Reactivity of nitrate and organic acids at the concrete–bitumen interface of a nuclear waste repository cell

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To cite this version :

Bertron, Alexandra and Jacquemet, Nicolas

and Erable, Benjamin and Sablayrolles, Caroline and Escadeillas,

Gilles and Albrecht, Achim Reactivity of nitrate and organic acids

at the concrete–bitumen interface of a nuclear waste repository cell.

(2014) Nuclear Engineering and Design, vol. 268 . pp. 51-57. ISSN

0029-5493

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administrator:

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Reactivity

of

nitrate

and

organic

acids

at

the

concrete–bitumen

interface

of

a

nuclear

waste

repository

cell

A. Bertron

a,∗

,

N. Jacquemet

a

,

B. Erable

b

,

C. Sablayrolles

c,d

,

G. Escadeillas

a

,

A. Albrecht

e

a_Université_de_Toulouse;_UPS,_INSA;_LMDC_(Laboratoire_Matériaux_et_Durabilité_des_{Constructions);}_135,_avenue_de_Rangueil;_F-31_077;_Toulouse_Cedex_04,

France

b_Université_de_Toulouse;_INPT,_UPS;_CNRS;_Laboratoire_de_Génie_Chimique;_4,_Allée_Emile_Monso,_F-31030_Toulouse,_France

c_Université_de_Toulouse;_INP;_LCA_(Laboratoire_de_Chimie_{Agro-Industrielle);}_ENSIACET,₄_allée_Emile_Monso,_BP₄₄_362,₃₁₄₃₂_Toulouse_Cedex_4,_France d_INRA;_LCA_(Laboratoire_de_Chimie_{Agro-Industrielle),}_F-31029_Toulouse,_France

e_Andra,_1-7,_rue_Jean-Monnet,₉₂₂₉₈_{Châtenay-Malabry,}_France

h

i

g

h

l

i

g

h

t

s

•Interactionsofcementpasteandorganicacid–nitratesolutionswereinvestigated.

•CementleachingimposedalkalinepH(>10)veryrapidlyintheliquidmedia.

•Aceticacidactiononcementpastewassimilartothatofclassicalleaching.

•OxalicacidattackformedCa-oxalatesalts;organicmatterinsolutiondecreased.

•Nitratewasstableunderabioticconditionsandwithorganicmatter.

a

b

s

t

r

a

c

t

Thisstudyinvestigatesthefateofnitrateandorganicacidsatthebitumen–concreteinterfacewithin repositorycellforlong-lived,intermediate-level,radioactivewastes.Theinterfacewassimulatedbya multiphasesysteminwhichcementitiousmatrices(CEMVcementpastespecimens)wereexposedto bitumenmodelleachatesconsistingofnitratesandaceticacidwithandwithoutoxalicacid,chemical compoundslikelytobereleasedbybitumen.Leachingexperimentswereconductedwithdailyrenewalof thesolutionsinordertoacceleratereactions.Theconcentrationsofanions(acetate,oxalate,nitrate,and nitrite)andcations(calcium,potassium)andthepHweremonitoredovertime.Mineralogicalchanges ofthecementitiousmatriceswereanalysedbyXRD.Theresultsconfirmedthestabilityofnitratesinthe abioticconditionsoftheexperiments.Theactionofaceticacidonthecementitiousmatrixwassimilarto thatofordinaryleachingintheabsenceoforganicacids(i.e.carriedoutwithwaterorstrongacids);no specificinteractionwasdetectedbetweenacetateandcementitiouscations.Thereactionofoxalicacid withthecementitiousphasesledtotheprecipitationofcalciumoxalatesaltsintheouterlayerofthe matrix.Theconcentrationofoxalatewasreducedby65%insidetheleachingmedium.

1. Introduction

InFrance,asignificantfractionoflong-lived,intermediate-level, radioactivewastes (bituminous wastes) consist ofa mixture of inorganicsaltsimmobilisedinabitumenmatrix.Theyarepoured incylindricalsteelcontainerscalledprimarypackages. Depend-ing onthedisposal concept, thesecontainers are groupedinto cuboidreinforcedconcreteoverpacks.Thesesecondarypackages

∗ Correspondingauthorat:LMDCINSAUPSGénieCivil,135avdeRangueil,31077 ToulouseCedex4,France.Tel.:+33561559931;fax:+33561559949.

E-mailaddresses:bertron@insa-toulouse.fr,alexandra.bertron@insa-toulouse.fr

(A.Bertron).

arethenplacedinsidewastecellsbuiltatadepthof450–550m within a Callovo-Oxfordian clay rock formation, using signifi-cantamountsofadditionalconcrete(i.e.lining,shotcrete)(Andra, 2005).

