CRITICAL FACTORS LIMITING MICROBIAL FE(III)- AND MN(IV)-OXIDE REDUCTION: OXIDE SURFACE AREA, DISSOLVED CONCENTRATION OF PRODUCT ION, AND ARSENIC ADSORPTION

We investigated, the kinetics for the microbial reduction of ferrihydrite (Fe(III)) and manganese oxide (Mn(IV)) under anoxic conditions by measuring the production of Fe(II) or Mn(II) over time in sealed batch reactors. Production of Fe(II) and Mn(II) were accurately fit during exponential and stationary microbial growth phases with a kinetic model combining a first-order and zero-order model. Maximum Fe(II) production rates, Rmax, were linearly correlated with ferrihydrite concentrations in reactors containing 10, 20, and 30 mM of ferrihydrite yielding Rmax values of 0.010, 0.015, and 0.019 mM of Fe(II)/hr, respectively. The presence of 1 mM of arsenic, 100% adsorbed onto 30 mM of ferrihydrite, greatly reduced Fe(II) production indicating that microbial growth was limited (Rmax=0.012 mM of Fe(II)/hr). This decrease in the initial production rate and the extent of Fe(II) production during exponential growth was correlated with the calculated loss of 257 mM of surface reduction sites per mole of ferrihydrite due to arsenic adsorption. In addition, the transition from exponential to stationary microbial growth was dependent on the ratio of cell density to dissolved Fe(II) suggesting that microbial tolerance to dissolved ions may be a growth limiting factor.

The maximum production rate in reactors containing 10 mM MnO2 was calculated as 0.021 mM of Mn(II)/hr, approximately two times greater than the Rmax for Fe(II) production on 10 mM of ferrihydrite. However, the extent of Mn(II) and Fe(II) production during exponential growth was approximately equal due to a much shorter exponential growth phase associated with MnO2 reduction. Based on our calculation that the reduction of one mole of MnO2 produces approximately 11.5 times more free energy as the reduction of one mole of ferrihydrite, and assuming that increased energy yield translates to increased cell population, we suggest that the shorter exponential growth time is the result of saturation of surface reduction sites with respect to cell density. We suggest that the three critical factors controlling microbial Fe(III) and Mn(IV) oxide reduction are: 1) dissolved metal concentration, 2) blockage of surface reduction sites by ion adsorption and, 3) saturation of surface reduction sites with respect to cell density.