Degradation of trinitrophenol by sequential catalytic wet air oxidation and solar TiO2 photocatalysis

ContentslistsavailableatScienceDirect

Chemical

Engineering

Journal

j ourna l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c e j

Degradation

of

trinitrophenol

by

sequential

catalytic

wet

air

oxidation

and

solar

TiO

2

photocatalysis

Athanasia

Katsoni

_,

_Helder

_T.

_Gomes

_,

_Luisa

_M.

_{Pastrana-Martínez}

_,

_Joaquim

_L.

_Faria

_,

José

L.

Figueiredo

_,

_Dionissios

_Mantzavinos

_,

_Adrián

_M.T.

_Silva

a_{DepartmentofEnvironmentalEngineering,TechnicalUniversityofCrete,Polytechneioupolis,GR-73100Chania,Greece}

b_{DepartamentodeTecnologiaQuímicaeBiológica,EscolaSuperiordeTecnologiaeGestão,InstitutoPolitécnicodeBraganc¸a,CampusdeSantaApolónia,Apartado1134,5301-857}

Braganc¸a,Portugal

c_{LaboratóriodeCatáliseeMateriais(LCM),LaboratórioAssociadoLSRE/LCM,DepartamentodeEngenhariaQuímica,FaculdadedeEngenharia,UniversidadedoPorto,RuaDr.Roberto}

Friass/n,4200-465Porto,Portugal

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received29March2011

Receivedinrevisedform9June2011 Accepted10June2011 Keywords: Watertreatment CWAO Activatedcarbon Processintegration Solarphotocatalysis

a

b

s

t

r

a

c

t

Catalyticwetairoxidation(CWAO)andsolarTiO2photocatalysiswereinvestigatedasadvancedoxidation processestodegradetrinitrophenol(TNP)inmodelaqueoussolutions.Anactivatedcarbon(AC)treated withsulphuricacidofdifferentconcentrations(5,10and18M)attwodifferenttemperatures(353and 423K)wasinvestigatedasametal-freeCWAOcatalyst,whileacommerciallyavailableP25TiO2 pow-derwasusedasaphotocatalyst.CWAOexperimentswereconductedat448K,0.7MPaoxygenpressure (4.7MPaoftotalpressure),1.3gL−1_{ACloadingand270mgL}−1_{TNPconcentration,whilephotocatalytic} experimentswereconductedatambienttemperature,1gL−1photocatalystloading,500–1000Wm−2 irradianceprovidedbyasolarsimulatorand32–270mgL−1_{TNPconcentration.Treatmentefficiency} wasassessedbymeasuringtheconcentrationsofTNPandnitrates,totalorganiccarbon(TOC)and bio-chemicaloxygendemand(BOD5).Upto90%TNPdegradationwasattainedduringCWAOover120min, fromaninitialconcentrationof270mgL−1_{.ForthesameTNPconcentration,TiO2photocatalysisgives} only13%conversionoverthesame120min.However,forTNPconcentrationsbelow144mgL−1, pho-tocatalysiscanbeeffectivelyused:100and80%TNPdegradationobtainedin120minofirradiationfor initialTNPconcentrationsof64and144mgL−1_{,respectively.Inthisrespect,CWAOand} photocataly-siswereemployedsequentiallytotreatTNP;completeTNPconversionbeingachievedafter120minof CWAOfollowedby60minofphotocatalysisat1000Wm−2irradiance,andthiswasaccompaniedby82% TOCreduction,aswellasanincreaseofBOD5/TOCratiofrom0to2.28.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Advanced oxidation processes (AOPs) are remediation solu-tionsbasedonthegenerationofnon-selectiveandhighlyreactive radicals,suchashydroxyl (HO•)and hydroperoxyl(HOO•) radi-cals,appliedtooxidisepersistentorganicpollutantsintocarbon dioxideandwater,oralternativelyintoeasilybiodegradable by-products.Briefly, heterogeneous photocatalysisis based onthe irradiationofa semiconductor materialwithphotonsofenergy equal toor greater than itsband-gap energy, toproduce reac-tiveelectron–hole(e−/h+_{)pairsand,subsequently,highlyoxidising}

species[1,2].Titaniumdioxide(TiO2)isthemostcommonlyused

∗ Correspondingauthor.Tel.:+351225081582;fax:+351225081449. ∗∗ Correspondingauthor.Tel.:+302821037797;fax:+302821037852.

