- Email: [email protected]
The influence of citric acid, EDTA, and fulvic acid on U(VI) sorption onto kaolinite
Applied Geochemistry 26 (2011) S158–S161
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
The influence of citric acid, EDTA, and fulvic acid on U(VI) sorption onto kaolinite Michelle Barger ⇑, Carla M. Koretsky Department of Geosciences, Western Michigan University, Kalamazoo, MI, USA
a r t i c l e
i n f o
Article history: Available online 12 April 2011
a b s t r a c t Uranium(VI) sorption onto kaolinite was investigated as a function of pH (3–12), sorbate/sorbent ratio (1 10 6–1 10 4 M U(VI) with 2 g/L kaolinite), ionic strength (0.001–0.1 M NaNO3), and pCO2 (0–5%) in the presence or absence of 1 10 2–1 10 4 M citric acid, 1 10 2–1 10 4 M EDTA, and 10 or 20 mg/L fulvic acid. Control experiments without-solids, containing 1 10 6–1 10 4 M U(VI) in 0.01 M NaNO3 were used to evaluate sorption to the container wall and precipitation of U phases as a function of pH. Control experiments demonstrate significant loss (up to 100%) of U from solution. Although some loss, particularly in 1 10 5 and 1 10 4 M U experiments, is expected due to precipitation of schoepite, adsorption on the container walls is significant, particularly in 1 10 6 M U experiments. In the absence of ligands, U(VI) sorption on kaolinite increases from pH 3 to 7 and decreases from pH 7.5 to 12. Increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7, whereas 10% pCO2 greatly diminishes U(VI) sorption between pH 5.5 and 11. Addition of fulvic acid produces a small increase in U(VI) sorption at pH < 5; in contrast, between pH 5 and 10 fulvic acid, citric acid, and EDTA all decrease U(VI) sorption. This suggests that fulvic acid enhances U(VI) sorption slightly via formation of ternary ligand bridges at low pH, whereas EDTA and citric acid do not form ternary surface complexes with the U(VI), and that all three ligands, as well as carbonate, form aqueous uranyl complexes that keep U(VI) in solution at higher pH. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The mobility of aqueous U(VI) in near surface environments can be significantly influenced by sorption on solids. For example, kaolinite, a widespread weathering product present in many soils, may affect U(VI) transport (Payne et al., 2004; Arda et al., 2006). Organic acids may also modify U(VI) mobility and bioavailability, potentially enhancing U(VI) sorption by formation of ternary complexes or diminishing it via the formation of aqueous U(VI)–ligand complexes (Sachs and Bernhard, 2008). For example, naturally occurring organic matter (NOM), including fulvic and humic substances (HS), is present in most sediments and soils and has been shown to modify the sorption of many metals, both by binding to clay surfaces or interlayers and forming ternary complexes, and also by forming strong aqueous uranyl complexes (Wood, 1996). In near surface soils, the exudation of organic acids from plants, fungi and microorganisms can also have an impact on metal transport (Ryan and Delhaize, 2001; Jones et al., 2003). For example, it has been established that citric acid forms strong aqueous complexes with U(VI) and for this reason it has been employed in the remediation of soils contaminated with U (Kantar and
⇑ Corresponding author. Tel.: +1 361 813 1199; fax: +1 269 387 5513. E-mail address: [email protected] (M. Barger). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.03.092
Honeyman, 2006). Synthetic organic acids, such as EDTA, which is a chelating agent that forms very strong complexes with metals (Knepper, 2003), can be found in U-bearing wastes (Cartwright et al., 2007). It is well established that organic acids can have a high affinity to complex and increase the mobility of metals in solution, but what is less well understood is how the presence of these ligands will affect cation sorption to clay. The major objective of this study is to characterize U(VI) sorption on kaolinite over a wide range of solution conditions (e.g. pH, ionic strength, sorbate/sorbent ratio, pCO2) in the presence or absence of citric acid, EDTA or fulvic acid. The data are used to assess the influence of ternary complexes and aqueous complexes on uranyl sorption to the kaolinite.
