A natural deep eutectic solvent-based aqueous biphasic system coupled with MoS₂ photocatalytic reduction for green recovery of gold from thiosulfate solution

Aqueous two-phase system (ATPS) extraction combined with a natural deep eutectic solvent (NADES) is an environmentally friendly separation process. In this work, ATPSs were constructed for the first time using NADESs, inorganic salts, and water to extract gold thiosulfate from alkaline solutions. The crucial parameters were optimized, including the types of NADESs and salts, equilibrium pH, contact time, and the initial concentration of Au(I). Thermodynamics studies showed that Au(S2O3)23− was effectively extracted into a NADES-rich phase, and this was mainly driven by an increase in entropy. Studies on the extraction mechanism indicated that the anion Au(S2O3)23− replaces the anion Cl− in NADES molecules to generate stable ion pairs with N+ through electrostatic interaction; simultaneously, new hydrogen bonds form with the acidic component. Also, amorphous MoS2 was used directly for the photocatalytic reduction of gold in the NADES-rich phase. The results showed that MoS2 can be used for efficient recovery of Au(I) and it simultaneously regenerates the NADES without destroying the structure of the extractant. Finally, a continuous cyclic experimental model is constructed. After three cycles, the extraction rate of Au(S2O3)23− by NADES–ATPS is slightly reduced from 96.2% to 93.0%, and the reduction rate of Au(S2O3)23− by MoS2 is decreased from 100.0% to 95.0%; this indicates that NADES–ATPS had excellent cycling performance. The strategy of coupling NADES–ATPSs with photocatalytic reduction with amorphous MoS2 to recover gold from thiosulfate solution is expected to contribute to the sustainable and green industrial processes of gold.