E-mail addresses: mantzavi@mred.tuc.gr (D. Mantzavinos), adrian@fe.up.pt

(A.M.T.Silva).

photocatalystduetoitshighefficiencytoproducee−/h+_pairsand

alsobecauseitisachemicallystable,non-toxicandlowcost mate-rial[1,3,4].

Ambient conditions of temperatureand pressure are typical inphotocatalysis. Incontrast,wetair oxidation(WAO)operates at harsh conditions, which canbecome milder in thepresence ofactivecatalysts(400–523K,0.5–5.0MPa)[5–15].Several het-erogeneouscatalystsbased onsupportedor unsupportedmetal oxides (e.g.,Cu, Zn, Mn,Fe, Co, and Bi)and noble metals (e.g., Ru, Pt, Pd and Rh) have been tested in the last four decades for catalytic wet air oxidation (CWAO) [5–11,13,14]. However, deactivationphenomenaarefrequent,suchasleachingofactive metals to the liquid phase. For this reason, metal-free carbon materialshavebeentestedascatalystsinmany CWAO applica-tions,includingactivatedcarbons[15–24],carbonxerogels[20,25], multi-walled carbon nanotubes [26–28] and carbon foams and fibres enriched with nitrogen [29]. Carbon materials are very versatile catalysts, since their surface chemistry can be easily

1385-8947/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.06.022

modified[30]inordertoprovideadequateactivesitesforthe reac-tion.

2,4,6-Trinitrophenol(TNP),alsoknownaspicricacid,isa by-productoftheindustrialsynthesisofnitrobenzene.Theresulting wasteisanaqueousstreamattheprocessingtemperature(above 348K)andasolidwhenleftatambienttemperature.Theraw efflu-entresultingfromthenitrobenzenesynthesiscanbetreatedinsitu byCWAO,takingadvantageofthehightemperatureatwhichitis releasedandallowingforimportantenergysavings.Indeed,recent works showthat carbon materials withno added metalphase canbeusedashighlyefficientcatalystsfordegradationofTNPby CWAOat473K[23,25].IncomparisonwithotherAOPs,CWAOis alsothemostappropriatetechnologytotreatwastewaterswith moderateor highorganiccontent(chemical oxygendemandof 10–100gL−1)[12].However,sincelargescaleCWAOiscapitalcost intensivewhencomparedtootherAOPs,couplingsmallcost effec-tiveCWAOunitswithanotherAOPpost-treatmentcouldrepresent aneconomicallyfavourablesolutionformanyspecificinstances.

AmongstAOPsoperatingatlesssevereconditionsthanthose usedinCWAO,heterogeneousphotocatalysisisearningstronger relevancesincethedegradationprocesscanbedrivenbysolarlight. Additionally,itwasrecentlyreportedthatphotocatalysiscouldbe beneficialwhenappliedasasecondarytreatmenttoWAO[31].

Inthepresentwork,anactivatedcarboncommonlyusedfor wastewater treatment (Norit ROX 0.8), with modified surface chemistry,wasstudiedasametal-freeCWAOcatalyst.Then,TiO2

heterogeneousphotocatalysisdrivenbyartificialsolarlightwas investigatedasapossiblepost-treatmenttototallydegradeTNP.

2. Experimental

2.1. Reagentsandmaterials

2,4,6-Trinitrophenol (C6H3N3O7, 98%), acetic acid (99.8%),

sodiumdihydrogenphosphate(≥99%)andphosphoricacid(85%) were purchased from Sigma–Aldrich. Acetonitrile (99.8%) and methanol(99.8%)wereofHPLCgrade(Chromanorm). Ultrapure waterwasproducedinaDirect-Qmilliporesystem.

2.2. CWAOexperiments

DifferentactivatedcarbonsamplesweretestedasCWAO cata-lysts,namelytheas-receivedoriginalactivatedcarbonNoritROX 0.8(AC),andchemicallymodifiedcarbonmaterialsobtainedfrom thesameACbyliquidphasetreatments,followingtheprocedure reportedelsewhere[32].Samplesweretreatedwiththree differ-entsulphuricacidconcentrations(5,10and18M)attwodifferent temperatures(353 and 423K),and labelled as ACX-Y, where X referstothesulphuricacidconcentrationandYtothe tempera-ture(e.g.,AC5-353referstoasampletreatedwith5Msulphuric acidat353K).Inatypicaltreatment,10gofACwereimmersedin 200mLofthesulphuricacidsolutionfor3hina500mL round-bottomflaskheated byan oilbathatthedesired temperature. Then,therecoveredsampleswerethoroughlywashedwith dis-tilledwateruntiltheneutralityoftherinsingwaterswasreached, andfurtherdriedin anovenfor 18hat383K.Thespecific sur-facearea(SBET),thenon-microporoussurfaceareadeterminedby

