Effects of Solid-to-Solution Ratio on Uranium(VI) Adsorption and Its Implications

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1 Environ. Sci. Technol. 2006, 40, Effects of Solid-to-Solution Ratio on Uranium(VI) Adsorption and Its Implications TAO CHENG, MARK O. BARNETT,*, ERIC E. RODEN, AND JINLING ZHUANG Department of Civil Engineering, 238 Harbert Engineering Center, Auburn University, Auburn, Alabama 36849, and Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, Wisconsin U(VI) adsorption onto goethite-coated sand was studied in batch experiments at a solid-to-solution ratio (SSR) ranging from 33.3 to 333 g/l. Batch kinetic experiments revealed that the presence of 10-4 M phosphate increased both the initial rate and ultimate extent of U(VI) adsorption compared with phosphate-free systems. Our experimental U(VI) adsorption isotherms were independent of SSR in phosphatefree systems. However, the U(VI) adsorption isotherm became dependent on SSR in phosphate-containing systems (with a lower SSR resulting in stronger U(VI) adsorption). A surface complexation model (SCM) was used to conceptualize the interactions in systems containing U(VI), phosphate, and goethite contributing to this SSR effect. The SCM accounted for the effects of SSR on U(VI) adsorption reasonably well. This study implies that the extrapolation of batch-measured adsorption parameters of U(VI) (and potentially other radionuclides and metal- (loid)s as well) to field conditions should be done with caution, especially in the presence of strongly interacting ligands. Introduction The migration of uranium(vi), as well as other radionuclides and metal(loid)s, in subsurface porous media is strongly influenced by their adsorption/desorption reactions at the solid/solution interface. The standard approach to predict the subsurface transport of these contaminants is through the use of a retardation factor (R F), which is directly related to the partition coefficient (K D). A higher partition coefficient (K D) will lead to a larger retardation factor (R F), and thus less mobility. Batch reactors have been widely used to investigate the adsorption of U(VI) to subsurface media (1-9) and to derive adsorption isotherms (or K D), which may then be used to model U(VI) transport in porous media (10-14). When the K D approach is used, it is assumed that the surface coverage is well below saturation (i.e., the adsorption isotherm is linear) and that adsorption is independent of solid-to-solution ratio (SSR). In most batch experiments, the typical SSR used are generally much lower than that of porous media in the environment. In addition, batch experiments are often conducted over a long enough time frame to reach adsorption equilibrium. Yet, in subsurface media, U(VI) * Corresponding author phone: +1 (334) ; fax: +1 (334) ; barnettm@eng.auburn.edu. Auburn University. University of Wisconsin-Madison. adsorption and transport can be kinetically controlled (15). As a result, the standard approach using K D obtained from batch experiments may have only limited applicability to predict the adsorption and transport of radionuclides and metal(oid)s in porous media (e.g., ref 16). Phosphate is often present in subsurface systems and is important in governing the mobility of U(VI) (2, 5, 17-19). At low ph, formation of a U(VI)-phosphate ternary surface complex can greatly increase U(VI) adsorption (2, 5). However, the complicated interactions between U(VI) and phosphate and their effects on U(VI) adsorption and transport are not fully understood. In this investigation, we studied the effects of phosphate on U(VI) adsorption kinetics and the effects of SSR on U(VI) adsorption isotherms in batch reactors in the presence and absence of phosphate. Our major goal was to examine the effects of SSR on batch-measured U(VI) adsorption isotherms and the related implications to predicting U(VI) adsorption in porous media. Materials and Methods Experimental. All chemicals used were certified ACS grade and were obtained from Fisher Scientific (Pittsburgh, PA). The acids were trace metal grade and the uranium standards and stock solutions were prepared from plasma grade uranium standard (depleted uranium). Synthetic goethite coated sand was used as the adsorbent in experiments. Crystalline iron oxides such as goethite (R-FeOOH), which have been shown to adsorb U(VI) and phosphate (e.g., refs 6, 11, 20), are a common coating material for subsurface particles. Similar