2-chlorophenol oxidation kinetic by photo-assisted Fenton process

Jovmal of Emiomenal Seienes Vol. I5.No.4.p.475- 481 2003Arirke D: 10107074200)0475-407aLC number: X703Documeat code: A2-chlorophenol oxidation kinetic by phot-assisted Fenton processXU Xin-hua" ，ZHAO Wei-rong, HUANG Yan -qiao, W ANG Da-hui(Drpartment of Entionmcenal Engioeeing. Zrejin Liresiyr. Hangrhou 310027. China. E-mil: txhw mail .hz.j.cn)Abrtract: Exprinental da ar preented 1o let and alidae s kinetire model fr the oidation of 2clopheonl wastewate by peoasededFenion process. The data showed thu this process had produced good fecth under aridue conductions.. Up to 90% 2-bormphenl was renovedafer 90- mioute reartion time wih 1,02 of 25处CODo. 。while in UV/H2Oz systrm only 16.8% 2 chlorophenol was renmoved sher one hourtrotment. The opind pH in this reaction ceurd brtwern pH 3.0 and pll 4.0. The rartion kineties for paloasisted Fenton pmessesperimrnted in this research was invriated. Knetic models sere prpornd for tbe trcatnent ol 2-chlomphenld wastcwater. The rextion w联tound to llo th 2nd order. The equtions af reacton kineties are an flws: -出= Keu| H|. HhO,Joexp( - Kn以1):-d( CODK [Cop.J[H0, loexp(- K't). The pediction of the models a found。be in a god greonent wih rprinetaleults. thus onfrmring be proposed reation merhanism.Keywords: 2-chorupheneal wastem ater; polrasterd; Fentom reagent; rmartion kinetieIntroductionThe o-chlorophenol is a very toxic and poorly biodegradable pollutant， which is used as anintermecdiale in the manufacture of higher chlorophenol und phenol resins and for extracting sulphur andnirogen compounds from coals. It is also used in organie synthesis( dlyes and pesticide). Oxidation of 0-chlorophenol wastewaler in catalytic wel air oxidation is of practical interest since a wastewater containingo-chlorophenol over 200 ppm may not be treated eeetively by direet biological methods.Advanced oxidation processes( AOPg) have been sucessfully used in degrading many of the organic-comound-contaminated wastewalers. Among all of the chemical oxidation methods. the dark reaction ofH,02 with frrous salts( known as Fenton's reagent) and the photo- asisted Fenton' s reaction( photo Fentonprocess) is a possible source of hydroxyI radicals in water and the oxidation of organic compounds by theseAOPs have been the subject of numerous studies during the last decade.Many hazardous organic-compoun-pololulants can be mineralised bhy the hydroxyl radicals generated byFenlon' s reagent or Fenton-like reactions. UV/H20,/Fe* reaction can convert most of the organic orinorganie compounds into end products such as minerals, formie acid, carbon dioxide and water( Sadana，1974; Huang, 1989; 1990; 1991; 1996). Practically , the hydroxyl radicals produced during the reactioncan detoxify the contaminants. The hydroxyI redicals are generated by HO2 reacted with a small amount ofiron，concentration as low as 0.05 mmol/L，and also with other transition metals than iron. Numerouscompeting reactions, such as Fe"，Fe"。H2O2，hydruxyI radicals, super-oxide radicals, and radicalsderived from the substrate, may exist during Fenton's reagent treatment. This process has beensuccessfully applied in the treatment of wastewaler discharged from various sources such as medicament andchemical engineering industries. The photo asted Fenton process has become one of the most atractive :destruction methods for hazardous wastewaler Irealment. which is too dilute to incinerate, or tooconcentraled or toxic for conventional biological lrealment. In this paper, different oxidation