Uranium(VI) Removal by Nanoscale Zerovalent Iron in Anoxic Batch

Sep 21, 2010 - Sciences, Lincoln University of Missouri, Jefferson City,. Missouri 65102 ... Introduction. Uranium (U) is a common contaminant at many...
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Environ. Sci. Technol. 2010, 44, 7783–7789

Uranium(VI) Removal by Nanoscale Zerovalent Iron in Anoxic Batch Systems S E N Y A N , †,‡ B I N H U A , ‡,§ Z H E N G Y U B A O , † JOHN YANG,§ CHONGXUAN LIU,| AND B A O L I N D E N G * ,‡ Key Laboratory of Bio-geology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan, Hubei 430074, China, Department of Civil & Environmental Engineering, University of Missouri, Columbia, Missouri 65211, Department of Agriculture & Environmental Sciences, Lincoln University of Missouri, Jefferson City, Missouri 65102, and Pacific Northwest National Laboratory, Richland, Washington 99354

Received December 1, 2009. Revised manuscript received June 29, 2010. Accepted September 3, 2010.

This study investigated the influences of pH, bicarbonate, and calcium on U(VI) removal and reduction by synthetic nanoscale zerovalent iron (nanoFe0) particles under anoxic conditions. The results showed that the rates of U(VI) removal and reduction by nanoFe0 varied significantly with pH and concentrations of bicarbonate and/or calcium. For instance, at pH 6.92 the pseudo-first-order rate constants of U(VI) removal decreased by 78.5% and 81.3%, and U(VI) reduction decreased by 90.3% and 89.3%, when bicarbonate and calcium concentrations were increased from 0 to 1 mM, respectively. X-ray photoelectron spectroscopy (XPS) analysis confirmed the formation of UO2 and iron (hydr)oxides as a result of the redox interactions between U(VI) and nanoFe0. The study demonstratedthepotentialofusingnanoFe0 forU(VI)-contaminated site remediation and highlighted the impacts of pH, bicarbonate, and calcium on the U(VI) removal and reduction processes.

Introduction Uranium (U) is a common contaminant at many nuclear energy and weapon-related waste sites and U mining locations (1, 2). U can exist in several oxidation states [e.g., U(0), U(III), U(IV), and U(VI)], but in the ambient environment, the predominant oxidation states are U(VI) and U(IV) (3, 4). U(VI) dominates under oxidizing conditions, mainly as complexed, adsorbed, or precipitated uranyl (UO22+) species. Under reducing conditions, uranium exists commonly as insoluble uranium(IV) oxide (4). Many studies have demonstrated that the fate of uranium in the environment is significantly affected by its redox transformation between U(VI) and U(IV) (5-16). Reduction of soluble U(VI) to insoluble uranium(IV) oxide has been proposed as an important approach to remediate U-contaminated sites. U(VI) can be reduced by various reductants such as organic compounds (e.g., lactate and acetate) mediated by * Corresponding author phone: 573-882-0075; fax: 573-882-4784; e-mail: [email protected]. † China University of Geosciences. ‡ University of Missouri. § Lincoln University of Missouri. | Pacific Northwest National Laboratory. 10.1021/es9036308

 2010 American Chemical Society

Published on Web 09/21/2010

dissimilatory metal-reducing bacteria (DMRB) and sulfatereducing bacteria (SRB), reduced sulfur species, sorbed/ structural Fe(II), and zerovalent iron (ZVI) including iron filings and nanoFe0 (5-16). NanoFe0 has unique properties, such as large surface area to volume ratio, high reactivity, and high mobility for groundwater remedial applications (17). Although nanoFe0 could be oxidized and form aggregates that presumably affect its applications in a negative way, it has been applied with success to remediate chlorinated solvents in the subsurface and to immobilize toxic metals and radionuclides such as Cr, Pb, Tc, and U (16, 17). The use of ZVI to immobilize U(VI) in groundwater through reduction has been demonstrated (8, 9, 16). The impact of various common groundwater constituents (e.g., bicarbonate, calcium and natural organic matter) on the reduction process, however, has not been fully understood. Bicarbonate and calcium are ubiquitous in groundwater and are known to influence U(VI) speciation (3, 18). Recent studies have shown that microbial U(VI) reduction is strongly inhibited by bicarbonate and calcium, because U(VI) forms stable calcium-uranyl-carbonato complexes that are apparently less efficient electron acceptors than uranyl hydroxyl complexes (11, 15). Consequently, understanding the potential impact of bicarbonate and calcium on U(VI) removal and reduction by nanoFe0 is important for its use in site remediation. Here we report our investigation on the effects of bicarbonate and calcium on the rate and extent of U(VI) reduction by synthetic nanoFe0 particles. Effort was made to differentiate the U(VI) adsorption and U(VI) reduction by nanoFe0 through batch experiments with bicarbonate concentration from 0 to 10 mM and calcium concentration from 0 to 1 mM. The pH range of typical natural groundwater is from 5.0 to 9.0. We chose a starting pH at 6.92 because nanoFe0 was too reactive and unstable under more acidic conditions. A kinetic model was developed to describe the observed kinetics of U(VI) removal and reduction and provided insights into the mechanisms of U(VI) reduction by nanoFe0.

