Environ. Sci. Technol. 2008, 42, 8930–8934
Enhanced Solubilization of a Metal-Organic Contaminant Mixture (Pb, Sr, Zn, and Perchloroethylene) by Cyclodextrin MAGNUS E. SKOLD* Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401 GEOFFREY D. THYNE Hydrologic Science and Engineering Program, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401 JOHN W. DREXLER Geological Sciences, University of Colorado at Boulder, Boulder, Colorado 80309 DONALD L. MACALADY Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401 JOHN E. MCCRAY Hydrologic Science and Engineering Program, Environmental Science and Engineering Division, Colorado School of Mines, Golden, Colorado 80401
Received July 2, 2008. Revised manuscript received September 25, 2008. Accepted September 26, 2008.
Prior work has suggested that (carboxymethyl)-β-cyclodextrin (CMCD) is capable of simultaneously enhancing the solubility of organics and metals, but sparse experimental data and no theoreticalmodelshavebeenpublishedonthisprocess.Preciously, a geochemical model for metal complexation by CMCD was formulated using PHREEQC on the basis of conditional stability constants measured in experiments using single-metal salts. In this study, the model is expanded to simultaneous metal and organic (perchloroethylene, PCE) complexation by CMCD. Experiments to verify the application of the formulation to mixedwaste systems were performed using solutions containing multiple metal ions (Pb, Sr, and Zn) and in a separate experiment introducing PCE with multiple metal ions. These experimental results show simultaneous solubility enhancement of metals and PCE. For solutions up to about 50 g/L CMCD, the model accurately predicted the simultaneous solubility enhancement for PCE, Pb, and Zn, while the difference between the measured and predicted Sr concentrations was accurate to within 15%. At CMCD concentrations greater than 50 g/L, the observed metal solubilities were greater than predicted (10% for Pb and Zn), probably due to the difficulty in accurately representing the activity and the effect on the ionic strength of functional groups on large organic molecules at higher concentrations. * Corresponding author current address: ARCADIS U.S., 630 Plaza Drive Suite 100, Highlands Ranch, CO 80129; phone: (303) 471-3482; e-mail:
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Introduction A large number of sites in the United States are contaminated with both metals and organic pollutants (1). The different properties of the two contaminant classes make remediation of mixed-waste sites challenging. One approach that has been suggested is to modify an existing successful technique for one class of contaminants so it can be applied to both contaminant classes; changing the composition of a flushing solution can allow for solubility enhancement of both metals and organic contaminants. In this study we investigate (carboxymethyl)-β-cyclodextrin (CMCD) for the simultaneous solubility enhancement of metals and PCE (perchloroethylene, C2Cl4). CMCD is a nontoxic, glucose-based molecule that has been previously shown to simultaneously enhance the solubilities of metals and organic contaminants (2, 3). The circular structure of the cyclodextrin (CD) molecule results in a nonpolar cavity into which organic molecules can partition (4), and carboxyl groups on the outside of the CMCD molecules can complex metal ions. The functional groups on the outside of the CD molecule affect the properties of the compound and can give the molecule specific properties. Polar groups, such as carboxyl groups, render the molecule more polar than do nonpolar groups. One study indicates that (hydroxypropyl)-β-cyclodextrin (HPCD), a less polar CD than CMCD, does not sorb significantly to soil particles (5), and another study shows that HPCD does not experience pore exclusion in porous media (6). The behavior of CMCD is expected to be similar to that of HPCD. These properties make CMCD an attractive complexing agent for in situ remediation of mixed-waste sites, i.e., sites cocontaminated with organic and inorganic contaminants. In comparison to synthetic complexing agents such as EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid), CMCD forms weaker metal complexes, and the metal removal efficiency of CMCD has been questioned (7). Other researchers argue that because CMCD does not sorb significantly to soil surfaces, it may still be a viable complexing agent for in situ remediation of metals (8). However, CMCD mobility can be retarded by a high content of iron hydroxide (9). Thus, site-specific conditions, including metal speciation, will determine the feasibility of CMCD-enhanced aquifer flushing. A geochemical model that describes metal complexation by CMCD would aid in screening the viability of CMCD for enhanced dissolution of metal ions. To make CMCD-enhanced solubilization an economically feasible remediation technique, the reagent