Afterclosureofthecells,waterresaturationthatprobablyfills thewastecellsfromthebottomup(bathtubeffect),reaching near-saturationwithinafewthousandyears,shouldenhancetherelease ofchemicalspecies.Amongthecompoundsprobablyreleasedinto theinterstitialaqueousmediumaresolublesalts(notablynitrates) andorganicmattersuchasorganicacids,phenols,etc.contained withinthebituminousmatrix(Walczaketal.,2001),togetherwith gas(mostlyH2)producedviaradiolysisoforganicmatterand/or

anaerobicsteelcorrosion(blackandstainlesssteelpresentin pri-marycontainersandconcretereinforcement).

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Table1

CEMV/Acementcomposition.l.o.i.:lossonignition.

CaO SiO2 Al2O3 Fe2O3 MgO TiO2 Na2O K2O MnO SO3 l.o.i.

CEMV/A42.5N 46.4 30.0 11.2 3.6 2.8 0.6 0.2 1.2 0.1 2.8 2.1

Thepresenceofnitratesinthevicinityofwastepackagesmay resultinoxidisingconditionsfavourabletothemobilityofaseries ofradionuclides(Se,U,Tc,Pu,Np,etc.)Albrechtetal.(2013). How-ever,inthegeochemicalconditionsprevailinginthecell,different redoxreactionsarelikelytooccur,includingnitratereduction driv-ingthesystembacktoareducingenvironmentfavourabletosafe repository.Reductionofnitrate(NO3−)mayoccur(i)fromabiotic

processes,withironfromthesteeland/orH2 actingaselectron

donors,andsurface catalysisprovidedby thedifferenttypesof steelandcorrosionproductspresentinthewastecell,and/or(ii) frombiologicalcatalysisthroughbacterialactivity.Thereactions mayleadtotheformationofnitrite(NO2−),gaseousnitrogen(N2)

and/orammonium(NH4+),dependingonavarietyofparameters

notyetwellunderstood,particularlyinconcrete-dominated sys-tems(Devlinetal.,2000;Trucheetal.,2013a,b;Alquieretal.,2014; Libertetal.,2011; Bertronetal.,2013).Bothtypesofreactions (abioticandbiotic)involveelectrondonorsandnumerous inor-ganicandorganiccandidatesareavailableinthewastecellorin thehostrock(organicacids,H2,zero-valentmetals,etc.)Albrecht

etal.(2013).Allreactionsoccurinanenvironmentinfluencedby thealkalineconditionsimposedbytheconcretephases.Theoverall project,ofwhichthispaperisapart,aimstoinvestigatethe reduc-tionofnitrateswithinasystemcomparabletoa“real”waste-cell wherebacterialactivityislikelytooccur(denitrifyingalkaliphilic bacteria)andnotablytodeterminethephenomenologyand kinet-icsofreactionsandtheroleoftheelectrondonor(i.e.organicacids releasedbybitumen,suchasaceticoroxalicacids)inthereactions (Alquieretal.,2012,2013,2014).

However,previousstudieshaveshownthatstronginteractions occurbetweencementitiousmaterialsandorganicacids(Bertron etal.,2005a,b;Bertronetal.,2007;Larreur-Cayoletal.,2011a,b; OueslatiandDuchesne,2012;BertronandDuchesne,2013).These reactionscouldmodifynotonlythebioavailabilityoforganic mat-ter – and thus influence thereactivity of nitrates under biotic conditions–butalsothecompositionoftheconcreteanditsability tobufferthechemicalsystem(i.e.pH).

An important prerequisite was therefore to investigate the interactionsoforganicacidswiththecementitiousmatrixinthe presenceofnitratesunderaerobicconditionsandwithoutbacteria. Reactionsandtherelatedequilibriaofmultiphasesystems repre-sentativeofthebitumen–concreteinterfacewereinvestigatedin simplifiedbatchsystemsandinshorttermexperiments.CEMV pastespecimenswereimmersedinsolutionsmadeofaceticand/or oxalicacidsandnitratesthatsimulatedbitumenmodelleachates. Changesinthecementpastesandliquidphasecompositionswere investigated.

