Contaminant loading and competitive access of Pb, Zn and Mn(III) to vacancy sites in biogenic MnO2

The scavenging properties of MnO2 are largely attributed to sorption on layer vacancy sites. However, co-occurrence of Mn(II, III) with MnO2 can change mineral reactivity by modifying its Mn(III) content or number of free vacancy sites. These processes are critical in biogenic MnO2 because nascent precipitates are in contact with aqueous Mn(II) and redox processes involving the biomass can modify the Mn(III) content of the oxide. Here we studied the mechanism of Zn(II) and Pb(II) sorption by the biogenic MnO2 precipitated by Pseudomonas putida GB-1. Sorption isotherms on biogenic MnO2 at pH 5.2 ± 0.3 showed considerably higher loadings for Pb(II) (0.49 mol Pb mol−1 Mn) than for Zn(II) (0.12 mol Zn mol−1 Mn). For loadings above 0.1 mol mol−1 Mn, Zn(II) and Pb(II) sorption was concomitant, albeit to different extents, with Mn(II) accumulation in solution. Wet chemical measurements and analysis of Zn K–edge and Pb L3-edge EXAFS spectra showed that the difference in metal loadings on the oxide and extent of Mn(II) release to solution originates from the ability of the cations to displace interlayer Mn(III), which then undergoes disproportionation and accumulates as Mn(II) in solution. In addition, the formation of nanoscale precipitates rich in Pb-P-Cl in the biomass matrix, as shown by energy dispersive X-ray analysis, leads to greater accumulation of Pb than Zn on the biomass. Our findings show how the reactivity of biogenic MnO2 towards co-occurring trace or contaminant metals in natural systems depends directly on the mineral Mn(III) content and competitive sorption processes.