive than the atoms in the interior of the basal planes, and hence they

represent active sites for oxygen chemisorption. Therefore, surface oxygen groups are

predominantly located on the edges. In a study conducted by Boehm it was shown that basal

planes of graphite (graphite consisting of mostly organized basal planes unlike the disorganized

basal planes for activated carbon) were attacked by molecular oxygen only at their outer edge

(Boehm, 1994).

Adsorption onto activated carbons is mainly of dispersive interaction type where surface

chemistry can play an important role. In addition, surface chemistry of activated carbons can

determine its moisture content, catalytic properties, acid–base character, and may likely impact

the adsorption of polar and inorganic species (Salame and Bandosz, 2001). The surface

chemistry can be described by these heteroatoms bound to the edges of the graphite-like layers

15

and the oxygen functional groups formed are known to include carboxylic acids, lactones,

phenols, carbonyls, aldehydes, ethers, amines, nitro compounds, and phosphates (Salame and

Bandosz, 2001). More broadly, activated carbons, can have basic or acidic behaviors in aqueous

dispersions and are thus classified as amphoteric in nature. The acidic surface properties are due

to the presence of acidic surface groups. Activated carbons with low oxygen contents result in a

basic surface (Boehm, 1994).

Acidic Functional Groups

Oxygen is chemically bound to the activated carbon surface most typically when the

carbon cools after its normal thermal activation at about 850 °C. It is to be expected that most of

the oxygen is bound by covalent bonds in the form of functional groups that are known from

organic chemistry (Figure 2-1 (Boehm, 2008)). As shown in Figure 2-1, the functional groups

include carboxyl groups (a) which might form carboxylic anhydrides (b) if they are close

together. In close proximity to hydroxyl groups or carboxyl groups, carbonyl groups might

condense to lactone groups (c) or form lactols (d). Single hydroxyl groups (e) are on the edge of

“aromatic” layers those would be of phenolic character. The existence of carbonyl groups is very

plausible; they could come either isolated (f) or arranged in quinone-like fashion (g). Finally,

oxygen could simply be substituted for edge carbon atoms (h); such xanthene- or ether-type

oxygen which is very difficult to detect (Boehm, 1994).

Since the acidity constants of carboxyl groups, lactones, or phenols differ over several

orders of magnitude, it was established that the various types of groups can be distinguished by

their neutralization behavior (Boehm, 1994). In other words, the acidity of these groups opens a

convenient way for their determination by titration with aqueous or alcoholic bases by

neutralization adsorption (Boehm, 2008). However, the remaining oxygen functional groups lack

characterization techniques.

16

In solution, carboxyl groups will be dissociated to carboxylate. Boehm concluded that the

most convenient approach to determine the concentration of free carboxyl groups on carbon

surfaces is to perform a neutralization adsorption experiment with 0.05 M NaHCO3 solution,

separate the solution from the carbon, and titrate the remaining Na+ ions (by adding excess 0.05

M HCl to an aliquot, and back-titrate with standardized NaOH) (Boehm, 1994).

Lactones on the other hand, are weaker acids than free carboxyl groups. This is

demonstrated by the fact that the lactone ring is disrupted by sodium carbonate (Na2CO3), but not

by sodium bicarbonate (NaHCO3). Therefore, 0.05 N Na2CO3 solution is suitable for the

determination of carboxyl and lactone groups (Boehm, 1994; Boehm, 2008).

Basic Functional Groups

The activated carbon surface can carry basic and acidic surface groups simultaneously.

The nature of basic sites on the surface of activated carbons is still very unclear. There are a few

theories that try to explain the basic nature of activated carbon, one of which include the

relationship between chemisorption of oxygen and basic structure development. Papirer et al.

(1987), analyzed the gases of thermal decomposition of basic carbons and concluded that

pyrone-like structures are responsible for the basic nature. Much earlier, Garten and Weiss

(1957) attributed the basicity of carbons to the formation of chromene-type structures.

Obviously, the cause of the surface basicity of carbons is still not satisfactorily understood. Most

likely, basicity of oxygen-containing surface functionality exists in addition to the π basicity of

basal faces (Boehm, 2008; Radovic et al., 1997).

Although basic groups are not well understood in its effect to the activated carbon’s

functionality, Salame and Bandosz (2001) concluded that the concentration of basic surface sites

decreases with increasing surface oxidation and creation of acidic surface functional groups. The

17

cause of the decrease was associated to withdrawing effects of electronegative surface groups on

the π electron resonance system of the graphene layers (Boehm, 2008).

In cases where researchers want to understand the impact of heteroatoms on adsorption,

they focus on the acidic functional groups. However, the role of basic functional groups may

play in adsorption and/or catalysis systems, cannot be ruled out.

Boehm Titrations

Surface functional groups on activated carbons can be determined using “wet” and “dry”

methods of analysis. One of the “wet” techniques used is the Boehm titrations. Boehm titrations

provide qualitative and quantitative information on the acidic nature of the carbon surface.

However, the information on acidic groups is limited to compounds such as phenols, lactones,

and carboxylic acids, neglecting any other groups present (Salame and Bandosz, 2001).

The Boehm Method was originally developed in 1964 by Hans Peter Boehm for

quantifying acidic oxygen surface functional groups of carbon blacks (Boehm et al., 1964;

Boehm, 1994). The original procedure called for equilibrating 0.05 M solutions of the Boehm

reactants (NaHCO3, Na2CO3, and NaOH) with various samples of carbon black. The solid

sorbent was separated from the solution, and the isolated solution would be acidified and boiled

to remove CO2, this latter step often neglected or replaced with an inert gas bubbling. Finally,

aliquots of the prepared acidified solutions were back-titrated with a standardized NaOH solution

to determine the quantity of the Boehm reactants that were neutralized during the initial

equilibration with the solid (Fidel et al., 2013). Later on it was established that acidification is

done by titrating with HCl and then CO2 is removed by purging with N2 (Goertzen et al., 2010;

Oickle et al., 2010). This process, called for the need of using Blanks in calculation (Fidel et al.,

2013).