2. Materialsandmethods 2.1. Materials

2.1.1. Cementpastes

ThespecimenswereCEMV/A42.5(S-V)NCEPM-ES-CP1cement pastes(slagandflyashcementfromtheAirvaultCalciafactory, chemicalcompositiongiveninTable1)withawater/cementratio of0.40.Theywerecastincylindricalplasticmoulds50mmhighand 27mmindiameter(volume=28.6cm3_and_surface_area₌_53.9_cm2₎

withoutdemouldingoilandwerevibratedtoevacuateairvoids. Thespecimensweretakenoutoftheirmoulds24hafterpouring andstoredinwaterat20◦_C_for₂₈_days._They_were_then_subjected_to

theleachingtestsdescribedbelow.Inparallel,somecontrol speci-menswerekeptinwaterat20◦_C.

2.1.2. Bitumenmodelleachatesolutions

Uncertainties remain on the release of organic and inor-ganicmatterbybitumen–saltmixtures.Experimentalstudieshave shownthatthedegradationofbitumengivesrisetotherelease oforganicmatter(naphthalene,alcohols,linearcarboxylicacids, aromaticsandglycols)andsalts(NaNO3,Na2SO4,etc.)(Walczak

etal.,2001;VanLoonandKopajtic,1990;LibertandWalczak,2000; Nakayamaetal.,2003;Marienetal.,2008;Walczak,2000;Kagawa etal.,2000).

Onthebasisofthisliteraturedata,leachingofbitumenwas sim-ulatedbyaqueoussolutions madeoforganicacidsandnitrates. Linearcarboxylic acids were considered(i) since theyare eas-ily assimilatedby (Ma etal., 2012;Daniel etal., 2007) and(ii) becauseoftheirprobablestronginteractionswithcementphases. Experimentsofbitumendegradationbywateruptakeand/or radi-olysishaveshownthepresenceofacetic,formicandoxalicacids in the leachates (Walczak et al., 2001; Van Loon and Kopajtic, 1990;Walczak,2000;Kagawaetal.,2000).Amongexperimental parametersinfluencingthequantityandnatureofacidsare:pH oftheleachingsolution,irradiation,temperatureanddurationof experimentand presenceor lackofoxygen. Inleaching experi-mentsperformedatambientormoderatetemperature(≤45◦_C),

concentrations ofacids of theorder of tenmilligrams perlitre havebeenfoundintheleachates.Oxalicacidwasnotablydetected afterradiolysisofbitumeninthepresenceofoxygen;incoherence withexperimentsyieldingoxalateproductionduringirradiationof formicacidunderoxygen.VanLoonandKopajtic(1990)concluded thattheproductionofoxalateintheabsenceofoxygenwas ques-tionable.Oxygeninitiallypresentinthewastecellsafterclosure willberapidlyconsumedeitherreactingwithcomponentsofthe clayhostrockorwithmetalspresentinthereinforcedconcrete, butitmustbekeptinmindthatthewastecontainerswillbestored forseveraldecadesinanoxygen-bearingatmosphere.

Calciumsaltsofformic andacetic acidsbeinghighlysoluble inwater(LangeandDean,1985),itislikelythatbothacidshave similareffectsonthecementitiousmatrix(BertronandDuchesne, 2013).Incontrast,calciumoxalatesaltsareonlyslightlysolublein water(Seidell,1919;Streitetal.,1998)andarethuslikelytobe producedduringtheattackofcementitiousmatrixbyoxalicacid. Aceticandoxalicacidswerethereforeconsideredinthepresent study.

Twotypesofmodelleachatesolutionswereconsidered.Thefirst typewasmadeofaceticacid(0.50mMto30mg/L)andsodium nitrate(32.3mM).TheinitialpHofthissolution,notedasN+OA1, was4.Thesecondtypewasamixofaceticacid(0.33mM),oxalic acid(0.17mM)(sametotalconcentrationofacidsof0.50mM)and sodiumnitrate(32.3mM).ThepHofthissolution,notedasN+OA2, was3.6.Thenitrateconcentration(32.3M)wasonlyanestimateof thelikelyconcentrationinawastecell(resultsofcoupled chemi-calandtransfermodelling)(Sercombeetal.,2006;Gwinneretal., 2006).Table2summarisesthecompositionofthetestsolutions.

2.2. Experimentalset-upandleachingprocess

TheexperimentaldeviceisshowninFig.1.Batchsystemswere implemented.

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Table2

Compositionsofbitumenmodelleachates.

Solutions Concentration pH

Aceticacid Oxalicacid NitrateNO3−

mg/L mM mg/L mM mg/L mM

N+OA1 30 0.50 – – 2000 32.3 4.0

N+OA2 20 0.33 15 0.17 2000 32.3 3.6

Fig.1.Experimentalset-up.

