Fast removal of toxic hexavalent chromium from an aqueous solution by high-density Geobacter sulfurreducens

Hexavalent chromium (Cr(VI)) is a carcinogenic compound that can be removed from contaminated sites by the activity of metal-reducing 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 x 10(8) cells ml(-1), 99.0 +/- 0.8% of 100 mg l(-1) Cr(VI) was removed after 20 min. With a higher starting Cr(VI) concentration of 200 mg l(-1), G. sulfurreducens with a density of 11.4 x 10(8) cells ml(-1) 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 re-oxidation. 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. (C) 2020 Elsevier Ltd. All rights reserved.

Automated summary

Geobacter sulfurreducens can efficiently remove hexavalent chromium from contaminated sites by reducing it to less toxic chromium (III) with the help of extracellular proteins. The high-density cultures of G. sulfurreducens have shown great potential for bioremediation applications, such as the treatment of industrial wastewaters.