Reductive removal of selenate by zero-valent iron: The roles of aqueous Fe2+ and corrosion products, and selenate removal mechanisms

Abstract

Batch tests were conducted to investigate the roles of dissolved Fe2+ and corrosion products, and the involved mechanisms in selenate (SeVI) removal by zero-valent iron (ZVI). The results showed that insignificant SeVI removal (4–7.5%) was observed in the presence of ZVI or Fe2+ alone. However, external supply of dissolved ferrous ion dramatically enhanced SeVI removal in the presence of ZVI. Selenate removal efficiency increased with increasing Fe2+ concentration. Selenate removal sustained only if Fe2+ was supplied continuously. Both sequential extraction experiments and XPS analysis showed that selenate was reduced step by step, with elemental selenium and adsorbed selenite as the dominant reductive products. Selenite and elemental selenium could be further reduced to selenide, with continuous Fe2+ supply and sufficient reaction time. In the ZVI-SeVI-Fe2+ system, ZVI was the major electron donor for selenate reduction. Fe2+ functioned as electron donor as well and was consumed with a Fe2+:Se stoichiometry of ∼1:1. It also facilitated the transformation of the passive layer of iron coatings to a medium (e.g., magnetite) favoring electron transfer and thus enhanced selenate reduction. Iron corrosion products were media for electron transfer and reactive interfaces for selenium adsorption and reduction. These findings provided a new approach to overcome ZVI surface passivation for long-term application.