needs to be reused. Volatile organic contaminants can be stripped from the cyclodextrin solution (10). Because the complex between the CMCD molecule and metal ions is weaker than, for instance, the complex between EDTA and metal ions, it appears possible to break the complex and remove the metal ions from the solution. No study has yet been published detailing the separation of metal ions from the CMCD solution. Previously, we have estimated conditional formation constants for complexation between CMCD and eight metal ions (11). These formation constants were derived in the presence of a single-metal salt. The model used to derive the conditional formation constants assumes identical complexation sites for all metals. However, in field conditions where several metal ions are present, competition between metal ions may occur for these complexation sites. The complexation between ionic solutes and functional groups attached to large organic molecules, such as the carboxyl 10.1021/es801835x CCC: $40.75
2008 American Chemical Society
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groups on CMCD, is not well understood. For instance, activity coefficients are generally calculated on the basis of the assumption of point charges, which may not be correct for functional groups attached to a large molecule. Thus, it is not clear whether thermodynamic models can adequately simulate competition between several metal ions complexing with large organic molecules containing multiple functional groups. In this study, one goal is to validate a multicomponent mixed-contaminant complexation model by comparing predicted and measured metal concentrations in the presence of three metals and an organic contaminant at various concentrations of CMCD. A second goal is to incorporate complexation of an organic compound into a geochemical model and to attempt to predict the concurrent solubility enhancement of organics and metals. We assumed that dissolved metal ions and organic compounds interact with different moieties of the CMCD molecule. Prior data suggest the interactions operate separately (2), but another study (3) suggested that metal complexation neutralizes charged groups on CMCD, which may enhance the partitioning of organic compounds into the CD cavity. Potentially, charged moieties on the CMCD molecule may repel the nonpolar organic contaminant. PCE was chosen as the model organic compound because of the large number of locations that are contaminated with this solvent. Lead and zinc were used in the study because of their toxicity and occurrence at contaminated sites, and enhanced dissolution of strontium was investigated because it has a radioactive isotope but can be studied using a stable isotope. Therefore, the enhanced solubilities of metal salts (lead oxalate, strontium sulfate, and zinc oxalate) were measured in the absence and presence of PCE to evaluate the full range of metal-PCE-CMCD interactions.
Materials and Methods Materials. Enhanced metal and PCE solubility experiments were performed using industrial grade CMCD, which was provided as a Na salt by Wacker, Inc. The impurities of the CMCD are unknown but are expected to decrease the metal and organic solubilization capacity. Lead oxalate (PbC2O4) and strontium sulfate (SrSO4) were 98+% pure, and zinc oxalate (ZnC2O4) was made from analytical grade oxalic acid and zinc nitrate. PCE was 99.9+% pure, and potassium nitrate (KNO3), sodium hydroxide (NaOH), and nitric acid (HNO3) were analytical grade. Batch Experiments. All solubility enhancement experiments were performed in glass vials with Teflon-lined caps at 25 ( 0.5 °C in 50 mM KNO3. Prior to the experiments, the pH in CMCD standard solutions was adjusted to 6.0 with sodium hydroxide or nitric acid. Enhanced solubilities of metal salts and liquid-phase PCE were measured in 12 different CMCD solutions (0-100 g/L CMCD). The glass vials were rotated top-over-bottom for 7 days before being sampled. CMCD-enhanced PCE solubility was investigated in the absence and presence of three metal salts (PbC2O4, SrSO4, and ZnC2O4). The samples were prepared and analyzed in a random order. In the batch experiments, 250 µL of liquid PCE was added to glass vials with a total volume of 4.5 mL that contained CMCD standard solutions ranging from 0 to 100 g/L CMCD. No head space was allowed. To half of the samples was added approximately 15 mg of each metal salt prior to addition of CMCD and PCE. After rotation for 6.5 days at 25 ( 0.5 °C all samples were centrifuged at 2000g for 5 min and left standing overnight before 100 µL of the aqueous phase was extracted into 2 mL glass vials containing 1.00 mL of hexane. These vials were rotated top-over-bottom for 3 h followed by centrifugation (2000g for 5 min) and immediate analysis on a gas chromatograph (Shimadzu GC-17A)
