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Journal of Environmental Radioactivity 131 (2014) 40e46

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate / jenvrad

Retention and chemical speciation of uranium in an oxidized wetlandsediment from the Savannah River Site

Dien Li a,*, John C. Seaman b, Hyun-Shik Chang b, Peter R. Jaffe c, Paul Koster van Groos c,De-Tong Jiang d, Ning Chen e, Jinru Lin f, Zachary Arthur d, Yuanming Pan f,Kirk G. Scheckel g, Matthew Newville h, Antonio Lanzirotti h, Daniel I. Kaplan a

a Savannah River National Laboratory, Aiken, SC 29808, USAb Savannah River Ecology Laboratory, University of Georgia, Aiken, SC 29802, USAcDepartment of Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544, USAdDepartment of Physics, University of Guelph, Guelph, ON N1G 2W1, CanadaeCanadian Light Source Inc., Saskatoon, SK S7N 2V3, CanadafUniversity of Saskatchewan, Saskatoon SK S7N 5E2, CanadagUS Environmental Protection Agency, National Risk Management Research Laboratory, Cincinnati, OH 45224-1702, USAhUniversity of Chicago, Chicago, IL 60637, USA

a r t i c l e i n f o

Article history:Received 21 June 2013Received in revised form18 October 2013Accepted 21 October 2013Available online 14 November 2013

Keywords:Wetland sedimentsUraniumChemical speciationRetentionSpectroscopy

* Corresponding author. Tel.: þ1 803 725 7520; faxE-mail address: Dien.Li@srs.gov (D. Li).

0265-931X/$ e see front matter Published by Elseviehttp://dx.doi.org/10.1016/j.jenvrad.2013.10.017

a b s t r a c t

Uranium speciation and retention mechanisms onto Savannah River Site (SRS) wetland sediments wasstudied using batch (ad)sorption experiments, sequential extraction, U L3-edge X-ray absorption near-edge structure (XANES) spectroscopy, fluorescence mapping and m-XANES. Under oxidized conditions,U was highly retained by the SRS wetland sediments. In contrast to other similar but much lower naturalorganic matter (NOM) sediments, significant sorption of U onto the SRS sediments was observed atpH < 4 and pH > 8. Sequential extraction indicated that the U species were primarily associated with theacid soluble fraction (weak acetic acid extractable) and organic fraction (Na-pyrophosphate extractable).Uranium L3-edge XANES spectra of the U-bound sediments were nearly identical to that of uranyl ace-tate. Based on fluorescence mapping, U and Fe distributions in the sediment were poorly correlated, Uwas distributed throughout the sample and did not appear as isolated U mineral phases. The primaryoxidation state of U in these oxidized sediments was U(VI), and there was little evidence that the highsorptive capacity of the sediments could be ascribed to abiotic or biotic reduction to the less soluble U(IV)species or to secondary mineral formation. Collectively, this study suggests that U may be strongly boundto wetland sediments, not only under reducing conditions by reductive precipitation, but also underoxidizing conditions through NOM-uranium bonding.

Published by Elsevier Ltd.

1. Introduction

Uranium (U) accumulates in wetlands in areas where U-richrock formations are present (Zielinski et al., 1987). Natural andconstructed wetlands have been used as a cost-effective means toimmobilize U from U mines and processing facilities (Noller et al.,1994). Thus, understanding the biogeochemical processes of U inthe wetlands is important for predicting risk associated with Ucontamination (e.g., Umine andmilling sites, nuclear waste storagesites, and fuel cell rod storage basins). Possible pathways for long

: þ1 803 725 7673.

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term U immobilization in wetlands include the chemical and/ormicrobial reduction of soluble U(VI) ions to fairly insoluble U(IV)species (Kalin et al., 2005; Lovley et al., 1991; Noubactep et al.,2006; Turick et al., 2008), mineral precipitation, sorption ontoNOM (Wan et al., 2011), ion exchange or sorption to sedimentminerals (Akber et al., 1992; Dong et al., 2012; Grybos et al., 2007),accumulation in plants (Hinton et al., 2005; Knox et al., 2008), andincorporation into secondary minerals (Lind et al., 2013a).

