Hexavalent chromium (Cr(VI)) is a carcinogenic compound that can be removed from contaminated sites by the activity of metal-educing bacteria. The model bacterium Geobacter sulfurreducens reduces Cr(VI)
to less toxic Cr(III) and accumulates Cr ions intracellularly. However, this process is usually slow with
small concentrations of Cr(VI) removed in a matter of days. Here, high-density G. sulfurreducens cultures were tested for the capacity to remove Cr(VI) readily. With an initial G. sulfurreducens density of
5.8  108 cells ml1, 99.0 ± 0.8% of 100 mg l1 Cr(VI) was removed after 20 min. With a higher starting
Cr(VI) concentration of 200 mg l1, G. sulfurreducens with a density of 11.4  108 cells ml1 removed
99.0 ± 0.4% Cr(VI) after 2 h. Experiments performed with cell-free spent medium indicate that extracellular proteins are major contributors for the reduction of Cr(VI) to Cr(III). Furthermore, results show
that most Cr(III) ions ultimately end up inside the bacterial cells where they are less susceptible to reoxidation.
The fast Cr(VI) removal rates observed with high-density G. sulfurreducens demonstrate the potential of this bacterium for bioremediation applications such as the cleaning of industrial
wastewaters.