18

The titration’s underlying principle is that strong acids and bases will react with all bases

and acids, respectively, whereas weak acids will only donate protons to the conjugate bases of

acids with higher pKa values. Researchers generally assume that (i) NaOH accepts protons from

all Brønsted acids (including phenols and carboxylic acids) while hydrolyzing lactones and

lactols, (ii) Na2CO3 accepts protons from functional groups with pKa values <10.3 (carboxylic

acids) while hydrolyzing lactones and lactols, and (iii) NaHCO3 accepts protons from functional

groups with pKa values <6.4 (carboxylic acids). This differentiation allows for calculation of

functional group quantities in discrete pKa ranges via subtraction (Goertzen et al., 2010; Oickle

et al., 2010) (Figure 2-2).

This paper briefly looks into determining the basic functional group concentration, in the

form of Total Basicity based on a similar neutralization principle, where an acid (HCl) is treated

with carbon which is then titrated with a base. Here, back titration was not required. This is

because the carbon was directly treated with an acid that automatically prevents the formation of

alkaline H2CO3, and so the calculation avoided the use of a blank.

Standardizations in Boehm Titration Methodology

To help standardization and comparison among research groups worldwide, Goertzen et

al. (2010) and Oickle et al. (2010) established that the carbon to base ratio was chosen, based on

the requirement that at least 10% of the reaction base should react with the carbon.

To simplify the process of removing carbonate ions from the extracts before acidification,

the samples are treated as recommended by Goertzen et al. (2010) and Oickle et al. (2010). To

lower the pH to <2, 0.05 M HCl was added at a 2:1 (v:v) ratio to the NaHCO3 and NaOH

extracts and at a 3:1 ratio to the Na2CO3 extracts. Further, to remove CO2, before titration, the

acidified samples are sparged with N2 for 2 h. Additionally, degassing using either N2 or Ar

would continue during the titration. All treated extracts were titrated with standardized 0.05 M

19

NaOH to an endpoint pH of 7. All treatments were also performed on blanks, and the blank

values were subtracted from those of the samples (Fidel et al., 2013).

Since activated carbon’s surfaces have both acidic and basic surface functionalities as

agreed on in previous sections, these scientists found it is important to remove (filter before

titration) all of the activated carbon from the reaction mixture in the Boehm titration. It was

found that accurate quantification is only possible, when all of the functional groups are retained

in the solid phase (Fidel et al., 2013). This was believed necessary as basic functional groups of

any leftover activated carbon have a tendency to react with the HCl used in the acidification step

of the titration, leading to a negative bias for the amount of acidic functional groups determined

through this method (Goertzen et al., 2010; Oickle et al., 2010).

Fourier Transform InfraRed (FTIR) Spectroscopy

In order to obtain a comprehensive understanding of the surface structure of activated

carbon, different analytical methods, such as spectroscopy have been used. As previously stated

the activated carbon surface is very complex as (1) it can have a high surface area, (2) can

exhibit differences in the physicochemical parameters of heterogeneous surface sites, (3) may

have partial delocalization of the π electrons and (4) have the ability of surface sites to react with

water and other solvents. The comparison and possible correlation of analytical methods may be

crucial to this study as it has been with others in the past (Barkauskas and Dervinyte, 2004).

Infrared spectroscopy (IR) is a technique that measures the vibrations of a functional

group. In IR spectroscopy, a functional group absorbs light that induces vibrational excitation of

its covalent bonds. A functional group can have various vibrational modes which are

characteristic of its components. Since the energy of the absorbed light corresponds to these

vibrational modes, it has been possible to determine the structure of a compound with IR (Kim,

2010).

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IR can identify carboxylic groups in a compound by C=O stretch and O-H stretch bands

at ~1730 cm-1 and 3234 cm-1 respectively. Similarly, a peak at 1710 –1717cm−1 has been

associated to carboxyl groups (Boehm, 2008).

The unique IR spectrum of a compound (peak position, intensity, and shape) depicts a

unique configuration of covalent bonds which allows for the qualitative analysis of unknown

materials by this type of fingerprinting. However, there are some disadvantages of IR when

applied to surface functional group analysis (Kim, 2010).

Firstly, the accurate assignment of absorption bands is hard to achieve because IR

measures the absorption by different molecules and functional groups simultaneously. As a

result, an absorption band may contain contributions from not only surface functional groups, but

also the carbon substrate, which allows only limited qualitative and quantitative analyses

(Langley et al., 2005), showing difficulty in assigning the peaks.

Secondly, due to the low concentration of surface functional groups on activated carbon,

the peaks from substrate and other molecules can dominate the IR spectrum.

Therefore, detection sensitivity has been an issue for IR in terms of total oxygen content,

and quantitative analysis of IR data is essentially not possible (Langley et al., 2005).

Activated Carbon Oxidation Treatment

It was concluded by Salame and Bandosz (2001), after experimenting with various

oxidants that oxidation leads to an increase in the number of acidic groups and a simultaneous

decrease in the number of basic sites on the activated carbon sample.

