posted on 2014-10-07, 00:00authored byKatrin Ehlert, Christian Mikutta, Ruben Kretzschmar
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; Fetot: 32.1 mM) by Shewanella sp. ANA-3 (108 cells/mL) in the presence of different concentrations of
birnessite (Mntot: 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
(kinitial = 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 (kinitial = 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.