coatings on quartz sands have been shown to control adsorption of U(VI) (1, 21) and metals (22) to natural subsurface materials. The use of synthetic iron oxide-coated sand allowed us to more closely approximate real subsurface materials while still maintaining a chemically well-controlled system (e.g., no preadsorbed phosphate). The preparation of the goethite coated sand and its Fe content and specific surface area were described previously (2). The adsorption kinetics and equilibrium adsorption isotherms of U(VI) were studied in batch reactors. In all the batch experiments, the ph of the solution was fixed at 4.2 (( 0.1 ph unit). The suspension of the batch experiments was bubbled with humidified 0.1 M NaOH scrubbed N 2 gas (P CO2 ) 0 atm) while the ph was adjusted with freshly prepared NaOH and HNO 3. The resulting ph was stable, changing by <0.1 unit over the remainder of the experiments. We used ph 4.2 in our experiments for several seasons: (1) groundwater with ph ranging from 4.1 to 4.7 has been reported at U(VI)-contaminated Department of Energy (DOE) sites (10, 18), (2) at ph 4.2, goethite is a more stable phase than ferric phosphate and the U(VI)-phosphate ternary surface complex formed on goethite surface could have important effects on U(VI) adsorption and transport, (3) U(VI) adsorption to reaction vessel was minimal, (4) the uptake of CO 2 (g) from the atmosphere was minimized at this ph, as we had difficulty in excluding CO 2 (g) at higher ph, even when the samples were bubbled with N 2 (g) and sealed with gastight lids. Equilibrium aqueous U(VI) concentrations in the adsorption kinetic experiments and the adsorption isotherm experiments were 10-8 to 10-6 M, within the ranges used by other workers to represent typical U(VI) concentrations in contaminated groundwater (5-7, 19). The total phosphate concentration was 10-4 M in experiments with phosphate. The fixed phosphate concentration simulates subsurface environments where the total ligand (dissolved and adsorbed) concentration is constant. The environmental relevance of using a total phosphate concentration of /es051771b CCC: $ American Chemical Society VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY Published on Web 04/08/2006

2 TABLE 1. Experimental Conditions for Batch Experiments: a U(VI) Adsorption Kinetics and Adsorption Isotherms expt ID solid/solution ratio (g/l) total Fe(III) (M) total phosphate (M) results shown in kinetics b Figure 1 2 c Figure 1 isotherm 3 c Figure 2 4 d Figure 2 5 c Figure 2 6 c Figure Figure Figure 2 a Ionic strength was 0.1 M, ph was 4.2 for all batch experiments. b Total U(VI) concentration was M in all kinetics experiments. c U(VI) added immediately after phosphate. d Phosphate added 72 h before U(VI). M was discussed previously (2). Aqueous U(VI) concentration was analyzed by a kinetic phosphorescence analyzer and phosphate concentration by ion chromatography as previously described (2). The experimental conditions of the batch experiments are summarized in Table 1. The protocol of the batch experiments was similar to that described previously (2). In all batch experiments, except for Experiment 4, phosphate was added (when required) immediately before the U(VI). The reaction time for kinetic experiments was from a few minutes to one week. In the adsorption isotherm experiments, the samples were spiked with varying amount of U(VI) stock solution. The reaction time was 48 h for the isotherm experiments unless otherwise noted. Our kinetics studies on U(VI) adsorption established that 48 h is adequate for U(VI) adsorption to approach equilibrium. In Experiment 4, to examine the effects of preadsorbed phosphate on U(VI) adsorption, phosphate was added to the system and allowed to react for 72 h, then U(VI) was added. The ph dropped 0.2 to 1 ph unit after U(VI) addition and freshly prepared 1 and 0.1 M NaOH solution was added to compensate for the ph decrease. Our kinetics experiments established that 72 h of reaction time was adequate for phosphate adsorption to approach equilibrium (Figure S1, Supporting Information). While solution phosphate concentration was observed to decrease slightly beyond 72 h both by our experiments and other research (23, 24), it is common to allow h contact time in batch adsorption experiments because a reasonably constant solution concentration of phosphate is achieved by this time (25, 26). Modeling. A previously developed surface complexation model (SCM) (2) was