Degradation of simulated organic w astewater by advanced oxidation with oxidants generated from oxygen reduction☆

Yi Ouyang ,Qing Xu ,Yang Xiang *,Wei Liu ,*,Junqi Du

1 Beijing Key Laboratory of Membrane Science and Technology,Beijing University of Chemical Technology,Beijing 100029,China

2 School of Chemical Engineering,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:AOPs Degradation Advanced oxidants Monoaromatic compounds Microw ave

ABSTRACT The degradation capacity of advanced oxidants generated from oxygen reduction w as investigated in model effl uent containing chlorobenzene,aniline and benzene through the advanced oxidation processes(AOPs).Intermediate products of the degradation process w ere determined by GC-MS,and they contributed to specify the degradation pathways of monoaromatic compounds.The study particularly focused on the in fl uence of the dosage of the oxidant,p H and the initial concentration of organic compounds on the degradation effectiveness.When the dosage of oxidant was4 w t%and the p H was 7,the maximum degradation ratesof 74.83%chlorobenzene,70.32%aniline and 37.69%benzene were achieved.Furthermore,microwave was applied to intensify the oxidation processunder optimal operation conditions,and the degradation rates were increased to 87.85%chlorobenzene,89.11%aniline and 39.03%benzene,respectively.

1.Introduction

Monoaromatic compounds,a kind of toxic substances,are a hazardous threat to the environment and human health even at very low concentrations[1,2].Three common monoaromatic compounds(benzene,chlorobenzene and aniline)are w idely used in national defense,printing,plastics,pesticides and pharmaceuticals industries[3].Moreover,these are also w ell-know n to be carcinogenic,teratogenic or mutagenic substances.Some characteristics such as long-term persistence,bioaccumulation and carcinogenicity make them to be listed as the priority control pollutants by the Environmental Protection Agency[4].

With the development of modern industry,more and more w astewater containing monoaromatic compoundsw ere produced in production process.Methods for monoaromatic compound removal mainly involve adsorption[2],biological method[5],advanced oxidation process[6-8]and the cavitation based on AOPs,as reported recently[9-11].Adsorption has high treatment ef fi ciency.The adsorbent may show different processing results for various adsorbates due to the selectivity of the medium[2].The biological way is stable but has certain requirements on reaction conditions.In some cases,it show s slow degradation rate on monoaromatic compounds,caused by the interaction between aromatic azo and microbes[12].

In recent years,a novel approach that has broad prospects,the advanced oxidation processes(AOPs),w as developed to sewage treatment.The hydroxyl radical species produced via different approaches have strong ability of oxidation and play a signi fi cant role in the degradation process of organic compounds.Other technologies such as electrochemistry treatment,ultrasonic degradation,photocatalytic oxidation and the cavitation[11,13,14]are all related to AOPs due to their high degradation ef fi ciency,non-selectivity to constituents of sew age and low toxicity.Brillas[15]found that the degradation rate of aniline w as 95%in the presence of Fe2+and H2O2at p H 3.Mascia[16]show ed that the removal of monoaromatic compounds w as controlled by the masstransfer towardstheanode,and the degradation rate of the pollutants could be higher than 95%.Hong[17]studied the photochemical degradation of 2-chlorobiphenyl(2-CB)w ith the photo catalyst TiO2in aqueous solution,showing that 2-CBadsorbed on the TiO2could be degraded by oxidative dechlorination,and 2-CB w as completely degraded after 30 min lighting.It is worth mentioning that most of the previous research focused on w ater that only contains onecomponent,instead of mononuclear aromatic compounds.

