Cr(VI) remediation in groundwater aquifer media using natural organic matter as carbon source

Abstract

Cr(VI) is toxic to biological systems due to its strong oxidizing potential. However, some microorganisms possess mechanisms that enable them to reduce the toxic of chromium [Cr(VI)] to the less toxic trivalent state [Cr(III)] either as a survival mechanism aimed at reducing toxicity around the cell or as a means of deriving metabolic energy for cell growth. Although Cr(VI) can be reduced by algae, fungi and phyto- and zooplanktons, bacteria have been demonstrated to be the most efficient Cr(VI) reducing agents. Bacteria can reduce Cr(VI) to Cr(III) either aerobically or anaerobically through a variety of biochemical pathways. In this study, a culture comprising of mostly Bacilli sp. and Enterococcus sp. achieved high levels of Cr(VI) removal in a continuous flow process under near anaerobic conditions. Up to 93 % removal was obtained under a continuous feed of 40 to 60 mg/L after operation for 45 days. No additional nutrients and carbon sources were added to the feed water as it was assumed that organic compounds produced by decaying roots and organic matter in the soil could serve as carbon sources. Organics in the soil were characterized using the HPLC, TOC Analyser and GC-MS and were determined to be mostly comprised of breakdown compounds of humic acid. The study demonstrated the feasibility of in situ bioremediation of Cr(VI) contaminated sites using selected cultures of Cr(VI) reducing bacteria while avoiding secondary pollution from additional nutrients.