Study on the adsorption of precious metals from wastewater by XAD-2 resin loaded with functionalized deep eutectic solvent

This study prepared a novel adsorption material XAD-2-HDES with fast adsorption rate by impregnating XAD-2 resin with quaternary ammonium hydrophobic deep eutectic solvent (HDES). It was applied to the enrichment and recovery of various precious metal ions in aqua regia solution. The results showed that the adsorption efficiencies of Ir(IV), Pt(IV), Ru(III) and Rh(III) were 95.3 %, 99.8 %, 94.1 % and 43.9 %, respectively, under the following conditions: trioctylmethylammonium chloride (N263) to menthol (Men) molar ratio of 1:2, adsorbent dosage of 20 g L⁻¹, initial aqueous phase pH of 0.1, adsorption time of 2 h, and adsorption temperature of 298.15 K. Under optimal process conditions, the maximum loading of Ir(IV), Pt(IV), Ru(III), and Rh(III) were 19.4 mg g⁻¹, 20.6 mg g⁻¹, 4.6 mg g⁻¹ and 4.3 mg g⁻¹, respectively. The adsorption process of precious metal ions obeys the Langmuir model and quasi-second-order kinetic model, which is mainly controlled by the chemical reaction of the monomolecular layer. The surface loaded HDES of XAD-2-HDES provides abundant adsorption reaction sites, and the adsorption process is mainly driven by electrostatic attraction. The complex ions of precious metals undergo an anion-exchange reaction with Cl⁻ in N263 to form a stable hydrophobic type neutral complex species with N atoms as a bridge with HDES through ligand bonding, and form a new hydrogen-bonding network with the –OH group of Men to facilitate the dissolution of the complex.