2. Methods Uranium(VI) sorption on kaolinite was measured in the presence and absence of organic acids using batch experiments. Precipitation of U-bearing solids and potential sorption of U(VI) to the container wall was first investigated by conducting experiments in the absence of solid. Polypropylene tubes (50 mL) were filled solutions containing 1 10 6, 1 10 5, or 1 10 4 M U(VI) in 0.01 M NaNO3. These were agitated with a stir bar while the pH of each tube was titrated to span a pH range from 3 to 11. Once
M. Barger, C.M. Koretsky / Applied Geochemistry 26 (2011) S158–S161
the desired pH was achieved, the tube was tightly capped and equilibrated for 24 h on a rotator, after which the pH was remeasured. Tubes were then centrifuged at 4000 rpm for 10 min and a 10 mL aliquot of the supernatant was pipetted off, syringe-filtered, and prepared with 5% HNO3 and an internal standard (1 lg/L Y) for analysis of total U by ICP–OES. Uranium(VI) sorbed was calculated as the difference between the U(VI) added and the concentration measured in the supernatant solution. Sorption of U(VI) to the polypropylene tube wall was further investigated by pouring out all remaining solution from each tube used in no-solid control experiments and then filling each container with 25 mL of 0.5 M NaHCO3. The tubes were tightly capped, rotated for 24 h, and then prepared as described above for ICP–OES analysis. Uranium(VI) sorbed onto the tube wall was assumed to equal the amount of U recovered in the NaHCO3 wash solution. Uranium(VI) sorption to kaolinite in the presence or absence of organic acids was similarly completed using batch reactors. Natural kaolinite (KGA-1b) was obtained from the Clay Minerals Society Source Clays. Suwanee River Fulvic Acid (SRFAS1) was purchased from the International Humic Substances Society. EDTA and citric acid solutions were prepared from NaEDTA and HCitrate salts. For each experiment, 0.1 g kaolinite (2 g/L) was weighed into each batch reactor and then 50 mL of solution containing uranyl (1 10 6–1 10 4 M U), background electrolyte (0.001–0.1 M NaNO3), and organic acid (if included) was added to each tube. A control 50 mL batch was set up with each experiment by adding this initial solution to a solid free tube and titrating to a pH of 3. All other tubes were titrated, as described above, using trace metal grade HNO3 or NaOH to span the desired pH range and were
Fig. 1. (A) Precipitation of U from control experiments. (B) Comparison of 10 precipitation of U using the speciation code JCHESS.
5
S159
subsequently equilibrated for 24 h, after which the pH was remeasured, the tubes centrifuged and the supernatant syringe-filtered and analyzed for total U(VI) using ICP OES. For most of the experiments with added organic acids, the concentration of total organic C (TOC) in the filtered supernatant was measured using a Shimadzu 5000 TOC analyzer. Experiments investigating U(VI) sorption on kaolinite as a function of pCO2 were conducted under a controlled atmosphere (N2/CO2) inside a Coy anaerobic chamber. 3. Results and discussion Control experiments without added solids demonstrate loss of U from solution for all U(VI) concentrations explored (Fig. 1A). At circumneutral pH, experiments containing 1 10 5 and 1 10 4 M U(VI) have up to 90% loss of U(VI) from solution; U(VI) sorption reaches 100% for 1 10 6 M U(VI) experiments. Less U(VI) is sorbed in all experiments at higher or lower pH, although for 1 10 6 M U(VI) experiments between 40% and 50% remains sorbed across the tested pH range (3–10). Speciation calculations completed using JCHESS with the NEA thermodynamic database (Van der Lee and De Windt, 2000) predict precipitation of schoepite at a pH of 4.7 and precipitation of Na2U2O7 at a pH of 8.5 for experiments containing 1 10 5 and 1 10 4 M U(VI). However, no precipitation is predicted for 1 10 6 M U(VI). The disagreement between JCHESS predictions and experimental data suggest that U(VI) is lost to the container walls, filters or syringes. This hypothesis was tested by rinsing tubes with 0.5 M NaHCO3, which should complex and remove any U(VI) bound onto the polypropylene container walls. The wash solution typically released
M U(VI) precipitation with U(VI) recovered from 0.5 M NaHCO3 rinse. Lines indicate calculated
Fig. 2. U(VI) sorption on 2 g/L kaolinite as a function of (A) ionic strength under atmospheric conditions and (B) varying pCO2, 0.01 M NaNO3.
