As(III) adsorption on Fe-Mn binary oxides: Are Fe and Mn oxides synergistic or antagonistic for arsenic removal?

Fe-Mn binary oxides are ubiquitous in the natural environment and have attracted increasing interest due to their high removal capacity for As(III), as well as their important role in the natural cycling of arsenic. Although numerous studies have characterized the respective roles of Fe and Mn oxides in As(III) removal, the working relationship between Fe and Mn oxides in different Fe-Mn binary oxides for As(III) removal has not been fully explored. In this study, three Fe-Mn binary oxides containing either ferrihydrite, hematite or goethite, were used to evaluate their adsorption capacities for As(III) in comparison with their corresponding single Fe and Mn oxide forms. The dynamics and speciation transformation of As(III) within mixed Fe and Mn oxides systems were investigated by using spectroscopic techniques of in situ flow ATR-FTIR and XPS combined with a Donnan reactor, where Fe and Mn oxides were isolated by a semi-permeable membrane through which arsenic could pass. The result showed that the synergistic effect, as well as antagonistic effects, between Fe and Mn oxides, was present in Fe-Mn binary oxides for As(III) removal. An obvious increase in As(III) removal by hematite containing Fe-Mn binary oxide, and a decrease by ferrihydrite containing Fe-Mn binary oxide, was attributed to As(III) oxidation mediated by Mn oxide as well as the difference in arsenic adsorption affinity in Fe oxides.