equipped with a flame ionization detector (FID). Standards were analyzed prior to the samples. The instrumental response was linear with respect to PCE concentration over the entire range (R2 ) 0.997, data not shown). All PCE batch experiments were performed in duplicate, and each sample was analyzed in separate triplicate vials. Thus, for each CMCD concentration six chromatographic analyses for PCE were performed with metals and six analyses without metals. A separate set of batch experiments was conducted to investigate the CMCD-enhanced solubility for the inorganic contaminants with and without PCE. The only difference between these samples and the ones used for PCE solubilization was the presence of 250 µL of liquid PCE in half of the samples; all samples contained the same amount of metal salts. Zinc oxalate was precipitated from 0.1 M solutions of oxalic acid and zinc nitrate as described elsewhere (12). X-ray diffraction (XRD) analysis confirmed that crystalline ZnC2O4 was precipitated (data not shown). At the end of the batch experiments each solution was filtered (0.45 µm) to isolate the aqueous phase. Solid metal salts remained in the vials at the end of all experiments. Immediately after filtering, the pH of the aqueous phase was measured, and metal concentrations were subsequently analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). Two separate enhanced metal solubility experiments were performed, each in both the presence and absence of PCE. In the first experiment the CMCD concentration ranged from 0 to 20 g/L and in the second from 0 to 100 g/L. All other conditions were the same in the two experiments. The results of the experiments were analyzed using the geochemical equilibrium program PHREEQC (13) in combination with the parameter estimation program UCODE_2005 (14). Mass action expressions postulated for the reactions between CMCD and the metals and PCE were incorporated into PHREEQC, and the equilibrium constants were estimated using UCODE_2005. PHREEQC calculates chemical speciation of the aqueous solutions and corrects activities as a function of ionic strength. UCODE_2005 optimizes unknown equilibrium constants by comparing PHREEQC-estimated total aqueous metal and PCE concentrations to measured concentrations.
Theory We hypothesize that PCE and metal complexations by CMCD are independent. Therefore, in our model, metal and PCE complexation reactions with CMCD are formulated as separate mass action equations. We model interactions between PCE and CMCD as formation of an inclusion complex (CDcavity-PCE) between PCE and the single CMCD cavity (CDcavity): PCE + CDcavity a CDcavity-PCE
(1)
The stability constant for the reaction, KCD-PCE, has units of liters per mole and is defined by eq 2. Values within curved brackets represent aqueous activities. We assume ideal interactions between the CD cavity and PCE. Thus, the activity coefficients for CDcavity, PCE, and CDcavity-PCE are unity. KCD-PCE )
{CDcavity - PCE} {PCE}{CDcavity}
(2)
An alternative formulation for CMCD-enhanced solubility of sorbed-phase or non-aqueous-phase liquid (NAPL) contaminants is partitioning of the organic contaminant between the CD cavity and water. This partitioning is often reported as a CD-water partitioning constant, KCD-W, which has units of liters per kilogram (15): VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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KCD-W )
PCECD PCEw
(3)
PCECD is the mass of PCE that partitions into the CD cavity (g of PCE/kg of CMCD), and PCEw is the aqueous solubility (measured to be 0.280 g/L at 298 K) of PCE in the absence of CMCD. To facilitate comparisons to other studies, we convert our estimated stability constant to a CD-water partitioning constant according to eq 4, where MWCD and MWPCE are the molar masses of CMCD (1387 g/mol) and PCE (165.8 g/mol), respectively. KCD-W )
(
MWCD
1000 PCEw 1 + MWPCE KCD-PCE
)
(4)
To model the metal complexation reactions, we use standard mass action formulations. The model assumes one metal complexation site per CMCD molecule and a metal ion to CMCD stoichiometry of 1:1. Assuming two sites per CMCD molecule does not simulate metal solubilization better than the one-site model, and complexes with metal ion to CMCD stoichiometry greater than 1:1 are not probable at CMCD concentrations above 1 g/L (16). An example using Pb is provided below: PbC2O4(s) a C2O42- + Pb2+
(5)
CMCD2- + Pb2+ a CMCD-Pb
(6)
Solubility products and conditional formation constants for CMCD-metal ion complexes determined in earlier studies are presented in Table 1 (11). Previous evaluation of CMCD-Pb complexation showed that the activity coefficient for CMCD2- produces the best fit to the experimental data when coupled to activity calculation using the WATEQ equation (17) with the coefficients a and b equal to 3.50 and 0.65, respectively (16). The constants A and B in eq 7 depend on temperature and pressure only, γ represents the activity coefficient, and µ represents the ionic strength of the solutions.
