Environ. Sci. Technol. 2001, 35, 3849-3856
Surface Complexation Modeling of Carbonate Effects on the Adsorption of Cr(VI), Pb(II), and U(VI) on Goethite MARIO VILLALOBOS,* MAYA A. TROTZ, AND JAMES O. LECKIE Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305
Dissolved carbonate species are known to affect the sorption behavior of trace species. The macroscopic description of these interactions with a thermodynamic approach has been limited by the lack of data on the binary interaction between carbonate and relevant mineral surfaces. This work follows from two detailed studies of carbonate adsorption on goethite (4, 13). It shows that independent triple-layer surface complexation modeling (TLM) of carbonate adsorption allows successful descriptions of carbonate-trace element ternary sorption on this oxide, using relatively simple and optimal stoichiometries. Carbonate adsorption was considerably enhanced in the presence of Pb(II), despite an invariant total Pb(II) sorption to equilibration with up to 1% CO2(g). Both the Pb(II)carbonate system behavior and the anion-like pH adsorption behavior of U(VI) in the presence of CO2 were successfully modeled using binary and ternary metalbound surface complexes. The significant reduction of Cr(VI) adsorption edges to lower pH values in the presence of CO2 was accurately simulated and explained via site competition and surface electrostatic repulsion effects on the predicted inner- and outer-sphere Cr(VI) surface complexes formed. The results of this research are highly relevant to modeling of metal transport field data and of potential soil remediation schemes using carbonate.
I. Introduction Contamination by heavy metals is a concern in many subsurface environments, and researchers have investigated the geochemical factors that control the fate of these species. Mineral oxide surfaces have been identified as major sinks for these metals. Various geochemical conditions, including pH, ionic strength, and the presence of other ions, particularly carbonate, can affect the sorption behavior of heavy metals. The carbon dioxide-carbonate system is prevalent in subsurface environments, yielding total aqueous carbonate concentrations that range from 0.5 to 8 mM (1-3). Carbonate sorbs to mineral oxide surfaces over a wide pH range (4) and may influence the behavior of trace metals. It may reduce anion adsorption by directly competing for binding sites on the mineral oxide (5-7), but more complex mechanisms may * Corresponding author phone: (510)643-9951; fax: (510)643-2940; e-mail:
[email protected]. Current address: Department of Environmental Science, Policy and Management, Ecosystem Sciences Division, Hilgard Hall, Room 235, University of California, Berkeley, CA 94720-3110. 10.1021/es001748k CCC: $20.00 Published on Web 08/31/2001
2001 American Chemical Society
apply to cationic species (e.g., heavy metals), involving formation of aqueous carbonato complexes with different sorbing abilities and formation of precipitates (8-12). Detailed surface complexation modeling (SCM) of available sorption data on ternary systems involving carbonate have been limited by the scarcity of binary carbonate adsorption studies. Recently, we investigated (4) and modeled (13) the adsorption behavior of aqueous carbonate on goethite, under conditions applicable to natural settings. Results show that carbonate binds in an inner-sphere monodentate fashion to the surface but is strongly influenced by pH and ionic strength. Simulations yielded a potential coverage of more than half the total number of sites (assuming goethite surfaces with 2.3 sites/nm2 (14)), at PCO2 values common in subsurface environments (1-10%) and neutral pH. This suggests that carbonate may considerably affect trace metal sorption and, thus, their mobility in the environment. To incorporate adsorption models to describe transport and fate of trace metals in aqueous systems, the role of CO2 must be understood. We present in this work new data on the Pb(II)-carbonate-goethite system. Using these data as well as previously reported sorption data of U(VI) and Cr(VI) open CO2 systems on goethite we examined the ability of the triple layer model (TLM) (14) to describe the ternary systems using simple surface stoichiometries and independently derived binary carbonate-goethite surface parameters. SCM coupled with spectroscopic investigations have described