Morphology and structure of in situ FeS affect Cr(VI) removal by sulfidated microscale zero-valent iron with short-term ultrasonication

The properties of sulfidated zero-valent iron (S-ZVI) are considered to be determined by the entire structure of Fe0 and FexSy as a whole, but few studies focus on the influence of the morphology and structure of the external FexSy layer on the performance of S-ZVI. In this study, after the sulfidation of microscale ZVI in acetate (HAc-NaAc) and 2-(N-morpholino) ethanesulfonic acid (MES) buffer solution, the S-mZVIHAc-NaAc surface presented the in situ growth of the FeS nanosheet, while the S-mZVIMES surface was dominated by agglomerated FeS sub-micron particles. Under short-term ultrasonication, S-mZVIHAc-NaAc was superior to removing Cr(VI) than S-mZVIMES, and the clearance of the passivation layer by ultrasound maximized the conductivity of the FeS nanosheet to strengthen the sulfidation contribution. However, agglomerated FeS particles were easily separated from S-mZVIMES by ultrasonication, resulting in the suppression of its sulfidation contribution. The removal of Cr(VI) by S-ZVI increased linearly with FeS content, and the chemical combination of FeS with ZVI had more significant synergy than their physical mixture. The FeS nanosheet with excellent conductivity and large vertical space benefited the generation of dissolved and surface-associated Fe(II) as electron donors and structural Fe(II) as the electron shuttle. Understanding the relationship between FeS structure and S-ZVI performance will pave a way for optimizing the synthesis of S-ZVI.