skold et al 2009 jch

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Research article

Solubility enhancement of seven metal contaminants using

carboxymethyl--cyclodextrin (CMCD)

Magnus E. Skold ,a, Geoffrey D. Thyne b, John W. Drexler c, John E. McCray d

a Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, United Statesb Hydrologic Science and Engineering Program, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, United Statesc Geological Sciences, University of Colorado at Boulder, CO 80401, United Statesd Hydrologic Science and Engineering Program, Environmental Science and Engineering Division, Colorado School of Mines, Golden, CO 80401, United States

a r t i c l e i n f o a b s t r a c t

Article history:

Received 5 December 2008

Received in revised form 25 February 2009

Accepted 16 April 2009

Available online 20 April 2009

Carboxymethyl--cyclodextrin (CMCD) has been suggested as a complexing agent for

remediation of sites co-contaminated with metals and organic pollutants. As part of an attempt

to construct a geochemical complexation model for metal-CMCD interactions, conditional

formation constants for the complexes between CMCD and 7 metal ions (Ba, Ca, Cd, Ni, Pb,Sr, and

Zn) are estimated from experimental data. Stable metal concentrations were reached after

approximately 1 dayand estimated logarithmic conditional formation constants range from 3.2

to 5.1 with condence intervals within 0.08 log units. Experiments performed at 10 C and

25 C show that temperature affects the solubility of the metal salts but the strength of CMCD-

metal complexesare notaffected by thistemperaturevariation. The conditional stability constants

and complexation model presented in this work can be used to screen CMCD as a potential

remediation agent for clean-up of contaminated soil and groundwater.

2009 Elsevier B.V. All rights reserved.

Keywords:

Cyclodextrin

Solubility enhancement

Metals

Remediation

1. Introduction

Organic and inorganic contaminant mixtures present at

contaminated sites is one of the major challenges in

remediating soil and ground. Due to the fundamentally

different physiochemical properties of these contaminants,

research has to a large extent focused on remediating organic

and inorganic contaminants separately rather than with a

single treatment. Two basic approaches to remediation are to

wash (ex-situ) or ush (in-situ) the contaminated soil withwater (pump and treat) or with an agent to increase solubility

(enhanced ushing). If the complexing agent, or mixture of

complexing agents, is chosen appropriately both organic and

inorganic contaminants can be removed simultaneously. For

instance, biosurfactants (Mulligan et al., 2001) and solutions

of strong acids mixed with isopropyl alcohol (Semer and

Reddy, 1996) have successfully been used in bench scale

experiments. In both studies heavy metals were removed in

the presence of organic compounds from a contaminated

sand and sandy silt, respectively.

One potential complexing agent to clean up mixed waste is

cyclodextrin (CD), a compound produced by microorganisms

consisting of 6, 7 or 8 glucose molecules in a ring structure. The

most common CD is the-cyclodextrin which is composed of 7

glucose molecules.The ringstructure creates a non-polar cavity

enabling inclusion complexation of organic compounds (Wang

and Brusseau, 1993). Chemical properties of interest such as

aqueous solubility and metal complexation potential can bealtered by substituting functional groups to the outside of the

cyclodextrin. Laboratory and eld studies have demonstrated

that hydroxypropyl--cyclodextrin (HPCD) has high aqueous

solubility, strongly enhances solubility of organic compounds

and is not retained by soil (Boving and McCray, 2000).

A eld test at Hill Air Force Base, Utah demonstrated that

addition of 10 wt.% HPCD signicantly accelerated the removal

of organic contaminants via pump and treat (McCray et al.,

1999; McCray and Brusseau,1999). The aquifer consisted ofne

to coarse sandinterbedded with gravel and some silt. More TCE

was removed over 10 days than would have been removed in

Journal of Contaminant Hydrology 107 (2009) 108113

Corresponding author. Tel.: +1 303 471 3482; fax: +1 303 471 3482.

E-mail address:[email protected](M.E. Skold).

