Effect of soil PH on the transport, fractionation, and oxidation of chromium (iii)

Ecotoxicology and Environmental Safety 195 (2020) 110459 ContentslistsavailableatScienceDirect Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv ff E ect of soil pH on the transport, fractionation, and oxidation of chromium T (III) TengXua,FengNanb,XiaofengJianga,YulingTangb,YunhangZengb,WenhuaZhanga,b,∗ ,BiShia aKeyLaboratoryofLeatherChemistryandEngineeringofMinistryofEducation,SichuanUniversity,Chengdu,610065,China bNationalEngineeringLaboratoryforCleanTechnologyofLeatherManufacture,SichuanUniversity,Chengdu,610065,China ARTICLE INFO ABSTRACT Keywords: ThisworkwasconductedtostudytheeffectofsoilpH(4.0,6.0,and8.0)onthetransport,fractionation,and Trivalentchromium oxidationoftrivalentchromium[Cr(III)].VariationinpHalteredsoilchemicalandmineralogicalproperties Hexavalentchromium suchaszetapotential,cationexchangecapacityandredoxpotentialofnaturalsoil.Breakthroughcurvesand SoilpH batchsorptionexperimentscoupledwithfouriertransforminfraredspectroscopy(FTIR)andX-rayphotoelectron FTIR spectroscopy(XPS)analysesdemonstratedthattheeasymobilityofCr(III)inpH4.0soilwasdominatedbythe XPS limitedcoordinationeffect.ThehighretentionofCr(III)inpH8.0soilwasmainlyascribedtothehydrolysis. IncubationexperimentsindicatedthattheproportionsofCrinexchangeablefractiondecreasedwithincreasing of soil pH and incubation time, and kinetics analysis revealed that the time dependent transformation was controlledbymasstransferandchemicalprocesses(e.g.,hydrolysis,ionassociation).TheXPSconfirmedthe oxidationofCr(III)inpH8.0soilduringtheincubationperiod.Furthermore,thecontentoftoxichexavalent chromium[Cr(VI)]waspositivelyassociatedwithtimeandinitialconcentrationofCr(III)released.Theseresults revealedthehazardousnessofCr(III)insoilcontaminatedsimultaneouslybyinorganicacidandalkali. 1. Introduction California(USA)hastotalCrof130–22,000mgkg−1,Cr(VI)of1<to 187 mg kg−1, and pH of 4.6–7.8 (Makdisi, 1991). These site in- Soilcontaminationbyanthropogenicchromium(Cr)isaworldwide vestigations reveal that soil in tannery sites is contaminated simulta- problem (Shahid et al., 2017). Several industrial activities such as neouslybyCr(III)andinorganicacidandalkali,suchassulfuricacidin leather tanning, wood preservation and metal finishing are the main the pickling and sodium hydroxide in the soaking during leather sourcesofCrpollution(Choppalaetal.,2018;Faridetal.,2018).Inthe manufacture(Covington,2009).Assessmentoftheenvironmentalrisk leather industry, large amounts of Cr-containing effluents, shavings, ofCrinsoilisofutmostimportanceanddependsonthoroughunder- and sludge are produced due to the common use of basic chromium standing ofthetransport, solid-phase fractionation, and oxidation be- sulfate(Zhangetal.,2017).Environmentalseepagemayoccurthrough haviorofCr(III)insoilswithdifferentpHlevels. multiple pathways, such as inadvertent dripping of Cr tanning liquor TheverticaldistributionofCr(III)insoilscanbestronglylocalized (containing Cr 3000 mg L−1) (Zhou et al., 2012), leaking of tannery by transport limitations, and laboratory column experiments of Cr- wastewater(upto1500mgL−1)(Religaetal.,2011),andpoorstorage containingliquidflowingthroughsoilmediaarecommonlydesignedto or indiscriminate dumping and landfill of sludge (up to simulate Cr transport and retention (Banks et al., 2006; Zhang et al., 40,000 mg kg−1) (Alibardi and Cossu, 2016). The released Cr would 2018b). Scholars have extensively investigated factors influencing Cr eventuallyentersoil,andthespatialdistributionofCrandsitepHare transport and retention particularly for cases involving soil structure, closelyrelatedtoleatherproduction.OnlyCr(III)isusedintheleather soilminerals,organicmatter,microbialexopolymeric substances,and industry,whichisconsideredtobelesstoxicandmobilethanCr(VI).Cr flow rate (Akhtar et al., 2011; Hu et al., 2010; Jardine et al., 1999; (III)isthemaincontaminantinthesoiloftannerysites,butCr(VI)has Kantaretal.,2011).However,informationabouttheeffectofsoilpH alsobeendetected.Soilwithinthevicinityofthetannerywastewater onthebehaviorofCr(III)transportislacking. dischargesiteintheSialkotofPakistanhastotalCrof21–675mgkg−1 ThegeochemicalfractionationofCriscrucialinunderstandingthe and pH of 7.1–10.6 (Ali et al., 2015). Soil in a tannery room in mobilityandbioavailabilityofCrinsoils(Ertanietal.,2017),among ∗Correspondingauthor.TheKeyLaboratoryofLeatherChemistryandEngineeringofMinistryofEducation,SICHUANUniversity,No.24SouthSection1,Yihuan Road,Chengdu,610065,China. E-mailaddress:zhangwh@scu.edu.cn(W.Zhang). https://doi.org/10.1016/j.ecoenv.2020.110459 Received19December2019;Receivedinrevisedform25February2020;Accepted7March2020 Available online 14 March 2020 0147-6513/ © 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/). T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 whichtheexchangeableCrisconsideredtobethemosteasilymobile dryweightbasisbyovendrying(3gofsoilat105°Cfor～15h). and bioavailable fraction using sequential extraction method (Tessier The soils used for column experiment were divided into three etal.,1979).Acevesetal.