A frequently used mechanism to oxidize the surface of activated carbons is that of nitric

acid (HNO3) treatment. It is advantageous as its oxidizing effect can be easily controlled by the

solution concentration, reaction temperature, and reaction time. One disadvantage of treating

21

with nitric acid is, however, that the pore structure of the carbon may be considerably changed.

The micropores become wider and the micropore volume is reduced (Boehm, 2008).

The chemistry behind nitric acid’s interaction with the activated carbon’s surface can

follow either an oxidative route or a nitrative route. A coal based activated carbon, such as the

one treated in this work, which has fewer aliphatic chains and subject to greater aromatization

has been known to follow the oxidative route (Salame and Bandosz, 2001).

Mercury Removal

Experimental studies have been carried out to understand the mechanism of mercury

binding on activated carbon surfaces. Hutson et al. (2007) reported the factors that play a role in

determining the rate and mechanism of mercury binding, to be gas-phase speciation of mercury,

flue gas temperature, and the presence and type of active binding sites on the sorbent. However,

understanding the mechanism by which mercury adsorbs on activated carbon is crucial to the

design and fabrication of effective capture technologies (Padak and Wilcox, 2009).

Early experiments using X-ray absorption fine structure (XAFS) spectroscopy indicated

that mercury bonding on the carbon surface appears to be associated with oxygen, and that

surface oxygen complexes are most likely to provide the active sites for Hg0 bonding. However,

it is not clear what particular surface functional groups are participating in Hg0 adsorption due to

the lack of chemical characteristic information of the carbons tested. No known research has

been done to understand the role of carbon–oxygen surface complexes in Hg0 adsorption (Li et

al., 2003).

A study by Zhang et al. (2015) found that increasing the active sites of activated carbons

was more effective than increasing the surface area for improving the mercury removal

efficiency.

22

Some have suggested that Hg0 adsorption on activated carbon surfaces seems to follow

an oxidation–reduction mechanism. It was concluded that the oxygen surface complexes,

possibly lactone and carbonyl groups, are the active sites for Hg0 capture. The carbons that have

a low phenol group concentration tend to have a higher Hg0 adsorption capacity, suggesting that

phenol groups may inhibit Hg0 adsorption or affect the equilibrium concentrations of lactone or

carbonyl groups. The high Hg0 adsorption capacity of a carbon sample is also found to be

associated with a low ratio of the phenol/carbonyl groups (Li et al., 2003).

23

Figure 2-1. Possible functional groups on carbon surfaces: (a) carboxyl groups, (b) carboxylic

anhydrides, (c) lactones, (d) lactols, (e) phenolic hydroxyl groups, (f) carbonyl

groups, (g) o-quinone-like structures, and (h) ether-type (or pyran- or xanthene-like)

oxygen atoms (H.P. Boehm, 2008)

Figure 2-2. Illustration of the approach used to calculate concentrations of reactive functional

groups in discrete pKa ranges using Boehm titrations (Fidel et al., 2013)

24

CHAPTER 3

MATERIALS AND METHODS

Chemicals and Materials

All chemicals used in this work were analytical grade and were applied without further

purification. Solutions were prepared using ultrapure Type I water with a resistivity of 18.2 MΩ

and a conductivity of 0.055 μS. A commercially available activated carbon (F820) was ground to

a particle size of <45 µm and oven-dried at 110 °C for about 12 hours for all the analyses and

experiments detailed below. The activated carbon was steam-activated made from coal with an

approximate BET surface area of 713 m2/g.

Material Synthesis: Nitric Acid Treatment

Three different concentrations of nitric acid (HNO3) solutions were prepared using a 70%

concentrated trace metal stock solution and ultrapure water. The solutions were 1 M, 5 M and 10

M. 20 g of the activated carbon was treated with each solution by raising the solution

temperature to 80 °C in 500 mL round bottom flasks. A simple funnel and watch glass system

was used to cover the top of the flask, to prevent any loss of acid. After mixing for a period of 24

hours, the treated activated carbon samples were denoted as F820 1 M, F820 5 M and F820 10 M

respectively. Each mixture was filtered using a vacuum pump setup and Whatman #1 paper, and

washed with boiling distilled water until a constant pH was achieved. It was then dried at 110 °C

for 12 hours.

Activated Carbon Physical Characterization Methods

Surface Area, Pore Volume and Average Pore Size

The physical characteristics of all samples discussed in this work were analyzed using

nitrogen adsorption desorption via a Quantachrome NOVA 2200e instrument. Each sample was

degassed at 110 °C under vacuum for 3 h prior to analysis. Samples were then immersed into the

25

liquid nitrogen bath where nitrogen gas was introduced in finite volumes at specific pressures at

a constant temperature of approximately 77 K. The quantity of adsorbed gas was plotted against

the relative equilibrium pressure to determine surface characteristics including surface area, pore

volume and average pore sizes.

The surface area of each sample was calculated by the Brunauer–Emmett–Teller (BET)

Equation 3-1. The P/P0 = 0.1 to 0.3, in which W is the weight of the adsorbed gas at P/P0, Wm is

the weight of the adsorbed gas at monolayer coverage, and C is the BET constant.

1

W[(P0

P⁄ )−1]=

1

WmC+

C−1

WmC (

P

P0) (3-1)

It is important that the following parameters meet their requirements to reduce error in

analyzing BET data. The C constant, calculated from the slope and y-intercept, must not be

negative, the correlation coefficient (R2) should be no less than 0.9975, and the upper limit for

the multipoint BET range, should be the P/P0 value with the maximum single point BET value. A

best fit set of four to five, data points were used in the multipoint BET calculation

(Quantachrome Instruments, 2008).

Assuming that all pore space of the activated carbon is filled with the adsorbate, total

pore volume is measured from the amount of gas adsorbed at the limiting pressure, P/P0 = 0.99.