Therelation betw een thetreatment ef fi ciency of AOPsand the physicochemical property of target organics should be carefully considered.If the target organics and intermediates are radical chain reaction promoter,it has positive in fl uence on degradation ef fi ciency.Therefore,studying the degradation pathways of monoaromatic compounds has great signi fi cance.Under the effect of acoustic wave,Kruus[18]carried out the chlorobenzene degradation experiment,indicating that fi nal products were chlorophenols,chloronaphthalene,single chloride and dichlorobiphenyl by an ion chromatography analysis.A recent study[19]show ed that nitrobenzene w as identi fi ed using LC-MSas one of the major intermediates of aniline oxidation by persulfate.The aniline radical generated from the interaction of aniline and hydroxyl can be oxidized to nitrobenzene by the attack of hydroxyl radicals,and it may be further oxidized to more stable small molecules such as oxalic acid.Tezuka[20]studied the aniline degradation guided by plasma,show ing that aniline converted into inorganic substance and the process had three steps:(1)hydroxylation of benzene ring;(2)breaking of the benzene rings to produce carboxylic acids;and(3)formation of carbon dioxide.For the UVprocess[21],intermediatesw ere identi fi ed as phenol,chlorobiphenyl isomers and benzaldehyde.According to the identi fi cation of intermediate products,an established degradation pathw ays[22]show ed that the combination betw een phenols and·Cl w ould occur to form chlorophenol.Moreover,it has been reported that benzene can be fi rstly oxidized by·OH to form phenol,and eventually form p-benzoquinone[23].Hydroxylation w as an important reaction process,as Liu[24]show ed in the study of degradation kinetics and mechanism of oxytetracycline.In addition,combining w ith other approaches,such as microwave[25],acoustic cavitation[26],AOPs could be more effective due to the synergistic effect.

AOPs have good effects on the monoaromatic compound degradation,w hich has high reaction rate and is also easy to control.In this w ork,oxidants generated from oxygen reduction w ere prepared.The degradation capacity of this advanced oxidant w as investigated in model ef fl uent(simulated w astewater w ith given concentrationsof different aromatics)containing chlorobenzene,aniline and benzene through the AOPs.The in fl uence of the dosage of oxidant,p H and the initial concentration of organic compounds on degradation process w asinvestigated.GC-MSw asused to determine theintermediate productsgenerated in theprocessof oxidative degradation,and the common rule of degradation mechanism for monoaromatic compounds was deduced.Moreover,the oxidation degradation processcombined w ith microwave was studied.

2.Materials and Methods

2.1.Chemicals

All chemicals used in the experiment were of analytical(chlorobenzene,aniline and benzene)or chromatographical grades(methyl alcohol and acetonitrile).Sodium hydroxide,sulfuric acid,dichloromethane,chlorobenzene,aniline and benzene w ere purchased from Beijing Chemical Reagent Co.Ltd.(China).Methyl alcohol and acetonitrile w ere purchased from Sinopharm Chemical Reagent Co.Ltd.(China).The aqueous solution was prepared w ith MilliQ w ater(18.25 MΩ).

2.2.Analysis

Concentrationsof chlorobenzene,anilineand benzene in all aqueous solutions w ere analyzed w ith the Dionex Ultimate3000 HPLCequipped w ith Ultraviolet detector set at 210 nm,254 nm and 210 nm,respectively.The C18column was used w ith diameter of 4.6 mm.Mobile phase w as a mixture of methanol-w ater(85:15,V:V)for chlorobenzene,methanol-w ater(75:25,V:V)for aniline and acetonitrile-w ater(75:25,V:V)for benzene.The fl ow rate w as set at 1.0 ml·min-1.The peak identi fi cation w as based on the retention time and the UV spectrum of external standards.

The intermediateswere identi fi ed on GCMS-QP2010SEand API5000 w ith Rtx-5MScolumn.The samples w ere analyzed after extraction by dichloromethane at a pressure of 100 k Pa and total helium fl ow rate of 50 ml·min-1(purge fl ow=3 ml·min-1).ES-scan model w as used to obtain the mass spectrum of small molecular acids.

2.3.Preparation of the oxidant

Our group used a generator to prepare the oxidants generated from oxygen reduction[27].The core part of the device was an electric catalytic membrane reactor,the electrode of which was prepared through the technology of fuel cell.The electrochemical oxidation potential of·OHproduced was2.8 V,which isa little lower than fl uorine.These features give the advantages of high degradation ef fi ciency,reaction rate and degrading under mild conditions.Also as an energy-saving and environment-friendly device,it fi ts w ell with the concept of green chemistry.

