当前位置:文档之家› 2009Microbial reduction of iron(III)-rich nontronite and uranium(VI)

2009Microbial reduction of iron(III)-rich nontronite and uranium(VI)

Microbial reduction of iron(III)-rich nontronite and uranium(VI)

Gengxin Zhang a ,John M.Senko b ,Shelly D.Kelly c ,Hui Tan a ,

Kenneth M.Kemner c ,William D.Burgos a,*

a

Department of Civil and Environmental Engineering,The Pennsylvania State University,212Sackett Building,University Park,

PA 16802-1408,USA

b

Department of Geological and Environmental Sciences,University of Akron,Akron,OH,USA

c

Biosciences Division,Argonne National Laboratory,Argonne,IL,USA

Received 27October 2008;accepted in revised form 23March 2009;available online 2April 2009

Abstract

To assess the dynamics of microbially mediated U-clay redox reactions,we examined the reduction of iron(III)-rich non-tronite NAu-2and uranium(VI)by Shewanella oneidensis MR-1.Bioreduction experiments were conducted with combina-tions and varied concentrations of MR-1,nontronite,U(VI)and the electron shuttle anthraquinone-2,6-disulfonate (AQDS).Abiotic experiments were conducted to quantify U(VI)sorption to NAu-2,the reduction of U(VI)by chemi-cally-reduced nontronite-Fe(II),and the oxidation of uraninite,U (IV)O 2(s),by nontronite-Fe(III).When we incubated S.oneidensis MR-1at lower concentration (0.5?108cell mL à1)with nontronite (5.0g L à1)and U(VI)(1.0mM),little U(VI)reduction occurred compared to nontronite-free incubations,despite the production of abundant Fe(II).The addition of AQDS to U(VI)-and nontronite-containing incubations enhanced both U(VI)and nontronite-Fe(III)reduction.While U(VI)was completely reduced by S.oneidensis MR-1at higher concentration (1.0?108cell mL à1)in the presence of nontr-onite,increasing concentrations of nontronite led to progressively slower rates of U(VI)reduction.U(VI)enhanced nontro-nite-Fe(III)reduction and uraninite was oxidized by nontronite-Fe(III),demonstrating that U served as an e?ective electron shuttle from S.oneidensis MR-1to nontronite-Fe(III).The electron-shuttling activity of U can explain the lack or delay of U(VI)reduction observed in the bulk solution.Little U(VI)reduction was observed in incubations that contained chemically-reduced nontronite-Fe(II),suggesting that biologic U(VI)reduction drove U valence cycling in these systems.Under the con-ditions used in these experiments,we demonstrate that iron-rich smectite may inhibit or delay U(VI)bioreduction.ó2009Elsevier Ltd.All rights reserved.

1.INTRODUCTION

Uranium pollution is a consequence of the nuclear age and is problematic because of its complicated speciation and varied sorption a?nity for sediment minerals.Poorly soluble U(IV)minerals and highly soluble U(VI)species govern uranium mobility in groundwater and sediments (Grenthe et al.,1992).Soluble U(VI)species can be biolog-ically or chemically reduced to the sparingly soluble U(IV)mineral uraninite to retard uranium transport.Bacterial reduction of U(VI)to U(IV)may be exploited for in situ

remediation of contaminated sites (Anderson et al.,2003;Istok et al.,2004;Wu et al.,2006a,b ).The low solubility of uraninite and relatively high rates of bacterial U(VI)reduction and its potential cost-e?ectiveness makes biore-mediation an attractive approach for large sites with rela-tively low levels of contamination (Brooks et al.,2003).Dissimilatory metal-reducing bacteria (DMRB)gain en-ergy by utilizing a variety of oxidized metals such as Cr(VI),Fe(III),Mn(VI),Tc(VII)and U(VI)as terminal electron acceptors (TEAs)with organic acids,alcohols or H 2as elec-tron donors (Lovley,1993;Nealson and Sa?arini,1994).Iron is the fourth most abundant element and the most dominant redox active metal in the Earth’s crust.Iron-bear-ing clay minerals are widely distributed in soils and sedi-ments (Schwertmann and Cornell,2000),and represent a

0016-7037/$-see front matter ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.gca.2009.03.030

*

Corresponding author.Fax:+18148637304.E-mail address:wdb3@https://www.doczj.com/doc/9815050659.html, (W.D.Burgos).

https://www.doczj.com/doc/9815050659.html,/locate/gca

Available online at https://www.doczj.com/doc/9815050659.html,

Geochimica et Cosmochimica Acta 73(2009)

3523–3538

large mass of electron acceptor Fe(III)for DMRB(Kostka et al.,2002).In subsurface sediments containing U(VI), DMRB may encounter multiple electron acceptors,and when evaluating the potential for in situ uranium remedia-tion,it is important to understand how the presence of alternative electron acceptors such as nitrate,Mn(III/IV) oxides,and Fe(III)oxides a?ect U(VI)remediation(Wiel-inga et al.,2000).

Smectite is a dioctahedral layer phyllosilicate that is ubiquitous in soils and sediments.Structural Fe in smectite often accounts for about half of the Fe pool in soils and sediments(Favre et al.,2006).The Fe(II)/(III)redox couple generally has a higher half-cell reduction potential than the U(IV)/(VI)redox couple(Table1),such that Fe(III)may be a more thermodynamically favorable TEA than U(VI) (Fredrickson et al.,2000).However,the actual potentials are dependent upon several factors such as mineralogy,spe-ciation,solubility,concentration,and pH.For example, based on reduction potentials,Fe(III)(oxyhydr)oxides may be respired in preference to uranyl carbonate com-plexes that may in turn be respired in preference to struc-tural Fe(III)in iron-rich smectites.However,bioreduction of soluble U(VI)species may be physiologically easier (and therefore kinetically favored)compared to bioreduc-tion of solid-phase ferric oxides or structural Fe(III)in phyllosilicates.In addition to thermodynamic and kinetic constraints,di?erences in the mass of Fe(III)(e.g.,%-levels) and U(e.g.,l M-levels)will exert an e?ect on TEA utiliza-tion.At present,it is di?cult to predict the fate of U(VI)as microbial metal reduction proceeds in iron(III)-rich clay-containing sediments.

In addition to‘‘direct”microbial reduction of U(VI),the ‘‘indirect”abiotic reduction of U(VI)by various reactive forms of Fe(II)(or other reductants)may be an equally important process.Experiments have demonstrated that U(VI)can be reduced by Fe(II)sorbed to a variety of iron oxides(Liger et al.,1999;Fredrickson et al.,2000;Jeon et al.,2004b;Jang et al.,2008),Fe(II)-containing natural sediments(Behrends and Van Cappellen,2005;Jeon et al., 2005),Fe(II)-containing carboxyl-functionalized micro-spheres(Boyanov et al.,2007),structural Fe(II)in green rust (O’Loughlin et al.,2003),and structural Fe(II)in magnetite (Missana et al.,2003).Experiments have also demonstrated that structural Fe(II)in specimen and natural smectites can reduce nitroaromatic compounds(Hofstetter et al.,2003, 2006),chlorinated aliphatics(Sorensen et al.,2005),pesti-cides(Elsner et al.,2004),Cr(VI)(Gan et al.,1996;Taylor et al.,2000),and Tc(VII)(Peretyazhko et al.,2008).

The objective of this research was to study the interac-tions between uranium(VI)and the iron(III)-rich smectite

Table1

Standard-state reduction potentials(E0)for iron oxides,nontronite,uranium,technetium,and experimental components.

Redox couple(eq#)E0(V) Ferrihydrite,Fe(OH)3

Fe(OH)3(s)+2H+(aq)+eà+HCOà3(aq)?FeCO3(s)+3H2O(l)0.971a Goethite,a-FeOOH(s)

a-FeOOH(s)+2H+(aq)+eà+HCO3(aq)?FeCO3(s)+2H2O(l)0.848a Pertechnetate,TcO4à

TcO4à(aq)+4H+(aq)+3eà?TcO2án H2O(s)+(2àn)H2O(l)0.748b Nontronite SWa-1(Eq.(2))

mFe3+(s)+w0Na+(s)+n i(OHà)(s)+meà+pNa+(aq)+n i H+(aq)?mFe2+(s)+wNa+(s)+n i H2O(l)0.70c Uranyl carbonate,UO2(CO3)34à

0.5UO2(CO3)4à3(aq)+1.5H+(aq)+eà?0.5UO2(s)+1.5HCOà3(aq)0.687a Ca–U(VI)–CO3complex,CaUO2(CO3)32à

0.5CaUO2(CO3)32à(aq)+1.5H+(aq)+eà?0.5UO2(s)+1.5HCOà3(aq)+0.5Ca2+0.576d Nontronite NAu-2(Eq.(1))

Na.71Mg.05Fe3.83Al.5Si7.55O20(OH)4(s)(NAu-2)+3.83eà+11.8H+(aq)?0.71Na+(aq)+0.05Mg2+(aq)

+3.83Fe2+(aq)+0.5Al(OH)4à(aq)+7.55SiO2(aq)+6.9H2O(l)

0.60e

Ca2–U(VI)–CO3complex,Ca2UO2(CO3)3(aq)

0.5Ca2UO2(CO3)32à(aq)+1.5H+(aq)+eà?0.5UO2(s)+1.5HCOà3(aq)+Ca2+0.424c Anthraquinone-2,6-disulfonate,AQDS

0.5AQDS(aq)+H+(aq)+eà(aq)?0.5AH2DS(aq)0.230f Acetate/lactate,CH3COOà/C2H5COOà

0.25CH3COOà(aq)+0.25HCOà3(aq)+1.25H+(aq)+eà(aq)?0.25C2H5COOà(aq)+0.5H2O(l)0.121f

a Fredrickson et al.(2002).

b Peretyazhko et al.(2008).

c Favre et al.(2006).

d Brooks et al.(2003).

e Jaisi et al.(2007b).

f Fredrickson et al.(2000).

3524G.Zhang et al./Geochimica et Cosmochimica Acta73(2009)3523–3538

nontronite NAu-2during their concomitant biological reduction.Bioreduction experiments were designed to mea-sure the e?ects of variable concentrations of either U(VI)or nontronite-Fe(III)and the presence/absence of the electron shuttling compound anthraquinone-2,6-disulfonate (AQDS)on the net reduction of all TEAs.Abiotic reduc-tion experiments were conducted to measure the reduction of U(VI)by chemically-reduced nontronite-Fe(II),and the oxidation of biogenic uraninite by unaltered nontronite-Fe(III)to determine the relative importance of other oper-ative redox processes.The sorption of U(VI)to nontronite was measured as a function of pH and ionic strength to quantify the distribution of aqueous and nontronite-sorbed U(VI)in these systems.Wet chemical methods were used to measure reaction kinetics and reaction products were char-acterized by scanning electron microscopy,X-ray di?rac-tion,and X-ray absorption spectroscopy.

2.EXPERIMENTAL

2.1.Cell cultivation

Shewanella oneidensis MR-1(referred to hereafter as MR-1)was cultured in a chemically de?ned minimal med-ium(McDonough,2006;Burgos et al.,2008).MR-1cells were harvested by centrifugation(15min and20°C at 3500g),washed three times with anoxic30mM NaHCO3 bu?er(pH6.8,prepared under an80:20%N2:CO2atm) and resuspended in the same bu?er.Cell density was mea-sured by absorbance at600nm and correlated to cell num-ber by acridine orange direct counts.

2.2.Mineral preparation

Nontronite NAu-2,an iron-rich variety of smectite from Uley graphite mine,South Australia(Keeling et al.,2000) was purchased from the Source Clays Repository(West Lafayette,IN).The formula for NAu-2is M+0.72-(Si7.55Al0.16Fe0.29)(Al0.34Fe3.54Mg0.05)O20(OH)4where M may be Ca,Na or K(Gates et al.,2002).A clay fraction (0.5–2.0l m)was suspended in1M NaCl for one week,sep-arated in distilled water by centrifugation,washed repeat-edly until no Clàwas detected by silver nitrate,and then air-dried.This Na-saturated nontronite was pure without any other mineral phases(as determined by XRD)and con-tained23.4%total iron(4.2mmol Fe/g)in its structure with almost all(99.4%)iron as Fe(III)(Keeling et al.,2000).The majority of structural Fe(III)in NAu-2is contained within octahedral layers(92%Fe T)and the remainder in tetrahe-dral layers(8%Fe T)(Gates et al.,2002;Jaisi et al.,2005). The BET surface area of the air-dried nontronite was deter-mined to be42.5m2/g based on N2adsorption.