0169-7722/$ see front matter 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jconhyd.2009.04.006

Contents lists available at ScienceDirect

Journal of Contaminant Hydrology

j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j c o n h y d
mailto:[email protected]://dx.doi.org/10.1016/j.jconhyd.2009.04.006http://www.sciencedirect.com/science/journal/01697722http://www.sciencedirect.com/science/journal/01697722http://dx.doi.org/10.1016/j.jconhyd.2009.04.006mailto:[email protected]


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1 year with regular pump and treat. In addition, biotracer

studies indicated that ushing with HPCD enhanced the

biodegradation potential for ethanol and hexanol (Alter et al.,

2003). Vertical ushing with HPCD of a contaminant source

zone was attempted at another superfund site (Air Force Plant-

44, Tucson, Arizona) where the subsurface consisted of silt

and clay layers. Despite rate limited transfer of TCE to the

aqueous phase in this setting, 20 wt.% HPCD increased the TCE

concentration in the extractedgroundwater threefold (Blanford

et al., 2001). In a thirdeld trial, enhanced ushing with HPCD

increased efuent TCE concentration up to 22 times compared

to waterush and the HPCD solution wasrecovered and reused

after removing 99.99% of TCE with an air stripper (Tick et al.,

2003). The study was conducted in an unconned aquifer

consisting of medium to ne sand with interbedded strings of

gravelandsilt andclay lenses at the Dover Air Force Base, Dover,

Delaware. These studies show thatHPCD is a potentialagent for

remediation of organic contaminants.

Laboratory studies have shown that carboxymethyl--

cyclodextrin (CMCD) has the ability to complex heavy metals

such as cadmium, nickel, strontiumand mercury in thepresence

of various organic contaminants (Wang and Brusseau, 1995;

Brusseau et al., 1997; Wang et al., 2004). A comparison between

HPCD and CMCD showed that both CDs enhanced the solubility

of organic compounds but that HPCD was the more effective

(Brusseau et al., 1997). Because HPCD does not complex metal

ions but CMCD does, the ability of the latter to simultaneously

enhance the solubility of organic and inorganic contaminants

is evaluated.

A literature review revealed only one study where the

stability constant for a complex between a single metal (Cd)

and CMCD was investigated (Wang and Brusseau, 1995),

making quantitative evaluation of potential remediation

performance and comparison to other complexing agents

difcult. If formation constants would be available for the

formation of aqueous complexes and metal complexation

agents such as CMCD, the solubility enhancement of metal

contaminants could be estimated and the use of each

complexation agent for in-situ remediation could be evalu-

ated. This study conducted a series of batch equilibrium

experiments using industrial grade CMCD to derive condi-

tional stability constants for aqueous complexes between

CMCD and Ba, Ca, Cd, Ni, Pb, Sr, and Zn. These elements were

selected to represent a suite of common metal contaminants

(Ba, Cd, Ni, Pb, Sr, and Zn) as wells as provide insight into

potential competition of elements present in most aquifers

(group 2 elements). In addition, Sr has a radioactive isotopeand is of concern at some sites. These constants can be

incorporated into a geochemical equilibrium model to simulate

the competition for complexation sites on CMCD between

common cations and metal contaminants and investigate

the effect of aqueous metal speciation and solid composition,

i.e. the composition of the mineral phases hosting the metal

contaminant.

2. Materials and methods

2.1. Experimental techniques

Industrial grade CMCD was provided by Wacker Company.The compound was delivered as a sodium salt with unspecied

purity. All other chemicals were +98% pure unless other-

wise noted. The background ionic strength was controlled

by KNO3 in the multi-day experiments, while NaNO3 was

used in the 2-h experiments. In all titrations, the solutions

were purged with nitrogen gas (ultra high purity) prior to

reaction and the pH was adjusted with sodium hydroxide

solution.

All metal salts (Ba, Ca, Cd, Ni, Pb, and Zn) were oxalates,

except for SrSO4. The purity of Cd oxalate was not reported. Zn

and Ni oxalates were formed via precipitation from solutions

of metal nitrates and oxalic acid as described inDonia (1997).

XRD analysis showed that crystalline Ni and Zn oxalates

formed (data not shown).

The acidbase behavior of CMCD was investigated by

potentiometric titrations in a CO2(g) free atmosphere. The

titrations were performed in a glass vessel. Temperature was

kept constant at 25 C and background ionic strength was

50 mM KNO3while pH was increased by adding incremental

volumes of 0.1 M NaOH. The pH was measured with a

combination glass electrode. Before titrating cyclodextrin

with NaOH, the CMCD-Na salt was converted to its acidic form

by passing 50 ml of a 100 g/l CMCD solution twice through a

column containing an ion exchange resin. Ion-exchange resin

(Rexyn 101H) was purchased from Fisher Scientic. The

CMCD solution was bubbled with nitrogen gas for at least 1 h

at pH near 3.3 prior to each titration in order to remove any

carbon dioxide.

Batch experiments aimed at determining conditional

stability constants for complexation between CMCD and

metal ions were performed in sealed 15 ml centrifuge tubes

submerged in a water bath to control temperature at 100.5 or

25 0.5 C. Two separate sets of experiments were performed.

