A P204/Guaiacol-Based Hydrophobic Deep Eutectic Solvent Modulating Proton Exchange for Highly Selective In(III)/Fe(III) Separation

The recovery of In3+ from zinc hydrometallurgy residue leachates is challenging due to its low concentration, the high concentration of interfering Fe3+, and the poor selectivity of conventional extraction systems. To address this, a novel hydrophobic deep eutectic solvent (HDES) composed of bis(2-ethylhexyl) phosphate (P204) and guaiacol (GUA) was developed for the selective separation of In3+ from Fe3+. After process optimization, the HDES achieved exceptional separation performance. Under optimal extraction conditions, with the initial In3+/Fe3+ concentration ratio of 1:10, an In3+ extraction efficiency of 98.61% and an Fe3+ co-extraction of 5.52% were obtained, corresponding to a high separation coefficient of 1222. This HDES was characterized by FT-IR and 1H NMR techniques. Thermodynamic and kinetic analyses were conducted. Mechanistic studies indicated that extraction proceeds via cation exchange, wherein GUA suppresses the reactivity of the P204 extractant, thereby effectively inhibiting Fe3+ co-extraction. Furthermore, efficient stripping of In3+ was achieved using dilute hydrochloric acid. This HDES-based method presents a highly selective and practical alternative for In3+ recovery from Fe3+-rich solutions.