Optimizing Biogenic Cyanide Production Using Indigenous Cyanogenic Microorganisms for Bioleaching of Precious Metals

The present study investigated and optimized biogenic cyanide (bio-CN) production by using mine indigenous bacterial strains with the aim of extracting precious metals from Upper Group Two (UG-2) platinum group metals (PGM) ore. Among all the bacterial strains tested, under optimized conditions, Pseudomonas brassicacearum produced the highest bio-CN (14.1 ± 1 mg/L) and was therefore used in subsequent experiments. The study examined glycine consumption during cyano-genesis, as well as the stability and speciation of bio-CN. The findings confirmed that while glycine was consumed during cyanogenesis, the produced bio-CN was not stable and decreased over time, transforming into different cyanide species. The dose–response analysis for P. brassicacearum showed EC50 of 1785 g/L and 107.9 g/L against untreated and pre-treated PGM concentrate, respectively, suggesting higher toxicity for the latter. Two-step bioleaching of the pre-treated PGM concentrate showed extractions of 75.7, 18.7, 9.4, and 0.3% for Au, Pd, Rh, and Pt, respectively, at 10 g/L pulp density. The findings on bio-cyanide stability and bio-CN speciation presented in this paper are novel contributions. This study has established a foundation for developing a complete bio-based approach to PGM bioprocessing from UG-2 ores.