S160
M. Barger, C.M. Koretsky / Applied Geochemistry 26 (2011) S158–S161
quantities of U(VI) equivalent to, or slightly less than, those sorbed in the no-solid experiments. Lower recoveries, particularly for 10 5 or 10 4 M U(VI) experiments (e.g. Fig. 1B), are likely due to some loss of U(VI) precipitate with the initial supernatant. Uranyl sorption on kaolinite increases from pH 3 to 7, reaching nearly 100% at pH 8 (Fig. 2A). At higher pH, sorption decreases, presumably due to formation of strong aqueous U(VI)–carbonate complexes. Little ionic strength dependence is observed; increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7. In contrast, U(VI) sorption is greatly effected by pCO2 (Fig. 2B). In the absence of pCO2, U(VI)
sorption increases slightly compared to sorption under atmospheric conditions. In the presence of 5% pCO2, however, U(VI) sorption at pH > 5 decreases dramatically as U(VI)–carbonate aqueous complexes form in preference to U(VI) species on the kaolinite surface. At pH > 4, the addition of 1 10 4 or 1 10 2 M citric acid inhibits U(VI) sorption on kaolinite (Fig. 3A), consistent with the formation of U(VI)–citric acid aqueous complexes. The greater addition of citric acid, 1 10 2 M, results in less U(VI) sorption. A very small enhancement of U(VI) sorption is suggested at pH < 4, which could indicate formation of ternary uranyl–citric
Fig. 3. Sorption of (A) U(VI) and (B) citric acid on 2 g/L kaolinite in 0.01 M NaNO3 under atmospheric conditions.
Fig. 4. Sorption of (A) U(VI) and (B) EDTA on 2 g/L kaolinite in 0.01 M NaNO3 under atmospheric conditions.
Fig. 5. Sorption of (A) U(VI) and (B) fulvic acid on 2 g/L kaolinite in 0.01 M NaNO3 under atmospheric conditions.
M. Barger, C.M. Koretsky / Applied Geochemistry 26 (2011) S158–S161
acid complexes on the kaolinite. Analyses of TOC in solution for experiments with citric acid suggest that a small quantity of the citric acid may bind to the solid (up to 10%). However, there is no clear evidence of increased citric acid sorption at low pH (Fig. 3B). The addition of EDTA decreases U(VI) sorption at nearly all pH values; more inhibition of U(VI) sorption is observed with increasing concentration of EDTA (Fig. 4A). TOC analyses show very little loss of EDTA (610%) across the tested pH range (Fig. 4B). These data suggest that, as for citric acid, ternary complexes between uranyl, EDTA and kaolinite are not significant under the experimental conditions. Rather, the addition of EDTA leads to the formation of aqueous U(VI)–EDTA complexes, inhibiting uranyl sorption to kaolinite and suggesting that U(VI) mobility increases in the presence of EDTA. The addition of 10 or 20 mg/L fulvic acid enhances U(VI) sorption onto kaolinite between pH 3 and 5 but inhibits it at pH > 6 (Fig. 5A). This suggests that fulvic acid may form a ternary complex with U(VI) on the kaolinite surface at pH < 5. This is also consistent with the observed sorption envelope of fulvic acid on the kaolinite (Fig. 5B). TOC results show that 30% of 10 mg/L and 40% of 10 mg/L fulvic acid are sorbed at a pH of 2–3, and a steady decrease in fulvic acid sorption is observed as pH increases. Perhaps, surprisingly, significant quantities of fulvic acid remain sorbed above pH 5, when uranyl adsorption is inhibited by the addition of fulvic acid. This suggests that at these pH conditions, aqueous uranyl–fulvic acid complexes are stronger than ternary uranyl–fulvic acid complexes. 4. Conclusions Precipitation experiments demonstrate that loss of U(VI) from solution in the absence of kaolinite is significant. The NaHCO3 wash reveals that sorption to the container wall is responsible for much of this loss. Uranium(VI) sorption data in the presence of ligands suggest that fulvic acid binds to the kaolinite surface at low pH (65), possibly enhancing uranyl sorption via formation of ternary ligand bridges. In contrast, at higher pH values, fulvic acid, EDTA and citric acid all inhibit sorption of U(VI) to kaolinite, presumably due to the formation of strong aqueous U(VI)–organic acid
S161