Experimental Section Materials. All common chemicals used in this study [FeCl2, CaCl2, Ca(NO3)2, CaBr2, MgSO4, Na2SO4, Na2CO3, NaHCO3, KHCO3, and NaBH4] were of ACS reagent grade. U(VI) standard solution was purchased from Sigma-Aldrich Co. Tris(hydroxymethyl)aminomethane (Tris) (BioRad Laboratory) was used as a pH buffer. The stock solutions of 1000 µM U(VI), 500 mM NaHCO3, 100 mM CaCl2, and 250 mM Tris were prepared with Milli-Q water (18.2 MΩ cm, Millipore Co.), which was pretreated following a previously reported procedure (13) to eliminate dissolved O2 and CO2. The synthetic groundwater (GW) used in the study contained 0.286 mM Ca(NO3)2, 0.312 mM CaBr2, 0.529 mM MgSO4, 0.451 mM Na2SO4, 0.0111 mM Na2CO3, 0.604 mM NaHCO3, and 0.43 mM KHCO3 (19). NanoFe0 Preparation and Characterization. NanoFe0 particles were prepared using a method similar to Manning et al. (20) by reacting 250 mL of N2-purged 0.012 M FeCl2 (in 30% ethanol) with 5-10 mL of 0.53 M NaBH4 in a N2-purged beaker while stirring. The synthesized nanoFe0 particles were recovered by vacuum filtration and washed three times with N2-purged 30% ethanol to remove the unreacted reagents. NanoFe0 slurry was prepared by mixing a predetermined amount of nanoFe0 with pretreated Milli-Q water in an anoxic glovebox (Coy Laboratory Products Inc.) containing ∼5% H2 balanced with ∼95% N2. NanoFe0 particles were characterized VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Removal of U(VI) from the aqueous phase by both U(VI) sorption and reduction to U(IV) under different conditions ([U(VI)]0 ) 200.0 µM, [Fe0] ) 27.57 mg/L). In order to compare with the U(VI) reduction kinetics, the time scale for each diagram is the same as the corresponding diagram in Figure 2. by a FEI Quanta 600 FEG extended vacuum scanning electron microscope (SEM). Its specific surface area was measured using a PMI automated Brunauer-Emmett-Teller (BET) sorptometer (Porous Materials, Inc.) with N2 adsorption at -196 °C, yielding a value of 30.8 m2/g. Kinetic Experiments. A detailed procedure measuring the kinetics of U(VI) removal and reduction is presented as Supporting Information (section S1). Briefly, all experiments were conducted in duplicate at room temperature (25 ( 1 °C) in an anoxic glovebox free of CO2. Rates of U(VI) removal and reduction by nanoFe0 were examined in batch systems under various pHs (from 6.92 to 9.03 controlled by 12.5 mM Tris buffer), bicarbonate concentration (from 0 to 10 mM), and calcium concentration (from 0 to 1 mM) but fixed concentrations of U(VI) (200.0 µM) and nanoFe0 (27.57 mg/ L, i.e., 492.3 µM). Experiments began by transferring desired amounts of nano Fe0 slurry, Tris buffer, and NaHCO3 /CaCl2/ GW solution into a set of 50 mL tubes. U(VI) stock solution was then added to initiate the reaction. The reaction vessels were agitated on a rotator (Bellco Glass Inc.) at ∼60 rpm to maintain particle suspensions. At various time intervals, the reaction vessels were sacrificially sampled and aqueous U(VI) concentration was spectrophotometrically determined (21). U(VI) removal from the aqueous phase could result from both sorption and reduction (i.e., [U(VI)]removal ) [U(VI)]sorbed + [U(VI)]reduced) (7-9). A 40 mM bicarbonate extraction method adapted from Gu et al. (8) and Liger et al. (10) was used to determine the total residual U(VI) (i.e., [U(VI)]residual ) [U(VI)]sorbed + [U(VI)]aqueous) in suspension samples. The difference between the total residual U(VI) and aqueous U(VI) concentrations was considered as the adsorbed U(VI). The difference between the initial U(VI) and the total residual U(VI) concentrations was considered as the reduced U(VI) (i.e., [U(VI)]initial ) [U(VI)]reduced + [U(VI)]sorbed + [U(VI)]aqueous). X-ray Photoelectron Spectroscopy (XPS) Analysis. XPS analysis was conducted on a KRATOS model AXIS 165 XPS spectrometer with an argon sputtering KRATOS Minibeam 7784

