A newly isolated indigenous metal-reducing bacterium induced Fe and Mn oxides synergy for enhanced in situ As(III/V) immobilization in groundwater

• Reducing bacteria FMR enhanced in situ As fixation in groundwater by Fe-Mn oxides. • Mn oxide components prevented the bio-desorption of adsorbed As on Fe-Mn oxides. • Bio-reduction of Mn(III/IV) in Fe-Mn oxides created more fresh sites for As adsorption. • Biological passivation of the Mn component inhibited As oxidation but not As fixation. • Bacteria-induced synergy between Fe and Mn oxides promoted the fixation of As(III/V). Iron-manganese binary oxides (Fe-Mn oxides) have the potential to immobilize arsenite/arsenate [As(III/V)] in situ in natural environments. However, the As(III/V) immobilization performances of Fe-Mn oxides in the presence of indigenous metal-reducing bacteria in reducing groundwater systems have received limited attention. Here, As(III/V) immobilization by Fe-Mn oxides and the interactions of Fe and Mn oxide components in groundwater in the presence of a newly isolated indigenous metal-reducing bacterium ( Bacillus sp. FMR) were investigated. Microcosm experiments were performed with artificial groundwater and mineral phases (Fe, Mn and Fe-Mn oxides) with As(III) or As(V) in the presence or absence of strain FMR under anoxic conditions. The Fe oxides in anaerobic cultures without bacteria exhibited better immobilization performances for As(III) [69% As(T) removal efficiency] and As(V) (70%) than Mn oxides (19% and 17%, respectively). Comparably, the involvement of bacteria induced the re-release of arsenic adsorbed on the single Fe and Mn oxides under anoxic conditions, whereas this shift in arsenic adsorption–desorption was not observed in Fe-Mn oxide systems because the Mn oxide components inhibited the reductive dissolution of the Fe oxide components. The As(T) removal efficiencies of Fe-Mn oxides in the presence of bacteria increased by 17% for As(III) and 16% for As(V) compared with anaerobic incubation without bacteria. The interaction of Fe and Mn oxides in Fe-Mn oxides resulted in a slightly increased mobilization of As(III/V) in the absence of bacteria, whereas there was a synergistic effect in the presence of bacteria. The favorability of indigenous bacteria for As(III/V) immobilization and the synergy between Fe and Mn oxides provide novel insights for in-situ As(III/V) immobilization in high arsenic-containing groundwater.