complexes. No evidence is found for enhancement of U(VI) adsorption by EDTA or citric acid through formation of metal–ligand–sorbent complexes for the conditions considered here. Thus, at high pH (P5) the presence of organic acids is expected to enhance U(VI) mobility, whereas at lower pH organic acids may inhibit or enhance U(VI) mobility depending on their specific interactions with clay mineral surfaces. Acknowledgements The National Science Foundation CAREER program (NSF EAR 0348435), the WMU Graduate College Research Fund and the WMU Gwen Frostic Endowment provided funding to support this work. TOC analyses were completed at the Center for Environmental Science and Technology at Notre Dame University with support from Jon Luftus, Dennis Birdsell, and Patricia Maurice. References Arda, D., Hizal, J., Apak, R., 2006. Surface complexation modeling of uranyl adsorption onto kaolinite based clay minerals using FITEQL 3.2. Radiochim. Acta 94, 835–844. Cartwright, A.J., May, C.C., Worsfold, P.J., Keith-Roach, M.J., 2007. Characterisation of thorium–ethylenediaminetetraacetic acid and thorium–nitrilotriacetic acid species by electrospray ionization–mass spectrometry. Anal. Chim. Acta 590, 125–131. Jones, D.L., Dennis, P.G., Owen, A.G., van Hees, P.A.W., 2003. Organic acid behavior in soils – misconceptions and knowledge gaps. Plant Soil 248, 31–41. Kantar, C., Honeyman, B.D., 2006. Citric acid enhanced remediation of soils contaminated with uranium by soil flushing and soil washing. J. Environ. Eng. 141, 246–247. Knepper, T., 2003. Synthetic chelating agents and compounds exhibiting complexing properties in the aquatic environment. Trends Anal. Chem. 2, 708–724. Payne, J.A., Davis, J.A., Lumpkin, G.R., Chisari, R., Waite, T.D., 2004. Surface complexation model of uranyl sorption on Georgia kaolinite. Appl. Clay Sci. 26, 151–162. Ryan, P.R., Delhaize, E., 2001. Function and mechanism of organic anion exudation from plant roots. Ann. Rev. Plant Physiol. Mol. Biol. 52, 527–560. Sachs, S., Bernhard, G., 2008. Sorption of U(VI) onto an artificial humic substance– kaolinite–associate. Chemosphere 72, 1441–1447. Van der Lee, J., De Windt, L., 2000. CHESS Tutorial and Cookbook. User’s Guide Nr. LHM/RD/99/05 Fontainebleau. CIG-Ecole des Mines de Paris, France. Wood, S.A., 1996. The role of humic substances in the transport and fixation of metals of economic interest (Au, Pt, Pd, U, V). Ore Geol. Rev. 11, 1–31.
Contents lists available at ScienceDirect
Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem
The influence of citric acid, EDTA, and fulvic acid on U(VI) sorption onto kaolinite Michelle Barger ⇑, Carla M. Koretsky Department of Geosciences, Western Michigan University, Kalamazoo, MI, USA
a r t i c l e
i n f o
Article history: Available online 12 April 2011
a b s t r a c t Uranium(VI) sorption onto kaolinite was investigated as a function of pH (3–12), sorbate/sorbent ratio (1 10 6–1 10 4 M U(VI) with 2 g/L kaolinite), ionic strength (0.001–0.1 M NaNO3), and pCO2 (0–5%) in the presence or absence of 1 10 2–1 10 4 M citric acid, 1 10 2–1 10 4 M EDTA, and 10 or 20 mg/L fulvic acid. Control experiments without-solids, containing 1 10 6–1 10 4 M U(VI) in 0.01 M NaNO3 were used to evaluate sorption to the container wall and precipitation of U phases as a function of pH. Control experiments demonstrate significant loss (up to 100%) of U from solution. Although some loss, particularly in 1 10 5 and 1 10 4 M U experiments, is expected due to precipitation of schoepite, adsorption on the container walls is significant, particularly in 1 10 6 M U experiments. In the absence of ligands, U(VI) sorption on kaolinite increases from pH 3 to 7 and decreases from pH 7.5 to 12. Increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7, whereas 10% pCO2 greatly diminishes U(VI) sorption between pH 5.5 and 11. Addition of fulvic acid produces a small increase in U(VI) sorption at pH < 5; in contrast, between pH 5 and 10 fulvic acid, citric acid, and EDTA all decrease U(VI) sorption. This suggests that fulvic acid enhances U(VI) sorption slightly via formation of ternary ligand bridges at low pH, whereas EDTA and citric acid do not form ternary surface complexes with the U(VI), and that all three ligands, as well as carbonate, form aqueous uranyl complexes that keep U(VI) in solution at higher pH. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The mobility of aqueous U(VI) in near surface environments can be significantly influenced by sorption on solids. For example, kaolinite, a widespread weathering product present in many soils, may affect U(VI) transport (Payne et al., 2004; Arda et al., 2006). Organic acids may also modify U(VI) mobility and bioavailability, potentially enhancing U(VI) sorption by formation of ternary complexes or diminishing it via the formation of aqueous U(VI)–ligand complexes (Sachs and Bernhard, 2008). For example, naturally occurring organic matter (NOM), including fulvic and humic substances (HS), is present in most sediments and soils and has been shown to modify the sorption of many metals, both by binding to clay surfaces or interlayers and forming ternary complexes, and also by forming strong aqueous uranyl complexes (Wood, 1996). In near surface soils, the exudation of organic acids from plants, fungi and microorganisms can also have an impact on metal transport (Ryan and Delhaize, 2001; Jones et al., 2003). For example, it has been established that citric acid forms strong aqueous complexes with U(VI) and for this reason it has been employed in the remediation of soils contaminated with U (Kantar and
⇑ Corresponding author. Tel.: +1 361 813 1199; fax: +1 269 387 5513. E-mail address: [email protected] (M. Barger). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.03.092
Honeyman, 2006). Synthetic organic acids, such as EDTA, which is a chelating agent that forms very strong complexes with metals (Knepper, 2003), can be found in U-bearing wastes (Cartwright et al., 2007). It is well established that organic acids can have a high affinity to complex and increase the mobility of metals in solution, but what is less well understood is how the presence of these ligands will affect cation sorption to clay. The major objective of this study is to characterize U(VI) sorption on kaolinite over a wide range of solution conditions (e.g. pH, ionic strength, sorbate/sorbent ratio, pCO2) in the presence or absence of citric acid, EDTA or fulvic acid. The data are used to assess the influence of ternary complexes and aqueous complexes on uranyl sorption to the kaolinite.
2. Methods Uranium(VI) sorption on kaolinite was measured in the presence and absence of organic acids using batch experiments. Precipitation of U-bearing solids and potential sorption of U(VI) to the container wall was first investigated by conducting experiments in the absence of solid. Polypropylene tubes (50 mL) were filled solutions containing 1 10 6, 1 10 5, or 1 10 4 M U(VI) in 0.01 M NaNO3. These were agitated with a stir bar while the pH of each tube was titrated to span a pH range from 3 to 11. Once
M. Barger, C.M. Koretsky / Applied Geochemistry 26 (2011) S158–S161
the desired pH was achieved, the tube was tightly capped and equilibrated for 24 h on a rotator, after which the pH was remeasured. Tubes were then centrifuged at 4000 rpm for 10 min and a 10 mL aliquot of the supernatant was pipetted off, syringe-filtered, and prepared with 5% HNO3 and an internal standard (1 lg/L Y) for analysis of total U by ICP–OES. Uranium(VI) sorbed was calculated as the difference between the U(VI) added and the concentration measured in the supernatant solution. Sorption of U(VI) to the polypropylene tube wall was further investigated by pouring out all remaining solution from each tube used in no-solid control experiments and then filling each container with 25 mL of 0.5 M NaHCO3. The tubes were tightly capped, rotated for 24 h, and then prepared as described above for ICP–OES analysis. Uranium(VI) sorbed onto the tube wall was assumed to equal the amount of U recovered in the NaHCO3 wash solution. Uranium(VI) sorption to kaolinite in the presence or absence of organic acids was similarly completed using batch reactors. Natural kaolinite (KGA-1b) was obtained from the Clay Minerals Society Source Clays. Suwanee River Fulvic Acid (SRFAS1) was purchased from the International Humic Substances Society. EDTA and citric acid solutions were prepared from NaEDTA and HCitrate salts. For each experiment, 0.1 g kaolinite (2 g/L) was weighed into each batch reactor and then 50 mL of solution containing uranyl (1 10 6–1 10 4 M U), background electrolyte (0.001–0.1 M NaNO3), and organic acid (if included) was added to each tube. A control 50 mL batch was set up with each experiment by adding this initial solution to a solid free tube and titrating to a pH of 3. All other tubes were titrated, as described above, using trace metal grade HNO3 or NaOH to span the desired pH range and were
Fig. 1. (A) Precipitation of U from control experiments. (B) Comparison of 10 precipitation of U using the speciation code JCHESS.
