Exploration of the reduction mechanism of Cr(VI) in anaerobic hydrogen fermenter.

The bio-reduction of hexavalent chromium (Cr(VI)) by anaerobic fermentation is considered as a promising, low-cost and environment-friendly way. However, it is unclear for the reduction mechanisms of Cr(VI) in an anaerobic hydrogen fermenter, such as reduction kinetics, related electron donors, migration and transformation, reduction site and key components, and related microorganisms. To clarify these issues, a hydrogen fermenter was designed to reduce Cr(VI) at 55 °C with glucose as initial substrate. Results show that 100 mg/L Cr(VI) can be completely reduced (99.5%) to trivalent chromium (Cr(III) through chemical and biological reactions. Bio-reduction dominates Cr(VI) removal in a first-order exponential decay mode with both glucose and its metabolites (volatile fatty acids) as electron donors. Moreover, volatile fatty acids are more suitable as electron donors for Cr(VI) bio-reduction than glucose. Bacilli, Clostridia and Thermotogae in the fermenter dominated the reduction of Cr(VI) by regulating the production and composition of extracellular polymers (EPSs), in which carboxyl and hydroxyl groups play an important role for Cr(VI) reduction by coordination. The results can guide us to regulate the bio-reduction of Cr(VI), and provide reference for the development of bio-reduction technology of Cr(VI).