Hybrid magnetic graphitic nanocomposites towards catalytic wet peroxide oxidation of the liquid effluent from a mechanical biological treatment plant for municipal solid waste

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ContentslistsavailableatScienceDirect

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,∗

a_Laboratory_of_Separation_and_Reaction_Engineering_–_Laboratory_of_Catalysis_and_Materials_(LSRE-LCM),_Escola_Superior_de_Tecnologia_e_Gestão,_Instituto

PolitécnicodeBraganc¸a,CampusdeSantaApolónia,5300-253Braganc¸a,Portugal

b_Laboratory_of_Separation_and_Reaction_Engineering_–_Laboratory_of_Catalysis_and_Materials_(LSRE-LCM),_Faculdade_de_Engenharia,_Universidade_do_Porto,

RuaDr.RobertoFrias,4200-465Porto,Portugal

c_CQVR_–_Centro_de_Química_–_Vila_Real,_Universidade_de_{Trás-os-Montes}_e_Alto_Douro,_5000-801_Vila_Real,_Portugal d_Resíduos_do_Nordeste,_EIM,_Rua_Fundac¸_ão_Calouste_Gulbenkian,_5370-340_Mirandela,_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_,_a_very_low_dosage_when_compared_to_the_high_chemical

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.

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:[email protected](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

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fromresources(e.g.recyclablesandcompost)andenergyrecovery [5].

Inordertoachieveoptimumprocessconditions,the biodegrad-ablefractionoftheMSWgoingtoanaerobicdigestionmustﬁrst besuspendedin,ormoistenedwithwater,leadingtosigniﬁcant freshwaterrequirements–from0.4to0.6m3_per_tonne_of_waste,

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,25␮m)priorto itsuseinthiswork,inordertoremovethesuspendedsolidsthat wouldinterfereinsubsequenttreatmentandanalyticalsteps. 2.3. Magneticnanoparticles

Magnetite(Fe3O4)wassynthesizedbyco-precipitationofFe2+

and Fe3+ _in _basic_solution, _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.

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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(100cm3_min−1₎_at₅₀₀◦_C_for₂_h

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

(100cm3_min−1₎_at_120,₄₀₀_and₆₀₀◦_C_during₆₀_min_at_each

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

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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(0.2␮m)priortothe analysis.Likewise,dissolvedNiandCocontentsweredetermined

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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

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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 _×104_CFU_mL−1

Conductivitya ₂₃₉₃₃_±₄₅₅₄ _␮S_cm−1

Bicarbonatesa ₁₄₃₅₀_±₅₀ _mg_L−1

Ammonianitrogena ₂₃₀₀_±₂₈₅ _mg_L−1

Totalhydrocarbonsa ₄_±₁ _mg_L−1 a_As_determined_in_quarterly_analysis_provided_by_the_{intermunicipal}_company.

at37◦C.Afterwards,MHAplateswereinoculatedwithstandardised inoculaofthetest microorganisms(0.5McFarland; correspond-ingto108_CFU_mL−1_)._Then,_antibiotics_testing_paper_discs₍₆_mm

diameter;FiltresFioroni)containing10␮Lofthetestingsolution, previouslysterilizedbyﬁltration(0.2␮m),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+_ions_in_the_inverse_spinel_structure

ofmagnetitetoFe3+_–_most_likely_during_the_Fe

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 _of_the_MGNC_materials_(cf._Fig._S2) _denote_the

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

(7)

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

0

t (h)

Fig.3. 4-NPandH2O2conversionsobtainedasafunctionoftimeinCWPOruns

performedwiththeMGNCmaterials.Experimentsperformedwith[4-NP]0=5gL−1,

[catalyst]=0.5gL−1_,_T₌₈₀◦_C,_pH₌₃_and_[H₂_O₂_]₀₌_[H₂_O₂_]_{Stoichiometric}₌_17.8_g_L−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

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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, Adsorption

X

/ X

0

t (h)

0 1 2 3 4 0.0 0.2 0.4 0.6 0.8 1.0

(b)

X

/ X

0

t (h)

4-NP, CWPO TOC, CWPO 4-NP, leaching test TOC, leaching test

0 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

]

0

t (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 1800

Co

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(14.7×104_CFU_mL−1_total_{heterotrophic}_bacteria_cultivable_at

(9)

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 4.3×109_L_mol−1_s−1_;_however,_the_hypochlorous_radical_(HOCl•−₎

formedbythereactiondescribedinEq.(5)isabletodissociateback toHO•radicalandchlorideion(Cl−),thedissociationrateconstant being6.1×109_s−1_[45]_._As_described_by_Eq.₍₆₎_,_HOCl•₋

canalsobe convertedintochlorineradicals(Cl•)andwaterthrougha protona-tionreactionwiththerateconstantof2.1_×1010_L_mol−1_s−1_,_in_this

casethereverseratereactionbeingmuchsmaller(1.3×103_s−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,

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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(0.2␮m), 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(14.7×104_CFU_mL−1_total_{heterotrophic}

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

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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

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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 Internacionalização (POCI) − and by national funds through FCT− Fundaçãopara 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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