Cr(VI) mediated hydrolysis of algae cell walls to release TOC for enhanced biotransformation of Cr(VI) by a culture of Cr(VI) reducing bacteria
Hexavalent chromium [Cr(VI)], the most toxic form of chromium, is frequently released into the environment from anthropogenic sources. Cr(VI) mainly occurs in the oxyanionic forms, CrO42− and Cr2O72−. It is highly oxidative and carcinogenic under chronic and subchronic exposure conditions. Conventionally, Cr(VI) pollution is remediated by reducing Cr(VI) to Cr(IIII). Cr(III) is naturally less toxic than Cr(VI) and is a 1000 times less mobile in the aquatic phase than Cr(VI). Biological reduction and detoxification of Cr(VI) are viewed as the most ecologically friendly process for remediation of Cr(VI) pollution. However, fast reduction of Cr(VI) mainly occurs under aerobic conditions in the presence of organic carbon sources. In the current research, freshwater algae are utilized as a carbon source for Cr(VI) reduction with using symbiotic bacterial cultures. The algal species, Chlamydomonas reinhardtii and Chlorococcum ellipsoideum, were tested in their ability to serve as or produce a carbon source for locally isolated bacteria to achieve reduction of Cr(VI) to Cr(III). Batch experiments were conducted under aerobic conditions at different concentrations of Cr(VI) to determine the kinetics of the biological reduction reaction. In the batch experiments, complete removal of up to 50 mg L−1 of initial Cr(VI) concentration was achieved within 24 h. At 100 mg L−1 initial Cr(VI) concentration, the system could remove 92% of the Cr(VI). Algae was found to be very sensitive to Cr(VI) toxicity. The Cr(VI) inhibited the algae growth and reduced the chlorophyll a content and by extension the algae’s ability to undergo photosynthesis.
KeywordsPhytoremediation Freshwater algae Cr(VI) reduction Bioremediation Batch kinetic
The authors would like to thank the University of Pretoria and the Water Utilization and Environmental Engineering Division at the University of Pretoria for the research support. The research was partially funded by the National Research Foundation (NRF) of South Africa through Grant No. CSUR180215313534 awarded to Prof Evans M. N. Chirwa of the Department of Chemical Engineering and Maria Roestorff (Grant No: 114172). Additional funding was provided by the Sedibeng Water Chair in Water Utilization Engineering in the Water Utilization Division at the University of Pretoria.
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