(
log γ ) -Az
õ 1 + Baõ
)
+ bµ
(7)
In the model, all metal ions form complexes only with the deprotonated cyclodextrin molecule, CMCD2-. This creates competition among the metal ions for the CMCD: CMCD2- + Sr 2+ a CMCD-Sr CMCD
2-
+ Zn
2+
(8)
a CMCD-Zn
(9)
TABLE 1. Previously Determined Solubility Products and CMCD-Metal Complexation Constants (9) element and salt
log Ksp
log KCMCD-Me
Sr (SrSO4) Zn (ZnC2O4) Pb (PbC2O4)
-6.69 -8.86 -10.46
3.55 3.64 5.18
TABLE 2. Summary of Partitioning Constants Derived in This Study system
KCD-W (L/kg)
95% CI
CMCD-PCE (no metals) CMCD-PCE (with metals)
71.4 72.9
70.7–72.1 72.1–74.9
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FIGURE 1. Comparison between observed and simulated enhanced PCE concentrations in the absence and presence of metals. Metals are believed to form complexes with the carboxyl groups on CMCD (2). However, the carboxyl groups also act as acids, producing or accepting protons. Therefore, the model must represent both potential reactions. We describe the acid/base reactions of CMCD as follows: CMCD-H2 a CMCD-H- + H+
(10)
CMCD-H- ) CMCD2- + H+
(11)
Titrations of CMCD show that each CMCD contains on average 4.27 carboxyl groups and that all the acid sites have identical strength with a pKa of 3.80 (16). Because the CMCD available for metal complexation reactions is represented as a diprotic acid, the additional acidity of the CMCD is separately represented by adding 2.27 separate carboxyl groups per CMCD to the model: COOH a COO- + H+
(12)
If the metal ions in a mixture interact in different ways or with different moieties of the CMCD molecule, this competition may not be correctly described by reactions 5 and 6 and reactions 8 and 9. We test for competition between the metals by comparing theoretical simulations and experimental metal concentrations.
