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.