adsorption processes at fundamental levels (14). This work makes use of the TLM, representative of electrostatic SCM, because of its satisfactory description of many sorbing systems, especially regarding changes in ionic strength and pH (13, 15, 16). Determination of surface stoichiometries in SCM, however, is often left to educated guesses, following trials for data simulation performance, especially in the absence of spectroscopic data that provide more detailed structural information on surface complexes formed. We have used a more systematic modeling approach, described in detail previously (13), to find simple and optimal surface stoichiometries by analyzing the modeling performance of all possible whole-unit charge interfacial configurations. One of the recent challenges of SCM is its ability to describe ternary sorption systems in a manner both selfconsistent with the corresponding binary systems and that agrees well with spectroscopic data available on the system of interest. This work shows that this is possible using the TLM for representative carbonate-trace metal sorption systems. Furthermore, in the absence of unequivocal spectroscopic structural data, optimization of SCM results of binary and ternary systems may contribute with valuable leads on possible surface configurations and interaction mechanisms between adsorbing species to guide future investigations. We also present new carbonate adsorption data on two goethite preparations of very different reactivity used for ternary experiments with heavy metals and carbonate. Modeling of the data was based on a TLM and Fourier transform infrared (FTIR) investigation of carbonate adsorbed on a different goethite preparation, mentioned above (13). In this we found evidence of a single predominant monodentate inner-sphere surface carbonate complex in which the resulting negative charge was placed in the outer-sphere plane. This configuration agrees well with a charge distribution concept from a SCM approach previously developed by Hiemstra and Van Riemsdijk (31) and was used for modeling on the new goethite preparations. The results, in turn, were VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Exponents for TLM Definition Equationa of Surface Species as a Function of Componentsb and Equilibrium Constants exponents for definition equationa [surface
species]a
a
b
c
d
e
f
g
SO-
1
-1
-1
SOH2+
1
1
1
h
goethitec
configurationd
log Kf -11.2 -10.9 7.2 6.9
B C B C
Electrolyte Binding Equations SO-‚‚‚Na+
1
-1
SOH2+‚‚‚An-
1
1
SO-0.2COO-0.8
1
1
SOCOONa
SOCOOH
1
1
1
2
1 1
1
1
-1
2.3 sites/nm2 -9.61 -8.73 9.10 8.93
B C B C
Carbonate and Metal Binding Equations -0.2 -0.8 B mononuclear binuclear
1
1
-1
1 2
SO-...PbOH+ (SO)2HPbOCOO-
1 1
-2 -1
1
1 1
(SO2)UO2 (SO2)2-UO2(CO3)22-
1 1
-2 -2
2
1 1
1 1
12.40 12.40
mononuclear binuclear
14.32
13.40 13.44
B
mononuclear binuclear
13.22
11.97 12.08
C
mononuclear binuclear
14.02
e 11.93
B
mononuclear binuclear
18.41
17.46 17.44
C
mononuclear binuclear
20.74
19.24 19.23
-1 -1
C C
mononuclear mononuclear
14.05 20.98
12.76 20.41
-1
1 -1
B B
mononuclear mononuclear binuclear
-8.77 6.20 6.22
-9.96
-2
-2
C C
binuclear binuclear
1
1 1
13.62
C 1
SOCrO3-β SOH2+...HCrO4-
10 sites/nm2 -10.37 -9.29 8.35 8.38
1
-4.71 10.57
a [Surface species] ) K [[SOH]a(H+)b(Na+)c(An-)d(CO 2-)e(Me)f exp{(-F/RT)(gΨ +hΨ )}, where An- ) electrolyte anion, and Me ) CrO 2-, Pb2+, f 3 0 β 4 or UO22+. ∆pKa)4, C2 ) 0.2 F/m2. b Thermodynamic information of all aqueous species formed has been included in the modeling but not listed here. Data sources: Smith, R. M.; Martell, A. E. NIST Standard Reference Database 46. NIST Critically Selected Stability Constants of Metal Complexes Database Version 4.0; U.S. Department of Commerce Technology Administration National Institue of Standards and Technology Standard Reference Data Program, 1997. Grenthe, I.; Lemire, R. J.; Muller, A. B.; Nguyen-Trung, C.; Wanner, H. NEA-TDB, chemical thermodynamics of uranium; Nuclear Energy Agency, organization for economic cooperation and development: 1992. c Goethite preparations and conditions were as follows: B: 94 m2/g, NaNO3 used as electrolyte, and optimal C1 ) 0.8 F/m2; C: 45 m2/g, NaClO4 used as electrolyte, and optimal C1 ) 1.4 (for 2.3 sites/nnm2) and C1 ) 1.2 (for 10/nm2). d Mononuclear complexes are bound to one surface site, and binuclear complexes are bound to two sites. e Does not contribute.
