New insight into adsorption and reduction of hexavalent chromium by magnetite: Multi-step reaction mechanism and kinetic model developing

Magnetite is a common mixed ferrous-ferric iron oxide in soils and sediments, which has both adsorption and reduction capacities for Cr(VI). Previous studies mainly focused on the application of magnetite in Cr(VI) removal from aqueous phase by adsorption, while few efforts has been devoted to the reduction process of adsorbed Cr(VI) on the solid phase. In this study, the kinetic and thermodynamic experiments were simultaneously conducted to verify the relationship between Cr(VI) adsorption and reduction, and SEM-EDS, XPS and Raman spectroscopies were utilized to reveal the mechanisms of Cr(VI) reduction and magnetite transformation. According to the results, Cr(VI) was found to be quickly adsorbed onto magnetite surface with a very high adsorption rate constant of 11.369 d(-1), and then the adsorbed Cr(VI) as an intermediate was directly reduced to Cr(III) by the active Fe(II) on magnetite surface with a fast reaction rate constant of 1.606 d(-1). Notably, the reduction rate constant gradually decreased as low as 0.032 d(-1) with time, which might be induced by the maghemite passive layer formed on the outer-sphere of magnetite, where the adsorbed Cr(VI) could only be reduced indirectly by the Fe(II) located at the inner-sphere of magnetite, and thus the electron transfer rate from internal Fe(II) to adsorbed Cr(VI) became the rate-limiting step of Cr(VI) reduction. Accordingly, a coupling mechanism of "fast adsorption-direct reduction-indirect reduction" was proposed, and a multi-step kinetic model was developed based on that, which had a much better fitting effect (0.936 < R-2 < 0.996) than the classical firstorder or second-order kinetic models (0.885 < R-2 < 0.956). This work highlighted the multi-step reaction mechanism of Cr(VI) with magnetite, especially the distinguished Cr(VI) reduction schemes by magnetite with different passivation extents, and these findings may favor the understanding of the interfacial processes of Cr (VI) on Fe(II) containing minerals in subsurface environment, where the minerals may exist in various passivation degrees due to natural oxidation.