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Applied
Catalysis
B:
Environmental
jo u r n al ho 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 / a p c a t b
Research
Paper
Hybrid
magnetic
graphitic
nanocomposites
towards
catalytic
wet
peroxide
oxidation
of
the
liquid
effluent
from
a
mechanical
biological
treatment
plant
for
municipal
solid
waste
Rui
S.
Ribeiro
a,b,
Raquel
O.
Rodrigues
a,b,
Adrián
M.T.
Silva
b,
Pedro
B.
Tavares
c,
Ana
M.C.
Carvalho
d,
José
L.
Figueiredo
b,
Joaquim
L.
Faria
b,
Helder
T.
Gomes
a,∗aLaboratoryofSeparationandReactionEngineering–LaboratoryofCatalysisandMaterials(LSRE-LCM),EscolaSuperiordeTecnologiaeGestão,Instituto
PolitécnicodeBraganc¸a,CampusdeSantaApolónia,5300-253Braganc¸a,Portugal
bLaboratoryofSeparationandReactionEngineering–LaboratoryofCatalysisandMaterials(LSRE-LCM),FaculdadedeEngenharia,UniversidadedoPorto,
RuaDr.RobertoFrias,4200-465Porto,Portugal
cCQVR–CentrodeQuímica–VilaReal,UniversidadedeTrás-os-MonteseAltoDouro,5000-801VilaReal,Portugal dResíduosdoNordeste,EIM,RuaFundac¸ãoCalousteGulbenkian,5370-340Mirandela,Portugal
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received11May2017
Receivedinrevisedform26July2017 Accepted2August2017
Availableonline3August2017
Keywords:
Core-shellnanocomposites HeterogeneousFenton-likeprocess Mechanicalbiologicaltreatment Processwater
Magneticseparation
a
b
s
t
r
a
c
t
Magnetite,nickelandcobaltferriteswerepreparedandencapsulatedwithingraphiticshells,resultingin threehybridmagneticgraphiticnanocomposites.Screeningexperimentswitha4-nitrophenolaqueous modelsystem(5gL−1)allowedtoselectthebestperformingcatalyst,whichwasobjectofadditional studieswiththeliquideffluentresultingfromamechanicalbiologicaltreatmentplantformunicipalsolid waste.Duetoitshighcontentinbicarbonates(14350mgL−1)andchlorides(2833mgL−1),controlling theinitialpHwasacrucialsteptomaximizetheperformanceofthecatalyticwetperoxideoxidation (CWPO)treatment.Thecatalystloadwas0.5gL−1,averylowdosagewhencomparedtothehighchemical
oxygendemand(COD)oftheeffluent−9206mgL−1.AttheoptimumoperatingpH(i.e.,pH=6),ca.95% ofthearomaticitywasconvertedandca.55%ofCODandtotalorganiccarbon(TOC)oftheliquideffluent wasremoved.ThebiodegradabilityoftheliquideffluentwasenhancedduringthetreatmentbyCWPO, asreflectedbythe2-foldincreaseofthefive-daybiochemicaloxygendemand(BOD5)toCODratio
(BOD5/COD),namelyfrom0.21(indicatingnon-biodegradability)to0.42(suggestingbiodegradabilityof
thetreatedwastewater).Inaddition,thetreatedwaterrevealednotoxicityagainstselectedbacteria. Lastly,amagneticseparationsystemwasdesignedforin-situcatalystrecoveryaftertheCWPO reac-tionstage.Thehighcatalyststabilitywasdemonstratedthroughfivereaction/separationsequential experimentsinthesamevesselwithconsecutivecatalystreuse.
©2017ElsevierB.V.Allrightsreserved.
1. Introduction
The organic fraction accounts for about 30–40wt.% of the municipalsolid waste (MSW)produced in Europe, correspond-ingtoover 70milliontonnesperyear[1].Asignificantportion ofthisbiodegradablewasteendsupinlandfills[1];however,this approachisnotreallysustainable[2].Inordertolimitthe envi-ronmentalimpactofdirectlandfilldisposal,theEUDirectiveon wastelandfilling(CouncilDirective99/31/EC)aimstoreducethe biodegradableMSWgoingtolandfills,namely65wt.%withinjust 15years[3].Asaconsequence,theconceptofhierarchicalwaste
∗ Correspondingauthor.
E-mailaddress:htgomes@ipb.pt(H.T.Gomes).
managementiswell-establishednowadays,mechanicalbiological treatment(MBT)plantsbeingasuitablealternativethatis grow-inginpopularityinmanyEuropeancountriesaswellasinother countriesworldwide[4,5].
MBTplantstypicallyoperateintwosteps:thefirststep com-prisesresiduepreparationandsortingintodifferentfractionsusing mechanicalmeans;thesecondstepenvisagesthebiological treat-mentoftheorganicfractionofMSWtoproduceastabilizedsolid outputforagronomicalapplications(compost) orultimatelyfor disposal to landfill [6]. Under this context, anaerobicdigestion isanenergyefficientbiologicaltreatmenttechniquethatallows using theorganicfractionofMSW asrenewable energy source (e.g.throughbiogasproduction)whilereducingthe environmen-talimpactofitsdirectlandfilldisposal[2,7].Therefore,MBTplants allowreducingthewastestreamgoingtolandfillwhilebenefiting http://dx.doi.org/10.1016/j.apcatb.2017.08.013
fromresources(e.g.recyclablesandcompost)andenergyrecovery [5].
Inordertoachieveoptimumprocessconditions,the biodegrad-ablefractionoftheMSWgoingtoanaerobicdigestionmustfirst besuspendedin,ormoistenedwithwater,leadingtosignificant freshwaterrequirements–from0.4to0.6m3pertonneofwaste,
fordryor wetfermentation,respectively [7].Inadditiontothe processwaterfrom(i)anaerobicdigestion,theothermainsources ofprocesswaterstreamsare:(ii)leachatefromintensiverotting, (iii)pressingwaterfromdigestatedewateringand(iv)condensates and/orscrubberwaterfromtheexhausttreatment[7].However, unlikethegaseousemissionsandthestabilizedsolidoutput,the literaturelacksoninformationregardingthecharacteristicsand subsequenttreatmentof theliquideffluentwithhighpollutant loadsresultingfromtheprocesswaterstreamsofMBTplantsfor MSW[7].
Bearingthisinmind,aliquideffluentwascollectedfromaMBT plantlocatedintheNortheastregionofPortugal(currently pro-cessing50000tofMSWperyear);afterdetailedcharacterization, catalyticwetperoxideoxidation(CWPO)–apromisingalternative technologytotheconventionalhomogeneousFentonprocess[8]– wasstudiedunderatmosphericpressureandmoderate tempera-ture(80◦C)forthetreatmentofthisliquideffluentpresentinglow biodegradability.Thisapproachisinlinewiththecurrenttrendfor theintensificationofCWPOprocesses,whichismovingtheprocess towardstheapplicationofhighertemperatures,rangingfrom80◦C to125◦C[9–11].
CatalystdesignplaysacrucialroleinCWPO,sincethiswater treatment technology relies on the catalytic decomposition of hydrogenperoxide(H2O2)viaformationofhydroxylradicals(HO•)
–whichareverypowerfulandeffectiveoxidantsforthe destruc-tionofahugerangeoforganicpollutants[12,13].Thisisconfirmed byseveralrecentreviewarticlesonthesynthesisandapplication ofheterogeneousmetal/magneticphasesinCWPO,eitherdirectly appliedascatalysts orincluded in verydistinct support/hybrid materials[8,13–21].Under this context, ourpreviousworkhas beenmainlyfocusedonthecombinationofactiveandmagnetically separableiron-basedmaterialswiththeeasilytunedproperties ofcarbon-basedmaterials[14].Asaresult,theoutstanding per-formanceofahybridmagneticgraphiticnanocomposite(MGNC) catalyst–composedbyamagnetite(Fe3O4)coreandagraphitic
shell–wasreportedwhenappliedintheCWPOofatypical refrac-toryorganicmodelpollutant(4-nitrophenol;4-NP)withhighload (5gL−1,correspondingtoatotal organiccarboncontentsimilar tothatoftheliquideffluentconsideredinthiswork)[9]. Specifi-cally,itwasconcludedthattheperformanceofbareFe3O4inCWPO
isenhancedwhenthismagneticmaterialisencapsulatedwithin agraphitic structure duringthe synthesisof MGNC,due tothe increasedadsorptiveinteractionsbetweenthecarbonphaseand thepollutantmolecules;whileatthesametime,theleachingofFe speciesfromFe3O4tothetreatedwaterisstronglylimiteddueto
theconfinementeffectcausedbythecarbonshell[9].
