Diuron abatement using activated persulphate: Effect of pH, Fe(II) and oxidant dosage

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Diuron abatement using activated persulphate: Effect of pH, Fe(II) and oxidant dosage
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  ChemicalEngineeringJournal 183 (2012) 357–364 ContentslistsavailableatSciVerseScienceDirect Chemical   Engineering    Journal  j   ournal   ho   mepage:www.elsevier.com/locate/cej Diuron   abatement   incontaminated   soil   using   Fenton-like   process Fernando   Vicente ∗ ,Aurora   Santos,   Elena   G.   Sagüillo,   Ángel   M.   Martínez-Villacorta,    Juana   MaríaRosas,Arturo   Romero DptoIngenieriaQuimica,FacultaddeCienciasQuímicas,UniversidadComplutenseMadrid,CiudadUniversitariaS/N,28040Madrid,Spain a   r   t   i   c   le   i   nf   o  Articlehistory: Received2September2011Receivedinrevisedform22   December2011Accepted3January2012 Keywords: CitrateDiuronFenton-likeSoilremediation a   b   s   t   ra   ct Theuseof    catalyzedH 2 O 2  propagations   (CHP)   by   the   Fenton-likeprocess   hasbeenevaluated   for   thetreatment   of    contaminated   soilby   diuron(( q Diuron ) o  =   40   mg   kg − 1 )   atnatural   neutral   pH.Theinteractionof    H 2 O 2  with   the   soil   inbatch   system   has   been   previously   studied   at   four   differentinitial   H 2 O 2  concen-tration   (20,000–40,000–60,000–120,000   mg   kg − 1 )   atroom   temperature   inorder   toevaluate   the   oxidantdecomposition   by   the   natural   reductants   species   (soil   organicmatter   and   iron   minerals).Theoxidation   of    diuronusingonlyH 2 O 2  at   aninitialconcentration   of    60,000mg   kg − 1 wasfirstlydetermined(  X  Diuron  =   55%).Anothertest   wasdeveloped   tostudythe   effectof    the   addition   of    citrate( C  Citrate  =3000mg   kg − 1 )   asa   chelating   agent   toincrease   the   removalof    the   contaminant   up   to73%   sug-gesting   that   the   concentration   of    iron   mobilized   by   citratefrom   the   soil( C  Fe ≈ 40mg   kg − 1 )promotesthe   Fenton’sreaction.   Finally,   athirdexperiment   wasassessedusinganextraamountof    ironsalt( C  Fe 3 +  =   600   mgkg − 1 )inaddition   tocitrate,   resulting   toan   increase   inthe   diuronremoval(  X  Diuron  >   80%)because   citratemakesstable   iron   chelateswith   Fe 3+ and   increases   the   generation   of    OH • in   aqueousphaseand   ahigherdiuronremovalis   obtained.Ecotoxicitymeasurements   in   the   soil   andliquid   phases   at   initial   and   after   the   Fenton’s   treatment   by   theMicrotox ® bioassay   demonstrated   that   low   toxicity   isobtained   inthe   treated   soiland   temporal   toxicityvalues   inthe   liquid   phaseisindirect   correlationwith   the   residualH 2 O 2 .   Thetoxicity   values   decreasewith   the   H 2 O 2  concentrationtonottoxic   values   with   afinal   total   degradation   of    the   oxidant. © 2012 Elsevier B.V. All rights reserved. 1.Introduction Herbicidesarethemostextensivelytypeof    pesticidesusedinagriculture.Duetotheirlimitedbiodegradability,highvapourpres-sureandhighlipidaffinity,haloaromaticherbicidesareconsidereda   persistentclassofchemicals[1].Diuronisoneof    theseherbicideswhichhasbeenfrequentlyusedinvineyardsandithasbeenfoundinnaturalwateratconcentrationsexceedingtheregulatorylimitof    0.1  gL  − 1 [2].   