Fig.1 outlinesthe components such as DCelectrical source,reactant supply,membrane catalytic reactor,recycle of electrolyte and product collection[28].KOH or NaOH,H2O and O2w ere pumped into reactor under appropriate temperature and pressure,and OH-w as converted into O2in the anodechamber w hile K+or Na+entered into thecathode chamber acrossthe membrane.With theinteraction between electrode and catalyst,O2w as reduced to HO2-.Hydroxyl radicals(·OH)w as produced from the conversion of HO2-in liquid phase.The electrode reaction is as follow s:

Fig.1.Preparation process of the oxidant.

2.4.Experiment

The mass concentration of the oxidant w as measured by the H2O2equivalent concentration.The reagent that contained reactive oxygen species was liquid.Speci fi cally,the H2O2equivalent concentration was obtained via potassium permanganate titration:fi rstly,the concentration of KMn O4w as determined via titration,w hich sodium oxalate w asregarded asthe basis material.Then,1 ml of reagent that contained reactive oxygen species w as added to a conical fl ask,follow ed by 20 ml deionized water and 10 ml 3 mol·L-1H2SO4,and the titration started.The endpoint of the titration w as identi fi ed w hen solution became reddish and did not fade in 30s.

w here c isthemolar concentration,V isthesolution volumeand M isthe mass fraction.

The oxidant w as added w ith the ultrasonic in order to degrade the model ef fl uent for further treatment.The ultrasonic iscreated by an ultrasound equipment(KQ-50B,Kunshan Ultrasonic Instrument Co.,Ltd.)w ith a power of 50 W.The degradation process took place in the ultrasound environment.The optimum conditionsw ere determined through discussion on thein fl uenceof oxidant dosage,p Hand theinitial concentration of chlorobenzene,aniline and benzene.The concentration of chlorobenzene,aniline and benzene w as 2 mg·L-1,w hich w as close to the real sew age located in Changzhou,China.

The degradation rate was de fi ned as follows:

whereηis the degradation rate,C is the concentration of the target organics at time t and C0is the initial concentration of the target organics.

3.Results and Discussion

3.1.Analysis of degradation pathway

Under the dosage of 4 w t%oxidant at p H 7 and the initial concentration of chlorobenzene,aniline and benzene being 100 mg·L-1respectively,the identi fi cation of intermediate products by GC-MS w as proceeded.For small molecule acids,electron impact ionization of MS w as applied to obtain the mass spectrum.The intermediates w ere determined(Table 1).

Table 1 Intermediates of degradation process

Degradation pathw ays of chlorobenzene,aniline and benzene are shown in Fig.2.As a key component,hydroxyl radicals(·OH)degrade chlorobenzene,aniline and benzene through electrophilic substitution(Fig.2a),addition reaction(Fig.2b),oxidation w ith bond breaking(Fig.2c)and so on.During the preliminary degradation process,πcomplexesandσcomplexes w ere generated through electrophilic substitution,and aromatic ring radical wasfurther produced.On the other hand,the conversion from monoaromatic compounds to cyclohexene radical occurred through addition reaction.Cyclohexene radical dehydrated,then also formed aromatic ring radical.Besides,phenyl radical w asgenerated by bond breaking of C--Cl and C--NH2.Further,after binding to·OH,aromatic ring radicals in the aqueous solution became polyhydric phenols,and this further formed benzoquinone via electron transfer.Oxalic acid,alcohols,acetic and other small molecule acids were generated from the ring-opening of benzoquinone due to the effect of·OH,and fi nally mineralized to CO2and H2O.

