Recovering precious metals from spent catalysts by peroxydisulfate-based advanced oxidation process

Abstract The recovery of precious metals from discarded wastes is an attractive potential remedy for their supply disruption risk. Nevertheless, the use of conventional solvents in metallurgical processes has significant negative environmental effects. Here, we report peroxydisulfate (PDS)-based advanced oxidation process (AOPs) to develop a novel and efficient leaching process for recovering precious metals from spent catalysts. The PDS/NaCl photochemical system could fully dissolve palladium (Pd) and gold (Au) in 300 min. By introducing Fe(II), the PDS/FeCl2·4H2O solution functioned as Fenton-like system, which enhanced the leaching efficiency and required no xenon (Xe) lamp light irradiation. Electron paramagnetic resonance (EPR) and 18O isotope tracing experiments revealed the reactive oxidation species of SO4·-, ·OH and Fe(IV)=O were responsible for the oxidative dissolution of precious metals. Density functional theory calculations showed that the total energy barrier for the three species were 7.62, 18.46 and 17.52 kcal mol-1, respectively. Solvent leaching and one-step electrodeposition recovered high-purity precious metals and maintained solvent dissolution and electrochemical stability after 5 cycles. Strong acids, poisonous cyanide, volatile organic solvents, light irradiation, and photocatalysts were not used during recovery. This work will enable a green and sustainable precious metal recovery approach and encourage AOPs technology for secondary resource recycling.