It   isconsideredapriorityhazardoussubstancebytheEuropeanCommission.Thedispersionof    thiscompoundinagricultureleadstopollutionoftheaquaticenvironmentbysoilleaching[3].Becauseof    itstoxicityforaquaticorganismsandsus-picionofbeingcarcinogenicforhumans,diuronistheobjectof growingenvironmentalconcern[4].Agriculturaluseof    herbicidesresultsinrelativelylowsoil   con-tamination(diffusecontamination).Inaddition,nearproduction ∗ Correspondingauthor.Tel.:+3491   3944171;fax:+34913944171. E-mail   addresses: fervicen@quim.ucm.es(F.   Vicente),aursan@quim.ucm.es(A.Santos),egsaguillo@quim.ucm.es(E.G.Sagüillo),angelmma@pas.ucm.es(Á.M. Martínez-Villacorta), jmrosas@quim.ucm.es(J.M.Rosas),aromeros@quim.ucm.es (A.Romero). orstorageoftheseherbicidesmay   causeepisodesof    concentratedcontaminatedsoil   causedbyspillageduringpesticidemixing,load-ingandrinsingoperationsordirectdumplignintosoildisposalsites.Thesepracticesarethoughto   beresponsibleforapproxi-mately45%ofcasesofgroundwatercontamination[5].In   situchemicaloxidation(ISCO)seemstobea   viabletechnologyforremediationofpesticides.Inthistechniquea   strongchemi-caloxidantisinjectedintothecontaminatedsurfaceto   destroythetargetedcontaminants[6].   Theoxidantsfrequentlyusedarehydrogenperoxide(H 2 O 2 ),permanganate(MnO 4 − ),   persulphate(S 2 O 82 − )   andozone(O 3 )   [6–14].   Eachoftheseoxidantshavelimi-tations,(e.g.,persistence,reactivity,etc.)withina   soilmatrix.H 2 O 2  has   longbeenusedin   industrialapplicationsandinwatertreatmentprocesses.Itis   particularlyeffectivewhenitis   appliedincombinationwitha   catalystsuchasferrousiron(Fe 2+ )toproduceFenton’sreagent.Thereactionchemistryiscomplex,but   poten-tiallycapableof    degradinga   widerangeoforganiccontaminantsdependingonconditions[15].Thefollowingreactionsmayoccur:H 2 O 2 + Fe 2 + →   Fe 3 + + OH − +   HO • (1)Fe 3 + + H 2 O 2 → Fe 2 + + H + + HO 2 • (2)Fe 3 + + HO 2 • → Fe 2 + + O 2 + H + (3) 1385-8947/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.010  358  F.Vicenteetal./    ChemicalEngineeringJournal 183 (2012) 357–364 HO • + Fe 2 + → Fe 3 + + OH − (4)HO • + H 2 O 2 → HO 2 • + H 2 O(5)HO 2 • + Fe 2 + + H + → Fe 3 + + H 2 O 2  (6)2H 2 O 2 → 2H 2 O +   O 2  (7)ThecombinationofsolubleFe 2+ saltsandH 2 O 2  produceshydroxylradicals(OH • )   accordingto   thereactionin   Eq.   (1).   ThisEquationimpliesafastdecompositionof    H 2 O 2  producinghydroxylradicalswhiletheregenerationofthecatalyst(Eq.(2))   isveryslow,beingthislasttheratecontrollingstep.ThisprocessofusingH 2 O 2  forISCOhasbeentermed“CatalyzedHydrogenPeroxidePropagation”(CHP)[16].Therearealsoanothertermssuchas“ModifiedFenton’sreagent”or“Fenton-like”reac-tionsthathavebeenusedtoreferto   ISCOapplicationwithCHPwhensomemodificationsof    ClassicalFentonreactionareappliedinordertosolvethelimitationsof    theclassicalprocessasacidpHrequierement.ModifiedFenton’sreactionsapplyconcentratedH 2 O 2  solutions[17],ironchelatingagents[18–20],   H 2 O 2  stabilizingagents[21]orironcontainingminerals[22–26].   H 2 O 2  concentra-tionishigherthantheoreticaldemanddosagerequiredfortheremediationofwastewatersbyFenton’sreagentduetotheunpro-ductiveconsumptionoftheoxidantbythenaturalcomponentsof thesoils.Constituentsinthesoilrepresentthebulkof    interferencesthatmusthaveto   takeintoaccountto   evaluatethemassofH 2 O 2 required.Giventhevariouschemicalstructuresthatpesticidesexhibit,thetransformationpathwaysandmechanismsforindividualcom-poundsmay   notbewellestablished.Thus,incaseswherethechemistryassociatedwithcontaminanttransformationsis   notwellknown,treatabilitystudieswillbeneeded.OnecommonthemefoundinstudiesofCHPdegradationofpesticidesisthatsolubilityandsorptiondegreeaffectstotherateandextentof treatmentachievable.Inthissensehighersolubility,lessstronglysorbedpesticidesmay   be   degradedmorerapidlyandtogreaterextents[22,27].   