Impact of Birnessite on Arsenic and Iron Speciation during Microbial Reduction of Arsenic-Bearing Ferrihydrite.

Elevated solution concentrations of As in anoxic natural systems are usually accompanied by microbially mediated As(V), Mn(III/IV), and Fe(III) reduction. The microbially mediated reductive dissolution of Fe(III)-(oxyhydr)oxides mainly liberates sorbed As(V) which is subsequently reduced to As(III). Manganese oxides have been shown to rapidly oxidize As(III) and Fe(II) under oxic conditions. Here, we investigated the microbial reduction of As(V)-bearing ferrihydrite (molar As/Fe: 0.05; Fe_tot: 32.1 mM) by Shewanella sp. ANA-3 (10⁸ cells/mL) in the presence of different concentrations of birnessite (Mn_tot: 0, 0.9, 3.1 mM) at circumneutral pH over 397 h using wet-chemical analyses and X-ray absorption spectroscopy. Additional abiotic experiments were performed to explore the reactivity of birnessite toward As(III) and Fe(II) in the presence of Mn(II), Fe(II), ferrihydrite, or deactivated bacterial cells. Compared to the birnessite-free control, the highest birnessite concentration resulted in 78% less Fe and 47% less As reduction at the end of the biotic experiment. The abiotic oxidation of As(III) by birnessite (k_initial = 0.68±0.31/h) was inhibited by Mn(II) and ferrihydrite, and lowered by Fe(II) and bacterial cell material. In contrast, the oxidation of Fe(II) by birnessite proceeded equally fast under all conditions (k_initial = 493±2/h) and was significantly faster than the oxidation of As(III). We conclude that in the presence of birnessite, microbially produced Fe(II) is rapidly re-oxidized and precipitates as As-sequestering ferrihydrite. Our findings imply that the ability of Mn-oxides to oxidize As(III) in water-logged soils and sediments is limited by the formation of ferrihydrite and surface passivation processes.