Results and Discussion Organic Contaminant and CMCD. The computer program PHREEQC (13) coupled to UCODE_2005 (14) was used to estimate the stability constant for the CMCD-PCE inclusion complex. In the absence of metal salts, the optimized log stability constant is 2.075 with an estimated 95% confidence interval of 2.070-2.080 (Figure 1). Our experimentally determined stability constant (Table 2) compares well to the published stability constant for PCE and HPCD. Other researchers (18) have reported a partitioning constant of 79.4 L/kg, and our constant, measured in the absence of metals, converts to 71.4 L/kg. In the reported experiments, analytical grade HPCD was used; our experiments were performed using industrial grade CMCD. Other researchers have found that less polar CD derivatives are more efficient at complexing organic contaminants (19-23). The slightly lower KCD-W measured in this study for the more polar CD derivative compared to the constant for PCE-HPCD reported in ref 18 conforms to this trend. Measured and Predicted Metal Concentrations in Multiple-Metal Solutions. The inorganic portion of the complexation model was evaluated for multiple-metal solutions by first comparing the measured total aqueous concentrations of Pb, Sr, and Zn from the metals only experiments to
those predicted with PHREEQC using the theory presented in eqs 5-12 (Figure 1). Conditional formation constants for CMCD-metal complexation previously determined from single-metal experiments (11) were used to predict metal concentrations for CMCD solutions containing the three metal salts. In prior experiments, the metal salt solubility product, Ksp, was determined from the blank solutions (without CMCD). This method produces a consistent experimental data set and avoids problems that arise from chemical impurities in reagents and analytical and experimental uncertainties associated with prior determinations of salt solubility. The same methodology was used in this study with the multiplemetal solution blank. Because the measured solubility of SrSO4 was slightly lower in 0.1 g/L CMCD than in the blank KNO3 solutions, the Ksp for SrSO4 was based on Sr concentrations in this low-CMCD solution. The reason for the observed decrease in Sr concentration at low CMCD concentrations cannot be determined with certainty from our data, but a phase transfer of SrSO4 to a less soluble phase in the presence of CMCD would explain this observation. Sr was used as a sulfate salt, while the counterion in the Pb and Zn salts was the oxalate ion. The model accurately simulates the observed lead and zinc concentrations at CMCD concentrations up to about 50 g/L CMCD (Figure 2a,c). However, above 60 g/L predicted metal concentrations are less than the observed values. The simulated concentration curves flatten out, whereas experimentally measured Pb and Zn increase in nearly linear fashion with CMCD. However, the model still predicts the metal concentrations to within 10%. We believe this minor discrepancy between measured and simulated metal concentrations at high CMCD concentrations is related to uncertainties in ionic strength and activity coefficient calculations. In the model, ionic strength calculations include the CMCD2- ligand, which is the principle component of ionic strength at CMCD concentrations above 20 g/L. For instance, the calculated ionic strength increases from 50 mM in the pure electrolyte blank solution (KNO3) to above 400 mM in the 95 g/L CMCD solution. This causes the activity coefficients for Pb and Zn to decrease from 0.44 in the blank solutions to 0.26 in the 95 g/L CMCD solution. However, the model still performs surprisingly well considering the high ionic strength of the more concentrated CMCD solutions. In comparison, average seawater, which is challenging to simulate accurately with common geochemical models, contains 35 g/L total dissolved solids and has an ionic strength of 670 mM. Another possible cause for the increased solubility of Pb and Zn at high CMCD concentrations is the presence of bidentate mononuclear complexes among CMCD carboxyl ligands and metal ions. Since each CMCD molecule contains an average of more than four COOH groups, metal ion complexing involving one metal ion and two adjacent COOgroups becomes increasingly likely as the relative concentration of CMCD is increased. These bidentate complexes are expected to be stronger than monodentate complexes. In addition, binuclear bidentate complexes become increasingly probable as the CMCD concentration increases. Both effects would enhance solubilities above those calculated neglecting such possibilities. The model simulates CMCD-enhanced Sr solubilities less accurately than Pb and Zn. At low CMCD concentrations, enhanced Sr solubility is simulated well, but at CMCD concentrations above 10 g/L simulated enhanced Sr solubility exceeds the measured solubility. Not surprisingly, Sr competes less strongly than Pb and Zn for complexation sites, and the observed solubility is lower than predicted from the single-salt experiments. The difference between simulated and measured Sr concentrations increases with CMCD. At