(2007)foundtheexchangeableCrintannery groups and chemically adjusted to pH 4.0 ± 0.1, 6.0 ± 0.1, and sludge-amendedsoildecreasedwithprolongedincubationtime.How- 8.0 ± 0.1, respectively, which represented a range of soil pHs ac- ever,TaghipourandJalali(2015)reportedtheexchangeableCrinsoil cordingtotheactualinvestigationresultsattannerypollutionsites(Ali with 5% leather factory waste increased from 7.4 to 201.6 mg kg−1 etal.,2015;Makdisi,1991).SoilpHwasloweredbyadditionofH SO 2 4 when incubation lasting from 15 to 90 days. Different phenomena solution(1molL−1)andelevatedbyNaOHsolution(0.5molL−1)to suggest that the factors affecting the fractionation of Cr in soil are obtaintargetpHs.H SO andNaOHwereusedtoadjustsoilpH(Obia 2 4 complicated, and previous studies have mainly focused on applying et al., 2015) for their common use in pickling, soaking, and liming substancestoaffectthefractionationofCr.Variousamendments,such during leather manufacture (Covington, 2009; Font et al., 1998). Soil ascitricacid,polyasparticacid,soilorganic,andzerovalentiron,affect pHwascontinuouslyadjustedfortwoweekstoensurepHstabilityand fractionationbymobilizationorimmobilizationofCrinsoil(Balasoiu airdriedbeforeuse.Thepre-adjustedpHsoilsamplesamendedwithCr et al., 2001; Fu et al., 2017; Li et al., 2017; Su et al., 2016). Never- (OH)SO ,whichwaswidelyusedintannery(Pandaetal.,2016),were 4 theless, limited studies have focused on the effect of soil pH on the usedforincubationexperimentandmaintainedattargetpHvaluesby fractionationofCr. periodicallyadditionofH SO orNaOHsolution. 2 4 Theconversion ofCr(III)intoCr(VI)isundesirable andhazardous from an environmental perspective because Cr(VI) is a known carci- 2.2. Designoftheexperiments nogen(HausladenandFendorf,2017).TheoxidationofCr(III)toCr(VI) was highly concerned and extensively studied in solution system. 2.2.1. Columntransportandbatchsorptionexperiments However,thebehaviorofmetalinhomogenoussystemisverydifferent Transport of Cr(III) solutions was examined with a plexiglass fromthatinheterogenoussystemespeciallyinmediaofsoil.Theeffects column (12 cm long, 1.6-cm i. d.) packed with pH-preadjusted soil ofpHonthefateofCr(III)insoilwasmainlycenteredontheoxidation samples without addition of Cr(III). 23.2 g air-dried soil sample was according to existing literature, and studied by using soil/water sus- packedinthecolumnwithapproximately8cmheight.Thecolumnwas pensiontosimulatethebehaviorofCrinsoil.Forexample,Bartlettand fittedwithquartzsand(2-cmthicklayer)andnylonsieve(50-μmpore James (1979) devised a suspension system containing 1:2000 of su- size)atthebottomandtoptoreducesoildeformationandsoilcolloid persoil-to-solutionofCrCl tostudytheeffectsofpHonCr(III)oxida- 3 releasecausedbysolutioninfiltration.Thecolumnwaspreconditioned tionbyadjustingliquidphasepHfrom3.0to10.0.Theyfoundlower by pumping 5 mmol L−1 NaCl solution in the upward direction. The pH facilitated the formation of Cr(VI), and at pH 3.2 all Cr(III) was flow rate was set as 1 mL min−1and controlled by a calibrated peri- oxidized to Cr(VI). Reijonen and Hartikainen (2016) used soils in- stalticpump(siliconetubing).Atotalofatleast50porevolumeswere cubatedatpH4.4,5.5and6.2tostudytheoxidationofCrCl solution 3 elutedduringconditioning.Theequilibrationperiodensuredthatwater insuspensionsystemwithsoil/solutionratioof1:10,andnoCr(VI)was flowandionicstrengthinthesoilcolumnremainedstableduringthe foundinsoilwithoutadditionofMnO .Obviously,thesestudieswere 2 experiments. After soil colloid stabilization, the feed solution was re- verydifferentfromtherealenvironmentwhichwasconductedinsemi- placed with 3000 mg L−1 (57.69 mmol L−1) Cr(III) [as Cr(OH)SO ]. aridsoil.Hence,theeffectofpHontheoxidationofCr(III)insemi-arid 4 TheconcentrationwasselectedconsideringthetypicalCrcontentinCr soilrequiressystematicstudy. tanning liquor (Zhou et al., 2012) and was higher than the limit of Inthisstudy,weinvestigatedthesoilpH-dependentmechanismsof 1.5 mg L−1 defined by the Chinese Environmental Protection Agency thetransport,fractionation,andoxidationofCr(III)torevealtheriskof (GB 30486–2013). Following injection, column effluent was collected the release of Cr(III) from tannery sites. Experiments on soil column at intervals of 3 min by using an automatic collector to analyze Cr were carriedout to obtain theretention characteristics ofCr(III), and content. Br− (0.75 mmol L−1) of NaBr was injected as a tracer to the results were quantified and compared using sorption isotherms. confirmthecloselyidenticalhydrodynamicpropertiesofeachpacked Incubation experiments were performed to determine the behavior soilcolumn(Caldereretal.,2014).Theporevolumewasestimatedto underlying the transformation of Cr into various fractions and the beapproximately0.3mLbyusingthemethodsofKantaretal.