thet-method(SMES)and themicroporevolume(VMIC)were

cal-culatedfromtheN2 adsorptionisothermsthatwereobtainedin

aQuantachrome NOVA4200e analyser.Theamountsof surface groupsreleasedasSO2 weredeterminedasdescribedelsewhere

[32],byusingtemperatureprogrammeddesorptioninaAMI-200 apparatus (Altamira Instruments), equipped with a quadrupole massspectrometer(Dymaxion,Ametek).Thermogravimetric anal-ysis(TGA)wasalsoperformedforselectedsamplesbyusingaSTA

490PC/4/HLuxxNetzschthermalanalyserandheatingthesamples at20Kmin−1undernitrogenflow(50cm3_min−1_)upto1273K.

CWAOexperimentswereperformedina160mL316-stainless steelhighpressurebatchreactor(ParrInstruments)equippedwith atemperaturecontroller(Fig.1a).Theoperatingconditionswere typicallyasfollows:75mLofaTNPmodelsolution,448K,500rpm (confirmed asadequateto ensuretheabsenceof masstransfer limitations),4.7MPaoftotal pressure(0.5MPaofpurenitrogen, 0.9MPaofwatervaporpressureand3.3MPaofair,corresponding to0.7MPaofoxygenpartialpressure),naturalpHandacatalyst loadingof1.33gL−1.Inatypicalrun,themodelsolutionwasplaced intotheautoclave,flushedwithN2inordertoremovedissolved

oxygen,pressurisedwith0.5MPaofnitrogenandpre-heatedup tothedesiredtemperature.Theintroductionofair(3.3MPa)after thispre-heatingperiodwastakenast=0minforthereaction.A non-catalyticblankexperimentwasalsoperformedwithairinthe absenceofcatalyst(WAO).Sampleswereperiodicallywithdrawn andanalysedbyhighperformanceliquidchromatography(HPLC). 2.3. Solarphotocatalyticexperiments

A commercialTiO2 powder (AEROXIDE® TiO2 P25) supplied

byDegussa(nowEvonik)wasusedascatalystinphotocatalytic experiments,referred hereafteras P25 TiO2. Thephotocatalytic

experiments were performed in a solar simulator (Cofomegra SolarBox1500e,Fig.1b).FourdifferentinitialTNPconcentrations weretested(32,64,144and270mgL−1).Inatypicalrun,theTiO2

catalyst(1gL−1)wasaddedto75mLoftheTNPsolution.The sus-pensionwasfirststirredinthedarkfor30minandthenirradiated for120min.Airwascontinuouslyspargedinthesuspensionunder continuousstirringandthetemperaturemaintainedat303±3K. Onetrialexperimentat500Wm−2irradiancewasperformedwith 270mgL−1 ofTNP and then thepower wassetat 1000Wm−2 fortherestoftheexperimentsatdifferentTNPconcentrations.A blankexperimentwasalsoperformedfor64mgL−1ofTNPinthe absenceofTiO2.Sampleswithdrawnatregulartimeintervalswere

centrifugedfor15minand thesupernatantliquid wascarefully separatedforanalysis.

2.4. Analyticaltechniques

TheconcentrationofTNPandnitratesinliquidsampleswere monitoredby HPLCwith a Hitachi EliteLaChrom HPLC system equippedwithadiodearraydetector(L-2450)andasolvent deliv-erypump(L-2130)ataflowrateof1mLmin−1.TNPwasseparated onaPurospherStarRP-18column(250mm×4.6mm;5␮m parti-cles)withanisocraticmethodofanA:B(40:60)mixtureof3%acetic acidand1%acetonitrileinmethanol(A)and3%aceticacidinwater (B).Theconcentrationofnitrateswasanalysedwithanother iso-craticmethod(20mMNaH2PO4acidifiedwithH3PO4atpH2.8)and

usingaHydrosphereC18column(250mm× 4.6mm;5␮m parti-cles).Quantificationwasbasedonthechromatogramsbyusingthe EZChromElitechromatographydatahandlingsoftware(Version 3.1.7).TheconcentrationsofTNP andnitratesweredetermined atthemaximumabsorbancefor eachspecies (359and 201nm, respectively).Absorbanceandconcentrationswerefoundtobe lin-earoverthewholerangeofconcentrationsunderconsideration (maximumrelativestandarddeviationof2%).