In   aqueousphase,pesticideshavebeenmoreextensivelystudiedwithCHPoxidation.Oxidationof    herbicides2,4-dichlorophenoxyaceticacidand2,4,5-trichlorophenoxyaceticacidinaqueoussystem[28–30]byCHPsystems(acidicpH,low oxidantconcentration,Fe 2+ catalyst)ofteninvolvesthereleaseof mono-,di-,andtri-chlorophenols,afterwhichthereactionpro-ceedsasdescribedforchlorophenols.LittleinformationisavailableonCHPdegradationofotherpesticidesin   contaminatedsoil,suchasdiuron.Takingintoaccounttheabove,themainobjectiveofthestudypresentedhereis   toinvestigatetheuseof    Fenton-likereactionfordiurondegradationatroomtemperatureinasandyclayloamsoil.Firstly,aseriesofexperimentswereconductedwithuncontami-natedsoiltostudythesystemsoil-oxidantandto   selecttheproperconditionstoreactwithcontaminantofconcern.ExperimentsweredesignedtodeterminetheinfluenceofvariousinitialH 2 O 2  concen-trations,thepotentialuseof    citrateaschelatingagentandtheeffectofadditionalamountofFe 3+ ascatalyticspecie.Thereforesubsequentexperimentsweredevelopedin   contami-natedsoiltodeterminetheeffectivenessofCHPforthedestructionofdiuronascontaminantmodelbytheFenton-likeprocess.Dueto   theslowFe 3+ reductionreactionsrelativeto   therapidFen-tonreaction(Fe 2+ oxidation),alowerefficientandslowerrateof HO • productionoccursafterFe 2+ is   initiallyreactedwithH 2 O 2 .Consequently,onedisadvantageof    Fe 2+ amendmentis   thatstoi-chiometricquantitiesarerequired.SoFe 3+ hasbeenselectedascatalyticspecietopromotetheFenton’sreaction.Furthermore,theuseofcitrateaschelatingagentisrequiredtoenhanceeffectivenessof    Fenton’sreactionundernaturalneutralpHconditionswithoutacidifyingthesystem.Thedosageof    H 2 O 2  andcitratehasbeenselectedaccordingpreviousstudiesperformedbytheresearchgroupwithsoilsatnaturalneutralpHwithbuffercapacity[45]. 2.Materialsandmethods  2.1.Reagents Analytical-reagentgradewereusedin   theexperiments.Diuron98%andsodiumcitrate2-hydratewerepurchasedfromSigma–Aldrich.H 2 O 2  30%(w/w)   was   providedfromRiedeldeHaën.FerricchloridehexahydrateobtainedfromFlukawasusedasthecatalyticFe 3+ specie.Potassiumpermanganateandsul-phuricacid95–98%fromPanreacwereusedinthedeterminationof H 2 O 2 .MethanolusedassolventandacetonitrileformobilephaseinHPLCwereobtainedfromScharlab.Naphthalene>99%fromSigma–Aldrichwas   usedasinternalstandardinHPLCmeasure-ments.Allof    thesuspensionsandsolutionswerepreparedwithMilli-Qwater(>18M  cm)   purifiedwitha   deionizingsystem.  2.2.Characterizationofsoilsample Thesoilselectedforthisstudywas   categorizedassandyclayloamatnaturalneutralpH.   Thepropertiesof    thesoilsamplearelistedinTable1.   Themeasurementsofthepropertieswereper-formedbytriplicatetoobtaina   averagevalue.Standarddeviationwasbetween4and7%.Thesoilselectedcorrespondedtosubsurfacehorizon(Bt)atanagriculturezonecharacterizedbyloworganiccarboncontent.ThepHwasmeasuredwithapHelectrode(Basic20-CRISON)in1:2.5soil/watersuspensions.Thesoilorganicmatter(SOM)contentin   thesoilwas   determinedbyincinerationofa   knownweightsam-pleplacedina   ceramiccrucibleinanelectricmufflefor3hat550 ◦ CobtainingthecorrespondingSOMcontentbymassdifference(NEN5754Method)[31].Soilcontainstwodifferentformsof    