used to simulate the U(VI) adsorption isotherms in this study. In this model the adsorption of phosphate to goethite coated sand was modeled using three monodentate surface complexes. U(VI) adsorption in the absence of phosphate was modeled using an inner-sphere, mononuclear, bidentate surface complex. In the presence of phosphate, a ternary surface complex between U(VI) and phosphate was also included. The surface species included in the model and the justification of these species are based on previous spectroscopic data and thermodynamic modeling of adsorption data (2, 5, 7, 18, 27-33), which are described in detail in the Supporting Information. FITEQL4.0 (34) was used to calculate U(VI) adsorption isotherms and related surface and aqueous species. MINTEQA2 (35) was used to calculate the saturation index for the aqueous solution. Results and Discussion Effects of Phosphate on Kinetics of U(VI) Adsorption. Both in the presence and absence of phosphate, the loss of U(VI) FIGURE 1. The effects of phosphate on U(VI) adsorption kinetics: (a) showing aqueous U(VI) concentration from 0 to 172 h and (b) showing aqueous U(VI) concentration from 0 to 8 h for the same experiments. SSR ) 33.3 g/l, U(VI) ) M, ph ) 4.2, I ) 0.1 M, T ) 295 K. (no phosphate (O); total phosphate ) 10-4 M (b). In the experiments with phosphate, the aqueous U(VI) concentration for the entire duration of the experiment was higher than 10-9 M, well above the detection limit (10-10 M) of our analytical method (2). from solution was characterized by an initial rapid decrease in aqueous concentration followed by a slower rate of decrease (Figure 1). In the absence of phosphate, after 8 h, the aqueous concentration of U(VI) became constant (relative change <3% over the next 160 h), indicating that adsorption equilibrium was approached after 8 h. In the presence of 10-4 mol/l phosphate, after 2 h, the aqueous concentration of U(VI) became very low ( 10-8 mol/l). Although, the relative change in aqueous U(VI) concentration after 2 h was relatively large (ranging from to mol/l), the absolute change in U(VI) concentration was very small (< mol/l). Additionally, no systematic decrease in U(VI) concentration was observed after 2 h, so it is reasonable to conclude that U(VI) adsorption equilibrium was approached after 2hinthepresence of phosphate. The presence of phosphate thus not only increased the amount of U(VI) adsorbed at equilibrium ( 98% vs 50% of the total U(VI) added) but also reduced the time required for U(VI) adsorption to reach equilibrium ( 2 hvs 8 h). The initial U(VI) adsorption rate was enhanced in the presence of phosphate. From 0 to 5 min, the average adsorption rate was mol/l/hr in the presence of phosphate vs mol/l/hr in the absence of phosphate. From T ) 5 to 20 min, the average rate was mol/l/hr (with phosphate) vs mol/l/hr (with no phosphate), respectively. When phosphate is present, the initial conditions of the system is further from U(VI) adsorption equilibrium (compared with systems with no phosphate), which might contribute to the observed rate ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 10, 2006

3 FIGURE 2. Adsorption isotherm of U(VI) at different SSR. Open symbols represent data for systems with no phosphate (SSR ) 33.3 g/l (]); SSR ) 333 g/l (4)), filled symbols represent data for systems with 10-4 M phosphate (SSR ) 33.3 g/l ([); SSR ) 42.0 g/l (9); SSR ) 333 g/l (2)). The ( ) represents data for systems with preadsorbed phosphate. I ) 0.1 M, T ) 295 K, ph ) 4.2. The solid line (#1) is the model predicted isotherm at all SSR in the absence of phosphate. The dashed lines are model predicted isotherms in the presence of 10-4 M phosphate (SSR ) 33.3 g/l (#2); SSR ) 42.0 g/l (#3); SSR ) 333 g/l (#4)). increase. The real mechanism of this increased adsorption rate and any related kinetic models, however, still need further investigation. Nonetheless, our kinetics study demonstrated that when other conditions (e.g., SSR, ph, total U(VI)) are the same, the presence of phosphate increases both the initial rate and the ultimate extent of U(VI) adsorption. This observation has important implications in U(VI) transport in subsurface environments since U(VI) adsorption and transport in the subsurface have been reported to be kinetically controlled (15). Our kinetic study also established the time required for U(VI) adsorption to approach equilibrium for use in the adsorption isotherm experiments. Effects of