Thereactor,filledwith1Lofnitrateandorganicacidsolution (N+OA1orN+OA2)wasequippedwithanoutletforsolution sam-pling,agasinletwithcheckvalve(usedforN2bubblingtoimpose

anoxicconditionsinsidethereactorsimilartothosethatwill pre-vailinsidearepositorycell),agastighttapforapHprobe,anda hermeticallyclosedlidfittedwithagasvent.

ThepHprobewasconnectedtoadataacquisitionsystem (Con-sort,D230DataAcquisitionSystem,v1.1.13).Thesolutioninthe reactor was continuously stirred using a magnetic barrel. The cementpastespecimenwassuspendedinthesolutionbyaPTFE thread. The solid/liquid volume ratio wasabout 3% (similar to thatimplementedin thebiotic partof thestudy(Alquieretal., 2012)).Theexperimentaldevicewaskeptinanair-conditioned room(20◦_C)_during_the_whole_experiment.

N+OA1 (acetic acid 0.5mM; NO3− 32.23mM) and N+OA2

(acetic0.33mMandoxalic0.17mMacids;NO3−32.23mM)

solu-tionswerereneweddailyfor5days.Theseshorttermexperiments werechosenbecausethekeyphenomenaimpactingbacterial reac-tion(exploredinthebioticpartoftheoverallstudyofwhichthis paper is partof (Alquier et al., 2013,2014)) occurin the very shorttermoftheexperiment(becauseoftheverysevere condi-tionsrapidlyimposedbythecementitiousmaterialsintheliquid medium).Moreover,itshouldbenotedthattheseshortterm leach-ingexperimentswerefavouredwiththeviewtolimittheseverityof themediaandensuremicrobialgrowth(andthusmicrobial activ-ity)inthecorrespondingbiotictesting(Alquieretal.,2013,2014).

Table4

Detectionandquantificationlimits(DLandQL)ofions(mmol/L)(S/N=Signal/Noise). Acetate Oxalate Nitrite Nitrate Potassium Calcium DL(3<S/N) 0.68 0.79 1.74 2.42 0.402 0.764 QL(10<S/N) 2.04 2.61 6.09 8.06 1.339 2.546

Duringthefirstdayofexposure,thesolutionwassampled4times (20–25mLeachsample).Thesolid/liquidratiochangedverylittle. Ondays2–5,aliquidsamplewastakenfromthesolutionjustbefore renewal.ConcentrationsofCa,K,acetate,nitrateandnitritewere measuredoneachsample.

Controlexperimentswerecarriedouttoinvestigatethe possi-blesorptionofnitratesonthecementpasteinthesameconditions (NO3−32.3mM,solid/liquidratio3%,durationoftheexperiment:

15dayswithnorenewalofthesolution).Nodecreaseofnitrate concentrationwasobservedinthesolution.Moreover,the cemen-titiousspecimenswererinsedthreetimesandtherinsingsolutions wereanalysedbyHPICtodeterminenitrateandnitrite concentra-tion.Theconcentrationswerelowerthanthedetectionlimitsofthe equipment(Table3).

2.3. Analyticalmethods

2.3.1. Methodsfordeterminingionconcentrationsofsolutions Concentrationsofanions(acetate,oxalate,nitrateandnitrite) andofcations(calciumandpotassium)weremeasuredbyHigh PerformanceIonChromatography(HPIC)coupledtoa conducti-metricdetectorfittedwithchemicalsuppressor(DionexICS-2000 andICS-3000).TheanalyticalconditionsaresummarisedinTable3. Quantificationwasperformedbyexternalcalibrationoverthe concentration range 1–40mg/L witha regression coefficient of 0.999calculatedfrompeakareas.Repeatabilitywasassessedon 5injectionsandwasbetterthan98%.Biaserrorwasbetween5and 10%.Thedetectionlimit(definedasinjectedquantitygivinga sig-nal/noiseratioof3)andquantificationlimit(definedasinjected quantitygivingasignal/noiseratioof10)werecalculatedforeach compound(Table4).

Liquidsampleswerefilteredat0.2mm(MinisartPES,Fisher Sci-entific)toremovesuspendedsolidmatter.

2.3.2. Mineralogicalanalysesofcementitiousspecimens

To explore the mineralogical changes, cement paste sam-pleswereanalysedbyX-Raydiffraction(SiemensD5000;copper cathode;anodevoltage40kV;currentstrength30mA)afterthe leachingtests.Themeasurementswereperformedaccordingtothe distanceinwardsfromtheplanecylindersurfaceincontactwiththe solutions.Thefirstanalysiswasmadeontheplaneexternalfaceof thespecimen,whichwasthenabradedwithapolishingdiscand subjectedtothenextanalysis.Thedepthofanalysiswasmeasured usingacallipersquare.Thelastanalysis,whichwasonthecoreof thespecimen,wascarriedoutatadepthof5mm(afterthe speci-menhadbeensawnperpendicularlytoitsaxis).Acontrolspecimen wasalsoanalysed5weeksafterpouring.