necessary in order to self-consistently model the ternary sorption systems studied in the present work. Adsorption of Cr(VI) to Fe oxides is relatively weak and decreases in the presence of carbonate (5, 7). The data of Van Geen et al. (7) were used in this study and show that the sorption edge of Cr(VI) on goethite appears at decreasing pH values when equilibrated with PCO2 values increasing from atmospheric (450 µatm) to 40 matm. They used SCM to describe the data but failed to accurately simulate the behavior at 40 matm CO2(g) using the TLM. Their simulations using the diffuse double layer model (DDLM) were more satisfactory, but their model description of the binary carbonate sorption system was relatively unsuccessful for both models, limiting the self-consistency of their modeling results. Pb(II) binds more strongly to oxide surfaces (16, 17), and enhanced Pb(II) sorption onto goethite has been observed at very high CO2 concentrations, where FTIR and X-ray absorption (XAS) spectroscopic data have suggested the formation of a Pb(II)-CO3 ternary surface complex (18). No modeling of the ternary sorption system has been attempted previously, and the results of this study with environmentally relevant carbonate concentrations agree well with the spectroscopic evidence obtained at higher concentrations. In the case of uranyl (U(VI), the presence of CO2 causes a 3850
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considerable decrease in the pH region of adsorption on goethite, through formation of competing U(VI)-CO3 aqueous complexes and showing anion-like sorption behavior (8). Spectroscopic evidence has shown the formation of ternary complexes on the surface of ferrihydrite (9) and hematite (26), suggesting a similarity of behavior with the goethite surface. Previous modeling attempts have required incorporation of these ternary complexes (8, 9, 12), to avoid overestimation of the CO2 effect. However, the lack of detailed description and accurate modeling of the binary interaction of carbonate with the goethite surface have limited the selfconsistency and accuracy of these modeling results.
II. Materials and Methods Preparation and Characterization of a 94 m2/g Goethite Suspension. The procedure of Hiemstra et al. (19) was used to synthesize a high-surface area goethite suspension. BET surface area determinations using nitrogen adsorption yielded a value of 94 ((2) m2/g. A previous goethite preparation of 45 m2/g (20) was additionally used. Goethite Potentiometric Titrations. Acid-base titrations of aqueous 94 m2/g goethite suspensions were done using NaNO3 as background electrolyte (0.01, 0.05, and 0.24 M). Titrations were conducted from pH 4 to pH 11 (“carbonatefree dilut-it” NaOH) and back to pH 4 (HNO3), to verify
FIGURE 1. Pb(II) sorption as a function of pH onto 4.8 g/L, 94 m2/g goethite: (a) fractional adsorption for two different total Pb(II) loadings in the absence of CO2. Experimental data and TLM simulations for optimum charge allocations are shown. Parameters are given in Table 1. Optimizations at 10 sites/nm2 were not successful in simulating Pb(II) adsorption, and the model suggests that a low site density may be a more appropriate average representing the Pb-adsorbing sites on this goethite preparation. The optimum charge configuration was for complex M - 1 + 1, best represented with an outer-sphere configuration. Insert: Pb(II) adsorption isotherm at pH 5 as a function of total Pb(II) loading density, in the absence of CO2 (symbols ) experimental data, provided by John Ostergren (18)) and TLM simulations. (b) Concentration adsorbed for [Pb(II)]tot ) 1.1 µmol/m2 goethite in the presence and absence of an open system of 398 µatm CO2(g). Total sorption is unaffected by atmospheric CO2, but predicted TLM speciation is gradually shifted to the ternary complex in the carbonate system (thin lines) as pH increases. reversibility. Acid/base additions during titrations were computer-controlled (21), and equilibrium was defined as a pH drift < 0.01 mV/min (0.01 pH units/h). The potentiometric titration data of the 45 m2/g (7) were remodeled using the new value of ∆pKa of 4 for self-consistency purposes. The optimized modeling parameters are listed in Table 1. Pb(II) Adsorption on 94 m2/g Goethite. A goethite slurry at 0.1 M NaNO3 was acidified to pH 4.4 and subsequently purged with humidified N2(g) for 12 h. The pH of the slurry was adjusted to 3.2, and Pb(II) stock was added to give a final Pb(II) concentration of 0.48 mM (1.06 µM Pb/m2 goethite) or 0.048 mM (0.106 µM Pb/m2). The pH was then gradually raised using 1 M NaOH, and samples were withdrawn in 10 mL polycarbonate tubes. A nitrogen headspace was maintained throughout the whole procedure. Samples were shaken end-over-end in a 25 °C chamber. After 24 h the pH of the samples was measured. The aqueous concentration of Pb(II)
