Development and evolution of biocyanidation in metal recovery from solid waste: a review

Solid waste, especially electronic waste is increasingly considered as secondary sources of base, critical, precious, rare and heavy metals. Some microorganisms that possess specific metabolic pathways, adapted to the recycling of these materials, have been shown to be a cost-effective resource in the bioleaching of such secondary sources. Bioleaching not only provides an efficient alternative to extract and recuperate metals, but it also provides a green approach for tackling environmental challenges. As such, while environmental concerns have limited the use of chemical cyanidation, biocyanidation is a more sustainable approach that utilises cyanogenic organisms that are able to solubilise gold and other noble metals via cyanogenesis and through the production of specific metabolites and siderophores. To illustrate this process, this review describes cyanogenesis and explores its use and associated challenges through the cyanogenic metabolic pathways (i.e. enzymes and intracellular functions), biocyanidation and metal complexation. In addition, the following process of metal-cyanide complex formation is also summarised. Finally, recent biotechnological developments, which promote recovery and provide guidance for the improvement of downstream processes for recovery from pregnant solutions, will be described.