The pH-dependent role of different manganese oxides in the fate of arsenic during microbial reduction of arsenate-bearing goethite.

Manganese oxides reduce arsenic (As) toxicity by promoting aqueous-phase As(III) oxidation and immobilization in natural aquatic ecosystems. In anaerobic water-sediment systems, arsenic exists both in a free state in the liquid phase and in an adsorbed state on iron (Fe) minerals. However, the influence of different manganese oxides on the fate of As in this system remains unclear. Therefore, in this study, we constructed an anaerobic microbial As(V) reduction environment and investigated the effects of three different manganese oxides on the fate of both aqueous-phase and goethite-adsorbed As under different pH conditions. The results showed that δ-MnO had a superior As(III) oxidation ability in both aqueous and solid phase due not only to the higher SSA, but also to its wrinkled crystalline morphology, less favorable structure for bacterial reduction, structure conducive to ion exchange, and less interference caused by the formation of secondary Fe-minerals compared to α-MnO and γ-MnO. Regarding aqueous-phase As, δ-MnO, α-MnO, and γ-MnO required an alkaline condition (pH 9) to exhibit their strongest As(III) oxidation and immobilization capability. For goethite-adsorbed As, under microbial-reducing conditions, all manganese oxides had the highest As immobilization effect in neutral pH environments and the strongest As oxidation effect in alkaline environments. This was because at pH 7, Fe(II) and Mn(II) formed hydrated complexes, which was more favorable for As adsorption. At pH 9, the negatively charged state of goethite hindered As adsorption but promoted the adsorption and oxidation of As by the manganese oxides. Our research offers new insights for optimizing As removal from water using various manganese oxides and for controlling the mobilization of As in water-sediment system under different pH conditions.