Solid-to-Solution Ratio on U(VI) Adsorption Isotherm. The experimental U(VI) adsorption isotherms in the absence of phosphate (open symbols) are shown in Figure 2. The data points at different SSR (33.3, and 333 g/l) superimposed on one another, indicating the isotherm was relatively independent of the SSR. Similar observations have been reported for many other simple systems, e.g., cadmium adsorption on quartz (36), silicate adsorption on goethite (23), phosphate adsorption on Cecil Clay (23), and U(VI) adsorption on quartz (12). In the absence of phosphate, the U(VI) adsorption isotherms were also approximately linear, indicating that the U(VI) concentration was also well below the total surface site concentration. The experimental U(VI) adsorption isotherms in the presence of 10-4 M total phosphate (filled symbols) are shown in Figure 2. At an SSR of 33.3 g/l in a system with phosphate, U(VI) adsorption greatly increased compared with U(VI) adsorption in a phosphate-free system at the same SSR. At an SSR of 33.3 g/l the increase in U(VI) adsorption in the presence of phosphate was consistent with previous results and was attributed to the formation of a U(VI) phosphate ternary surface complex (2, 5). However, at an SSR of 333 g/l in a system with phosphate, U(VI) adsorption decreased compared with U(VI) adsorption in a phosphate-free system at the same SSR, which seemed to contradict previous results (e.g., Figure 1). Comparing the U(VI) adsorption isotherm in systems with phosphate at different SSR (Figure 2), it is observed that these U(VI) adsorption isotherms are dependent on SSR, an increase in SSR in the presence of phosphate resulted in less adsorption systematically. It should also be noted that the presence of phosphate at SSR of 33.3, and 42.0 g/l caused nonlinear adsorption, an indication of approaching site saturation and that the K D approach is no longer valid, while at a SSR of 333 g/l, U(VI) adsorption was still approximately linear in the presence of phosphate. We used MINTEQA2 to check the saturation index of the aqueous solution for possible U(VI) solids and found these solutions were under saturated with respect to U(VI) solids (an example of calculated saturation index is provided in Table S2, Supporting Information). The effects of preadsorbed phosphate on U(VI) adsorption were studied at an SSR of 33.3 g/l. Compared with a phosphate-free system, U(VI) adsorption significantly increased when the goethite was pretreated with phosphate (Figure 2, vs ]). Based on previous studies (2, 5), it is reasonable to conclude that the enhanced U(VI) adsorption was due to the formation of a ternary surface complex with preadsorbed phosphate on the synthetic goethite surface. The U(VI) adsorption isotherm with preadsorbed phosphate was very close to, but slightly lower than, the isotherm obtained when U(VI) and phosphate were added simultaneously (Figure 2, vs [). The results indicate that the macroscopic reactions in the two systems (phosphate preadsorbed system and U(VI)-phosphate coexisting system) may be similar. The similarity of U(VI) adsorption in these two systems added strength to our previously proposed structure (2) for the U(VI) phosphate ternary surface complex (Type B complex in the form of >FePO 4UO 2, instead of a Type A complex in the form of >FeUO 2PO 4, see details in Supporting Information). While U(VI) adsorption isotherms are independent of SSR in the absence of phosphate, our model calculation indicates that phosphate adsorption isotherms are also independent of SSR in the absence of U(VI). Therefore, the observed dependence of U(VI) adsorption isotherms on SSR in the presence of phosphate is caused by the interactions between U(VI) and phosphate. These interactions can be conceptualized and explained by surface complexation modeling (described below). Surface Complexation Modeling. A previously developed surface complexation model (SCM) (2) was used to simulate the U(VI) adsorption isotherms for our experimental systems and the model simulation results are also shown in Figure 2. In the absence of phosphate, although U(VI) adsorption was under-predicted by the model, indicating the stability constant of U(VI) surface complex may have originally been under-estimated as discussed below, the calculated isotherms at SSR of 33.3 and 333 g/l were linear and superimposed on one another (Figure 2, solid line), consistent with our experimental results. Kohler et al. found that the