Table3

HPICtestconditions.

Eluant(1mL/min) Precolumn Chromatographic

column

Suppressor

Anions KOH(1×10−3_mol/L)

elutiongradient:10%mobilephaseto 60%mobilephasein25min

NG1(4mm×50mm, Dionex)+IonPacAG11-HC (4mm×50mm,Dionex) IonPacAS11-HC (4mm×250mm, Dionex) ASRS300(4mm, Dionex)+CRD200 (4mm,Dionex) Cations Methylsulfonicacid(3010−3_mol/L)

isocratic(30min) NG1,(4mm×50mm, Dionex)+IonPacCG16 (4mm×50mm,Dionex) IonPacCS16 (4mm×250mm, Dionex) SuppressorCSRS300 (4mm,Dionex)

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Fig.2.Macroscopicobservationofthecementpastespecimens.(a)Control spec-imenkeptinwater,(b)specimenafter5daysofleachingbyN+OA1(aceticacid 0.50mM,nitrate32.3mM)solutionand(c)specimenafter5daysofleachingby N+OA2(acetic0.33mMandoxalic0.17mMacids,nitrate32.3mM)solution.

3. Resultsanddiscussion 3.1. Macroscopicobservations

Thecementitiousmatrix,initiallygrey,hadturnedtoayellowish colourattheendoftheleachingexperiments(Fig.2).Thisis charac-teristicoftheleachingofacementitiousmatrixbyacidicsolutions whereFe(III)ofhydratedandor/anhydrousphasesprecipitatesin theformoforangey-colouredironhydroxide.InCEMV-type anhy-drouscement,ironiscontainedinflyashesandintheC4AF-phase

(brownmillerite)ofclinker.Iron-bearingphasesofcementhydrate moreslowlythansilico–calcicphases.Inahydratedcementitious matrix,ironisfoundinanhydrousresidualphasesandaspartial substitutioninhydrates.TheouterlayerofleachedCEMIandCEM Vpastesisenrichediniron(relativeenrichment)(Bertronetal., 2007;Moudilou,2000;Pavlík,1994).Ironintheouterlayeris prob-ablycontainedinresidualanhydrousflyashes,inC4AF(thisphase

ismoreresistanttoleachingthanotherhydratedandanhydrous phasesofPortlandmatrix(Bertronetal.,2005a)andinthesilica gel,issuedfromleachedcalciumsilicatehydrates(mainhydrated phaseofcementitiousmatrices).

3.2. CompositionandpHofthetestsolutions

Concentrationsof K+ _and _Ca2+_of _N₊_OA1 _and _N₊_OA2

solu-tionsincementpasteimmersiontestsareshownversustimein

Figs.3and4.ConcentrationsofOH−_ions,_calculated_from

contin-uousmeasurementofpHarealsorepresented.Concentrationsof

Fig.3. ConcentrationsofK+_,_Ca2+_and_OH−_in_N₊_OA1_(acetic_acid_0.50_mM;_nitrate

32.3mM)leachingsolution.Thesolutionwasreneweddaily.Solidarea/liquid vol-ume≈50cm2_/L._{Concentration}_of_OH−_was_calculated_from_the_pH_values._For_days

2–5,variationsofKandCaconcentrationsarebasedonthefinalconcentrationand anevolutionsimilartoday1.

Fig.4.Concentrations ofK+_, _Ca2+ _and_OH−

in N+OA2 (acetic 0.33mM/oxalic 0.17mMacids;nitrate32.3mM)leachingsolution.Thesolutionwasreneweddaily. Solidarea/liquidvolume≈50cm2_/L._{Concentration}_of_OH−_was_calculated_from_the

pHvalues.Fordays2–5,variationsofKandCaconcentrationsarebasedonthefinal concentrationandanevolutionsimilartoday1.

nitrate,nitrite,acetateand oxalateofthesolutionsaregiven in

Table5.

ThevariationsofpHweresimilarforthefive24-hcycles:from theinitialvaluesof4.0forN+OA1and3.6forN+OA2,pHincreased rapidlyduringthefirst6hofleaching,toreachabout9.5, then increasedslowly toreach 10.6 in N+OA1 and 10.5 in N+ OA2 after24h(Figs.3and4).LongerreactiontimeswouldallowpHto increaseevenfurther.Alkalineconditionswerethusveryrapidly imposedintheleachingmedium.TheincreaseinpHwasmostly duetothereleaseofhydroxideionsbythedissolvingcementpaste matrix.