Totalorganiccarbon(TOC)wasdeterminedwithaShimadzu 5000ATOCanalyser.Thisequipmentfirstdeterminesthetotal car-bon(TC)bysamplecombustionat973KoveraPtcatalystbed,and thenthetotalinorganiccarbon(TIC)ismeasuredusingphosphoric acid.TOCwascalculatedbysubtractingTICfromTC.The uncer-taintyinthisparameter,quotedastherelativedeviationofthree separatemeasurements,wasnevergreaterthan2%.Lowmolecular

a

b

Fig.1. Experimentalset-upforCWAO(a)andphotocatalysis(b).

weightcarboxylicacidswerequantifiedbyionicchromatography (DionexDX-600,IonPacAS11-HCcolumn).

Thebiochemical oxygendemandBOD5 wasdeterminedin a

WTWequipment(inoLabBSB/BOD740withaStirrOXelectrode) bymeasuringtheoxygenconsumptionofamicroorganismculture (obtainedfromgardensoil)after5daysofincubation,inaccordance withstandardmethods[33].

3. Resultsanddiscussion

TotesttheperformanceoftheCWAOprocess,severalACswere usedtotreatthemodelsolutioncontainingTNP.Sometexturaland chemicalpropertiesofthesematerialsareshowninTable1.The texturalpropertiesareverysimilarfortheACstreatedaccording todifferentmethods:SBET=845±25m2g−1,SMES=185±5m2g−1,

andVMIC=0.33±0.01cm3g−1.Thesetexturalpropertiesarealso

similar to those of the original material (SBET=850m2g−1,

SMES=190m2g−1,andVMIC=0.33cm3g−1).However,theSO2

con-tentsignificantlychangeswiththeconditionsappliedtoprepare eachAC(120,420,460,480,520, 600and680␮molg−1 for AC, AC5-423,AC5-353,AC10-423,AC18-423,AC10-353andAC18-353, respectively).Therefore,thecarbonmaterialstestedasmetal-free CWAOcatalystshavesimilartexturalpropertiesbutdifferent sur-facechemistry.

Fig.2showstheTNPconcentrationasafunctionoftimeina blankexperimentperformedintheabsenceofcatalyst(WAO)and intheCWAOexperimentsperformedwiththeACandACX-Y cata-lysts.ItisobservedthattheremovalofTNPisstronglyfavoured when thecarbon materialsare usedas catalysts.However,the markeddecreaseintheTNPconcentrationduringthepre-heating periodindicatesthatTNPwasremoved,toasubstantialdegree,by pureadsorptionduringthisperiod(56±7%).TheevolutionofTNP concentrationasafunctionoftimewasalsodeterminedin experi-mentsperformedbyintroducing3.3MPaofnitrogen(insteadofair) afterthepre-heatingperiod(notshown).Forinstance,theamount ofTNPremovedbypureadsorptionwiththeuntreatedACwas26% from0to30min(thisexperimentyieldingnonitratesthroughout thecourseoftheexperiment)while44%ofTNPwasremovedin

Table1

TexturalandchemicalcharacterizationofACs.

SBET(m2g−1) SMES(m2g−1) VMIC(cm3g−1) SO2(␮molg−1)

AC 850 190 0.33 120 AC5-353 820 180 0.32 460 AC10-353 850 180 0.33 600 AC18-353 870 190 0.34 680 AC5-423 840 180 0.33 420 AC10-423 840 180 0.32 480 AC18-423 850 190 0.33 520

thesameperiodoftimeintheCWAOexperiment(nitratesbeing

detected). Sinceboth processes(adsorptionand reaction)occur

simultaneouslyinCWAO,itisquitedifficulttodiscriminatewhich

fractionisremovedbypureadsorptionorpurereactionwhen

oxy-genisinjectedinthesystem,butitisevidentthatTNPisstrongly

removedbybothprocessesoccurringsimultaneously.

