Bacterial surfaces inhibit the oxidation of Fe(II)

International audienceBacteria are known to affect the fate of metals through sorption reactions. Secondary minerals, such as iron-oxides, are another important phase scavenging metals in the environment. It has often been suggested that bacterial surfaces favor the precipitation of iron-oxide phases. In this study, we demonstrate that this is not the case. We have mimicked the processes occurring at an oxic interface, which lead to the formation of iron oxides, by introducing progressively a fixed total amount of Fe2+ ions at a fixed pH of 6.5 in an aerobic reactor in the absence and in the presence of increasing concentrations of Bacillus subtilis bacterial cells. Alternatively, increasing amounts of Fe2+ ions were added to a fixed concentration of Anoxybacillus flavithermus bacterial cells. The kinetics of the reaction were monitored by measuring the rate of addition of the base needed to maintain the pH constant. In another set of experiments, we added Fe2+ ions at once in anaerobic conditions and oxygenated the suspension afterwards. The rate of oxidation of the Fe(II) was then measured directly using the ferrozine colorimetric method. In both types of experiments, it was clearly observed that the kinetics of oxidation of the Fe(II) are dramatically reduced by the presence of the bacterial cells. This is an important result, as the oxidation state and the speciation of the sorbed Fe impact the immobilization by the bacterial-iron composite suspensions of other metals. For instance, we have observed that a given amount of Fe reduces the Cd sorption ability of the cells, and it does this more effectively when it occurs as a sorbed Fe(II) species as when it is precipitated as adhering iron oxide nanoparticles