Co-metabolism of thiocyanate and free cyanide by Exiguobacterium acetylicum and Bacillus marisflavi under alkaline conditions.

The continuous discharge of cyanide-containing effluents to the environment has necessitated for the development of environmentally benign treatment processes that would result in complete detoxification of the cyanide-containing wastewaters, without producing additional environmental toxicants. Since biological detoxification of hazardous chemical compounds has been renowned for its robustness and environmental-friendliness, the ability of the Exiguobacterium acetylicum (GenBank accession number KT282229) and Bacillus marisflavi (GenBank accession number KR016603) to co-metabolise thiocyanate (SCN(-)) and free cyanide (CN(-)) under alkaline conditions was evaluated. E. acetylicum had an SCN(-) degradation efficiency of 99.9 % from an initial SCN(-) concentration of 150 mg SCN(-)/L, but the organism was unable to degrade CN(-). Consequently, B. marisflavi had a CN(-) degradation efficiency of 99 % from an initial concentration of 200 mg CN(-)/L. Similarly, the organism was unable to degrade SCN(-); hence, this resulted in the evaluation of co-metabolism of SCN(-) and CN(-) by the two microbial species. Optimisation of operational conditions was evaluated using response surface methodology (RSM). A numeric optimisation technique was used to evaluate the optimisation of the input variables i.e. pH, temperature, SCN(-) and CN(-) concentrations. The optimum conditions were found to be as follows: pH 9.0, temperature 34 °C, 140 mg SCN(-)/L and 205 mg CN(-)/L under which complete SCN(-) and CN(-) degradation would be achieved over a 168-h period. Using the optimised data, co-metabolism of SCN(-) and CN(-) by both E. acetylicum and B. marisflavi was evaluated, achieving a combined degradation efficiency of ≥99.9 %. The high degradative capacity of these organisms has resulted in their supplementation on an active continuous biological degradation system that is treating both SCN(-) and CN(-).