Alsoofinterestistonote(Fig.3)thecorrelationbetweenthe

amountofsulphur-containinggroupsinthecarbonmaterialsand TNPadsorptionatt=0min.TheamountofTNPadsorbedduringthe pre-heatingperiodincreaseswiththesulphur-containinggroupsas follows:9.6,10.7,11.1,11.4,11.6and12.9mgTNPg−1ACforSO2

contentsintheACof420,460,480,520,600and680␮molg−1, respectively.

RegardingTNPreductionalong30minofreaction,itisdifficult to distinguishdifferences in the efficiency of the tested mate-rialsbecause thecontributionof adsorption isquite significant in alltheexperiments.Additionally, thefinal TOCremoval was alsoquite similarfor thetreated ACs(70–74%) but remarkably differentfromtheTOCremovalwiththeoriginalAC(51%), deter-minedwithreferencetotheinitialTNPconcentration.Therefore, theeffectofthesulphur-containing groupsonthecatalyst effi-ciencyisnot clear,butcarbon materialstreated withsulphuric acidareingeneralmoreefficientthantheoriginalAC,notonly for TNPremoval butalsofor TOC removal.Althoughit is quite difficultto distinguishthedifferencesamongst the variousACs treatedat423K,adecreasingorderofefficiencyintermsofTNP removalwasfoundfortheACstreatedat353K;inparticularafter

Time (min) 30 20 10 0 [TNP] (mg L -1 ) 0 40 80 120 160 200 240 280 WAO AC AC5-353 AC10-353 AC18-353 AC5-423 AC10-423 AC18-423 0 mi n

Fig.2.Evolutionoftrinitrophenolconcentrationinnon-catalyticandcatalyticWAO experimentsusingtheoriginalandtreatedactivatedcarbons(448K,0.7MPaoxygen pressure,1.3gL−1_{catalystloading).}

SO₂ (µmol g-1₎ 700 650 600 550 500 450 400 TNP 0 (mg g -1 ) 9 10 11 12 13 14

Fig.3. AmountofTNPadsorbedondifferentACsasafunctionofsulphur concen-trationintherespectivecarbonmaterials.

15minofreaction:AC18-353>AC10-353>AC5-353,whichseems toberelatedwiththeamountofsulphur-containinggroups,i.e., 680␮molg−1>600␮molg−1>460␮molg−1,respectively.

Theamountofnitratesformedduringthereactioncangivean indicationoftheextentofoxidation.Onceagain,theresultswere quitesimilarforthematerialstreatedat423Kbutdifferencescould beobservedforthosetreatedat353K.Forthisreason,Table2shows resultsforthesetofmaterialstreatedat353K;forcomparative purposes,datafromtherespectiveCWAOrunswithAC5-423and theoriginalACmaterialarealsogiven.

Theefficiencyofthesetofmaterialstreatedat353Kdecreases in the order: AC5-353>AC10-353>AC18-353. This shows that smalleramountsofsulphur-containinggroupscorrespondtolarger productionofNO3−.Sincetheintroductionofsulphur-containing

groupsincreasestheacidityofthematerials,theprevious observa-tionalsosuggeststhattheheterogeneousoxidationofTNPoccursto higherextentoncarbonmaterialswithloweracidity(thepHatthe pointofzerocharge(pHPZC)is5.8,4.1and2.4forAC5-353,

AC10-353andAC18-353,respectively).However,theuntreatedactivated carbon,althoughitexhibitsthelowestacidity(pHPZC=7.6),yields

thesmallestamountofnitratesafter30minofreaction,indicating thattherearetwooppositeeffectsplayingaroleinthisreaction: theintroductionofsulphur-containinggroupsincreasestheAC effi-ciency,but theACefficiencyisalsofavouredbymaterialswith loweracidity.Thebetterperformance of carbonmaterialswith loweraciditywasalreadyreportedwhenmulti-walledcarbon nan-otubeswereusedascatalystsforthedegradationofoxalicacid [28].

TwoconsecutiveCWAOrunswereperformedinordertoassess theprofileofTNPoxidationwiththeusedcarbonmaterial.The secondrunwasperformedwithfreshTNPsolutionandwiththe carbonmaterialrecoveredafterthefirstrun.Forthissetof exper-iments,theAC5-423catalystwasrandomlyselectedamongstall thetreatedsamples,sincetheobservedefficienciesweresimilar.