carbon:TotalOrganicCar-bon(TOC)andInorganicCarbon(IC).Totalcarbon(TC)   of    thesoilwasdeterminedusinga   TOCanalyzerasdescribedinthestandardprocedureEN13137.Firstly,thesamplewasdividedintotwosub-samplesandonesub-samplewasoxidizedwithanoxygenstream(500mLmin − 1 )   usingTOCanalyzer(TOC-VCSHShimadzu)witha   solidstatemodule(SSM-5000A)at900 ◦ C   wheretheformedCO 2  was   analyzedbynon-dispersiveinfraredabsorbance(NDIR).Thesecondsub-samplewaspre-treatedwith4mLofconcentratedH 3 PO 4  toremovetheinorganiccarbon(IC)andthemeasurementswastakenat200 ◦ C.   ThiscarbondioxideisdetectedbytheNDIR andthesampleIC   concentrationismeasuredinthesamewayasTC.TheIC   isa   combinationof    carbonateandbicarbonate.TheTOCconcentrationis   determinedbydifference:TOC = TC −   IC(8)  Table1 Averagevalueofthepropertiesof    the   soilsample(standarddeviation=   4–7%).pHSOM(%)TC(%)   TOC(%)   SOMfractionsIC(%)Mn(mg   g − 1 )Fe(mgg − 1 )   Fe Amorphous (mg   g − 1 )Fe Crystalline (mg   g − 1 ) S  BETg  (m 2 g − 1 )LPI(%) LPII(%)   R(%)7.220.3650.1980.1960.1140.0120.070.0020.1718.20.5256.71023  F.Vicenteetal./ChemicalEngineeringJournal 183 (2012) 357–364 359  Table   2 Operatingconditionsforrunscarriedoutinbatchtests. T  =   20 ◦ C;   V  L  / W  =2mLg − 1 .RunContaminantLoadcontaminantInitialconcentrationReactantdosage q Diuron  (mg   kg − 1 )   C  H 2 O 2  (mg   L  − 1 ) C  Fe 3 +  (mg   L  − 1 ) C  Citrate  (mg   L  − 1 )   C  H 2 O 2  (mgkg − 1 )   C  Fe 3 +  (mgkg − 1 )   C  Citrate  (mg   kg − 1 )1Withoutcontaminant–10,000––20,000––2   –20,000––40,000––3   –30,000––60,000––4 – 60,000 – –120,000––5 –   30,000–150060,000–30006   –30,000300   150060,00060030007   Diuron4030,000––60,000––8   4030,000–150060,000–30009   4030,000300   150060,0006003000 ToquantifylabileandrecalcitrantfractionsofSOMa   two   stepacidhydrolysiswithH 2 SO 4  wasperformed.Inthistechnique20mL    of5NH 2 SO 4  wasaddedto   0.5gsoil,andthesampleswerehydrolyzedfor30min   at105 ◦ CinsealedPyrextubes.Thesampleswerecentrifugedandthesupernatantwas   takenasthelabilepoolI(LPI).Theremainingresiduewas   hydrolyzedwith2mLof26NH 2 SO 4  atroomtemperatureundercontin-uousshaking.Theconcentrationof    theacidwas   thendilutedwithde-ionizedwaterto2Nandthesamplewas   keptfor3hat105 ◦ Cwithoccasionalshaking.ThesupernatantobtainedwastakenaslabilepoolII   (LPII)[32].Bothsamples(LPIandLPII)and theresidualsoilwereanalyzedfollowingWalkey-BlackMethodwhichconsistonthewetoxidationat125 ◦ C   witha   mixtureof 1NK 2 Cr 2 O 7  and96%H 2 SO 4  [33].Theoxidationstepwas   followedbytitrationof    theexcessdichromatesolutionwith0.5Nferroussulphate.TotalFeandMncontentin   soilsweredeterminedbyacidextrac-tion/atomicabsorptionspectroscopy(EPA3050BMethod)[34].Crystallineandamorphousironoxyhydroxidesweredeterminedbycitrate–bicarbonate–dithioniteextraction[35].Theporousstructureof    thesampleswascharacterizedbyN 2 adsorption–desorptionat − 196 ◦ C,performedina   SA3100surfaceareaanalyzer(Coulter).Sampleswerepreviouslyoutgassedforatleast8hatroomtemperatureto   avoidsoil   degradation.FromtheN 2 isotherm,theapparentsurfacearea( S  BETg  )wasdeterminedapplyingthe   BETequation.  2.3.StabilityofH   2 O  2  inthepresenceofsoil H 2 O 2  degradationin   contactwithuncontaminatedsoilwasstudiedbyduplicatethroughbatchexperiments.Runswereper-formedin20mL    glassvials,keptin   continuousagitation(50rpm)on   ashakingwaterbath,suppliedbySelecta.Thetemperaturewascontrolledandremainedat20 ◦ CandthesoilpHwasnotadjusted.Theliquidphasevolumetouncontaminatedsoilweightratio( V  L  / W  )   was2mL    g − 1 .Firstly,theexperimentswerecarriedoutatdifferentinitialconcentrationsof    H 2 O 2  (20,000–40,000–60,000and120,000mgkg − 1 ),runs1–4inTable2.   