Concentrationsof K+ _and _Ca2+ _varied_the _same_way._During

thefirstleachingcycle,variationsfollowedthoseofpHand con-centrationsreached[Ca2+_]_≈_0.25_mmol/L _and_[K+_]_≈_0.15_mmol/L

in N+OA1 and [Ca2+_]_≈_0.33_mmol/L _and _[K+_]_≈_0.17_mmol/L _in

N+OA2. For leaching days 2–5, concentrations at the end of thecycle decreased progressivelytoreach [Ca2+_]_≈_0.25_mmol/L

and [K+_]_≈_0.09_mmol/L _in _N₊_OA1_and _[Ca2+_]_≈_0.22_mmol/L _and

[K+_]_≈_0.08_mmol/L_in_N₊_OA2_at_the_end_of_the_5th_leaching_cycle.

Concentrations of nitrates in N+OA1 and N+OA2 (Table 5) remainedequivalenttotheinitialvalue(32.3mM).Nitriteswere notdetectedineithersolution.Moreover,cationchromatograms didnot reveal anypeak due toammonium NH4+ produced by

abioticreductionofnitratesunderanoxicconditions.Ammonium hasnotablybeenobservedby(Alquieretal.,2012;Trucheetal., 2013a,b)inthepresenceofvarioustypesofsteelactingas reac-tioncatalysts.Resultsobtainedinthisstudyindicatethatnoabiotic reductionofnitrateoccurredintheconditionsoftheexperiment.

Table5

Evolutionofconcentrations(mmol/L)ofanionsinN+OA1andN+AO2solutions withtimeofcementpasteimmersion.Solutionreneweddaily.Solidarea/liquid volume≈50cm2_/L._<DL:_lower_than_the_detection_limit.

Time(day) N+OA1solution N+OA2solution

[Ac] [NO3] [NO2] [Ac] [Ox] [NO3] [NO2]

0.00 0.57 31.6 <DL 0.37 0.17 31.8 <DL 0.08 0.53 31.9 <DL 0.30 0.05 32.1 <DL 0.67 0.62 31.4 <DL 0.26 0.07 31.7 <DL 0.79 0.60 31.7 <DL 0.34 0.07 31.7 <DL 1.00 0.61 31.6 <DL 0.34 0.06 31.8 <DL 2.04 0.55 31.8 <DL 0.32 0.05 31.7 <DL 3.04 0.52 31.7 <DL 0.28 0.06 31.9 <DL 3.99 0.47 31.7 <DL 0.31 0.05 31.7 <DL 4.97 0.56 31.6 <DL 0.30 0.05 31.8 <DL

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70 68 66 64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4

2 θ

-

C

₀

K

_α Ca

Core

or zone1

Zone 2

150 => 1000

μm

Ca Ca Ca Ca Ca Ca Ca Ca Ca Et A Po Et Et Po : portlandite (04-0733) Et : e=ringite (41-1451) A : C4AHX(02-0077) Hy : hydrotalcite (35-0964) Po + C3S Po Po Po Ca + Et Po C2S + C3S Mu+ Et C3S+ C2S+ Mu+ Et Q C3S (31-0301) β-C2S (33-0302) Mu : mullite (15-0776) Q : quartz (33-1161) Ca : calcite (05-0586) Mu Mu Q A Q Q Q Et Et Et Ca Ca + Et _Mu+ Et C3S+ C2S+ Mu+ Et Ca Ca Q C2S + C3S Ca _Ca Hy Mu Q Hy

Zone 3

Surf. => 150

μm

Fig.5. XRDmineralogicalanalysesofCEMVpastespecimenafter5-dayleachinginN+OA1(aceticacid0.5mM,nitrate)solutionatvariousdistancesinfromthesurfaceof thespecimen.

ConcentrationsofacetateinN+OA1andN+OA2werealmost constant throughouttheexperiment and weresimilartoinitial concentrations(Table2).Incontrast,theconcentrationofoxalate, whichwasinitially0.17mmol/L,haddecreasedatthefirst sam-plingtime(0.08day∼2h)andremainedrelativelystable(equalto 0.06±0.01mmol/L)inthefollowingsamples.

