Sonodegradation of amitriptyline and ibuprofen in the presence of Ti

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Sonodegradation of amitriptyline and ibuprofen in the presence of Ti

₃

C

₂

T

_x

MXene

Minjung Jeon

^a

, Byung-Moon Jun

^b

, Sewoon Kim

^a

, Jaeweon Cho

^c

, Chang Min Park

^d

, Choe Earn Choong

^e

, Min Jang

^e,

*, Yeomin Yoon

^a,

*

aDepartmentofCivilandEnvironmentalEngineering,UniversityofSouthCarolina,Columbia,300MainStreet,SC,29208,USA

bRadwasteManagementCenter,KoreaAtomicEnergyResearchInstitute(KAERI),111Daedeok-Daero989beon-gil,Yuseong-Gu,Daejeon,34057,RepublicofKorea

cSchoolofUrbanandEnvironmentalEngineering,UlsanNationalInstituteofScienceandTechnology,UNIST-gil50,Ulju-gun,Ulsan,RepublicofKorea

dDepartmentofEnvironmentalEngineering,KyungpookNationalUniversity,80Daehak-ro,Buk-gu,Daegu,41566,RepublicofKorea

eDepartmentofEnvironmentalEngineering,KwangwoonUniversity,447-1,Wolgye-DongNowon-Gu,Seoul,01897,RepublicofKorea

ABSTRACT

This study,which investigated thesonodegradation of selected pharmaceutical active compounds (PhACs) (amitriptyline(AMT)andibuprofen(IBP))withMXene,wascarriedoutinanaqueoussolution.Toinvestigatethe practicalityofthedegradationprocess,theexperimentswereconductedinvariouswaterqualityconditions, includingpH,temperature,naturalorganicmatter,andionicstrength.Basedontheexperimentalresults,the producedhydrogenperoxide,whichcouldbearepresentativeoftheproducedOHradicals,wasavitalfactorthat affectedthedegradationperformanceofbothPhACs.ToconfirmtheimportanceofOHradicals,theeffectofaOH radical promoter (H2O2) and scavenger (t-BuOH) was also studied. In addition, the synergism between ultrasonication(US)andMXenewasevaluatedwiththerateconstantsofUSonly,MXeneonly,andaUS/MXene combinedsystem.MineralizationofthePhACswasalsoinvestigated,andremovalofAMTwashigherthanthatof IBP,whichcouldbeattributedtothephysicochemicalpropertiesofthecompoundsandenhancedadsorptionbythe well-dispersedMXene.Overall,utilizationofMXenebymeansofultrasonicationcouldenhancetheremoval performanceofPhACsinwater.

ARTICLE INFO Keywords:

Ti3C2TXMXene Pharmaceuticals Sonodegradation Adsorption Watertreatment

1. Introduction

Therehavebeenadvancesinthepharmaceuticalindustryasaresultof the increasing need for medication for various purposes, such as treatment of a wide range of diseases, prevention of diseases, and, ultimately, improvementof the quality of life (Hester et al., 2020).

Endocrine-disrupting compounds (EDCs) and pharmaceutical active compounds(PhACs),whicharenoweasilyaccessibletothepublic,have savedanumberofpeople,buttheyhavehadanegativeimpactonthe environmentaswell.Overthelasttwodecades,alargeamountofEDCs andPhACshavebeendischargedintotheenvironmentwithoutbeing properly treated, and accordingly, concentrations of the discharged compoundsinwatersystemshaveincreased(Al-Hamadanietal.,2017b;

Junetal.,2019;Kimetal.,2018).Furthermore,severalstudieshave emphasizedthatconventionalwatertreatmentprocessesarenotvery efficienttocompletelyremovethereleasedEDCsandPhACspresentin influents(Cerretaetal.,2020;Manolietal.,2019;Yangetal.,2017).

Throughwatersystems,suchassurfacewaterandgroundwater,the concentrated discharged compounds could ultimately take a toll on

animals,plantsandhumanbeings(Kolpinetal.,2002;Lietal.,2019).

Specifically, ibuprofen (IBP), which is frequently used as an anti- inflammatoryanalgesic, hasbeenfound tobe dischargedfromwater treatmentplantsataconcentrationof0.002–24.6

m

^g/L^(Buser^et^al.,^1999;

Méndez-Arriaga etal., 2008).Theotherselected PhAC, amitriptyline (AMT),whichiswidelyprescribedasanantidepressant,hasbeendetected insurfacewater,leadingtolong-termexposureoflivingorganismstothe compoundinwatersystems(BakerandKasprzyk-Hordern,2011;Baker etal., 2012;Kasprzyk-Hordern etal.,2008; Lajeunesseetal., 2012).