The Savannah River Site (SRS) in Aiken, SC includes several(former) uranium processing facilities. As a result of their opera-tions, U has entered the surrounding environments through acci-dental and purposeful discharges. For example, approximately45,000 kg of depleted U was released into Tims Branch and itsassociated wetlands between 1958 and 1980 and approximately

Table 1Characteristics of the SRS contaminated and uncontaminated sediments.

Contaminatedsediment

Uncontaminatedsediment

pHa 4.53 4.16Organic carbon, mg/kg 1427 1395Fe oxide,b wt% 0.09 0.01Sand, wt% 53 79Silt, wt% 22 16Clay, wt% 19 6238U, mg/kg 285 0.57238U desorption Kd, mL/g 1297 n/a

n/a ¼ not available.a 1:1 solid/deionized water ratio.b Analyzed using Na dithionite-sodium citrate-citric acid extraction, reported as

Fe2O3.

D. Li et al. / Journal of Environmental Radioactivity 131 (2014) 40e46 41

70% of the U still remains in the stream and associated wetlandsediments of this waterway (Bertsch et al., 1994; Evans et al., 1992;Pickett, 1990).

The U desorption distribution coefficients (Kd values) of SRScontaminated wetland sediments has been measured to be quitelarge, ranging from 170 to 6493 mL/g (Kaplan and Serkiz, 2001).NOM has been demonstrated to control the availability of U in thecontaminated sediments within Tims Branch (Sowder et al., 2003).Bertsch et al. (1994) reported the in situ characterization of chem-ical speciation of U in the sand fraction of a SRS sediment sampleusing micro X-ray absorption spectroscopy, which appeared toindicate the presence of U(VI). Given these initial observations, theremediation technologies of the contaminated SRS sediments withapatite (Ca10(PO4)6(OH, F)2) (Arey et al., 1999; Kaplan and Knox,2004; Seaman et al., 2001), humic acid (Wan et al., 2011), nativetrees (Hinton et al., 2005), natural hyper-accumulators (e.g., thenetted chain fern (Woodwardia areolata)) (Knox et al., 2008), andmicrobial metabolites (Turick et al., 2008) have been evaluated.

However, the chemical speciation of U and the molecularmechanisms responsible for its high retention in the SRS wetlandsediments reported by Kaplan and Serkiz (2001) are not sufficientlywell understood. What remains unclear is the strong U retentionmechanism, even under oxidizing conditions where U(VI) oxida-tion state is expected to predominate. The tendency of U to enterthe mobile phase and become available to local biota or be trans-ported from the contaminated area has not been well quantified.Additional information is also needed to facilitate the developmentof in situ treatment and remediation strategies of U-contaminatedSRS wetland sediments. The objective of this work was to identify(1) the chemical speciation of U; and (2) the possible molecularmechanisms responsible for the high U retention in SRS wetlandsediments under oxidizing conditions.

2. Materials and methods

The general approach was to conduct laboratory batch tests tomeasure U sorption as a function of pH with SRS uncontaminatedwetland sediments, and then to use sequential extraction, U L3-edge XANES spectroscopy, fluorescence mapping and m-XANES toevaluate the U speciation of spiked laboratory samples and U-contaminated wetland sediments.

2.1. Artificial groundwater

The recipe for an artificial groundwater (AGW) solution wasbased on the monitoring survey results for uncontaminatedgroundwater samples at the SRS (Strom and Kaback, 1992). Briefly,the AGW has a pH of w6.0, electrical conductivity of 0.026 mS/cm,turbidity of <1 NTU (Nephelometric Turbidity Units), 1.25 mg/L Na,0.25mg/L K. 0.93mg/L Ca, 0.66 mg/LMg, 5.51mg/L Cl, and 0.73mg/L SO4, which mimics the non-impacted SRS subsurface ground-water conditions (Li et al., 2012).