was determined by centrifuging at 10 000 rpm for 30 min, filtering (0.2 µm Nalgene), and analyzing the supernatant by Graphite-Flame AAS (Varian, Spectra AA640) at 217 nm wavelength. The TLM charge allocation analysis for Pb(II) adsorption on 94 m2/g goethite assumed mononuclear complexes based on the spectroscopic results of Bargar et al. (17). Therefore, σo was varied exclusively from -1 to +1, while σβ was varied from +2 to 0. Carbonate Adsorption at Atmospheric CO2. Open CO2 adsorption experiments, to determine carbonate adsorption parameters of the different goethite preparations, were performed for the following: (1) 9.6 g/L 94 m2/g and (2) 12.8 g/L 45 m2/g goethite preparations, at a PCO2 of 400 µatm in 0.1 M NaNO3 and NaClO4, respectively; and (3) systems at a PCO2 of 400 µatm in 0.1 M NaNO3 with (a) 4.8 g/L 94 m2/g goethite in the presence and absence of 1.1 µmol/m2 total Pb(II) and (b) 0.48 mM solid-free Pb(II) solution. VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Carbonate adsorption experimental data (symbols) and TLM (lines) on 94 m2/g goethite in an open 398 µatm CO2(g) system and with or without [Pb(II)]tot ) 1.1 µmol/m2 goethite. The presence of Pb(II) enhances carbonate adsorption considerably (filled versus open squares), suggesting ternary Pb-CO3 surface complex formation. TLM carbonate and Pb(II) surface complexes used were those optimized in independent binary experiments (Figure 1). An additional Pb-bound ternary carbonate complex (B0-1) was invoked (the modeling parameters of which are given in Table 1). A TLM simulation of carbonate adsorption without invoking the Pb-ternary complex is shown in the dashed line and predicts a decreased carbonate adsorption compared to that in the absence of Pb(II) (compare to open symbols) due to site competition. The predicted contribution to total carbonate adsorption shifts almost exclusively to the Pb-ternary complex (other contributions not shown). TLM simulations in the Pb-free system are indicated in the thin lines, showing contribution of two surface complexes according to ref 13. An apparatus, adapted to maintain a constant PCO2 with pH variations, was utilized, and the procedure has been described in detail elsewhere (4). General TLM Modeling Scheme. Modeling of each sorption system was preceded by optimizing the charging behavior (titration data) of each goethite/electrolyte system, using FITEQL3.2 (22). Two different site densities representing low (2.3/nm2) and high (10/nm2) values reported for goethite were considered (14, 23, 24), and inner-layer capacitance (C1) and electrolyte-binding constant values were obtained (listed in Table 1) (see ref 25 for details) and used for the subsequent modeling of adsorption behavior. Carbonate adsorption modeling was performed by deriving independent sorption parameters for the various goethite preparations used. The surface complexation stoichiometries derived in a previous work (13) were used, and the resulting optimized equilibrium constants are listed in Table 1 (main contribution is from complex SO-0.2COO-0.8). These values were used to model trace metal adsorption. Relevant thermodynamic data for aqueous species formed in each system considered were included in the modeling procedure but have not been listed in Table 1 for purposes of brevity. The TLM performance in simulating the available sorption data for the three trace elements was evaluated. Electrostatic SCM ultimately describes surface reactions in terms of interfacial charge interactions; therefore, analysis of the simulation performance of an exhaustive stoichiometry list of whole unit-charge allocations at the different surface planes was performed for each system studied. All valence charges considered reasonable were placed on the inner (0-) and on the outer (β-) adsorption planes of the model and analyzed by pairs. FITEQL3.2 (22) was used to optimize the equilibrium constant for each configuration, assuming formation of only one complex at a time, and an analysis diagram (13) was constructed for each. For this, the FITEQL optimization error parameter WSOS/DF (weighted sum of squares/degrees of freedom) (22) was plotted versus the charge on the 0-plane (σ0), for the different designated values of charge on the β-plane (σβ). Fractional charges were considered subse3852
