In situ bioremediation of hexavalent chromium by permeable reactive barrier using wastewater sludge bacteria

Abstract

Environmental pollution is a global problem that affects both developed and developing countries by contaminating soil and water, threatening biodiversity, ecosystems, and human health. South Africa holds the largest chrome ore reserves in the world, and it is one of the largest producers of ferrochrome. During steel and chromate production, enormous quantities of ferrochrome wastes are generated and discarded in dumps. This waste has been shown to contain significantly higher levels of Cr(VI) than the maximum acceptable risk concentration that is allowed for waste disposal in South Africa, which becomes a serious concern for soil and groundwater pollution. There are various conventional technologies available for minimizing the environmental impact of Cr(VI), including chemical reduction, ion exchange, electrochemical treatment, membrane separation, etc. However, most of these technologies are often ineffective and very expensive, especially for low concentrations of metals. Additionally, the use of chemical reagents produces an enormous amount of hazardous sludge that requires further treatment. The bioreduction of toxic Cr(VI) to less toxic Cr(III) using microbial organisms is considered a valuable, promising, and cost-effective approach for Cr(VI) remediation. In this study, using batch and continuous flow bioreactor systems, the efficiency of the indigenous culture of bacteria from the local wastewater treatment plant located near the contaminated site was evaluated for Cr(VI) reduction potential. The Cr(VI) reduction capability and efficiency of the isolated bacteria were investigated under a range of operational conditions, i.e., pH, temperature and Cr(VI) loading in a batch system. The culture showed great efficiency in reduction capability, with 100% removal in less than 4 h at a nominal loading concentration of 50 mg Cr(VI)/L. The culture showed resilience by achieving total removal at concentrations as high as 400 mg Cr(VI)/L. The consortia exhibited considerable Cr(VI) removal efficiency in the pH range from 2 to 11, with 100% removal being achieved at a pH value of 7 at a 37 ± 1 °C incubation temperature. The ability of the mixed bacterial consortium to treat Cr(VI) may be explored further in a continuous flow process for practical application. The effectiveness of bioremediation of Cr(VI) contaminated water using biological permeable reactive barrier technology was evaluated through bench-scale studies. Successful Cr(VI) reduction was achieved with 95.9% removal over the 90 days operational period of the BPRB system. When glucose was used as the carbon source, a drastic decline in effluent pH from 6.91 to below 5.5 was observed in the effluent. The decrease in pH values was ascribed to the oxidation of glucose forming several types of organic acids by different Bacillus species and other bacterial species which result in a subsequent drop in medium pH. However, it did not influence the overall reactor performance. These results could also be effective in optimizing and improving the operation and performance of in situ bioremediation of Cr(VI) at target sites. Cr(VI) reduction kinetic parameters in both batch and continuous-flow systems were estimated using a modified non-competitive inhibition model with a computer program for simulation of the aquatic system (AQUASIM 2.0). Further studies are required to understand the interaction of bacteria with other heavy metals that co-exist with Cr(VI) in the environment and also to evaluate the effect of operating the BPRB under various HRTs while occasionally backwashing or dislodging the accumulated precipitate from the system. Finally, experiments should be conducted with real contaminated groundwater to study the effect of different chemical compositions and conditions of contaminated water on the Cr(VI) removal efficiency by bacteria and the hydraulic behaviour of the used mixtures.