(2011). oxidation of Cr(III) into Cr(VI) in semi-arid soils. The kinetics of The diagram of the soil column experimental device is shown in transformation and oxidation of Cr(III) were also investigated in soil SupplementaryFig.S1,andtheparametersofthesoilcolumnarelisted withthreepHlevels. inTableS1. Two different sets of sorption experiments were conducted to un- 2. Materialsandmethods derstandthemechanismsofCr(III)retentioninsoilcolumns.Thefirst set of experiments aimed at understanding the general sorption equi- 2.1. Materials libriumandtheeffectofsoilpHonsorptioncapacity.Inthesecondset of experiments, the contact time of the Cr-soil suspension was varied 2.1.1. Chemicals from 1 min to 24 h to test the sorption kinetics of Cr(III) in soils of All chemicals including magnesium chloride (MgCl ), sodium 2 different pH levels. The experimental procedures are detailed in sup- acetate (NaOAc), hydroxylamine hydrochloride (NH OH·HCl), acetic 2 plementarydatasection2. acid (HOAc), nitric acid (HNO ), ammonium acetate (NH OAc), sul- 3 4 furicacid(H SO ),sodiumhydroxide(NaOH),basicchromiumsulfate 2 4 [Cr(OH)SO ], sodium chloride (NaCl), sodium bromide (NaBr), and 2.2.2. Incubation 4 diphenylcarbazidewereanalyticalreagentgrade,andpurchasedfrom Incubation experiment was used to study the fractionation and Aladdin(Shanghai,China).Milli-Qwaterwasusedforthepreparation oxidation of Cr in soil at three pH levels. Cr(III) was added to soil at ofsamplesandstandardsolutions. 600 mg kg−1. The dose was selected as a criterion near the value of typicalCrcontentintannerysite(Alietal.,2015)andwashigherthan 2.1.2. Soil the limit of 350 mg kg−1 defined by the Chinese Environmental Pro- Thetestsoilusedintheexperimentwascollectedfromthetopsoil tectionAgency(GB15618–2018).Theincubationsoilsamplesretained (0–20 cm) of bare arable land in eastern China (37°22′N 118°02′E), watercontentequivalentto15%ofthesoilweightbyperiodicaddition homogenized, air dried, crushed, and sieved (stainless steel sieve, Ø ofdistilledwater.Thesoilsampleswereincubatedforupto150daysat 2 mm). Sieved soil was stored at 4 °C until use as natural soil. The 303KandthesoilswerecollectedfordeterminationoftotalCrcontent resultsofthecontentofeachsubstanceinthesoilwereexpressedona ineachfractionandCr(VI)contentatcertaintimeintervals. 2 T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 2.3. Analyticalmethods 2.4.2. Mobilityindex(MI) The mobility of Cr could be associated with the amount of ex- Soil properties including soil texture, soil pH, cation exchange ca- changeable Crinthesoil, whichcouldbecalculated according tothe pacity (CEC), organic matter (OM), redox potential (Eh), Cr(VI), and equationasfollowing(TaghipourandJalali,2015): soil digestion were analyzed using Chinese standard methods. The standardmethodnumbersarelistedinSupplementaryTableS2.Cr(VI) MI=∑ n CeiTi wasextractedfromthesoilsamplesbyusingthemethodofBartlettand i=1 n (2) James(1979),asdescribedindetailinsupplementarydata(section4). whereC istheCrconcentrationinexchangeablefraction(mgkg−1),T The extracts were then analyzed by spectrophotometric colorimetric istheto e talconcentrationoftheCr(mgkg−1),andnisthenumberof method. thesoilsamples. Metal elemental analyses were performed by inductively coupled plasma optical emission spectroscopy (ICP-OES, OPTIMA 8000DV, PerkinElmer,USA),λ =267.7/283.5nm,λ =238.2/259.9nmand 2.4.3. Kineticmodels Cr Fe λ =257.6/259.3nm.Totalcarbon,inorganiccarbon(IC)andtotal Mn (1) Diffusionkineticsmodel nitrogen(TN)insoilsweredeterminedbyusingacarbonandnitrogen analyzer (Primacs100, Skalar, Netherlands). Zeta potential of the soil InordertoanalyzethetransformationprocessofexchangeableCr, samples was measured by an autotitrator and zeta potential analyzer diffusionkineticsmodelwasappliedtofitthetransformationkinetics (NanoBrook Omni, Brookhaven, USA). The concentration of tracer bromide ion (Br−) in the column effluent was determined by ion data.ThedecreasesofCrinexchangeableformscouldbesimulatedina linearrelationshipwithtimeasthefollowingequation(Luetal.,2005): chromatography (ICS-600, Thermo Fisher, USA) with IonPac CS12A column(4×250mm)and30mmolL−1KOHasthemobilephaseata lnC =A+Bt (3) e flow rate of 1 mL min−1. The main mineral of the natural soil was determined by X-ray diffractometer (XRD, DX-2000, Aolong, China). wheretisthecontacttime(day),AandBaretheconstantofdiffusion