Table2

Concentrationofnitrates inCWAOexperimentsperformedwithdifferentACs (448K,0.7MPaoxygenpressure,1.3gL−1_{catalystloading).}

[NO3−]15min(mgL−1) [NO3−]30min(mgL−1)

AC 3.9 6.5 AC5-423 7.3 11.7 AC5-353 8.5 13.2 AC10-353 8.2 12.9 AC18-353 6.6 11.3 Time (min) 120 100 80 60 40 20 0 [TNP] (mg L -1 ) 0 50 100 150 200 250 300 1st CWAO Run 1st CWAO Run (rep) 2nd CWAO Run 2nd CWAO Run (rep)

0 m

in

Fig.4. EvolutionoftrinitrophenolconcentrationinCWAOexperimentswithsample AC5-423incyclic(1stand2ndrun)andrepeated(rep)runs.

Betweeneachoxidation run,therecoveredcatalystwaswashed

withwateranddriedintheovenat383Kfor18h.These

experi-mentswereperformedfor120insteadof30min.Fig.4showsthe

obtainedresults,togetherwiththerespectiveduplicatesforeach experiment (uncertaintyof the meanfor repeatedexperiments lowerthan5%).Forthefreshcatalyst,theTNPconcentrationafter thepre-heatingperiod(t=0min)wasabout135mgL−1,whichis halfoftheinitialconcentration(270mgL−1),whiletheTNP con-centrationatt=0minfortheusedcatalystwasalmostthesame astheinitialTNPconcentrationloadedintothereactor.Therefore, TNPremovaldecreasesnoticeablyfromthefirst(freshcatalyst)to thesecondrun(usedcatalyst),becauseafterthefirstrunthe cat-alystissaturatedwithTNPand/orwithitsreactionintermediates thatremainadsorbedonthecarbonmaterialafterthefirstrun.In thesecondrun,theTNPconcentrationafter60,90and120minof CWAOtreatmentwasabout113,75and45mgL−1,respectively, while TOC removal after 120min wasnearly 50%. The original untreatedTNPsolutionwasaerobicallybiorecalcitrantwithaBOD5

valueequal to zero;nonetheless, CWAOtreatmentfor 120min increasedBOD5to10.2mgL−1,whichcorrespondstoaBOD5/TOC

ratioof0.24.

In order to study possiblechanges produced onthe carbon materialduringtheCWAOexperiments,thetreatedACwas char-acterizedbyN2adsorptionat77KbeforeandafterthecyclicCWAO

experiments(firstandsecondrun).Theobtainedresultsare sum-marizedinFig.5.

TheN2adsorptionisothermofthefreshsampleis

characteris-ticofmicroporousmaterials(asconfirmedbythelargemicropore volumeandBETsurfaceareacollectedinTable3)withsome meso-porosity.Thus,theactivatedcarbonhasaporousstructureformed bymicro-andmesopores.Theisothermsobtainedforthesamples usedinCWAOexperiments(firstandsecondrun)showthatthere isaclearinfluenceoftheCWAOprocessonthecarbonmaterial, namelyamarkeddecreaseofSBET(from840to391m2g−1),VMIC

(from0.33to0.16cm3_g−1_{)andSMES(from180to88m}2_g−1_)after

thefirstrun.Alesspronounceddecreaseoftheseparameterswas observedinthesecondrun(from391to280m2_g−1_forSBET,from

0.16to0.12cm3_g−1_{forVMICandfrom88to58m}3_g−1_forSMES).The

evolutionofthetotalvolumeofpores(VT),determinedfromthe

N2uptakeatP/P0=0.95,aswellastheweightloss(WL)obtained

byTGAarealsopresentedinTable3.Theseresultsindicatethat adsorptionofTNPorintermediatecompoundsoccurs,asalready reportedforCWAOstudiesforTNPdegradationusingactivated

car-Table3

TexturalcharacterizationandweightlossoftheAC5-423material,beforeandaftercyclicCWAOruns(firstandsecondrun).

SBET(m2g−1) VMIC(cm3g−1) SMES(m2g−1) VT(cm3g−1) WL(%)

AC5-423:fresh 840 0.33 180 0.46 8.3

AC5-423:1strun 391 0.16 88 0.22 11.1

AC5-423:2ndrun 280 0.12 58 0.16 –

bonsproducedbychemicalactivationofolivestonesascatalysts

[23].