Freundlich isotherm parameters calculated from their experimental data of U(VI) adsorption to quartz were also independent of the SSR (12). In modeling the transport of U(VI) and other reactive contaminants in porous media, the adsorption of reactive contaminants is often modeled using adsorption isotherms measured by batch experiments (often with a much lower SSR compared with porous media). The implicit assumption in this widely implemented approach is that the adsorption isotherm is independent of the SSR. The assumption is true for simple systems, and good agreement between batch and column transport experiments has been reported for such systems (12, 36). The model-predicted isotherms for systems with phosphate accounted for our experimental data reasonably well and predicted several features not seen in the U(VI) adsorption isotherms of phosphate-free systems (Figure 2): (1) the isotherm was dependent on SSR, (2) at SSR of 33.3 and 42.0 g/l, the presence of phosphate caused nonlinear U(VI) adsorption, and more U(VI) was adsorbed compared with that of a phosphate-free system at the same SSR, (3) at a SSR of 333 g/l, U(VI) adsorption was still linear in the presence VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

4 of phosphate, however, less U(VI) was adsorbed compared with that of a phosphate-free system at the same SSR. At SSR of 33.3 and 42.0 g/l, the model generally under-predicted U(VI) adsorption in the low U(VI) concentration range and over-predicted U(VI) adsorption in the high U(VI) concentration range. The discrepancy between model predictions and experimental measurements is probably due to the extrapolation to a wider range of total U(VI) concentrations in our current model calculation. In originally calibrating the model, the total U(VI) surface loading was log Γ U(VI) (mol U(VI)/mol Fe) )-2.80 (2), whereas the total U(VI) surface loading in our current experiments ranged from log Γ U(VI) (mol U(VI)/mol Fe) )-4.50 to -2.40, so site heterogeneity may, therefore, have become more important. The fit to this new set of data could undoubtedly be improved by including more than one type of site in the model. However, even the existing one-site model agreed qualitatively with our experimental results, and correctly predicted the effects of SSR on U(VI) adsorption and the shape (linear or nonlinear) of the isotherms, both in the presence and absence of phosphate. At an SSR of 333 g/l, the difference in experimentally measured U(VI) adsorption between the phosphate-containing and phosphate-free systems (difference between filled and open triangles, Figure 2) was greater than that observed in the model-predicted curves (difference between line 1 and 4, Figure 2). This is probably caused by over estimation of the electrostatic effects of phosphate adsorption, since the parameters used to account for the electrostatics effects in the model might be inaccurate. Mechanistic Cause of U(VI) Adsorption Dependence on SSR in the Presence of Phosphate. The consistency between the experimentally observed and model-predicted features of the U(VI) adsorption isotherms as a function of SSR in the presence of phosphate suggests an underlying mechanistic cause. To examine this cause, model calculated speciation in the U(VI)-phosphate systems was examined in order to understand the potential importance of these different interactions between U(VI) and phosphate. Conceptually, four types of U(VI)-phosphate interactions may have taken place in our batch experiment systems (2, 5) (refer to Table S1 in Supporting Information): (1) formation of U(VI)- phosphate aqueous complexes, (2) competition between U(VI) and phosphate for surface sites, (3) coadsorption of U(VI) and phosphate as a ternary surface complex, and (4) an increase in U(VI) adsorption caused by the reduction in goethite surface charge (and potential) due to phosphate adsorption. Interactions (1) and (2) would act to decrease U(VI) adsorption, while interactions (3) and (4) would act to increase U(VI) adsorption. The relative importance of each interaction depends on the relative concentrations of total U(VI), total phosphate, and total surface sites. However, compared with interactions (2) and (3), interaction (4) is usually secondary and negligible, based on our model simulation and experimental data (discussed below). Model simulation showed that in our U(VI) adsorption isotherm experiments in systems with phosphate, the predominant aqueous