The average pore size is estimated from the pore volume, distributed over various pore

sizes, represented by a pore size distribution.

Pore size calculations were based upon the Kelvin Equation 3-2, which relates the vapor

pressure above a liquid to the pore diameter (Marsh and Rodriguez, 2006) where γ is the surface

tension, υ is the molar volume of the liquid, R is the molar gas constant (8.314 x 107 J/mol•K),

and rk is the effective radius of curvature:

ln (P

P0) = −

2γυ

rkRT (3-2)

26

Tap Density

The bulk density of a material is the ratio of the mass to the volume (including the inter-

particulate void volume) of an untapped powder sample. The tapped density is obtained by

mechanically tapping a graduated cylinder containing the sample until no volume change is

observed (Particle Analytical, 2015). The tapped density is calculated using Equation 3-3 as

mass (M) divided by the final volume (V) recorded of the powder measured in g/mL:

𝜌 = 𝑀

𝑉 (3-3)

To determine the tapped density of activated carbon, a mass of around 3 g was fed into a

10 mL graduated cylinder of 0.2 mL precision and tapped for 15 minutes. The volume taken was

recorded by reading the upper meniscus when no further change in volume was observed after 15

minutes.

Activated Carbon Chemical Characterization Methods

Contact pH

The pH meter used is first calibrated, using the standard buffers typically of pH 4.01, pH

7.00 and pH 10.01. Once standardized, the pH of deionized water to be used is recorded. In a 50

mL beaker, 1 g of sample is stirred along with 10 mL of water for ten minutes. Stirring continues

till sufficient turbulence has been achieved to fluidize the sample in the beaker. Immediately,

without delay or filtering, the pH of the suspension is measured as recorded (ASTM

International, 2003).

Boehm Titrations Methodology

The four Reactant (R) solutions used to carry out Boehm titrations were 0.05 M solutions

of Sodium Hydroxide (NaOHR), Sodium Carbonate (Na2CO3R), Sodium Bicarbonate (NaHCO3

R)

and Hydrochloric Acid (HClR). Of the four, Sodium Hydroxide and Hydrochloric Acid were also

27

used as Titrant (T) solutions, denoted by (NaOHT) and (HClT) respectively. Prepared solutions

were subjected to Nitrogen (N2) gas purging for two hours.

Each batch of carbon sample preparation and testing was divided into two parts. While one

was acidic (HClR) used to test for basic oxygen functional groups using direct titration, the other

was basic (NaOHR, Na2CO3R and NaHCO3

R) used to test for acidic oxygen functional groups

using back titration. Each carbon sample was run in duplicates along with its corresponding

blank (no carbon) sample.

Carbon samples were treated in two different ways before and during titration. One was

using “Unfiltered Aliquots” (Method 1) while the other was “Filtered Aliquots” (Method 2).

Through the use of appropriate equations the number of acidic functional groups and hence

the total acidity and total basicity of the carbons were determined (refer to APPENDIX for

sample calculations).

Unfiltered Aliquots (Method 1)

The carbon sample contents were 25 mL of Reactant (R) solution, 0.5 g of dried carbon,

and 0.2 g of dried KCl.

Once prepared and sealed, the acidic samples were purged with N2 gas and directly

titrated against NaOHT to a pH of around 7 and the volume of NaOHT was noted. The basic

samples were purged with N2 gas and titrated against HClT first, till the pH reached around 2 and

then back titrated with NaOHT to a pH of around 7. Here too, the volume of NaOHT was noted.

Filtered Aliquots (Method 2)

The carbon sample contents were 25 mL of Reactant (R) solution and 0.5 g of dried

carbon. Once prepared and sealed, the carbon samples were subjected to rotation for 72 hours.

The samples were then filtered using a 45 μm Whatman filter paper and 20 mL of filtrate was

28

retained for titrations. The titration methodology that follows is similar to what was followed for

the Unfiltered Aliquots.

Acidic Functional Group Concentrations

These were calculated as the difference between the micromoles of NaOH needed to

titrate the acidified blanks and extracts using Equation 3-4. Here Fx is the concentration of

functional groups donating protons to Boehm Reactant with a pKa of x, μmol/g; vex is the

volume of titrant used to titrate base/acid in presence of carbon, mL; vbx is the volume of titrant

used to titrate the blank base/acid (carbon is absent), mL; m is the mass of sample, g; Mt is the

molarity of titrant (NaOH), M and DF is the Dilution Factor (Fidel et al., 2013):

𝐹𝑥 =𝑣𝑒𝑥−𝑣𝑏𝑥

𝑚∗ 𝑀𝑡 ∗ 𝐷𝐹 ∗ 1000 (3-4)

The concentration of functional groups in each discrete pKa range, Fx1-x2, (x1 to x2 are the

pKas of Boehm reactants, where x1 < x2) was calculated using Equation 3-5. Here Fx2 is the

concentration of functional groups with pKas < x2 while Fx1 is the concentration of functional

groups with pKas < x1.

𝐹𝑥1−𝑥2= 𝐹𝑥2

− 𝐹𝑥1 (3-5)

Basic Functional Group Concentrations

Equation 3-6 was used to determine total basicity, as direct titration was involved when

using HClR as the Reactant solution (Goertzen et al., 2010; Oickle et al., 2010). Here [B] and VB

represent concentration and volume of the reaction acid mixed with the carbon.