Different functional groups on the phenyl ring have different effects on the activity of the benzene ring:(1)--NH2,as a group belonging to the electron-donating group,increases electron density of carbon atomson the benzenering,w hich makesit moreappropriate for electrophilic reaction of·OH.(2)--Cl is a w eak passivation group.Compared with the C--H on the benzene ring,it has the conjugated structure,so there is more chance to let the breaking and addition of·OH occur.As a result,chlorobenzene and aniline are more easily degraded by hydroxyl radical than benzene.In addition,--NH2of aniline can easily be oxidized by hydroxyl radical,and the--NO2which isstrong passivation group can reduce the activity of the benzene ring.Meanw hile,hydroxyl radical would be consumed when--NH2on the aniline is oxidized.

3.2.Effect of experimental parameters on degradation rate

3.2.1.Dosage of the oxidant

After the measurement via potassium permanganate titration,the H2O2equivalent concentration of the oxidant used in this w ork w as speci fi ed as 39.63 g·L-1.Therefore,mass fraction w as used to quantify the dosage.Fig.3 depicts the time-varying degradation rate of chlorobenzene,aniline and benzene at the oxidant dosage values of 2 w t%,4 w t%,6 w t%and 8 w t%,respectively.After dosing the oxidant,ultrasound w as equipped for synergistic oxidation to make the oxidant quickly turn to·OH.The degradation rate of chlorobenzene and aniline increased with the degradation time,and then reached a constant value(Fig.3a and b).The degradation rate of chlorobenzene and aniline increased asthedosageincreased,but the fi nal degradation rateof chlorobenzene and aniline waslower at the dosage of 8 wt%.The reason may be ow ing to the dynamic equilibrium betw een the hydroxyl radical and hydrogen peroxide.H2O and O2w ill be produced w ith·OH and excess H2O2,w hile ·OH w ill convert to H2O2if·OH is overdosed[29].That is,the low er degradation ef fi ciency could be due to the recombination of hydroxyl radicals.

During the aniline degradation process,substitution reaction occurred to replace amidogen with·OH.Amidogen could also be oxidized by·OH as nitrobenzene w as detected in the intermediate products.With regard to chlorobenzene,·OH substituted--Cl directly,and then both phenol and chlorophenol w ere generated.How ever,during the benzene degradation process,either substitution or oxidation took place not as easily as degrading chlorobenzene and aniline,because of the stable structure of benzene ring.

The fi nal degradation rate of benzene increased as the dosage increased under the range of experimental conditions,w hich show ed that the maximum degradation rate of benzene w as 40.19%(Fig.3c).The benzene ring w as relatively stable,and it w as dif fi cult to make ring-open oxidation happen,leading to the low est degradation rate for benzene among three organics.

3.2.2.pH value of simulated wastewater

The p H value has a great in fl uence on the AOPdegradation process[30].Experiments involving acid-base property have been carried out to determine its in fl uence on degradation,revealing that the low est degradation rate for chlorobenzene,aniline and benzene emerged at p H 11 w hen the dosage and ultrasonic is fi xed(Fig.4).The conversion form HO2-to·OH w ill be inhibited under the alkaline condition,and CO2w ill be generated simultaneously and further convert to HCO3-and CO32-.Two substances are·OH scavengers,which w ill hinder the degradation[31,32].

In addition,hydroxyl radical has different effects on tw o organic states:dissociation state and molecular state.It show s higher activity on degrading the former kind.Aniline is a dissociable substance,and dissociation hasmore chance to occur in alarger p Hvalue environment.Therefore,the degradation rate of aniline is higher at p H 7 than p H 3.Anyhow,the fi nal degradation rate of chlorobenzene and aniline barely changed under acidic and neutral conditions,show ing that the oxidant is capable of degrading them over a w ide range of p H value.

Asfor benzene,except for theinhibiting effect from HCO3-and CO32-,it'snot a dissociable substance,and the highest degradation rate of benzene wasshown in Fig.4c under the neutral condition.