SeekingforaMGNCcatalystoptimizationbasedonthe mag-netic core,nickel ferrite(NiFe2O4)and cobalt ferrite(CoFe2O4)
werepreparedinthepresentworkandthenencapsulatedwithin graphiticshells,inadditiontoFe3O4.Duetotheeasier
manipula-tion,simplicityandincreasedreproducibilityoftheexperimental results, the4-NP aqueous model system(5gL−1) wasused for theinitial screening of theMGNC catalysts, as well as for the developmentofanin-situmagneticseparation systemfor cata-lystrecoveryaftertheCWPOreactionstage.Afterwards,theMGNC catalystwithhigheractivityfortheCWPOof4-NPwasemployed inthetreatmentoftheliquideffluentcollectedfromaMBTplant. TheperformanceoftheCWPOprocesswasthoroughlyevaluated bysystematicmeasurementsofchemicaloxygendemand(COD), totalorganiccarbon (TOC), aromaticity,H2O2 consumption and
dissolvedmetalspecies,inordertooptimizetheoperating parame-ters.Five-daybiochemicaloxygendemand(BOD5)wasdetermined
inordertoestimatetheeffectofCWPOonthebiodegradabilityof theliquideffluent.Totalheterotrophicbacteriawereestimatedin ordertoassesstheeffectofCWPOontheautochthonousmicrobial populationoftheliquideffluent.Additionalmicrobiologicalassays wereperformedinordertoevaluatetheantimicrobialactivityof theliquideffluentbeforeandaftertreatmentbyCWPO.
2. Materialsandmethods
2.1. Chemicals
4-Nitrophenol,4-NP(O2NC6H4OH,Mr139.11,98wt.%),
hydro-genperoxide(H2O2,30%w/v)andpotassiumphthalatemonobasic
(99.5wt.%)werepurchased from AcrosOrganics andFluka and Riedel-de Haën, respectively. Iron (III) chloride hexahydrate (FeCl3·6H2O, 97wt.%), phenol (99.5wt.%), formaldehyde
solu-tion (37wt.%,stabilized with methanol), potassium dichromate (99.5wt.%), mercury (II) sulphate (HgSO4,99wt.%), ammonium
hydroxide solution(25wt.%)and silver nitrate(99.8wt.%)were obtainedfromPanreac.Titanium(IV)oxysulphate(TiOSO4·xH2O,
15wt.% in diluted sulphuric acid, 99.99%), iron (II) chloride tetrahydrate (FeCl2·4H2O, 99wt.%), copolymer pluronic F127,
ethanolabsolute(99.8wt.%)andsodiumsulphite(Na2SO3,98wt.%)
were purchased from Sigma-Aldrich. Nickel (II) chloride hex-ahydrate(NiCl2·6H2O,95wt.%), cobalt(II) chloridehexahydrate
(CoCl2·6H2O,99wt.%), sodiumhydroxide(NaOH, 98.7wt.%),
sul-phuricacid(H2SO4,95wt.%),hydrochloricacid(HCl,37wt.%),silver
sulphate(95wt.%),methanol(HPLCgrade),glacialaceticacid (ana-lyticalreagentgrade),tert-butanol(99.8wt.%),acetonitrile(HPLC grade)andpotassium chromate(99.6wt.%)wereobtainedfrom FisherChemical.Platecountagar(PCA),MullerHintonagar(MHA) andnutrientbroth(NB)werepurchasedfromLiofilchem.
Allchemicalswereusedasreceived,withoutfurther purifica-tion.Distilledwaterwasusedthroughoutthework.
2.2. Liquideffluentfromamechanicalbiologicaltreatmentplant formunicipalsolidwaste
The liquid effluentused in this work was collected from a MBTplantforMSWlocatedinNorthernPortugal.Theliquid efflu-ent,whosepropertiesaresummarizedinTable1,gathersallthe wastewaterproducedintheplant(mainlycomposedbya mechan-icalunitforresiduesorting,followedbyananaerobicdigestionunit forbiogasproductionfromtheorganicfractionoftheMSW).The liquideffluentwasfiltered(analyticalfilterpaper,25m)priorto itsuseinthiswork,inordertoremovethesuspendedsolidsthat wouldinterfereinsubsequenttreatmentandanalyticalsteps. 2.3. Magneticnanoparticles
Magnetite(Fe3O4)wassynthesizedbyco-precipitationofFe2+
and Fe3+ in basicsolution, at 30◦C and under N2 atmosphere,
aspreviouslydescribed[9].Forthatpurpose,13.44mmolofiron (II) chloride tetrahydrate and 26.88mmol of iron (III) chloride hexahydrateweredissolvedin250mLofdistilledwaterand trans-ferredintoa500mLglassreactor,equippedwithacondenserand immersedinanoilbathwithcontrolledtemperature.Whenthe desiredtemperaturewasreached,themixturewasdeaerated dur-ing10minwithN2undervigorousstirring,andfurtherkeptunder
inertatmosphere. Atthispoint,10mLofammoniumhydroxide solution(25wt.%) wasquickly added,a black precipitatebeing instantlyobtained.Afterwards,possibleresiduesoftheprecursors werewashed-outwithdistilledwater,thesamplebeingthendried inanovenat60◦Cfor24h,resultingintheFe3O4material.
Cobaltferrite(CoFe2O4)wassynthesized byco-precipitation
of Co2+ and Fe3+ in basic solution at 75◦C, adapting the
pro-cedure described elsewhere[22]. For that purpose, 67mmol of cobalt(II)chloridehexahydrateand134mmolofiron(III)chloride hexahydrateweredissolvedin100mLofdistilledwaterand trans-ferredintoa250mLglassreactor,equippedwithacondenserand immersedinanoilbathwithcontrolledtemperature.Whenthe desiredtemperaturewasreached,80mLofammoniumhydroxide solution(1molL−1)wasaddeddropwiseusingaperistalticpump, underconstantvigorousstirring.Afterthecolourofthesolution turnedtodark-brown,themixturewaskeptundervigorousstirring foranadditional30min,inordertoensurethecompleteformation offerritecrystals.Possibleresiduesoftheprecursorswere washed-outwithdistilled waterandabsoluteethanol,thesamplebeing thendriedinovenat60◦Cfor24h,groundintofinepowderand treatedunderapurifiedairflow(100cm3min−1)at500◦Cfor2h
withaheatingrampof2◦Cmin−1,allowingcrystallizationofthe materialswiththeinversespinelstructure[22,23],andresultingin theCoFe2O4sample.Nickelferrite(NiFe2O4)wassynthesizedusing
thesameprocedure,exceptthatthecobaltprecursorwasreplaced bynickel(II)chloridehexahydrate.
2.4. Hybridmagneticgraphiticnanocomposites
TheMGNCmaterialswerepreparedbyhierarchicalco-assembly ofmagneticnanoparticlesandcarbonprecursors,followedby ther-maltreatment,adaptingtheprocedurepreviouslydescribed[9]. Forthatpurpose,5gofcopolymerpluronicF127wasdissolvedin 50mLofH2O,inaroundbottom500mLglassreactorequippedwith
acondenserandimmersedinanoilbathwithtemperaturecontrol. Then,5mLofmagneticnanoparticlessuspension(17mgmL−1, pre-viouslyobtainedbydispersionofFe3O4,NiFe2O4 orCoFe2O4,in
H2Oinanultrasonicbath)wasadded,theresultingsolutionbeing
stirredduring2hat66◦Cforhomogenization.Afterthat,≈60mL ofa phenol/formaldehyde resolsolutionwasadded, the result-ingmixturebeingkeptunderstirring(400rpm)at66◦Cfor72h andthenat70◦Cforanadditional24h.Thephenol/formaldehyde resolsolutionwaspreparedbydissolutionof 2.0gofphenolin 7.0mLofformaldehyde37wt.%solution,towhich50.0mLofNaOH 0.1molL−1wasadded,thesolutionbeingthenkeptunderstirring at70◦Cfor30min.
Ineachcase,therecoveredsolidswerewashedwithdistilled water in order to promote the washing-out of some possible residuesoftheprecursorsandthendriedovernightinovenat60◦C. Afterwards,eachsamplewasthermallytreatedunderaN2 flow
(100cm3min−1)at120,400and600◦Cduring60minateach
tem-peratureandthenat800◦Cfor240min,definingaheatingramp of2◦Cmin−1.Finally, each samplewaswashedwith1L of dis-tilledwaterat50◦Cundervacuumfiltration,andthenwith1Lof HClsolution(pH=3),alsoat50◦Cundervacuumfiltration,being thendriedovernightinovenat60◦C,resultingintheFe3O4/MGNC,
NiFe2O4/MGNCorCoFe2O4/MGNCmaterials.
2.5. Characterizationtechniques
X-raydiffraction(XRD)analysiswasperformedinaPANalytical X’PertMPDequippedwithaX’Celeratordetectorandsecondary monochromator(CuK␣=0.154nm; datarecordedata0.017◦ step size). The crystallographic phases present were identified using HighScore software and Crystallography Open Database. RietveldrefinementoftheXRDdiffractionpatternswasperformed usingPowderCellsoftwareallowingphasequantification. Crystal-litesizesweredeterminedbytheWilliamson-Hallmethod.
