Salt aqua regia as a green solvent for recovering precious metals

Spent catalysts are valuable secondary resources of precious metals. Traditional methods of recovering precious metals typically use highly corrosive aqua regia, which is extremely dangerous and leads to severe environmental pollution. Here, we report an alternative route to recover precious metals from spent catalysts using purely inorganic salt solutions consisting of Al(NO3)3·9H2O and NaCl, which we name “salt aqua regia.” The precious metals palladium and gold can be completely dissolved in it. In simulated and real recovery processes, high-purity palladium and gold products are obtained through leaching and one-step electrodeposition. The solvent keeps almost the same dissolution properties and electrochemical stability even after three cycles. As salt aqua regia is safe and non-volatile and has good solubility for precious metals and electrochemical stability, it has excellent prospects for application in precious metals recovery.