Recovery of precious metals from ammoniacal thiosulfate solutions by hybrid mesoporous silica: 2 – A prospect of PGM adsorption

• Novelty in the application of hybrid mesoporous adsorbents for precious metals in thiosulfate medium.
• First report on successful recovery of PGM in neutral-pH solution by adsorption.
• Commercial attractiveness due to the facile and economically-favored hybrid mesoporous silica preparation.
• Bringing metal recovery solutions for a safer and cleaner extraction of precious metals.

Thiosulfate hydrometallurgy is known as a safer and less hazardous alternative to harsher and unsafe technologies (e.g. using cyanide or halides) mainly from environmental aspects. Thiosulfate leaching have long been known and practised in the research on alternative gold ore extraction. In this study, gold adsorption from simulated thiosulfate leach solution is expanded to platinum group metals (Pt, Pd and Rh) for the purpose of solution concentration/purification. Hybrid mesoporous MSU-X silica functionalized with thiol and amine groups were prepared and used as adsorbents. It was found at equimolar (namely ⩾0.10 mM) concentrations of PGM and gold in neutral-pH thiosulfate solutions, similar adsorption capacities and metal recoveries (as high as 80 wt%) on thiol and amine-functionalized adsorbents were achievable, disregarding the ranges of material’s pore size and pore structure. Adjusting the redox potential (toward +300 mV, SHE) helped preventing PGM precipitation in the neutral pH thiosulfate solution. Adsorption recoveries of both gold and platinum on thiol (–SH) ligands were also improved at such higher redox potential in neutral equilibrium pH. Similar improvements were obtained in the presence of ammonia in alkaline pH simulated conditions.