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quently when necessary for simulation improvement. The WSOS/DF is an error parameter and is inversely related to a “goodness-of-fit”; therefore, those FITEQL-optimized simulations yielding the lowest WSOS/DF values were considered the optimal charge allocations. Cr(VI) Adsorption Data. A previously published (7) experimental data set of 5 × 10-6 M (0.011 µmol/m2) total Cr(VI) adsorbed onto 10 g/L samples of the 45 m2/g goethite, equilibrated at three different PCO2’s (5 µatm, 0.45 matm, and 40 matm) in 0.1 M NaClO4, was used. Modeling of these data was reconsidered using the stoichiometries of our detailed carbonate adsorption modeling work recently reported (13). This procedure assumed a maximum of two sites occupied per chromate adsorbed. It should be mentioned that the carbonate adsorption data reported in ref 7 do not agree with the behavior we found (13) and was not possible to simulate using the stoichiometries optimized in the latter. U(VI) Adsorption Data. A data set similar to the Cr(VI) data was available for adsorption of 1 µM (0.022 µmol/m2) total uranyl (U(VI)), on 1 g/L samples of the 45 m2/g goethite, in the absence and in the presence of two different PCO2’s (0.32 matm and 20 matm) in 0.1 M NaClO4 (8). Optimizing the system with a purely binary U(VI) surface complex was unsuccessful in adequately describing the data, and modeling was performed using ternary surface complexes. This is consistent with previous spectroscopic evidence of ternary carbonate surface complex formation on other oxides (9, 26).
III. Results and Discussion Pb(II) Binary Adsorption. Optimal simulations for the binary Pb(II)-goethite system (minimum WSOS/DF values) were obtained for a charge configuration of σβ ) +1, and σ0 either of -1 for a site density of 2.3/nm2, or a range of 0 to -1 for 10 sites/nm2. Figure 1a shows the experimental adsorption data and TLM simulations for three different optimal configurations. The only adequate simulation was obtained assuming a low site density, i.e., for complex M - 1 + 1.
FIGURE 3. Possible structures and charge allocations for chromate surface complexes formed from one- and two-proton stoichiometries. Simulations with mononuclear complexes M0-1 and M1-1 (yielding a charge configuration average of +0.5-1 per Cr(VI), found optimal in the charge allocation analysis) are shown in Figure 4a. An equal combination of mononuclear M1-2 and binuclear B00 gives also an average of +0.5-1; however, model optimizations with these complexes did not simulate the data adequately. This optimum charge allocation is best represented by complex SO- PbOH+. This is consistent with reported proton release due to both Pb(II) binding to hydroxyl sites and Pb hydrolysis (17, 29). Most studies of Pb(II) adsorption to goethite have found evidence for inner-sphere bonding (27, 28, 17), although evidence of outer-sphere bonding at low sorption densities on the (0001) face of R-alumina has been obtained using grazing incidence (GI) XAFS spectroscopy (30). Complex M - 1 + 1 obtained here may be an innersphere complex with a charge distribution (31) of such nature. A test of the model was conducted with isotherm data of Pb(II) adsorption on the same goethite, at pH 5 (insert Figure 1a). Excellent predictions were obtained, and the model proved adequate for maximum Pb(II) sorption densities e 1.1 µmol/m2 of 94 m2/g goethite. Also, model predictions of the ionic strength effect were tested for this configuration, and a very small effect was obtained (not shown), consistent with previously reported data (27), for a maximum Pb(II) sorption density of 1.3 µmol/m2, and normally associated with inner-sphere bonding. Pb(II)-Carbonate Ternary Systems. Carbonate adsorption in open atmospheric (398 µatm) PCO2 was enhanced in the presence of 0.48 mM Pb(II) (1.1 µmol/m2) (Figure 2), but total Pb(II) sorption was unaffected upon equilibration with up to 1% CO2(g) (Figure 1b). TLM simulations using independently derived carbonate and Pb(II) surface complexes (Table 1) could not explain the carbonate adsorption increase; they predicted instead an opposite effect (Figure 2, dashed line) due mainly to site competition, including electrostatic effects: the stronger Pb(II) binding at relatively higher concentrations than carbonate (Figure 1b) decreased the concentration of sites available for carbonate binding. Successful simulations were obtained by invoking an additional Pb-CO3 ternary surface complex. This is a reasonable result considering that the aqueous speciation of Pb(II) in open atmospheric CO2 conditions shows a rapidly increasing contribution of Pb-CO3 aqueous complexes to total dissolved Pb(II) with pH. Various reasonable Pb-CO3 surface complexes were analyzed using FITEQL3.2 at two site densities, in mononuclear and binuclear (bound to 1 and 2 sites, respectively) configurations, Pb- or CO3-bound, and with different allocations of the resulting charge. The only acceptable simulations of the data were obtained for species with allocations -10 and 0-1 (Pb-bound complexes), of which only the latter, at 2.3 sites/nm2, was able to reproduce the change of slope occurring beyond pH 7 (Figure 2). This
