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).

Anaerobic bioremediation of tetrachloroethylene (PCE) under high concentration conditions is difficult. Anaerobic dechlorination of PCE occurs with synergetic reactions, fermentation, and sulfate-reduction; however, the way in which high concentrations of PCE affects these reactions is still poorly understood. This study aims to elucidate how high concentrations of PCE affect fermentation and sulfate-reduction, as well as PCE dechlorination. Laboratory dechlorination tests were performed using a wide concentration range of PCE between 2 and 125 mg/L added to a microbial consortium that had been continuously cultivated in the laboratory and completely dechlorinated PCE for over 4 years. Fermentation of lactate, reduction of sulfate, and dechlorination of PCE were monitored in addition to microbial activities based on RNA. All three reactions, fermentation, sulfate-reduction, and PCE dechlorination were observed to be inhibited. The inhibition for fermentation, sulfate-reduction, and dechlorination occurred when PCE concentrations were higher than 125, 75, and 30 mg/L, respectively. The fermenter, Anaerotignum, and the sulfate-reducer, Desulfosporosinus, were active when the dechlorination was inhibited with 30 mg/L of PCE. These findings suggest that there is interference of PCE dechlorination, despite the occurrence of fermentation and sulfate reduction. Bioaugmentation with a PCE dechlorinator that is tolerant to high PCE concentrations can be a possible solution for bioremediation of PCE when its concentrations are greater than 30 mg/L.