Dual-function thiourea for photochemical recovery of precious metals in an Fe(iii)-oxalate based system†

It is urgent to bring out an eco-friendly and high-efficiency strategy for efficient hydrometallurgical recovery of precious metals (PMs) to avoid the use of highly corrosive and strongly poisonous solvents. In this work, it has been found that PMs of Au, Pt, and Pd can be almost completely recovered at room temperature (RT) via a simulated-sunlight (SSL) irradiation route. Maximum recovery amounts of Au, Pt, and Pd are 6.0, 1.2, and 5.0 mg L⁻¹ in an appropriate FOC-T system including 0.02/0.05/0.05 mol L⁻¹ Fe(iii)-oxalate complexes (FOC) and 40/60/60 g L⁻¹ thiourea, respectively. Thiourea plays a dual-function role in increasing the amount and prolonging the lifetime of reactive oxygen radicals in the photo-dissolution system by the generation of sulfate radicals from thiourea oxidation during the photodegradation of FOC, and constructing the coordination of PMs during the photochemical dissolution process using thiocarbonyl groups of thiourea as the active coordination sites. As a result, the FOC-T system exhibits a superiority in the recovery performance of PMs with a low FOC concentration compared with the FOC-Cl system (∼100% versus <20%), and the obtained PM-containing lixivium can act well as a precursor similar to commercial H₂PtCl₆, HAuCl₄, and (NH₄)₂PdCl₄ for catalyst preparation. In brief, this work provides a new approach for the photochemical recovery of PMs in a green FOC-T dissolution system and enriches the development of novel recovery methods for PMs.