Catalytic bio-oxidation of toxic Arsenite (As(III)) to Arsenate (As(V)) in a mixed culture of bacteria strains isolated from cow dip in Tzaneen (Limpopo Province South Africa) was investigated under anaerobic condition. Anaerobic bio-treatment of As(III) is thermodynamically feasible since facultative microbes perform a significant role in the biogeochemical cycling of (As) under anaerobic environment, while deriving energy for cell growth and metabolism. If this metalloid is not efficiently treated before disposal of wastewater, it could result in cancer even at very low concentrations. An experiment was conducted and conditions were enhanced by varying different parameters such as pH, carbon source, oxidation-reduction potential (ORP),and As(III) concentration levels (20, 30, 50,100, 300, 500, and 1000 mg/L), under anaerobic condition, utilizing HCO3 as a carbon source. As(III) oxidizing activity was optimum at temperature of 32±0.3oC, ORP in the range of -60 to -180 MV and pH of 7±0.3, while incomplete oxidation was observed at pH of 1, 4 and 10.
Average of 50 % As(III) oxidation efficiency was observed at pH 1, followed by 30 %, 16 % and 99 % at pH 4, 10 and 7.3. At different initial As(III) concentrations, results showed that at concentrations = 500 mg/L, near complete As(III) oxidation was achieved with corresponding increases in As(V) concentrations. Inhibition effect was observed when As(III) concentration was set at = 1000 mg/L. This study suggests that the isolated bacteria strains developed a mechanism to resist and detoxify As(III) in the arsenic contaminated site under anaerobic condition. Initial evaluation of the bacteria using 16S rRNA partial sequence method showed that cells in the mixed culture comprised predominantly of the Gram-positive species: Bacillus sp., and Exiguobacterium sp.