equation. TheXRDpatternsofnaturalsoilandmineralcomponentsinnaturalsoil areshowninsupplementarydata(Fig.S2andTableS3).Theproperties (2) Reactionkinetics-basedmodels ofsoilsamplebeforeandafterpHadjustmentarelistedinTable1. The sequential extraction method (supplementary data, Table S4) Thepseudosecond-orderkineticmodelwasusedtofitthesorption wasusedtoclassifyandquantifyindividualCrfraction(Tessieretal., data. The pseudo second-order kinetics model is given as (Sun et al., 1979). Fourier transform infrared spectroscopy (FTIR, IS10, Thermo Fisher Scientific, USA) was used to characterize functional group 2011): changesinthesoilsamplesafterpHadjustmentandCr(III)treatment. t 1 t = + Surface elemental analysis of natural and Cr(III)-amended soils was q t k 2 q e 2 q e (4) conducted by X-ray photoelectron spectroscopy (XPS, XSAM800, Kratos,UK).ThesoilwithaninitialCr(III)contentof5000mgkg−1and where q e and q t are the sorption capacity (mmol kg−1) of Cr(III) at after 90 days of incubation was used for XPS analyses because XPS equilibriumandattimet(h),respectively,andk 2 [kg(mmolh)−1]is spectra cannot display characteristic peaks if Cr content was low thepseudosecond-orderrateconstant. (Boursiquotetal.,2002). TheElovich,first-orderandsecond-orderkineticmodelshavebeen widely usedto describe thekineticsofsoil chemicalprocesses, inthe presentexperiment,thesemodelswereappliedtofitthetransformation 2.4. Dataanalysis of exchangeable Cr (Sparks, 2003). These equations can be expressed as: 2.4.1. Retardationfactor Elovichmodel: Theretardationfactor(R)ofthreesoiltypeswasdeterminedfrom 1 1 the breakthrough curves according to the following equation (Godoy q t = β lnαβ+ β lnt (5) etal.,2019): Firstordermodel: PV R= i kt PV (1) logq =logq − 1 t 0 2.303 (6) where PV i is the corresponding pore volume number when outflow Secondordermodel: concentration (C) of Cr(III) reaches half of the initial concentration 1 1 ( w C h 0 e ), n i. t e h . e ,C tr / a C c 0 e = rb 0 ro .5 m , i a d n e d i P o V n i o s u t t h fl e o c w or c r o e n sp ce o n n t d r i a n t g io p n o C re /C vo 0 l = um 0 e .5 n . umber q t = q 0 +k 2 t (7) Table1 PropertiesofsoilsamplebeforeandafterpHadjustment. pH(H2O) Eha CECa OMa ICa OCa TNa Cr Fe Mn Soiltexture mV cmol+kg−1 gkg−1 % 8.64(natural) 476 6.79 7.96 10.26 3.22 1.21 0.041 23.11 0.42 C6.20I29.73A64.07 b 8.0 ± 0.1 491 6.71 7.83 9.32 3.09 0.46 0.047 24.09 0.41 C6.05I28.45A65.50 6.0 ± 0.1 540 5.97 7.37 1.94 2.85 0.55 0.038 24.92 0.45 C10.48I27.71A61.81 4.0 ± 0.1 557 5.36 7.72 0.19 2.89 0.76 0.045 22.15 0.41 C4.26I23.67A72.07 a Eh,redoxpotential;CEC,cationexchangecapacity;OM,organicmatter;IC,inorganiccarbon;OC,inorganiccarbon;TN,totalnitrogen. b Numericalsubscriptsrefertoweightpercentageofeachtextureinnaturalsoil.C,clay;I,silt;A,sand.Accordingtointernationalsoiltextureclassification,the soilusedinthisexperimentissandyloam.Allsoilpropertiesweremeasuredaftertwoweeksofincubation. 3 T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 Fig.1.BreakthroughcurvesofCr(III)invariouspHsoilscolumn:(A)soilpH4.0;(B)soilpH6.0;(C)soilpH8.0.(D)Cr(III)sorptiononsoilsampleasafunctionof pH.(E)KineticsofCr(III)sorptiononsoilswiththreepHlevels.Pointsrepresenttheexperimentaldata.Continuouslinesrepresentthefittingdata. whereq andq (mgkg−1)aretheamountsofCrintheexchangeable that is difficult to leach from soil, which was determined to be 5.36, t 0 fraction per unit mass of soil at time t (day) and initial time, respec- 5.97, and 6.71 cmol+/kg for soils with pH 4.0, 6.0, and 8.0, respec- tively;αandβareconstantsduringtheexperiment;k (day−1)isthe tively.ThehigherCECvalueofsoilshowedstrongerretentionforso- 1 constantinthefirst-ordermodel;andk (kgmg−1day−1)isthecon- luble cationic Cr(III) in soil columns. Meanwhile the point of zero 2 stantinsecond-ordermodel.Thefirst-orderkineticmodelwasalsoused chargeofthesoilsurfacewasdeterminedas2.48(SupplementaryFig. to describe the oxidation kinetics of Cr(III). Then the q and q (mg S3), indicating the soil surface charged negatively at the threepH le- t 0 kg−1)aretheamountofCr(III)perunitmassofsoilattimet(day)and vels.TheelectrostaticattractionofsoilwithcationicCr(III)speciesalso initial time, respectively, k (day−1) is the apparent reaction rate resultedintheretentionofCr(III)insoilcolumns.Intheory,themain 1 constant.Cr(III)contentwasdeterminedbysubtractingCr(VI)fromthe species of soluble inorganic Cr(III) varied from mononuclear Cr ions, totalCrcontent. such as free Cr3+,CrOH2+, and CrSO +, to polynuclear Cr ions with 4 more positive charges (e.g., Cr (OH) 4+ and Cr (OH) 5+), and the 2 2 3 4 precipitationCr(OH) becameeventuallydominantaccordingtoVisual 3 3. Resultsanddiscussion MINTEQver.3.0(Lövetal.,2017)withincreasingpH(Supplementary Fig.S4andTableS5).Theelectrostaticattractionandcationexchange 