Transmissionelectronmicroscopy(TEM)wasperformedina LEO906Einstrumentoperatingat120kV,equippedwitha4Mpixel 28×28mmCCDcamerafromTRS.Atleast100countswere
per-formedbyusingImageJsoftwareinordertoestimatethesizeof themagneticnanoparticles.Atleast65countswereperformedto estimatethesizeofthemetalcoreoftheMGNCmaterials.
The textural properties were determined from N2
adsorption–desorption isotherms at −196◦C, obtained in a
Quantachrome NOVA 4200e adsorption analyser, as previously described [24]. The pH at point of zero charge (pHPZC) was
obtainedbypHdrifttests[24].Thermogravimetricanalysis(TGA) wasperformedusingaNetzschTG209F3Tarsusequipmentunder oxidativeatmosphere,uponheatingthesamplesfrom30to950◦C at20◦Cmin−1.
2.6. Catalyticwetperoxideoxidationexperiments
BatchCWPOexperimentswith4-NPasmodelpollutantwere performedinawell-stirred(600rpm)glassreactorequippedwith a condenser, a temperature measurement thermocouple, a pH measurementelectrodeandasamplecollectionport.Thereactor wasloadedwith4-NPaqueoussolutions(5.0gL−1)andheatedby immersioninanoilbathatcontrolledtemperature.Upon stabiliza-tionat80◦C,thesolutionpHwasadjustedto3bymeansofH2SO4
andNaOHsolutions,andtheexperimentswereallowedto pro-ceedfreely,withoutfurtherpHconditioning.Acalculatedvolume ofH2O2(30%w/v)wasinjectedintothesystem,inordertoreachthe
stoichiometricamountofH2O2neededtocompletelymineralise
4-NP(17.8gL−1).Thecatalystwasaddedaftercomplete homoge-nizationoftheresultingsolution,thatmomentbeingconsidered ast0=0min.Pureadsorptionrunswerealsoperformedinorder
toassessthepossibleadsorptioninfluenceonthe4-NPremoval byCWPO,but,inthiscase,theamountofH2O2wasreplacedby
distilledwater.Blankexperiments,withoutanycatalyst,werealso carriedouttoassesspossiblenon-catalyticoxidationpromotedby H2O2.
Inordertoshowthepredominantroleofheterogeneous catal-ysispromotedbyCoFe2O4/MGNC,aleachingtestwasperformed
aspreviouslydescribed[9].Forthatpurpose,theCoFe2O4/MGNC
catalystwasremovedafter30minofreactionatthereaction tem-perature(80◦C),andthereactionsolutionwasallowedtoprogress further.TheparticipationoftheHO•radicalsintheCWPOprocess wasindirectlyshownbyaddingtert-butanol(tBuOH)–astrong HO•scavenger[25].
Batch CWPO experiments withthe liquid effluentdescribed in Section 2.2 were performed under the same experimental conditionsdescribedfortheCWPOrunsperformedwith4-NP solu-tions, except that theconcentration of H2O2 wasin the range
13.9–34.7gL−1(correspondingto0.6and1.6-foldthe stoichiomet-ricamounttheoreticallyneededtoreducetheeffluentCODandto reactwiththeeffluentchlorides),andtheoperatingpHintherange 2.5–8.
Thein-situmagneticseparationofthecatalystwasperformed afterthereactionstage,bycouplingaMitutoyo7033Bswitchable magneticstand(clampingforce600N)withtheglassreactorused forCWPO.Briefly,themagneticstandiscomposedbyfourparts: anon-ferrousmetalspacerplacedbetweentwoplatesoriron,and themagnetatthecentre;whenthemagnetpolesarealignedwith theferrousplatesthemagneticstandisON,whereasthemagnetic standisOFFwhen themagnetpoles arealignedwiththe non-ferrousspacer.Whentheroundbottomglassreactorwasplaced onthemagneticstand,itwasimmediatelyswitchedONandthe magneticseparationwasallowedtoproceedduring5min. After-wards,thetreatedwaterwascollectedwithapipette.Inorderto evaluatethestabilityoftheCoFe2O4/MGNCcatalystintheCWPO
oftheliquideffluentfromtheMBTplant,reusabilitycycleswere performedasdescribed:aftereachrun,in-situmagnetic separa-tionwasemployedforcatalystrecoveryandthetreatedwaterwas collected.Afterwards,23wt.%oftheinitialcatalystloadwasadded
Fig.1.XRDdiffractionpatternsof(a)Fe3O4andFe3O4/MGNC,(b)NiFe2O4and
NiFe2O4/MGNC,and(c)CoFe2O4andCoFe2O4/MGNC.Standardreferencepatterns
ofmagnetite(crystallographyopendatabasecode:9005840),nickelferrite (crys-tallographyopendatabasecode:2300289)andcobaltferrite(crystallographyopen databasecode:1535820)arealsogivenforcomparison.
(i.e.,0.115gL−1;correspondingtothemassfractionlostduetothe samplingprocedureandtothetreatedwatercollection)inorderto ensureequalcatalystdosagethroughoutallthereusabilitycycles (0.5gL−1),andthecatalystwasreusedinCWPOupontheaddition offreshliquideffluent.
Selectedexperimentswereperformedinduplicate,inorderto assessreproducibilityanderroroftheexperimentalresults.Itwas foundthatthestandarddeviationofthe4-NPandH2O2
determina-tionswasneversuperiorto1%intheexperimentsperformedwith themodelpollutant,whileintheexperimentsperformedwiththe liquideffluentfromaMBTplant,thestandarddeviationoftheCOD, TOC,H2O2andaromaticitydeterminationswasneversuperiorto
2%,1%,4%and1%,respectively. 2.7. Analyticalmethods
Theparentcompound4-NPandpossibleoxidationby-products were determined by high performance liquid chromatography (HPLC),usingamethodpreviouslydescribed [26].Totalorganic carbon(TOC)wasdeterminedusingaShimadzuTOC-LCSN anal-yser.Forquantificationpurposes,smallaliquotswereperiodically withdrawn fromthe reactor and an excess of sodium sulphite wasimmediatelyaddedinordertoconsumeresidualH2O2 and
toinstantaneouslystopthereaction.
The concentration of H2O2 was followed by a
colorimet-ricmethodwithtitanium(IV)oxysulfate[26].Chemicaloxygen demand(COD) wasdetermined by a closed refluxcolorimetric method,adaptingtheproceduredescribedelsewhere[27].A10:1 wtratioofHgSO4:Cl−wasensuredinalltheanalysisinorderto
avoidtheinterferenceofchlorideion[27].TheapparentCODvalue obtained(CODapp)wasthencorrectedconsideringthetheoretical
interferenceofresidualH2O2,asdescribedinEq.(1),whichwas
givenelsewhere[28],andconfirmedinthepresentstudy (consider-ingtheconcentrationrange0≤[H2O2]≤692mgL−1).Absorbance
spectraintherange200–660nmwereobtainedwitha0.5nm sam-plinginterval, using a T70spectrometer(PG Instruments, Ltd.). Aromaticitywasestimatedbymeasuringtheabsorbanceat254nm – a wavelength at which most aromatic compounds typically presenta maximum valueofabsorbance [29].The contribution ofresidualH2O2onaromaticitywasexperimentallydetermined
usingH2O2standardsolutionsintherange10–14000mgL−1.
After-wards,theapparentaromaticityvalueobtained(Aromaticityapp)
wascorrectedconsidering theinterferenceofresidual H2O2,as
describedinEq.(2).
Thefive-daybiochemicaloxygendemand(BOD5)was
deter-minedbythestandardisedrespirometricOxiTop®method(WTW, Weilheim,Germany).Forthatpurpose,anappropriatevolumeof sample was added into a brownglass bottle (nominal volume 510mL)equippedwithamagneticstirrerandacarbondioxidetrap intheheadspace(NaOHpellets).Eachbottlewassealedwithan OxiTop®headandthenplacedinanincubatorboxatconstant tem-perature(20◦C)duringthefive-dayincubationperiod.TheBOD5
valuewascalculatedfromthepressuredecreaseintheclosed ves-sel,asrecordedviathepiezoresistiveelectronicpressuresensors oftheOxiTop® measuringsystem.Inthesamplescollectedafter CWPO,theapparentBOD5valueobtained(BOD5,app)wasthen
cor-rectedconsideringthetheoreticalinterferenceofresidualH2O2,as
describedinEq.(3).
The samples collected for the determination of H2O2, COD,
absorbance spectra, aromaticity and BOD5 were immediately
placediniceinordertostopthereaction,andkeptat3◦Cuntil theanalysis.Appropriatedilutionsweremadewhennecessary.
