Recovery of catalytic metals from leaching solutions of spent automotive catalytic converters using plant extracts

This study investigates the potential of hydroalcoholic extracts of Cistus ladanifer L., Erica Andevalensis and Rubus idaeus L. as a green method for the recovery of platinum group metals (PGMs) from both synthetic unimetallic solutions and multimetallic solutions obtained from the leaching of two different spent automotive catalytic converters (SACC). Experiments with unimetallic solutions revealed that E. andevalensis and R. idaeus extracts could separate about 70% of Pd and less than 40% of other tested metals (Al, Ce, Fe and Pt) from the solutions. Then, application of the plant extracts to two different SACCs leachates showed that E. andevalensis and R. idaeus extracts can induce high precipitation (> 60%) of Pd and Pt with co-precipitation of less than 20% of other metals. UV-Visible spectra analysis confirmed the bio-reduction of Pd2+ ions into Pd-0 nanoparticles by R. idaeus extract, and Fourier transform infrared spectroscopy (FTIR) analysis revealed the contribution of functional groups of the phytochemicals present in the extract (such as phenols, flavonoids and anthocyanins) in the Pd2+ bio-reduction and stabilization. Afterward, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) analysis of the precipitate obtained from one leachate with R. idaeus extract demonstrated the presence of Pd particles along with organic compounds and particles containing other metals. Therefore, particles were subjected to a washing step with acetone for further purification. Finally, scanning transmission electron microscopy with energy dispersive X-ray spectroscopy (STEM-EDX) analysis showed the high purity of the final Pd particles and high-resolution STEM allowed to determine their size variation of 2.5 to 17 nm with an average Feret size of 6.1 nm and confirmed their crystalline structure with an interplanar lattice distance of similar to 0.22 nm. This green approach offers various benefits including simplicity of Pd separation from the leachates as valuable nanoparticles that makes the process more feasible from economic and environmental standpoints. A process cost of similar to 20 $/g of Pd particles recovered was estimated (excluding manpower). [Graphical abstract]