Concurrent sorption of As(V) and Mn(II) during biogenic manganese oxide formation

We report the concurrent sorption of As(V) and Mn(II) during biogenic manganese oxide (BMO) formation by Acremonium strictum strain KR21-2. Mn(II) sorption experiments with preformed BMOs in N2-purged HEPES buffer solutions showed that immature BMOs formed with shorter cultivation times have a higher Mn(II) sorption capacity. The sorption capacity for Mn(II) decreased as the BMOs matured. Average oxidation state analysis and two-step extraction revealed that the contents of sorbed Mn(II) and structural Mn(III) were higher in immature BMOs and decreased as the BMOs matured. A positive relationship between sorbed Mn(II) and structural Mn(III) throughout BMO formation suggested that the latter, which is predominant in immature BMOs, acts as the sorption site for Mn(II). The sorption of co-existing As(V) was positively related to sorbed Mn(II), implying the concurrent sorption of As(V) and Mn(II). XANES analyses showed a cultivation-time dependent change in the shape of the post-edge spectrum of As(V), suggesting change in As(V) sorption mode during BMO formation. The concurrent sorption of As(V) and Mn(II) on immature BMO possesses higher capacity for As(V) sequestration (As(V)/Mn ratio up to 0.13), leading to its potential applicability for As(V) remediation. The results presented here provide new insights on As(V) cycles in the environment, as the reactivity of BMOs makes them one of the most important classes of compounds that affect the fate of a variety of elements.