Thesampleswerecol-lectedatdifferentreactiontimesandimmediatelycentrifugedfor5min   at5000rpmina   CentrolitSelectacentrifuge.ThesupernatantwasanalyzedforH 2 O 2 ,asdescribedintheAnalyticalmethodssec-tion.Eachreactionvialrepresentsonetimepointandallreactionswereconductedinduplicate.Vialsweresacrificedat1,   8,   24and48h.Twoadditionalexperimentsatthesameexperimentalcondi-tionswereperformedmixingH 2 O 2  withcitrateandFe 3+ /citratecomplex,runs5and6respectivelyshowedin   Table2,   toevalu-atetheeffectof    metalchelatingagentandsolubleironin   theH 2 O 2 kineticdecomposition.Themolarratioof    H 2 O 2 tocitratewas   176:1,andthecitrateto   ironratiowas1:1accordingtotheexcessrequire-mentforFenton’streatmentin   soilsystemandstudiedinapreviouswork[45].  2.4.Fenton-likeoxidationexperiments Someexperiments,runs7,8,and9in   Table2,wereperformedbyduplicatetoevaluatetheeffectivenessof    Fenton-likeoxidationforremediatingcontaminatedsoilbydiuronatthesamepreviousconditions.Contaminatedsoilwasartificiallypreparedusinguncontam-inatedcontrolsoil.Thesoilwas   spikedwitha   solutionof diurondissolvedin   methanol,initialconcentrationof    diuronwas100mg   L  − 1 .Then,themixturewas   completelystirredandblendedtoensurehomogeneityat20 ◦ Cforabout   72hwitha V  L  / W  =3mL    g − 1 .Finally,themixturewaslaiduntilthemethanolwascompletelyevaporated.Duringtheventilation,themixturewasstirredeachday.Afterthesoilsamplewas   washedwithMilli-Qwaterwitha V  L  / W  =3mL    g − 1 stirredaround1htoelimi-natetheresidualmethanolremaininginthesoil.Thesamplewascentrifugedandtheliquidphasewasinjectedina   HPLCtodeter-minethepossiblediuronextractedto   evaluatethelosswiththewaterwashing.Thetotalremainingamountof    thecontaminantsorbedin   thesoilwasdeterminedbyultrasonicsolventextrac-tionusingmethanolasthesolventby3consecutiveextractions.Eachstepaccuratelyweighedspikedsoilwassonicated15min   withmethanolinanultrasonicbath(PowerSonic505)atroomtempera-turefollowedbya   5min   centrifugation.Therelationsoil/methanolwas1/5gmL  − 1 .Theamountof    totalextracteddiuronwas   deter-minedbyHPLC.Thetargetconcentrationof    diuroninthespikedsoilwas40mg   kg − 1 .Diuronoxidationexperimentswereperformedinbatchreactorsystemsevaluatingtheapplicabilityof    Fenton-like.Firstly,itwasdeterminedtheoxidationof    diuronusingonlyH 2 O 2  ataninitialconcentrationof    60,000mgkg − 1 .Then,anothertestwas   devel-opedtostudytheeffectof    addingcitrate(initialconcentrationof 3000mgkg − 1 )aschelatingagenttoextractthenaturalironpresentinthesoiltocatalyzeFenton’sreactionandto   promotetheremovalofthecontaminant.Finally,a   thirdexperimentwasassessedusinganadditionalamountof    ironsalt(( C  Fe 3 + ) o  = 600mgkg − 1 )in   addi-tiontocitrate.Soilsamples(1g)wereputincontactwith2mL    of    thechelatingagentor   complexiron/citratesolutionabout24hpriortotheadditionofH 2 O 2 .   