Insight into pH dependent Cr(VI) removal with magnetic Fe3S4

In this study, an magnetically separable iron sulfide (greigite, Fe3S4) was synthesized by solvothermal method and employed for effective removal of Cr(VI). The Cr(VI) removal process followed a pseudo-first-order kinetic model that was highly dependent on the initial Fe3S4:Cr(VI) molar ratio. The total Cr(VI) removal was involved in surface adsorption/reduction and solution reduction/precipitation processes via electrostatic attraction, electron transfer and co-precipitation mechanisms. The apparent Cr(VI) removal rate constants decreased from 0.061 to 0.017 min(-1) with solution pH increased from 3.5 to 10.0, which resulted from the higher reducibility and surface electropositivity of Fe3S4 at low pH. The 1,10-phenanthrolin inhibitory experiment revealed that the Cr(VI) reduction process was mainly mediated by ferrous ions rather than sulfides. By means of analyses on the iron and sulfur species, the efficient Cr(VI) removal with Fe3S4 was ascribed to the surface Fe(III)/Fe(II) cycles induced by the sulfide ions. As for the reusability of Fe3S4, the Cr(VI) removal efficiency after 3 cycles was decreased to ca. 50% under the same conditions, which may be caused by the generation of Cr (oxide) hydroxyl and sulfide. These findings provide new insights into the concerned chromate transfer mechanisms mediated by magnetic iron sulfides, and have great prospects in construction of highly efficient systems for the Cr(VI) removal.