FIGURE 4. Cr(VI) 5 µM () 0.011 µmol/m2)- goethite 10 g/L, 45 m2/g - open CO2 system in 0.1 M NaClO4. (a) Chromate adsorption: Experimental data taken from ref 7 (symbols) and optimized TLM simulations (lines) using a two-surface complex combination: σ0 ) 0 σβ ) -1, and σ0 ) +1 σβ ) -1. Broken lines show the individual contribution of each surface complex. Optimal simulations resulted for a site density of 10/nm2 (with bi- or mononuclear complexes), and the parameters used are listed in Table 1. Note the higher contribution of the inner-sphere over the outer-sphere Cr(VI) surface complex for the two lower values of PCO2 but the opposite prediction at 40 matm CO2. (b) Carbonate adsorption TLM predictions for the Cr(VI) systems at the three CO2 partial pressures and experimental data at atmospheric CO2 with no Cr(VI) added. Note that surface carbonate concentrations predicted are orders of magnitude higher than those of Cr(VI) and are therefore unaffected by the latter. supports a low site density assignment as more accurate for this goethite preparation. Complex B0-1 (binuclear configuration) yielded a slightly better simulation than the mononuclear complex (M0-1). These results agree well with recent uptake experiments where an increase in Pb(II) sorption on goethite was observed in the presence of very high concentrations of CO2(g), and FTIR and XAS spectroscopic evidence suggests formation of Pb-bound and monodentate Pb-CO3 ternary surface complexes (18). Consistent with the experimental data, the TLM construct predicted no effect of carbonate on total Pb(II) adsorption at the ratio studied (Figure 1b). However, the Pb(II) surface speciation shifted to a predominance of the ternary complex above pH 7, and surface carbonate was dominated by this complex across most of the pH range studied (Figure 2). This system represents a situation where the presence of carbonate causes a change in the surface speciation of the adsorbed element. It does not decrease the total concentration adsorbed but, on the contrary, has the potential for increasing it, by forming strongly binding ternary complexes. The enhancement of Pb(II) adsorption in the presence of 1 atm CO2, found by Ostergren et al. (18), could not be simulated because the resulting extremely high carbonate concentraVOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Uranyl adsorption onto 1 g/L, 45 m2/g goethite in open systems, and 0.1 M NaClO4, for total [U(VI)] ) 1 µM (0.022 µmol/m2). Experimental data (symbols - taken from ref 8) and optimized TLM simulations using a binary U(VI)-goethite complex (broken lines) and both binary and ternary carbonate-U(VI)-goethite complexes (thick full lines). Individual model contributions are shown for the 20 matm CO2 system (thin full lines). Equilibrium constants are given in Table 1. The model predicts a large predominance of the ternary complex from low pH values. tions were beyond the extrapolation capabilities of the model construct of this work. Cr(VI) Binary and Ternary Adsorption. Analysis of wholecharge optimization results for Cr(VI) sorption on the 45 m2/g goethite showed a minimum in the region between σo of 0 and 1 for σβ of -1. Subsequent analysis allowing for fractional charges yielded a minimum at σo ) 0.5 and σβ ) -1 valence units (species M(B)0.5-1) for both site densities considered. The high site density, however, simulated the data considerably better. Representation of such combination in one single complex would necessarily involve a fractionalcoefficient proton stoichiometry. The possibility of the result being a weighted average of the combination of two surface complexes’ charge contributions was considered. Figure 3 shows some reasonable Cr(VI) surface structures for proton stoichiometries involving one and two protons. Two complex pairs would yield an average represented by species 0.5-1 weighted equally across the pH range studied: species M0-1 + M(B)1-1 and species M1-2 + M(B)00. TLM simulations were optimized for both Cr(VI)-surface complex combinations, and only the combination of species M0-1 + (B)M1-1 simulated the data adequately (Figure 4a) (with a resulting WSOS/DF error similar to that for species M(B)0.5-1). As is the case for carbonate surface complexation, complex M0-1 would probably be represented by an inner-sphere configuration where the negative charge extends to the β-plane, justified by size considerations of the chromate anion. The simulation results of the optimal configuration are shown in Figure 4a. Excellent predictions were obtained for the high site density consideration, whereas in the low