Complexation mechanism of crown ether with indium in the presence of KI: Toward efficient recovery of indium from secondary resources

Abstract

The strong hydration of In3+ in the aqueous solution is a main factor which hinders the recovery of indium through a solvent extraction process. In the present study, KI was added to the indium-containing solution to form the complex ion of InI4− on the purpose of weakening the hydration of indium. The crown ethers (CEs) of benzo-12-crown-4 (B12C4), benzo-15-crown-5 (B15C5), and benzo-18-crown-6 (B18C6) were successfully synthesized, and the effects of KI concentration, CE cavity size, and CE/In mole ratio on the indium solvent extraction process were systematically investigated. The interactions of indium with water molecules and CEs during the solvent extraction process were theoretically analyzed through density functional theory (DFT) calculations from the aspects of geometric structure optimization, thermodynamics, as well as electrostatic potential. The complexation mechanism between CE and indium in the presence of KI was further clarified with Mass spectrum, 1H NMR spectrum, and SEM-EDS. Moreover, B18C6 showed excellent extraction kinetic and selectivity to indium that nearly 100% indium could be extracted in 0.5 min and the separation coefficients over the competing ions of Sn2+, Cu2+, Fe3+, and Al3+ were 67.03, 79.06, 73.41, and 70.60, respectively, showing the excellent application prospect of B18C6 for efficient indium recovery in the complex system.