3.3. Mineralogicalchangesinthecementpastes

Figs. 5 and 6 present the mineralogical characterisation by XRD ofthe CEM Vpaste specimens atthe end of theleaching experimentsbyN+OA1andN+OA2solutionsrespectively. Diffrac-togramsarepresentedaccordingtothedistancefromthesurfacein contactwiththetestsolution.Theanalysesshowedthatalterations ledtoamineralogicalzonationofthespecimens.

Diffractogramsofcontrolzones(“coreorzone1”onFigs.5and6) performedat5mmdepthpresenttypicalpeaksofanhydrous(C3S

or3CaO·SiO2,C2Sor2CaO·SiO2,mullite3Al2O3·2SiO2)andhydrated

(ettringite 3CaO·Al2O3·3CaSO4·32H2O, portlandite or Ca(OH)2,

hydratedcalciumaluminatesC4AHxor4CaO·Al2O3·×H2O)

cemen-titiousphasesandofquartz.

The peripheral part of cement pastes immersed in N+OA1 (aceticacid)solutionshowedtwozoneswithdifferent mineralog-icalcompositions (zones 2and 3 inFig. 5).Thetotal thickness of thetwo zoneswas about1mm. In zone 2(from 150mmto 1000mm from the surface) portlandite had disappeared and a hydrotalcite-type (Mg4Al2(OH)14·3H2O) compound had

precipi-tated.Precipitation of hydrotalcite(carbonated ornot)hasalso beenobservedbyFauconetal.(1996)indegradedzonesofcement pastespecimensleachedatpH7.Calcitealsoprecipitatedinzone 2,probablybecauseofa re-combinationofcalciumreleasedby portlanditedissolutionwithcarbonatereleasedbythedissolution of carbonatedAFm.Zone 3(from thesurface to150mm deep), wasmorestronglycarbonated.Allcrystallisedcementitiousphases

haddisappeared.Thisthincarbonatedlayerwasalsoobservedon controlspecimenskeptinwater(carbonationofspecimen’sskin becauseofambientCO2 dissolvedin repositorywater).Neither

Ca-acetatenorNa-acetatecharacteristicpeakswerefoundonthe diffractogram.

The peripheral zone of the specimen immersed in N+OA2 (acetic/oxalicacids)solutionshowed3zoneswithdifferent min-eralogicalcharacteristics(zones2–4onFig.6).Thesethreezones togethermadeup atotal a thicknessof approximately1.3mm. As for specimens immersed in N+OA1, portlandite had disap-peared in zone 2 (from 400mm to 1300mm deep) where a hydrotalcite-likecompoundhadprecipitatedtogetherwithcalcite. Zone3(300–400mmdeep)wascharacterisedbyettringite disso-lution.Calciteand calciumoxalatemono-hydrateorwhewellite (CaC2O4·H2O)hadprecipitated.Theouterlayerofthespecimenor

zone4(fromthesurfaceto300mmdeep)showedtypicalpeaks ofcalcite,whewelliteand calciumoxalatetri-hydrateorcaoxite (CaC2O4·3H2O).NeitherCa-orNa-acetatenorNa-oxalatesaltpeaks

werefoundinthediffractograms.

Precipitation of whewellite in cement paste specimens immersed in oxalic acid solutions (0.28M) has been observed experimentallyby Larreur-Cayolet al.(2011a,b).Ca-oxalate tri-hydrate, mentioned by Streit et al. (1998) is not very stable (Deganelloet al., 1981)and is veryeasily transformedinto di-or mono-hydrate Ca-oxalate (Echigo et al., 2005; Tomazic and Nancollas,1979).Nevertheless,theprecipitationofCa-oxalate tri-hydrate jointly with mono-hydrate has already been observed (Heijnenetal.,1985;Opalkoetal.,1997).

Finally,theresultsofcementpastemineralogicalanalyses cou-pled with those of chemical analyses of the solutions showed thattheexposureofCEMVpastespecimenstoN+OA1solution wassimilartoaleachingofcementpaste(BertronandDuchesne, 2013;Fauconetal.,1996;Moranvilleetal.,2004;Duchesneand Bertron,2013).Acetateiondidnotcombinewithcationsreleased

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70 68 66 64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4

2 θ

-

C

₀

K

_α

Core

zone 1

or

Et A Po Et Et Po : portlandite (04-0733) Et : e=ringite (41-1451) A : C4AHX(02-0077) Hy : hydrotalcite (35-0964) Po + C3S Po Po _Po Ca + Et Po C2S + C3S Mu+ Et C3S+ C2S+ Mu+ Et Q Ca Ca Ca+ Wh+ Co Ca Ca Ca Ca + Wh Ca Ca Q Et Et Et Ca Ca + Et Ca Ca Ca Ca Ca + Wh Ca Ca Ca Ca + Wh Ca Ca Q Ca Wh Wh Wh+ Co _Wh Wh Wh Wh Wh Wh Wh Co Co Co Co Co Q Co C3S (31-0301) β-C2S (33-0302) Mu : mullite (15-0776) Q : quartz (33-1161) Ca

Zone 2

=> 1300

(400

μm)

Zone 3

(300 =>

40

0 μm)

Zone 4

(Surf.

