Can microbial Bio-CN be a sustainable alternative to the chemical cyanidation of precious metals? An update and way forward

The global demand for raw materials in technology, combined with environmental regulations and growing social awareness about the negative impacts of traditional metal extraction processes, has led to a shift towards green technology. Green technology has been defined as the use of science and technology to create less harmful products that protect the environment. Since biogenic cyanide (bio-CN) is naturally produced by microorganisms and is biodegradable, its use in precious metals (PMs) recovery from metal-bearing resources can be considered a viable and green alternative to chemical cyanidation. Several microorganisms are known to produce bio-CN, however, only a few such as Chromobacterium violaceum (C. violaceum), Pseudomonas fluorescence (P. fluorescence), Bacillus megaterium (B. megaterium), Pseudomonas aeruginosa (P. aeruginosa), Pseudomonas chlororaphis (P. chlororaphis) have been quantified. The present review summarizes research on cyanogenic microorganisms known to produce bio-CN, the mechanism and metabolic pathways involved in bio-CN production, and the genetics of cyanide production. This paper also discusses the factors influencing bio-CN production and the methods used for its quantification. The application of bio-CN in leaching PMs i.e., gold (Au), palladium (Pd), platinum (Pt), and rhodium (Rh) from primary and secondary resources, pre-treatment approaches used, and the mechanism of residual bio-CN detoxification are systematically and comprehensively reviewed and provided. Further, the review provides essential insights into challenges faced during bio-CN production and its application in PMs leaching and offers insight into new ways of thinking to move the process towards commercialization.