COD= CODapp−0.4706[H2O2]/mgL−1 (1)
Aromaticity= Aromaticityapp−0.0005[H2O2]/mgL−1 (2)
BOD5= BOD5,app+0.4706[H2O2]/mgL−1 (3)
Total Fe content was determined by a colorimetric method with1,10-phenantroline,according toISO 6332 and measuring theabsorbanceat510nm[30].Thesameanalyticalprocedurewas employedforthedeterminationofthedissolvedFecontent,except thatinthiscasethesampleswerefiltered(0.2m)priortothe analysis.Likewise,dissolvedNiandCocontentsweredetermined
Fig.2. TEMmicrographs:(main)of(a)Fe3O4/MGNC,(c)NiFe2O4/MGNCand(e)CoFe2O4/MGNC;and(inset)of(a)Fe3O4,(c)NiFe2O4and(e)CoFe2O4.Histogramofparticle
sizedistribution:(main)ofthemetalcoreof(b)Fe3O4/MGNC,(d)NiFe2O4/MGNCand(f)CoFe2O4/MGNC;and(inset)of(b)Fe3O4,(d)NiFe2O4and(f)CoFe2O4,asdetermined
byTEMmeasurements.
byusingaPerkinElmerPinAAcle900atomicabsorption spectrom-eter,employing a multi-element hollowcathodelamp (Lumina N3050214).Theconcentrationofchloridesdissolvedintheliquid effluentwasdeterminedbytheMohrmethod(titrationwithsilver nitrate,usingpotassiumchromateasindicator).
2.8. Microbiologicalassays
Heterotrophicplatecountbythespreadplatemethodwasused inordertoestimatethenumberofliveheterotrophicbacteriainthe liquideffluentbeforeandaftertreatmentbyCWPO[31].Forthat
purpose,0.1mLofstartingsampleandserial10-folddilutionswere spreadontoPCAplatesunderasepticconditionsandincubatedat 28◦Cduring5days;countingwasperformedforplatesdisplaying 10–100colony-formingunits(CFU),theresultsbeingreportedas CFUpermillilitre(CFUmL−1).Alltheprocedurewasperformedin triplicate.
Theagardisk-diffusionmethodwasusedfortheantimicrobial susceptibilitytestingoftheliquideffluentbeforeandafter treat-mentbyCWPO[32].Klebsiellapneumoniae(Gram negative)and Bacilluscereus(Grampositive)wereselectedastest microorgan-isms.Ineachcase,thebacteriawereinitiallygrownovernightinNB
Table1
CharacterizationoftheliquideffluentfromtheMBTplantforMSWlocatedin North-ernPortugal,asdeterminedintriplicatemeasurements.
Parameter Value Units
Chemicaloxygendemand(COD) 9206±284 mgL−1
Biochemicaloxygendemand(BOD5) 1933±153 mgL−1
Totalorganiccarbon(TOC) 2046±16 mgL−1
Chlorides 2833±14 mgL−1
pHat25◦C 8.20±0.01 Sørensenscale
TotalFecontent 6.4±0.1 mgL−1
DissolvedCocontent 0.12±0.01 mgL−1
Totalheterotrophicbacteriacultivableat28◦C 14.7±2.1 ×104CFUmL−1
Conductivitya 23933±4554 Scm−1
Bicarbonatesa 14350±50 mgL−1
Ammonianitrogena 2300±285 mgL−1
Totalhydrocarbonsa 4±1 mgL−1 aAsdeterminedinquarterlyanalysisprovidedbytheintermunicipalcompany.
at37◦C.Afterwards,MHAplateswereinoculatedwithstandardised inoculaofthetest microorganisms(0.5McFarland; correspond-ingto108CFUmL−1).Then,antibioticstestingpaperdiscs(6mm
diameter;FiltresFioroni)containing10Lofthetestingsolution, previouslysterilizedbyfiltration(0.2m),wereplacedontheagar surface.Finally,possibleinhibitiongrowthzonessurroundingthe paperdiscswereevaluatedafter16and24hofincubationat37◦C. Thewholeprocedurewasperformedintriplicate.
3. Resultsanddiscussion
3.1. Materialscharacterization
BoththemagneticnanoparticlesandtheresultingMGNC mate-rials developed for this work were extensively characterized. XRD and TEM analysis were performed in order to assess the molecularstructureandmorphologyofthesematerials,the corre-spondingresultsbeinggiveninFigs.1and2,respectively.Fe3O4
exhibitsthetypicaldiffractionpattern ofmagnetite, witha lat-ticeparametera=8.357Å and acrystallite sizeof 16.6±0.2nm (cf.Fig.1a).AsobservedinFig.1b,animpurephase wasfound inthediffractionpatternofNiFe2O4 inadditiontonickelferrite
(a=8.330Å,crystallitesizeof10.2±0.5nm),whichwasascribedto thethermaltreatmentrequiredforthecrystallizationofthe amor-phousmaterialswiththeinversespinelstructure.Thisadditional phasewasidentifiedashematite(a=5.015Å,c=13.75Å, crystal-litesizeof16.6±1.0nm),correspondingto21wt.%ofNiFe2O4,as
determinedbyXRDanalysis.AsobservedinFig.1c,asimilar phe-nomenonoccurredduringthesynthesisofCoFe2O4.Inthiscase
hematite(a=5.043Å,c=13.78Å,crystallitesizeof24.2±0.5nm) wasfoundinadditiontocobaltferrite(a=8.387Å,crystallitesize of14.3±0.2nm),theimpurephase correspondingto24wt.%of CoFe2O4.
Asdetailedinourpreviouswork[9],Fe3O4nanoparticleswere
successfullyencapsulatedwithinagraphiticstructureduringthe synthesisofFe3O4/MGNC,resultinginacore-shellstructure(cf.
Fig.2a).AsobservedinFig.1a,somechangesinthemagnetitephase occurred, maghemite (a=8.379Å), iron (a=2.868Å) and traces ofhematite(proto)beingidentifiedinthediffractionpatternof Fe3O4/MGNC,inadditiontomagnetite(a=8.343Å)andgraphite.
Sincemagnetitenanoparticlesareverysensitivetooxidationinthe presenceofoxygen,thesephasemodificationscanbeascribedto theprogressiveoxidationofFe2+ionsintheinversespinelstructure
ofmagnetitetoFe3+–mostlikelyduringtheFe
3O4/MGNCsynthesis
stepperformedinliquidphase(i.e.,inoxidativemedia),resulting initspartialoxidationtomaghemiteandhematite[33,34].
During the hierarchical co-assembly mechanism in the Fe3O4/MGNC synthesis, Fe3O4 nanocrystals grow and
sponta-neouslyco-assemblebypartiallyreplacingF127/resolmicelles;at
thesametime,carbon–carbonbondsareformedwiththe poly-merizationofresolsand,uponthermaltreatment,thecopolymer F127iseliminated,graphiticcarbonframeworksbeingobtained withtheparticipationofFe3O4asgraphitizationcatalyst[35].The
averagesizeofthemagneticcoreofFe3O4/MGNC,109±35nm,as
determinedfromTEMmeasurements(cf.Fig.2b),suggeststhatthe coresaremainlycomposedbyagglomeratesofmagnetic nanopar-ticles(withcrystallitesizesintherange23–165nm,dependingon thephase,asdeterminedbyXRDanalysis).Theseobservationsare inaccordancewiththehierarchicalco-assemblymechanismofthe Fe3O4/MGNCsynthesis.
NiFe2O4/MGNCandCoFe2O4/MGNCwerepreparedbythesame
procedure used for the synthesis of Fe3O4/MGNC, except that
Fe3O4wasreplacedbyNiFe2O4andCoFe2O4,respectively.
There-fore,theencapsulationofthesemagneticnanoparticleswithina graphiticshellisexpectedtooccurthroughasimilarroutetothat reportedfor Fe3O4.As observedin Fig.2c ande, both NiFe2O4
andCoFe2O4nanoparticlesweresuccessfullyencapsulatedwithin
graphiticstructures duringthesynthesis ofNiFe2O4/MGNC and
CoFe2O4/MGNC, respectively, resulting in core-shell structures.
Regardingthemolecularstructure,nickel (a=3.575Å,crystallite size of 33±5nm) was identified in the diffraction pattern of NiFe2O4/MGNC,inadditiontonickelferrite(a=8.348Å,
crystal-litesizeof19±3nm)andgraphite(cf.Fig.1b);iron(a=2.858Å, crystallitesizeof44±1nm)wasidentifiedinthediffraction pat-ternofCoFe2O4/MGNC,in additiontocobalt ferrite(a=8.363Å,
crystallitesizeof33±2nm)andgraphite(cf.Fig.1c).Onceagain, whentheaveragesizeof themagneticcoresof NiFe2O4/MGNC
andCoFe2O4/MGNC(36±15nmand56±18nmrespectively,as
determinedfromTEMmeasurements,cf.Fig.2dandf),are com-paredtothesizeofparentNiFe2O4 andCoFe2O4 (11±3nmand
14±4nmrespectively,cf.theinsetsofFig.2dandf),itissuggested thatthecoresoftheseMGNCmaterialsaremainlycomposedby agglomeratesofmagneticnanoparticles.
ThetextureandsurfacechemistryoftheMGNCmaterialswere furthercharacterizedbyTGAanalysis,N2 adsorption-desorption
isothermsandpHPZC.TheTGAanalysisofFe3O4/MGNC(cf.Fig.S1a,
insupplementarymaterial)reveals27.3wt.%ofashes, correspond-ingtothemassfractionofFe3O4 encapsulatedinFe3O4/MGNC.
