Microbial chromium (VI) immobilization in saturated aquifer columns using culture inoculated soil

Abstract

Chromium exists in two oxidation states in the environment namely; Cr(VI) which is highly toxic, carcinogenic and mutagenic in nature and Cr(III) which is less toxic, mobile and is required in trace elements with human diets. South Africa holds approximately 72 % of the currently documented chromium ore reserves. Areas where chromium is mined, experience severe Cr(VI) contamination due to poor treatments of Cr(VI) by products. Cr(VI) is exclusively released through anthropogenic activities into wastewaters. Biological reactors have been investigated in reducing chromium successfully. Bioremediaion of Cr(VI) to Cr(III) has increasingly gained interest as it is seen as economic and ecological. In this study, in situ bioremediation of treating Cr(VI) aquifers is evaluated. Firstly, dried sludge containing indigenous chromium reducing bacteria (CRB) from a dried sludge was collected from Brits Wastewater Treatment Plant, North West Province (South Africa). Batch experiments by isolated bacteria were done under aerobic conditions. Cr(VI) reduction was recorded at 50, 100, 200 and 400 mg/L of Cr(VI) concentrations. Pseudomonas putida showed complete Cr(VI) reduction within 5 h of batch analysis. The continuous batch experiments were done using columns with five equally spaced sampling pots along the column length. Cr(VI) reduction profile was thus observed. At steady state there was complete reduction of Cr(VI) on all the columns including; sludge, saw dust and carbon source at 40 mg/L. The continuous batch system appeared to have certain amount of algae emerging after 30 d of operation, from the bottom of the column where there’s reduced Cr(VI) concentration and so it was cultivated and characterized. The presence of algae may show a symbiosis relationship involving a complex relationship of detoxification survival of algae or a food source for bacteria. Cr(VI) reducing effects of the columns was further tested against higher Cr(VI) concentrations at 60 mg/L. Near complete Cr(VI) reduction was observed within a couple of days of operation at this concentration. This study demonstrates the potential of biological Cr(VI) reduction using Cr(VI) reducing bacteria that has been isolated from polluted sites in South Africa.