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ion source. To collect enough solid products for XPS analysis, the sample was prepared by mixing 40 mL of 1000 µM U(VI) with 160 mL of 5.0 g/L nanoFe0 at pH 6.92 in a 250 mL polyethylene bottle for 24 h. The measured pH (6.95) and Eh (-491.9 mV) in the system for the XPS sample preparation were close to those in the kinetic experiments (pH 6.85-6.99; Eh -490.2 to -492.6 mV), suggesting that the XPS sample should be representative of the reaction products. The particles were then collected by filtration through a 0.2 µm nylon filter and freeze-dried in an airtight container for over 24 h. The XPS analysis was made by tapping the particle sample onto a sample supporting plate, which was then placed in a chamber and vacuumed to ∼10-9 Torr. XPS measurements were conducted with magnesium at 1253.6 eV with pass energies between 20 and 80 eV. The XPS spectra peak energies were calibrated with adventitious C 1s (285.0 eV), and the data were processed by CasaXPS software (version 2.3.14).

Results NanoFe0 Material. The nanoFe0 particles prepared by NaBH4 reduction method were in the 30-50 nm size range (Supporting Information, Figure S2a), similar to those reported in the literature (20, 22). Elemental analysis using energy dispersive X-ray spectroscopy (Supporting Information, Figure S2b) showed the presence of a small amount of chlorine, most likely as a residue of the unreacted FeCl2. Furthermore, the small oxygen peak was observed, which indicated the partial oxidation of nanoFe0 particles. U(VI) Removal by NanoFe0. U(VI) removal from the aqueous phase by nanoFe0 could come from U(VI) sorption and/or reduction. As shown in Figure 1, the aqueous U(VI) concentration as a function of time varied with the solution chemical composition. The removal rate decreased with increasing pH, and bicarbonate and calcium concentrations (Supporting Information, Table S1). For example, in the control systems without bicarbonate and calcium, over 93%

FIGURE 2. Reduction of U(VI) as indicated by changes of [U(VI)]residual/[U(VI)]initial with time under different conditions ([U(VI)]0 ) 200.0 µM, [Fe0] ) 27.57 mg/L). of U(VI) was removed from the aqueous phase within 10 min at pH 6.92, within 20 min at pH 8.03, and within 90 min at pH 9.03 (Figure 1a). When the bicarbonate concentration was increased to 5 mM, it required 24 h to remove 98.4%, 95.8%, and 94.4% of U(VI) at pH values 6.92, 8.03, and 9.03, respectively. When bicarbonate concentration was further increased to 10 mM, only 81.9%, 75.0%, and 70.8% of U(VI) were removed after 96 h at pH values 6.92, 8.03, and 9.03, respectively. The impact of pH was less pronounced with higher bicarbonate concentrations (Figure 1 and Supporting Information, Table S1). For instance, when bicarbonate concentration was 1 mM, the pseudo-first-order removal rate constants were 6.2 ( 1.0 and 0.76 ( 0.13 h-1 at pH values 6.92 and 9.03, respectively, while in the presence of 5 mM bicarbonate, the corresponding rate constants were 0.41 ( 0.05 and 0.27 ( 0.04 h-1 at pH values 6.92 and 9.03, respectively. The rate of aqueous U(VI) removal could also be affected by calcium concentration. In the presence of 0.5 mM CaCl2 the pseudo-first-order removal rate constants were comparable to those in the control systems (Supporting Information, Figure S3 and Table S1). In 1 mM CaCl2 solutions, however, the rate constants were 5.3, 3.6, and 1.3 times lower than those in the control systems at pH values 6.92, 8.03, and 9.03, respectively (Figure 1c and Supporting Information, Table S1). The inhibitive effect of bicarbonate and calcium on the rate of aqueous U(VI) removal in the synthetic groundwater was comparable to those in the systems containing 1 mM bicarbonate or calcium (Figure 1b,c,e). U(VI) removal proceeded to near completion within 1 h at pH 6.92 and 4 h at pH values 8.03 and 9.03. U(VI) Reduction by NanoFe0. The observed rates of U(VI) reduction, as indicated by the residual U(VI) in the suspensions (Figure 2), also decreased with increasing pH, bicarbonate, and/or calcium concentrations (Supporting Information, Table S2). In the control systems without bicarbonate and calcium, the reduction rates were the fastest, and the