3.1. TransportofCr(III)insoilsatthreepHlevels of soil with soluble cationic Cr(III) species contributed part of the re- tention of Cr(III) in soil columns with increasing pH. Indeed, pre- Fig.1A–CshowsthebreakthroughcurvesoftracerBr−asNaBrand cipitation would occur at pH 4.4 with the initial Cr(III) of Cr(III) as Cr(OH)SO 4 in the soil columns at pH 4.0, 6.0 and 8.0, re- 57.69 mmol L−1 in soil column experiments based on the solubility spectively. The breakthrough curves of Br− were similar in the three productconstantofCr(OH) (k =6.3×10−31)(Dean,1999).Thena 3 sp soiltypes.However,themigrationofCr(III)wasweakenedcompared large amount of Cr(III) would be retained as immobile precipitate in withBr−anddelayedwithincreasingsoilpH.Approximately3.01PV soil with pH 6.0 and 8.0 because of the ion product beyond k , re- sp wasnecessarytoreachC/C 0 =0.5forpH4.0soil,4.85PVforpH6.0 sultingindelayofbreakthroughcurves.Thecharacteristicsofsoilsat soil,and54.21PVforpH8.0soil,indicatingsignificantCr(III)retention thethreepHlevelsandtheprecipitationprocesswouldconstitutethe inthecolumnwithpH8.0soil. mechanismoftransport,whichcouldbefurtherelucidatedbysorption According to Eq. (1), the retardation factors (R) were 1.18, 1.90, processofCr(III)insoilwithvariouspHlevels. and21.26,forsoilswithpHof4.0,6.0,and8.0,respectively.Ahigh value of R suggested low transport, indicating that Cr(III) was more mobileinacidicsoilthaninalkalinesoil,consistentwiththeprevious 3.2. SorptionofCr(III)insoilsatthreepHlevels survey (Makdisi, 1991). The breakthrough curves also indicated that thepollutionrangeshouldeasilyexpandwhenCr(III)solutionentered SorptionexperimentscanintuitivelyexplainthemigrationofCr(III) acidicsoil,whereaslargeamountsofCr(III)wereretainedinalkaline in soil columns with different pH levels. Sorption capacity of Cr(III) soil. This result may be ascribed to the difference in pH-dependent showedapositivecorrelationwithsoilpH(Fig.1D).Thecapacitywas surfacecharacteristicsofsoils(e.g.,cationicexchangecapacityandzeta characterized by a sorption edge (Sparks, 2003), which significantly potential)andtheprecipitationofCr(III)asCr(OH) athighpH. increasedfrom3.6mmolkg−1toapproximate46.6mmolkg−1within 3 ThesoilCECvaluesignifiedthecapacitytoretaincationinaform anarrowpHrange(about2units).ThesorptionstartedataroundpH 4 T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 4.0(Fig.1D).Intheory,precipitationwouldoccuratpH4.8forinitial Exchangeable fraction was considered to be the most mobile and Cr(III) concentration of 1.92 mmol L−1 in the batch sorption experi- bioavailableportionandthemostimportantformofCrthatcouldcause ments,suggestingthatsorptionwasmainlytriggeredbytheadsorption environmental risks(Ertanietal.,2017).Thehigh MIvalues ofCrin (chemical complexation) (Flogeac et al., 2005) (confirmed further by soilsindicatedhighenvironmentalrisk.Atthebeginningofincubation, FTIR results, section3.5), electrostatic attraction, cation exchangera- theMIvalueswere0.66forpH4.0soil,0.10forpH6.0soil,and0.024 therthanhydrolysisatpH4.0.AsmallamountofCr(III)wasfixedin forpH8.0soil(Fig.2C).MIvaluesgenerallydecreasedovertimeinthe acidicsoilsduetothelimitedquantityoffunctionalgroupsonthesoil threesoiltypes.Attheendoftheincubation(150days),theMIvalues surfacetocoordinate,lowcationexchangecapacityandweakelectro- werereducedto0.025(pH4.0),0.0035(pH6.0),and0.0029(pH8.0), staticattraction.WithincreasingsoilpH,thesorptionofCr(III)onsoil respectively.Moreover,thevalueofMIwasgenerallyintheorderofpH mainlyinvolvedhydrolysis,whichplayakeyroleinthesharpincrease 4.0 > pH 6.0 > pH 8.0 soil during the entire incubation period, in the sorption edge. Hydrolysis equilibrium would shift toward Cr consistent with the order of the retardation factor. Hence, soil acid- (OH) ,leadingtomoreCr(III)wasprecipitatedonthesoilwithhighpH ificationwouldspreadthepollutionrangeandenhancebioavailability 3 values. The batch sorption experiments illustrated that alkaline soils ofCr(III),therebyposinghighriskstotheenvironment,particularlyat hadhigher affinityforCr(III) thanacidicsoil, therebyconfirmingthe theearlystageofCr(III)pollution. highretentionofCr(III)inalkalinesoilcolumn. InordertofurtherstudythetransformationofEXCrwithincuba- Fig.1EshowsthesorptionkineticsofCr(III)onsoilsatthethreepH tiontime.TransformationkineticsofCrinexchangeable fractioninto levels.ThesorptionofCr(III)bypH8.0soilalmostreachedequilibrium otherstableformswerefittedtothelinearizeddiffusion-basedkinetic within5min,whereasbysoilwithpH4.0whichneededmorethan4h. model and reaction kinetics-based empirical models such as first-or- On account of the slow formation of complex, the findings further deredequation,second-orderedequation,andElovichmodeltoexplore speculatedthatadsorption(chemicalcomplexation)shouldbethemain