=>

30

0 μm)

Wh Wh Q Ca : calcite (05-0586) Wh : whewellite (13-0601) Co : Calcium oxalate hydrate (20-0232)

C3S C2S + C3S Ca _Ca Hy Mu Mu Mu Mu Mu A Q Q Q Q Q Hy Mu+ Et C3S+ C2S+ Mu+ Et

Fig.6. XRDmineralogicalanalysesofCEMVpastespecimenafter5-dayleachinginN+OA2(acetic0.33mM/oxalic0.17mMacids,nitrate)solutionforvariousdistancesin fromthesurfaceofthespecimen.

bythecementitiousmatrix.Thisresultisinaccordancewiththose obtainedbyotherauthorswhohaveworkedontheattackofacetic acidbutwithdifferentconcentrationsofacids(Bertronetal.,2007; OueslatiandDuchesne,2012;Pavlík,1994).Moreover,itshouldbe notedthatanumericalanalysisbyLarreur-Cayoletal.(2011a,b)

suggestedthatnoCa-acetateaqueouscomplexformsinsolutions withacetateconcentrationsrangingfrom10−5_M_to_0.3_M._Thus,

regardingtheframeofthestudy,thereactionsbetweenaceticacid releasedbybitumenshouldnotmodifyacetatebioavailabilityfor bacteriathatmaybepresentinsidethewastecells.

Incontrast,whenoxalicacidwaspresentinsolution,reactions ofoxalateanionwithcementitiouscations(calciumatleast)did occurandledtotheformationofvariousformsofcalciumoxalate salts.Precipitationofcalciumoxalatesaltinoxalicacidsolution containingcalciumhasbeenconfirmednumericallyby Larreur-Cayol(2012)foralargerangeofoxalateactivity(10−5_M–0.3_M).

Inthepresentstudyconductedwithoxalicacidconcentrationof 0.17mM,65%ofoxalatewascombinedwithcementitiouscations. Consequently,withrespecttobioticstudiesconductedinthe pres-enceofcementitiousmatrices,oxalatecouldbelessbioavailableto bacteria.

4. Conclusion

The overall study, of which this paper is a part, aimed to investigatethereactivityofnitrateunderbioticconditions(biotic catalysis of nitrate reduction) in the context of repository of long-lived,intermediate-level,radioactivewastes.Theaimofthe presentpaperwastoinvestigatetheinteractionsbetweenthe var-iouschemicalspecies(nitrates,bitumenmodelleachates)andthe cementitiousmatrix underabiotic conditions,withtheviewto determiningthechemicalconditionstowhichbacterialcandidate speciescouldbeexposedinthebioticpartofthestudy.Leaching experimentswereconductedonbatchsystemscomprisingCEM

Vpastespecimensincontactwithsolutionsmadeofnitrateand aceticacidoraceticandoxalicacids(initialpHof4and3.6).Results showedthatanalkalinepH(>10)isveryrapidly(lessthan24h) establishedintheaqueousmedium.Theimpactofbitumenmodel leachatesmadeofaceticacid(0.50mM)wassimilartothosefound inclassicalleachingtestsofcementitiousmatrixasnoparticular interactionbetweencementitiouscationsandacetateanionswas observed.Thus,inexperimentsinvolvingbacteriaanda cementi-tiousmatrix,organicmatterwouldremaininsolutionandwould thereforemoreprobablybeavailableforoxidationandasacarbon sourceformicroorganisms.Incontrast,thestudyconductedwith themodelleachatemadeofacetic(0.33mM)andoxalic(0.17mM) acidsshowedastronginteractionbetweenoxalateanionsand cal-ciumcationsreleasedbythecement paste.65% ofoxalatewas combinedwithcalciumtoprecipitateascalciumoxalate mono-andtri-hydratesaltsintheperipheryofthematrix.Oxalatewould thusbelessavailableforbacteria.Finally,thestudyconfirmedthe stabilityofnitratesunderabioticconditionsinthepresenceofa cementitiousmatrixandorganicmatter.

Acknowledgements

TheauthorsthankAndra(FrenchNationalRadioactiveWaste ManagementAgency)foritsscientificandfinancialsupport.

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