pseudo-first-order reduction rate constants were 6.1 ( 0.8, 1.3 ( 0.2, and 0.92 ( 0.15 h-1 at pH values 6.92, 8.03, and 9.03, respectively (Supporting Information, Table S2). The U(VI) reduction rate constant then exponentially decreased with increasing bicarbonate concentrations from 0 to 10 mM, with a much faster exponential decrease at a lower pH (Supporting Information, Table S2 and Figure S4). In the presence of 0.5 mM CaCl2, the U(VI) reduction rates were essentially the same as those in the control systems (Supporting Information, Figure S3 and Table S2). With the addition of 1 mM CaCl2, the reduction rate constants were 9.4, 6.5, and 11.8 times lower than those for the control systems at pH values 6.92, 8.03, and 9.03, respectively. Nevertheless, about 86.4% of initial U(VI) was still reduced after 6 h at pH 6.92 and 68.7% and 67.7% after 24 h at pH values 8.03 and 9.03, respectively (Figure 2c). The U(VI) reduction rates in the simulated groundwater were slower than those for the control system and the system containing 0.5 mM CaCl2 but faster than those in the systems containing only 1 mM bicarbonate or calcium (Figure 2). For instance, at pH 6.92 the reduction rate constant with GW was 1.1 ( 0.2 h-1, while the corresponding rate constants for the control system, the system with only 1 mM NaHCO3, and the system with only 0.5 mM or 1 mM CaCl2 were 6.1 ( 0.8, 0.59 ( 0.13, 6.0 ( 0.5, and 0.65 ( 0.18 h-1, respectively (Supporting Information, Table S2). Reaction Products. The XPS survey spectra for the reaction sample prepared at pH 6.92 are shown in Figure S5a (Supporting Information), with the red line representing the surface scan and the pink and yellow lines denoting the scans after argon etching approximately 10 and 50 nm from the surface, respectively. Scans after argon etching were assumed to represent samples free of alterations during sample handling. The major identified peaks for the reaction sample were iron, oxygen, carbon, and uranium (Supporting Information, Figure S5a), while only iron, oxygen, and carbon peaks were identified for the pure nanoFe0 sample (Supporting Information, Figure S5b). VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. U 4f and Fe 2p XPS spectra for nanoFe0 and U reaction sample prepared at pH 6.92: (a) surface spectra, (b) 10 nm depth spectra, and (c) 50 nm depth spectra. Points are the experimental data and solid lines are the results of quantitative Gaussian-Lorentzian curve fitting showing individual contributions from components. The high resolution U 4f and Fe 2p peaks along with fitting curves are presented in Figure 3. The binding energies of U 4f7/2 for 0, 10, and 50 nm depths were 380.3, 380.2, and 380.3 eV, respectively, and the binding energies of U 4f 5/2 peaks were 390.9, 391.0, and 391.1 eV, respectively. All these peaks agreed with the values reported for U(IV) (UO2) in the NIST XPS spectral database and other literature (9, 16), indicating the reductive precipitation of uranium. In the XPS spectra of Fe 2p, iron (hydr)oxides were the predominant species and the Fe0 (with binding energy of 706.7 eV (23)) was only present as a minor component at the 50 nm depth. The curve fitting results showed that 50.8%, 52.5%, and 55.6% of iron were present in the form of ferrous oxide at 0, 10, and 50 nm depths, respectively. The Fe 2p3/2 peaks at 712.4-714.1 eV were indicative of ferric iron (24), which may be present in any one of several possible species including hydrated ferric oxide (FeOOH) and hematite (Fe2O3). The presence of Fe(III) revealed Fe(II) could reduce U(VI). Furthermore, the Fe 2p1/2 peaks with binding energy from 722.1 to 723.3 eV indicated the magnetite component (25).