Likewise,theashcontentsofNiFe2O4/MGNCandCoFe2O4/MGNC
are10.4wt.%and14.4wt.%,respectively(cf.Fig.S1bandc).Itwas alsofoundthatalltheMGNCmaterialsarestableupto400◦Cunder oxidizing atmosphere. The N2 adsorption-desorption isotherms
at−196◦C oftheMGNCmaterials(cf.Fig.S2) denotethe
pres-ence of mesoporosity (as revealed by the progressive increase oftheamountofN2 adsorbedathigher relativepressures) and
microporosity(asrevealedbytheamountofN2 adsorbedatlow
relativepressure).Thedetailed texturalpropertiesoftheMGNC materialsaregiveninTable2.Asobserved,similartextural prop-ertiesareobtainedregardlessofthecompositionofthemagnetic core.Nevertheless,asreflectedbythepHPZCvaluesalsogivenin
Table2,theoverall surfacechemistry is slightlyaffected.Ifthe valuesof pHPZC obtainedfor Fe3O4/MGNC(7.1), NiFe2O4/MGNC
(8.7)andCoFe2O4/MGNC(9.0)arecomparedwiththoserecently
reportedintheliteraturefortheirmainmetaloxideconstituents [36,37],namelyFe3O4(6.2–7.4),NiFe2O4(6.7–10.2)andCoFe2O4
(7.5–10.1),itissuggestedthatthepHPZCoftheMGNCmaterials
aremainlydeterminedbythedifferentcontributionsofthemetal oxidesdetectedbyXRD.
3.2. Screeningofthehybridmagneticgraphiticnanocomposites inCWPO
TheperformanceoftheMGNCmaterialswhenappliedinCWPO wasinitiallyevaluatedthroughexperimentsperformedwithhighly loaded4-NP solutions (5gL−1, correspondingtoa TOC content
Table2
PropertiesoftheMGNCmaterials:specificsurfacearea(SBET),non-microporousspecificsurfacearea(Smeso),microporevolume(Vmicro),totalporevolume(Vtotal),average
porediameter(dpore)andpHatthepointofzerocharge(pHPZC).
Material Parameter
SBET(m2g−1) Smeso(m2g−1) Vmic(cm3g−1) Vtotal(cm3g−1) dpore(nm) pHPZC
Fe3O4/MGNC 330 170 0.07 0.31 3.75 7.1 NiFe2O4/MGNC 345 135 0.10 0.29 3.36 8.7 CoFe2O4/MGNC 330 170 0.07 0.31 3.75 9.0 0 2 4 6 8 0.0 0.2 0.4 0.6 0.8 1.0 4-NP H2O2 Fe3O4/MGNC CoFe2O4/MGNC NiFe2O4/MGNC Non-catalytic 4-NP H2O2
C
/ C
0t (h)
Fig.3. 4-NPandH2O2conversionsobtainedasafunctionoftimeinCWPOruns
performedwiththeMGNCmaterials.Experimentsperformedwith[4-NP]0=5gL−1,
[catalyst]=0.5gL−1,T=80◦C,pH=3and[H2O2]0=[H2O2]Stoichiometric=17.8gL−1.
similartothat oftheliquid effluentconsideredin Section3.4), under theexperimental conditions described in Section 2.6. In order to optimize the efficiency of catalyst usage, the catalyst dosagewaskeptverylowwhencomparedtothepollutant con-centration,witha fixedpollutant/catalyst massratioashighas 10. As observed in Fig.3,the 4-NP removal obtained after8h inthenon-catalyticexperimentisnegligiblewhencomparedto thatobtainedinthepresenceoftheMGNCcatalysts.Amongthe compositematerials,thecatalystresultingfromtheinclusionof CoFe2O4withinacarbonshell(CoFe2O4/MGNC)revealsthe
high-estactivity forthe CWPO of 4-NP. In this case, complete 4-NP conversionisobtainedin3h, correspondingtoaveryhigh pol-lutantmassremovalof3333mgg−1h−1.Thisenhancedactivityin CWPOcanbeascribedtothepresenceofCospecies,asobserved inpreviousworks[24,38].Theapparentcatalyticactivityofthe material resulting from the inclusion of NiFe2O4 within a
car-bonshell(NiFe2O4/MGNC)isalsosuperiortothatobtainedwith
the material with the Fe3O4 core (Fe3O4/MGNC). However, by
comparingthe leaching of Fe species at theend of the CWPO runsperformed with Fe3O4/MGNC (1.8mgL−1), NiFe2O4/MGNC
(3.2mgL−1)andCoFe2O4/MGNC(0.9mgL−1),itisobservedthatthe
NiFe2O4/MGNCcatalystexhibitstheloweststability.Onthe
con-trary,theCoFe2O4/MGNCcatalystrevealsanenhancedresistance
totheleachingofFespecies,whichcanalsobeascribedtothe pres-enceofCospecies,asdetailedinapreviouswork[38].However,as CoisoxidizedduringCWPO,itssusceptibilitytoundergoleaching tothetreatedwatersisexpectedtoincrease[38].Inorderto con-firmthishypothesis,theleachingofCofromtheCoFe2O4/MGNC
catalystwasdeterminedattheendoftheCWPOrundepictedin Fig.3.AlthoughundertheseconditionstheleachingofCoamounts to10.7mgL−1,therearealackofstandardsfortreated wastew-aterandevenfor drinkingwater [39].Likewise,theleachingof
Nispecies attheendof theCWPOexperiment performedwith NiFe2O4/MGNCwasalsodetermined(4.2mgL−1).
Duetothebestoverallperformance inthescreening experi-ments,CoFe2O4/MGNCwas objectof additionalstudies.A pure
adsorption run was performed in order to assess the possible adsorptioninfluenceontheremovalof4-NPbyCWPO.Asobserved inFig.4a, theremoval of4-NPobtainedinthepureadsorption runperformedwithCoFe2O4/MGNCisnegligible,corresponding
to 0.7% of the initial content of 4-NP. This negligible percent adsorption removal of 4-NP can be ascribed to the very low CoFe2O4/MGNCdosagewhencomparedtothepollutant
concen-tration, as reflected by the4-NP/catalyst mass ratioof 10. The evolutionofCOD,TOCandH2O2 consumptionduringtheCWPO
run performed with CoFe2O4/MGNC is also shown in Fig. 4a.
As observed, TOC and COD removalsof 54.4% and 74.2% were respectivelyobtained.Atthesametime,theconsumptionofH2O2
amountsto71.0%,representinghighefficienciesofTOCandCOD removalsperunitofH2O2decomposed.
Aleachingtestwasperformedinordertoevaluatethe heteroge-neousnatureoftheCoFe2O4/MGNCcatalyst,asdescribedinSection
2.6.AsobservedinFig.4b,whentheCoFe2O4/MGNCcatalystis
removedafter30minofreaction,thereactionsolutionreveals neg-ligibleactivityintheCWPOof4-NP,consideringboth4-NPand TOCremovals.Thisobservationconfirmsthepredominantroleof heterogeneousCWPOpromotedbyCoFe2O4/MGNC.
TheparticipationofHO•radicalsintheprocesswasindirectly evaluated.Forthatpurpose,tert-butanol(tBuOH)−astrongHO• scavenger[25],wasaddedbeforeaCWPOexperimentperformedin thepresenceofCoFe2O4/MGNC.WhentheCWPOrunsperformed
inthepresenceandabsenceoftBuOHarecompared(cf.Fig.4c), itisobservedthattheremovalof4-NPisgreatlysuppressedby tBuOH.Althoughnotdirectlysupportedbyquantificationofthe HO•radicalsformedduringtheprocess,thisindirectresultsuggests theabilityofCoFe2O4/MGNCtoefficiently promotethe
decom-positionof H2O2 viaHO• formation.Furthermore,the aromatic
by-productsdetectedduringtheCWPOof4-NPin thepresence ofCoFe2O4/MGNC(cf.Fig.5a)areinagreementwithareaction
mechanismincludingtheattackofthe4-NPmoleculebyHO• rad-icals,asreportedinourpreviousworks[9,26].Furtherattackof HO•radicalsonthearomaticintermediatecompoundsleadstothe openingofthearomaticring,andthustotheformationofaseries oflowmolecularweightcarboxylicacids(cf.Fig.5b).Inaddition, NO3−canbeproducedfromthe NO2groupsubtractionfromthe
4-NParomaticringundertheoxidationconditionsemployed[26]. BasedontheNO3−concentrationsshowninFig.5b,itcanbe
con-cludedthatatleast50.8%ofthetotalnitrogeninitiallypresentinthe 4-NP5gL−1solutionwaseffectivelysubtractedfromthemain aro-maticringafter4hofCWPOinthepresenceoftheCoFe2O4/MGNC
catalyst.
