Thermodynamic and kinetic constraints on microbial iron reduction.

Nanoparticulate, colloidal iron oxides showed a high reactivity in microbial reduction experiments. Surface-normalized reduction rates revealed that colloidal iron oxides are more reactive per unit surface as compared to their respective bulk materials. When colloidal iron oxides were added to a larger volume of bulk iron oxide, the reactivity of the bulk ferric iron was catalytically enhanced. These findings were validated on a column scale and demonstrated the high reactivity of colloidal iron oxides under flow and porous medium conditions. This showed the particle-size dependent reactivity of iron oxides in microbial reactions and allows the hypothesis that microbial iron reduction in the environment might have been underestimated so far. Furthermore, a concomitant physiological study showed that iron reducing bacteria can adjust their biomass formation to the available energy of the ferric electron acceptor