3.2.3.Initial concentration of organic matters

The in fl uence of the initial mass concentration of monoaromatic compounds on the degradation rate is show n in Fig.5.The degradation ratesof thechlorobenzene,anilineand benzenew ere in theorder asfollow s:chlorobenzene＞aniline＞benzene.The result show ed that the degradation rate signi fi cantly decreased w ith the increasing of initial concentration of chlorobenzene,aniline and benzene,especially w hen initial concentration was 10 mg·L-1,the degradation rate was reduced to 58.16%,53.57%and 23.06%,respectively.

Fig.2.(a)Electrophilic substitution reaction;(b)additional reaction;(c)bond breaking;(d)proposed pathways for monoaromatic compounds degradation by hydroxyl radical.

Fig.3.In fl uence of the dosage on degradation rate:T=25 °C;p H=7;C0=2 mg·L-1.

Fig.4.In fl uence of p H on degradation rate:T=25 °C;C0=2 mg·L-1;4 w t%of dosage.

The higher the initial concentration w as,the more the intermediate products were generated,and·OH tended to react with intermediate productsinstead of monoaromatic compounds.For instance,asthedegradation of chlorobenzene proceeded,the concentration of chloridion increased gradually.It is a strong scavenger agent for·OH.Moreover,the total amount of organics in the w astew ater increased as the initial concentration increased,leading to an increase of the absolute amount of degradation but at low er degradation rate.

Fig.5.In fl uence of initial concentration on degradation rate:T=25°C;p H=7;4 wt%of dosage.

3.3.Effect of microwave on the degradation rate

Microw ave has the heating and non-heating effects,and can accelerate the chemical reaction.In this w ork,a microw ave oven(Midea KD23B-DE)w as used to create microw ave,w hich has 27 L effective volume and maximum 700 W pow er.When the experiments w ere carried out,the microw ave power was set as 80 W.After dosing the oxidant,the reagent w as put into the microw ave environment for 50s.When it cooled dow n,the concentrations of samples w ere measured.And continuously,the reagent was put into the microwave environment for another 50 s.This operation w as repeated w ith a period of 10 min.Further,the degradation rate results can be obtained every 10 min of the degradation process(Fig.6a).As an auxiliary mean,the non-heating effect of microw ave is usually used to advance oxidation technology in the liquid phase.By changing the degradation reaction kinetics of organic matters in simulated w astewater,it decreases the reaction activation energy and speeds up the process of reactive oxygen species to·OH[25,33].The experimental results show ed that the fi nal degradation rates w ith microw ave of chlorobenzene,aniline and benzene w ere 87.85%,89.11%and 39.03%,respectively(Fig.6a).Compared w ith the ultrasonic w ave results in Fig.6b,microwave improved the effect of the oxidant,and increased the degradation rate of chlorobenzene and aniline,but had no obvious effect on benzene.

Fig.6.In fl uence of microw ave or ultrasonic w ave on degradation rate:T=25°C;p H=7;C0=2 mg·L-1;4 w t%of dosage.

4.Conclusions

Advanced oxidantsgenerated from oxygen reduction w ereprepared by a membrane catalytic reactor.The degradation pathw ays and degradation rate w ere investigated in model ef fl uent containing chlorobenzene,anilineand benzenethrough AOPs.Thequalitativeanalysisof intermediatesindicated that thedegradation pathwaysof chlorobenzene,aniline and benzene w ere determined by the effectsof·OHon target organics.The results show ed that the degradation rate increased with the increase of the oxidant dosage,and p H 7 was the optimal conditions to carry out the degradation reaction.How ever,if the dosage wasexcessive,degradation rate of benzene and aniline would decrease.At a constant dosage,the degradation rate decreased w ith higher initial concentration of organic matter.Eventually,the treatment effect w as ascertained:chlorobenzene＞aniline＞benzene.Moreover,microwave intensi fi ed the oxidation process under the optimal operation conditions,and the degradation rates w ere increased to 87.85%chlorobenzene,89.11%aniline and 39.03%benzene respectively.

On the w hole,the oxidants generated from oxygen reduction prepared by the generator in this w ork show ed a typical AOPs technique for organic sew age treatment w ith industrial application prospects.