VarioustreatmentprocesseshavebeenusedtoremovePhACsandEDCs fromwater,suchasadsorption(Guillossouetal.,2020;Jungetal.,2013), coagulation/sedimentation (Adams etal., 2002),membraneprocesses (Heoetal.,2013,2012),disinfection(Vienoetal.,2007),UV/H2O2(Yuan etal.2009),andFentonoxidation(Mohapatraetal.2013).However, efficienciesoftheconventionaltreatmentsareconsideredlow,anditis known that the disinfection process could produce by-products (Al- Hamadanietal.,2017a;Westerhoffetal.,2005).

Ultrasonic(US)treatmenthasbeenstudiedasoneoftheadvanced technologies.Itisprominentforremovingcontaminantswithcomplex

* Correspondingauthor.

E-mailaddresses:[email protected],[email protected](M.Jang),[email protected](Y.Yoon).

http://doi.org/10.1016/j.hazl.2021.100028

Received15April2021;Receivedinrevisedform24May2021;Accepted25May2021 Availableonline27May2021

JournalofHazardousMaterialsLetters2(2021)100028

ContentslistsavailableatScienceDirect

Journal of Hazardous Materials Letters

j o urn a lh ome pa ge : w ww . e l se v i e r . com / l oca t e / ha z l

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structure.UStreatmentis basicallyconductedbyhighlyreactiveOH radicalsbydissociatingwatermoleculesthroughultrasonication.Water molecules are dissociated through a series of processes: formation, growth,andcollapseofcavitationbubblescausedbysonicationinwater (Im etal., 2014).Acoustic cavitationcan form and growcavitation bubblesand,whenthebubblescollapse,extremelyhightemperatures (5000K)andpressures(1000atm)canbeproduced,resultinginthe productionofreactivefreeradicalssuchasOH,O,andHO2(Al-Hamadani etal.,2016,2017a).Thefreeradicalscanoxidizevariouscontaminants, thus degrading the contaminants in water. Due to its advantages, including high efficiency, convenient operation, and environmental- friendliness,UStreatmenthasbeenwidelystudiedforadaptability in watertreatment(Chuetal.,2017).However,UStreatmentgenerally causeshighusageof energy,whichmaylead topoor costefficiency comparedtootherwatertreatments.SinceUStreatmenthasthiscritical drawback,avarietyofstudieshavebeenconductedonsonocatalystsin ordertoimprovetheirUSdegradationrates(Imetal.,2015;Junetal., 2019;Leeetal.,2018).

ByaddingsonocatalystsinUSreactions,theycanplayanimportant roleofpromotingtheproductionofcavitation bubblesbyproviding additionalnucleitothebubbles,thussupportingthegenerationofmore freeradicals(Al-Hamadanietal.,2018; Imet al.,2015; Junetal., 2019b).Severalsonocatalystshavebeenusedtoenhancetheefficiency of US degradation, such as single-walled carbon nanotubes (Al- Hamadanietal.,2016,2017a;Imetal.,2013),grapheneoxide(Al- Hamadanietal.,2018),powderedactivatedcarbon(Imetal.,2014), metal–organic framework (Jun et al., 2019b), glass beads (Al- Hamadani etal.,2016),flyashes(Al-Hamadaniet al.,2017b),and TiO2nanotubes(Imetal.,2015).Addingtheabovementionedmaterials as sonocatalysts could improve the sonodegradation of organic pollutants by increasingthe productionof freeradicals andmostly dispersingthecatalysts,whichcouldboosttheiradsorptioneffect(Chu et al., 2017). MXene is a new family oftwo-dimensionalmaterials fabricatedbyetching“A”layersfroma“MAX”structure,thusobtaining compoundswiththeformulaMn+1XnTx,suchasTi3C2Tx,where“A” represents an A-group element, such as Si, Al, or Sn, “M” is a transitionalmetal(e.g.,V,Nb,orTi),and“X”isNorC(Jeonetal., 2020).Tothebestofourknowledge, only afew studieshave been

conductedontheutilizationofMXenetoimprovethedegradationof organicpollutants,suchasPhACs,byultrasonication.

