Bacterial Cr(VI) reduction with internal carbon recirculation using freshwater algae as primary producers

Abstract

Chromium exits naturally in two oxidation states: hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)). Cr(VI) is carcinogenic and mutagenic to living organisms and Cr(III) is 100 times less toxic than Cr(VI). Conventional treatment of Cr(VI) involves the reduction of Cr(VI) to Cr(III), followed by the precipitation of Cr(III) as chromium hydroxide [Cr(OH)3(s)]. Several species of bacteria have been shown to reduce Cr(VI) to Cr(III) either as metabolic necessity or as detoxification strategy for survival. Biological reduction processes can be engineered to reduce cost. In this study, a well-studied locally isolated culture of bacteria is utilised to reduce Cr(VI) to Cr(III) while growing on carbon sources produced as algal metabolites. The algae are also tested in their ability to adsorb the reduced Cr(VI), thereby reducing the total chromium in solution. Locally isolated bacteria combined with algae achieved 100 % removal of Cr(VI) in 24 hours without additional carbon sources, whereas bacteria with added glucose also achieved 100% removal in less than 7 hours. The algae were not able to adsorb the reduced Cr(VI), but was instead used as a carbon source for Cr(VI) reduction. Without carbon sources the bacteria could only reduce 29 % of the Cr(VI) and up to 53 % in cell free spent algae media which contained external metabolites produced by algae. Utilizing carbon sources produced by algae would be more practical to implement in the real world than adding glucose. This demonstrates the potential of combining locally isolated Cr(VI) reducing bacteria and green algae to decontaminate Cr(VI) polluted sites in South Africa.