TheCWAOmechanismisheterogeneousinnature(asopposed tothehomogeneousnon-catalyticwetairoxidation),butitcantake placeinporesofdifferentsizes;thus,theextentofreactiondepends ontheACtexturalproperties.TheoxidationofTNPtakesplaceinto widemicropores(>0.5nm)whenhighlymicroporousACsareused [23],becauseallTNPisadsorbedinthistypeofporosity.The degra-dationofTNPwillalsooccurontheexternalsurfacearea(meso-and macropores)whenACswithlowmicroporosityareused,because thereisenoughTNPavailableintheliquidphase[23].SincetheACs testedinthepresentworkhavemicro-andmesopores,the hetero-geneousoxidationmechanismshouldoccurinthewidemicropores andlargerpores.

Insubsequentexperiments,thephotocatalyticdegradationof TNP wasinvestigated. Fig. 6 shows changes in TNP concentra-tionasafunctionofirradiationtimeforexperimentsperformed at initialTNP concentrations in therange 32–270mgL−1. First, itisimportanttoobservethat:(i)increasedTNPconcentrations (270mgL−1)areverystableregardlessofthelevelofirradiance employed(500or1000Wm−2)inphotocatalyticexperiments,(ii) TNPisnotdegradedwithoutTiO2evenwhentheTNP

concentra-tionislow(64mgL−1)and(iii)lightpenetrationwasnotreducedat theemployedcatalystloadof1gL−1,thephotoactivityat2gL−1of TiO2beingsimilartotheoneobtainedat1gL−1.Infact,TNPcould

onlybeeffectively degradedbythephotocatalytictreatmentat lowerconcentrationsand1000Wm−2ofirradiation.Forinstance, TNPwasreducedbyabout78%after120minataninitial concen-trationof144mgL−1,whiletotaldegradationwasachievedin60 and45minwhentheinitialconcentrationwas64and32mgL−1, respectively.Theseobservationsconfirmthatthephotocatalytic processisnotappropriateforthedegradationofhighinitialTNP concentrations,butitcaneffectivelybeusedatlowerTNP concen-trations;inparticular,totaldegradationwasachievedafter60min

P/P₀ 1.0 0.8 0.6 0.4 0.2 0.0 Vad s (cm 3 g -1 ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Fresh 1st CWAO Run 2nd CWAO Run

Fig.5.N2adsorptionisothermsat77KoftheAC5-423material,beforeandafter

cyclicCWAOruns(1stand2ndrun).VadsistheamountofN2adsorbedexpressed

asvolumeofliquid.

forconcentrationslowerthan64mgL−1.Theseresultsalso indi-catethatphotocatalysiscanbeusedasapost-treatmenttopolish thestreamgenerated fromtheCWAOof higherTNP concentra-tions.

Toevaluatethe eventualprocessintegration,the final efflu-entfromtheCWAOtreatmentwiththedifferentmodifiedcarbon materials,wastreatedbyphotocatalysis.Theobtainedresultsare quitesimilarregardlessofthecatalystused,namelycompleteTNP degradationin60minofphotocatalytictreatmentandfinalTOC removalsof79–87%.SinceTOC removalsbysingleCWAOwere 70–74%,itispossibletoenhancetheTOCremovalbyabout10–15% withphotocatalysis.

It iswellknownthat lowmolecularweightcarboxylicacids arethemainby-productsofAOPs.Forthisreason,organicacids werequantifiedafterCWAOexperimentsperformedwiththe mod-ified carbonmaterialsas wellasaftertheintegrated treatment comprisingphotocatalysis.Maleicacidwasalwaysdetectedafter theCWAOexperimentsinconcentrationsnear39.7±0.3mgL−1, decreasingto33.5_±1.7mgL−1afterthephotocatalytictreatment. MalonicacidwasnotdetectedafterCWAO,butseemstobeformed byphotocatalysis(6.5±0.3mgL−1)probablyduetodegradationof maleicacid.Otheracids,suchasformicandaceticacidsareformed byCWAO(12.6±0.8mgL−1and7.4±0.4mgL−1,respectively)but canbeeffectivelydegradedbyphotocatalysis.Onlytraceamounts ofoxalic,citric,pyruvicandvalericacidsweredetectedinsome cases (<0.4mgL−1).The presenceof low molecularweight car-boxylicacidsinthetreatedsolutionscanalsoexplainthefactthat onlyaslightlypHincrease(fromnaturalsolutionpHof3.1upto amaximumof3.6)wasobservedaftertheCWAOand photocat-alytictreatments.Itisimportanttoreferthateveniflowmolecular weightcarboxylicacidsareingeneralbiodegradablecompounds, itisknownthatsomeotherspeciesthatwerenotdetectedcanbe harmfultothebiologicaltreatmentevenwhentheirconcentrations arelowerthan1mgL−1.