site density case (not shown), an underestimation of the carbonate effect in reducing Cr(VI) adsorption was obtained. This may be explained by a lower predicted absolute value of the adsorbed carbonate concentration for the low site density, which would translate into a lower absolute negative electrostatic field to repel anionic Cr(VI) approach to the surface. The individual contributions of each complex proposed to the total Cr(VI) sorbed are also shown in Figure 4a. At the two lower PCO2 values, the inner-sphere Cr(VI) surface complex dominated the adsorption behavior, while the outer-sphere complex was prevalent at 40 matm CO2. Figure 4b shows the experimental carbonate adsorption data for this goethite preparation at atmospheric PCO2 and the corresponding TLM 3854
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simulations using the surface charge stoichiometries determined in a previous investigation (13). The mechanism proposed by the model seems to involve site competition and electrostatic effects from swamping of the surface by orders of magnitude more abundant carbonate ions, predicted to be unaffected by the presence of Cr(VI). This mechanism may define the interaction of carbonate with other environmentally relevant anions, e.g., selenate, arsenate, and small organic acids, on metal oxide surfaces, showing potential variations that would depend on the relative concentrations present and specific surface affinity ratios. Van Geen et al. (7) suggested formation of an innersphere Cr(VI) surface complex SOCrO3- with the resulting negative charge placed on the 0-plane. However, size considerations of bulky oxyanions have prompted development of more realistic models that allow for spatially distributed interfacial charges (31). Optimization results allowing for simple charge distribution in the TLM place all resulting charge of the Cr(VI) surface complex on the outer oxygen atoms, i.e., on the β-plane, which may be represented as SOCrO3-β. This is analogous to the representation of the carbonate inner-sphere surface complex (SOCOO-β) developed in a previous TLM and FTIR investigation (13). Nevertheless, the model required the contribution of an additional outer-sphere Cr(VI) surface complex to explain the variations observed with pH and total carbonate loading. While spectroscopic investigations are necessary to confirm the model results, the prediction of predominance reversal from the inner-sphere to the outer-sphere complex upon increasing PCO2 (Figure 4a) may provide the experimental conditions necessary to probe for required spectral differences. Finally, in view of the abundance and pervasive nature of aqueous carbonate species in the environment, the mechanism proposed may account for a potential enhancement of anion mobility in aqueous settings. U(VI) Binary and Ternary Adsorption. Uranyl shows a cation-like adsorption edge similar to that of Pb(II) in the absence of carbonate. However, in open systems of CO2, a strong negative interference in adsorption occurs in the high pH region (Figure 5), showing anion-like behavior, similar to the adsorption edges of anionic Cr(VI) (Figure 4). As observed with chromate, the desorption edges of U(VI) appear at lower pH values for increasing PCO2. TLM simulations,
using the independently derived carbonate adsorption constants and a binary U(VI) complex of the form (SO)2UO2, yielded the dashed lines in Figure 5, after an optimization analysis similar to that performed for the above systems. Due to the appearance of strong aqueous carbonato complexes as pH is raised, desorption of U(VI) occurred because of competition between surface sites and carbonato ligands for binding of uranyl. However, the model overestimated the extent of this desorption when it assumed occurrence of these two competing mechanisms exclusively (dashed lines, Figure 5). The TLM simulations were significantly improved by additionally allowing some of the dominant aqueous carbonato species to show sorption behavior. The thick lines in Figure 5 show such simulations for a ternary complex formed with either one or two carbonate ions: (SO)2UO2(CO3)2- (with a charge distribution (CD) of -1-1) or (SO)2UO2(CO3)24- (with a CD of -2-2), which yielded identical results. The adsorbing U(VI)-carbonato species showed an anion-like behavior and dominated adsorption above pH 4.2 (Figure 5, thin lines); this is in excellent agreement with recent FTIR and XAS spectroscopic evidence suggesting predominance of the adsorbed carbonato complexes on hematite in the range of pH values studied of 5-8 (26), a wider predominance pH range than the corresponding one for aqueous complexes. It also agrees with evidence of similar complexes on the surface of ferrihydrite (9), although