𝐹𝑥 =[𝐵]𝑉𝐵−(𝑣𝑒𝑥∗𝑀𝑡∗𝐷𝐹)

𝑚∗ 1000 (3-6)

FTIR Methodology

IR spectra were collected by using a Nicolet Magna 760 FT-IR instrument equipped with

a diffuse reflectance unit. The instrument’s resolution was set at 4 cm-1.

29

Samples were weighed out at ~0.0009 g and mixed/milled with 0.35 g of KCl in a Wig-

L-Bug for ten seconds. Before each measurement, the instrument was run to collect the

background (using ground KCl), which was then automatically subtracted from the sample

spectrum. The peaks are all inverted, as expected with highly absorbing material.

Gas Phase Mercury Adsorption

Elemental mercury (Hg0) adsorption by activated carbons was performed using a bench-

scale mercury test stand at an elevated temperature of 150 °C. Prior to conducting the

experiments, the activated carbons were oven-dried at 110 °C, and then cooled in a desiccator

before use.

The mercury bonding process was examined through the use of a mercury vapor analyzer

with the carbon sample heated to 150 °C under the influence of N2 gas. The experiments

involved a 30 mg of carbon sample mixed with 1 g of sand and loaded into the mercury test

stand. Industrial grade N2 was first sent through the test stand and then the Hg0 was introduced

through the bypass, into the gas stream in order to establish the baseline Hg0 concentration prior

to adsorption. Once the carbon sample was introduced into the test stand, the N2 flow was

switched from the bypass to the flow containing the carbon sample.

A three-hour testing time was chosen as a basis for comparison and the Hg0 adsorption

capacity was calculated from the area between the baseline Hg0 concentration and the

breakthrough curve.

30

CHAPTER 4

RESULTS AND DISCUSSION

Activated Carbon Oxidation

The virgin activated carbon sample was treated with nitric acid as previously described,

and as expected the water contact pH for all acid treated samples decreased substantially when

compared to the untreated activated carbon (Table 4-1). Although the acid concentration spanned

one order of magnitude, the water contact pH for all treated samples were about the same, pH 3.

This suggests that the total acidity of these samples might be the same, and is discussed in more

detail below when Boehm titration results are reviewed.

Treatment with 1 M and 5 M nitric acid had very little impact on total surface area, and

slightly a positive one because the surface area was larger than the virgin sample perhaps some

of the carbon was etched or digested by the acid. The 10 M treated sample had a lower surface

area, likely because this higher concentration of nitric acid digested more of the carbon than

desired. When comparing these samples for Hg capture, it is important to keep in mind that the

surface area of the 10 M treated carbon is about 25% lower than the 1 M and 5 M treated

samples.

Boehm Titrations Comparison for Method 1 and Method 2

In order to understand the variability in precision of functional group quantification,

Boehm titrations were carried out via two distinct methods as detailed in the experimental

section. The significant differences between the two methods took place in the preparation steps

where in “Unfiltered Aliquots” (Method 1) the activated carbon was stirred and in contact with

the reactant for 30 minutes, while in “Filtered Aliquots” (Method 2) a contact time between

activated carbon and solution was 72 hours. The other difference was that in Method 1, the

titration was conducted directly in the presence of the unfiltered activated carbon, while the

31

Method 2 titration was conducted on the aliquot obtained after filtration. Lastly, Method 1 used

about 0.2 g of Potassium Chloride, to increase the ionic strength of the solution, which was

absent in Method 2.

Results obtained from running Boehm titrations on the untreated sample (F820) in

duplicates were averaged and analyzed to determine the efficacy of the two methods and one was

adopted for further experimentation. In Figure 4-1, the two titration methods were graphed (x-

axis) against the concentration in µmol/g AC (y-axis) of the detectable functional groups

(Carboxyl, Phenol and Lactone) and total acidity and basicity. The data shows that Method 2 was

highly inconsistent in comparison to Method 1 for various functional groups plotted. For

example, the carboxyl and phenolic group error bars for Method 2 are far reaching each extreme

of the averaged results in comparison to the error bars from Method 1. Method 2 displayed

negative functional group counts of 291 µmol/g AC on phenolic groups.

Again, Method 2 titrated the filtrate. For this methodology to work, either the functional

groups would need to dissociate from the carbon surface and/or during mixing for 72 hours there

would need to be deprotonation of the functional groups. Neither was likely. The functional

groups are chemisorbed to the activated carbon. Furthermore, when the activated carbons were

wetted, the pH was driven to about 3 (lower than the pKa for the three acids). Therefore, Method

2 is most likely titrating water soluble ash.

Although there was sufficient reasoning to discard Method 2, another factor considered

was the total basicity obtained from both methods, which was compared to the contact pH of ~

7.5 for F820. Here, Method 2 assessed the basicity as very low in comparison to the acidity

where for Method 1 total acidity and total basicity, were much more comparable at 304 and 437

µmol/g AC, respectively. Since the water contact pH for the virgin carbon was 7.47, one would

32

expect that total acidic and basic functional groups would be about the same. Therefore Method

1 was solely used going forward.

Unfiltered Aliquots (Method 1) Boehm Titration Comparison for the Acid Treated Carbon

The concentration of oxygen functional groups for the virgin activated carbon and three

acid treated activated carbons are shown in Figure 4-2. The carboxyl group concentration shows

a proportional increase with increasing concentration of acid, albeit the 5 M and 10 M error bars

overlap. This could suggest that nitric acid treatments of activated carbons primarily produce

carboxyl functional groups.

On average, the lactone functional group for the three acid treated samples decreases, but

the concentration of these functional groups (< 100 µmol/g AC) and error bars associated with

these measurements suggest drawing conclusions scientifically was questionable. Nonetheless on

average one could conclude that lactonic functional groups are converted to other functional

groups with nitric acid treatment.