2.3.Uranium–nontronite adsorption experiments

Uranyl nitrate was dissolved in distilled deionized water to produce a stock concentration of1.0mM U(VI).Sorp-tion of uranyl onto nontronite was measured as a function of pH at10–14duplicated pH values(pH2.5–9.0)and four ionic strengths(0.001,0.01,0.1and1M NaNO3).For each batch system(50mL Te?on centrifuge tubes),uranyl ni-trate stock solution was added to achieve an initial concen-tration of0.01mM U(VI)that reacted with a?nal nontronite concentration of3.0g Là1.Variable amounts (l L quantities)of0.01M HNO3or0.01M NaOH were added to each tube to give the desired range in equilibrium pH values.All experiments were conducted at room tem-perature under atmospheric conditions.All tubes(ca. 20mL suspension,30mL air)were equilibrated with atmo-spheric P CO

2

through a combination of passive mixing and active purging throughout the10–14days sorption period. Tubes were mixed by end-over-end rotation(7rpm)except when periodically placed on a shaker table(80rpm)with loosened caps.Tubes were also periodically purged with?l-ter-sterilized air during the sorption period.After equilibra-tion,suspensions were centrifuged for10min at14,100g and room temperature.The pH of the supernatant was measured using a combination electrode(Fisher Scienti?c), and the quantity of U(VI)sorbed was calculated from the di?erence between initial total and?nal aqueous U(VI)con-centrations(measured by kinetic phosphorescence analysis (KPA)as described below).No loss of U(VI)from solution was measured in nontronite-free controls.

Sorption of uranyl onto nontronite was measured as a function of[U(VI)]in anoxic30mM NaHCO3to quantify the distribution of U(VI)in the bioreduction experiments. Sorption was measured in duplicate at10initial total U(VI)concentrations ranging from0.025to1.5mM with a constant nontronite concentration of5.0g Là1.Serum bottles were equilibrated for10days(under80:20% N2:CO2atm),and the quantity of U(VI)sorbed was calcu-lated from the di?erence between initial total and?nal aqueous U(VI)concentrations(measured by KPA).

2.4.Bacterial reduction experiments

MR-1bioreduction experiments were conducted in the presence or absence of U(VI),nontronite or anthraqui-none-2,6-disulfonate(AQDS)to examine the e?ects of mul-tiple electron acceptors on concomitant U(VI)and Fe(III) reduction.Uranyl acetate was dissolved in anoxic30mM NaHCO3to produce a stock concentration of25mM U(VI).Speciation modeling using PHREEQC(Parkhurst and Appelo,1999)con?rmed that U(VI)was undersatu-rated with respect to schoepite(UO2(OH)2áH2O(schoepite)+ 2H+=UO22++3H2O;log K=5.39)for concentrations >25mM U(VI)in this bu?er.Sterile nontronite suspensions (100g Là1)were prepared in anoxic30mM NaHCO3and served as stock suspensions for subsequent experiments. Nontronite suspensions were sterilized by a5-min exposure to microwave radiation(Keller et al.,1988),and sterility was con?rmed by lack of bacterial growth in tryptic soy broth following a48h aerobic incubation at30°C.AQDS was dissolved in anoxic30mM NaHCO3to produce a stock concentration of1.0mM.Variable amounts of uranyl acetate,nontronite or AQDS stock solutions were added to anoxic30mM NaHCO3in120mL glass serum bottles. Nontronite and uranyl acetate were equilibrated for two weeks before inoculation with MR-1.Nontronite concentrations ranged from0to5.0g Là1,uranyl acetate

Bioreduction of nontronite and uranium3525

concentrations ranged from0to1.5mM,and AQDS con-centrations were0or0.10mM depending on the experi-ment.MR-1was inoculated at either0.5?108or 1.0?108cells mLà1(?nal concentrations)with sodium lac-tate(5mM)provided as the electron donor.Cell-free abi-otic controls were prepared along with every experiment. All incubations were conducted in anoxic30mM NaHCO3 at room temperature in the dark with no mixing except for sample collection.After cells were added,samples were periodically removed with sterile needle and syringe and 0.5N HCl-extractable Fe(II),ferrozine-extractable Fe(II), aqueous Fe(II),NaHCO3-extractable U(VI),and aqueous U(VI)concentrations were quanti?ed as described below. All sample manipulations were performed inside an anoxic chamber(95:5%N2:H2atm;Coy Laboratory Products Inc.; Grass Lake,MI).

2.5.Experiments with biogenic uraninite and nontronite

Biogenic uraninite(UO2(s))precipitates produced by MR-1in experiments conducted with1.0mM uranyl ace-tate as the sole electron acceptor(5mM sodium lactate in 30mM NaHCO3bu?er)were collected after a50days incu-bation period.The cell-uraninite precipitates were pasteur-ized(70°C for30min)to deactivate biological activity, concentrated by centrifugation,and resuspended in anoxic 30mM NaHCO3.Nontronite was dispensed into anoxic 30mM NaHCO3in20mL glass serum bottles(5.0g Là1, 99.4%Fe(III)).Abiotic uranium(IV)oxidation experiments were initiated by the addition of0.14mM U(IV)as urani-nite.Reactors were not mixed mixing except for sample col-lection.Samples were periodically removed with sterile needle and syringe(in anoxic chamber)and NaHCO3-extractable U(VI)and aqueous U(VI)concentrations were measured.A nontronite-free control was prepared along with this experiment.

2.6.Experiments with chemically-reduced nontronite

Uranyl acetate was reacted with chemically-reduced nontronite to evaluate the rate and extent of U(VI)reduc-tion by nontronite-Fe(II).Low-temperature oxygen traps were used to remove trace concentrations of dissolved oxy-gen from all reactant solutions and experimental reactors (Jeon et al.,2004a).Nontronite was reduced using the so-dium citrate,bicarbonate,and dithionite(CBD)method as described by Stucki et al.(1984).Chemically-reduced nontronite was washed three times with anoxic distilled water as described by Hofstetter et al.(2003)and resus-pended in anoxic30mM NaHCO3to yield a CBD-reduced nontronite stock suspension of25g Là1.The CBD reduc-tion period was purposefully controlled(Komadel et al., 1990)such that27%of the Fe(III)in nontronite was re-duced to0.5N HCl-extractable Fe(II),a reduction extent comparable to the maximum extent in our bioreduction experiments.CBD-reduced nontronite was dispensed into anoxic30mM NaHCO3in20mL glass serum bottles (2.5g Là1?nal concentration,2.8mM0.5N HCl-extract-able Fe(II)),and equilibrated at least3days before uranium addition.Abiotic uranium(VI)reduction experiments were initiated by the addition of0.23mM uranyl acetate to these bottles.Samples were periodically removed with sterile nee-dle and syringe(in anoxic chamber)and0.5N HCl-extract-able Fe(II),ferrozine-extractable Fe(II),aqueous Fe(II), NaHCO3-extractable U(VI),and aqueous U(VI)concentra-tions were measured.All experiments were conducted in the dark at room temperature with no mixing except for sample collection.A uranium-free control was prepared along with this experiment.

2.7.Analytical techniques

Aqueous Fe(II)was measured after centrifugation for 10min at14,100g and20°C,and analyzed using the ferro-zine assay(Stookey,1970;Royer et al.,2002).Ferrozine-extractable Fe(II)was measured after0.1mL of well-mixed suspension was added to0.9mL of anoxic ferrozine solu-tion for2h,centrifuged,and analyzed using the ferrozine assay.The ferrozine reagent(1.96mM ferrozine in50mM Hepes,pH8.0)does not dissolve nontronite(data not shown).We assume that ferrozine can extract Fe(II)ad-sorbed to clay surfaces,a fraction of the Fe(II)cation-ex-changed in the clay interlayers,and the Fe(II)adsorbed to cell surfaces.Previous studies have shown that ferro-zine-extractable Fe(II)includes both dissolved Fe(II)and adsorbed Fe(II)(Sorensen and Thorling,1991;Myers and Myers,1994;Jeon et al.,2003).We have operationally de-?ned the di?erence between ferrozine-extractable and aque-ous Fe(II)concentrations as‘‘surface-adsorbed Fe(II).”

0.5N HCl-extractable Fe(II)was measured after0.5mL of suspension was added to0.5mL of1N HCl for24h, centrifuged,and analyzed using the ferrozine assay.In pre-vious studies it has been assumed that0.5N HCl can ex-tract essentially all of the Fe(II)from the clay structure (Kostka et al.,1999a,b;O’Reilly et al.,2006;Furukawa and O’Reilly,2007;Jaisi et al.,2007a;Zhang et al., 2007a,b),however,Jaisi et al.(2007b)have reported that 0.5N HCl extracted only73%of total biogenic Fe(II)in NAu-2compared to a HF/H3PO4total dissolution method. Therefore,using select split samples,we compared the extraction e?ciency of0.5N HCl to valence state determi-nations based on Fe X-ray absorption near edge structure (XANES)spectroscopy(described below).For bioreduced and reoxidized samples(NUR and NUO),the Fe(II)/ {Fe(II)+[Fe(III)]}ratios based on0.5N HCl extraction were equal to approximately74%of the Fe(II)ratios deter-mined by linear combination?tting Fe XANES analysis (Table2).Therefore,we assume that0.5N HCl can extract only ca.74%of the Fe(II)from the clay structure,in good agreement with Jaisi et al.(2007b).

Aqueous U(VI)was measured after centrifugation for 10min at14,100g and20°C.NaHCO3-extractable U(VI) was measured in samples of well-mixed suspensions that were placed in1M anoxic NaHCO3(pH8.4)(all sample collection and manipulations performed in anoxic cham-ber)(Elias et al.,2003;Zhou and Gu,2005).After extrac-tion for1h,solids were removed by centrifugation and U(VI)was measured in the supernatant.U(VI)was mea-sured by kinetic phosphorescence analysis on a KPA-11 (ChemChek Instruments,Richland,WA)(Brina and Mill-

3526G.Zhang et al./Geochimica et Cosmochimica Acta73(2009)3523–3538

er,1992).Adsorbed U(VI)was operationally de?ned as the di?erence between NaHCO3-extractable and aqueous U(VI)concentrations,divided by the nontronite concentra-tion.U valence state determinations based on U XANES spectroscopy were in good agreement with the1M NaH-CO3extraction data(Table2).

2.8.Electron microscopy

Scanning electron microscopy(SEM)samples were pre-pared in an anoxic glovebox following a previously pub-lished procedure(Zhang et al.,2007a).Brie?y,cell-mineral suspensions were?xed in anoxic2.5%glutaralde-hyde,placed on a glass cover slip,and mineral particles were allowed to settle onto the cover slip for15min.The particle-coated cover slips were gradually dehydrated in an ethanol series followed by critical point drying(CPD). All sample preparation,except CPD,was performed in an anoxic chamber.Cover slips were mounted onto a SEM stub and Au coated for observation using a Zeiss Supra 35FEG-VP SEM at an accelerating voltage of10–15kV.

A short working distance(6–10mm)and low beam current (30–40mA)were used to achieve the best image resolution.

A longer working distance(8.0mm)and higher beam cur-rent(50–70mA)were used for energy dispersive spectros-copy(EDS)analysis.

