Concurrent transformation of Ce(III) and formation of biogenic manganese oxides

We examined the changes in the chemical states of Ce(III) during the formation of manganese oxide occasioned by Mn(II)-oxidizing bacteria, BY86 strain, isolated from the Yunotaki Mn mine, Hokkaido, Japan. The bacteria were incubated with (i) 0.1 mM Ce(III), (ii) 1 mM Mn(II), and, (iii) 0.1 mM Ce(III) and 1 mM Mn(II) for 288 h at 30 °C, and the oxidation states of Ce and Mn then were measured by X-ray Absorption Near Edge Structure (XANES). We also determined the elemental distributions in the bacteria and precipitates by Scanning-Proton Induced X-ray Emission (S-PIXE). In the absence of Mn(II), the concentration of Ce rapidly decreased to 0.03 mM, increased to 0.06 mM, and then remained there. In the presence of Mn(II), the concentration of Ce fell rapidly to approximately 0.015 mM, and then rose to 0.05 mM, and finally declined to 0.005; a black precipitate containing Mn was formed during this last phase. Mn-bearing precipitates also formed in the medium containing only Mn. XANES analysis of the precipitate revealed the presence of Mn(III) and (IV), and Ce (III) and (IV). Ce (III) was detected in solution, and with the bacteria. S-PIXE analyses of the precipitates showed that Ce was associated with Mn, and not with the bacteria. These results suggest that the precipitation of Ce is preceded by its accumulation by the bacterium, followed by its oxidization to Ce(IV) by the Mn(III, IV)-containing precipitates that the bacteria generate.