5
S159
subsequently equilibrated for 24 h, after which the pH was remeasured, the tubes centrifuged and the supernatant syringe-filtered and analyzed for total U(VI) using ICP OES. For most of the experiments with added organic acids, the concentration of total organic C (TOC) in the filtered supernatant was measured using a Shimadzu 5000 TOC analyzer. Experiments investigating U(VI) sorption on kaolinite as a function of pCO2 were conducted under a controlled atmosphere (N2/CO2) inside a Coy anaerobic chamber. 3. Results and discussion Control experiments without added solids demonstrate loss of U from solution for all U(VI) concentrations explored (Fig. 1A). At circumneutral pH, experiments containing 1 10 5 and 1 10 4 M U(VI) have up to 90% loss of U(VI) from solution; U(VI) sorption reaches 100% for 1 10 6 M U(VI) experiments. Less U(VI) is sorbed in all experiments at higher or lower pH, although for 1 10 6 M U(VI) experiments between 40% and 50% remains sorbed across the tested pH range (3–10). Speciation calculations completed using JCHESS with the NEA thermodynamic database (Van der Lee and De Windt, 2000) predict precipitation of schoepite at a pH of 4.7 and precipitation of Na2U2O7 at a pH of 8.5 for experiments containing 1 10 5 and 1 10 4 M U(VI). However, no precipitation is predicted for 1 10 6 M U(VI). The disagreement between JCHESS predictions and experimental data suggest that U(VI) is lost to the container walls, filters or syringes. This hypothesis was tested by rinsing tubes with 0.5 M NaHCO3, which should complex and remove any U(VI) bound onto the polypropylene container walls. The wash solution typically released
M U(VI) precipitation with U(VI) recovered from 0.5 M NaHCO3 rinse. Lines indicate calculated
Fig. 2. U(VI) sorption on 2 g/L kaolinite as a function of (A) ionic strength under atmospheric conditions and (B) varying pCO2, 0.01 M NaNO3.
S160
M. Barger, C.M. Koretsky / Applied Geochemistry 26 (2011) S158–S161
quantities of U(VI) equivalent to, or slightly less than, those sorbed in the no-solid experiments. Lower recoveries, particularly for 10 5 or 10 4 M U(VI) experiments (e.g. Fig. 1B), are likely due to some loss of U(VI) precipitate with the initial supernatant. Uranyl sorption on kaolinite increases from pH 3 to 7, reaching nearly 100% at pH 8 (Fig. 2A). At higher pH, sorption decreases, presumably due to formation of strong aqueous U(VI)–carbonate complexes. Little ionic strength dependence is observed; increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7. In contrast, U(VI) sorption is greatly effected by pCO2 (Fig. 2B). In the absence of pCO2, U(VI)
sorption increases slightly compared to sorption under atmospheric conditions. In the presence of 5% pCO2, however, U(VI) sorption at pH > 5 decreases dramatically as U(VI)–carbonate aqueous complexes form in preference to U(VI) species on the kaolinite surface. At pH > 4, the addition of 1 10 4 or 1 10 2 M citric acid inhibits U(VI) sorption on kaolinite (Fig. 3A), consistent with the formation of U(VI)–citric acid aqueous complexes. The greater addition of citric acid, 1 10 2 M, results in less U(VI) sorption. A very small enhancement of U(VI) sorption is suggested at pH < 4, which could indicate formation of ternary uranyl–citric
Fig. 3. Sorption of (A) U(VI) and (B) citric acid on 2 g/L kaolinite in 0.01 M NaNO3 under atmospheric conditions.
Fig. 4. Sorption of (A) U(VI) and (B) EDTA on 2 g/L kaolinite in 0.01 M NaNO3 under atmospheric conditions.
Fig. 5. Sorption of (A) U(VI) and (B) fulvic acid on 2 g/L kaolinite in 0.01 M NaNO3 under atmospheric conditions.