the time-dependent process. The calculated parameters are given in transportmechanismofCr(III)atpH4.0(Gustafssonetal.,2014;Zhou Table2.Thevaluesofcorrelationcoefficient(R2)forthediffusionki- et al., 2019). With increasing soil pH, the quick sorption equilibrium neticmodelwerehigh(>0.9342),indicatingthatthereductionofEX involved the fast formation of precipitate, because when the ion pro- Crwaspartlyattributedtothediffusiveprocessthroughthemacropores ductfarexceededK ,thesupersaturatedsolutionisextremelyunstable and micropores of the soil. The absolute values of the transformation sp andhastoprecipitatetolowertheconcentrations(Bradyetal.,1990). rateconstantBwereintheorderofpH6.0soil > pH4.0soil > pH ThehydrolysisofCr(III)couldbethemainfactorforthefastsorption 8.0soil,indicatingeasierdiffusionofEXCrinacidsoilsthaninalkaline onalkalinesoilandthusledtothetransportofCr(III)waslimitedinthe soils. alkaline soil column due to the dominant deposition of Cr(OH) pre- Thereactionkinetics-basedempiricalmodelsalsowelldescribedthe 3 cipitate on the soil. The sorption kinetics can be well fitted by the ratedatabasedoncorrelationcoefficients.ThevaluesofR2indicated pseudo second-order kinetic equation, and the fitting parameters are thattheElovichequationwasthebestfitforpH4.0soil(0.9973),the listedinTableS6(supplementarydata). first-orderedequationforpH6.0soil(0.9645),andthesecond-ordered equationforpH8.0soil(0.9967).Thesuitabilityofaparticularkinetic 3.3. TransformationofCrfractioninCr(III)-amendedsoilsatthreevarious model to each pH was due to the fact that the study was conducted pH before the transformation equilibrium. The difference inthe mostap- propriate model for soils at the three pH levels indicated various re- Soilsamendedby600mgkg−1Cr(III)wereincubatedfrom1dayto action-controlled steps during the transformation of exchangeable Cr. 150daysatthethreepHlevels.TheeffectsofsoilpHandincubation The good fit of the diffusion and reaction models indicated that ex- time on the percentage of Cr fraction are shown in Fig. 2A, and the changeableCrmasstransferandchemicaltransformationoccurredsi- contentsofCrineachfractionarelistedinSupplementaryTableS7.In multaneouslyandtheseweredifficulttoseparate. natural soils with threepH levels (Fig. 2B), Cr was associated mainly withresidual(RS)fraction(>68.4%)andverylowexchangeable(EX) 3.4. OxidationofCrinCr(III)-amendedsoils fraction(<1.8%)inthefollowingorder:RS > OX(fractionboundto Fe–Mnoxides)>OM(fractionboundtoorganicmatter)>CB(fraction The oxidation of Cr(III) in soil would result in hazardous Cr(VI), bound to carbonates)>EX. However, the release of Cr significantly which is considered more mobile and toxic than the former. Cr(VI) changedthefractionationofCrinthethreesoiltypes,andsoilpHhad contentwasmeasuredfrom1dayto90daystostudyCr(III)oxidation considerableinfluenceonthefractiondistributionattheearlystageof in the soils at the three pH levels. Cr(VI) was undetectable in all soil incubation.Forexample,theproportionofEXCrwas66.1%forpH4.0 samples after 1 day of incubation, however significantly increased in soil,10.1%forpH6.0soil,and2.4%forpH8.0soiljustafteraddition pH8.0soilwithprolongedincubationtime.Meanwhile,noCr(VI)was ofCr(III)(control).Withtheextensionofincubationtime,thepercen- detected in soils with pH 4.0 and 6.0 soil during the experimental tage ofEX and CB Cr fractions decreased significantly over time, and period.After90daysofapplication(Fig.3A),theconcentrationofCr theOXfractionincreasedevidently.Meanwhile,RSandOMfractions (VI)inpH8.0soilincreasedto5.74mgkg−1,whichwashigherthan varied slightly during the 150-day incubation in the three soil types. thelimitationofChineseSoilEnvironmentalQuality(<3.0mgkg−1, Afterincubationfor150days,OXCrbecamethepredominantfraction GB36600–2018)inspiteoftheoxidationratioofCr(III)onlyaslowas inallCr(III)-amendedsoilsandreachedashighas75.4%inpH4.0soil, 0.96%. Therefore, the release of Cr(III) into alkaline soil would pose 78.8%inpH6.0soil,and81.1%inpH8.0soil.ThepercentageofEXCr environmentalriskswithprolongedincubationtime. reducedto2.5%,0.35%,and0.29%insoilswithpH4.0,6.0,and8.0, The behavior of Cr(III) and Cr(VI) in environment is closely de- respectively.Thesefindingscouldbeattributedtothetransformationof pendedonpHandEhofthesoil.Fig.3BdisplaystheEhmeasurements Crfromlooselyboundfractionstostronglyboundfractionsduringin- insoilsatthethreepHlevelsamendedwithCr(III).AsshowninFig.3B, cubationandconsistentwiththepracticalsurveyofthecontaminated theEhvaluesofsoilswithpH4.0,6.0,and8.0weretypicallywithinthe site(Castilloetal.,2012).Therefore,transformationofCrfractionwas rangesof540–556mV,529–539mVand480–491mV,respectively.A spontaneous and proceeded once Cr(III) was released into soil. In ad- significant shift to high potentials occurred with decreasing soil pH. dition, no significant increase in RS Cr fraction indicated that the Theseresultswerelikelytobeexplainedbytheequationproposedby amendedCrwasunlikelytoenterthecrystallinelatticeduringthein- Patricketal.