U(VI) species was free uranyl ion. Aqueous U(VI)-phosphate complexes were negligible for all SSR (model simulation for SSR 42 g/l is shown in Table S3 as an example), indicating that interaction (1) (formation of U(VI)-phosphate aqueous complexes) is not likely to have significantly affected U(VI) adsorption. This is further confirmed by both our experimental data and model simulations that showed that at any SSR (33.3, 42.0, and 333 g/l), phosphate adsorption to the goethite-coated sand was almost complete (i.e., the measured dissolved phosphate concentration was near or below the detection limit of our instrument ( 3 µm), and our model predicted practically 100% phosphate adsorption). At an SSR of 33.3 and 42.0 g/l, both our experimental data and model calculations demonstrated that U(VI) adsorption greatly increased in comparison to phosphatefree systems at the same SSR (Figure 2). These results indicate that interaction (3) (U(VI) and phosphate coadsorption as a ternary surface complex) was the dominant process that controlled U(VI) adsorption at those SSR. This was verified by our speciation calculation, which revealed that at SSR of 33.3 and 42.0 g/l, the strongly adsorbed ternary surface complex in the form of (>FePO 4UO 2) was the dominant U(VI) surface complex (Figure S2, panels (a) and (b), Supporting Information). In contrast, at an SSR of 333 g/l, both our experimental data and the model simulation showed that U(VI) adsorption decreased compared to that in the absence of phosphate (Figure 2), suggesting that interaction (2) (competition between U(VI) and phosphate for surface sites) became the dominant interaction. This was verified by our model simulation which indicated that the ternary U(VI)- phosphate complex was negligible and more than 99% of the surface U(VI) species was in the form of ((>FeO 2)UO 2) at a SSR of 333 g/l (Figure S2, panel (c), Supporting Information). At an SSR of 333 g/l, the decreased U(VI) adsorption in the presence of phosphate (as compared with that in the absence of phosphate) also confirms that interaction (4) was only secondary compared with interaction (2), since the surface potential in the phosphate containing system was lower (which would favor U(VI) cation adsorption) compared with that of phosphate-free systems, as indicated by our model calculation. Environmental Implications. This study demonstrates that solid-to-solution ratio (SSR) is a potentially important parameter governing U(VI) adsorption in the presence of phosphate at ph 4. This low ph is encountered in some of the U(VI) contaminated DOE sites (10, 18). However, in more typical subsurface environments (aquifer or vadose zone), the ph is usually near neutral-alkaline and the CO 2(g) partial pressure is much greater than that in the atmosphere. Carbonate ions, humic substances, and phosphate are common in natural environments, and these constituents all interact with U(VI) (e.g., 1, 2, 5-7, 37). Under certain conditions (e.g., low ph with phosphate present), such interactions can make U(VI) adsorption dependent on SSR and affect the kinetics of U(VI) adsorption. As a result, U(VI) adsorption in porous media under flow conditions can be very different from that in batch experiments where the SSR is usually much lower and adsorption equilibrium can be assumed. The extrapolation of batch-measured adsorption characteristics of U(VI) (and potentially other radionuclides and metal(loid)s as well) to field conditions should be done with caution, especially in the presence of strongly interacting ligands. Acknowledgments This research was supported by grant DE-FG07-ER6321 from the U.S. Department of Energy Environmental Management and Science Program. We thank the three anonymous reviewers and Associate Editor David Dzombak whose insightful comments helped to greatly improve the paper. Supporting Information Available Description of the surface complexation model (SCM) used in modeling experimental U(VI) adsorption isotherms. Model calculated saturation index of U(VI) minerals and aqueous and surface species in U(VI) adsorption isotherm experiments in the presence of phosphate. This material is available free of charge via the Internet at Literature Cited (1) Barnett, M. O.; Jardine, P. M.; Brooks, S. C. U(VI) adsorption to heterogeneous subsurface media: Application of a surface complexation model. Environ. Sci. Technol. 2002, 36, ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 10, 2006