3.3. Developmentofanin-situmagneticseparationsystemfor catalystrecovery
In order to take advantage of the magnetic properties of CoFe2O4/MGNC, a lab-scale magnetic separation system was
0 1 2 3 4 0.0 0.2 0.4 0.6 0.8 1.0
(a)
4-NP, CWPO H2O2, CWPO TOC, CWPO COD, CWPO 4-NP, AdsorptionX
/ X
0t (h)
0 1 2 3 4 0.0 0.2 0.4 0.6 0.8 1.0(b)
X
/ X
0t (h)
4-NP, CWPO TOC, CWPO 4-NP, leaching test TOC, leaching test0 1 2 3 4 0.0 0.2 0.4 0.6 0.8 1.0
(c)
[tBuOH] = 0 g L-1 [tBuOH] = 60 g L-1[4-N
P
]/
[4-N
P
]
0t (h)
Fig.4.(a)4-NP,COD,TOCandH2O2conversionsasafunctionoftimeintheCWPO
runperformedwithCoFe2O4/MGNC;4-NPremovalsbyadsorptionarealsoshown
forcomparison.(b)4-NPandTOCremovalsobtainedduringthe“leachingtest” per-formedwithCoFe2O4/MGNC(i.e.wherethecatalystwasremovedfromthesolution
after30minofreaction).(c)Effectoftert-butanol(tBuOH)ontheCWPOremoval of4-NPwhenusingCoFe2O4/MGNC.Experimentsperformedundertheconditions
giveninFig.3.
stage.Forthatpurpose,aswitchablemagneticstandwascoupled withthereactorafterthereactionstage(asdescribedinSection 2.6),allowingtoperformreaction/separationsequentialstagesin asinglevessel,thusavoidingtheseparationoftheheterogeneous catalystsbyfiltrationand/orcentrifugation.AsobservedinFig.6, thistechniquewassuccessfullyemployedfortherecoveryofthe CoFe2O4/MGNCcatalystaftertheCWPOrunperformedwith4-NP
0 1 2 3 4 0 200 400 600 800 1000 1200 1400 1600 1800
(a)
Co
n
c
e
n
trati
on
(mg
L
-1)
t (h)
4-Nitrocatechol Hydroquinone 1,4-Benzoquinone 0 1 2 3 4 0 200 400 600 800 1000 1200 1400 1600 1800Co
n
c
e
n
tra
ti
on
(mg
L
-1)
(b)
t (h)
Oxalic acid Formic acid NO3
-Malic acid Malonic acid
Acetic acid Maleic acid
Fig.5.Evolutionofaromatic(a)andnon-aromatic(b)by-productsof4-NP oxi-dation,whenusingCoFe2O4/MGNCintheCWPOprocessdevelopedunderthe
conditionsgiveninFig.3.
(5gL−1),ca.98%ofthetreatedwater beingcollected. Animage oftheglassreactorcoupledwiththemagneticstandisprovided in Fig.6a. In orderto estimatetheefficiencyof catalyst recov-ery,theCoFe2O4/MGNCcollectedaftertheCWPOrunwasdried
overnightat60◦Candthenweighed.Reaction/separation sequen-tialexperimentsperformedintriplicateallowedtodeterminethe percentageofcatalystrecovery.Itwasfoundthat77.0±6.1wt.%of theinitialCoFe2O4/MGNCloadisrecoveredbyimplementingthe
proposedin-situmagneticseparationsystem,avaluewellabove the57.9±2.4wt.%recoveredbyconventionalfiltration.Thedirect benefit of magnetic separation for therecovery of the catalyst inthesameunit thatis usedintheCWPOexperiments isthus demonstrated.Withthisinmind,in-situmagneticseparationwill befurtherexploredinthereusabilitycyclesperformedinSection 3.4.3.
3.4. CWPOoftheliquideffluentfromaMBTplantformunicipal solidwaste
Herein,thesuitabilityofthemostactiveand stablecatalytic systemobtainedinSection3.2fortheCWPOoftheliquid efflu-entcollectedfromaMBTplantisevaluated.AsdetailedinTable1, this effluent contains a high pollutant load, due to the pres-enceoforganic(9206mgL−1 COD; 2046mgL−1 TOC),inorganic (14350mgL−1 bicarbonates;2833mgL−1 chlorides)and biologi-cal(14.7×104CFUmL−1totalheterotrophicbacteriacultivableat
28◦C)species.AccordingtotheBOD5/CODratio,whichiswidely
used asan indicatorof the biodegradabilityof liquid effluents, awastewater isconsideredeasilytreatablebybiological means if theBOD5/CODratio is 0.5 or larger[31].On thecontrary, a
BOD5/CODratio below0.3 suggeststhe presenceof toxic
com-ponents,thewastewaterbeingnotbiodegradable oracclimated microorganismsbeingrequiredforitsbiologicaltreatment[31]. The liquid effluent considered in this work is not expected to beproneto degradationby conventional biological treatments, accordingtothelowBOD5/CODratioof0.21.Itisalsoknownthat
bothbicarbonatesandchloridescanactasHO•radicalscavengers. Althoughseveralreactionsinvolvingbicarbonateandchlorideions mayoccurinthebulk,theHO•radicalscavengingeffectpromoted bytheseinorganicspeciesresultsmainlyfromtheirdirectreaction withHO•radicals.ThereactionofbicarbonateionswithHO• radi-calscanbedescribedbyEq.(4)[40],whilethereactionofchloride ionswithHO•radicalsproceedsthroughthemechanismdescribed byEqs.(5)and(6)[41].Therefore,thepresenceoftheseinorganic speciesis expectedtohindertheperformanceof CWPOforthe treatmentoftheliquideffluentconsideredinthiswork.Bearingthis inmind,theoperatingconditionsthatmaximizetheperformance ofCWPOforthetreatmentoftheliquideffluentfromtheMBTplant inthepresenceofCoFe2O4/MGNCwerethoroughlyinvestigated.
HO• +HCO3−→ CO3•−+H2O (4)
HO• +Cl−→HOCl•− (5)
HOCl•−+H+→Cl• +H2O (6)
3.4.1. CWPOprocessoptimization:thecrucialroleofthe operatingpH
The operating conditions used in Section 3.2 were initially employedinthisSection,thecatalystdosagebeingkeptverylow whencomparedtotheeffluentCOD.Takingintoaccountthe pos-siblecatalyticcontributionoftheFespeciespresentintheliquid effluentcollectedfromtheMBTplant(6.4mgL−1,cf.Table1), a preliminaryexperimentwasperformedwithoutaddedcatalyst.In spiteoftheFecontentandthehighcomplexityoftheliquid efflu-ent,theCODremovalobtainedafter24hofreactionunderthese conditionswasonly12.8%,representinglessthanathirdofthat obtainedinthepresenceofCoFe2O4/MGNC(0.5gL−1).Thisresult
highlightstheroleofCWPOpromotedbyCoFe2O4/MGNC.
SeekingforCWPOprocessoptimization,theinfluenceofH2O2
dosage was then evaluated. Based on the results obtained in CWPO runs performed with H2O2 concentrations in the range
13.9–34.7gL−1 (results not shown), it wasfound that thebest performanceisachievedwhenemploying[H2O2]0=27.7gL−1
(cor-responding to 1.2-fold the stoichiometric amount theoretically neededtoreducetheeffluentCODandtoneutralizetheHO• radi-calscavengingeffectpromotedbythechloridesdissolvedinthe effluent, asdescribed byEqs.(5) and(6)).Therefore, additional experimentswereperformedwiththisH2O2dosage.
Itisnoteworthythatthebicarbonateequilibriumconcentration inwaterdependsonthepH,asdescribedbytheacidionization reactions,Eqs.(7)and(8),andcorrespondingacid-dissociation con-stants[42].AtthenaturalpHoftheliquideffluentconsideredin thiswork(8.2)andgivenconcentrationsofdissolvedmatterand ions,theprevailingspeciesisthebicarbonateion(HCO3−).In
addi-tiontotheHO•radicalscavengingdescribedbyEq.(4),thenegative effectofHCO3−isparticularlysignificantinthecaseofCWPO,since
H2O2 canbedirectlydecomposedthroughtheparasiticreaction
describedbyEq.(9)[43],evenbeforeHO•formation.However,this parasiticreactioncanbeavoidedbydecreasingthesolutionpHto valuesbelow6.35;in thiscase,HCO3− isconverted tocarbonic
acid(H2CO3)−which is subsequentlydecomposed intocarbon
dioxide(whichescapesthesolutionforthegasphase)andwater
(cf.Eq.(10))[44].Therefore,basedonthepresenceofbicarbonate species(14350mgL−1),operatingatpH<6.35isexpectedtofavour theperformanceofCWPOforthetreatmentoftheliquideffluent consideredinthiswork.
