Biogenic Manganese Oxides Facilitate Iodide Oxidation at pH <= 5

Radioactive I-129, a byproduct of nuclear power generation, can pose risks to human health if released into the environment, where its mobility is highly dependent on speciation. Based on thermodynamic principles, 129I should exist primarily as iodide (I-) in most terrestrial environments; however, organo-I-129 and (129)iodate are also commonly detected in contaminated soils and groundwater. To investigate the capability of biogenic manganese oxides to influence iodide speciation, 17 manganese-oxidizing bacterial strains, representing six genera, were isolated from soils of the Savannah River Site, South Carolina. The isolates produced between 2.6 and 67.1 nmole Mn oxides (ml(-1) media after 25 days, pH 6.5). Results from inhibitor assays targeting extracellular enzymes and reactive oxygen species indicated that both play a role in microbe-induced Mn(II) oxidation among the strains examined. Iodide oxidation was not observed in cultures of the most active Mn-oxidizing bacteria, Chryseobacterium sp. strain SRS1 and Chromobacterium sp. strain SRS8, or the fungus, Acremonium strictum strain KR21-2. While substantial amounts of Mn(III/IV) oxides were only generated in cultures at >= pH 6, iodide oxidation was only observed in the presence of Mn(III/IV) oxides when the pH was <= 5. Iodide oxidation was promoted to a greater extent by synthetic Mn(IV) O-2 than biogenic Mn(III/IV) oxides under these low pH conditions (<= pH 5). These results indicate that the influence of biogenic manganese oxides on iodide oxidation and immobilization is primarily limited to low pH environments.