A preliminary investigation at 10 and 25 C lasting 2 h was

performed prior to a second set of experiment at 25 C lasting

7 days. A small amount (2050 mg) metal salt was added to

10 ml solutions containing up to 20 g/l CMCD in 50 mM

background ionic strength. In the preliminary experiments,

background ionic strength was varied from 0 for 20 g/l CMCD

solutions, to 50 mM at the lower CMCD concentrations. In

the longer time experiments, the CMCD concentration ranged

from 0 to 100 g/l and background ionic strength was held

constant at 50 mM KNO3. Hydrogenion activity was adjusted to

pH 6.000.05 in all CMCD solutions before adding the metal

salts. The pH was notcontrolled during the experiment to avoid

interference with buffering compounds; the nal pH varied

from 5.3 to 7.3. Effects of pH variations on the metal speciation,

such as formation of metal carbonates and hydroxideswas taken into account by geochemical modeling as explained

below. The centrifuge tubes were slowly rotated top-over-

bottom throughout the experiment to ensure constant and

gentle mixing.

The solid metal salt was separated from the aqueous

solution by 0.45 m ltering and the pH was measured

immediately. The aqueous metal concentration was subse-

quently analyzed with inductively couple plasma-optical

emission spectrometry (ICP-OES).

2.2. Modeling techniques

In our batch experiments, the free metal ion activity, Me

2+

for all divalent cations investigatedin thisstudy, is controlled by

109M.E. Skold et al. / Journal of Contaminant Hydrology 107 (2009) 108113


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the presence of a sparingly soluble metal salt. CMCD complexes

the free metal ion which causes more of the salt to dissolve

(Eqs.(1) and (2)). Previous systematic evaluation of experi-

mental data for lead complexation by CMCD indicated the data

is best modeled by representing 3 sets of mass action equations

(Skold et al., 2007b). The CMCD has two protonation reactions

and a complexation reaction with a 1:1 (metal ion:CMCD)

stoichiometry (Eqs.(1)(4)). The previous evaluation investi-

gated conceptual models based on various Pb:CMCD ratio,

different acid strength (pKa) for the individual carboxyl groups

that are part of the CMCD molecule, different methods for

calculating the activity coefcients of these carboxyl groups,

and methods of estimating the effect on ionic strength of high

CMCD concentrations. Titrations indicated that each CMCD

molecule has on average 4.27 titrable groups. Additional CMCD

acidity(2.27carboxylgroups perCMCD)is modeled as separate

by carboxylic acid reactions (Eq. (5)). It is important to include

the acid/basebehaviorof CMCD in themodelbecause hydrogen

ions and metal ions will compete for sites on CMCD at low pH.

Also, the buffering capacity of CMCD will affect pH dependent

dissolution of minerals in both lab experiments and potential

eld applications.

MeC2O4 s = Me2+ +C2O24 1

CMCD2

+Me2+

=CMCDMe 2

CMCDH2 = CMCDH

+H + 3

CMCDH

=CMCD2

+H + 4

COOH= COO

+H + 5

The activity coefcient for the species CMCD2 is

calculated using the WATEQ DebyeHckel equation with

a0,CMCD2-=3.50 and bCMCD2-=0.65 (Eq. (6)). The Davies

equation was used to calculate the activity coefcients of all

other species for specic ionic strength of the solution ()

(Stumm and Morgan, 1995).

logCMCD2 = A z 2

CMCD2

ffiffiffi

p1 +B a0;CMCD2

ffiffiffi

p !

+bCMCD2 6

The conditional stability constant for metal-CMCD com-

plexation is calculated according to:

KCMCDMe= CMCDMef g

CMCD2

Me2+ 7

Values within curved brackets represent activities. The

geochemical modeling program PHREEQC (Parkhurst and

Appelo, 1999) was used to model metal complexation by

CMCD and the conditional stability constants were derived

with the parameter optimization program UCODE_2005

(Poeter and Hill, 1998; Poeter et al., 2005). The optimization

process is described in Skold et al. (2007a). Eqs. (1)(7)

were included in the PHREEQC model and the standardPHREEQC database was amended with thermodynamic data

for aqueous complexes between oxalate and metal ions as

shown inTable 1(Martell and Smith, 1977).

