Study On The Functions Of Sulfides On The Reduction Of Ferric In Advanced Oxidation Processes

The advanced oxidation processes（AOPs）can achieve a high mineralization efficiency of pollutants in a relatively short duration.Furthermore,heterogeneous catalysts in AOPs can be recovered and reused.Therefore,AOPs are becoming popular in wastewater treatment.In AOPs,the mostly used catalyst is Fe.However,the reaction rate of Fe-based AOPs is always limited by the reduction process of Fe^III.Herein,a series of AOPs experiments were carried out in this paper to explore the reduction process of Fe^III while achieving a high degradation efficiency of pollutants.First,a hybrid catalyst composed of hematite and selenium disulfide（Fe/Se-3）was prepared by a hydrothermal method.Methyl orange was selected as a modeling pollutant.The iron cycle in both hydrogen peroxide（hydrogen peroxide）-AOPs and persulfate（PS）-AOPs were investageted.It was found that the ratio of Fe^II/Fe increased from 68.4%in fresh sample to 205.8%after using in H₂O₂-AOPs and 125.4%in PS-AOPs.Due to the promoted regeneration of Fe^II,Fe/Se-3 achieved 100.0%degradation efficiency within 12 min in both systems,significantly higher than that of its two individual components.The main active species was identified as hydroxyl radicals in H₂O₂-AOPs and sulfate radicals in PS-AOPs.The possible mechanism is that hematite can provide soluble and surface-bound iron species for H₂O₂ or PS activation,while selenium disulfide can reduce Fe^III to Fe^II through Se（IV）.Reducing sulfur sites function as electron donors to participate in the reduction of Fe^III directly or indirectly.In the experiments with Fe/Se-3,the optimal reaction condition was p H 3.0 and the cycle of Se was not proved by characterizations.Therefore,a hybrid catalyst(FC₇₅)composed of hematite and cuprous sulfide was prepared to improve the degradation efficiency of pollutants and accelerate the iron cycle in persulfate system.The characterization results of XRD,Raman and TEM showed that FC₇₅ was successfully synthesized.Due to the promoted regeneration of Fe^II,AOP reaction with FC₇₅ achieved 100.0%removal of the surfactant sodium dodecyl sulfate under a neutral condition,significantly higher than 22.9%with hematite or 39.6%with cuprous sulfide.FC₇₅ also showed good pollutant degradation performance in recycle experiments and simulated wastewater.The regeneration of Fe^II was confirmed by the increase of Fe^II from 39.5%in the fresh sample to 42.6%in the used catalyst,and the proportion of higher-valent copper and sulfur in cuprous sulfide was increased correspondingly.The main active species in the system was detected to be sulfate radicals under acidic conditions,and hydroxyl radicals under alkaline conditions.In the hybrid catalyst,hematite provides Fe^II to activate persulfate into free radicals while being oxidized to Fe^III,which is then reduced to Fe^II by Cu^I in cuprous sulfide.In addition,the sulfur element in cuprous sulfide can directly or indirectly promote the redox reaction of iron as an electron donor.These results indicate that the synthesized hybrid catalyst can effectively promote the regeneration of Fe^II and degradation efficiency.In the process of ferric ion reduction by sulfide,it was found that ferric ions can be quickly reduced to ferrous ions,and the conversion efficiency was higher than 90%.Therefore,a study on the promotion of ferric reduction and pollutant degradation by sulfide in AOPs was conducted.Using ferric or ferrous ions had similar degradation behavior（including degradation efficiency and reaction rate）,and the ratio of ferrous ions to total iron after experiments was also similar.These results suggest that molybdenum disulfide can achieve rapid and complete reduction of ferric iron in a short period of time,so that the use of ferric iron or ferrous iron becomes no longer different.Additionally,the fed-batch experiment results imply that the system has a possibility for a long-term operation.The current research shows that sulfide can effectively promote the iron redox cycle,thereby greatly improving the catalytic ability of iron-based catalysts or iron-based AOPs,and facilitating the degradation efficiency of pollutants.