Promotion effect of Mn2+ and Co2+ on selenate reduction by zero-valent iron

• Co2+ and Mn2+ significantly enhanced selenate removal by zero-valent iron (ZVI).
• Co2+ and Mn2+ provided 69% and 30% electron for selenate reduction, respectively.
• The dominant reductive product of selenate by ZVI depending on added metal ion.
• Adsorbed Se(VI) and Se(IV) could be further reduced to Se(0) and even Se(-II).
• The ratio of Co2+ and selenate consumption was ∼1.5, but ∼1.2 for Mn2+.

Our previous work demonstrated that Fe2+ could overcome the passivation of zero-valent iron (ZVI or Fe0) and sustain iron reactivity for rapid selenate reduction. Due to the similar chemical properties to Fe2+ and co-existence in ZVI material as impurities, Mn2+ and Co2+ were evaluated as an alternative in selenate removal by ZVI. The results of batch experiments showed that Co2+ and Mn2+, especially the former, significantly enhanced selenate removal by ZVI. Co2+ promoted Fe2+ release and the formation of a bimetallic (Co0–Fe0) catalytic system by redox reaction. Co2+ and Mn2+ provided around 69% and 30% electron for selenate reduction, respectively. Selenite (SeIV) was the predominant reductive product in the presence of Co2+; however, selenite and elemental Se (Se0) were the main reductive products in the presence of Mn2+. Adsorbed selenate and selenite could be further reduced to elemental Se and even selenide by ZVI in the presence of Co2+ and Mn2+, identified by XPS analysis and sequential extraction test. The roles and evolution of Co2+ and Mn2+, and selenate removal mechanism were also comprehensively investigated by spectroscopic and chemical analysis. This study indicates that the co-existent of Co2+ and Mn2+ would be positive for selenate removal.