Influence of electron donor/acceptor concentrations on hydrous ferric oxide (HFO) bioreduction

Dissimilatory metal-reducing bacteria (DMRB) facilitate the reduction of Fe and Mn oxides in anoxic soils and sediments and play an important role in the cycling of these metals and other elements such as carbon in aqueous environments. Previous studies investigating the reduction of Fe(III) oxides by DMRB focused on reactions under constant initial electron donor (lactate) and electron acceptor (Fe oxide) concentrations. Because the concentrations of these reactants can vary greatly in the environment and would be expected to influence the rate and extent of oxide reduction, the influence of variable electron acceptor and donor concentrations on hydrous ferric oxide (HFO) bioreduction was investigated. Batch experiments were conducted in pH 7 HCO3 buffered media using Shewanella putrefaciens strain CN32. In general, the rate of Fe(III) reduction decreased with increasing HFO:lactate ratios, resulting in a relatively greater proportion of crystalline Fe(III) oxides of relatively low availability for DMRB. HFO was transformed to a variety of crystalline minerals including goethite, lepidocrocite, and siderite but was almost completely dissolved at high lactate to HFO ratios. These results indicate that electron donor and acceptor concentrations can greatly impact the bioreduction of HFO and the suite of Fe minerals formed as a result of reduction. The respiration driven rate of Fe(II) formation from HFO is believed to be a primary factor governing the array of ferrous and ferric iron phases formed during reduction.