Reduction of iron oxides enhanced by a sulfate-reducing bacterium and biogenic H2S

Interactions between bacteria and minerals at low temperatures often lead to accelerated alteration and transformation of mineral phases through dissolution and precipitation. Here we report the reductive dissolution of ferrihydrite, goethite, hematite, and magnetite by the sulfate-reducing bacterium Desulfovibrio desulfuricans strain G-20. The goal of this study was: (1) to investigate iron reduction by G-20 using iron as the sole electron acceptor and (2) to determine whether iron reduction could be enhanced during bacterial sulfate reduction. In the absence of sulfate, G-20 was capable of enzymatically reducing structural Fe3+ from different iron-oxide phases including ferrihydrite (4.6% of total iron reduced), goethite (5.3%), hematite (3.7%), magnetite (8.8%) and ferric citrate (23.0%). Enzymatic reduction of goethite and hematite was comparable to abiotic reduction by N2S using the same medium. Within 3 weeks, the maximum cells-density increased 13-fold in the magnetite culture and 5-fold in the ferric-citrate culture compared to cell densities at the beginning. In the presence of sulfate, iron reduction was significantly enhanced in all bacterial cultures. The amount of reduced iron was 64.3% of total iron for hematite, 73.9% for goethite, 97.3% for magnetite, and nearly 100% for ferric citrate and ferrihydrite after incubation for 156 hours. The accelerated dissolution of the iron oxides under sulfate-reducing conditions was due to strong interplay between cell growth and redox-reactions between ferric iron and biogenic sulfides. Analysis by transmission electron microscopy and electron-dispersion spectroscopy indicated extensive alteration of the crystals of goethite, hematite, and magnetite, and revealed changes in stoichiometry of iron sulfides after 1 year's incubation.