(Sparks,2003): cubationperiod. ThemobilityofCrinsoilscanbeevaluatedbythemobilityindex Eh=E0− 59 log (Red) −59 m pH (MI),whichcanbecalculatedfromtheexchangeablefraction(Eq.(2)). n (Ox) n (8) 5 T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 Fig.2.(A)EffectofincubationtimeontheCrfractionation(%)inthreesoiltypesafteradditionofCr(III).ThecontentofCrinsoilswas600mgkg−1.(B)The fractionationofCrinnaturalsoilswithdifferentpHlevels(withoutadditionofCr(III)).ThecontentofCrinnaturalsoilwas41mgkg−1.(C)MobilityindexofCrin variouspHsoilswithadditionofCr(III)asinfluencedbyincubationtime. Table2 Parametersofdiffusionmodelandreactionkinetics-basedmodel. Diffusionequation Elovichequation First-orderequation Second-orderequation SoilpH A B R2 α×10−2 β R2 k1 R2 k2×10−4 R2 pH4.0 5.58 −0.021 0.9427 −40.13 −0.015 0.9973 0.021 0.9427 4.5 0.9252 pH6.0 3.91 −0.023 0.9645 −3.81 −0.10 0.8000 0.023 0.9645 30.7 0.9245 pH8.0 2.34 −0.014 0.9254 −0.46 −0.50 0.9384 0.014 0.9254 34.7 0.9967 whereEh(mV)istheelectrodepotential,E0(mV)isthestandardpo- wasdependentontheinitialconcentrationofCr(III)releasedintosoils tential,nisthenumberofelectronsexchangedinthereaction,misthe according to the kinetic analysis. These findings may help to under- numberofprotonsexchanged,andparenthesesincludetheactivitiesof stand the serious environmental risks associated with alkaline soils theoxidizedandreducedspecies.Qualitativelyanalysisindicatedthat contaminatedbyCr(III)foralongtimeduetothecontinuousconver- Eh increased with decreasing pH. However, Eh did not vary linearly sionofCr(III)intohazardousCr(VI). withpH.Thus,m/nintheaboveequationwasnotconstantinthesoil. Thisinconsistencycouldbeascribedtocomplexsoilcomponents,such as silicates and oxides. In accordance with the Eh-pH diagram (Kaur 3.5. SpectralcharacterizationofsoilsamendedwithCr(III)atthreepH andCrimi,2014),Cr(VI)/Cr(III)couplefavoursCr(VI)stabilizationin levels soilwith pH8.0.Incontrast, Cr(VI)/Cr(III) couplefavoursCr(III) sta- bilizationinsoilswithpH6.0and4.0. SoilwithpreadjustedpH(pH4.0,6.0,and8.0)wereamendedwith Thefirst-orderequationwasusedtofitCr(III)oxidationdatainpH Cr(III)(600mgkg−1)andincubatedfor90daysat303KbeforeFTIR 8.0soil(Fig.3C).ThehighcorrelationcoefficientR2(0.9891)indicated analysis.Fig.4(A-C)displaysthecomparisonoftheFTIRspectraofsoils thattheoxidationprocesswasofthefirstorder,andtheapparentrate beforeandafteramendmentwithCr(III).BeforetheapplicationofCr constantwas1.04×10−4day−1.ThetimerequiredforhalfofCr(III) (III),abroadintensebandappearednear3437cm−1,whichcouldbea tobeconvertedintoCr(VI)was6357daysbasedonthehalf-time(t ) characteristic of hydroxy (Ramrakhiani et al., 2011). The bands then 1/2 of(ln2)/k forthereaction.OxidationofCr(III)totoxicCr(VI)byMnO shiftedto3415cm−1aftertheacidificationofsoilduetotheformation 1 2 has been identified as probably the important oxidation pathway in of new H-bonded hydroxyl groups. The band around 1430 cm−1 re- soils(HausladenandFendorf,2017).InpH8.0soil,theactiveoxidizing presentedcarbonateinthespectraofpH8.0soil(Ghorbel-Abidetal., sites on MnO may be partly covered by the precipitation Cr(OH) 2009).Meanwhile,thepeakdisappearedforpH4.0and6.0soilsdueto 2 3 whichlimitedtheoxidationprocessandledtothelowoxidationratio thedecompositionofcarbonate,resultinginthelowerproportionofCB ofCr(III)(MilacicandStupar,1995).However,thequantityofCr(VI) Cr in acid soil than in alkaline soil (Fig. 2A). The decomposition of carbonate was also related to the decrease of the content of soil 6 T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 Fig.3.(A)KineticofformationofCr(VI)inthreetypesofsoil,(B)redoxpo- tentialinsoilswithCr(III)treatmentasafunctionofincubationtimeatthree pHlevels,(C)first-orderkineticmodelfittingofCr(III)oxidationkineticsdata inpH8.0soil.Errorbarsrepresentedtheresultsofthreemeasurements.The initialconcentrationofCr(III)inthesoilwas600mgkg−1andwithoutCr(VI). inorganiccarbon(Table1).Thepeaksaround1032cm−1inthethree spectrarepresentedthestretchingvibrationofSi–OandC–O(Lietal., 2011; Zhang et al., 2018a). The bands for soils with pH 4.0 and 6.0 appearedanewpeakandcenteredat1140cm−1and1141cm−1,re- Fig.4.FT-IRspectraof(A)pH8.0soilbeforeandafterCr(III)treatment,(B)pH spectively,possiblybecausethequartzwasreallyhadtobebrokenand 6.0soilbeforeandafterCr(III)treatment,(C)pH4.0soilbeforeandafterCr(III) some phyllosilicates were changed due to the soil acidification. The treatment.TheinitialcontentofCr(III)inthesoilwere600mgkg−1andthe resultsshowedthatsoilpHresultedinvariationsinsoilcompositions incubationtimewere90days. 7 T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 Fig.5.TheCr2pXPSspectraofsoilswithorwithoutadditionofCr(III)at(A)pH4.0,(B)pH6.0,and(C)pH8.0.TheO1sXPSspectraofsoilswithorwithout additionofCr(III)at(D)pH4.0,(E)pH6.0,and(F)pH8.0.TheinitialcontentofCr(III)insoilswere5000mgkg−1andtheincubationtimewere90days. and mineralogical properties, thereby altering soil surface properties, fractionation and retention. In addition, the peak intensity at suchasCECandcharge. 