Green synthesis of platinum and palladium nanoparticles from spent automotive catalyst leachate using bioreduction

Metal-bearing solid waste is considered a secondary source of precious metals. Spent automotive catalyst (SAC) contains significant quantities of platinum group metals (PGM). The biorecovery of these metals from SAC is gaining widespread attention due to its economic and environmental advantages. However, most of the studies focused on the bioextraction of these metals and ignored their separation and precipitation from leach liquor. The few studies that investigated their separation used model synthetic solutions instead of real waste to recover these metals. This study used the bioreduction pathway to recover Pt and Pd from SAC leach liquor. A Gram-negative, metallophillic, and heavy metal-resistant bacterium Cupriavidus metallidurans is used to biosynthesize Pt and Pd nanoparticles. C. metallidurans undergo bioreduction which is an enzymatically-assisted metal precipitation process to biofabricate the Pt and Pd nanoparticles intracellularly, on the cell surface, and extracellularly. The viable cells of C. metallidurans showed a bioreduction efficiency of 65 % and 52 % of Pt (II) and Pd (II), respectively, from SAC leachate. Overall, this study shows the potential and efficacy of the biorecovery of Pt and Pd and the green synthesis of Pt and Pd nanoparticles from metal-bearing solid waste.