Discussion Comparison Between NanoFe0 and Other Reductants. To compare the efficiencies of various reductants capable of U(VI) removal and reduction, the initial surface area of nanoFe0 was used to normalize the observed U(VI) removal and reduction rate constants. On the basis of U(VI) removal, 7786

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the normalized rate constants of nanoFe0 without calcium and bicarbonate at pH 6.92 (Supporting Information, Table S3) were much higher than those of conventional iron filings and sulfide species (6-9). To illustrate, the normalized removal rate constant at pH 6.92 was 33.9 L h-1 m-2 by nanoFe0, which was 678, 1696, and 5 times larger than those by metallic iron filing (7), galena (6), and iron sulfide (at pH 6.90) (14), respectively. Among these three reductants, nanoFe0 was comparable to iron sulfide and significantly more reactive than iron filing and galena. The high reactivity of nanoFe0 observed in this study toward U(VI) removal was consistent with the results reported by Riba et al. (16). For U(VI) reduction by nanoFe0, the normalized rate constants (Supporting Information, Table S4) were over 38 and 340 times larger than those by iron sulfide (14) and Fe(II) sorbed on hematite (10) under comparable pH conditions, respectively. For example, the normalized reduction rate constant by nanoFe0 at pH 6.92 was 7.18 L h-1 m-2, while that for Fe(II) sorbed on the hematite at pH 6 was 3.6 × 10-4 L h-1 m-2. It is worth noting that at pH 6.92 the observed reduction rate constants for nanoFe0 with 1 or 5 mM bicarbonate (0.59 and 0.27 h-1, respectively) were lower than that by hydrogen sulfide in the presence of 4 mM bicarbonate at pH 6.89 (1.61 h-1). At higher pH values of 8.01 and 9.06, however, over 82.7% and 78.3% U(VI) could be reduced by nanoFe0 even in the presence of 10 mM NaHCO3, whereas less than 10% U(VI) was reduced by hydrogen sulfide in the presence of 2 mM NaHCO3 at pH 9.06 (13). Compared to the

FIGURE 4. Temporal changes of [U(VI)]aqueous/[U(VI)]initial, [U(VI)]adsorbed/[U(VI)]initial, and [U(VI)]reduced/[U(VI)]initial and the kinetics model fitting curves at pH ) 6.92. U(VI) reduction mediated by Shewanella putrefaciens, the observed reduction rate constant for nanoFe0 in the presence of 10 mM bicarbonate at pH 6.92 was larger than that reported by Stewart (15), but lower than those by Liu et al. (26) and Brooks et al. (11), which could be due to the differences in the initial bacteria concentration, the nature and concentration of electron donor, and calcium and/or bicarbonate concentrations. Overall, nanoFe0 is very efficient to remove and reduce U(VI), which could be attributed to its nano size, high reactivity, large surface area, and reactive Fe(II) produced by nanoFe0. Modeling U(VI) Reduction. On the basis of the rates of U(VI) reduction by nanoFe0 as well as the product analysis described above, any reaction scheme proposed to represent U(VI) reduction by nanoFe0 should account for the observations that (i) U(VI) removal results from both sorption and reduction (8, 9) and (ii) U(VI) reduction and subsequent precipitation (as UO2) are triggered by sorption of U(VI) on the nanoFe0 surface (16). We hypothesize that U(VI) reduction by nanoFe0 occurs following a two-step process as being conceptualized by Laidler (27) for surface reactions: Step 1: sorption process with forward and backward rate constants k1 and k-1 tFe + U(VI) h tFeU(VI)

(1)

Step 2: reduction process with rate constant k2 k2

tFeU(VI) 98 tU(IV) + iron (hydr)oxides

(2)

where t designates the species associated with nanoFe0 surfaces. Since nanoFe0 has a large specific surface area (the

estimated surface area per uranium molecule is approximately 10 nm2/molecule), the surface concentration of tFe is considered as a constant in the model. Then the time evolution of aqueous U(VI) depletion, U(VI) sorbed, and U(IV) production in the system can be described as d[U(VI)] ) -k1[U(VI)] + k-1[tFeU(VI)] dt

(3)

d[tFeU(VI)] ) k1[U(VI)] - (k-1 + k2)[≡FeU(VI)] dt

(4)

d[U(IV)] ) k2[tFeU(VI)] dt

(5)

