Iron-sulphur transformation control for enhancing Cr(VI) removal in flake and nanoscale porous pyrrhotite (Fe7S8) added wastewater

Hexavalent chromium (Cr(VI)) contaminated wastewater should be addressed efficiently in the environmental field. In previous applications, nano iron sulfides amendment has not been well controlled for iron-sulfur transformation. In this study, the novel flake and nanoscale porous pyrrhotite (Fe7S8) (FNPP) amendment was synthesized. The iron-sulphur transformation of FNPP was controlled and optimized for enhancing Cr(VI) removal. The specific surface area and average pore diameter of the FNPP amendment reached 115.7 m2/g and 2.1 nm. The maximum adsorption capacity of total chromium reached 66.3 mg/g. The optimized iron-sulphur transformation condition was an initial FNPP and Cr(VI) molar ratio of 8, pH at 5.6, in which the Cr(VI) removal reached 96.5% and all producing S2- was utterly consumed. It is confirmed that S2- fast induced Fe3+/Fe2+ circulation and FNPP has a speedier adsorption rate for Cr(III) than Cr(VI). Fe2+ and S2- mediated the Cr(VI) reduction to Cr(III), thus, much faster Cr(VI) removal was achieved. High efficiency removal mechanism of Cr(VI) was combined with surface adsorption/reduction and solution reduction/precipitation. The research demonstrated that controlling and optimizing the iron-sulphur transformation of Fe7S8 amendment can significantly enhance Cr(VI) removal.