Enhanced bio-reduction of hexavalent chromium by an anaerobic consortium using henna plant biomass as electron donor and redox mediator

The effects of henna (Lawsonia inermis) flower biomass on the enhanced bio-reduction of Cr(VI) were investigated in this study. The average Cr(VI) removal rate in the anaerobic system with the addition of henna flower powder achieved 2.04 mg L−1 h−1, which was much higher than the rates in the activated sludge control, autoclaved sludge control, and adsorption control. Under anaerobic conditions, the organic compounds in henna flower powder could be leached out from the solid phase to the mixed solution, and then fermented to volatile fatty acids and H2, acting as effective electron donors for the rapid bio-reduction of Cr(VI). The released and/or fixed lawsone not only shuttles electrons from the donor to the final acceptor of Cr(VI), but might also inversely improves the fermentation process of henna flower biomass.