The phenolic groups on average appear to remain about the same, regardless of acid

concentration. Therefore it appears that nitric acid has the biggest impact on the carboxyl and

lactonic functional groups.

The total acidity of the activated carbon samples gradually increased as the concentration

of HNO3 increased except for the F820 10 M, whose acidity slightly decreased. One explanation

for this result maybe the low surface area and pore volume of F820 10 M, which was

comparatively lower at 646 m2/g and 0464 cc/g when compared to F820 1 M and F820 5 M as

seen in Table 4-1. The lower surface area may be responsible for the concentration decrease of

functional groups as these groups are found on the edges of the planes as discussed previously.

Although the basicity of some samples show negative values, this is an artifact of the titration

33

analysis whereby, for example, residual acid is being titrated by the base. Since the residual acid

is not present in the virgin carbon, negative values are calculated.

FTIR Results

FTIR results uses the peaks corresponding to a particular wavelength shown in Table 4-2

to determine possible oxygen functional groups present on activated carbon. In Table 4-2, m, w,

s, b, sh, represent the type or strength of bond to be medium, weak, strong, broad, or sharp

respectively. The two techniques used to obtain the IR spectra are DRIFT and Transmission

Analysis. DRIFT is more surface sensitive and therefore as seen in Figure 4-3 does not appear to

show any significant differences between the virgin and treated samples peak heights. However

the Transmission Analysis is representative of the structure from its interiors as well, and a slight

variation can be seen in peak heights of the four samples (Figure 4-4).

Comparing Figure 4-3 to Table 4-2, it can be established that the first peak seen between

3500-3200 cm-1 corresponds to O-H stretch found in phenols, closely following this peak is the

small one at 1730-1715 cm-1 which corresponds to C=O stretch of α, β–unsaturated esters

(lactone), after this, there is a peak at 1760-1690 cm-1 which can be attributed to C=O stretch of

carboxylic acids (carboxyl). Further lower, the peak at 1320-1000 cm-1 is representative of the C-

O stretch found in both carboxylic acids and esters and finally the peaks form 950-910 cm-1 is for

the O-H bend seen in carboxylic acids.

Using the Transmission analysis, the full scale spectrum can be studied (Figure 4-4).

While Figure 4-3 gives an overall qualitative information regarding the possible oxygen

functional groups that can be found on the sample, Figure 4-4 attempts to provide a quantitative

comparison between them. When Figure 4-4 is zoomed in and all graphs are clubbed over a

common baseline, it produces Figure 4-5. Figure 4-5 can be used to correlate the trends in

34

amount of the three acidic oxygen functional groups detected, with that of Boehm titration

results.

Comparison between Boehm and FTIR Results

The trends generated from Boehm titrations for the three acidic functional groups were

compared to those seen from the FTIR spectrum. The closest comparison in trends was found to

be for the 1760-1690 cm-1 band that represents the C=O stretch in carboxylic groups. While

Boehm shows a trend of F820 < F820 1M < F820 5M < F820 10M, FTIR was F820 < F820 1M

< F820 10M < F820 5M. Interestingly, when the carboxyl and lactone groups obtained from

Boehm titrations were summed up (Figure 4-6) and compared to the band of 1320-1000 cm-1, it

was found that the two analytical methods produced identical results of F820 < F820 1M < F820

10M < F820 5M.

Mercury Adsorption Performance Analysis

After identifying the quantity and types of functional groups on the virgin and treated

activated carbon samples, each sample was analyzed to determine the amount of elemental

mercury (Hg0) removed from the gas phase by the sorbent. In Figure 4-7, mercury removal is

graphed on the y-axis in µg Hg/g AC for the four samples studied above. The results demonstrate

an initial removal of 556 µg Hg/g AC for the virgin sample of activated carbon. This

concentration was calculated after a 3 hour period where the activated carbon was exposed to 2

LPM flow rate of nitrogen gas containing 76 µg Hg/m3 at a temperature of 150 °C. The

concentration of mercury removed by the treated samples were of greater magnitude than that of

the virgin sample. A trend was detected in the treated sample results, depicting that with

increasing HNO3 concentration there is a decreasing mercury removal concentration in the order

of F820 10 M < F820 5 M < F820 1 M.

35

When the four components of Boehm titrations (μmol/g AC) were compared to the

amount of Hg removed (at 170 min), the Carboxyl group (Figure 4-8) followed the order F820 <

F820 1M < F820 5M < F820 10M, Lactone group (Figure 4-9) followed an exactly opposite

trend of F820 10M < F820 5M < F820 1M < F820, Phenol group followed F820 1M < F820

10M < F820 5M < F820 (Figure 4-10) and finally Total Acidity trends were F820 < F820 1M <

F820 10M < F820 5M (Figure 4-11). Clearly, one cannot point out which of the functional

groups is more predominantly responsible for the Hg0 adsorption trend seen. As a result, further

investigation in the future will be required.

