Research On The Sulfur Precursors Affecting The Morphology,Reactivity And Sulfidation Mechanism Of Sulfidated Micron Zero-valent Iron

Zero-valent iron(ZVI)has been widely used in groundwater remediation,soil remediation and wastewater treatment due to its wide availability,low acquisition cost,high environmental compatibility and strong reducibility.However,there are some limitations in the practical application of ZVI,for example,low selectivity of target pollutants,limited treatment effect of pollutants in some high pH scenarios limit its application.As a modification technology of ZVI,vulcanization technology improves the stability,reactivity and electron selectivity of ZVI,and also widens the range of pollutant removal,which has become a research hotspot at present.The key to the vulcanization technique is to load iron sulfide(FeSx)onto the ZVI to form a reliable FeSx layer.Different sulfur precursors will lead to different FeSx forms,which will affect the properties of sulfide micron zero-valent iron(S-mZVI),such as hydrophobicity,conductivity,etc.So the sulfur precursor is closely related to the reactivity of S-mZVI.If we can understand the role of different FeSx in improving the removal process of Cr(Ⅵ)from sulfide S-mZVI,it will be beneficial to prepare S-mZVI with certain characteristics for specific pollutants,and carry out reasonable design of S-mZVI for specific application scenarios.In this study,S-mZVI was prepared with sodium thiosulfate(Na2S2O3),sodium hyposulfite(Na2S2O4)and sodium sulfide(Na2S)as Sulfur precursor,and are labeled as S-mZVIS2O32-,S-mZVIS2O42-and S-mZVIS2-,and investigated the role of two factors,including the sulfur dosage and the sulfidation time.On this basis,we studied the interaction between pH value and sulfur source,and the effect of pre-corrosion of mZVI on the formation and loading of FeSx during vulcanization.So as to improve the preparation environment and determine the best preparation conditions.The optimum preparation conditions of S-mZVIS2O32-are as follows:S/Fe=0.056,sulfidation time=12 h,pH=6.5;The optimum preparation conditions of S-mZVIS2O4-are as follows:mZVI requires pre-corrosion,S/Fe molar ratio=0.056,sulfidation time=12 h,pH=6.0;The optimum preparation conditions of S-mZVIS2-are as follows:mZVI without pre-corrosion,S/Fe molar ratio=0.056,sulfidation time is about 8-12 h,pH=6.5.The reaction rates constant of S-mZVIS2O32-,S-mZVIS2O42-and S-mZVIS2-for Cr(Ⅵ)are as follows:0.1987 min-1,0.0697 min-1,0.1778 min-1,the removal rate of Cr(Ⅵ)is improved.On the basis of the above optimal preparation conditions,the research analyzed the effect of sulfur precursors on the physicochemical properties and pollutant removal effect of S-mZVI.With the help of a series of kinetic experiments and analytical techniques such as X-ray Photoelectron Spectroscopy(XPS),Scanning Electron Microscopy(SEM-EDS),Raman Spectroscopy,Contact Angle Measurement.The conclusion can be expressed as follows:(1)S-mZVIS2O32-has a large specific surface area and a concentrated distribution of sulfur elements,the main FeSx morphology is FeS;S-mZVIS2O42-has a relatively uniform distribution of sulfur elements and a large specific surface area,the main FeSx morphology is FeS2.The distributionof sulfur elements on the surface of S-mZVIS2-is relatively concentrated,and the specific surface area is the largest,the contents of FeS and FeS2 are relatively average.(2)FeS is more hydrophilic than FeS2,so it is easy to adsorb Cr(Ⅵ).This allows Cr(Ⅵ)to use electrons more efficiently,resulting in S-mZVIS2O32-and S-mZVIS2-being better at removing Cr(Ⅵ).(3)Redox reaction of Na2S2O3 with mZVIto produce S(-Ⅱ)on the surface of mZVI,which is a key step for Na2S2O3 to achieve sulfidation;The key to the sulfation of Na2S2O4 is hydrolysis and disproportionation in a slightly acidic environment to produce S0;While Na2S mainly react with Fe(Ⅱ),for efficient vulcanization,they must react with Fe(Ⅱ)in situ generated by mZVI.This paper explores the relationship between different sulfur precursors and the reactivity and selectivity of S-mZVI,which can provide a design idea of S-mZVI for specific pollutants to improve the electron utilization efficiency.