FIGURE 2. Comparison between observed and simulated enhanced metal concentrations in the absence and presence of PCE. 95 g/L, the difference is approximately 15%. For the purpose of this model, to screen CMCD as a remediation agent, this accuracy is considered sufficient. Mixed-Waste Systems: Effect of Metals on Enhanced PCE Solubility. The solubility enhancement of PCE in the presence and absence of the three metal salts is shown in Figure 1. Each point on the graph represents the average PCE concentration from triplicate analyses (error bars are smaller than the symbols). Consistent with prior work, the solubility enhancement is directly proportional to the CMCD concentration (4). The presence of metal salts did not significantly affect the solubility enhancement of PCE; the optimized log stability constant for the metal + PCE experiments is 2.086, and the 95% confidence interval is 2.080-2.100. As mentioned above, the log stability constant in the absence of metals is 2.075 ( 0.005. Thus, there is no evidence that the presence VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of metals affects CMCD-enhanced PCE solubility. The average partitioning constant is 72.1 L/kg. Mixed-Waste Systems: Effect of PCE on Enhanced Metal Solubility. The enhanced Pb and Zn concentrations in the presence of PCE do not differ from the concentrations measured in its absence (Figure 2). Thus, PCE does not affect the CMCD-enhanced solubility of these metals. The effect of PCE on the solubility enhancement of SrSO4 is less clear. The Sr concentration in experiment 1 is independent of the presence of PCE, whereas the Sr concentration in experiment 2 decreases in the presence of PCE. At 95 g/L CMCD, the difference in Sr concentration in samples with and without PCE is approximately 10%, which is larger than the observed experimental uncertainty based on duplicate samples. Thus, the influence of PCE on Sr solubility is practically negligible. The results presented herein show that our CMCDmetal complexation model accurately predicts enhanced solubility of metal salts. Nonideal interactions between CMCD and metal ions appear to be simulated accurately up to at least 60 g/L CMCD. Also, competition between the two transition elements Pb and Zn is simulated properly by the model. While the model overpredicts the enhanced Sr solubility in the presence of Pb and Zn, the predicted metal concentrations are generally within 10-15% of the measured concentrations. We consider this discrepancy acceptable for screening CMCD flushing as a remediation technology for contaminated aquifers. The presence of NAPL PCE did not appreciably influence the CMCD-enhanced solubilities of the three metal salts. Similarly, the presence of metal salts did not affect the solubility enhancement of PCE by CMCD. Thus, the solubility enhancement of metals and organic contaminants can be modeled simultaneously, and the CMCD complexation model presented in this work appears promising for screening CMCD as a complexing agent for remediation of sites cocontaminated with metals and organics. In situ remedation is more complex than solubility enhancement in a controlled laboratory environment, however. During in situ remediation competition for complexing sites will occur between metal contaminants and native metal ions in the aquifer. For instance, cations in the aqueous phase will form complexes with CMCD, and introducing a complexing agent such as CMCD may enhance dissolution of solids such as iron hydroxides and calcite. Therefore, to accurately simulate competition for complexing sites, conditional formation constants for all involved metals would be needed. Thus, while the model appears promising, further testing using aquifer material and/or field testing is recommended to validate the model.
Acknowledgments This study was sponsored by the Environmental Science and Engineering Division, the Department of Geology and Geological Engineering at the Colorado School of Mines, and the Laboratory for Environmental and Geological Studies (LEGS) at the University of Colorado at Boulder. We thankfully appreciate the effort by the editor and four anonymous reviewers who greatly enhanced the quality of this manuscript.
Literature Cited (1) EPA. Comprehensive Environmental Response, Compensation, and Liability Information System. http://www.epa.gov/ superfund/sites/cursites/.