2.9.X-ray absorption spectroscopy

XAS measurements were made at the Materials Re-search Collaborative Access Team(MRCAT)sector10-ID beamline(Segre et al.,2000)of the Advanced Photon Source at Argonne National Laboratory(ANL).The XAS spectra were collected in transmission mode using quick-scanning of the monochromator.XANES spectra from the cell-uranium-nontronite precipitates were used to determine the average valence state of uranium and iron within the samples.For uranium and iron,the X-ray absorption edge energy was calibrated by collecting the ref-erence spectrum from hydrogen uranyl phosphate(U(VI) Std)and an iron foil(Fe(0)Std),respectively,during the collection of each spectrum.All data sets were accurately aligned in energy using the derivative of the edge of the U(VI)or Fe(0)standard.A linear combination?tting (LCF)of the U(VI)Std and a biogenic nanoparticulate UO2standard(U(IV)Std)(O’Loughlin et al.,2003)was used to determine the approximate U(VI)to U(IV)ratio in the sample spectra.This approach assumes that the stan-dard spectra represent the uranyl species in the samples. This assumption can lead to a systematic uncertainty for these samples because we do not know the U(VI)species. Similarly,the Fe XANES spectra were modeled with linear combinations of an Fe(II)standard spectrum from a fully reduced Fe(II)-nontronite NAu-2and an Fe(III)standard spectrum from an unaltered fully oxidized Fe(III)-nontro-nite NAu-2.The Fe(II)–NAu-2standard was produced by CBD reduction which can reduce100%of the Fe in nontr-onite(Jaisi et al.,2007b).To further validate the valence state of the Fe in the Fe(III)-nontronite,the spectrum was modeled with a combination of Fe(III)oxide standards.

Samples(approximately40mg cell-uranium-nontronite precipitates)were mounted as a moist paste on?lter paper and covered with Kapton?lm and sealed with Kapton tape. All sample preparation was performed in a Coy anoxic chamber and all samples were stored in the chamber prior to analysis.Samples mounted in the holders were exposed to the atmosphere for less than1min before being mounted for XAS measurements in a free-?owing N2environment to limit possible sample oxidation.These XAS sample holders have been shown to maintain anoxic integrity when ex-posed to an oxic environment for at least8h(O’Loughlin et al.,2003).Uranium and iron XANES spectra were col-lected in30s intervals consecutively for5min.No radia-tion-induced changes to the XANES spectra were observed at the30s intervals of data collection.

3.RESULTS

3.1.U(VI)speciation and nontronite dissolution

The speciation of uranyl(VI)is sensitive to the concen-trations of dissolved calcium and magnesium(Fig.1; Brooks et al.,2003;Dong and Brooks,2006,2008).While nontronite NAu-2was prepared in a Na-saturated form, Ca and Mg were detected in nontronite suspensions

(5.0g NAu-2Là1;anoxic30mM NaHCO3bu?er,pH

6.8)before and after inoculation with Shewanella putrefac-iens CN32in previous experiments very similar to those

Table2

Average valence states of Fe and U within uranium-nontronite samples as determined by wet chemical methods and linear combination?tting of the XANES spectra.Sample names correspond to XANES spectra presented in Fig.9.

Sample name/description%Fe(II)in nontronite%U(IV)in precipitates

0.5N HCl extraction Fe XANES1M NaHCO3extraction U XANES NUC0.38±3.00±10à3.5±4.10±10

5g Là1nontronite NAu-2+1mM uranyl(VI)acetate

+0MR-1(abiotic control)incubated for52days

NUR28.3±1.638±1082.7±2.790±10

5g Là1nontronite NAu-2+1mM uranyl(VI)acetate

+1?108cell mLà1MR-1incubated for52days

NUO13.3±0.218±1030.6±0.820±10 NUR sample reacted with dissolved oxygen for20h

Bioreduction of nontronite and uranium3527

presented here(Table3;Zhang et al.,2007a).PHREEQC (Parkhurst et al.,1999)was used to model U(VI)speciation in30mM NaHCO3with variable concentrations of Ca and Mg–0mM Ca and0mM Mg assumed in the absence of nontronite;0.06mM Ca and0.68mM Mg measured with nontronite at the start of the bioreduction period,and;

0.27mM Ca and1.16mM Mg measured with nontronite after a23days bioreduction period with CN32.Details of the U(VI)speciation modeling are included in Electronic Annex EA-1.Ca and Mg were not measured as part of our current experiments conducted with S.oneidensis MR-1.With the lower concentrations of Ca and Mg mea-sured before bioreduction,UO2(CO3)34àwas the predomi-nant U(VI)species accounting for39–67%of dissolved U(VI)species for total U concentrations of10–1000l M, respectively(Fig.1b).With the higher concentrations of Ca and Mg measured after bioreduction,CaUO2(CO3)32àbecame the predominant U(VI)species below ca.600l M U(VI)(Fig.1c).

0.5N HCl-extractable Fe(II)concentrations were mea-sured along with soluble Ca,Mg,Si,Al and Fe T during nontronite bioreduction by CN32(Table3;Zhang et al., 2007a).Based on these evolving concentrations of soluble Fe,Mg,Si and Al during bioreduction,congruent reductive dissolution of nontronite(Eq.(1)in Table1)likely did not occur.The?nal reduction extent reported in Table3is very similar to reduction extents measured in our current exper-iments(Figs.3–5).While a certain amount of Fe(II)was re-

3528G.Zhang et al./Geochimica et Cosmochimica Acta73(2009)3523–3538

leased into solution,the majority of Fe(II)was retained in the clay structure.As noted in Section2.7,we assume that 0.5N HCl can extract ca.74%of the Fe(II)from the non-tronite structure as supported by Fe XANES spectroscopy measurements for select samples(Table2).

3.2.U(VI)sorption onto nontronite

The sorption of uranyl onto nontronite was dependent on both the pH and ionic strength of the background elec-trolyte(Fig.2a).The pH e?ect was caused by both changes in uranyl speciation and nontronite surface charge as a function of pH.While U(VI)speciation is sensitive to Ca and Mg,because these metals were not measured as a func-tion of pH during the sorption experiments,Ca–U(VI)–CO3and Mg–U(VI)–CO3species were not included in reac-tion networks used for speciation modeling of the sorption edges(Electronic Annex EA-1).For a solution of10l M U(VI)in0.01M NaNO3(in equilibrium with atmospheric CO2),U(VI)speciation modeling demonstrated that cat-ionic uranyl species were predominant up to ca.pH5.5, and that anionic uranyl species were predominant at pH values above ca.pH6.0.Nontronite contains both a perma-nent negative layer charge and a pH-dependent edge charge.Structural Fe(III)in the octahedral layer(92% Fe T)imparts no layer charge(Fe3+substituted for Al3+), however,structural Mg(II)substituted for Al3+imparts a permanent negative charge.Structural Fe(III)in the tetra-hedral layer(8%Fe T)imparts a permanent negative charge (Fe3+substituted for Si4+)to basal siloxane surfaces.

The pH-dependent edge charge is located at the layer edges and is regulated by surface hydroxyl groups.The pH at zero point of charge(pH zpc)for nontronite has been reported to range from pH6.5(Merola et al.,2007)to pH 7.0(Jaisi et al.,2007a),therefore,nontronite was likely pre-dominantly positively charged below ca.pH5.5and pre-dominantly negatively charged above ca.pH8.0. Complexation to edge sites is not favorable at low pH be-cause H+can out-compete UO22+for sorption sites.Fur-thermore,the predominant sorption mechanism onto nontronite at low pH is presumed to be cation exchange to basal siloxane surfaces(Boult et al.,1998;Kowal-Fou-chard et al.,2004),consistent with our results that showed higher Na+concentrations suppressed UO22+at low pH. The sorption of uranyl to nontronite was relatively constant between pH6.0and8.0and independent of ionic strength, suggesting that neutral(UO2CO3)and polymeric ((UO2)2CO3(OH)3à)species could sorb strongly(e.g.,as in-ner sphere complexes)under these conditions.Catalano and Brown(2005)used EXAFS spectroscopy to show that UO2(CO3)4à3(aq)was preferentially bound to[Fe(O,OH)6] octahedral sites over[Al(O,OH)6]octahedral sites on layer edge sites of smectite.Because nontronite has very low alu-minum content in octahedral sites(Keeling et al.,2000),we speculate that[Fe(O,OH)6]octahedral layer edge sites play a predominant role in uranyl sorption to nontronite.

The extent of uranyl sorption onto nontronite at pH6.8 in anoxic30mM NaHCO3was measured to quantify the distribution of U(VI)under the conditions used in the bio-reduction experiments(Fig.2b).We assumed that0.06mM Ca and0.68mM Mg dissolved from nontronite under this one pH condition(Table3).Depending on the initial U(VI)concentration(0.025–1.5mM),13.3–24.6%of the to-tal U(VI)was sorbed onto nontronite(5.0g Là1).The pre-dominant uranyl species for all the sorption experiments in 30mM NaHCO3was UO2(CO3)34à(Fig.1b).Uranyl sur-face coverage achieved a maximum value of 1.73?10à6mole/m2(equivalent to1.04sites nmà2)at the highest U(VI)concentration tested.The maximum sorption density of Fe(II)onto nontronite(5.0g Là1)at pH6.8in anoxic30mM PIPES bu?er was reported to equal 17sites nmà2(Jaisi et al.,2007b),where surface area was measured by N2-BET.The di?erences in sorption density of Fe(II)versus U(VI)was likely due to the strong complex-ation of UO22+by carbonate under these conditions.Based on our review of the literature,the sorption of uranyl onto nontronite has been reported only one other time(Ames et al.,1983)(Table4).From results reported by Ames et al.(1982),we calculated a uranyl sorption density of 3.34?10à6mole/m2for their experiments conducted at pH7.7in Hanford groundwater with a natural smectite (experiments in equilibrium with atmospheric CO2).For uranyl sorption onto montmorillonite SWy-2,a sorption density of9.57?10à6mole/m2was reported for experi-ments conducted at pH7.1in100mM NaNO3(Pabalan and Turner,1997;Prikryl et al.,2001).

3.3.Bioreduction of U(VI)and nontronite

Shewanella oneidensis MR-1was able to reduce both uranyl(VI)and structural Fe(III)in nontronite when both of these TEAs were present(Fig.3;0.5?108cell mLà1). In the absence of nontronite or AQDS,U(VI)was removed relatively slowly from solution but was e?ectively removed to<5l M after21days.Cell viability was not measured over the course of these experiments,however,AQDS addi-tion demonstrated that cells were metabolically active after 21days(Fig.3a),and Fe(III)and U(VI)continued to be re-duced over50days incubation periods(Figs.4and5).The

Table3

Concentrations of select dissolved metals measured before and after bioreduction of nontronite by S.putrefaciens CN32(Zhang et al.,2007a).Experiments conducted with5.0g Là1nontronite NAu-2and5.0mM sodium lactate in anoxic30mM NaHCO3(pH 6.8;80:20%N2:CO2atm)incubated with0.5?108cell mLà1CN32 for23days.

Reaction time(days)023

Ca(mM)a0.060.27 Mg(mM)0.68 1.16 Si(mM)0.830.79 Al(mM)<0.001<0.001 Fe T(mM)0.0210.036

0.5N HCl-extractable Fe(II)(mM)reduction extent(%Fe(II)/Fe T)0.10 4.3 0.5%20.5%

pH 6.8 6.8

a Mg,Ca,Si,Al and Fe

T

measured by ICP-MS where samples

were?rst?ltered(0.2l m)and acidi?ed with conc.HNO3.Zhang

et al.(2007a).

Bioreduction of nontronite and uranium3529

addition of the soluble electron shuttle AQDS increased the rate of U(VI)removal.In the presence of nontronite(but absence of AQDS),U(VI)was not e?ectively removed at a cell density of0.5?108cell mLà1even though substan-tial Fe(II)had evolved(Fig.3b).However,when AQDS was later added to U(VI)-with-nontronite suspensions, complete removal of U(VI)occurred rapidly.In the pres-ence of U(VI),nontronite and AQDS,bioreduction of non-tronite occurred most rapidly and to the greatest extent, and U(VI)removal kinetics were faster compared to sus-pensions of U(VI)-with-nontronite and U(VI)alone.The addition of AQDS was previously shown to signi?cantly in-crease the rate and extent of nontronite bioreduction(Dong et al.,2003).

