Critical factors limiting microbial iron(III)- and manganese(IV)-oxide reduction: Oxide surface area, dissolved concentration of reduced ion, and arsenic absorption

The critical factors affecting microbial Fe3+ and Mn 4+ reduction were studied by measuring the production of Fe 2+ and Mn2+ in batch reactors containing ferrihydrite and MnO2, respectively. In support of this study, pre-existing methods for creating reduced growth media, determining total reduced ion concentrations, and modeling reduced ion production were significantly modified for application to the experimental conditions of this study. The model results showed that the growth of iron and manganese reducing bacteria is limited by the concentration of reduction sites provided by the oxide surface and by the concentration of metal ions in solution. Maximum productions rates were found to be linearly correlated with oxide concentration while the culture's tolerance to dissolved ions was determined to be the major growth limiting factor causing the transition from exponential to stationary growth. Adsorption of arsenic was found to significantly reduce the concentration of surface reduction sites thereby limiting microbial growth.