Using the experimental data obtained in this study and a computer software (Scientist) for simultaneous solution of this set of kinetic equations, we obtained adsorption, desorption, and reduction rate constants k1, k-1, and k2 as the best fitting parameters under various experimental conditions (Figure 4 and Supporting Information, Table S5). The results indicated that in the control system without bicarbonate and calcium, the adsorption rate constants (k1) were about 17 orders of magnitude larger than the desorption rate constants (k-1) over the examined pH range, suggesting that the sorption process (step 1) was irreversible. Furthermore, k1 was larger than the reduction rate constant (k2), indicating that the overall reaction was controlled by the reduction process (step 2). With the addition of bicarbonate, the adsorption and reduction rate constants decreased, while the desorption rate constant increased. More importantly, the rate-determining step for the overall reaction VOL. 44, NO. 20, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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shifted from reduction (step 2) to sorption (step 1) with increasing NaHCO3 concentration from 0 to 10 mM. For instance, at pH 6.92, when the bicarbonate concentration was 0 mM, the sorption, desorption, and reduction rate constants were k1 ) 19.5 ( 3.5 h-1, k-1 ) (6.0 ( 0.5) × 10-17 h-1, and k2 ) 11.4 ( 1.7 h-1, respectively, whereas when 10 mM bicarbonate was added, the rate constants were k1 ) 0.03 ( 0.00 h-1, k-1 ) 0.1 ( 0.0 h-1, and k2 ) 2.0 ( 0.2 h-1, respectively. The results indicated a strong inhibition of bicarbonate on U(VI) sorption and reduction. Furthermore, U(VI)adsorptionratespositivelycorrelatedwithuranyl-hydroxyl species but negatively correlated with uranyl-carbonato species (Supporting Information, Figure S6). The inhibition of bicarbonate was attributed to the formation of stable U(VI)-carbonate aqueous complexes, as indicated by the calculated distribution of U(VI) species under the experimental conditions used in this study (Supporting Information, Table S6). Formation of stable U(VI)-carbonate aqueous complexes stabilized U(VI) in the aqueous phase (3), decreased U(VI) adsorption (28), and consequently suppressed the process of U(VI) reduction. In the presence of 0.5 or 1 mM calcium, the sorption process was also irreversible, with the adsorption rate constants being much larger than the desorption rate constants. The adsorption rate constants were also larger than the reduction rate constants (Supporting Information, Table S5), suggesting that the overall reaction was controlled by the reduction process. The suppression of calcium on U(VI) removal and reduction likely resulted from the combined effects of (1) competitive sorption and electrostatic repulsion between Ca2+ and UO22+ and (2) when bicarbonate is present, formation of the stable ternary calcium-uranyl-carbonato species in the aqueous phase that inhibited the U(VI) removal and reduction (11, 15). The fitting results for synthetic groundwater containing bicarbonate and calcium were complex. At pH 6.92, the adsorption rate constant was larger than reduction rate constant, while the results were reversed at pH values 8.03 and 9.03. The lower adsorption rate constant at higher pH was likely due to the electrostatic repulsion between the dominant and negatively charged CaUO2(CO3)32- species (Supporting Information, Table S6) and nanoFe0 surface with a pH of zero-point charge around 8.1 (29). The reason for higher reduction rate constants at pH values 8.03 and 9.03 in GW is unclear. It is probably related to the iron corrosion process in synthetic groundwater media that is different from what occurred in the more pure systems. Also the ferric oxides identified by XPS data (Figure 3) indicated Fe(II) can reduce U(VI), and Fe(II) was more reactive at higher pH (10). Environmental Implications. Bicarbonate and calcium are ubiquitous components in natural and contaminated groundwater. Both batch experiments and spectroscopic studies herein show that nanoFe0 is effective in removing U(VI) from water in the presence of bicarbonate and/or calcium under the anoxic condition. The major reaction pathway is the reduction of U(VI) by nanoFe0 to form insoluble U(IV) species. This study has demonstrated the potential of using nanoFe0 to immobilize U(VI) for groundwater remediation. To deploy this technology, however, further study is needed to evaluate the effect of other organic/inorganic complexing ligands (e.g., natural organic matter) and oxidants [e.g., Fe(III) (hydr)oxides], potential aggregation and oxidation of nanoFe0, and deliverability of nanoFe0 in heterogeneous subsurface.

Acknowledgments We would like to thank the China Scholarship Council (CSC) for sponsoring S.Y. to conduct this cooperative research at the University of Missouri. Reviews by Dr. Armelle Braud on 7788

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an earlier draft of this paper are greatly appreciated, as well as constructive comments by four anonymous reviewers and the ES&T associate editor, Dr. Michelle Scherer. This work was partially supported by the United States Department of Energy (DOE)’s Joint EPA, NSF, and DOE program on Nanotechnology in the Office of Biological and Environmental Research (BER).

Supporting Information Available Figures S1-S6 and Tables S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org.

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