2.2. SRS wetland sediments

Contaminated and uncontaminated surface sediment samples(0e0.15m depth below the leaf-litter level) were collected from thewetland site, which is adjacent to a pilot-scale nuclear facility thatreleased processing wastes including U and Th into an unlinedseepage basin between 1958 and 1980. The basin contents enteredthe adjacent wetland by subsurface flow and overland flow. Thecharacteristics of the contaminated and uncontaminated sedimentsare summarized in Table 1 (Kaplan and Serkiz, 2001). The miner-alogy of both the contaminated and uncontaminated sediments

was characterized by primary silica sand, lesser kaolinite, hydroxy-interlayered vermiculite, illite and gibbsite, and little Fe oxides.

2.3. Batch experiments

Batch U(VI) sorption experiments as a function of pH (2.5e10)were set up at a constant total U(VI) concentration (2 � 10�5 M) inAGW solution with or without 0.1 M NaNO3 under ambient atmo-spheric CO2 (PCO2 ¼ 10�3.5 atm) and temperature (22 �C). For eachset of experiments, a solid-free control was included in doublet. Thepurpose of these controls was to determine the initial U concen-tration for Kd calculation (described below) and to provide anindication if any U sorption to the tube walls occurred during theexperiment. About 0.1 g of the uncontaminated sediment and10 mL solution were added into a 15 mL polypropylene centrifugetube while exposed to air. The resulting solid concentration was10 g/L. Uranyl nitrate (238U, Electron Microscopy Sciences, Hatfield,PA) was used to make the U stock solution (2 � 10�3 M, pH 3.6, Eh433 mV). After spiking 0.1 mL of the U stock solution and then pHadjustment using 0.1 M NaOH or 0.1 M HNO3 solution, the sus-pensions were placed on a shaker for 96 h sorption reaction. Forsome batch experiments, all tubes were open to atmospheric CO2twice per day and 30 min each time to promote equilibration withatmospheric CO2. The pH values were adjusted daily until the pHshifts were <0.1 pH unit (a Radiometer Copenhagen PHM 95 pHmeter). Each suspensionwas filtered using 0.2 mmnylonmembranesyringe filters. The filtrate was acidifiedwith 2% HNO3 and analyzedfor U by inductively coupled plasma mass spectrometry (ICP-MS)according to EPA Method 200.8, Rev. 5.4 (USEPA, 1994). The ICP-MSanalyses had an uncertainty of �10%, but our repeatability testindicated that this uncertainty was often within �5%. The extent ofU sorption to the sediments was calculated using a distributioncoefficient, Kd value, which is a U concentration ratio of sediment tosolution. The Kd values (mL/g) were calculated using Equation (1):

Kd ¼ C0 � CC

� VM

(1)

where C0 is the initial U concentration in the control samples and Cis the final U concentration in solutions, V is the volume of thesolution (mL) and M is the mass of the sediment (g).

2.4. Sequential extraction

Sequential extraction of contaminated and U-sorbed uncon-taminated sediments was carried out using the method of Milleret al. (1986), which was slightly modified in American Society forTesting and Materials (ASTM) D3974-81 (ASTM, 1999) and otherliterature protocols (Baston et al., 1996; Kaplan and Serkiz, 2001;

Table 2Retention of U in the whole contaminated sediments from sequential extraction.

Contaminatedsediment#1

Contaminatedsediment#2

235U 238U 235U 238U

Acid extractable(% fraction)

44 36 27 25

Organic fraction(% fraction)

51 37 70 57

AmorphousFe/Al oxides (% fraction)

0 6 0 3

CrystallineFe/Al oxides (% fraction)

5 3 3 2

Structural(% fraction)

0 17 0 14

Total Uconcentration, (mg/kg)

2.13 285 1.81 329

Fig. 1. U L3-edge XANES spectra of contaminated sediment (<53 mm; filled circles), itsblack NOM (<53 mm; empty squares), in comparison with the spectra of uraninite(UO2; U(IV)) (solid line) and uranyl acetate (U(VI)) (dash line).

