Fe(II) oxidation after microbial Fe(III) reduction does not cause Cd release

The flooding and drainage of soil can cause the reduction and oxidation of iron (Fe), as well as the immobilization and mobilization of cadmium (Cd). However, the impact of Fe(II) oxidation following microbial Fe(III) reduction on Cd mobility remains unclear. We examined the behavior of Cd during microbial reduction of Fe(III) oxides and subsequent chemical re-oxidation of Fe(II) using batch reactor systems. The bacterium Shewanella oneidensis MR-1 was incubated with ferrihydrite, lepidocrocite, goethite, or hematite anaerobically and then aerobically in media containing 212 μg L-1 Cd, with or without pH buffering (initial pH = 7.0). Compared to control systems without MR-1, microbial Fe(III) reduction significantly promoted the immobilization of dissolved Cd, as well as the conversion of dissolved and adsorbed Cd to strongly bound Cd that could not be extracted by 0.4 mol L-1 HCl. The mechanisms of Cd immobilization were different during the microbial reduction of different Fe(III) oxides. The buffering of system pH affected the phase of Fe oxide formed during the reduction and oxidation stages in the systems containing MR-1 and ferrihydrite or lepidocrocite. Nevertheless, in all the systems containing MR-1 and Fe oxide—irrespective of pH buffering—the concentration of dissolved Cd dropped to < 1 μg L-1 after 35 d of anaerobic incubation and remained < 4 μg L-1 after subsequent 72 h of aerobic incubation. This suggests that Fe(II) oxidation after microbial Fe(III) reduction (generating 0.025–0.22 g L-1 Fe(II)) does not result in Cd release.