An investigation for biogenic cyanide distillation for gold recovery and cyanide bioremediation by Bacillus megaterium

Abstract

First time, we report the application of cyanide distillation process to selectively isolate biogenic cyanide (Bio-CN) generated by Bacillus megaterium for gold leaching, and to study the mechanism involved in the bioremediation of cyanide. We aimed to design a sustainable process for gold leaching that starts without chemical cyanide and ends with minimum hazardous waste. Results showed that Bio-CN generated by the bacteria reaches to 113 mg/L after 24 h. Baseline distillation experiments showed that almost entire cyanide from chemical solution (Chem-CN) is recoverable; however, only about 18% of the Bio-CN could be retrieved. Investigation about bioremediation property of B. megaterium at different ratios of Bio-CN and Chem-CN suggested that the presence of bacteria have negligible effect on cyanide conversion while the enzymes generated through bacteria activity play role in reducing the concentration of cyanide from expected concentration of 105 mg/L to less than 29 mg/L. Hydrolytic and substitution pathways are suggested as the main mechanisms of cyanide conversion/remediation by the bacteria. Potentiometric titration using AgNO3 was used to analyze the cyanide concentration and findings are also supported by UV–visible technique. Distillation experiments by using mixture of Bio-CN and Chem-CN solutions suggested that the partial recovery of cyanide could be associated with the complexation of CN with the organic materials in the biogenic solution, which needs further study. At similar cyanide concentration i.e., 15 mg/L, gold recovery from distilled cyanide (3.22 mg/L) was as high as from Chem-CN (3.15 mg/L), suggesting the successful functionality of distilled cyanide in gold leaching.