1140 cm−1 or 1032 cm−1 representing the stretching bands of Si–O After the application of Cr(III), the bands at 3415 cm−1 split into increasedobviouslyaftertheamendmentofCr(III),justasthefinding twopeaksaround3544cm−1and3406cm−1forsoilswithpH4.0and ofFathimaetal.(2012).ThismaybeascribedtotheinitialCr(III)so- 6.0,indicatingtheinteractionofCr(III)withthehydroxylofsoils,such lutionwithlowpH(about4.0)resultinginphyllosilicateschanged. as silanol or carboxyl. In the meantime, a new peak appeared at SoilswithpreadjustedpHwereamendedwith5000mgkg−1Cr(III) 1681cm−1forpH4.0soiland1684cm−1forpH6.0soilrespectively, and incubated for 90 days prior to XPS analyses. The wide-scan XPS signifyingthechelationofcarboxylwithCr(III)(Fathimaetal.,2012), spectraofsoilsbeforeandafteramendedwithCr(III)atthreepHlevels whichisverydifferentfromthatofsoilwithpH8.0.Whereasthein- were displayed in supplementary date (Fig. S5). Fig. 5A–C shows the tensityofpeaksat774to776cm−1slightlyincreasedinallsoilsdueto comparisonofCr2pXPSspectraofsoilswithorwithoutadditionofCr Cr–O vibration (Mishra et al., 2012), indicating the binding of the (III)atthreepHlevels.BeforeaddingCr(III)tothesoil,therewereno functional groups in soils with Cr(III). The different combinations of obviousCrpeakinthespectrumduetothelowCrcontentinthenatural anthropogenicCr(III)withfunctionalgroupsinsoilsmayaffectCr(III) soil. The Cr 2p peaks clearly appeared in the spectrum of soils after 8 T.Xu,etal. Ecotoxicology and Environmental Safety 195 (2020) 110459 addition of Cr(III). The 2p lines were located at 577.3, 576.9 and environmentalrisksofCr(III)inalkalinesoilwererelatedtotheinitial 3/2 576.6 eV for soils with pH of 4.0–8.0, respectively. These lines were concentration of Cr(III) released and increased with incubation time. characteristicofCr(III),andthebindingenergydecreasedwithsoilpH. Thisworkprovidesabasisforunderstandingtheenvironmentalimpact Accordingtoliterature(Jungetal.,2007),theCr2p bindingenergies ofanthropogenicCr(III)affectedbysoilpH. 3/2 ofchromichydroxideswerewithin576.5–576.9eV.Then2p peaks 3/2 indicatedchromichydroxidesasdominantspeciesinpH8.0soilrather CRediTauthorshipcontributionstatement thaninpH4.0soil,verifyingtheprecipitateretentionmechanismofCr (III)inalkalinesoilsuggestedinbatchsorptionexperiments.Further- TengXu:Conceptualization,Methodology,Writing-originaldraft. more,thebindingenergyofthe2p signalwascenteredat579.4eV FengNan:Investigation,Resources.XiaofengJiang:Formalanalysis. 3/2 for pH 8.0 soil, which was characteristic of Cr(VI) (Boursiquot et al., Yuling Tang: Software. Yunhang Zeng: Writing - review & editing. 2002). This was consistent with the presence of Cr(VI) in incubation Wenhua Zhang: Writing - review & editing, Supervision. Bi Shi: experiments(section3.4).Noevidentshoulderwasfoundnear579eV Supervision. for soils with pH 4.0 and 6.0. Accordingly, the use of a component attributable to Cr(VI) did not achieve a satisfactory fit. The con- Declarationofcompetinginterests centration-enhancedincubationsamplesfurtherconfirmedthatCr(III) inalkalinesoilswasconvertedintoCr(VI)incontrasttothatinacidic Theauthors declarethattheyhavenoknown competingfinancial soils.Thisresultwasconsistentwiththeoxidationkineticsexperiment. interests or personal relationships that could have appeared to influ- Fig.5D–FdisplaystheO1sspectraofsoilswithvariouspHlevels encetheworkreportedinthispaper. before and after amended with Cr(III). The O 1s spectra could be re- solved into four peaks. Before adding Cr(III) to soils, the binding en- Acknowledgement ergiesofthesignalswerelocatedat530.7,531.3,531.8,and532.4eV inpH8.0soil,representingmetal-oxide,metal-hydroxide(e.g.,Fe–OH), This work was funded by the National Key Research and C–O,andO–Hbonds,respectively(Lyuetal.,2018).Asillustratedin DevelopmentProgramofChina(No.2018YFC1802201). Fig.5D–F,anobviousincreaseoftherelativepeakareaassociatedwith metal-OHbondscouldbefoundinsoilswithpH8.0(about3.68%)after AppendixA. Supplementarydata the addition of Cr(III), indicating the formation of Cr–OH and corro- borating chromic hydroxide as dominate species in alkaline soil Supplementary data to this article can be found online at https:// showing in Cr 2p spectra. 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