Unraveling the neglected role of elemental sulfur in chromate removal by sulfidated microscale zero-valent iron

Elemental sulfur (S0), as an oxidation product of low-valent sulfur, is widely believed to inhibit the reactivity of sulfidated zero-valent iron (S-ZVI). However, this study found that the Cr(VI) removal and recyclability of S-ZVI with S0 as the dominant sulfur species were superior to those FeS or iron polysulfides (FeSx, x > 1) dominated ones. The more S0 directly mixed with ZVI, the better Cr(VI) removal obtained. This was ascribed to the formation of micro-galvanic cells, the semiconductor properties of cyclo-octasulfur S0 with sulfur atom substituted by Fe2+, and the in situ generations of highly reactive iron monosulfide (FeSaq) or polysulfides precursors (FeSx,aq). The Cr(VI) sequestration of FeSx,aq was 1.2–2 times that of FeSaq, and the reaction rate of amorphous iron sulfides (FexSy) in the removal of Cr(VI) by S-ZVI was 8- and 66-fold faster than that of crystalline FexSy and micron ZVI, respectively. The interaction of S0 with ZVI required direct contact and needed to overcome the spatial barrier caused by FexSy formation. These findings reveal the role of S0 in Cr(VI) removal by S-ZVI and guide the future development of in situ sulfidation technologies to utilize the highly reactive FexSy precursors for field remediation.