processesemployed in the oxidation of 2-chlorophenol in aqueous solutions were compared. The efect of diferent pHvalues and concentrations of H202 and iron compounds was evaluated. The reaction kinetics for photo-”Corresponding author中国煤化工MHCNM HG76XIXinhuaetal.Vol.I5assisted Fenton procrsses experimented in this research was also inuvestigated .1 Experiments.1 MaterialsAll reagents used in this work were analytical reagent grade and used without furher purification.Feric sulfate( FeSO,●7H2O, 99.5% ), hydrogen peroxide (30%， w/w, unstabilized) ，2-chlorophenol .1.2 Experimental set upAll experiments were: performed in a batch reactor. and the schemalie diagram of the experimental set-叩used in this study is shown in tig.1. The radiant power of the high-pressure mercury iamp(250 W )employed in the photo:hemical experiment is described there. The tolal volume of the photolysis solutionwas 1.0 L.The pH was adjusted using H2SO, lo the initial value of 6.0. Because the producls of oxidation werecarbon dioxide, oxalice and other carbonic acids, the pH value would drop from 6.0 to 3.0- -4 .0 during ;the teaction. No significant dependence of the reaction rates of 2-chlorophenol-degraudation was observedbeween pH 2.0 and 4.0. The analysis was performed imnediately after the samples were taken from thereactor and filtered using nylon luer-lock- membrane filters.As pre-treatmenl of the sample prior to the analywis of2-chlorophenol，2.0 ml“ reduction and preipitationagent"，which was composed of0.10 mol/L Na,PO,0.10 'Ai.UV lightmol/L Kl and 0.10 mol/I. NaxS03, was added to the 5.0ml samples laken from the batch reaclor al variousphotolysis intervals. This procedure led to cormpleledec:omposing of the residual H2O2 as well as the removal ofmost of the iron( I) by precipitation. The precipitateswere removed by the nylon filters( Roth) .Water bathReyetle puapThe detextion of pssible low molecular weightintermnedistes produced during plotochemical enhancedFig. ! Sctbenatic diagan of plato asied rectonFenton reaction， such as oxalic acid, acetic acid,farilityacetaldehyde and ethanol，was perforned using an HPLC.2 Results and discussion2.1 Comparison of differemt oxidation processes1100A series of experiments with different oxidation processes1000were carried out in the reactor. The results are compared inFig. 2. As expected, among all the systems the COD。of 2-p 90chlorophenol wastewaler renoval was highest in UV/H202/800Fe* system, in which about 40% COD was renoved after0060 min of reaction. Within 5 min a nearly complete600-destnuction of 2-chlorophenol could be observed using photo-0204060 100 Fenton processes. But in H2O2 system only 3.6% COD。was●Fe2*+UV ▲H2O2+Fe2+H2O+UV removed after 60 min.■HO+UV▼FEe2-+H2.2 Effect of HO2 concentrationFig.2 Comparison of UV/H20/Fe2- with otherFig. 3 presentes the efect of the initial hydrngenarxidation procesrs( 1/40Qth.25C)peroxide level on the removal of COD& and 2-chlorophenowith the use of photo asted Fenton reaction process. As中国煤化工MYHCNMHGNu.42-chloropherul oxidation kinetic by photr astedl Fentun process477expected. degradaion of 2-chlorophenol and COD。 was expedied hy clevating the initial H2O2c:oncentration. Reaction with H2O2 at 25% of its required stoichiometric amounl lo mineralise organiccarbons in 2-chlorophenol wastewaler(C0D.) gave a 2-chlorophenol removal of 90% alter 90 min in thisreaxtion. Reaction with H,O, of 6.25% C0D.。also had 45% 2-chlorophenol removed afler 90 minreaction. This can be explained by the effect of the presence of extra amount of 0H radicals. Therefore,H2O2 should be added at an optimal concentration to achieve the best degnadation.