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-.