Themainpurposeofthisstudyistoinvestigatetheeffectofseveral experimentalconditionsonsonodegradationoftheselectedPhACswith MXene.ThesynergismbetweenultrasonicationandMXenewasalso evaluated.Synergeticindiceswerefirstevaluatedbycalculationwith pseudo-first-orderkineticconstantsofUSonly,MXeneonly,andthe US/MXene combined process. All the other experiments were conducted in a US/MXene combinedsystem, and the experimental conditionsincluded pHandtemperatureofsolution,ionic strength, naturalorganicmatter(NOM),andexistenceofOHradicalpromoter andscavenger.

2. Materialsandmethods

Detailsregardingchemicals, usedinthis study,andmeasurement method of PhAC and H2O2 concentration are provided in the supplementaryinformation(SI,Section1).

2.1. USreactorandspecificexperimentalconditions

AlltheUS experimentswereconductedwithacustom-designedUS systemcomprisingawatercirculationchiller(HS-28A,22240V,50Hz, HAILEA)andacirculationpump (HX6530,50W,220240V,HAILEA).The USreactiontank(

F

¹¹⁰170mm,effectivevolume1L)consistedofdual stainless-steelwalls,mountingplates,andcirculationfittingstocontrolthe watertemperature.Astainless-steeltransducermodule(Fulltech,Taiwan) wasattachedtoanultrasonicgenerator(FlexonicMiraeUltrasonicTech., modelFX-4FREQ,Korea). Mechanicalstirringwascontrolledusingan overheadstirrer(HS-50A,WiseStir¹)ataconstantspeed.

TheUSfrequencyandpowerofthissystemwereoptimized,withthe combinationof35kHzand50WL¹,whichwasfoundtoproducethe highestamountofH2O2,selectedforthisstudy.45mgL¹ofMXenewas usedforeachexperiment,anddetailedexperimentalconditionswereset asfollows:(i)initialpHofPhACsolutionof3.5,7,or10.5;(ii)additionof 0.2,1,or5mMofH2O2asaOHradicalpromoterandoft-BuOHasaOH radicalscavenger;(iii)solutiontemperatureof293,303,or313K;(iv) additionof3or9mgL¹ofHA;and(v)ionicstrengthofNaClof300,600,

Fig.1.InfluenceofpHonsonodegradationperformanceof(a)AMTand(b)IBP,(c)degradationrate,and(d)H2O2productionatdifferentpHlevels(US/MXenesystem, 35kHz,50WL¹,and2931K).

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or1200

m

^S^cm¹^.^The^initial^pH^andtemperatureweresetto7and293K, respectivelyunlessotherwisestated.

2.2. Calculationofsynergeticindex

Thesynergeticeffectwasevaluatedbycalculationofthesynergetic index,accordingtoEq.(1),whichhasbeendescribedinpreviousstudies (Madhavanetal.,2010)and(Junetal.,2020a,2020b).

Synergeticindex¼ k1ðUS=MXeneÞ

k1ðUSonlyÞþk1ðMXeneonlyÞ (1)

Eachoftheobtaineddegradationrateconstantsofapseudo-first-order kineticmodelwasappliedtoEq.(1).TheUS/MXenecombinedprocess was considered more efficient compared to the singly conducted processesifthesynergeticindexvaluewasgreaterthan1.0.

3. Resultsanddiscussion

3.1. InfluenceofsolutionpH,temperature,NOM,andionicstrength

ThesolutionpHisanimportantfactorthataffectsthedegradationrate ofPhACs.TheinfluenceofpHcouldaffectsonodegradationefficiencydue tothedifferentoxidationpotentialsofOHradicals,pKaoftheselected compounds,andsurfacechargeofMXene.Sincetheoxidationpotentialof

OHradicalscouldchangedependingonthepHlevel,thequantityofOH radicalscouldalsochange,whichmightinfluencethesonodegradation efficiency(Balachandranetal.,2016;Chuetal.,2017).Also,atdifferent pHlevels,thepKaoftheselectedPhACsandthepHpzcofMXenecould affectthemobilityofthePhACsandthesurfacechargeofMXene,thus resultingin differentsonodegradationperformances (Im etal.,2014, 2013;Zhangetal.,2019).

