Interactions Of Sulfidation With Oxic Aging For Mediating The Reactivity Of Zerovalent Iron Toward Cr(Ⅵ)

Zero-valent iron（ZVI）is widely used in the field of water pollution control due to its strong reducibility,low price and environmental friendliness.However,the oxide film on the surface of ZVI will hinder the mass transfer and the accumulation of corrosion products will aggravate the passivation leading the decrease of reaction activity.Meanwhile,the electron of inner Fe⁰ cannot transfer to the target contaminants,and the reduction decontamination efficiency is poor.The hydrophobicity and electron conductivity of ZVI can be mediated by sulfidation under anaerobic system to improve its decontamination performance.However,the dynamic compatibility of sulfidation and aging based on ZVI under aerobic condition and the importance of sulfur have not been clarified.Therefore,Cr（Ⅵ）was selected as the target contaminant to clarify the reduction efficiency and reaction mechanism of sulfidation coupled with oxic aging on contaminant removal by ZVI.Firstly,it was confirmed that sulfidation and oxic aging could synergistically enhance the reactivity of Cr（Ⅵ）removal by ZVI.The coupling of sulfidation and oxic aging changed the morphology and structure of ZVI.The surface corrosion products of the ZVI of aging by sulfidation（AZVI^S）and the ZVI of sulfidation by aging（SZVI^A）were obviously deposited and accompanied by cracks.The different coupling modes of sulfidation and aging could both improve the decontamination activity of ZVI.The reaction rate constants of Cr（Ⅵ）by AZVI^S and SZVI^A were increased by 1.25 and 1.44times respectively compared with the pristine ZVI.Under different oxic aging time,the maximum reaction rates of Cr（Ⅵ）removal by AZVI^S and SZVI^A were 0.0269 min^-1and 0.020 min^-1.Simultaneously,the decontamination performance of AZVI^S and SZVI^A was different under various reaction conditions.In the consecutive tests,the cumulative Cr（Ⅵ）removal capacity of AZVI^S and SZVI^A was determined to 19.1mg/g and 13.2 mg/g.The experiments of 1,10-phenanthroline and ferrous iron release initially proved that the coupling of sulfidation and oxic aging could promote the corrosion of ZVI to form Fe（II）,which played an important role in the reduction of Cr（Ⅵ）.Secondly,the mechanisms of sulfidation coupled with aerobic aging driving the removal of Cr（Ⅵ）by ZVI were elucidated.XPS was used to deeply analyze the spatial compositions of iron and sulfur.The specific contents of reduced sulfur(i.e.,S^2-,and S₂^2-)in the sub-surficial of AZVI^S and SZVI^A samples were less,and the more inside the particle,the more the specific contents.Combined with particle interface characterization and electrochemical tests,it was shown that the Cr（III）contents on the surface of SZVI^A particles were higher,the polarization impedance was lower.The correlation coefficients（R²）between the corrosion current density,ferrous iron release rate and the specific surface area normalized rate constant(lgk _SA)was both superior than that of AZVI^S,demonstrating that AZVI^S/SZVI^A could separate Cr（Ⅵ）through adsorption and reduction together.The analysis results of the spatial reduced sulfur compositions and lgk _SA shown that the R² between the S^2-and/or S₂^2-contents of AZVI^Sparticles below their surface at 5 nm and lgk _SA were all greater than 0.63,and the S^2-contents at sputtering depth of 20 nm was also positively correlated with it.The corresponding R² between the S^2-contents at 20 nm and lgk _SA of SZVI^A was as high as0.76.It was further confirmed that sulfidation and oxic aging could synergistically adjust the reduction efficiency of ZVI decontamination by spatial S^2-/S₂^2-.In addition,the positive roles of spatial sulfur were closely associated with their species of iron（hydr）oxide shells.the electroconductive shell of magnetite can couple with the sub-shell Fe S at 20 nm below the particle surface to the electron transfer of ZVI toward Cr（Ⅵ）.While the aging shell such as lepidocrocite is beneficial to the Cr（Ⅵ）adsorption but non-electroconductive,both the sub-shell Fe S at 20 nm and shell Fe S/Fe S₂ at 5 nm should be the main electron shuttle of ZVI for reduction of Cr（Ⅵ）.In summary,this research is expected to provide a solid theoretical foundation and technical support for the practical application of SZVI decontamination technology.