Then,198  Lofthestock30%H 2 O 2  solutionwasadded,givinga   finalH 2 O 2  concentrationof    60,000mg   kg − 1 .TheFenton-likereactionwasjuststarteduponadditionof    H 2 O 2 .Vialsweresacrificedatdifferenttimes.Eachreactionvialrep-resentsonetimepoint.Inorderto   avoidpossibleexplosionsduetogasaccumulationinthevials,thecapswerenotsealedduringreac-tiontimetopermitevacuationofthegeneratedgas.Thesoilandaqueousphasewereseparatedbeforetheextraction.Aftercentrifu-gation,thesupernatantandsoilextractsbyultrasonicextractionmethodpreviouslyexplainedwereanalyzedto   determineresid-ual   diuronconcentrationbymeansofHPLC.DeterminationoftheremainingH 2 O 2  insupernatantwas   performedasexplainedbelow.Thecitrateandsolubleironin   theliquidphasewerealsomonitoredthroughtheexperiments.Ecotoxicityof    theaqueousphaseandsoil  360  F.Vicenteetal./    ChemicalEngineeringJournal 183 (2012) 357–364 weremeasuredbyusingtheMicrotox ® bioassaytodeterminethepotentialtoxiceffectsof    theoxidationprocess.  2.5.Analyticalmethods Inordertodeterminetheresidualdiuronconcentrationandindicatethepresenceofitsdegradationproductswithtime,20  Lofsamplewasinjectedintoahigh-performanceliquidchromatographycolumn.Theinitialconcentrationofthesub-strateanditstransformationwasmonitoredbyHPLC(Agilent,mod.1100).APoroshell120SB-C182.7  mPerformancecol-umn   (100–4.6mm)   wasusedasthestationaryphase.Themobilephasewasacetonitrile–water(40:60,v/v)witha   flow-rateoff 1.00mL    min − 1 fordetectionandquantificationof    diuronwithaAgilent1290InfinityDiodeArrayDetector.Theeffluentwas   mon-itoredat210nm.   Evaluationandquantificationweremadeonachromatographydatasystemmethod(detectionlimit20ngmL  − 1 ).Theconcentrationof    H 2 O 2  in   solutionisdeterminedbytitra-tionusingasolutionofpotassiumpermanganate,KMnO 4 ,   of    knownconcentration,in   sulphuricacidusinga   titrimetricvolumetricana-lyzersuppliedbyMetrohm.Ionchromatographywasusedtodeterminecitrateconcentra-tion   usingananionchromatographycolumn(MetrosepASupp5-250/40.Metrohm)anda   mobilephasecontaininganbuffer(HCO 3 − /CO 32 − )andacidsuppressor.Solubleironconcentrationswereanalyzedusinga   DR/890colorimeterwiththeFerroVerIronReagent,suppliedbyHACH.SolutionpHwasmeasuredwithapHelectrode(Basic20-CRISON).EcotoxicitywasdeterminedbyusingtheMicrotox ® bioassay.TheMicrotoxbioassayis   basedonthedecreaseof    lightemissionby Photobacteriumphosphoreum resultingfromitsexposuretoatoxicant,usingaMicrotox ® M500Analyzer(AzurEnvironmental).Theinhibitionofthelightemittedbythebacteriawas   measuredafter15min   contacttime.TheEC 50  is   definedastheeffectivenom-inalconcentrationof    toxicant(mg   L  − 1 )   thatreducestheintensityoflightemissionby50%.TheparameterIC 50  isdefinedastheper-centageoftheinitialvolumeofthesampletothevolumeof    thesampleyielding,aftertherequireddilution,a50%reductionof    thelightemittedbythemicro-organisms.Thetoxicityunits( TU  )   ofthewastewaterarecalculatedas: TU  = 100IC 50 (9)Iftheaqueoussampleonlycontainsa   compound i ata   con-centration Ci thetoxicityunitscanbepredictedbythenextrelationship. TU  i  = C  i EC 50 i (10)Beforemeasuringthetoxicity,thepHvaluesofthesampleswerere-adjustedtobetween6and7,in   ordertopreventthepHeffect.ABasicSolidPhase-Test(BSPT)wasperformanceaccordingtheAzurEnvironmentalprocedure.A20%(w/v)solutionof    thesoilwasstirredandafter10min   whilestirringitssamplefromthecorrectregionofthebeaker2mL    ofthesolutionand9serialdilu-tionswereperformed.Theluminescencewasmeasuredbeforesampleadditionand30min   afterthesampleadditiontheEC 50  val-ueswereobtained.All   thechemicalsusedwerepurchasedfromSigma–Aldrichandthemicro-organismswereMicrotox ® AcuteReagentsuppliedbyI.O.Analytical. 