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(2) Wang, X.; Brusseau, M. L. Simultaneous complexation of organic compounds and heavy metals by a modified cyclodextrin. Environ. Sci. Technol. 1995, 29, 2632–2635. (3) Brusseau, M. L.; Wang, X.; Wang, W.-Z. Simultaneous elution of heavy metals and organic compounds from soil by cyclodextrin. Environ. Sci. Technol. 1997, 31, 1087–1092. (4) Wang, X.; Brusseau, M. L. Solubilization of some low-polarity organic compounds by hydroxypropyl-β-cyclodextrin. Environ. Sci. Technol. 1993, 27, 2821–2825. (5) Brusseau, M. L.; Wang, X.; Hu, Q. Enhanced transport of lowpolarity organic compounds through soil by cyclodextrin. Environ. Sci. Technol. 1994, 28, 952–956. (6) Hu, Q.; Brusseau, M. L. Effect of solute size on transport in structured porous media. Water Resour. Res. 1995, 31, 1637– 1646. (7) Neilson, J. W.; Artiola, J. F.; Maier, R. M. Characterization of lead removal from contaminated soils by nontoxic soil-washing agents. J. Environ. Qual. 2003, 32, 899–908. (8) Chatain, V.; Hanna, K.; de Brauer, C.; Germain, P. Enhanced solubilization of arsenic and 2,3,4,6 tetrachlorophenol from soils by a cyclodextrin derivative. Chemosphere 2004, 57, 197–206. (9) Vuluva, V. M.; Seaman, J. C. Mobilization of lead from highly weathered porous material by extracting agents. Environ. Sci. Technol. 2000, 34, 4828–4834. (10) Tick, R. T.; Lourenso, F.; Wood, A. L.; Brusseau, M. L. Pilot-scale demonstration of cyclodextrin as a solubility-enhancement agent for remediation of a tetrachloroethene-contaminated aquifer. Environ. Sci. Technol. 2003, 37, 5829–5834. (11) Skold, M. E. Ph.D. Thesis, Colorado School of Mines, Golden, CO, 2006. (12) Donia, A. M. Synthesis, identification and thermal analysis of coprecipitates of silver-(cobalt, nickel, copper and zinc) oxalate. Polyhedron 1997, 16, 3013–3031. (13) Parkhurst, D. L.; Appelo, C. A. J. User’s Guide to PHREEQC (version 2)sA Computer Program for Speciation, Batch Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations; U.S. Geological Survey WRI Report 99-4259; Reston, VA, 1999. (14) Poeter, E. P.; Hill, M. C.; Banta, E. R.; Mehl, S.; Christensen, S. UCODE_2005 and Six Other Computer Codes for Universal Sensitivity Analysis, Calibration, and Uncertainty Evaluation; U.S. Geological Survey Techniques and Methods 6-A11; Reston, VA, 2005. (15) Boving, T. B.; McCray, J. E. Cyclodextrin-enhanced remediation of organic and metal contaminants in porous media and groundwater. Remediation 2000, 10, 59–83. (16) Skold, M. E.; Thyne, G. D.; Drexler, J. W.; McCray, J. E. Determining conditional stability constants for Pb complexation by carboxymethyl-β-cyclodextrin (CMCD). J. Contam. Hydrol. 2007, 93, 203–215. (17) Truesdell, A. H.; Jones, B. F. WATEQ, a computer program for calculating chemical equilibria of natural waters. J. Res. U.S. Geol. Surv. 1974, 2, 233–248. (18) McCray, J. E.; Boving, T. B.; Brusseau, M. L. Cyclodextrinenhanced solubilization of organic contaminants with implications for aquifer remediation. Ground Water Monit. Rem. 2000, 20, 94–103. (19) Shixiang, G.; Liansheng, W.; Qingguo, H.; Sukui, H. Solubilization of polycyclic aromatic hydrocarbons by β-cyclodextrin and carboxymethyl-β-cyclodextrin. Chemosphere 1998, 37, 1299– 1305. (20) Boving, T. B.; Wang, X.; Brusseau, M. L. Cyclodextrin-enhanced solubilization and removal of residual-phase chlorinated solvents from porous media. Environ. Sci. Technol. 1999, 33, 764– 770. (21) Fenyvesi, E.; Szeman, J.; Szetli, J. Extraction of PAHs and pesticides from contaminated soils with aqueous CD solutions. J. Inclusion Phenom. Mol. Recognit. Chem. 1996, 25, 229–232. (22) Sheremata, T. W.; Hawari, J. Cyclodextrins for desorption and solubilization of 2,4,6-trinitrotoluene and its metabolites from soil. Environ. Sci. Technol. 2000, 34, 3462–3468. (23) Boving, T. B.; Brusseau, M. L. Solubilization and removal of residual trichloroethene from porous media: Comparison of several solubilization agents. J. Contam. Hydrol. 2000, 42, 51–67.
ES801835X