3. Results and discussion

3.1. Potentiometric titrations

Titration end-points and pKas were evaluated with half-

point analysis; the pKa of the hydroxyl groups on the CMCD

was determinedas the pH at a pointin the titration where half

of the groups were titrated and the total concentration of

carboxylic acid groups were determined at the inection

point when the change in pH was the greatest per addition of

acid or base (Stumm and Morgan, 1995). The analysis

indicated an average carboxyl concentration of 4.27 func-

tional groups/CMCD molecule. This is equivalent to 0.610

substituted carboxymethyl groups per glucose ring. The 95%

condence interval ranges from 0.601 to 0.619. The molecular

weight of CMCD was calculated to 1390 g/mol based on

substitution ratio and its molecular structure. The average pKafrom ve titrations was 3.80 with 95% condence interval

ranging from 3.77 to 3.84. This value compares well with

published pKas of metoxyacetic acid and etoxyacetic acid,

which have similar structure to functional groups on CMCD:

3.57 and 3.67, respectively (CRC, 2004). The calculations were

performed assuming pure CMCD. If impurities containing less

carboxyl groups than CMCD were present in the experiments

the calculated concentration of functional groups would be

underestimated. However, it is difcult to know the composi-

tion of impurities.

The carboxymethyl groups are separated from each other

by several single carbon\

carbon bonds. Very little electronsharing is expected and we assume above that all carboxyl

groups have identical strength. This assumption was investi-

gated by comparing experimental data to a model based on

experimentally derived acidity constants and assuming that

all carboxyl groups have the same strength. The model ts

experimental data well. The correlation coefcient (R2)

between observed and simulated pH for all 5 titrations

modeled simultaneously was 0.996. Experimental data show

a slightly smoother pH transition than do calculated data.

Subsequently, a model using two different pKavalues for the

carboxyl groups was used to evaluate the data. This two-site

model enhanced the t somewhat but the improvement was

not considered great enough to warrant the use of a morecomplicated model (Fig. 1).

Table 1

Logarithmic constants for aqueous metal-oxalate complexes used in

modeling (Martell and Smith, 1977).

MeC2O4 Me(C2O4)22 Me(C2O4)3

4 MeHC2O4+ Me(HC2O4)2

Ba 2.31 NA NA NA NA

Ca 2.54 2.69 NA 1.84 NA

Cd 3.89 5.66 5.06 NA NA

Ni 5.16 NA NA NA NA

Pb 4.91 6.76 NA NA NASr NA NA NA NA NA

Zn 4.87 7.65 NA 1.72 3.12

110 M.E. Skold et al. / Journal of Contaminant Hydrology 107 (2009) 108113
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3.2. Transient metal solubilization

In all experimental solutions, metal salts reached stable

concentrations within approximately 1 day (Fig. 2). IR

spectroscopy of CMCD solutions indicate that the large

molecule does not decompose during the remaining experi-

mental duration (data not shown). Ca, Sr, Ba, Pb, Ni, and Zn

trends are stable over 7 days duration. In contrast, Cd shows

concentrations that decline with time in the presence of

CMCD. The dissolution of all the salt was at equilibrium in less

than a day except Ca-oxalate dissolution, which reached

equilibrium after 2 days.

Cadmium oxalate dissolved in a different manner than

the other metal salts. For all salts equilibrium was approached

from undersaturation by adding the metal salt to a solution

without the metal. The metal concentration was therefore

expected to increase until stabilizing at equilibrium. This

pattern was observed for all metals but Cd. The concentration

of Cd was 60.7 and 60.5 mg/l after 1 and 2.5 h, respectively,

in the blank solutions. The concentration then started to

decrease (58.6 and 58.7 mg/l after 4.5 and 19.5 h) and reached

44.9 mg/l after 7 days. The most plausible explanation for this

behavior is a change in the metal salt structure resulting in a

change in solubility. Cadmium oxalate exists as a hydrous salt

(CdC2O43H2O) and as an anhydrous salt (CdC2O4) (CRC, 2004).

It is postulated that the salt used in the experiments transi-

tioned from one phase to a thermodynamically more stable

phase. A literature did not reveal any studies investigating the

solubility of the anhydrous form of cadmium oxalate.

3.3. Measured conditional stability constants

Results from batch experiments show that CMCDenhances the solubility of all the investigated metal salts.

The conditional formation constants were optimized by

systematically varying their values such that the model

calculated aqueous metal concentration matched the

Fig. 1.Base titrations at 0.518, 0.698, 0.977, 1.22, and 1.49 g/L of CMCD. Solid

lines represent single-site model and dotted lines represent two-site model.

Fig. 2. Aqueous metal concentrations as a function of time. Each symbolrepresents a single measurement.

Table 2

Measured conditional formation constants and solubility products.