3.Resultsanddiscussion Thechemicalpropertiesof    thesoilstudiedarepresentin   Table1.Ascanbeseenthesoilis   categorizedasasandyclayloamwithaneutralpHvalueandloworganicmattercontent.Thelabilepool 50403020100 0.00.20.40.60.81.0  Run (C H2O2 ) o  (mg·kg -1 ) 1 20,000 2 40,000 3 60,000 4 120,000    (   C   /   C   o    )    H   2   O   2 time (h) Fig.1. OxidantdecompositionatdifferentinitialH 2 O 2  concentrations. T    =20 ◦ C; V  L  / W  =   2   mL    g − 1 . fractioninorganicmatterrepresentsthelargestproportionofthisparameterbeingtheeasierdegradablefraction(carbohydratesandproteins)[36].Thehighvalueof    crystallineironmeasuredimpliesthatFenton-likereagentcanbesuccessfullyappliedbecauseitcanactasheterogeneouscatalyst,whichispreviouslydemonstratedinliterature[22–24,37]. Thehighspecificsurfacevaluemay   indicatethat,despitetheloworganicmattercontent,thesoilis   abletoretaincontaminantsduetoitsporousstructure.Furthermore,thesurfaceareaof    iron-oxidemineralsisalsoanimportantfactorinfluencingthedegradationof organicpollutantsbytheFenton-likereaction[38].  3.1.H   2 O  2  decompositionbysoil H 2 O 2  decomposition,withoutpHadjustment,was   measuredinordertocharacterizethereactivityofthesoil.H 2 O 2  concentrationwasmonitoredwithtimeatfourdifferentvaluesof    initialoxidantconcentration(runs1–4inTable2).AscanbeseeninFig.1,H 2 O 2 decompositiondependsontheinitialconcentrationof    theoxidant.Whenusinga   lowH 2 O 2  concentrationahigheroxidantconversionisreached.H 2 O 2  decompositionrateswererapidinthefirsthoursanddecreasedwithtime.Experimentsperformedontheaqueousphasewithoutsoilat30,000mgL  − 1 ofH 2 O 2  and20 ◦ CshowedthattheH 2 O 2  concentrationremainsconstantat24h,sothedecreaseoftheconcentrationisduetotheinteractionoxidant-soil.Thenat-uralreductantspeciesof    soilsuchastotalorganiccarbon(TOC)andmetallicsubstancescanactasalimitingreagentinH 2 O 2  decom-positionandcouldberesponsibleforthe“non-productive”H 2 O 2 decompositionpathways,suchasthedisproportionto   O 2  andH 2 O.Thiscouldexplainthehigherslopeobtainedatinitialstagesin   Fig.1.Otherwisethepresenceof    ironminerals(e.g.,goethite,magnetite,andhematite)orothertransitionmetals,suchasMn,   in   a   peroxidesystemcancatalyzethehydrogenperoxidedecompositionandalsofacilitatecontaminantoxidationvia   catalysis[39–42].The   catalyticdecompositionof    hydrogenperoxideduethesetransitionmetalscouldexplaintheslopenoticedattimeshigherthan10hinFig.1.Finally,theinfluenceof    addinga   chelatingagent(sodiumcit-rate)andthecomplexFe 3+ /citrate,werestudiedatinitialH 2 O 2 concentrationof    60,000mg   kg − 1 .Theresults,showninFig.   2,indi-catethattheinfluenceofthecitrateis   somewhatcomparablewiththatobtainedwithoutaddingchelatingagent.Whenaddingcit-ratethedecompositionofH 2 O 2  isslightlyhigherthanthecasewhenonlyH 2 O 2  isadded.Thiscouldbeexplainedbythesolubi-lizationof    ironcationfromthesoilto   theaqueousphasedueto   thechelatingagent.Besides,thiseffectismorenotoriouswhena   saltof Fe 3+ isaddedwithcitrate.TheironcationismaintainedinsolutionbythechelatingagentatneutralpHandthecatalyticdecompo-sitionof    H 2 O 2  increases.ThereactionofFe 3+ withH 2 O 2  leadsto  F.Vicenteetal./ChemicalEngineeringJournal 183 (2012) 357–364 361 50403020100 0.00.20.40.60.81.0  Run (C   )   (mg·kg   ) (C   )   (mg·kg   ) (C   )   (mg·kg   ) 3 60,000 -   - 5 60,000 - 3,000 6 60,000 600 3,000    (   C   /   C   o    )    H   2   O   2 time (h) Fig.2. H 2 O 2  decompositioninsoilSAforruns3,   5   and6. T  =20 ◦ C; V  L  / W  =2mLg − 1 . regenerationofFe 2+ ionsandsubsequentformationofhydroxylradicals(OH • ).Inanycase,itwasnoticedthat   thissoil,withlowcarbonateandSOMcontent,producesa   muchmoreslowerhydrogenper-oxidedecompositionthatthesoilwithhigherSOMandcarbonateamountsusedelsewhere[43].InordertoevaluatetheeffectsofusingH 2 O 2 asanoxidantonthesoilquality,thefinalSOMcontentwasmeasured48hafterH 2 O 2 additioninalltherunscarriedout.Theresultsshowedthat ≥ 90%of theSOMinitiallypresentinthesoilwasdegraded.It   iscloserelatedtothehighestlabile(LPI)contentof    thissoilwhichisconstitutedbyeasilybiodegradablecompounds(ofwhichcarbohydratesarethemosttypical).  