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MINTEQA2/ PRODEFA2, A Geochemical Assessment Model for Environmental Systems: Version 3.0 User s Manual; U.S. EPA: Athens, GA, (36) Burgisser C. S.; Cernik, M.; Borkovec, M.; Stlcher, H. Determination of nonlinear adsorption isotherms from column experiments: An alternative to batch studies. Environ. Sci. Technol. 1993, 27, (37) Lenhart, J. J.; Honeyman, B. D. Uranium(VI) sorption to hematite in the presence of humic acid. Geochim. Cosmochim. Acta 1999, 63, Received for review September 6, Revised manuscript received March 8, Accepted March 10, ES051771B VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

6 Supporting Information Summary: This Supporting Information contains 9 pages (including cover sheet), 2 figures, and 3 tables, prepared in October, Journal: Environ. Sci. Technol. Manuscript title: Effects of Solid-to-Solution Ratio on Uranium(VI) Adsorption and Its Implications Authors: Tao Cheng, Mark O. Barnett*,, Eric E. Roden, Jinling Zhuang Authors affiliations: Department of Civil Engineering, 238 Harbert Engineering Center, Auburn University, Auburn, Alabama 36849, and Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W. Dayton St., Madison, WI * Corresponding author phone: +1 (334) ; fax: +1 (334) ; barnettm@eng.auburn.edu. Auburn University. University of Wisconsin-Madison. S1

7 Surface Complexation Model (SCM) Based on our previous data, quartz in the goethite coated sand was insignificant with respect to adsorption, so the surface complexation reactions of phosphate and U(VI) can be assumed to occur only on the goethite coating (1). Surface complexation reactions included in the model are shown in Table S1. In surface complexation modeling, the adsorption of both the U(VI) cations and phosphate anions onto the goethite surface is considered to be the result of their reactions with the surface >FeOH groups (2, 3). Therefore U(VI) cations and phosphate anions may compete for surface sites of goethite (See Equations (3), (4), (5) vs. Equation (6), Table S1). The adsorption of phosphate to goethite coated sand was modeled using three mononuclear, monodentate surface complexes, which were described in detail previously (4, 5). The structure of the mononuclear, monodentate phosphate surface complex was confirmed by Fourier transformed infra-red spectroscopy (FTIR) and atomic force microscopy (AFM) studies (3, 6). However, it has also been suggested that phosphate can be adsorbed to iron oxide surfaces in a binuclear, bidentate manner (7-9). The model predicted U(VI) adsorption isotherms based on the binuclear, bidentate phosphate surface complex were practically the same as those predicted by the model based on the mononuclear, monodentate phosphate surface complex (data not shown). U(VI) adsorption was modeled using an inner sphere, mononuclear, bidentate surface complex in the absence of phosphate based on the extended X-ray absorption fine structure spectroscopy (EXAFS) studies by Waite et al. (2). The selection of the surface species used in our model is based on these spectroscopic studies and is consistent with thermodynamic modeling of our adsorption S2

8 data. However, the selection of these surface species in our model does not suggest these are the only possible species. To model U(VI) adsorption onto goethite in the presence of phosphate, besides the inner-sphere mononuclear, bidentate surface complex, a ternary surface complex between U(VI) and phosphate was also required (1, 10). In addition to the formation of ternary U(VI)-phosphate surface complexes, the reduction in surface charge due to phosphate adsorption might also cause the observed increase in U(VI) adsorption in the presence of phosphate at low ph. However, our model calculations indicated that this mechanism alone is not sufficient to explain the observed U(VI) adsorption increase. Previous extended X-ray absorption fine structure spectroscopy (EXAFS) studies by Bostick et al. (11) suggest that U(VI)-phosphate ternary surface complexes could form in acidic natural subsurface media. Bostick et al. postulated a Type A complex in the form of >FeUO 2 PO 4. We originally adopted this Type A surface complex in our modeling. However, model calculations based on a Type A complex failed to account for our experimental data (1). Therefore, a Type B complex in the form of > FePO 4 UO 2, was considered. Model calculations based on a Type B complex agreed well with our experimental data, so a Type B complex was adopted in our modeling work (1). In our experiments with pre-adsorbed phosphate, the U(VI) adsorption isotherm was almost identical to that when U(VI) and phosphate were added simultaneously, further indicating that the formation of a Type B complex is more plausible in our systems. Payne et al. (10) also proposed a Type B U(VI)-phosphate surface complex to model their U(VI) adsorption data in systems with phosphate and ferrihydrite. S3

