Elemental sulfur generated in situ from Fe(III) and sulfide promotes sulfidation of microscale zero-valent iron for superior Cr(VI) removal.

Herein, we compared the effect of different extra iron and sulfur precursors on the sulfidation efficiency, physicochemical properties, and reactivity of post-sulfidated microscale zero-valent iron (S-ZVI). S@ZVI was synthesized from in situ S generated via reaction of Fe(III) with S, which resulted in 23-fold higher Cr(VI) removal compared with S/ZVI synthesized using commercial S. The direct formation of FeS film via reaction between S and ZVI played a crucial role in enhancing the removal of Cr(VI) by S@ZVI, with 16- and 12-fold faster rates compared with FeS@ZVI and FeS@ZVI prepared via precipitated reaction of Fe(II) with S and sulfur mixtures, respectively. The incorporated sulfur, sulfidation sequence, and sulfidation time determined the performance of S@ZVI. A combination of batch experiments and kinetic models was used to determine the chemical composition of reduced Cr(VI) products. S@ZVI immobilized Cr(VI) as FeCr(OH) via surface heterogeneous reactions, and partial Cr(VI) was homogeneously reduced to soluble Cr(acetate) or FeCr(OH)(aq) by dissolved Fe(II). The insights gained from this study will facilitate the fabrication of highly reactive S-ZVI and elucidate the mechanism of Cr(VI) removal.