Influence And Mechanisms Of Different Preparation Methods On Reactivity And Long-Term Performance Of Sulfidized Nanoscale Zerovalent Iron

Trichloroethylene（TCE）is widely used in industrial production,and has been released into the environment due to improper storage and disposal,resulting in high detection rates in soil and subterranean water.It poses a serious menace to human health and ecosystem stability due to the toxicity and non-biodegradation.Sulfidized nanoscale zerovalent iron（S-nZVI）has excellent reactivity and electron efficiency for TCE,which is considered as a promising reductant.There are plentiful ways for the preparation of S-nZVI,and physicochemical properties of the prepared particles are different.But the underlying mechanism remains to be studied.In this study,we explored the influence and mechanism of sulfur reagents（Na₂S,Na₂S₂O₃,Na₂S₂O₄）and sulfidation sequence（co-sulfidation and post-sulfidation method）on the reactivity and long-term performance of S-nZVI in simulated groundwater.The main contents and conclusions of the study are as follows:Part one:The influence and mechanism of different sulfidation methods on the reactivity of S-nZVI were delved.The findings demonstrated that the sulfidation sequence has a significant effect on the reactivity.The co-sulfidized nZVI(S-nZVI_co)has higher reactivity than S-nZVI_post due to the stronger reduction capacity,hydrophobicity and electron transfer capacity,which derive from the higher content of Fe⁰ and reductive sulfur species.The degradation pathway in nZVI system for TCE was β-elimination,and sulfidation sequences and sulfur precursors did not change the degradation pathway of TCE.And the higher content of acetylene in S-nZVI_co system is due to the inhibition of the hydrogenation reaction,deriving from the higher sulfur content.The sulfur reagents have slight effects on the reactivity and product distribution of S-nZVI for TCE.Part two:The comparative study on the long-term performance and physicochemical properties evolution of different S-nZVI particles in simulated groundwater was conducted.The degradation rate and electron efficiency of S-nZVI_co and S-nZVI_post for TCE increased slightly during the 30 d of aging.This was attributed to the formation of cracks in the passivation film,which contributed to the anodic and cathodic reactions occuring on locally separated sites.However,after 60-90 d of aging,the degradation rate by S-nZVI_co dropped sharply to almost 0%,while that by S-nZVI_post was constant.During aging,the reactivity of S-nZVI_co would be lost more rapidly than S-nZVI_post,due to the faster corrosion of Fe⁰ and more oxidation of reductive sulfur species.Thus,the long-term performance of S-nZVI_post was superior.Sulfur precursors can control the electron efficiency of S-nZVI by affecting the sulfur speciation on the particle surface.The proportion of reductive sulfur species on S-nZVI_post synthesized by Na₂S was higher than S-nZVI_post synthesized by Na₂S₂O₃ or Na₂S₂O₄,resulting in a higher electron efficiency of the former S-nZVI_post than the latter S-nZVI_post.