D. Li et al. / Journal of Environmental Radioactivity 131 (2014) 40e4642

Sowder et al., 2003). In brief, 0.44 M acetic acid (CH3COOH) þ 0.1 MCa(NO3)2, 0.1 M Na pyrophosphate (Na4P2O7), 0.175 M ammoniumoxalate (NH4)2C2O4 þ 0.1 M oxalic acid (H2C2O4), 0.29 M Nadithionite in 0.15 M sodium citrateþ 0.05 M citric acid buffer, and 1part HNO3 þ 3 parts HCl þ 1 part H2O were used for sequentialextraction of uranium associated with acid soluble fraction, organicfraction, amorphous Al/Fe oxides, crystalline Al/Fe oxides andstructural fractions of the sediments, respectively.

2.5. U L3-edge XANES measurements

U L3-edge XANES spectra of the contaminated sediments(<53 mm fraction) and the U-sorbed uncontaminated sediments(<53 mm batch) were collected using the Hard X-ray MicroAnalysisbeamline (06ID-1) at the Canadian Light Source (Jiang et al., 2007).The beamline was equipped with Si(111) double crystal mono-chromators and Rh mirrors. The monochromators were calibratedusing the first inflection point at 17,038 eV of the K-edge of yttrium(Y) metal foil that was mounted between two N2-filled ionizationchambers downstream of the sample. The 50e100 mg air-driedpowder sample was pressed into a disk pellet with a 6.3 mmdiameter and sealed by Kapton tape. The U L3-edge XANES spectraof all sediment samples and uranyl acetate standard (Electron Mi-croscopy Sciences, Hatfield, PA; mixed with silica powder tow500 mg/kg U) were collected using a 32-element solid state Gefluorescence detector. The spectrum of UO2 standard (Alfar Aesar,Ward Hill, MA) was collected using transmission mode. Eight layersof aluminum foils were used to eliminate Fe signals, and a Sr-3 solidstate filter to screen other impurity signals near U L3-edge. Thestorage ring was operated at 140e200 mA during the experimentalmeasurements. The U L3-edge XANES spectra were recorded in theenergy range of 17,000e17,470 eV (beam spot size of 0.8� 1.5 mm2,energy increment of 0.5 eV, and counting time of 2 s per data pointnear the edge region) at room temperature. All the collected spectrawere processed using Athena and the presented spectra werebackground removed and normalized (Ravel and Newville, 2005).

2.6. Fluorescence mapping and U L3-edge m-XANES

Fe and U fluorescence maps and U L3-edge m-XANES of the SRScontaminated sediment were obtained at the beamline 13ID-E atthe Advanced Photon Source (Argonne National Laboratory,Argonne, IL). The same bulk sediment sample as above was placedinto the Peltier-cooled sample stage (�18 �C) for this experiment.The beam sizes of 3 � 3 mm were used for both fluorescencemapping and m-XANES measurement. Monochromatic X-ray en-ergies were scanned from 17,000 to 17,500 eV using 0.4 eV stepswithin the main edge, and coarse (2e5 eV) steps over the pre-edgeand post-edge regions. The double crystal monochromator Si(111)calibration was done with Y foil, with its absorption edge taken as17038.0 eV. The storage ring was operated at 120� 5mA during theexperimental measurements.

3. Results

3.1. Retention and chemical species of U in the contaminated SRSwetland sediment

The sequential extraction results of the contaminated (285 mg/kg 238U) and uncontaminated (0.57 mg/kg 238U) sediments aresummarized in Table 2, where the percentage of each fraction wascalculated relative to the total U concentration. For the contami-nated sediment, U was primarily associated with acid soluble andorganic carbon fractions, and to a much smaller extent with the

amorphous Fe/Al oxide, crystalline Fe/Al oxide, and structuralfractions.