In order to obtain a better picture of the integrated treat-ment,theresultsofthephotocatalytictreatmentoftheresulting stream after120min of CWAOwiththe used AC5-423sample and 270mgL−1 TNP (Fig. 4) are shown in Fig.6 (pre-treated). Asseen,completedegradationoftheresidualTNPwasachieved after 60min, together with 82% TOC reduction, while the BOD5 of the final solution increased to 35mgL−1 (this

cor-responds to a BOD5/TOC ratio of 2.28). Therefore, since (i)

photocatalysis is not effective in treating high TNP concen-trations, and (ii) the actual industrial waste would be solid at ambient temperatures but liquid at temperatures typically employedin wetoxidation, CWAO(60–120min)couldbeused first followed by photocatalysis (30–120min) for the complete removalofTNPandeffectiveincreaseofeffluent biodegradabil-ity.

4. Conclusions

This work proposes an integrated treatment comprising catalyticwetairoxidationand solar-drivenheterogeneous pho-tocatalysisfortheefficientremovaloftrinitrophenolfromaqueous streams.Relativelymildoperatingconditions(e.g.,upto120min at448Kand0.7MPaoxygenpressure)inthepresenceofmodified

Time (min40 60) 80 100 120 20 0 [TNP] (mg L -1 ) 0 50 100 150 200 250 300 270 mg L-1 - 500 W m-2 (1.0 g L-1 TiO₂) 270 mg L-1 _{- 1000 W m}-2 _{(1.0 g L}-1 _TiO 2) 144 mg L-1 _{- 1000 W m}-2 _{(1.0 g L}-1 _TiO 2) 64 mg L-1 - 1000 W m-2 (0.0 g L-1 TiO₂) 64 mg L-1 _{- 1000 W m}-2 _{(1.0 g L}-1 _TiO 2) 64 mg L-1 _{- 1000 W m}-2 _{(2.0 g L}-1 _TiO 2) 32 mg L-1 _{- 1000 W m}-2 _{(1.0 g L}-1 _TiO 2) pre-treated - 1000 W m-2 _{(1.0 g L}-1 _TiO 2) 0 min

Fig.6.Evolutionoftrinitrophenolconcentrationinphotocatalyticexperimentswith1gL−1_{ofTiO2-P25atvariousinitialconcentrationsandirradiancevalues,andblank}

experimentswithoutcatalystandwith2gL−1_{ofTiO2-P25byusing64mgL}−1_{oftrinitrophenol.}

metal-free carbon materials lead to the removal of a signifi-cantamountofthecontaminantbyCWAOandpartiallyimprove the aerobic biodegradability of the stream. In addition, TiO2

basedphotocatalysiscanpolishtheresultingwaters,thus remov-ingcompletelytheresidualcontaminantand furtherimproving biodegradability.

Sincethedevelopmentofcost-effectiveandgreenprocessesis criticaltoachieve sustainablewater treatment,couplingCWAO withTiO2 photocatalysismayserve thispurpose. Theuseof(i)

materials that can act as low cost and stable CWAO catalysts (activatedcarbons)andphotocatalysts(TiO2),and(ii)renewable

energy,aswellasselectingtherighttreatmentsequenceand con-ditions,isevidentlyastepintherightdirection.

Acknowledgements

LSRE/LCM LA is supported by “Programa de Financiamento Plurianual de Unidades de I&D/Laboratórios Associados” by Fundac¸ãopara a Ciência e a Tecnologia (FCT). AMTS acknowl-edgesthefinancialsupportfromPOCI/N010/2006.AKthanksthe ResearchCommitteeof TUC forsubsidising hervisit to Univer-sityofPorto.ThefinancialsupportfromtheEuropeanCommission (CleanWater –GA n◦ 227017,Seventh FrameworkProgramme (FP7/2007–2013)) is gratefully acknowledged. Clean Water is a Collaborative Project co-funded by the Research DG of the EuropeanCommissionwithinthejointRTDactivitiesofthe Envi-ronmentandNMPThematicPriorities.Theworkwasalsopartially fundedbyprojectsPTDC/AAC-AMB/110088/2009and NANO/NTec-CA/0046/2007,approvedbyFCTandco-supportedbyFEDER.

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