Effect of oxidation reduction potential (Eh) on the biocatalytic oxidation of As3+ to As5+ in a mixed culture of chemoautotrophic bacteria

Abstract

Biocatalytic oxidation of Arsenite (As3+) to arsenate (As5+) in a mobilized and immobilized chemoautotrophic bacterium has been a challenging area of metallic bioremediation research. This is because arsenic in its trivalent form (As(III)) is a major water pollutant and a threat to the human health and the environment. Previous studies have shown that Gram-positive species: Bacillus sp., and Exiguobacterium sp, has the tendency to biologically catalyse or mediate the oxidation of As3+ to As5+. The redox process is believed to be thermodynamic feasible because of the Mo6+ content of the microbial cell sub unit, acting as an electron acceptor for As3+ oxidation with adequate amount of energy generated for cell growth and metabolism. The changes in arsenic oxidation state (i.e. +3 to +5) is a natural process, and it is not well understood. However, the current study investigates or validate the changes in oxidation state of arsenic from +3 to +5 in a mobilized cell at various Oxidation Reduction Potential (ORP) and pH subjected to different arsenic concentrations ranging from 50 mg/L to 100 mg/L. It was observed that there exists a proportional relationship between increasing the arsenic concentration and achieving high ORP values. The maximum ORP achieved in the anaerobic batch experiment increased from -15.9 mV at 50 mg/L, to -14.8 mV at 80 mg/L and to -12.5 mV at 100 mg/L. It was evident that the ORP increased at 100 mg/L from -16.1 mV to -12.5 mV, indicating possible electron switch. Arsenic oxidation was highly favoured in anaerobic conditions than in aerobic conditions.