▲1/16Qth1000▼Qh .1/4Qth■1/82th▼1/4Qh400-i 600300-B 400200-200020406080100000203040506t, minFig.3 Efeet of H202 adioni in UV/ Fenon' s reaction syterm(Fe' : H202= 1:20. 25C)2.3 Effect of Fe* concentration▲1:5 ◆1:20Iron in its ferous and ferie forms acts as a photocatalyst900▼1:10 ●1:40and requires a working pH below 4.0. To oblain the oplimal Fe800. II) concentration, experiments were done with various arnount点700of the irn salt. Fig.4 shows thut COD changes as a functionuf the added Fe( I). Apparenly, the degradation rale of 2-600-chlorophenol and CODa went up with increasing amount of iron5000尚20304050600市80100 sal when Fe2* to H2O, ratio was below 1:20. But adding more1 ainFig.4 Efee o Fe2* addion in UV/Fenlon'iron salt resuled in higher brownish turbidity that hinderedadsorption of the UV light required for pholuolysis and causedreaction sysem( 1/4Quh, 25C)re:ombination of OH radicals. In this case, Fe2 ”reacted with0H radicals as a scavenger. It is desirable to have the Fe(II) to H202 raio as small as possible, so therecombination can be avoided and the sludge production from iron complex is also reduced . The optimal Fe(1I) to H2O2 ratio should be less than 1:20.2.4 ffect of pHpH value afects the oxidation of organice substances both dire:tly and indirecly. Phoua-asistedFenton process is strongly afected by the pH of wastewaler.650The pH value inluences the generation of OH radicals and the00oxidation eficiency. Fig.5 shows the efet by the pH valuesofduring photo-agsisted Fenton process. As expected, pH valuei 500-will decline during the reation proces . When the initial pHis。450-■pH4.30 .. . pH6.00greater than 6.5，the pH of wastewaler does not decrease 县▼pH 6.65signifcantly. But when the initial pH is less than 6.5, the pHs0-will go down to 3.0 -4.0, and also give the best performance.00-More than 45% of the 2-chlorophenol is removed afler 90 min0102030405060reactin with H202 at 12.5% CODcain. The optimal pH in the4 mireaction course occurs between pH 3.0 and pH 4.0.Fig.5 Efeet of pll in UV/ Fenlon's reactionsysemThe second aspect of pH deals with its shift as the reaction(1/80h.25七)中国煤化工MYHCNMHG78XU Xin-hua吐al .Vol.15progresses. Providing an initial waslewater pH of 6.0, the fllowing profile is typical to Fenton reactions .'The finst inflection is caused by adding FeSO, calalyst ，which usually contains residual H2 SO4. 'The secondand more pronounced drop in pH slarls as H2O2 is added, and continues at the rale that is largelydependent on the catalyst concentration. This drop in pH is llrbuted to the ragmentation of organicmaterials into organic acids. The pH change is often monitored to ensure that the reaction is progressing asplanned bccause dropping in pH value may indicale an inhibition on reaction and a potentially hazardousbuild- up of HO2 in the reaction mixture .2.5 Effeet of carry gasA series of experiments with different camy gases were1000-lone in lab scale reactors. The experimental resuls are_ 900TAircompared in Fig.6. As expected, the largest COD。renoval800was observed with pure oxygen in which more than 41% of 。CODa was removed afer 90 min reaction with H202 of 25% C700CODa，in, while the COD removal with pure nitrogen was600orly 26% after the same reaction time.- δ204060801002.6 Effect of temperaturet, mioFig. 