With a near-constant initial concentration of U(VI) (0.7–0.8mM),increasing concentrations of nontronite in-creased the lag time prior to U(VI)reduction but did not inhibit the ultimate removal of U(VI)(Fig.4;

1.0?108cell mLà1).The extent of nontronite reduction in our experiments was measured only after27and52days. After a52days incubation with variable nontronite concen-trations,the extent of U(VI)removal was near-constant and complete,while the extent of nontronite reduction(ex-pressed as%of Fe T)decreased with increased clay concen-trations(Fig.4b).The total concentration of electrons (eàeq/L)reduced was calculated from the sum of U(VI)

3530G.Zhang et al./Geochimica et Cosmochimica Acta73(2009)3523–3538

consumed and Fe(II)produced.The varied extent of nontr-onite reduction was likely controlled by the thermodynamic capability of nontronite-Fe(II)to reoxidize U(IV)O2.As dis-cussed below in Section4.3,as soon as uraninite cannot be oxidized by nontronite-Fe(II),U can no longer serve as an electron shuttle.This endpoint for the thermodynamic favorability of U valence cycling apparently is not con-trolled by the solid-state[structural-Fe(III)]/[structural-Fe(II)]ratio,otherwise nontronite reduction extent would have been constant in these experiments with variable non-tronite concentrations.Instead,the thermodynamic end-point for U valence cycling is likely controlled by soluble or sorbed Fe(II)species.The near-constant extent of U(VI)reduction re?ected the ability of MR-1to continue to bioreduce U(VI),after U valence cycling ceased,until U?nally precipitated from solution.

With a constant initial concentration of nontronite (5.0g Là1),increasing concentrations of U(VI)increased the rate of nontronite reduction,however,this enhance-ment e?ect began to decline at U(VI)concentrations above 800l M(Fig.5;1.0?108cell mLà1).The extent of U(VI) reduction in these experiments was measured only after 27and52days.After a52days incubation with variable U(VI)concentrations,the extent of nontronite reduction was near-constant(ca.30%)except for0and1500l M U(VI)concentrations,while the extent of U(VI)removal was relatively low at the lowest(25–100l M)and highest (1500l M)U(VI)concentrations but essentially complete at intermediate U(VI)concentrations(400–1000l M) (Fig.5b).The near-constant reduction extent of nontronite was also likely controlled by the thermodynamic capability of nontronite-Fe(II)to reoxidize U(IV)O2.In this case,by using the same nontronite concentration in all incubations (and possibly producing equal?nal Fe2+(aq)concentra-tions),the thermodynamic endpoint for U valence cycling also occurred at near-constant solid-state[structural-Fe(III)]/[structural-Fe(II)]ratios.

3.4.Abiotic oxidation of biogenic uraninite by nontronite

Biogenic uraninite was e?ectively oxidized by structural Fe(III)in nontronite when reacted in anoxic30mM NaH-CO3(Fig.6).We presume that direct solid-to-solid contact must occur for electrons to be transferred from UO2parti-cles to structural Fe(III)in https://www.doczj.com/doc/9815050659.html,plete reoxida-tion of uraninite to U(VI)occurred in ca. 1.2days. Structural Fe(III)in nontronite oxidized biogenic uraninite at rates comparable to uraninite oxidation by poorly crys-talline Fe(III)(oxyhydr)oxides(Senko et al.,2005; Ginder-Vogel et al.,2006).The rate of uraninite oxidation was quanti?ed as the rate of soluble U(VI)production according to:

R ox?d?UO2 =dt?àk oxáU TOT

? àUeVITesolT

?

eTe1Twhere k ox is the pseudo-?rst-order oxidation rate constant (dà1)and[U TOT]is the total uranium concentration(both U(IV)and U(VI))in the reactor.The rate constant for non-tronite-Fe(III)oxidation of uraninite was 1.14dà1 (R2=0.96),and was measured using identical procedures developed in our previous experiments on the oxidation of biogenic uraninite(Burgos et al.,2008).In those experi-ments,uraninite was produced by S.oneidensis MR-1in 30mM NaHCO3,pasteurized,and subsequently oxidized by dissolved oxygen.The values for k ox for uraninite oxida-tion by dissolved oxygen were found to vary from16to27 dà1.While these two sets of experiments were not con-ducted side-by-side with identical materials,it appears that dissolved oxygen will react more rapidly than nontronite-Fe(III)in the oxidation of uraninite.This is not unexpected considering that oxygen is a stronger oxidant than struc-tural Fe(III)and,as a dissolved constituent,would have greater access to uraninite particles.

3.5.Abiotic reduction of U(VI)by chemically-reduced nontronite

Uranyl was only partially removed from solution in the presence of citrate-dithionite-bicarbonate(CBD)-reduced nontronite(Fig.7),even in the presence of a large stoichiom-etric excess of Fe(II)relative to U(VI).In preparing the CBD-reduced nontronite,the extent of reduction,27%Fe(II) (equivalent to2.8mM0.5N HCl-extractable Fe(II)),was

Bioreduction of nontronite and uranium3531

purposefully controlled(Komadel et al.,1990)to be compara-ble to maximum reduction extents achieved in the bioreduc-tion experiments.The CBD-reduced nontronite was washed three times with anoxic distilled water such that Ca concentra-tions were likely low(e.g.,<<0.06mM).Surface-adsorbed

Fe(II)concentrations remained relatively constant,ranging from75to100l M(equivalent to2.7–3.6%of0.5N HCl-extractable Fe(II)in system),and aqueous Fe(II)concentra-tions ranged from10to30l M.Structural Fe(II)was the predominant reductant in this system.The NaHCO3-extractable U(VI)concentration decreased from230to 180l M between days1and4in conjunction with a decrease in the of0.5N HCl-extractable Fe(II)concentration.How-ever,the decrease and subsequent increase in the0.5N HCl-extractable Fe(II)concentration could not be solely

Table4

Comparison of uranyl(VI)sorption to nontronite NAu-2(current study)with other smectites.

Reference Solid-phase Solution chemistry Sorption characteristics Ames et al.(1983)Nontronite NG-110mM NaHCO3C max=1.21?10à10mole/m2

CEC=95meq/100g pH8.5K d=4.1mL/g

SA=861m2/g

Ames et al.(1983)Nontronite NG-110mM NaCl C max=6.55?10à10mole/m2

CEC=95meq/100g pH7.0K d=300mL/g

SA=861m2/g

Ames et al.(1983)Montmorillonite10mM NaHCO3C max=8.99?10à10mole/m2

CEC=120meq/100g pH8.5K d=1.8mL/g

SA=747m2/g

Ames et al.(1983)Montmorillonite10mM NaCl C max=1.18?10à9mole/m2

CEC=120meq/100g pH7.0K d=542mL/g

SA=747m2/g

Ames et al.(1982)Smectite Hanford groundwater C max=3.34?10à7mole/m2

CEC=4.84meq/100g pH7.7K d=12–127mL/g

SA=31.2m2/g

Catalano and Brown(2005)Montmorillonite SWy-2100mM NaNO3C max=9.57?10à7mole/m2

CEC=85meq/100g pH7.1K d=8.2mL/g

SA=31.8m2/g

Current study Nontronite NAu-230mM NaHCO3C max=1.73?10à6mole/m2

CEC=69.7meq/100g pH7.0K d=64.9mL/g

SA=42m2/g

Ames et al.(1983).

Ames et al.(1982).

Catalano and Brown(2005).

3532G.Zhang et al./Geochimica et Cosmochimica Acta73(2009)3523–3538

attributed to changes in U(VI)concentrations,suggesting that other redox active components could have been present in these suspensions.The use of low-temperature O2-traps in the preparation and maintenance of all reactant solutions used in these experiments would lead us to believe that O2 was not signi?cant in these incubations.Regardless of the time-dependent behavior of0.5N HCl-extractable Fe(II), both structural and surface-adsorbed Fe(II)species were present and yet the total U(VI)concentration was not statis-tically di?erent from4to82days(t=à19.8;p=0.0001).

3.6.Characterization of mineral products

Mineral products from many of these experiments were characterized by XRD,SEM,and XANES spectroscopy. The presence of nontronite did not substantially a?ect the XRD patterns of biogenic uraninite(Electronic Annex EA-2),even though uraninite and nontronite were observed in close proximity in SEM images(Fig.8b and c).In the ab-sence of nontronite,a predominant portion of uraninite accumulated extracellularly of MR-1(Fig.8a).In the pres-ence of nontronite,MR-1cells were found to associate with both nontronite and uraninite.MR-1cells were not found preferentially associated with any speci?c facet of the non-tronite particles(e.g.,along layer edges or basal siloxane surfaces).Nontronite particles collected at the end of the bioreduction experiment exhibited severe dissolution fea-tures(Fig.8b).SEM–EDS analyses revealed increased ura-nium content with bioreduced nontronite particles, presumably nanoparticulate uraninite,that was not ob-served in abiotic U(VI)-with-nontronite controls.Broad di?raction peaks for biogenic uraninite were indicative of ?ne-grained nanoparticulate uraninite,consistent with our SEM observations.

The biological reduction of nontronite a?ected the XRD patterns of these clays compared to the initial unaltered material(Electronic Annex EA-2).Bioreduction of Fe(III) in nontronite resulted in the disappearance of the peak at 12.8A?(2h=6.9°)and appearance of peaks at15.2A?(2h=5.8°)and12.4A?(2h=7.1°).The dissolution of non-tronite caused by bioreduction was consistent with previous studies that,either directly or indirectly,observed collapsed nontronite layers upon reduction(Lear and Stucki,1989; Wu et al.,1989;Kostka et al.,1999a;Dong et al.,2003; Zhang et al.,2007b).

The average oxidation state of both Fe and U were determined by XANES spectroscopy for select samples col-lected during these experiments.XANES spectra were col-lected from abiotic controls(1.0mM U(VI), 5.0g Là1 nontronite)that had been incubated for52days,biotic con-trols inoculated with1.0?108cell mLà1MR-1and incu-bated for52days(Fig.4),and the biotic controls after they were oxidized by dissolved oxygen for20h.The Fe K edge XANES spectrum for the bioreduced sample was intermediate between our Fe(III)and Fe(II)nontronite NAu-2standards(sample NUR in Fig.9a).The Fe(II)/ Fe(III)ratio was determined by linear combination?tting (LCF)as described in Section2.9.The Fe reduction extent calculated as([Fe(II)]/{[Fe(II)]+[Fe(III)]})was38±10% compared to our estimate of28.3%based on Fe(II)mea-sured after0.5N HCl extraction(Table2).The Fe(II)/ Fe T ratio of the abiotic control was0±10%,in good agree-ment with the0.5N HCl extraction measurement (0.38±3.0%).The U L III edge XANES spectra for the

bio-Fig.8.(a)Scanning electron micrographs of unstained S.oneidensis MR-1cells and uraninite produced in the absence of nontronite NAu-2. Inset is SEM–EDS spectrum of uraninite showing typical elemental composition(Au peak from Au coating).(b)Secondary electron image of bioreduced nontronite NAu-2with uranium.Inset is SEM–EDS spectrum showing typical elemental composition.(c)Secondary electron image of uraninite produced in the presence of nontronite NAu-2.(d)Secondary electron image of abiotic control(no cells)containing nontronite NAu-2and U(VI).

Bioreduction of nontronite and uranium3533

reduced sample closely matched that of a U(IV)standard of biogenic UO 2nanoparticles (Fig.9b).The U(IV)/U(VI)ra-tio was determined by LCF of the XANES spectra.The U reduction extent calculated as ([U(IV)]/{[U(IV)]+[U-(VI)]})was 90±10%in good agreement with our estimate of 83±2.7%based on total U(VI)measured after 1M NaHCO 3extraction (Table 2).