36

Table 4-1. Physical and chemical characteristics of F820, F820 1 M, F820 5 M and F820 10 M

Sample BET

m2/g

Pore size

Å

Pore volume

cc/g Contact pH

Density

g/mL

F820 713 14.5 0.517 7.47 0.58

F820 1 M 802 14.4 0.579 3.33 0.55

F820 5 M 792 14.4 0.565 2.35 0.56

F820 10 M 646 14.4 0.464 2.95 0.60

Table 4-2. IR absorptions

Frequency, cm-1 Bond Oxygen Containing Functional Groups

3640–3610 (s, sh) O–H stretch, free

hydroxyl alcohols, phenols

3500–3200 (s,b) O–H stretch, H–bonded alcohols, phenols

3300–2500 (m) O–H stretch carboxylic acids

2830–2695 (m) H–C=O: C–H stretch aldehydes

1760–1665 (s) C=O stretch carbonyls (general)

1760–1690 (s) C=O stretch carboxylic acids

1750–1735 (s) C=O stretch esters, saturated aliphatic

1740–1720 (s) C=O stretch aldehydes, saturated aliphatic

1730–1715 (s) C=O stretch α,β–unsaturated ester

1715 (s) C=O stretch ketones, saturated aliphatic

1710–1665 (s) C=O stretch α,β–unsaturated aldehydes, ketones

1320–1000 (s) C–O stretch alcohols, carboxylic acids, esters, ethers

950–910 (m) O–H bend carboxylic acids

*This is an abridged version of an extended table provided by the University of Colorado (Table

of Characteristic IR Absorptions)

37

Figure 4-1. Boehm titrations methodology comparison of F820 - Method 1: (unfiltered PAC,

0.2g KCl, 0.5 g PAC, back- titrated HCl, paraffin sealed beaker, N2 purging), Method

2: (filtered PAC, 0.5 g PAC, back- titrated HCl, paraffin sealed beaker, N2 purging)

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1000

Carboxyl Lactone Phenolic Total Acidity Total

BasicityFu

nct

ion

al

Gro

up

Con

cen

trati

on

(μ

mol/

gA

C)

F820

Method 1

F820

Method 2

38

Figure 4-2. Boehm titrations of F820, F820 1 M, F820 5 M and F820 10 M following Method 1

(µmol/g AC)

-300

-200

-100

0

100

200

300

400

500

600

700

800

900

1000

Carboxyl Lactone Phenolic Total Acidity Total Basicity

Fu

nct

ion

al G

rou

p C

on

cen

tra

tio

n (μ

mo

l/g

AC

) F820

F820 Treated

with 1M HNO3

F820 Treated

with 5M HNO3

F820 Treated

with 10M

HNO3

39

Figure 4-3. DRIFT spectrum corrected for interference and baseline, ~0.0009 g in 0.35 g of KCl

40

Figure 4-4. Common scale transmission spectrum, corrected for water vapor, CO2, baseline and

inverted

41

Figure 4-5. Zoomed from 3500–800 cm-1- common scale transmission spectrum, corrected for

water vapor, CO2, baseline and inverted

42

Figure 4-6. Boehm titrations results for the sum of carboxyl and lactone groups of F820, F820 1

M, F820 5 M and F820 10 M following Method 1 (µmol/g AC)

0

100

200

300

400

500

600

Carboxyl + Lactone

Fu

nct

ion

al

Gro

up

Con

cen

trati

on

(μ

mol/

gA

C)

F820

F820 Treated with 1M

HNO3

F820 Treated with 5M

HNO3

F820 Treated with 10M

HNO3

43

Figure 4-7. Mercury removal performance measure on F820, F820 1 M, F820 5 M and F820 10

M (2 LPM of 76 µg/m3 Hg at 150 °C; 0.03 g sorbent in sand; w/o SnCl2)

0

100

200

300

400

500

600

700

800

900

1000M

ercu

ry R

emoval

(µg H

g/g

AC

)

F820

F820 Treated with 1M

HNO3

F820 Treated with 5M

HNO3

F820 Treated with 10M

HNO3

44

Figure 4-8. Compare μmol of carboxyl group/g AC of F820 samples against mercury removed

(μg Hg/g AC)

0

50

100

150

200

250

300

350

400

450

500

550

0 200 400 600 800 1000

Carb

oxyl

Gro

up

(μ

mol/

g A

C)

Mercury Removal (µg Hg/g AC)

F820

F820 Treated with 1M

HNO3

F820 Treated with 5M

HNO3

F820 Treated with 10M

HNO3

45

Figure 4-9. Compare μmol of lactone group/g AC of F820 samples against mercury removed

(μg Hg/g AC)

-100

-50

0

50

100

150

200

250

300

350

400

450

500

550

0 200 400 600 800 1000

Lact

on

e G

rou

p (μ

mol/

g A

C)

Mercury Removal (µg Hg/g AC)

F820

F820 Treated with 1M

HNO3

F820 Treated with 5M

HNO3

F820 Treated with 10M

HNO3

46

Figure 4-10. Compare μmol of phenol group/g AC of F820 samples against mercury removed

(μg Hg/g AC)

0

50

100

150

200

250

300

350

400

450

500

550

0 200 400 600 800 1000

Ph

enol

Gro

up

(μ

mol/

g A

C)

Mercury Removal (µg Hg/g AC)

F820

F820 Treated with 1M

HNO3

F820 Treated with 5M

HNO3

F820 Treated with 10M

HNO3

47

Figure 4-11. Compare μmol of total acidity/g AC of F820 samples against mercury removed (μg

Hg/g AC)

0

50

100

150

200

250

300

350

400

450

500

550

600

650

0 200 400 600 800 1000

Tota

l A

cid

ity (μ

mol/

g A

C)

Mercury Removal (µg Hg/g AC)

F820

F820 Treated with 1M

HNO3

F820 Treated with 5M

HNO3

F820 Treated with 10M

HNO3

48

CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS

Activated carbon is still very much one of the most applied technologies for air and water

treatment. Although activated carbon is viewed as a mature technology, and in some instances it

is since it has been in use for centuries, we still don’t have quantitative techniques to measure the

oxygen functional groups on the activated carbon surface. Indeed, Boehm titration, FTIR and not

explored in this research, temperature programmed desorption (TPD) are used. However, TPD is

purely speculative making the assumption that certain functional groups will desorb at separate

and discrete temperatures. FTIR measures the surface of activated carbon, but the adsorption

occurs within the pores. Boehm appears to still be the best at quantifying specific functional

groups, albeit the approach where the activated carbon in solution during titrations, appears to be

more appropriate for activated carbon.