H2CO3(aq) H+(aq)+HCO−3(aq) pKa1=6.35 (7)
HCO3−(aq) H++CO32−(aq) pKa2=10.33 (8)
HCO3−(aq)+H2O2(aq)HCO4−(aq)+H2O(aq) (9)
H2CO3(aq)CO2(g)+H2O(l) (10)
Nevertheless,thepresenceofchloridesshouldalsobetakeninto accountinthisanalysis.AsdiscussedelsewherefortheH2O2/UV
process, the solution pHcan also affect theextent of the HO• radical scavenging reaction described by Eq. (5) [45]. Specifi-cally, therate constant for the reaction described in Eq. (5) is 4.3×109Lmol−1s−1;however,thehypochlorousradical(HOCl•−)
formedbythereactiondescribedinEq.(5)isabletodissociateback toHO•radicalandchlorideion(Cl−),thedissociationrateconstant being6.1×109s−1[45].AsdescribedbyEq.(6),HOCl•−
canalsobe convertedintochlorineradicals(Cl•)andwaterthrougha protona-tionreactionwiththerateconstantof2.1×1010Lmol−1s−1,inthis
casethereverseratereactionbeingmuchsmaller(1.3×103s−1)
[45].Therefore,itisexpectedthattheformationofCl•bythe pro-tonationreactiondescribedinEq.(6)increasesasthesolutionpH decreases,thuspromotingthescavengingreactiondescribedbyEq. (5).Inthiscase,thecriticalpointaffectingtheextentofHO•radical scavengingisthepKaofthereversereaction(i.e.,the deprotona-tionreaction)describedbyEq.(6)(7.2).Thus,itcanbeconcluded thatCl•istheprevailingspeciesatsolutionpH<7.2,whileHOCl•− becomesthedominantspecieswhenpH>7.2,thusdecreasingthe consumptionofHO•radicalsbythereactiondescribedbyEq.(5). Therefore,basedonthepresenceofchloridespecies(2833mgL−1), operatingpH>7.2isexpectedtofavourtheperformanceofCWPO forthetreatmentoftheliquideffluentconsideredinthiswork.
Summarizing,thesolutionpHisexpectedtosignificantlyaffect theperformanceofCWPOforthetreatmentoftheeffluentfrom theMBTplant.Ononehand,pH<6.35limitsthenegativeeffectof bicarbonates;while,ontheotherhand,pH>7.2limitsthenegative effectofchlorides.Therefore,theselectionoftheoptimumpHmay beconsideredasthecrucialsteptoachievetheoperating parame-tersthatmaximizetheperformanceoftheliquideffluenttreatment byCWPOinthepresenceofCoFe2O4/MGNC.
Bearingthisinmind,theindividualeffectoftheoperatingpHon theefficiencyofCWPOwasevaluatedintherange2.5–8.Forthat purpose,COD,TOC,aromaticityandH2O2conversionswere
deter-mined,asshowninFig.7a.TheefficiencyofCWPOforthetreatment oftheliquideffluentfromtheMBTplanttendstoincreaseasthepH increasesintherange2.5–6,whereasitdramaticallydecreasesfor pH>6.Thisphenomenon–whichcanbeascribedtothepresenceof bicarbonatesandchlorides−confirmsthecrucialroleofthe oper-atingpHintheCWPOoftheliquideffluentconsideredinthiswork, aspreviouslydiscussed.AtpH>6,97%oftheinitialH2O2dosageis
consumedwithinthefirst30minofreaction,mostlikelyduetothe fastreactionwithHCO3−,asdescribedbyEq.(9).AtpH<6,theHO•
radicalscavengingbyCl−isincreasinglysignificant,thushindering theefficiencyoftheCWPOprocess.Therefore,pH=6canbe con-sideredtheoptimumoperatingpH,sinceitallowstomaximizethe performanceoftheliquideffluenttreatmentbyCWPOinthe pres-enceofCoFe2O4/MGNC.AdditionalinsightsontheCOD,TOC,H2O2
andaromaticityconversions,andabsorbancespectraevolutionas afunctionoftimeintheCWPOrunperformedundertheoptimized conditionsare giveninFig.7b andc,respectively. Asobserved, fastconversionsofCOD,TOC,aromaticityandH2O2areobtained
inthefirst8hofCWPO.Uptothatpoint,ca.93%oftheeffluent aromaticityisalreadyconverted;whilefrom8to24hofreaction,
Fig.6.In-situmagneticseparationofCoFe2O4/MGNCattheendoftheCWPOstageperformedwiththeaidofaMitutoyo7033Bswitchablemagneticstand(clampingforce
600N);(a)frontand(b)topviewoftheglassreactorimmediatelyaftertherecoveryofthetreatedwaterand(c)topviewofthereactorafterthedryingprocess(60◦C)for catalystrecovery.
anincreaseofonlyca.2%isobservedinthearomaticity conver-sion.However,theTOCcontentoftheeffluentdecreasesca.14% duringthesameperiod,whiletheabsorbancespectraalsoevolved favourably(cf.Fig.7c),revealingthatthetreatmentreactionsstill proceedalthoughatalowerrate.Theseobservationssuggestthat recalcitrantby-productsareformedwhenthearomaticcompounds areattackedbyHO•radicals(cf.demonstratedinSection3.2).This mechanismisalsosuggestedbythechanges inthesolutionpH observedduringthetreatmentbyCWPO(cf.Fig.7b).Specifically, pHdecreasesfrom6(i.e.,theinitialpH)to5.4inthefirst2h, sug-gestingtheformationoflowmolecularweightcarboxylicacids; afterwardsthepHgraduallyincreasesupto6.5attheendofthe reaction,suggestingthattheCWPOtreatmentisabletomineralize mostofthesecarboxylicacidsalthoughatanapparentlylowerrate whenbroadparameters,likeCODandTOCareconsidered.After 24hofCWPO,ca.95%oftheeffluentaromaticityisconvertedunder theseoperatingconditions,whileca.55%oftheinitialCOD and TOCareeffectivelyremoved.TheH2O2 consumptionduringthe
treatmentrepresentsca.98%oftheinitialdosage.
TheBOD5 ofthetreatedwaterwasalsodeterminedinorder
toestimatetheeffectofCWPOonthebiodegradabilityofthe liq-uideffluent.ItwasfoundthattheBOD5isslightlyaffectedduring
theprocess,a decrease from1933mgL−1 to1760mgL−1 being observed.AtthesametimetheCODdecreasedfrom9206mgL−1 to4164mgL−1.Accordingly, theBOD5/CODratioof thetreated
wateris0.42, representinga 2-foldincrease whencomparedto theBOD5/CODratiooftheliquideffluent(0.21).ABOD5/CODratio
in the range 0.3–0.5 suggests that the wastewater is treatable bybiological means[31].Itcanbethereforeconcludedthatthe biodegradabilityoftheliquideffluentisenhancedduringthe treat-mentbyCWPOinthepresenceofCoFe2O4/MGNC.
Inaddition,thedissolvedFecontentwasmeasuredattheend of the CWPO runs performed with operating pH in the range 2.5–6.Thehighestvaluewasfoundintheexperimentperformedat pH=2.5,correspondingtoaFeconcentrationof1.04mgL−1;onthe otherhand,thelowestvaluewasobtainedintheexperiment per-formedatpH=6(0.27mgL−1).However,itshouldbenotedthatthe liquideffluentconsideredinthisworkisverycomplex.Although thetotalFecontentpresentintheeffluentis6.4mgL−1,theamount ofdissolvedFespecies,i.e.,thoseobtainedafterfiltration(0.2m), actuallydependsonthepH.Forinstance,theinherentdissolved Fecontentoftheeffluentis0.96mgL−1 after24hatpH=3,but
thisvaluedecreasesto0.15mgL−1after24hatpH=6.Therefore, thedissolvedFecontentdeterminedattheendoftheCWPOruns cannotbefullyascribedtoleachingfromtheCoFe2O4/MGNC
cata-lyst.Likewise,thedissolvedCocontentattheendoftheCWPOrun performedatpH=6wasalsodetermined(0.58mgL−1);itcanbe consideredverylow,inparticularwhencomparedtotheinherently dissolvedCocontentintheeffluent(0.12mgL−1,cf.Table1).
3.4.2. Disinfectionandantimicrobialactivity
Regardingthebacterialpopulation,heterotrophicplatecount isaprocedurewidelyusedtoevaluatetheperformanceof treat-ment processes, since it allowsto estimatethe number oflive heterotrophicbacteria in a given water or wastewater [31].As described in Section 2.8, thespread plate methodwasused in thiswork.Theselectedincubationtemperaturewas28◦C,since itfavoursthegrowthofwaterbornebacteria,thusyieldinghigher bacterialplatecountswhencomparedtothatobtainedatlower orhigher incubation temperatures[46–48].Under this context, totalheterotrophicbacteriainthetreatedwaterwereestimatedin ordertoassesstheeffectofCWPOontheeffluentautochthonous microbialpopulation(14.7×104CFUmL−1totalheterotrophic
bac-teriacultivableat28◦C).Afterincubationat28◦Cduring5days, not asingle colonywasfoundin theplate countperformedin triplicate, even in the undilutedtreated water samples. There-fore, although this was not the main goal of the treatment proposed, it can be concluded that disinfection of the effluent wasachievedupon theCWPOtreatmentemployedinthe pres-enceofCoFe2O4/MGNCundertheoptimumoperatingconditions
considered(i.e.,[CoFe2O4/MGNC]=0.5gL−1,T=80◦C, pH=6 and
[H2O2]0=27.7gL−1).