4.DISCUSSION

4.1.Uranium speciation and U/Fe thermodynamics The redox behaviors of the U(VI)/U(IV)couple and the nontronite-Fe(III)/Fe(II)couple are relatively complex and strongly dependent on chemical activities and bulk geo-chemical conditions.While the thermodynamics of uranium geochemistry are well established,the thermodynamic properties of iron-rich smectites like nontronite are less constrained.Nontronite dissolution or cation exchange reactions can also lead to the introduction or alteration of dissolved concentrations of Ca and Mg.These two cations have particularly important e?ects on uranium speciation and reactivity (Brooks et al.,2003;Dong and Brooks,2006,2008).As shown in Fig.1,U(VI)speciation is signif-icantly altered with the addition of just 0.06mM Ca.The shift of the predominant U(VI)species from UO 2(CO 3)34àto CaUO 2(CO 3)32àcould a?ect sorption and redox reac-tions with https://www.doczj.com/doc/9815050659.html,ing published values of stan-dard-state reduction potentials (E 0)for these U(VI)species (Table 1),e?ective reduction potentials (E 0red )were calculated for the current experimental conditions.The fol-lowing conditions were selected for comparison purposes:25°C,pH 6.8,30mM NaHCO 3,500l M UO 2(CO 3)34àwith no Ca present or 500l M CaUO 2(CO 3)32àwith 0.27mM Ca.For these conditions,UO 2(CO 3)34àhas an E 0red of +0.12V,while CaUO 2(CO 3)32àhas an E 0red of +0.22V.Thus,while Ca does a?ect U(VI)speciation,it does not decrease the reduction potential of other predom-inant U(VI)species.

Selection of a redox couple for nontronite-Fe(III)/Fe(II)is more challenging compared to U(VI)/U(IV).On one hand,a redox couple that represents reduction without dis-solution for structural nontronite-Fe(III)/structural nontr-onite-Fe(II)(Eq.(2)in Table 1;Favre et al.,2006)is appealing because the majority of Fe(II)does indeed re-main in the clay structure.However,calculation of the reduction potential as a function of reaction extent is prob-lematic due to the assignment of chemical activities to

these

Fig.9.(A)Fe K edge normalized XANES spectra o?set vertically by 1.0,and (B)overlaid.(C)U L III -edge normalized XANES spectra o?set vertically by 0.8,and (D)overlaid.The measured spectra (symbols)from the samples are over-plotted by linear combination ?tting of the XANES spectra (solid lines).The reduced standards (Fe(II)and U(IV))are shown as blue lines while the oxidized standards (Fe(III)and U(VI))are shown as black lines.The samples shown from bottom to top in panels (A)and (C)are:NUC –5g L à1nontronite NAu-2,1mM uranyl(VI)acetate,and no cells;NUR –5g L à1nontronite NAu-2,1mM uranyl(VI)acetate,1?108cell mL à1MR-1and incubated for 52days;NUO –the NUR sample reacted with dissolved oxygen for 20h.(For interpretation of the references to color in this ?gure legend,the reader is referred to the web version of this paper.)

3534G.Zhang et al./Geochimica et Cosmochimica Acta 73(2009)3523–3538

solid-phase species.On the other hand,a redox couple that represents congruent dissolution for structural nontronite-Fe(III)/Fe 2+(aq)(Eq.(1)in Table 1;Jaisi et al.,2007b )is appealing because the reduction potential of the remaining nontronite-Fe(III)can be calculated directly from aqueous metal concentrations.However,as noted above,congruent dissolution of nontronite does not commonly occur.Regardless of these di?culties,we calculated E 0red for nontr-onite-Fe(III)/Fe(II)where the dissolution of nontronite was assumed to be quasi-congruent with respect to Fe 2+and all other dissolved components were calculated in terms of Fe 2+based on the proposed stoichiometry in Eq.(1)in Table 1.

The bioreduction of UO 2(CO 3)34àcoupled to lactate oxidation (R 1in Fig.10)has an e?ective overall reduction potential (E 0overall )of +0.54V with an initial U(VI)concen-tration of 1000l M,5mM lactate,2mM acetate (from UO 2(Ace)2),pH 6.8,30mM NaHCO 3.In comparison,the bioreduction of nontronite-Fe(III)(R 3in Fig.10)cou-pled to lactate oxidation has an E 0overall of +0.92V with an initial Fe(II)concentration of 1l M and a lactate/acetate ratio of 5/2.Therefore,the bioreduction of nontronite-Fe(III)was thermodynamically favorable compared to U(VI)bioreduction in these experimental systems.How-ever,in experiments conducted with sole electron acceptors,both the rate and extent of electron acceptor utilization was greater with U(VI)compared to nontronite-Fe(III).Due to kinetic constraints related to transferring electrons to a so-lid-phase electron acceptor,U(VI)was likely reduced be-fore nontronite-Fe(III).

The abiotic oxidation of uraninite coupled to nontro-nite-Fe(III)reduction (R 2in Fig.10)has an E 0overall of +0.47V with an initial U(VI)concentration of 1l M and an initial Fe(II)concentration of 1l M.The kinetics of this reaction were relatively fast (Fig.5b)and con?rm the capa-bility of U valence cycling at the start of the bioreduction experiments.At a ?xed initial U(VI)concentration of 1l M,the E 0overall of R 2will approach <0when the aqueous Fe(II)concentration is P 170l M,a value higher than any measured in these experiments.However,uraninite oxida-tion did become thermodynamically unfavorable at some point during bioreduction because U(VI)eventually was precipitated from solution (Figs.3–5).Therefore,our pro-

posed reaction stoichiometry for this reaction is incorrect.Elucidation of the true reaction stoichiometry and mecha-nism are topics of future research.

The abiotic reduction of U(VI)coupled to nontronite-Fe(II)oxidation (R 4in Fig.10)has an E 0overall of à0.15V with an initial U(VI)concentration of 230l M and an initial Fe(II)concentration of 20l M (initial conditions for experi-mental results shown in Fig.5a).At a ?xed U(VI)concentra-tion of 1l M,the E 0overall of R 4will approach >0only when the aqueous Fe(II)concentration is P 700l M.Therefore,from a thermodynamic perspective,R 4is non-spontaneous.4.2.Uranium valence cycling with nontronite

The biologic and abiotic reactions between DMRB,nontronite and uranium represent a series of complex,cou-pled processes (Fig.10).In the current experiments,we found that U(VI)enhanced the rate and extent of nontro-nite-Fe(III)reduction (Figs.3–5),and that uraninite oxida-tion by structural Fe(III)in nontronite was relatively fast (Fig.6).The relatively rapid reoxidation of U(IV)allows uranium to serve as an e?ective electron shuttle to nontro-nite-Fe(III).We also found that U valence cycling was dri-ven primarily by biological reduction because abiotic reduction of U(VI)by CBD-reduced nontronite-Fe(II)was thermodynamically unfavorable or kinetically limited (Fig.7).An important ?nding from this work with respect to engineered systems for in situ immobilization of uranium is that iron-rich clays may inhibit the goal of U removal from solution.Indeed we observed that increasing concen-trations of nontronite increased the lag time prior to U(VI)reduction (Fig.4).

A mechanistic description of the operative reactions (R 1–R 4depicted in Fig.10)in these experimental systems requires an understanding of U speciation and the reactive forms of Fe.In the case of uraninite oxidation by structural Fe(III)in nontronite (R 2),we presume that solid-to-solid contact is required for electron transfer to occur.In the abi-otic experiments presented in Fig.6,biogenic uraninite was produced by MR-1in separate incubations and then pas-teurized prior to reaction with unaltered nontronite.MR-1incubation conditions were essentially identical to a previ-ous study where we found uraninite nanoparticles located in the periplasm,on the outer membrane,and extracellu-larly (Burgos et al.,2008),consistent with another recent study on U(VI)reduction by MR-1(Marshall et al.,2006).While uraninite particles retained within the peri-plasm may be somewhat limited in their availability to react with other mineral particles (e.g.,Fredrickson et al.,2002),we did not ?nd periplasmic uraninite as the predominant physiological form in these incubations (Fig.8a).Further-more,in biotic experiments with nontronite and U(VI),loosely aggregated extracellular uraninite was a predomi-nant physiological form (Fig.8c),and this form should readily be available to make solid-to-solid contact with nontronite-Fe(III).Extracellular uraninite has been found in intimate association with extracellular polymeric sub-stances (EPS)(Marshall et al.,2006;Burgos et al.,2008)but it is unknown how EPS may a?ect solid-to-solid elec-tron

transfer.

Fig.10.Conceptualization of operative reactions in experiments conducted with nontronite NAu-2,uranium and dissimilatory metal-reducing bacteria (DMRB).

Bioreduction of nontronite and uranium 3535

In the case of uranyl(VI)reduction by nontronite-Fe(II) (R4in Fig.10),the reaction mechanism and thermody-namic driving force depend on both U(VI)speciation and the form of Fe(II).We presume that U(VI)must sorb to the clay surface before any electron transfer can occur,thus preferentially sorbed U(VI)species(e.g.,cationic,poly-meric)should promote R4.We envision that Fe(II)in these systems can exist in the octahedral and tetrahedral layers of the clay,complexed to layer edge sites,sorbed to basal siloxane surfaces,or dissolved in solution.Thus,Fe(II) can exist in a variety of forms in reduced nontronite suspen-sions,each of which may have a distinct but unknown reduction potential.

4.3.Biogeochemical implications

With these conceptual thermodynamic calculations and experimental observations,we propose that U valence cy-cling in these systems can be explained as follows (Fig.10):DMRB preferentially bioreduce soluble U(VI) over solid-phase nontronite-Fe(III);uraninite is rapidly reoxidized by nontronite-Fe(III)at the start of the experi-ments;as Fe2+(aq)or structural Fe(II)accumulate in the system,the reduction potential of nontronite-Fe(III)de-creases;at some elevated concentration of Fe(II),nontro-nite-Fe(III)can no longer oxidize uraninite;DMRB continue to bioreduce U(VI),however,now it remains as uraninite and?nally precipitates from solution.The lag phase for the onset of U(VI)loss from solution increased with increasing nontronite concentrations(Fig.4).Higher nontronite concentrations provide greater sorption capacity for Fe2+,which delays the increase in Fe2+(aq)and,there-fore,prolongs the lag phase.The important implication of this is that U valence cycling with iron-rich clays may inhi-bit U removal from solution in subsurface environments.

Our results and interpretations are consistent with other bioreduction experiments conducted with U(VI)and fer-rihydrite(Wielinga et al.,2000),goethite(Fredrickson et al.,2000;Wielinga et al.,2000),and manganese(III/IV) oxides(Fredrickson et al.,2002).Similar to our?ndings with nontronite,ferrihydrite and manganese(III/IV)oxides were shown to inhibit and/or delay U(VI)reduction,while goethite had little e?ect on U(VI)reduction.The presence of U(VI)was shown to enhance the bioreduction of manga-nese(III/IV)oxides through U valence cycling(Fredrickson et al.,2002),consistent with our current?ndings with iro-n(III)-rich nontronite NAu-2.Our results may also extend to the(bio)reduction of other redox active contaminants (e.g.,As,Cr,Tc)in soils and sediments with signi?cant amounts of Fe-rich phyllosilicates.

ACKNOWLEDGMENTS

This work was supported by the Natural and Accelerated Bio-remediation Research(NABIR)Program,O?ce of Biological and Environmental Research(BER),O?ce of Energy Research,U.S. Department of Energy(DOE)Grant No.DE-FG02-01ER631180 to The Pennsylvania State University,and by the National Science Foundation under Grant https://www.doczj.com/doc/9815050659.html,e of the MRCAT sector at the Advanced Photon Source(APS)was supported by the Environmental Remediation Science Program(ERSP),U.S.DOE,O?ce of Science,BER and the MRCAT member https://www.doczj.com/doc/9815050659.html,e of the APS was supported by the U.S.Department of Energy,O?ce of Science,O?ce of Basic Energy Sciences,under contract DE-AC02-06CH11357.We thank Ed O’Loughlin for pre-paring and providing nontronite-Fe(III)and nontronite-Fe(II) standards for XANES spectroscopy.We thank Je?Catalano and Joel Kostka and three anonymous reviewers for providing helpful comments that improved this manuscript.

APPENDIX A.SUPPLEMENTARY DATA Supplementary data associated with this article can be found,in the online version,at doi:10.1016/j.gca.2009.03. 030.

REFERENCES

Ames L.L.,Mcgarrah J.E.and Walker B.A.(1982)Sorption of uranium and cesium by hanford basalts and associated secondary smectite.Chem.Geol.35,205–225.

