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, but their net effect on the microbially mediated reductive release of As and Fe is still poorly understood. 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 reoxidized 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.