Fig. 1 shows the U L3-edge XANES spectra of the contaminatedsediment (<53 mm fraction), black NOM particles that were sepa-rated from the contaminated sediment and ground to <53 mm, andtwo standards, uraninite (UO2) and uranyl acetate, in the energyrange of 17,150e17,230 eV. With regard to the standards, threedistinctive features distinguish the uraninite (U(IV)) from the ura-nyl acetate (U(VI)) spectra as marked in Fig. 1: 1) the U L3-edge firstscattering maximum is sharper and shifts toward lower energy by2.5 eV for U(IV) than U(VI), which is similar to previously reportedspectra of uraninite and uranyl nitrate (Tokunaga et al., 2008; Wuet al., 2006); 2) there is a “shoulder” at w17,188 eV for U(VI); 3)the peak atw17,215 eV is sharper and stronger for U(IV) than U(VI).The spectra of the contaminated sediment (filled circles) and itsNOM particles (empty squares) are essentially indistinguishableand overlap directly on the uranyl acetate standard spectrum (dash

D. Li et al. / Journal of Environmental Radioactivity 131 (2014) 40e46 43

line). The linear combination fitting of the standard UO2 and uranylacetate spectra to the sediment spectra also indicated that the Uspecies in the contaminated sediment is U(VI), without detectableU(IV). The U(VI) uranyl species is likely bonding to organic carbonin NOM, which could be confirmed by the ongoing U L3-edgeextended X-ray absorption fine structure (EXAFS) data analysis.

Fig. 2 shows Fe fluorescence map (A), U fluorescence map (B), Uand Fe count correlation (C), and U L3-edge m-XANES spectra (D) ofthe SRS contaminated sediment. Yellow, red and blue colors indi-cated that the high, medium and low counts, respectively. In the Ufluorescence map (Fig. 2B), although there were several spots ofhigher counts (yellow), they were not isolated and their countswere just slightly higher (1.5e2 times) than the area in red.>95% ofuranium (red) was heterogeneously distributed across the detec-tion area (1 � 1 mm), indicating that U precipitation and/or sec-ondary Umineral formationwere not the major pathways for the Uretention in the SRS wetland sediments. The U distributionwas nothighly correlated with the Fe distribution (Fig. 2C). The U L3-edge m-XANES spectra of several spots (Fig. 2D) demonstrated that the Uspecies were primarily U(VI), and the linear combination fittingusing the UO2 and uranyl acetate spectra also indicated the pres-ence of little U(IV), in agreement with the U spectra of bulk sedi-ments (Fig. 1).

3.2. Retention of U onto the uncontaminated sediment

The Kd values of U onto the uncontaminated SRS wetland sedi-ment versus pH are shown in Fig. 3. The uncertainties of the pre-sented Kd values were estimated to be �30%. Fig. 3A shows that the

Fig. 2. Fluorescence micro-mapping and U L3-edge micro-XANES spectra of the SRS wetlandand Fe count correlation, D. U L3-edge m-XANES spectra.

U sorption profiles were repeated fairly well in two separate andidentical batch experiments (<2 mm uncontaminated sediment,AGW, not open except for pH adjustment). Fig. 3B shows that withthe AGW being amended with 0.1 M NaNO3, the sorption profile(filled square) was largely indistinguishable from the results inFig. 3A, indicating that ion exchange was unlikely a major mecha-nism for the U uptake. For the batch which was open twice a day topromote equilibrium conditions with atmospheric CO2, the Usorption profile (open square) was not notably different frombatches without daily opening (except for the pH adjustment).Fig. 3C compares the effect of different fractions of particle size onthe U sorption, which indicated a general expected trend thatgreater sorption occurred in the smaller fraction (<53 mm) (filledtriangle), and less sorption occurred in the larger size fraction (53e250 mm) (open diamond). This trend was especially apparent atlower pH levels.