7 shows he degradation efeet by changingtemperature during photo-assisted Fenton process.As Fig.cElret uf cary gas in UvV/fenon's rertionexpected, the degradation rate of COD increased with risingtemperature if it is not too high， which conforms to theArhenius equation. Since the activation energy is not t0o high, the efet of the lemperalure does not playa major role in this process. On the conlrary, with a higher temperature, such 出more than 60心，thelotal degradation of COD will decrease because 1oo high temperature causes decomposition of H,02. Theappropriate reacting temperalure is 20- -40C .1002.7Kinetics of photo- assisted Fenton oxidation in 2-chlorophencl wastewater，900●25飞The kinetic model for the oxidation rate of 2-40C000r 60Cchlorophenol by Fe ( I)/H2O2 involves the single stepdissociation of H202 into two hydroxyl radicals(OH). These .hydroxyl radicals can oxidise organic compounds ( RH)through proton extraction and thus generale organie radicals(R.), which are highly reactive and can be futher500。20 4的0一 前Toridisede. This mliple step reation, which ocus duringI。minUV/H202 oxidation, can be witten as fllows ( Marechal,Efer od recin tepepelure in UVFento s 1997; Blough, 1988; Venkatadi, 1993; Bigda, 1995):reaction sylem(1/2Qh, Fe2+ : H2Oh= 1:20)Fe: + H2O2-Fe2. + H0.+ HO~ ,(1)Fe* + H0."Fe'" + HO(2)HO.+ RH H,0+ R.,<3)Fe- +R.*R*+ Fe2*,(4)R*+ H20- ROH+ H',中国煤化工MYHCNMHG179No.42-chorophenol oxidation kinetic by phasisted Fenon prcessHO.+ H2O2→H0+ HO2.(6)R.+R.→R- R.(7)HO.+ HO.- HO.(8)Based on the above equations, RH reacion equation can be writen as fllows(l Yang, 2001):. dH. Kew[H][H20.]e^%g，(9)Jt「H202 loexp(- Rng1)In[RH] = Kmu+C(10)- Kn0According to the above equation in this sludy we can calculate Kau， Kny，and c ( integrationconstant)using theresourceful exploring model. Fig. 8 shows the plot of second-order kinetics for 2-chlorophenol, in which all the experimental data ft well with the sccond order kinetie model.COD。removals can also be deseribed by the fllowing equstions:d[ COD !dt= Kp,[COD.][H,02]oexp(- K't),(11)In[CODc] = Kooo[HQ0,]oexp(- K't) + c.(12)-K'Interestingly, note that the order of COD removal can also be simplifed as second order. Fig. 9displays the plots of the experimental dala and model-culculated data. The prediction of the models agreeswell with the experimental results, thus confirming the proposed reaction mechanisms .0.0060.005事12001000 Syubol-txperinental data0.004t dita"Line-nodel dataLine-model data1/16Qth5 800140th0. .003100 fH inqub1/80u12h0.00114Qth00 t°1020304050600”153045607590日, minI, min .Fig.8 The model $ of photo sited fention oridationFig.9 The model i of photo. sted Frtonoridation2.8 Mechanism of photo-assisted Fenton oxidation in 2-chlorophenol wastewaterAccording to the observed intermediates, the dechlorination process seemns much slower than thebreaking down of orthochlorophenol because the amount of chloride ion measured is only a fraction of thetheoretical value calculated by assuming 100% release of chloride ion by orthochlorophenol. Chlorinatedinternediates are formned which are sequentally oxidised by hydroxyI radicals. In other words ，chlorineatoms will not be released directly from the aromatie ring after hydroxylation. In terms of oxidalionkinetics, the nate conslants between hydroxyl radicals and chlorinated aliphatic compounds are severalmagnitudes smaller than that between hydroxyl radicals and aromatic compounds. In addition, hydroxylradieals usually utack the aromatic ring preferabl!y al the sites that are nol occupied by cthloine atorns.Thermodynanically, the dissociation energy of a CI-C benzene bond is 95 kcal/mol, whereas the highestbond disciation energies of CI-C in an aliphatic molecule are about 81 to 84 keal/mol. Therefore. it isthermodyoamically more dfficult for chorine atoms to dissociate diretly from the aromatic ring than from中国煤化工MHCNM HG480XU Xir-hua et al.Vonl. I5the chlorinated aliphatic internediates . The pathways of Fenton oxidation are as follows: .OH一Carboxylic acids 一> CO2 and H2O。