TheeffectofpHisshowninFig.1,which presentstheresultsof MXene-assistedsonodegradationofthetargetcompoundswithincreas- ingpHvalues.Basedonthesonodegradationperformances,theremoval efficienciesofbothtargetcompoundsweregreateratpH7,followedby pH3.5and10.5.Higherefficiencyattheneutralconditioncouldbe explainedbytheproductionofH2O2,whichcouldrepresenttheamount ofproducedOHradicals.InFig.1d,theamountofproducedH2O2was highestatpH7,followedbypH3.5and10.5,withthetrendsimilartothat of thesonodegradationperformances.Oneresearch showedthat less amountofH2O2wasproducedatlowandhighpHduetothescavenging effectofexistingionsandfastrecombinationofOHradicals(Jiangetal., 2002). Consideringthe research,it can be concludedthat more OH radicalswereproducedandhigherremovalefficiencywasachievedatpH 7with the USsystem used in this study. Also, in terms of AMTin particular,sinceitspKais9.76,AMTispositivelychargedatpH3.5and pH7duetotheaminegroupactinglikeanelectrondonor(Guillossou etal.,2020).Owingtoitspositivelychargedcharacteristic,AMTcouldbe adsorbedonnegativelychargedMXenehavingsurfacefunctionalgroups

Fig.2.InfluenceofOHradicalpromoter(H2O2)onsonodegradationperformanceof(a)AMT,(b)IBP,and(e)degradationrate,andinfluenceofOHradicalscavenger(t- BuOH)onsonodegradationperformanceof(c)AMT,(d)IBP,and(f)degradationrateatdifferentconcentrationsofOHradicalpromoterandscavenger(US/MXene system,35kHz,50WL¹,pH7,and2931K).

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including-O,-F,and/orOH(Jeonetal.,2020).SinceMXeneismore negativelychargedathigherpH,showninFig.S1,AMTcouldbemore adsorbed on MXeneat pH7 than pH 3due to higher electrostatic interaction;similarresultisshownintheresearchconductedbyKimetal.

(Kimetal.,2021).

TheinfluenceoftemperatureonthedegradationrateofthePhACsis showninFig.S2.InFig.S2c,thedegradationratesofAMTandIBPslightly increased up to303Kanddecreased at313K. Theresult showsthe abovementionedpositiveandnegativeeffectsoftemperature,anditis consistentwiththeresultofH2O2productioninFig.S2d,whichindirectly representstheconcentrationsoftheOHradicalatdifferenttemperatures.

ThetendencyforH2O2productioniswellmatchedbythedegradation rateconstantsofbothPhACs,soitcanbeconcludedthat,inthisstudy,the quantityofH2O2isanimportantfactorinUSdegradation.Theinfluence ofHAisshowninFig.S3,whichindicatesthattheremovalratesofboth PhACsincreasedwhenHAwasadded.Thisresultisinagreementwiththe results oftheresearch conductedbyapreviousstudy,whichshowed higherdegradationratesafteraddingHAasarepresentativeofNOM (Parketal.2011).TheydemonstratedthatHAcouldtrapHradicalswith backgroundions,therebypreventingOHradicalsfromrecombination withtheHradicals,althoughtheynotedthatmoredetailedstudiesare neededtoconfirmthisphenomenon.Theinfluenceofionicstrengthis showninFig.S4,whichpresentstheresultofthesonodegradationwith MXenewithadjustedconductivitylevelsofthesolution.Accordingto Fig.S4,therewasnosignificanteffectobservedbyadditionofNaClto increaseconductivity.Thisresultisingoodagreementwiththeresultofa researchconductedbyMineroetal.ontheinfluenceofinorganicanions onsonodegradationofacidblue40(Mineroetal.2008).Theyconducted theexperimentsusingultrasonicationwithseveralinorganiccompounds, includingNaHCO3,Na2CO3,andNaCl,andconcludedthattheadditionof NaClhaspracticallynoinfluenceonsonodegradation.Theyconfirmed thatthereisonlyamarginaleffectofionicstrengthwhenNaClisused.

Detailed influence of temperature, NOM, and ionic strength on degradationisdemonstratedinSI(Section2).

3.2. Influenceofthepromoter(H2O2)andscavenger(t-BuOH)

TheproductionofOHradicalswasfoundtobevitalfactorforUS degradation,whichis ingood agreementwithotherresearches(Chu etal.,2017;Imetal.,2015;Junetal.,2020a,2020b).H2O2andt-BuOH arewidelyknownasanOHradicalpromoterandscavenger,respectively, andtheywereessentialtoinvestigatehowinfluentialwastheroleofOH radicalsintheexperiments.ToconfirmtheeffectofthequantityofOH radicals, H2O2 andt-BuOH, ranging from 0to5mM, wereused for sonodegradationofthePhACs.