In general, the U sorption profiles with the wetland sedimentwere similar to those with other pure phase minerals (Cheng et al.,2004; Hsi and Langmuir, 1985; Payne et al., 1998; Waite et al., 1994;Wan et al., 2011) and sediments (Dong et al., 2012). However, thiswetland sediment had two distinguishing aspects with its U sorp-tion trends. First, at pH > 8, the U sorption percentages or Kd valueswere higher than those measured with other solid phases (Hsi andLangmuir, 1985; Payne et al., 1998; Waite et al., 1994). Decreasingsorption in this elevated pH range is commonly attributed to theformation of uranylecarbonate complexes (Bernhard et al., 2001;Brooks et al., 2003; Duff and Amrhein, 1996; Kalmykov andChoppin, 2000). In our batch experiments, the solutions insidethe 15 mL centrifuge tubes might be not in a complete equilibrium

sediment. A. Fe fluorescence map (1 � 1 mm), B. U fluorescence map (1 � 1 mm), C. U

Fig. 3. Kd values of U onto the uncontaminated wetland sediment versus pH.

Fig. 4. Sequential extraction of U-sorbed uncontaminated sediments.

D. Li et al. / Journal of Environmental Radioactivity 131 (2014) 40e4644

with atmospheric CO2, even though the tubes were open to at-mospheric CO2 twice per day and 30 min each time. Second, atpH< 4, the U sorptionwas also much higher than for other systems(Hsi and Langmuir, 1985; Payne et al., 1998; Waite et al., 1994). Thismay be attributed to the presence of high concentrations of NOM inthese wetland samples. Uranyl and NOM form strong bonds, evenat low pH levels (Wan et al., 2011). This may partially explainwhy atpH < 4, the U sorption was much higher onto the fraction of<53 mm sediment than onto the 53e250 mm fraction, because theformer may contain much more NOM than the latter, in addition tothe particle size effect.

The pH value of the SRS wetland sediment is 4.53 (1:1 sedimentand deionized water; Table 1). At this pH, the Kd values of U onto the<2mmsedimentswasw1000mL/g (Kaplan and Serkiz, 2001);whilethe Kd values of U onto the <53 fraction of the sediment was 1.7e2.0 � 105 mL/g. These results indicated that U species in the SRSwetland should have relatively low mobility and bioavailability, incontrast to the tailing pondwithin the Kadji Saimining area (pH 7.5e8.7), Kyrgyzstan, where the U Kd values are 0.5e3.5�102mL/g and Uhas a relatively high mobility and bioavailability (Lind et al., 2013b;Salbu et al., 2013).

3.3. Chemical speciation of U in the U-sorbed uncontaminatedsediment

The U-sorbed uncontaminated sediments were further studiedusing sequential extraction. In a separate batch, the total sorptionpercentage (open diamonds) and the sequential extraction profileof U sorbed onto each fraction of this sediment are shown in Fig. 4,where the percentage of each fraction was calculated against thetotal spiked U concentration. The retained U is primarily associatedwith acid soluble and organic fractions, and lesser amounts wereassociated with the aqueous soluble or structural fractions. Theseresults are in agreement with the sequential extraction results ofthe contaminated sediment (Table 2).

The U L3-edge XANES spectra of the U sorbed uncontaminatedsediments (batch sorption data in filled triangle; Fig. 3C), in com-parison with that of uranyl acetate (empty circles) and the uncon-taminated sediment without U sorption (dash line), are shown inFig. 5. A very weak signal was recorded for the uncontaminatedsediment (0.57 mg/kg 238U), indicating that U L3-edge XANES candetect the U signal to the sub mg/kg level in such sediments, but therecorded spectrum does not provide as much structural or chemicalinformation as spectra derived from samples with greater U con-centrations. All theU L3-edgeXANES spectra of the sediment sampleswith U sorption were nearly identical to the uranyl acetate spectra,and the linear combination fitting of the standard UO2 and uranylacetate spectra yielded little detectableU(IV). These results suggestedthat the sorbedUspecies bedominantly theU(VI) species that is likelybonding to carbon in its second coordination shell, and probably tothe sediment minerals, which is in agreement with the sequentialextraction data and could be more explicitly confirmed by theongoing EXAFS data analysis.