9 Literature Cited (1) Cheng, T.; Barnett, M. O.; Roden, E. E.; Zhuang, J. Effects of phosphate on uranium(vi) adsorption to goethite-coated sand. Environ. Sci. Technol. 2004, 38, (2) Waite, T. D.; Davis, J. A.; Payne, T. E.; Waychunas, G. A.; Xu, N. Uranium(VI) adsorption to ferrihydrite: application of a surface complexation model. Geochim. Cosmoschim. Acta 1994, 58, (3) Dideriksen, K.; Stipp, S. L. S. The adsorption of glyphosate and phosphate to goethite: a molecular-scale atomic force microscopy study. Geochim. Cosmochim. Acta 2003, 67, (4) Nilsson, N.; Lovgren, L.; Sjoberg, S. Phosphate complexation at the surface of goethite. Chem. Speciation Bioavailability 1992, 4, (5) Dzombak D. A.; Morel F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide. Wiley, New York, (6) Persson, P.; Nilsson, N.; Sjöberg, S. Structure and bonding of orthophosphate ions at the iron oxide aqueous interface. J. Colloid Interface Sci. 1996, 177, (7) Tejedor-Tejedor, M. I.; Anderson, M. A. Protonation of phosphate on the surface of goethite as studied by CIR-FTIR and electrophoretic mobility. Langmuir 1990, 6, (8) Parfitt, R. L.; Russell, J. D.; Farmer, V. C. Confirmation of surface structure of goethite and phosphated goethite by infrared spectroscopy. J. Chem. Soc. Faraday Trans. 1976, 172, S4

10 (9) Parfitt, R. L.; Atkinson, R. J.; Smart, R. S. C. The mechanism of phosphate fixation by iron oxides. Soil Sci. Soc. Am. J. 1975, 39, (10) Payne, T. E.; Davis, J. A.; Waite, T. D. Uranium adsorption on ferrihydrite - effects of phosphate and humic acid. Radiochim. Acta 1996, 74, (11) Bostick, B. C.; Fendorf, S.; Barnett, M. O.; Jardine, P. M.; Brooks, S. C. Uranyl surface Complexes formed on subsurface media from DOE facilities. Soil Sci. Soc. Am. J. 2002, 66, S5

11 Table S1. Surface Complexation Reactions Included in Model (T = 298 K, I = 0.1 M, constant capacitance model (CCM) with a specific capacitance of 1.28 F/m 2 ) log K Acid/base Properties of Goethite Surface a >FeOH + H + + >FeOH 2 (1) 7.47 >FeOH >FeO - + H + (2) Phosphate Surface Complexation Reactions a >FeOH + H 2 P Ο 4 + H + >FePO 4 H 2 + H 2 O (3) >FeOH + H 2 P Ο 4 >FePO 4 Η + H 2 O (4) 7.93 >FeOH + H 2 P Ο 4 >FeP Ο H+ + H 2 O (5) 2.16 U(VI) Surface Complexation Reaction b >Fe(OH) 2 + UO 2 2+ (>FeO 2)UO H + (6) Ternary Surface Complexation Reaction b >FeOH + UO H 2 PO 4 - >FePO 4UO 2 + H 2 O + H + (7) a from Nilsson et al. (5) b from Cheng et al. (2) Table S2. Saturation index of U(VI) minerals in an example solution of U(VI) adsorption isotherm experiments a (U(VI) = 1e-6 M, total phosphate = 0, ph 4.2) calculated by MINTEQ2 Mineral Saturation Index Gummite Schoepite UO2(OH)2 (beta) UO a The solution with the highest possible U(VI) concentration in the U(VI) adsorption isotherm experiments is given here as an example. All the solutions with lower U(VI) concentrations are also under saturated with respect to U(VI) solids. S6

12 Table S3. Model calculated speciation (log C (mol/l)) for U(VI) adsorption isotherm at SSR 42 g/l in the presence of 1e-4 M phosphate XPO4UO2 UO2HPO4 UO2PO4- PO4[3-] HPO4[2-] H3PO4 H2PO4[-] XPO4[2-] XPO4H[-] XPO4H2 UO2[+2] XOH XOH2+ XO XO2-UO2 UO2OH+ UO2(OH)2 UO2(OH)3- UO2(OH)4-2 (UO2)3OH5+ U4(OH)7+ U2(OH)22+ U3(OH)7- U2(OH)+3 U3(OH) S7

13 6.00E E-04 aqueous PO4 (mol/l) 4.00E E E E E time (hour) Figure S1. The adsorption kinetics of phosphate on goethite coated sand. SSR = 33.3 g/l, PO4 = M, ph = 4.2, I = 0.1 M, T = ~295 K. S8

14 log q (mol U(VI) / kg goethite) a. SSR = 33.3 g/l log q (mol U(VI) / kg goethite) b. SSR = 42 g/l log C (mol U(VI) / L) log C (mol U(VI) / L) 0 log q (mol U(VI) / kg goethite) c. SSR = 333 g/l log C (mol U(VI) / L) Figure S2. Model predicted U(VI) adsorption isotherm and U(VI) surface species at different SSR. Open squares represent model predicted total U(VI) adsorption. The solid lines represent model predicted ternary surface complex (>FePO 4 UO 2 ). The dashed lines represent model predicted binary U(VI) surface complex ((>FeO 2 )UO 2 ). S9