Identification,Influencing Factors And Mechanism Of Fe(Ⅳ) In The S-mZVI Activated Peroxydisulfate Process

Advanced oxidation processes(AOPs)employing peroxydisulfate(PDS)have received widespread attention for pollutant removal recently.Iron-based materials have become the most widely investigated activators due to their abundance,cost-effectivess and eco-friendless.Among them,microscale zero-valent iron(mZVI),regarded as a representative iron-based materials,exhibited a high reactivity compared with aqueous Fe(Ⅱ),because the release of Fe(Ⅱ)by mZVI corrosion is a slow stage,which declines the consumption of the reactive species by superfluous Fe(Ⅳ).It was demonstrated that ferryl ion(Fe(Ⅳ))is an important reactive species in the process of PDS activation by ZVI.Fe(Ⅳ)has a high-spin state,high reduction potential and long lifetime,which has a broad application prospect in sewage treatment.However,mZVI has low reactivity because of the inherent passivation layer that generates during manufacture and use.Sulfidation has developed to be an understanding approach to improve the reactivity of mZVI.Although sulfidation has been used to improve the activity of mZVI for persulfate activation in previous studies,they were mainly limited on the efficiency of target compounds degradation.In S-mZVI/PDS process,the effect of sulfidation on Fe(Ⅳ)production,unfortunely,is rarely reported,and whether sulfidation changes the formation pathway of Fe(Ⅳ)or not is unknown.In this study,it was found that Fe(Ⅳ),·OH and SO4·-were the reactive species in S-mZVI/PDS system by electron spin resonance,quenching experiments,PMSO degradation and HPLC-ion trap MS.According to the competition kinetic experiments,the contribution of Fe(Ⅳ),·OH and SO4·-for PMSO degradation was 75.21%,9.9%and 14.89%,respectively,suggesting that Fe(Ⅳ)was the dominant reactive species in S-mZVI/PDS system while ·OH and SO4·-were secondary intermediates.It noted that MeOH inhibited the production of Fe(Ⅳ)during reaction,which cannot be regarded as a quencher to distinguish Fe(Ⅳ)from radicals.Blocking the released Fe(Ⅱ),yet 0.1583 mM PMSO2(61%)were generated in S-mZVI/PDS system,so more Fe(Ⅳ)were formed through a heterogeneous process.Compared with mZVI,the formation of Fe(Ⅳ)was cdependent on the release of Fe(Ⅱ)completely.Sulfidation significantly regulates the formation pathway of Fe(Ⅳ),realizing the conversion from homogeneous to heterogeneous phase.The reaction of S-mZVI produced Fe(Ⅲ)(24.76 mg/L)was much higher that mZVI(12.32 mg/L)in the presence of PDS.Electrochemical impedance spectroscopy(EIS)indicated that S-mZVI was more conductive than mZVI owing to that S-mZVI had a smaller radius corresponding to the lower charge-transfer resistance.Thus,the improvement of Fe(Ⅳ)production caused by sulfidation was ascribed to the acceleration of Fe(Ⅱ)release as well as electron transfer.According to X-ray photoelectron spectroscopy,sulfur species especially S(-Ⅱ)was able to participate in the Fe(Ⅱ)/Fe(Ⅲ)cycle for the Fe(Ⅱ)regeneration,and underwent oxidation.The production of PMSO2 was always maintained at 0.2409-0.2786 mM when the pH ranging from 4 to 7,exhibiting a wide pH tolerance.The formation of PMSO2 in S-mZVI/PDS system was elevated with the increase of S-mZVI and PDS dosage,which are constructive to the Fe(Ⅳ)production.In addition,inorganic anions had an inhibitory effect on Fe(Ⅳ)production in S-mZVI/PDS system,and the order was as follows:HCO3->Cl->SO42-.With the increase of Cl-,the contribution of Fe(Ⅳ)for PMSO degradation was gradually decreased,implying the stronger capacity of Cl-on quenching Fe(Ⅳ)than free redicals.This study not only distinguishes the interface activation of PDS for Fe(Ⅳ)production from homogeneous process,but also provides a new insight for the sulfidation to enhance the performance of iron-based materials on advanced oxidation processes.