Application of enhanced bioreduction for hexavalent chromium-polluted groundwater cleanup: Microcosm and microbial diversity studies

Hexavalent chromium (Cr6+) is a commonly found heavy metal at polluted groundwater sites. In this study, the effectiveness of Cr6+ bioreduction by the chromium-reducing bacteria was evaluated to remediate Cr6+-contaminated groundwater. Microcosms were constructed using indigenous microbial consortia from a Cr6+-contaminated aquifer as the inocula, and slow-releasing emulsified polycolloid-substrate (ES), cane molasses (CM), and nutrient broth (NB) as the primary substrates. The genes responsible for the bioreduction of Cr6+ and variations in bacterial diversity were evaluated using metagenomics assay. Complete Cr6+ reduction via the biological mechanism was observed within 80 days using CM as the carbon source under anaerobic processes with the increased trivalent chromium (Cr3+) concentrations. Cr6+ removal efficiencies were 83% and 59% in microcosms using ES and NB as the substrates, respectively. Increased bacterial communities associated with Cr6+ bioreduction was observed in microcosms treated with CM and ES. Decreased bacterial communities were observed in NB microcosms. Compared to ES, CM was more applicable by indigenous Cr6+ reduction bacteria and resulted in effective Cr6+ bioreduction, which was possibly due to the growth of Cr6+-reduction related bacteria including Sporolactobacillus, Clostridium, and Ensifer. While NB was applied for specific bacterial selection, it might not be appropriate for electron donor application. These results revealed that substrate addition had significant impact on microbial diversities, which affected Cr6+ bioreduction processes. Results are useful for designing a green and sustainable bioreduction system for Cr6+-polluted groundwater remediation.