Cobalt(II) sequestration on fungal biogenic manganese oxide enhanced by manganese(II) oxidase activity

We examined the ability of biogenic manganese oxide (BMO) formed in the cultures of a Mn(II) oxidizing fungus, Acremonium strictum strain KR21-2, to sequester Co(II) and found that the newly formed BMO effectively sequestered Co(II) under aerobic conditions with virtually no release of Mn(II). Under anaerobic conditions, smaller amounts of Co(II) were sequestered and a significant amount of Mn(II) was released. Similar trends were observed when the BMOs were poisoned with 50 mM NaN₃ or heated at 85 °C for 1 h. X-ray absorption near-edge structure spectroscopy and two-step extraction confirmed that oxidation of Co(II) to Co(III) occurs with BMOs with higher oxidation efficiency under aerobic conditions. These results demonstrate that BMOs can reoxidize Mn(II) through the Mn(II) oxidase associated with the BMO phase and can subsequently provide a new reaction site for Co sequestration. The ability of BMO to sequester Co(II) was also found to be long lasting in 20 mM 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES) buffer (pH 7.0) containing no nutrients to maintain fungal growth, because sequential treatment of BMOs with the Co(II) solution every 24 h for at least 8 days led to Co(II) sequestration. In addition, Co accumulation in the solid phase was eventually 2.48-fold relative to the accumulation of Mn oxide (molar ratio). X-ray diffraction results suggest that the continuing Co(II) oxidation by newly formed BMOs results in the formation of heterogenite (β-CoOOH) aggregates. Assays using the concentrated Mn(II) oxidase crude solution showed that the preformed Mn oxide phase was important for further Mn(II) oxidation in coexisting Co(II). The fact that the coexisting Co(II) was less inhibitory to Mn(II) oxidation if the preformed Mn oxide phase was present suggests a possible electron path from Co(II) to the final electron acceptor O₂ through BMO and Mn(II) oxidase in BMO/enzyme aggregation. These results suggest that fungal BMOs supporting Mn(II) oxidase activity can serve as an effective Co(II) sequestering material, without the need for additional nutrients.     

Lepidocrocite-catalyzed Mn(II) oxygenation by air and its effect on the oxidation and mobilization of As(III)

Manganese (oxy)hydroxides (MnO_X) play important roles in the oxidation and mobilization of toxic As(III) in natural environments. Abiotic oxidation of Mn(II) to MnO_X in the presence of Fe minerals has been proved to be an important pathway in the formation of Mn(III, IV) (oxy)hydroxides. However, interactions between Mn(II) and As(III) in the presence of Fe minerals are still poorly understood. In this study, abiotic oxidation of Mn(II) on lepidocrocite, and its effect on the oxidation and mobilization of As(III) were investigated. The results show that MnO_X species are detected on lepidocrocite and their contents increase with increasing pH values ranging from 7.5 to 8.4. After 10 days, an MnOx component, groutite (α-MnOOH) was found on lepidocrocite. During the simultaneous oxidation of Mn(II) and As(III), and the As(III) pre-adsorbed processes, the presence and oxidation of Mn(II) significantly promotes the removal of soluble As(III). In addition, MnOx formed on lepidocrocite also contributes to the oxidation of soluble and adsorbed As(III) to As(V), the latter being subsequently released into solution. In the process where Mn(II) is pre-adsorbed on lepidocrocite, less As(III) is removed, given that the active sites occupied by MnOx inhibit the adsorption of As(III). In all experiments, the removal percentages of As(III) and the release of As(V) are correlated positively with pH values and initial concentrations of Mn(II), although they are not apparent in the Mn(II) pre-adsorbed system.