Recycling of platinum group metals from exhausted petroleum and automobile catalysts using bioleaching approach: a critical review on potential, challenges, and outlook

Platinum group metals (PGMs) are essential in developing the modern high-tech industry. Spent petroleum and spent automobile catalysts are vital sources of PGMs, which have relatively higher concentrations of PGMs (0.1-1%) than those found in primary sources. Therefore, recycling these secondary PGMs sources is an integral element of the PGM supply policy of many countries. In recent years, 'bioleaching' has emerged as a sustainable technique to extract PGMs from the spent catalysts. Bioleaching processes are based on the capability of different microorganisms to mobilize PGMs through the production of different lixiviants (cyanide, organic acids, etc.). In this review article, we aimed to explore the potential of different microorganisms (cyanogenic bacteria and fungi) in leaching PGMs (Pt, Pd, Rh, and Re) from spent petroleum refinery catalysts and spent automobile catalysts. We also shed light on the role and mechanisms of these microorganisms in extracting PGMs from these spent catalysts. A detailed discussion is also being made on the effect of various process parameters concerning cyanogenic bioleaching. We also identified different technical and environmental challenges associated with bioleaching processes that need to be addressed before their scale-up at the commercial level. Our analysis suggests that cyanogenic bioleaching (biocyanidation) has exciting potential; however, specific challenges (slower kinetics and yield, toxicity) need to be addressed to scale the process from existing lab-scale to commercial application. [GRAPHICS] .