Additionalmicrobiologicalassayswereperformedinorderto evaluatethe antimicrobialactivity of theliquid effluent before and aftertreatment byCWPO. As described in Section 2.8,the agardisk-diffusion methodwasusedfor theantimicrobial sus-ceptibilitytesting.Forthatpurpose,Klebsiellapneumoniae(Gram negative) and Bacillus cereus (Gram positive) were selected as testmicroorganisms.After16and24hofincubationat37◦C,the absenceofinhibitiongrowthzonessurroundingthetestingpaper discscontainingboththeeffluentandthetreatedwatersamples wasobserved.Thesequalitativeresultsrevealthatbothselected microorganismsareresistanttotheeffluent,eitherbeforeorafter treatmentbyCWPO,suggestingthatthetoxicityoftheliquid
efflu-Fig.7. (a)EffectofpHonCOD,TOC,H2O2andaromaticityconversionsobtainedafter
24hinCWPOrunsperformedwithCoFe2O4/MGNC.(b)COD,TOC,H2O2,aromaticity
[leftaxis],solutionpH[rightaxis]and(c)absorbancespectraevolutionasafunction oftimeintheCWPOrunperformedatpH6;Experimentsperformedwiththeliquid effluentcollectedfromaMBTplant,[CoFe2O4/MGNC]=0.5gL−1,T=80◦C,pH=6and
[H2O2]0=27.7gL−1.
entfromtheMBTplantisnotincreasedduringitstreatmentby CWPOundertheoptimumoperatingconditionsdeterminedin Sec-tion3.4.1.
3.4.3. Reusabilitycyclesimplementingin-situmagnetic separationforcatalystrecovery
Catalystseparationandlong-termstabilityarecrucialaspects forthefeasibilityoftheproposedwatertreatmentprocessinlarge scaleapplications.Therefore, once thecatalytic systemandthe
CWPOprocesswereoptimized,thebenefitsofmagnetic separa-tionfortherecoveryofthecatalystwereexploredbyperforming aseriesoffiveCWPOreaction/separationsequentialexperiments inthesamevesselwithconsecutivereuseoftheCoFe2O4/MGNC
catalyst,asdepictedinFig.8.Forthatpurpose,thein-situ mag-neticseparationsystemdevelopedinSection3.3wasemployed forcatalystrecoveryattheendofeachCWPOcycle,thetreated waterbeingcollectedafterwards.Inordertoensureequalcatalyst dosagethroughoutallthereusabilitycycles(0.5gL−1),23wt.%of theinitialcatalystloadwasadded(i.e.,0.115gL−1; correspond-ingtothemassfractionlostduetosamplingandtreatedwater collection),andanewCWPOrunwasperformeduponthe addi-tionoffreshliquideffluent.AsobservedinFig.9,theefficiencyof CWPOforthetreatmentoftheliquideffluentfromtheMBTplant ismaintainedthroughoutthefivecyclesperformedinthe pres-enceofCoFe2O4/MGNCundertheoptimumoperatingconditions
determinedinSection3.4.1.Undertheseconditions,theCOD,TOC, aromaticityandH2O2conversionsobtainedafter24hofreaction
withCoFe2O4/MGNCarenotparticularlyaffectedbythesuccessive
reuseofthecatalyst,thusrevealingitshighstabilityforCWPO,and highpotentialforlargescaleapplications.Thisstabilityfeaturefor CWPOcanbeascribedtotheresistanceoftheCoFe2O4/MGNC
cat-alystagainsttheleachingofFespecies–whichistypicallythemain causeofcatalystdeactivation[8,14],ashighlightedbymeasuring thedissolvedFecontentattheendofeachCWPOcycle.Specifically, thehighestvaluewasobtainedinthefirstcycle,correspondingto 0.27mgL−1.Fromthesecondtothefifthcycle,thedissolvedFe contentwasintherange0.13–0.17mgL−1,thelowestvaluebeing obtainedafterthefifthcycle.ThedissolvedFecontentobtained afterthefifthCWPOcycleissimilartothedissolvedFecontent inherenttotheliquideffluent(0.15mgL−1;asdiscussedin Sec-tion3.4.1),confirmingthehighresistanceoftheCoFe2O4/MGNC
catalysttotheleachingofFespecies.Regardingtheleachingof Cospecies,thelowestvaluewasobtainedinthefirstCWPOcycle (0.58mgL−1),whilethehighestvaluewasobtainedinthethird cycle(4.55mgL−1).Theleaching ofCospecies intheremaining cycleswasintherange1.92–2.64mgL−1,thelowestvaluebeing obtainedafterthefifthcycle.TheseresultssuggestthatCospecies aremoresusceptibletoundergoleachingfromthecatalysttothe treatedwatersthanFespecies,which isinlinewiththeresults previously reportedontheapplication ofbimetallic iron-cobalt magneticcarbonxerogelsinCWPO[38].
4. Conclusions
Allthehybridmagneticgraphiticnanocompositesdevelopedin thisworkrevealedcatalyticactivityintheCWPOof4-NP. Neverthe-less,thebestperformancewasachievedwiththematerialresulting fromtheinclusionofCoFe2O4intoagraphiticstructureduringthe
synthesisofCoFe2O4/MGNC.
TheefficiencyofCWPOforthetreatmentoftheeffluentfrom aMBTplantwasstronglyinfluencedbytheoperatingpH,a phe-nomenonwhichwasascribedtothepresenceofbicarbonatesand chlorides.AtpH>6,H2O2isreadilyconsumedduetothefast
reac-tionwithHCO3−;whileatpH<6,theHO•radicalscavengingby
Cl−hinderstheefficiencyoftheCWPOprocess.BasedontheCOD, TOC,H2O2andaromaticityconversionsobtained,pH=6was
con-sideredtheoptimumoperatingpHfortheliquideffluenttreatment byCWPOinthepresenceofCoFe2O4/MGNC.The
biodegradabil-ityoftheliquideffluentwasenhancedduringthetreatment,as reflectedbythe2-foldincreaseoftheBOD5/CODratioobtained.In
addition,disinfectionoftheliquideffluentwasachievedandthe treatedwaterrevealednotoxicityagainstselectedbacteria.
A magneticseparation systemwas developedfor thein-situ recovery of the CoFe2O4/MGNC catalyst after the CWPO
reac-Fig.8.ExperimentalprocedureduringtheCWPOreusabilitycyclesperformedwiththeliquideffluentcollectedfromaMBTplantandCoFe2O4/MGNC. 1st cycle 2nd cycle 3rd cycle 4th cycle 5th cycle 0 10 20 30 40 50 60 70 80 90 100
COD TOC H2O2 Aromaticity
C
on
ve
rsion (
%)
Fig.9.COD,TOC, H2O2 and aromaticityconversionsobtained after 24hin a
seriesoffiveCWPOrunsperformedwithconsecutivereuseofCoFe2O4/MGNC.
Experimentsperformed with theliquideffluent collected from aMBT plant, [CoFe2O4/MGNC]=0.5gL−1,T=80◦C,pH=6and[H2O2]0=27.7gL−1.
tionstage.ThehighstabilityofCoFe2O4/MGNCfortheCWPOof
theliquideffluentconsideredinthisworkwasdemonstratedby performingaseriesoffiveCWPOreaction/separationsequential experimentsinthesamevessel.
Theabilityofthedevelopedcatalyticsystemtoenablethe treat-mentoftheliquideffluentfromaMBTplantforMSWbyCWPO–in spiteofitsveryhighconcentrationofchloridesandbicarbonates, andinawiderangeofoperatingpH–opensfutureprospectsfor theapplicabilityofthiswastewatertreatmenttechnology.
Acknowledgments
Thisworkwasfinanciallysupportedby:Project POCI-01-0145-FEDER-006984–AssociateLaboratoryLSRE-LCMfundedbyFEDER throughCOMPETE2020−ProgramaOperacionalCompetitividade e Internacionalizac¸ão (POCI) − and by national funds through FCT− Fundac¸ãopara a Ciência ea Tecnologia; Project VALOR-COMP,fundedbyFEDERthroughProgrammeINTERREGV-ASpain −Portugal(POCTEP)2014–2020.R.S.RibeiroandR.O.Rodrigues acknowledgetheFCTindividualPh.D.grantsSFRH/BD/94177/2013
andSFRH/BD/97658/2013,respectively,withfinancingfromFCT and the European Social Fund (through POPH and QREN). A.M.T.SilvaacknowledgestheFCTInvestigator2013Programme (IF/01501/2013),withfinancingfromtheEuropeanSocialFundand theHumanPotentialOperationalProgramme.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound, intheonline version,athttp://dx.doi.org/10.1016/j.apcatb.2017. 08.013.
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