Microbial CR(VI) reduction in indigenous culture of bacteria: characterization and modelling

Abstract

South Africa currently faces multiple Cr(VI) contamination problems which are unsuccessfully remediated using available technologies. Cr(VI) is highly toxic, carcinogenic and mutagenic in nature and it is exclusively released through anthropogenic activities. A new treatment approach is proposed using locally isolated Cr(VI) reducing species of bacteria. This method is envisioned to be economical and ecologically friendly. Indigenous chromium(VI) reducing bacteria (CRBs) were isolated from a dried sludge consortium collected in the Brits Wastewater Treatment Plant, North-West Province (South Africa). Characterisation using 16S rRNA fingerprinting followed by taxonomic studies revealed a wide diversity of CRBs isolated under anaerobic conditions than under aerobic conditions. The consortium was determined to be predominantly gram-positive. The Cr(VI) reducing component of the culture was determined to be predominantly facultative, consisting predominantly of Bacillus sp., i.e. B. cereus, B. thuringiensis and B. mycoides. Batch experiments under both aerobic and anaerobic conditions showed a high Cr(VI) reducing performance under relatively high initial Cr(VI) concentrations. The reduction rate using this culture was 3 to 8 times higher than reduction rates reported in bacteria previously isolated and studied in North America and Europe. The culture performed best as a consortium with the different species operating cooperatively. The bacteria were acclimated to Cr(VI) toxicity through the long period of contact during the activated sludge treatment process at the source. A Monod like model was used to evaluate the rate of Cr(VI) reduction over a wide range of initial Cr(VI) concentrations. The model revealed that Cr(VI) reduction in the consortium culture followed quasi-first order kinetics with a Cr(VI) inhibitor term as a second exponential: C = C0 . exp [-p . exp (-q . C0 ) . t]. The parameter p and q for the semi-empirical first order model were statistically accurate with R2 values greater than 94% for all data ranges evaluated. Previous studies were not able to pick the variability of Monod coefficients, kmc and Kc, since at narrow ranges of initial Cr(VI) concentrations, the impact of the chromium toxicity variability was insignificant. This study demonstrates the potential of a biological approach using locally isolated Cr(VI) reducing bacteria to decontaminate Cr(VI) polluted sites in South Africa.