Microbial reduction of iron(III) oxyhydroxides: effects of mineral solubility and availability

Abstract

The rate of Fe(III) reductive dissolution by ascorbate has been shown by [Larsen, O. and Postma, D., 2001. Kinetics of reductive bulk dissolution of lepidocrocite, ferrihydrite and goethite. Geochim. Cosmochim. Acta, 65: 1367–1379.] to decrease in the order ferrihydrite>lepidocrocite>goethite>hematite. The abiotic rate of reductive dissolution thus correlates with the solubility of the iron oxyhydroxides. We investigated whether this also holds for the microbial reduction kinetics of Fe(III) iron oxyhydroxides. The solubilities of nanoparticulate hematite, lepidocrocite, ferrihydrite and amorphous Fe(III) oxyhydroxide were obtained from pe–pH titrations of oxyhydroxide/Fe 2+( aq) suspensions, in pH range 4 to 7. The solubility of low surface area (LSA) hematite was estimated from redox potential measurements at pH 2. The same solid phases and soluble Fe(III)-citrate were then reduced by the iron reducing bacterium Shewanella putrefaciens using lactate as the electron donor. In all cases, the microbial Fe(III) reduction rates exhibited saturation behavior with respect to the initial Fe(III) concentration. The maximum specific rate of reduction ( v max, in μmol cell −1 h −1) correlated positively with the solubility of the Fe(III) oxyhydroxides, with the highest values of v max for ferrihydrite and amorphous Fe(III) oxide, the lowest for LSA hematite. Hence, the solubility appears to be a rate-controlling parameter in both the abiotic and enzymatic reduction of Fe(III) oxyhydroxides.