In Fig.2a,b,ande,the effects oftheOH radicalpromoteron the sonodegradationareshown.Ataconcentrationofupto1mMofH2O2,the rateconstants(k1)ofbothPhACsclearlyincreased,anduntilaconcentration of5mMwasreached,theserateconstantstendedtoremainconstantor decrease.AlthoughH2O2isawell-knownOHradicalpromoter,thepossible reasonforthisresultcouldbebecauseitcanalsohavescavengingeffectsat highconcentrations,asshowninEqs.(2)and(3)(Adewuyi,2001).OH radicalscouldreactwithhydrogenperoxidetobeconvertedtohydroperoxyl radicalsand,eventually,watermolecules.SinceHO2

^is^considered^a^less

reactiveradicalthananOHradicalduetoloweroxidationpotential,therate constantat5mMofH2O2islikelytobedecreasedthanexpected.Thus,this phenomenoncouldstrengthentheconclusionthatOHradicalisthemain mechanismforthePhACsdegradationinthisstudy.

OH

þH2O2 ! HO2

þH2O (2)

HO2

þOH

!H2OþO2 (3)

InFig.2c,d,andf,theeffectsoft-BuOH,atconcentrationsranging from0to5mM,onthesonodegradationofthePhACsareshown.With

increasingconcentrationupto5mMoft-BuOH,therateconstantsofthe degradation of both compounds decreased accordingly during ultrasonication.Thisresultcouldbeattributedtothereactionoft-BuOHwith H2O2,competingwiththePhACs.Eq.(4)showstheconversionoft-BuOH tocarbondioxideandwatermolecules.Sincet-BuOHcanreactwithH2O2

insteadofthePhACs,therewouldbelesschanceforthePhACstobe degradedbyOHradicalsandH2O2,thushavingdecreasedrateconstant.

Similarresultcanbe achievedby usingmethanol(MeOH),shownin Eq.(5)(Imetal.,2014).Imetal.explainedthephenomenathatthe decreasedrateconstantofPhACsisattributedtotheadditionoft-BuOH andMeOH, indicating the sonodegradation of PhACs is OHradical- dependent.Thisresultisingoodagreementwithotherstudiesusingt- BuOHandMeOHasOHradicalscavengers(Imetal.,2014,2013;Jun etal.,2019b).

C4H10Oþ12H2O2 !4CO2þ17H2O (4)

CH3OHþ3H2O2! CO2þ5H2O (5)

ThepresenceoftheOHradicalpromoterandscavengercouldaffect theproducedOHradicalsandeventuallyaffectthedegradationofthe PhACs.ItcanalsobeconcludedthatthequantityofOHradicalsduring sonodegradationofPhACsisanimportantfactor.Theseresultsarein goodagreementwiththoseofsomepreviousstudies(Al-Hamadanietal., 2017b;Chuetal.,2017;Imetal.,2015;Junetal.,2020a,2020b,Jun etal.,2019b).

3.3. Evaluationofsynergeticindices

Toinvestigate thesynergetic effectof using MXeneduring ultrasonication,theexperimentswereconductedinaUSonlyprocess,MXene only process, and US/MXene combined process to obtain the rate constantsforeachprocess.Synergeticindices,summarizedinTable1, were calculated with the rate constants of the three experimental processes,andthedegradationperformancesareshowninFig.3.TheUS/

MXene process was observed to be the most effective of the three processes.ItisdescribedthatthesynergeticindicesoftheUS/MXene processforbothcompoundswereabove1(1.45and1.13forAMTand IBP,respectively).Thisresultshowsthattherewasasynergeticeffectof theUS/MXenecombinedsystemforthebothcompounds,andtheeffect wasmorepronouncedontheresultofAMTinparticular.Thissynergetic effectcouldbeattributedtotheenhancedadsorptionbyMXene,since MXene,asananomaterial,canbecomewelldispersedandacquireamore negativesurfacebybeingsonicated(Junetal.,2020a,2020b).During ultrasonication, the hydrodynamic diameter of MXene decreases by preventing aggregation, which could lead to enhancement of its adsorptioncapacity.Also,thezetapotentialofsonicatedMXeneismore negativethanthatofpristineMXene,whichcouldsupportelectrostatic interactionwithpositivelychargedcompounds(Junetal.,2020b;Kim etal.,2021).Overall,itcanbeconcludedthataUS/MXeneprocesshasa synergeticeffectthatisattributabletoenhancedadsorptionbysonicated MXene.

