Influence of Aqueous and Solid-Phase Fe(II) Complexants on Microbial Reduction of Crystalline Iron(III) Oxides

Abstract

The influence of aqueous (NTA and EDTA) and solid-phase (aluminum oxide, layer silicates) Fe(II) complexants on the long-term microbial reduction of synthetic goethite by Shewanella alga strain BrY was studied. NTA enhanced goethite reduction by promoting aqueous Fe(II) accumulation, in direct proportion to its concentration in culture medium (0.01−5 mM). In contrast, EDTA failed to stimulate goethite reduction at concentrations ≤1 mM, and 5 mM EDTA enhanced the final extent of reduction by only 25% in relation to nonchelator controls. The minor effect of EDTA compared to NTA, despite the greater stability of the Fe(II)−EDTA complex, likely resulted from sorption of Fe(II)−EDTA complexes to goethite. Equilibrium Fe(II) speciation calculations showed that Fe(II) aq should increase with NTA at the expense of the solid-phase Fe(II) species, whereas the opposite trend was true for EDTA due to Fe(II)EDTA adsorption. The presence of aluminum oxide and layer silicates led to a variable but significant (1.5 to > 3-fold) increase in the extent of goethite reduction. Speciation of Fe(II) verified the binding of Fe(II) by these solid-phase materials. Our results support the hypothesis that iron(III) oxide reduction may be enhanced by aqueous or solid-phase compounds which prevent or delay Fe(II) sorption to oxide and FeRB cell surfaces.