Ames L.L.,Mcgarrah J.E.and Walker B.A.(1983)Sorption of trace constituents from aqueous solutions onto secondary minerals.I.Uranium.Clays Clay Miner.31(5),321–334. Anderson R.T.,Vrionis H.A.,Ortiz-Bernad I.,Resch C.T.,Long P.E.,Dayvault R.,Karp K.,Marutzky S.,Metzler D.R., Peacock A.,White D.C.,Lowe M.and Lovley D.R.(2003) Stimulating the in situ activity of Geobacter species to remove uranium from the groundwater of a uranium-contaminated aquifer.Appl.Environ.Microbiol.69(10),5884–5891. Behrends T.and Van Cappellen P.(2005)Competition between enzymatic and abiotic reduction of uranium(VI)under iron reducing conditions.Chem.Geol.220(3–4),315–327.

Boult K.A.,Cowper M.M.,Heath T.G.,Sato H.,Shibutani T.

and Yui M.(1998)Towards an understanding of the sorption of U(V)and Se(IV)on sodium bentonite.J.Contam.Hydrol.

35(1–3),141–150.

Boyanov M.I.,O’loughlin E.J.,Roden E.E.,Fein J.B.and Kemner K.M.(2007)Adsorption of Fe(II)and U(VI)to carboxyl-functionalized microspheres:The in?uence of specia-tion on uranyl reduction studied by titration and XAFS.

Geochim.Cosmochim.Acta71(8),1898–1912.

Brina R.and Miller A.G.(1992)Direct detection of trace levels of uranium by laser-induced kinetic phosphorimetry.Anal.Chem.

64(13),1413–1418.

Brooks S.C.,Fredrickson J.K.,Carroll S.L.,Kennedy D.W., Zachara J.M.,Plymale A.E.,Kelly S.D.,Kemner K.M.and Fendorf S.(2003)Inhibition of bacterial U(VI)reduction by calcium.Environ.Sci.Technol.37(9),1850–1858.

Burgos W.D.,Mcdonough J.T.,Senko J.M.,Zhang G.X., Dohnalkova A.C.,Kelly S.D.,Gorby Y.and Kemner K.M.

(2008)Characterization of uraninite nanoparticles produced by Shewanella oneidensis MR-1.Geochim.Cosmochim.Acta 72(20),4901–4915.

Catalano J.G.and Brown G.E.(2005)Uranyl adsorption onto montmorillonite:evaluation of binding sites and carbonate complexation.Geochim.Cosmochim.Acta69(12),2995–3005. Dong H.,Kostka J.E.and Kim J.W.(2003)Microscopic evidence for microbial dissolution of smectite.Clays Clay Miner.51(5), 502–512.

Dong W.and Brooks S. C.(2006)Determination of the formation constants of ternary complexes of uranyl and carbonate with alkaline Earth metals(Mg2+,Ca2+,Sr2+, and Ba2+)using anion exchange method.Environ.Sci.

Technol.40(15),4689–4695.

3536G.Zhang et al./Geochimica et Cosmochimica Acta73(2009)3523–3538

Dong W.and Brooks S. C.(2008)Formation of aqueous MgUO2(CO3)32àcomplex and uranium anion exchange mech-anism onto an exchange resin.Environ.Sci.Technol.42(6), 1979–1983.

Elias D.A.,Senko J.M.and Krumholz L.R.(2003)A procedure for quantization of total oxidized uranium for bioremediation studies.J.Microbiol.Methods53(3),343–353.

Elsner M.,Schwarzenbach R.P.and Haderlein S. B.(2004) Reactivity of Fe(II)-bearing minerals toward reductive trans-formation of organic contaminants.Environ.Sci.Technol.

38(3),799–807.

Favre F.,Stucki J.W.and Boivin P.(2006)Redox properties of structural Fe in ferruginous smectite.A discussion of the standard potential and its environmental implications.Clays Clay Miner.54(4),466–472.

Fredrickson J.K.,Zachara J.M.,Kennedy D.W.,Du?M.C., Gorby Y.A.,Li S.M.W.and Krupka K.M.(2000)Reduction of U(VI)in goethite(alpha-FeOOH)suspensions by a dissim-ilatory metal-reducing bacterium.Geochim.Cosmochim.Acta 64(18),3085–3098.

Fredrickson J.K.,Zachara J.M.,Kennedy D.W.,Liu C.,Du?M.

C.,Hunter

D.B.and Dohnalkova A.(2002)In?uence of Mn

oxides on the reduction of uranium(VI)by the metal-reducing bacterium Shewanella putrefaciens.Geochim.Cosmochim.Acta 66(18),3247–3262.

Furukawa Y.and O’Reilly S.E.(2007)Rapid precipitation of amorphous silica in experimental systems with nontronite (NAu-1)and Shewanella oneidensis MR-1.Geochim.Cosmo-chim.Acta71(2),363–377.

Gan H.,Bailey G.W.and Yu Y.(1996)Morphology of lead(II) and chromium(llI)reaction products on phyllosilicate surface as determined by atomic force microscopy.Clays Clay Miner.44, 734–743.

Gates W.P.,Slade P.G.,Manceau A.and Lanson B.(2002)Site occupancies by iron in nontronites.Clays Clay Miner.50(2), 223–239.

Ginder-Vogel M.,Criddle C.S.and Fendorf S.(2006)Thermo-dynamic constraints on the oxidation of biogenic UO2by Fe(III)(hydr)oxides.Environ.Sci.Technol.40(11),3544–3550. Grenthe I.,Stumm W.,Laaksuharju M.,Nilsson A. C.and Wikberg P.(1992)Redox potentials and redox reactions in deep groundwater systems.Chem.Geol.98(1–2),131–150. Hofstetter T.B.,Neumann A.and Schwarzenbach R.P.(2006) Reduction of nitroaromatic compounds by Fe(II)species associated with iron-rich smectites.Environ.Sci.Technol.

40(1),235–242.

Hofstetter T.B.,Schwarzenbach R.P.and Haderlein S.B.(2003) Reactivity of Fe(II)species associated with clay minerals.

Environ.Sci.Technol.37(3),519–528.

Istok J.D.,Senko J.M.,Krumholz L.R.,Watson D.,Bogle M.A., Peacock A.,Chang Y.J.and White D. C.(2004)In situ bioreduction of technetium and uranium in a nitrate-contam-inated aquifer.Environ.Sci.Technol.38(2),468–475.

Jaisi D.P.,Dong H.L.and Liu C.X.(2007a)Kinetic analysis of microbial reduction of Fe(III)in nontronite.Environ.Sci.

Technol.41(7),2437–2444.

Jaisi D.P.,Dong H.L.and Liu C.X.(2007b)In?uence of biogenic Fe(II)on the extent of microbial reduction of Fe(III)in clay minerals nontronite,illite,and chlorite.Geochim.Cosmochim.

Acta71(5),1145–1158.

Jaisi D.P.,Kukkadapu R.K.,Eberl D.D.and Dong H.L.(2005) Control of Fe(III)site occupancy on the rate and extent of microbial reduction of Fe(III)in nontronite.Geochim.Cosmo-chim.Acta69(23),5429–5440.

Jang J.-H.,Dempsey B.A.and Burgos W.D.(2008)Reduction of U(VI)by Fe(II)in the presence of hydrous ferric oxide and

hematite:E?ects of solid transformation,surface coverage,and humic acid.Water Res.42(8–9),2269–2277.

Jeon B.H.,Dempsey B. A.,Burgos W. D.,Barnett M.O.

and Roden E. E.(2005)Chemical reduction of U(VI)by Fe(II)at the solid–water interface using natural and synthetic Fe(III)oxides.Environ.Sci.Technol.39(15),5642–5649.

Jeon B.H.,Dempsey B.A.,Royer R.A.and Burgos W.D.(2004a) Low-temperature oxygen trap for maintaining strict anoxic conditions.J.Environ.Eng.ASCE130(11),1407–1410.

Jeon B.H.,Kelly S.D.,Kemner K.M.,Barnett M.O.,Burgos W.

D.,Dempsey B. A.and Roden

E. E.(2004b)Microbial

reduction of U(VI)at the solid–water interface.Environ.Sci.

Technol.38(21),5649–5655.

Jeon B.H.,Dempsey B.A.and Burgos W.D.(2003)Kinetics and mechanisms for reactions of Fe(II)with iron(III)oxides.

Environ.Sci.Technol.37(15),3309–3315.

Keeling J.L.,Raven M.D.and Gates W.P.(2000)Geology and characterization of two hydrothermal nontronites from weath-ered metamorphic rocks at the Uley Graphite Mine,South Australia.Clays Clay Miner.48(5),537–548.

Keller M. D.,Bellows W.K.and Guillard R.R.L.(1988) Microwave treatment for sterilization of phytoplankton culture media.J.Exp.Mar.Biol.Ecol.117(3),279–283.

Komadel P.,Lear P.R.and Stucki J.W.(1990)Reduction and reoxidation of nontronite:extent of reduction and reaction rates.Clays Clay Miner.38,203–208.

Kostka J.E.,Dalton D.D.,Skelton H.,Dollhopf S.and Stucki J.

W.(2002)Growth of iron(III)-reducing bacteria on clay minerals as the sole electron acceptor and comparison of growth yields on a variety of oxidized iron forms.Appl.

Environ.Microbiol.68(12),6256–6262.

Kostka J.E.,Haefele E.,Viehweger R.and Stucki J.W.(1999a) Respiration and dissolution of iron(III)-containing clay miner-als by bacteria.Environ.Sci.Technol.33,3127–3133.

Kostka J.E.,Wu J.,Nealson K.H.and Stucki J.W.(1999b)The impact of structural Fe(III)reduction by bacteria on the surface chemistry of smectite clay minerals.Geochim.Cosmochim.Acta 63(22),3705–3713.

Kowal-Fouchard A.,Drot R.,Simoni E.and Ehrhardt J.J.(2004) Use of spectroscopic techniques for uranium(VI)/montmoril-lonite interaction modeling.Environ.Sci.Technol.38(5),1399–1407.

Lear P.R.and Stucki J.W.(1989)E?ects of iron oxidation state on the speci?c surface area of nontronite.Clays Clay Miner.37, 547–552.

Liger E.,Charlet L.and Van Cappellen P.(1999)Surface catalysis of uranium(VI)reduction by iron(II).Geochim.Cosmochim.

Acta63(19–20),2939–2955.

Lovley D.R.(1993)Dissimilatory metal reduction.Ann.Rev.

Microbiol.47,263–290.

Marshall M.J.,Beliaev A.S.,Dohnalkova A.C.,Kennedy D.W., Shi L.,Wang Z.M.,Boyanov M.I.,Lai B.,Kemner K.M., Mclean J.S.,Reed S.B.,Culley D.E.,Bailey V.L.,Simonson

C.J.,Sa?arini

D. A.,Romine M. F.,Zachara J.M.and

Fredrickson J.K.(2006)c-Type cytochrome-dependent forma-tion of U(IV)nanoparticles by Shewanella oneidensis.Plos Biol.4(8),1324–1333.

McDonough J.M.(2006).Kinetics of Biological Uranium(VI) Reduction and Subsequent Uranium(IV)Oxidation.MS Thesis, The Pennsylvania State University,University Park,PA. Merola R. B.,Fournier E. D.and Mcguire M.M.(2007) Spectroscopic investigations of Fe2+complexation on nontro-nite https://www.doczj.com/doc/9815050659.html,ngmuir23(3),1223–1226.

Missana T.,Garcia-Gutierrez M.and Fernndez V.(2003)Ura-nium(VI)sorption on colloidal magnetite under anoxic envi-

Bioreduction of nontronite and uranium3537

ronment:experimental study and surface complexation model-ling.Geochim.Cosmochim.Acta67(14),2543–2550.

Myers C.R.and Myers J.M.(1994)Ferric iron reduction-linked growth yields of Shewanella putrefaciens MR-1.J.Appl.

Microbiol.76(3),253–258.

Nealson K.H.and Sa?arini D.(1994)Iron and manganese in anaerobic respiration:environmental signi?cance,physiology, and regulation.Ann.Rev.Microbiol.48,311–343.

O’Loughlin E.J.,Kelly S. D.,Cook R. E.,Csencsits R.and Kemner K.M.(2003)Reduction of uranium(VI)by mixed iron(II/iron(III)hydroxide(green rust):formation of UO2 nanoparticles.Environ.Sci.Technol.37(4),721–727.

