Biocatalytic oxidation of Arsenic (III) with concomitant Chromium (VI) reduction by an autotrophic culture of bacteria

Abstract

Environmental sustainability has become a leading discussion in the world, coupled with a quest to minimise severe environmental pollution. The chrome mining and ferrochrome processing industrial revolution has contributed significantly to the world’s environmental pollution. It is of interest to note that there are about 14 ferrochrome smelter plants in South Africa, and each plant generates considerable amounts of wastewater, requiring efficient treatment before discharge. If not properly managed, it could find its way into the environment (e.g. underground water), rendering it unsuitable for human consumption. These industries often generate a wide range of toxic waste containing Cr(VI), As(III), cyanide and other co-pollutants that are either difficult or expensive to treat. This study proposed the biochemical treatment of multiple pollutants by detoxification of Cr(VI) to less toxic Cr(III), and this is used to provide energy and oxidizing potential to achieve detoxification of As(III) through oxidation to the less mobile As(V) species. Theoretically, the process is shown to be thermodynamically feasible with a Cr(VI) to As(III) stoichiometric mole ratio of 3:2. Simultaneous detoxification of Cr(VI) and As(III) was achieved by using a mixed culture of chemoautotrophic anaerobic bacteria isolated from a cow dip site previously contaminated with arsenic, and from a wastewater treatment plant that received high levels of Cr(VI) from a nearby abandoned chrome ore refinery. Earlier experiments indicated that Cr(VI) is non-inhibitive to cultures from the Brits Wastewater Treatment Plant at levels below 99 mg Cr(VI)/L. Results obtained in the current study showed for the first time: Cr(VI) reduction coupled to As(III) oxidation, with As(III) serving as the principal electron donor. Experiments conducted with As(III) concentration ranging from 60-500 mg/L at a constant Cr(VI) concentration of 70 mg/L showed that As(III) enhanced Cr(VI) reduction rate at non-inhibitive Cr(VI) concentration (<100 mg Cr(VI)/L). With increasing Cr(VI) concentration to values greater than 100 mg/L, the redox process was inhibited. No Cr(VI) reduction and As(III) oxidation was obtained in cell-free cultures (control 1) and killed cells (control 2), which conclusively showed that the observed As(III) oxidation/Cr(VI) reduction was metabolically mediated. The 16S rRNA genomic sequence analysis indicated the predominance of Exiquobacterium profundum, Bacillus licheniformis, and Staphylococcus epidermidis as predominant species in the mixed culture. A bench-scale study with immobilised glass and ceramic bead media bed reactors linked to anaerobic tank retrofit (CSTR) shows that both reactors were efficient in reducing Cr(VI) to a lower concentration, with Cr(VI) removal efficiency exceeding 90%. It was noted that the attached growth reactor was not only affected by system overload (>100 mg Cr(VI)/L and 170 mg As(III) /L), but also by low hydraulic detention time (<5 h), which was characterized by the increase in effluent Cr(VI) concentration. Biokinetic parameters of these strains in the batch and continuous-flow system were estimated using a modified non-competitive inhibition model with a computer program for simulation of the aquatic system AQUASIM 2.0.