M. Barger, C.M. Koretsky / Applied Geochemistry 26 (2011) S158–S161
acid complexes on the kaolinite. Analyses of TOC in solution for experiments with citric acid suggest that a small quantity of the citric acid may bind to the solid (up to 10%). However, there is no clear evidence of increased citric acid sorption at low pH (Fig. 3B). The addition of EDTA decreases U(VI) sorption at nearly all pH values; more inhibition of U(VI) sorption is observed with increasing concentration of EDTA (Fig. 4A). TOC analyses show very little loss of EDTA (610%) across the tested pH range (Fig. 4B). These data suggest that, as for citric acid, ternary complexes between uranyl, EDTA and kaolinite are not significant under the experimental conditions. Rather, the addition of EDTA leads to the formation of aqueous U(VI)–EDTA complexes, inhibiting uranyl sorption to kaolinite and suggesting that U(VI) mobility increases in the presence of EDTA. The addition of 10 or 20 mg/L fulvic acid enhances U(VI) sorption onto kaolinite between pH 3 and 5 but inhibits it at pH > 6 (Fig. 5A). This suggests that fulvic acid may form a ternary complex with U(VI) on the kaolinite surface at pH < 5. This is also consistent with the observed sorption envelope of fulvic acid on the kaolinite (Fig. 5B). TOC results show that 30% of 10 mg/L and 40% of 10 mg/L fulvic acid are sorbed at a pH of 2–3, and a steady decrease in fulvic acid sorption is observed as pH increases. Perhaps, surprisingly, significant quantities of fulvic acid remain sorbed above pH 5, when uranyl adsorption is inhibited by the addition of fulvic acid. This suggests that at these pH conditions, aqueous uranyl–fulvic acid complexes are stronger than ternary uranyl–fulvic acid complexes. 4. Conclusions Precipitation experiments demonstrate that loss of U(VI) from solution in the absence of kaolinite is significant. The NaHCO3 wash reveals that sorption to the container wall is responsible for much of this loss. Uranium(VI) sorption data in the presence of ligands suggest that fulvic acid binds to the kaolinite surface at low pH (65), possibly enhancing uranyl sorption via formation of ternary ligand bridges. In contrast, at higher pH values, fulvic acid, EDTA and citric acid all inhibit sorption of U(VI) to kaolinite, presumably due to the formation of strong aqueous U(VI)–organic acid
S161
complexes. No evidence is found for enhancement of U(VI) adsorption by EDTA or citric acid through formation of metal–ligand–sorbent complexes for the conditions considered here. Thus, at high pH (P5) the presence of organic acids is expected to enhance U(VI) mobility, whereas at lower pH organic acids may inhibit or enhance U(VI) mobility depending on their specific interactions with clay mineral surfaces. Acknowledgements The National Science Foundation CAREER program (NSF EAR 0348435), the WMU Graduate College Research Fund and the WMU Gwen Frostic Endowment provided funding to support this work. TOC analyses were completed at the Center for Environmental Science and Technology at Notre Dame University with support from Jon Luftus, Dennis Birdsell, and Patricia Maurice. References Arda, D., Hizal, J., Apak, R., 2006. Surface complexation modeling of uranyl adsorption onto kaolinite based clay minerals using FITEQL 3.2. Radiochim. Acta 94, 835–844. Cartwright, A.J., May, C.C., Worsfold, P.J., Keith-Roach, M.J., 2007. Characterisation of thorium–ethylenediaminetetraacetic acid and thorium–nitrilotriacetic acid species by electrospray ionization–mass spectrometry. Anal. Chim. Acta 590, 125–131. Jones, D.L., Dennis, P.G., Owen, A.G., van Hees, P.A.W., 2003. Organic acid behavior in soils – misconceptions and knowledge gaps. Plant Soil 248, 31–41. Kantar, C., Honeyman, B.D., 2006. Citric acid enhanced remediation of soils contaminated with uranium by soil flushing and soil washing. J. Environ. Eng. 141, 246–247. Knepper, T., 2003. Synthetic chelating agents and compounds exhibiting complexing properties in the aquatic environment. Trends Anal. Chem. 2, 708–724. Payne, J.A., Davis, J.A., Lumpkin, G.R., Chisari, R., Waite, T.D., 2004. Surface complexation model of uranyl sorption on Georgia kaolinite. Appl. Clay Sci. 26, 151–162. Ryan, P.R., Delhaize, E., 2001. Function and mechanism of organic anion exudation from plant roots. Ann. Rev. Plant Physiol. Mol. Biol. 52, 527–560. Sachs, S., Bernhard, G., 2008. Sorption of U(VI) onto an artificial humic substance– kaolinite–associate. Chemosphere 72, 1441–1447. Van der Lee, J., De Windt, L., 2000. CHESS Tutorial and Cookbook. User’s Guide Nr. LHM/RD/99/05 Fontainebleau. CIG-Ecole des Mines de Paris, France. Wood, S.A., 1996. The role of humic substances in the transport and fixation of metals of economic interest (Au, Pt, Pd, U, V). Ore Geol. Rev. 11, 1–31.