O’Reilly S.E.,Furukawa Y.and Newell S.(2006)Dissolution and microbial Fe(III)reduction of nontronite(NAu-1).Chem.Geol.

235(1–2),1–11.

Pabalan R.T.and Turner D.R.(1997)Uranium(6+)sorption on montmorillonite:experimental and surface complexation mod-eling study.Aquat.Geochem.2,203–226.

Parkhurst D.L.and Appelo C. A.J.(1999)User’s guide to PHREEQC(version2)–A computer program for speciation, batch-reaction,one-dimensional transport,and inverse geo-chemical calculations:U.S.Geological Survey Water-Resources Investigations Report99-4259,p312.

Peretyazhko T.,Zachara J.M.,Heald S.M.,Jeon B.H., Kukkadapu R.K.,Liu C.,Moore D.and Resch C.T.(2008) Heterogeneous reduction of Tc(VII)by Fe(II)at the solid–water interface.Geochim.Cosmochim.Acta72,1521–1539. Prikryl J.D.,Jain A.,Turner D.R.and Pabalan R.T.(2001) Uranium(VI)sorption behavior on silicate mineral mixtures.J.

Contam.Hydrol.47(2–4),241–253.

Royer R.A.,Burgos W.D.,Fisher A.S.,Jeon B.H.,Unz R.F.

and Dempsey B.A.(2002)Enhancement of hematite bioreduc-tion by natural organic matter.Environ.Sci.Technol.36(13), 2897–2904.

Schwertmann U.and Cornell R.M.(2000)Iron Oxides in the Laboratory;Preparation and Characterization.VCH-Wiley, Weinheim,Federal Republic of Germany(DEU).

Segre C.W.,Leyarovska N.E.,Chapman L.D.,Lavender W.M., Plag P.W.,King A.S.,Kropf A.J.,Bunker B.A.,Kemner K.

M.,Dutta P.,Druan R.S.and Kaduk J.(2000)Synchrotron Radiation Instrumentation:Eleventh U.S.Conference.American Institute of Physics,College Park,MD.pp.419-422.

Senko J.M.,Dewers T.A.and Krumholz L.R.(2005)E?ect of oxidation rate and Fe(II)state on microbial nitrate-dependent Fe(III)mineral formation.Appl.Environ.Microbiol.71(11), 7172–7177.

Sorensen J.and Thorling L.(1991)Stimulation by lepidocrocite (gamma-FeOOH)of Fe(II)-dependent nitrite reduction.Geo-chim.Cosmochim.Acta55(5),1289–1294.

Sorensen K.C.,Stucki J.W.,Warner R.E.,Wagner E.D.and Plewa M.J.(2005)Modulation of the genotoxicity of pesticides

reacted with redox-modi?ed smectite clay.Environ.Mol.

Mutagen.46(3),174–181.

Stookey L.L.(1970)Ferrozine–a new spectrophotometric reagent for iron.Anal.Chem.42,779–781.

Stucki J.W.,Golden D.C.and Roth C.B.(1984)E?ects of reduction and reoxidation of structural iron on the surface-charge and dissolution of dioctahedral smectites.Clays Clay Miner.32(5),350–356.

Taylor R.W.,Shen S.Y.,Bleam W.F.and Tu S.I.(2000) Chromate removal by dithionite-reduced clays:evidence from direct X-ray adsorption near edge spectroscopy(XANES)of chromate reduction at clay surfaces.Clays Clay Miner.48(6), 648–654.

Wielinga B.,Bostick B.,Hansel C.M.,Rosenzweig R.F.and Fendorf S.(2000)Inhibition of bacterially promoted uranium reduction:ferric(hydr)oxides as competitive electron acceptors.

Environ.Sci.Technol.34(11),2190–2195.

Wu J.,Low P.F.and Roth C.B.(1989)E?ects of octahedral-iron reduction and swelling pressure on interlayer distances in Na-nontronite.Clays Clay Miner.37(3),211–218.

Wu W.M.,Carley J.,Fienen M.,Mehlhorn T.,Lowe K., Nyman J.,Luo J.,Gentile M. E.,Rajan R.,Wagner D., Hickey R.F.,Gu B.,Watson D.,Cirpka O.A.,Kitanidis P.K.,Jardine P.M.and Criddle C.S.(2006a)Pilot-scale in situ bioremediation of uranium in a highly contaminated aquifer. 1.Conditioning of a treatment zone.Environ.Sci.

Technol.40(12),3978–3985.

Wu W.M.,Carley J.,Gentry T.,Ginder-Vogel M.A.,Fienen M., Mehlhorn T.,Yan H.,Caroll S.,Pace M.N.,Nyman J.,Luo J., Gentile M.E.,Fields M.W.,Hickey R.F.,Gu B.,Watson D., Cirpka O.A.,Zhou J.,Fendorf S.,Kitanidis P.K.,Jardine P.

M.and Criddle C.S.(2006b)Pilot-scale in situ bioremedation of uranium in a highly contaminated aquifer.2.Reduction of U(VI)and geochemical control of U(VI)bioavailability.

Environ.Sci.Technol.40(12),3986–3995.

Zhang G.,Kim J.,Dong H.and Andre J.S.(2007a)Microbial e?ects in promoting the smectite to illite reaction:role of organic matter intercalated in the interlayer.Am.Mineral.92, 1401–1410.

Zhang G.,Dong H.,Kim J.and Eberl D.D.(2007b)Microbial reduction of structural Fe3+in nontronite by a thermophilic bacterium and its roles in promoting the smectite to illite reaction.Am.Mineral.92,1411–1419.

Zhou P.and Gu B.H.(2005)Extraction of oxidized and reduced forms of uranium from contaminated soils:e?ects of carbonate concentration and pH.Environ.Sci.Technol.39(12),4435–4440.

Associate editor:Susan Glasauer

3538G.Zhang et al./Geochimica et Cosmochimica Acta73(2009)3523–3538

2016春教科版科学六上33《电磁铁的磁力》练习题

电磁铁的磁力(一) 一、判断 1、电磁铁吸引大头针的数量越多,说明磁力越大.( √ ) 2、电池数量相同,电磁铁线圈越多磁力越小。(× ) 3、研究磁力与线圈圈数关系时所用电池数量应一样。(√ ) 4、电磁铁的磁力大小就是可能改变的。( √ ) 5、研究电磁铁磁力大小实验时,不能长时间接通电磁铁,这样会使电池耗电太多,影响实验准确性.×) 二、选择 1、下列不能改变电磁铁的磁力大小的就是( B )。 A、增加电池节数 B、改变线圈缠绕方向 C、增加线圈缠绕圈数 2、下列不能改变电磁铁的磁极的就是( C )。 A 、改变电流的方向B、改变线圈缠绕的方向C 、增加线圈的圈数 3、只改变电磁铁电流的方向,电磁铁( A )。 A、南北极改变 B、南北极不变 C、磁力强弱发生变化 4、小明先改变电磁铁线圈的缠绕方向后,再改变电磁铁的电流方向,此时的电磁铁(B )。 A、南北极改变 B、南北极不变 C、磁力强弱发生变化 (二)

一、填空 1、(改变电池正负极接法)或(改变线圈绕线的方向)会改变电磁铁的南北极。 2、电磁铁的磁力大小与(线圈圈数多少)有关,圈数少磁力小,圈数多磁力大。 3、电磁铁的磁力大小与使用的(电池数量)有关,电池少则磁力小,电池多则磁力大。 4、电磁铁的磁力大小与(线圈粗细长短)、(铁芯粗细长短)等因素有一定关系。 5、绕电磁铁可以按(顺时针)方向与(逆时针)方向,朝一个方向绕。 6、电磁铁具有接通电流产生(磁性),断开电流磁性(消失)的基本性质. 二、判断 1、电磁铁吸引大头针的数量越多,说明磁力越大。(√ ) 2、电池数量相同,电磁铁线圈越多磁力越小.( × ) 3、研究磁力与线圈圈数关系时所用电池数量应一样。( √ ) 4、电磁铁的磁力大小就是可能改变的.( √ ) 5、研究电磁铁磁力大小实验时,不能长时间接通电磁铁,这样会使电池耗电太多,影响实验准确性.(×) 三、选择 1、下列不能改变电磁铁的磁力大小的就是( B )。 A、增加电池节数 B、改变线圈缠绕方向 C、增加线圈缠绕圈数 2、研究电磁铁的磁力大小与电池数量的关系时,要保持不变的就是(B)。

六年级上册科学试题-3.2电磁铁 教科版 含答案

教科版科学3.2《电磁铁》练习 一、填空题 1、电磁铁具有_____________________的基本性质。 2、改变电池______接法或改变线圈绕线的______会改变电磁铁的南北极。 3、我们可以用______检测电磁铁是否具有南北极。 4、磁铁的两极不可改变,_______的两极则是可以改变的。 二、判断题,对的打√,错的打?。 1、圈数太多,磁力虽强但需要的导线很长。() 2、电磁铁不能长时间接在电池上。() 3、实验中,各组电磁铁的两极是一样的。() 4、断电后,电磁铁仍能吸起大头钉。() 5、电磁铁和磁铁都有南北极之分。() 6、电磁铁能产生磁性,但不能指示方向。() 三、选择题,将正确的序号填写在括号内。 1、下列不能改变电磁铁的磁极的是( )。 A、改变电流的方向 B、改变线缠绕的方向 C、増加线圏的圈数 2、只改变电磁铁电流的方向,电磁铁() A、南北极改变 B、南北极不变 C、磁力强弱发生变化 3、在()上绕线圈可以制成电磁铁。 A、木棍 B、铁钉 C、塑料棒 4、在制作电磁铁的过程中,我们应该用导线在铁钉上沿一个方向缠绕( )。 A、5圈~8圈 B、50圈~80圈 C、500圈~800圈 5、电磁铁的南极会吸引指南针的( )。 A、南极 B、北极 C、南极和北极 四、排序题 请将下面的实验步骤进行排序。 (______)在导线两头留出10厘米~15厘米做引出线。 (______)用有绝缘皮的导线在铁钉上沿一个方向缠绕50圈~100圈。 (______)固定导线两头,以免松开。 (______)用小磁针的北极靠近钉帽一端,观察实验现象。 (______)记录实验数据,标出电磁铁南北极。

最新教科版六年级上册科学《电磁铁的磁力(二)》教案

【教材简析】 《电磁铁的磁力(二)》是《能量》单元第四节课。本节课是《电磁铁的磁力(一)》基础的延伸,是学生学习猜测影响电磁铁磁力大小因素后,证明了电磁铁磁力的大小与线圈圈数有关后想进一步证明电磁铁磁力大小还与其他因素是否有关的探究活动,根据教材内容编排本节课共有三个相关活动。 聚集板块,通过实验探究后知道电磁铁磁力大小与线圈圈数有关,分析上一节实验中如何改变线圈的圈数,达到实验成功的原因,同时根据上节课的设计,让孩子们懂得在本节课实验设计时,知道只改变一个条件的实验,从疑问导向让孩子们探究中间铁芯长短粗细、电磁节数有关的疑惑。 探索板块,活动由教师提供的准备材料中,让小组选择最想探究问题设计,选择不同的材料进行实验探索。实验中强调要控制变量,进行对比实验时需要改变的条件只有一个,实验相关要求同上一课,实验前认真填写完整研究计划,实验过程认真准确进行记录。 研讨影响电磁铁磁力大小的因素,通过实验后学生对影响电磁铁力大小因素的全面了解,让学生认识到线圈的圈数和电池的节数对电磁铁磁力大小的影响,同时也了解到铁芯的长短粗细等对电磁铁磁力的大小有一定的关系。 拓展板块,根据上面的实验探索后,设计制作一个强磁力电磁铁。用自己的研究成果来做一个强磁力电磁铁,肯定是每一个学生的心愿,探究活动以学生成功制作出强力电磁铁、体验到成功的喜悦而结束。 【学情分析】 学生对磁铁比较熟悉,但却对电磁铁比较陌生,通过上一课的探索,学生对影响电磁铁磁力大小有了初步的体验和大胆的猜测,因此本节课学生探究的积极性会很高,教师在教学设计中可以让学生自由选择感兴趣的活动内容进行长时间探究活动,但探究前一定要教育学生制定实验计划,培养学生选择合适有效的实验材料,为学生成功开展实验奠定基础。 【教学目标】 科学概念目标 1.电磁铁的磁力大小与串联的电池数量有关:串联电池少则磁力小,串联电池多则磁力大。