面.0OH C3 ConclusionsExperimental resuls showed thal the UV/H2O2/Fe* process is efetivee for the trcalment of 2-chlorophenol waslewater. Up to 90% of 2-chlorophenol is removed after 90- minute reaction tine with theH02 level of 25% CODc and the optimal Fe* to H2O2 ratio as 1:20. While in the UV/H2O2 system .(without adding ferrous ion)， only 16.8% of 2-chlorophenol is removed in one-hour reuaction. Asexpected, the CODa and 2-chlorophenol removals increase with higher initial H202 dosage. Reaction withH2O2 of 100% COD gives the best perfomance in which more than 75% C0OD。is removed afer 90 minreaction; while reaction with H02 of 25% CODu only 18% COD. is removed alter 90 min reaction. 2-chlomphenol removal yields the same result 88 CODa does. It is inleresting to note that the COD and 2-chlorophenol removals give the best performnance when the Fe2* to HO2 ratio is about 1:20. Higher .reaction lemperature can accelerate the degradation rale of 2-chlorophenol, but at too high temperature, thetrealment elect will be because of the deconposition of H02. The optimal pH in rea:tion process occursbetween pH 3.0 and pH 4.0，whereas, with the degradation of 2-chlorphenol, organie acids will beproduced simulaneously, then the pH of solution will drop . Therefore, the optimal initial pH wuld bechosen as 6.0.he reaction kinetics for photo asisted fenton processes experimented in this research wasinvestigated. Kinetic models are proposed for the treatment of 2-chlorophenol wastewater und Lextileindustry wastewater. The equations of reaction kinetics are as fllows:_d[RH]= Kuw[RH][H2O.]oexp(- Kg,t); -d[CODad!d== Kcop, [COD][H2O2]oexp(- K' 1).The predicin of the models agrees well with the experimental reuts, thus confiming the proposedreaction mechanisms.References :Bigde RJ 1995. Contider Fentn'日chemistny for waste waer terumem[J]. ChenicallBlough N.988.Elecuon prangnetice reonance measrements of potheraial rdial prodution in hunie substuncese. Ffer of 0, andcharge on radical sravenging by nitrides[). Enionmcnal Science Terhnlogy. 22: 77- -82.Huang C P.Davis AP, 1989. Removal of penolo from wester by a pnarinalrtire oidaion poe[s],. Water Ssienee Tcholog.21(6/7):45--464.HuangGC P,Daris P, 1990. The remoal of sbotiurd phenal ly日polalie oidaion proce wih endmiom sodelu. WarResearch, 24(5): 543-550.HangCP. TengJM. 191. Remoral odf cenoperlo fom waler by potaltire oini. J. Water Srine Thonky. 23(13):377 -387.HuangCP. Tang w z.1996. 2.4choropbeal oxidauionsion by Fenton' 8 regent[J]. Environmental Technology, 17(12): 1373- - 1378.Marchal M. Slokar Y M.Taufer T, 197. Decontio of coriren roeive田dye wib HQ/Uv[J]. Dyao and Pigmoant, (33):中国煤化工MYHCNMHGNn.42-chlouphenol oxidation kinetie by pholo- asisted Fenton process48281 - 298.Sadana A.Janes R K.1974. Catalyic oxidaion o[ pbeno in uqueous solution owrr copper oridelJ. Ind Fng Chem Fundam. 23(2);127-133.Venkalandri R. Peters R W. 19939. Chemical oxidaion lchundugjes:; ulravinle ligh hydrogen peroxide，Fenton reagent, and tiauium dioideasaistled phocalys[J]. Haurdous Waste & Haznrdous Matriale, 10(2): 107-149.YangYP, WuX Y. XnX I.2001. Teatnent of dyeing wastwater bhy poastods Fenton system[J}]. Joumal of Tnivenity of ChemicalEingineering, 15(3): 242- -247.(Recrited for reriew May 20, 2002. Arrepted Oetober 24. 2002)中国煤化工MYHCNMHG