Metal salt Temp. texperimental logKspa logKsp

b logKCMCDMe 95% C.I.c

BaC2O4 10 C 2 h 6.71 6.79d 3.60 3.543.66

25 C 2 h 6.55 3.60 3.583.64

25 C 7 days 6.59 3.58 3.553.60

CaC2O4 10 C 2 h 8.76 8.63e 3.64 3.583.70

25 C 2 h 8.49 3.67 3.593.75

25 C 7 days 8.03 3.56 3.473.64CdC2O4 10 C 2 h 7.99 7.85

f 5.14 4.765.52

25 C 2 h 7.80 4.78 4.495.07

25 C 4.5 h 7.80 4.95 4.755.16

25 C 19 h 7.80 4.40 4.304.50

25 C 7 days 7.80 4.04 3.914.18

NiC2O4 10 C 2 h NA 9.4g NA NA

25 C 2 h NA NA NA

25 C 7 days 9.73 3.48 3.443.51

PbC2O4 10 C 2 h 10.79 9.32h 5.21 5.135.30

25 C 2 h 10.46 5.27 5.215.34

25 C 7 days 10.46 5.18 5.145.23

SrSO4 10 C 2 h 6.81 6.46i 3.49 3.293.69

25 C 2 h 6.69 3.53 3.283.78

25 C 7 days 6.69 3.55 3.473.63

ZnC2O4 10 C 2 h NA 8.86j NA NA

25 C 2 h NA NA NA25 C 7 days 8.86 3.64 3.613.67

NA, not analyzed.a Experimentally determined solubility product,Ksp, was calculated from

aqueous metal concentration measured in experiments with metal salt and

deionized water.b Kspvalues reported inDean (1999).c 95% condence interval for experimentally determined complexation

constant (KCMCD-Me) for complexation reaction between the free metal ions

and CMCD.d Value for BaC2O4H2O.e Value for CaC2O4H2O.f Value for CdC2O43H2O.g Value for NiC2O4.h Value for PbC2O4.i

Value for SrSO4.j Value for ZnC2O42H2O.



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2002). Because anions do not form strong complexes with

carboxyl groups, the behavior of these complexes cannot be

interpolated based on other elements but needs to be

investigated separately. Indeed, Chatain et al. (2004) found

that Cu and Fe were removed more readily from a mining

contaminated soil with CMCD than was As.

The effectiveness of metal remediation will be inuenced

by the presence of non-target cations, such as Ca and Mg. The

extent of competition will depend on their concentrations

and strengths of CMCD complexes. The conditional formation

constants for the group two elements Ca, Sr, and Ba, range

from 3.55 to 3.58. There is no trend in complexation strength

with their period in the periodic table because the 95%

condence intervals overlap. For the transition elements,

however, there does appear to be a trend. The magnitude

of the formation constants increases with increasing group

and period. The period 4 elements Ni and Zn have similar

conditional formationconstants as the group 2 elements: 3.48

and 3.64, respectively. The measured constant for Cd, a period

5 transition element, ranged between 4.95 (4.5 h) and 4.04

(7-day). The strongest complexes were formed between

CMCD and Pb, a period 6 element, the formation constant

(5.18) was almost two orders of magnitude greater than the

formation constants for the group two elements. Based on the

data produced in this study, transition elements in higher

period and group are preferentially complexed to CMCD

compared to group 2 elements. High concentrations of Ca and

Mg or the presence of soluble minerals containing group 2

elements would, however, lessen the solubility enhancement

of heavier target metals such as Pb and Cd.

4. Conclusions

Carboxymethyl--cyclodextrin enhances the solubility of

common metal contaminants making CMCD a potential agent

for in-situ remediation of metals. The solubility enhancement

is greater for heavy metals (greater period and group) than

for group 2 elements, however, the presence of soluble

minerals composed of non-target elements would affect to

what extent the solubility of the target metal would be

enhanced. Conditional formation constants for the complexes

between CMCD and 7 metal ions (Ba, Ca, Cd, Ni, Pb, Sr, and Zn)

range form 103.2 to 105.1 with condence intervals

within 0.08 log units. Temperature variation between

10 C and 25 C did not affect the metal complexation by

CMCD, but signicantly affected the solubility of the investi-

gated metal salts. These conditional formation constantsincorporated into geochemical software may be used to

screen CMCD as a potential remediation agent. It is recom-

mended that the technique be tested undereld conditions to

further assess the applicability to in-situ remediation.

Acknowledgements

This work was in part sponsored by the Environmental

Science and Engineering Division and the Department of

Geology and Geological Engineering at the Colorado School

of Mines and by the Laboratory for Environmental and

Geological Studies at the University of Colorado at Boulder. The

authors wish to acknowledge the editor and three anonymousreviewers who greatly improved the quality of this manuscript.

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