Bioleaching of platinum, palladium, and rhodium from spent automotive catalyst using bacterial cyanogenesis

Bioleaching of platinum group metals (PGM) from spent automotive catalyst (SAC) was studied using two mesophiles, Pseudomonas fluorescens and Bacillus megaterium. Leaching studies under different conditions were carried out to investigate the formation of biocyanide and its role in PGM solubilisation. Both P. fluorescens and B. megaterium produce cyanide as a secondary metabolite that forms water-soluble complexes with PGM. Factors that affect bioleaching, namely initial pH of culture, free cyanide ion concentration, and pulp density were examined. Two leaching strategies - two-step bioleaching and spent medium leaching were applied. P. fluorescens showed higher extraction compared to B. megaterium. Using spent medium leaching for pretreated SAC at 0.5% (w/v) pulp density and initial pH 10, P. fluorescens yielded the highest extraction of platinum (58%), palladium (65%) and rhodium (97%) on day 1. This study demonstrates the bioleaching potential of cyanogenic bacteria for the extraction of PGM from SAC.