六上3-2电磁铁(教学反思)

六上电磁铁教学反思 小学科学课程是以培养学生的科学素养为宗旨的,学生是科学学习的主体,科学学习要以探究为核心。本节课正是按照这样的理念进行教学设计的,让学生充分地经历“发现问题——提出假设——制订计划——分组实验——得出结论——交流评价”这一完整的科学探究过程,使学生在探究过程中动手动脑、亲自实践,在感知、体验的基础上理解科学概念。 在研究电磁铁南北极与什么因素有关时,充分放手让学生按照自己的计划、自己的想法自主探究,进行小组合作学习,把学习的主动权还给学生,并且给学生们适当的引导。让学生感受到老师也是他们中的一员与他们共同观察。这样学生就有浓厚的兴趣想要成功、想要发现别人没有看到的现象的愿望,所以探究的热情也就异常的高涨。学生在分组活动时,观察、探究的很仔细,提出了很多我都没有想到的问题。每发现一个问题他们都会很兴奋的告诉我,让我参与到他们当中,在他们发现的问题中有很多缺少研究价值的,这时我也没有加以阻拦。而是引导他们发现其它有价值的问题,充分保护了学生的学习兴趣,让学生体验自己的经历、自己的成功。 在教学中,我尽量不过多地干扰学生,也不领着学生一问一答得齐步走,直奔结论,而是把有结构的材料交给学生,让他们尽可能地发现各种现象和规律,经历像科学家那样去认识事物的探索过程。比如在教给学生铁钉电磁铁制作方法后,我就把准备好的导线、铁钉、电池、大头针发给学生,让他们动手制作电磁铁,制作好后玩一玩,学生在玩的过程中发现了“通电产生磁性,断电磁性消失”、“电磁铁两端吸的大头针多”等现象,从而引发新的问题:磁铁两端磁性最强,一端叫南极,一端叫北极。电磁铁有没有南北极呢?使他们产生自主活动的冲动,激起了探究的欲望。接着,又发给学生指南针,进行电磁铁南北极的研究,从而使研究引向更深一层。

教科版科学六年级上册3.2 电磁铁练习卷

教科版科学六年级上册3.2 电磁铁练习卷 姓名:________ 班级:________ 成绩:________ 小朋友,带上你一段时间的学习成果,一起来做个自我检测吧,相信你一定是最棒的! 一、选择题 1 . 一根钢条靠近磁针的磁极,磁针被吸引过来。则() A.钢条一定具有磁性 B.钢条一定没有磁性 C.钢条可能有磁性,也可能没有磁性 D.条件不足,无法判断 2 . 小科同学自制电磁铁,切断电流后铁芯还能吸起一枚大头针,可能是()。 A.没有电流电磁铁也能产生磁性 B.铁芯被磁化了 C.上述分析都错误 3 . 当两块条形磁铁相互靠近时,不会发生排斥现象的是()。 A.B.C. 4 . 下列说法错误的是()。 A.把条形磁铁摔成两个半块,每个半块剩下一个磁极 B.球形磁铁也有两个磁极 C.不同形状的磁铁之间也是同极相斥、异极相吸 5 . 有6块磁铁,它们之间有的相互排斥,有的相互吸引。已经知道第一块磁铁的左边一面是N极,那么最后一块磁铁的右面的磁极是。

A.N极B.S极C.无法判断 二、填空题 6 . 我们把两个相同的磁极叫(_____),不相同的磁极叫(______)。 A、南极 B、北极 C、同极 D、异极 7 . 是(___________)磁铁。 8 . 磁极间的作用是(_______)的。 三、判断题 9 . 磁铁磁力最强的部分是磁极。(____) 10 . 一块磁铁摔成两段后,每段就剩下一个磁极了。(________) 11 . 悬浮列车是利用磁极的相互作用实现悬浮的。(____) 12 . 录音磁带、录像磁带,磁盘它们都与电磁铁没有关系。(______) 13 . 用线圈和铁芯制成的电磁铁不用通电也会吸引大头针。(______) 四、简答题 14 . 写出两种判断磁铁南北极的方法。 15 . 为什么磁铁放在收音机前会发生杂音?

教科版小学科学六年级上学期 3.4电磁铁的磁力(二)D卷

教科版小学科学六年级上学期 3.4电磁铁的磁力(二)D卷姓名:________ 班级:________ 成绩:________ 小朋友,带上你一段时间的学习成果,一起来做个自我检测吧,相信你一定是最棒的! 一、电磁铁的磁力 (共8题;共52分) 1. (10分)判断题。 (1)我们可以通过电磁铁吸起大头针的个数来判断电磁铁磁力的大小。 (2)在研究电磁铁磁力与线圈的关系时,我们要保持线圈的圈数不变。 (3)电磁铁由线圈、铁芯组成,要产生磁力就必须通电。 (4)测量电磁铁的磁力大小时,也可以选用较大的铁球。 (5)电磁铁磁力的大小与电磁铁的铁芯毫无关系。 2. (8分)选择题。 (1)电磁铁磁力的大小可能与()无关。 A . 线圈圈数 B . 是否使用电池盒 C . 电池数量 (2)为了实验数据的准确性,我们至少要进行()次实验。 A . I B . 2 C . 3

(3)在检验电磁铁磁力大小与线圈圈数是否有关的实验中,需要改变的条件是()。 A . 线圈圈数 B . 电池数量 C . 导线的长度 (4)研究电磁铁的磁力大小与电流大小的关系时,要保持不变的条件是()。 A . 电池的节数、线圈的圈数 B . 线圈的圈数、铁芯的大小 C . 电池的节数、铁芯的大小 (5)可以改变电磁铁的南北极的方法是()。 A . 增加铁钉上的线圈圈数 B . 增加连接的电池的节数 C . 改变导线线圈的绕向 (6)如图所示,弹簧测力计甲和乙的下面分别挂一个条形磁铁和铁块,已知通电后电磁铁的左侧为N极。则闭合开关后,甲、乙两个弹簧测力计的情况是()。 A . 甲的示数增大,乙的示数减小 B . 甲的示数减小,乙的示数增大

【精品教案】新教科版科学六上3-2电磁铁_

2.电磁铁 【教材简析】 本课是教科版小学《科学》六年级上册第三单元《能量》第二课。通过上一 课的学习,学生已经认识了电可以产生磁,并经历了用通电线圈做电生磁实验, 为理解电磁铁的原理打下了基础。本课主要对电磁铁这一由电生磁的最直接应用 的装置开两个方面的研究活动,制作电磁铁与研究铁钉电磁铁的南北极。学生在自主探究与小组合作学习中建立和完善科学概念。 【学情分析】 电磁铁在学生的生活中应用非常广泛,身边可以找到许多实例。但是对于大 部分同学来说在身边的哪些电器应用了电磁铁了解不够,因为学生根本不懂得什 么是电磁铁。而学生要想真正了解电磁铁的应用就要写认识电磁铁的构造和原理。然后再研究电磁铁的南北极。六年级的学生科学探究能力应有了一定的基础,因 此本节课就以在继续培养学生的科学探究能力的前提,在老师的指导下,让学生 自己制作电磁铁、自主去认识电磁铁的构造和原理,去研究电磁铁的南北极。 【教学目标】 科学概念目标 1.电磁铁具有接通电流产生磁性、断开电流磁性消失的基本性质。 2.改变电池正负极的连接方法或改变线圈缠绕的方向会改变电磁铁的南北极。 科学探究目标 1.学会制作铁钉电磁铁。 2.能通过重复实验来收集电磁铁南北极的证据,运用比较和归纳的方法从实 验证据中发现电磁铁南北极改变的规律。 科学态度目标 1.能养成认真细致、合作研究的品质。 2.能如实地记录和描述有电磁铁南北极改变的证据。 科学、技术、社会与环境目标 初步了解电磁铁南北极改变在日常生活中的应用。

【教学重难点】 重点:制作铁钉电磁铁;改变电池正负极接法或改变线圈缠绕的方向会改变电磁铁的南北极。 难点:控制一定变量,做研究电磁铁南北极的实验,通过分析推理,帮助学生建立“改变电池正负极的连接方法或改变线圈缠绕的方向会改变电磁铁的南北极”这一科学概念。 【教学准备】 1. 学生探究:每组1号电池一节,电池盒一只,长约8厘米的退磁铁钉1根, 长约100厘米的绝缘导线1根,大头针一盒、指南针一只;作业本。 2. 教师演示:1号电池一节,电池盒一只,大头针一盒,指南针一只,磁铁 一块,长约8厘米退磁的铁钉1根。电脑课件。 【教学过程】 一、直接导入,开门见山 通过直接揭示课题,知道本节课的主题,为整个探究埋下伏笔。 1.读课题《电磁铁》你们在生活中见过电磁铁吗?老师给大家把他其中的一员请到了现场,恩,就这么简单的一样东西竟然敢自称自己为电磁铁,关键里面有磁铁两个字,在三年级时我们学过要能成为磁铁要具备最基本的性质,有磁性。那怎么检验它有没有磁性? 2.这是一枚铁钉,怎么检验它有没有磁性? 3.上节课我们学过通电线圈能产生磁性,那如果把导线绕在这枚铁钉上,它能变得有磁性吗? 二、探究活动:带线圈的铁钉有磁性吗? 1.“在线圈上绕导线”的制作步骤说明 (PPT出示)“在线圈上绕导线”的制作步骤说明 (1)左手握钉尾,先打一个结,再把导线沿同一个方向绕在铁钉上。

教科版六年级科学上册3.2 电磁铁(教案)

2.电磁铁 【教学目标】 科学概念: 1.电磁铁具有接通电流产生磁性、断开电流磁性消失的基本性质。 2.改变通过电磁铁中的电流方向(电池的正负极连接和线圈绕线方向)会改变电磁铁的南北极。 过程与方法: 1.制作铁钉电磁铁。 2.做研究电磁铁的南北极的实验。 情感、态度、价值观:养成认真细致、合作进行研究的品质。 【教学准备】 1.学生自备:大头针、透明胶 2.教师准备:绝缘导线、大铁钉、砂纸、指南针 【教学过程】 一、导入 通过上节课的研究,我们知道了“电能够产生磁”。那么我们如果把导线绕在一枚大铁钉上,铁钉又会出现什么变化呢?(板书课题:电磁铁) 二、制作铁钉电磁铁 1. 阅读P50制作铁钉电磁铁的部分,按照书上的方法制作铁钉电磁铁(或者观察模仿铁钉电磁铁制作视频)。 (1)朝着同一个方向绕导线。 (2)要将绕在铁钉上的线圈2头固定好。 (3)制作完成后,要通电试一试是否制作成功。 2. 学生活动,教师巡视指导。 3. 电磁铁做好了没有?怎么知道自己制作的电磁铁是否具有磁性?(注意学生对实验过程和现象的描述,指导正确的实验方法。) 4. 电磁铁做好以后,在不通电的情况下具有磁性吗?为什么?(介绍磁化现象)

三、铁钉电磁铁的南北极 1. 你们发现电磁铁的磁性哪里比较强?哪里比较弱?怎么知道的? 2. 组织学生讨论:对于普通的磁铁来说,磁性强的地方是磁极。电磁铁有磁极吗?我们可以验证电磁铁是否有磁极吗?怎么做?(观察铁钉电磁铁的南北极视频) 3. 学生汇报,教师小结实验方法。 4. 学生实验,教师巡视指导。 5. 汇报实验发现:电磁铁是否有磁极?南北极各在那一端? 6. 各小组电磁铁的磁极位置一样吗?为什么? (1)比较磁极位置不一样的小组的电磁铁(注意电池的接法要与汇报前一致),找出2者之间的差别。 (2)讨论:电磁铁的南北极与什么因素有关? (3)交流发现。

相关主题
相关文档 最新文档