4. Conclusions

The objective of this study was to evaluate the sorption reactionresponsible for the strong U retention to wetland sediments underoxidizing conditions. Uraniumwashighly retained in the SRSwetland

Fig. 5. U L3-edge XANES spectra of uncontaminated sediment (dash line) and its U-sorbed samples in the pH range of 2.8e9.4 (sorption data in filled triangle; Fig. 3C). Thespectrum of uranyl acetate is shown as empty circles for comparison.

D. Li et al. / Journal of Environmental Radioactivity 131 (2014) 40e46 45

sediments, which was not affected by higher ionic strength (0.1 MNaNO3) solutions. Uranium existed in the SRS wetland sedimentsprimarily as U(VI), with negligible amounts of detectable U(IV).Chemical or microbial reduction, ion exchange with sediment min-erals and secondary mineral formation are unlikely responsible forthe high U retention in the wetland sediments. Instead the strongretention appeared to be the result of U associationwith acid solubleand organic fractions of the wetland sediments. U L3-edge XANESspectraof the retainedUwerenearly identical to thatofuranyl acetate(U(VI)). These results might indicate that the primary molecularmechanisms for thehighUretention onto the SRSwetland sedimentswould beeither chemical sorption tohumate-likeNOMcolloids (Wanet al., 2011) oraccumulation in thewetlandplants (Hintonetal., 2005;Knox et al., 2008) that constitute a significant source of the NOM inthe SRS wetland sediments. Therefore it appears that wetland sedi-ments may be effective in retaining U, not only under reducing con-ditions, but also under oxidizing conditions. Furthermore, thesefindings indicate that U ismuch less likely to be released as a result ofseasonal changes in the oxidation state of the wetlands, therebyproviding greater sustained U retention under a wide range of envi-ronmental conditions.

Acknowledgments

This work was supported by Department of Energy, Environ-mental Management and Office of Sciences. Workwas conducted at

the Savannah River National Laboratory under the U.S. Departmentof Energy Contract DE-AC09-96SR18500. Participation of Drs. J.C.Seaman and H.S. Chang in the current study was supported by theSavannah River Ecology Laboratory through a Financial AssistanceAward DE-FC09-07SR22506 from DOE to the University of GeorgiaResearch Foundation. Work was conducted at Princeton Universityunder the U.S. Department of Energy Contract DE-SC0006847.Although EPA contributed to this article, the research presentedwas not directly performed by or funded by EPA and was notsubject to EPA’s quality system requirements. Consequently, theviews, interpretations, and conclusions expressed in this article aresolely those of the authors and do not necessarily reflect orrepresent EPA’s views or policies. U L3-edge XANES spectral mea-surements were performed at the Canadian Light Source, which issupported by the Natural Sciences and Engineering ResearchCouncil of Canada, the National Research Council Canada, the Ca-nadian Institutes of Health Research, the Province of Saskatchewan,Western Economic Diversification Canada, and the University ofSaskatchewan. GeoSoilEnviroCARS is supported by the NationalScience Foundation’s Earth Sciences (EAR-0217473), Department ofEnergy’s Geosciences (DE-FG02-94ER14466), and the State of Illi-nois. Use of the APS was supported by the U.S. Department of En-ergy, Office of Science, Office of Basic Energy Sciences, undercontract no. W-31-109-ENG-38. The authors appreciate SavannahHarris and Diana Soteropoulos, University of Georgia, for theirassistance with ICP-MS analysis.

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