3.2.Oxidationofdiuronby   Fenton-likeprocess Operatingconditionsof    diurondegradationarecollectedinTable2.Theinitialconcentrationofdiuronasherbicideinsoil( q Diuron =40mg   kg − 1 )ishigherthanthestandardregulatoryestab-lishedbytheRD9/2005[44].InitialH 2 O 2  concentrationselectedfortheFenton-likeexperimentstooxidizediuronwas   60,000mg   kg − 1 .Thisvalueisexpectedtobe   enoughtostartthedegradationprocess.ThisvalueismuchhigherthanthetheoreticalH 2 O 2  demandforthetotalmineralizationofdiuronbecausetherearenon-specificlossescausedbyitsdecompositionin   thepresenceofreactivematerialsinsoils.ThetheoreticalH 2 O 2  demandforthetotalmineralizationof    thesorbeddiuronis109.82mg   kg − 1 accordingtothestoichiometricequation(Eq.(11))C 9 H 10 Cl 2 N 2 O + 19H 2 O 2 → 9CO 2 + 21H 2 O + 2Cl − + 2NH 3  (11)Residualdiuronconcentrationin   aqueousphaseandsoilduringFenton-likeoxidationtreatmentatnaturalsoilpHforsoilwithand 0.00.20.40.60.81.0  Run (C   )(mg·kg   ) (C   )   (mg·kg   ) (C   ) (mg·kg   ) 7 60,000   -   - 8 60,000   -   3,000 9 60,000   600   3,000    X    D   I   U   R   O   N (a) 50403020100 0.00.20.40.60.81.0    (   C   /   C   o    )    H   2   O   2 time (h) (b) Fig.4. Diuronconversion(a)   andH 2 O 2  decomposition(b)forruns7–9.(q Diuron ) o  =40mgkg − 1 ;   T    =20 ◦ C; V  L  / W    =   2mL    g − 1 . withoutchelatingagentandextrinsicFe 3+ areshownin   Fig.3(runs7–9in   Table2).Thediuronconcentrationdecreaseswithtimeandtheremovalrateof    diuronischaracterizedbya   veryfastinitialratefollowedbya   muchslowerdegradationrate.As   canbeseen,theFenton’sreactiontakesplaceinbothphasesbecausethismay   beassociatedtothefastoxidationofthemostaccessiblediuronandlowerentrappedcontaminantdesorptionto   theliquidphasefromsoil.For   run7,withoutcitrate,theresidualdiuronin   themediawasfoundin   theaqueousphaseascanbeseenin   theFig.3(a),anditremainedconstantafterthefirstminutes.Asnoironcationispresentinthisphase,thediurondesorbedisnot   furtheroxidized.Inaddition,theuseof    citrate(runs8and9inFig.3(b)and(c),respec-tively)24hpriortoadditionof    H 2 O 2  favorsthediuronextractionfromthesoilto   theaqueousphaseimprovingtheefficiencyofFen-ton’sreactionsbya   coupleddesorption–oxidationmechanism[45].Theextractionpercentageof    diuronintheliquidphaseaftertheadditionofchelatingagentandpreviousto   theadditionoftheH 2 O 2 isaround25%forbothrunsandthisfactorenhancestheFenton’sreaction.Theevolutionof    diuronconversioninthethreecasesstudied(runs7–9)is   comparedin   Fig.4(a).Upto55%diuronremovaleffi-ciencywasattainedin   soilat48hreactiontimewiththeaddition 50403020100 010203040   q   (  m  g .   k  g   -   1    ) time (h) (a) 50403020100  Aqueous phase Soil time (h) (b) 50403020100 time (h) (c) Fig.3. Residualdiuronconcentrationinsoilandaqueousphaseforruns7(a),8(b)and9(c).( q Diuron ) o  =   40mgkg − 1 ;   ( C  H 2 O 2 ) o  =   60 , 000mgkg − 1 ; T    =   20 ◦ C;   V  L  / W  =   2mLg − 1 .
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