Speciation And Structure Modulation Ofthe Sulfur Layer And Its Effects On Pollutant Removal By Sulfidated Zero-valent Iron

Sulfidated zero-valent iron(S-ZVI),which combines zero-valent iron(ZVI)with iron sulfides(FexSy),has attracted extensive attention in soil and groundwater remediation due to its ability to significantly enhance the reactivity and selectivity of ZVI.The structure and sulfur speciation of sulfur layer on S-ZVI play a crucial role in controlling the electron transfer of Fe0 core,the diffusion of substances,and the solidliquid interface reactions.During the synthesis and storage of S-ZVI,FeS is easily oxidized to form more stable pyrite(FeS2)and elemental sulfur(S0).These various sulfur species exhibit significant differences in physicochemical properties,including solubility,conductivity,and redox activity.Nonetheless,the current understanding of the structure and sulfur speciation modulation of sulfur layer on S-ZVI,as well as their mechanistic impacts on various contaminants,remains inadequate.Therefore,in this study,the sulfidation method was optimized by varying the sulfur and iron sources,as well as solvent conditions to improve the sulfidation efficiency and elucidate the regulation mechanisms of the structure and speciation of sulfur layer.The strong oxidizing Cr(Ⅵ)and the weak oxidizing chloramphenicol(CAP)were chosen for this investigation.The study elucidated the influence of lay structure and different sulfur species,such as FeS,FeS2,and S0,on the properties and performances of S-ZVI,thereby clarifying the mechanisms of direct electron transfer and synergistic enhancement with ZVI for different pollutants.Additionally,the study examined the impact of organic acid on the performance of S-ZVI,considering the potential interaction between S-ZVI and natural organic acids during field remediation.This study provides a theoretical basis and novel insights for the rational design of sulfar layer in S-ZVI,thereby offering technical support for its practical applications.The main research contents and conclusions are as follows:1.To investigate the effects of the sulfur layer structure on the physicochemical properties and Cr(Ⅵ)removal performance of S-ZVI,S-ZVIHAc-NaAc and S-ZVIMES particles were prepared in acetate buffer(HAc-NaAc)and 2-(Nmorpholino)ethanesulfonic acid biogenic buffer(MES),respectively.The findings indicate that,in comparison to the agglomerated FeS particles on the surface of SZVIMES,the FeS nanosheets that grew in-situ on the surface of S-ZVIHAc-NaAc exhibited stronger adhesion to the ZVI substrate,facilitating electron transfer more effectively.Short-term ultrasonication proved to be less effective in delaminating the FeS nanosheets from the surface of S-ZVIHAc-NaAc,resulting in reduced sulfur loss.Simultaneously,during the Cr(Ⅵ)removal process,ultrasonication could remove the passivation layer on the nanosheets,regenerating Fe(Ⅱ),and enhancing the sulfidation strengthening effect of S-ZVIHAc-NaAc.However,the ultrasonication process tended to disrupt the surface structure of S-ZVIMES,thereby suppressing its sulfidation contribution.In cases where FeS was physically adsorbed onto ZVI,it primarily acted as an electron donor for the direct reduction of Cr(Ⅵ).Upon the chemical bonding between FeS and ZVI,the predominant effect was the accelerated electron transfer that synergistically facilitated the Cr(Ⅵ)removal.The strong attachment of FeS nanosheets to the ZVI substrate augmented their collaborative function.2.To improve the sulfidation efficiency and investigate the mechanisms of FeS and FeS2 in the degradation of CAP by S-ZVI,(FeSx+ZVI)bm was prepared by mechanically ball-milling micron-sized ZVI with FeS or FeS2 minerals.Additionally,FeSx@ZVI was synthesized by aqueous precipitation,coating amorphous FeS or FeS2 precursors onto the surface of ZVI.The results showed that two sulfidation methods achieved a sulfidation efficiency close to 100%.Sulfidation enhanced the degradation of chloramphenicol by ZVI through multiple strategies,including increasing the specific surface area,reducing electron transfer impedance,forming galvanic cells,and inducing pitting corrosion or localized acidification.(FeSx+ZVI)bm exhibited superior CAP degradation performance and greater atomic hydrogen contribution compared to FeSx@ZVI.However,under oxic conditions,FeSx@ZVI exhibited lower electron utilization efficiency.Geochemical equilibrium simulation,density functional theory calculations,competitive kinetic models,and molecular dynamics simulations revealed that comparing with FeS,FeS2 was more resistant to acid corrosion and O2 oxidation,had stronger weak interactions with CAP and its products,arid could enhance the nitro reduction and dechlorination of CAP by S-ZVI to a greater extent.FeS2 also exhibited higher electron utilization efficiency and made a larger contribution to direct electron transfer.