Recovery of Gold by Solvent Extraction and Direct Electrodeposition Using Phosphonium-Based Ionic Liquids

Abstract

In this study, phosphonium-based ionic liquids (IL), i.e., triethyl-n-pentyl, triethyl-n-octyl, and triethyl-n-dodecyl phosphonium bis(trifluoromethyl-sulfonyl)amide, [P222X][NTf2], (X = 5, 8, and 12) were investigated for Au(III) extraction. The IL–Au complex was identified as [P2225][AuCl4] using UV–Vis–NIR and Raman spectroscopic analyses. Slope analyses with the concentration dependence of [P222X +] confirmed the anion-exchange mechanism of Au(III) extraction by [P222X +] (X = 5, 8, and 12). The enthalpy, entropy, and Gibbs free energy for Au(III) extraction were determined using thermodynamic analysis, indicating that lower temperatures had a positive effect on the Au(III) extraction. Electrochemical analysis revealed that extracted Au(III) can be reduced in two steps: (i) Au(III) + 2e− → Au(I), (ii) Au(I) + e− → Au(0)]. The diffusion coefficients of the extracted Au(III) species in [P222X][NTf2] (X = 5, 8, and 12) were evaluated from 323 to 373 K using semi-integral and semi-differential analyses. Because of the viscosity of the IL medium, the diffusion coefficient of the extracted Au(III) increases with increasing alkyl chain length. The 4f 7/2 spectrum based on X-ray photoelectron spectroscopy revealed that the Au electrodeposits obtained after 10 cycles of continuous extraction and electrodeposition were in the metallic state.