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). S0@ZVI was synthesized from in situ S0 generated via reaction of Fe(III) with S2-, which resulted in 23-fold higher Cr(VI) removal compared with S0com/ZVI synthesized using commercial S0. The direct formation of FeSx film via reaction between S0 and ZVI played a crucial role in enhancing the removal of Cr(VI) by S0@ZVI, with 16- and 12-fold faster rates compared with FeS@ZVI and FeS2@ZVI prepared via precipitated reaction of Fe(II) with S2- and sulfur mixtures, respectively. The incorporated sulfur, sulfidation sequence, and sulfidation time determined the performance of S0@ZVI. A combination of batch experiments and kinetic models was used to determine the chemical composition of reduced Cr(VI) products. S0@ZVI immobilized Cr(VI) as Fe0.5Cr0.5(OH)3 via surface heterogeneous reactions, and partial Cr(VI) was homogeneously reduced to soluble Cr(acetate)3 or Fe0.75Cr0.25(OH)3(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.