When treating a coal based activated carbon with nitric acid, the overall acidity of the

activated carbon increased. Moreover there was a strong correlation between the concentration of

nitric acid and carboxyl functional groups. If researchers can determine how to create specific

functional groups on activated carbon, it is likely that activated carbon could be better

engineered for specific applications.

Each of the acid treated activated carbons improved in adsorption capacity compared to

the virgin carbon. This suggests that oxygen functional groups may be responsible for Hg

oxidation and adsorption.

49

APPENDIX

BOEHM TITRATION CALCULATIONS

Boehm titration calculations used to determine the concentration of acidic functional

groups, total acidity and total basicity in μmol/g are given below:

Acidic functional group concentration is determined combining Equation 3-4 and

Equation 3-5. Equation 3-4 works on the principle of back titration. Total Basicity is determined

using Equation 3-6 that follows the principle of direct titration.

𝐹𝑥 =𝑣𝑒𝑥−𝑣𝑏𝑥

𝑚∗ 𝑀𝑡 ∗ 𝐷𝐹 ∗ 1000 (3-4)

𝐹𝑥1−𝑥2= 𝐹𝑥2

− 𝐹𝑥1 (3-5)

𝐹𝑥 =[𝐵]𝑉𝐵−(𝑣𝑒𝑥∗𝑀𝑡∗𝐷𝐹)

𝑚 ∗ 1000 (3-6)

Sample calculations showing concentration of Carboxyl, Lactone and Phenol groups and

Total Acidity and Total Basicity are given below:

Carboxyl: 𝐹𝐶𝑎𝑟𝑏𝑜𝑥𝑦𝑙 = 𝐹𝑁𝑎𝐻𝐶𝑂3=

𝑣𝑒NaOH−𝑣𝑏NaOH

𝑚⌋

𝑁𝑎𝐻𝐶𝑂3

∗ 0.05 ∗ 1

Lactone + Carboxyl: 𝐹𝑁𝑎2𝐶𝑂3=

𝑣𝑒NaOH−𝑣𝑏NaOH

𝑚⌋

𝑁𝑎2𝐶𝑂3

∗ 0.05 ∗ 1

Lactone: 𝐹𝐿𝑎𝑐𝑡𝑜𝑛𝑒 = 𝐹𝑁𝑎2𝐶𝑂3− 𝐹𝑁𝑎𝐻𝐶𝑂3

Phenol + Lactone + Carboxyl: 𝐹𝑁𝑎𝑂𝐻 =𝑣𝑒NaOH−𝑣𝑏NaOH

𝑚⌋

𝑁𝑎𝑂𝐻∗ 0.05 ∗ 1

Phenol: 𝐹𝑃ℎ𝑒𝑛𝑜𝑙 = 𝐹𝑁𝑎𝑂𝐻 − 𝐹𝑁𝑎2𝐶𝑂3

Total Acidity: 𝐹𝑇𝑜𝑡𝑎𝑙 𝐴𝑐𝑖𝑑𝑖𝑡𝑦 = 𝐹𝑁𝑎𝑂𝐻

Total Basicity: 𝐹𝑇𝑜𝑡𝑎𝑙 𝐵𝑎𝑠𝑖𝑐𝑖𝑡𝑦 =[0.05]𝑉𝐻𝐶𝑙−(𝑣𝑒𝑁𝑎𝑂𝐻∗0.05∗1)⌋𝐻𝐶𝑙

𝑚 ∗ 1000

50

LIST OF REFERENCES

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Carbon”, Designation D- 6851-02, (2003)

Barkauskas, J. and M. Dervinyte; “An Investigation of the Functional Groups on the Surface of

Activated Carbons”, J.Serb.Chem.Soc., 69, 363–375(2004)

Boehm, H. P.; “Chapter Thirteen - Surface Chemical Characterization of Carbons from

Adsorption Studies” Adsorptions by Carbon, 301-327 (2008)

Boehm, H. P.; “Some Aspects of the Surface Chemistry of Carbon Blacks and Other Carbons”,

Carbon, 32, 759-769 (1994)

Boehm, H. P., E. Diehl, W. Heck and R. Sappok; “Surface Oxides of Carbon”, Angewandte

Chemie International Edition, 10, 669 – 677 (1964)

Coughlin, R. W., F. S. Ezra and R. N. Tan; “Influence of Chemisorbed Oxygen in Adsorption

onto Carbon from Aqueous Solution”, Journal of Colloid and Interface Science, 28, 386-

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BIOGRAPHICAL SKETCH

Upasna Bhagwanth Rai is the daughter of Bhagwanth and Satyabhama Rai, born in 1991,

Mumbai, India. Inspired by her parents, Upasna grew up to become passionate about science and

technology. She completed her high school as a Science Major and secured her Bachelor’s in

Chemical Engineering from Rashtreeya Vidyalaya College of Engineering. During the four years

as an undergraduate she decided to apply her scientific inclination towards the betterment of

society and nature. This steered her to want to study abroad and in 2013 began her journey as a

master’s student in Environmental Engineering, at the University of Florida, USA. It was only in

December 2014 that Upasna was offered the opportunity to carry out research towards achieving

her master’s thesis degree under the guidance of Dr. David Mazyck. She received her master’s

degree from the University of Florida in the fall of 2015.