Table1

Degradationrate(k₁),coefficientofdetermination(R²)andsynergeticindexof PhACsin US only,MXene only,andUS/MXene processwithin120min at 2931KandpH7.

AMT IBP

Processes k1(min¹) R² k1(min¹) R²

USonly 0.0177 0.984 0.0138 0.998

MXeneonly 0.0007 0.418 0.00001 0.565

US/MXene 0.0267 0.986 0.0156 0.996

Synergeticindex 1.451 1.130

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3.4. MineralizationofPhACs

TOC measurement was conducted to evaluate the degree of mineralization of the PhACs in a USonly process anda US/MXene combinedprocess.TheresultofTOCmeasurementisshowninFig.S5, fromwhichitcanbeseenthattheresultofmineralizationoftheUSonly process turned out to be low (6 % and 10 % for IBP and AMT, respectively). This result is not consistent with the degradation performances discussedabove,whichcouldbeduetotheby-product ofthesonochemicaloxidationofthePhACs.Thedifferencebetweenthe removalratesofAMTandIBPcouldbeattributedtotheirlogKow,shown inTableS1.SinceAMTismorehydrophobicthanIBP,basedontheirlog KowatpH7,theremightbeahigherchanceofAMTdegradationinagas–

liquidinterfacethanIBP(Naddeoetal.,2010,2009).However,intheUS/

MXenecombinedprocess,theremovalratesofIBPandAMTincreasedup to15%and23%,respectively,whicharemorethandoublethevaluesin theUSonlyprocess.Thisresultcouldbeattributedtotheremovalofthe PhACsfromthesolutionduetoadsorptionbyMXene.Therefore,itcanbe concludedthatthemineralizationofthePhACsis notsignificantand MXeneplaysavitalroleintheremovalofthetargetcompoundsfrom solutions.

4. Conclusions

ThisstudyinvestigatedtheremovaloftwoselectedPhACs(AMTand IBP)byultrasonicationwithMXene.Itwasobservedthatthedegradation performances were affected by the quantity of H2O2 produced by ultrasonicationundertheexperimentalparametersofpHandtempera- ture.Theremovalrateswerehigherattheconditionsthatcausedmore H2O2production,andtheimportanceofOHradicalswasalsoconfirmed

withexperimentsconductedinthepresenceofOHradicalpromotersand scavengers.Additionally,theeffectofNOMandionicstrengthwasalso evaluated,andconfirmedtoberelativelyinsignificant.Fortheeffectof NOMinparticular,theremovalratesslightlyincreasedbyadditionof more HA; this resultcan be attributedto lessrecombination of OH radicals,butmoreresearchisneededtoconfirmthiseffect.Toconfirmthe synergeticeffectofusingMXeneduringUS,thesynergeticindiceswere calculated,andtheresultshowedthatusingMXenewithUScouldyield betterremovalratesforbothPhACsthanusingMXeneandUSseparately.

ThiscouldbeduetoenhancedadsorptionperformanceofMXeneduring sonication,whichleadstoasmallerhydrodynamicdiameterandlower zetapotential.Lastly,mineralizationofthePhACsinaUSonlyandUS/

MXeneprocess was evaluated,andthe resultdemonstrated that the mineralizationofAMTwasmorefavoredthanthatofIBPduetotheir different hydrophobicity,andtheUS/MXene processachieved better removalratesthantheUSonlyprocessduetotheadsorptioncapacityof MXene.SincetheMXene,usedinthisstudy,isconsistedoftransition metalsandcarbon,theremightbeapossibilityofleachingofthemetalsor soluble exfoliated nanosheets. The potential leaching has not been confirmedanditisnecessarytobeinvestigatedasfutureresearch.

DeclarationofCompetingInterest

Theauthorsdeclarethattheyhavenoknowncompetingfinancial interestsorpersonalrelationshipsthatcouldhaveappearedtoinfluence theworkreportedinthispaper.

Acknowledgments

ThisresearchwasfundedbytheNationalResearchFoundationof Korea(NRF)GrantfundedbytheMSIP(no.NRF-2015R1A5A7037825).

ThisstudyisalsopartiallyfundedbytheExcellentresearchersupport projectofKwangwoonUniversityin2021.

AppendixA.Supplementarydata

Supplementarymaterialrelatedtothisarticlecanbefound,inthe onlineversion,atdoi:https://doi.org/10.1016/j.hazl.2021.100028.

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