their predominance was suggested only above pH 6, based on the aqueous speciation. Kohler et al. (8) suggested dominance of these ternary complexes only above pH 5.5. The optimal parameters in the high site density case yielded better simulation of adsorption than those of the low site density. These results and those from the Cr(VI) system support the hypothesis of a high site density representing the 45 m2/g goethite surface, which, by comparison to the Pb(II) system, suggest an inverse relationship between surface area and site density of goethite. This may be due to a higher prevalence of a more reactive face (i.e., with higher site density) on the low surface area preparation. Inclusion of additional simultaneous U(VI) ternary surface complexes may further improve simulation of actual slopes of experimental data shown in Figure 5. This is not unreasonable judging from the rich U(VI)-carbonate speciation in the aqueous phase. The model, however, was kept simple since the bulk need for ternary complexation was demonstrated. The potential for occurrence of several surface complexes, however, should be investigated by spectroscopic methods. The results of this work, supporting recent spectroscopic evidence for predominance of ternary U(VI) complexes at lower pH values than their corresponding predominance in the aqueous phase (26), show the importance of accurate modeling of binary systems for self-consistency purposes in the modeling of ternary systems, to derive useful and accurate predictive results. Environmental Implications. The TLM analysis suggests that dissolved carbonate at concentrations found in natural environments influences trace metal adsorption by means of two main mechanisms: one involving competition for surface sites with other anions, including electrostatic repulsion effects, and the other, involving competition with surface sites for cationic metal binding via formation of aqueous carbonato complexes. In the latter case, variations in total metal binding will depend not only on relative abundances and surface affinity ratios but also on the specific surface binding affinities of the aqueous carbonato complexes formed. In this manner, trace Cr(VI) anions are effectively desorbed in the presence of carbonate, and the general modeling scheme using independent carbonate adsorption data may prove necessary in modeling studies of Cr(VI) reactive fate
and transport under field conditions. The mechanisms proposed may help explain the lower apparent affinity of aquifer solids for anion adsorption than that of pure iron oxides (32) or the slower reduction rates of total Cr(VI) via surface interactions in the field as compared to laboratory batch experiments (33). Competition using carbonate may be utilized as an effective in-situ soil remediation strategy, with relatively innocuous alteration of the soil environment. The mechanism proposed may also represent a dominant process influencing fate and transport of other important trace anions, such as Se(IV) and (VI), As(III) and (V) and Mo(VI). The formation of dominant aqueous carbonato species of cationic trace elements, in the presence of environmental levels of dissolved carbonate, suggests their analogous predominance in the adsorbed state. The TLM analysis was able to describe the adsorption behavior observed for both Pb(II) and U(VI), by considering only ONE adsorbing carbonato species for each in the equilibrium set, but predicted its appearance starting at a lower pH than that of any aqueous carbonato species. The adsorbing Pb(II)carbonato species was predicted to have a similar affinity for the goethite surface as the carbonate-free sorbing Pb(II) system, although it may prove necessary to consider the ternary complex in modeling of Pb(II) transport (34) or remediation (35) in soils for values of PCO2 higher than atmospheric. U(VI), on the other hand, was predicted to form more weakly adsorbing carbonato species, which yielded an overall decrease in the total adsorbed concentrations of U(VI). This may render U(VI) more mobile under field conditions and of utmost importance in modeling of transport, e.g., ref 36, or of remediation schemes, e.g., refs 11 and 37.
Acknowledgments The authors wish to thank John D. Ostergren, Stanford University, for the fruitful discussions and communication on the effects of carbonate on Pb(II) adsorption onto goethite. We are grateful to Jose Gabriel Perigault, for the valuable comments in the preparation of this manuscript. This research was supported by a Department of Energy Grant no. /DE-FG07-96ER 146981.
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Received for review October 10, 2000. Revised manuscript received June 22, 2001. Accepted June 27, 2001. ES001748K
