Biodegradation of free cyanide and subsequent utilisation of biodegradation by-products by Bacillus consortia: Optimization using Response Surface Methodology

Abstract

A mesophilic alkali-tolerant bacterial consortium belonging to the Bacillus genus was evaluated for its ability to biodegrade high free cyanide (CN- ) concentration (up to 500 mg CN- /L), subsequent to the oxidation of the formed ammonium and nitrates in a continuous bioreactor system solely supplemented with whey waste. Furthermore, an optimisation study for successful cyanide biodegradation by this consortium was evaluated in batch bioreactors (BB) using response surface methodology (RSM). The input variables, that is, pH, temperature and whey-waste concentration, were optimised using a numerical optimisation technique where the optimum conditions were found to be: pH 9.88, temperature 33.60 °C and whey-waste concentration of 14.27 g/L, under which 206.53 mg CN- /L in 96 h can be biodegraded by the microbial species from an initial cyanide concentration of 500 mg CN- /L. Furthermore, using the optimised data, cyanide biodegradation in a continuous mode was evaluated in a dual-stage packed-bed bioreactor (PBB) connected in series to a pneumatic bioreactor system (PBS) used for simultaneous nitrification, including aerobic denitrification. The whey-supported Bacillus sp. culture was not inhibited by the free cyanide concentration of up to 500 mg CN- /L, with an overall degradation efficiency of ≥99% with subsequent nitrification and aerobic denitrification of the formed ammonium and nitrates over a period of 80 days. This is the first study to report free cyanide biodegradation at concentrations of up to 500 mg CN- /L in a continuous system using whey waste as a microbial feedstock. The results showed that the process has the potential for the bioremediation of cyanide containing wastewaters.