(FeSx+ZVI)bm accelerated the transformatio of nitroso and azoxy products more effectively than FeSx@ZVI,resulting in lower ecotoxicity of CAP and its byproducts.Therefore,controlling the FexSy composition of sulfur layer in S-ZVI and its spatial distribution with Fe0 can achieve efficient removal of CAP,effective utilization of electrons,and profit-seeking controlled transformation pathways of CAP and its products.3.In the previous chapter,it was found that Fe(Ⅱ)can react with S2-or Sx2-(x>1)to form FexSy through non-oxidative precipitation.Fe(Ⅲ)can oxidize S2-to produce iron(poly)sulfides and S0,and S0 can oxidize Fe0 to form FeS.However,the influence of FeS interactions on sulfidation is not yet clear.Therefore,the interactions of Fe0,Fe(Ⅱ),and Fe(Ⅲ)with different sulfur sources(S2-Sx2-,and S0)were investigated to understand their effects on the sulfidation efficiency,structure,properties,and Cr(Ⅵ)removal of S-ZVI.The results showed that the in situ generation of S0 from the oxidation of S2-by Fe(Ⅲ)significantly promoted the post-sulfidation of micron-sized ZVI,and its Cr(Ⅵ)removal efficiency was 16,12,and 23 times higher than that of FeS@ZVI,FeS2@ZVI,and S0com/ZVI,respectively.Additionally,by changing the S2-:Fe(Ⅲ)stoichiometric ratio,S/Fe molar ratio,sulfidation sequence,and sulfidation time,the surface structure and distribution of FexSy in S-ZVI could be controlled,thereby enhancing the reduction of Cr(Ⅵ).Therefore,the Fe-S interaction is the key to regulating the sulfur layer of S-ZVI.4.To investigate the roles of S0 and FexSy in the removal of Cr(Ⅵ)by S-ZVI,this chapter was based on the differences observed in Cr(Ⅵ)removal among various S-ZVI samples in the previous chapter and elucidated the underlying factors.The influences of specific surface area,sulfur content,electrochemical properties,and relative content of surface FexSy were sequentially eliminated.The focus was then shifted to the roles of S0 and different forms of FexSy as electron donors in the removal of Cr(Ⅵ)by various SZVI samples,considering their different effects in the removal of Cr(Ⅵ)and CAP.It was found that S-ZVI dominated by S0 as the sulfur species exhibited superior reactivity and recyclability in the removal of Cr(Ⅵ)compared to those dominated by FeS and FeS2 precursors.S0 could enhance the removal of Cr(Ⅵ)by forming galvanic cell with Fe0,facilitating electron transfer,and generating highly active iron(poly)sulfide precursors.However,the interaction between S0 and ZVI needed to overcome spatial barriers caused by iron-containing minerals.Additionally,it was found that amorphous FexSy exhibited significantly better reactivity than crystalline FexSy during the reduction of Cr(Ⅵ)by S-ZVI.Therefore,controlling the sulfur species and crystal structure of FexSy can achieve efficient removal of Cr(Ⅵ)by S-ZVI.5.To elucidate the effects of organic acids on the removal of Cr(Ⅵ)by S-nZVI,Sx2-solution was used as the sulfur source to overcome the issues of low sulfidation efficiency using S2-solution aud uneven surface sulfur distribution with S0 solid in the previous two chapters.The results suggested that Sx2-enhanced the sulfidation efficiency of nZVI by oxidizing Fe0,increasing the thickness of the sulfur layer,and reducing electron transfer impedance.Furthermore,with an increase in the carbon chain length or a decrease in the hydroxyl group amount,the[H+]storage capacity of the organic acid solution increased,which predominantly determined the Cr(Ⅵ)removal performance of S-nZVI with high-sulfur-content.Additionally,the coordinating ability of organic acid molecules controlled the distribution of Fe and Cr species,as well as the solubility of FeS,thereby affecting Cr(Ⅵ)removal,Cr(Ⅲ)deposition,Fe(Ⅲ)leaching,Fe(Ⅱ)oxidation,Fe(Ⅲ)reduction,and the reductive activity of Sx2-.The weak interaction between organic acid solutions and the sulfur layer mediated the influence of the solution on the electrochemical properties of nZVI/S-nZVI.In succinic acid solution,S-nZVI prepared by polysulfides exhibited high efficiency in Cr(Ⅵ)reduction and immobilization,minimal leaching of metal ions,and high electron utilization efficiency.Therefore,controlling the thickness of the sulfur layer in S-nZVI and the composition of organic acids in the environment can fully utilize the reactivity of S-nZVI.