A Green and Sustainable Process for the Recovery of Gold from Low-Grade Sources Using Biogenic Cyanide Generated by Bacillus megaterium: A Comprehensive Study

The decreasing grade of gold deposits and environmental regulations concerning the use of cyanide, a conventional extraction agent used in gold recovery, has highlighted the challenge in the field of gold extraction. The bioleaching of gold using microorganisms for in situ cyanide production to recover gold is a promising new approach for prohibiting the use of conventional chemical cyanide. For the first time, this study reports a comprehensive investigation on the role of pH on individual steps of Bacillus megaterium bacterial growth, cyanide production, and gold recovery. This novel way of maximizing biogenic cyanide is highly efficient and demonstrates biocyanidation as a potentially viable technique for direct treatment of sulfidic gold ores, i.e., eliminating the step of biooxidation for the first time. The low-grade sources were treated with both chemical cyanide and biogenic cyanide generated by B. megaterium. Results indicate that the bacteria generated a maximum of 61.89 ppm cyanide, which correspondingly recovered over 87% and 43% gold from O1 and O2 sources, respectively, comparable to gold recoveries by chemical leaching. The pure gold powder leaching was also performed to evaluate the loss in gold recovery due to the biosorption of gold to bacterial cells. The feasibility of the work is also supported by green chemistry metrics, and a comparison has been made between conventional and biocyanide leaching. The results revealed the potential of biocyanidation for industrial applications.