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 4f7/2 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.
The extraction behavior of Ir(IV) in a phosphonium-based ionic liquid (IL) system, triethyl-n-pentyl phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][NTf2]), was investigated using an amine extractant. In addition, the electrochemical behavior of the extracted Ir(IV) species in [P2225][NTf2] during electrodeposition was studied. Voltammetry-based electrochemical analysis revealed that the reduction of Ir(IV) proceeded via an intermediate Ir(III) species in two steps, namely, Ir(IV) + e− → Ir(III) and Ir(III) + 3e− → Ir(0). Diffusion coefficients of the extracted Ir(IV) species in the IL were determined over the temperature range of 298–373 K using semi-integral and semi-differential analyses, and values obtained from the two analyses were consistent with each other. Furthermore, consecutive solvent extraction and electrodeposition of Ir metal in the IL bath were performed for 10 cycles. The entire Ir electrodeposit was in the metallic state, as is evident from the 4f 7/2 spectrum obtained by XPS.
It is important to develop solvent extraction and electrodeposition processes for platinum group metals in order to reduce the volume of secondary wastes. In this study, the electrodeposition behavior of the extracted Pt(IV) complex in a phosphonium-based ionic liquid (IL) was investigated by using an electrochemical quartz crystal microbalance (EQCM). The charge transfer reaction Pt(IV)+2e -→Pt(II) was observed at -0.53 V. The significant increase in the mass change (Δm) and the apparent molar mass (Mapp) of 193.7, as evaluated by the EQCM, confirmed that the electrodeposition reaction Pt(II)+2e -→Pt(0) proceeded at -1.65 V in this system. Moreover, consecutive solvent extraction and electrodeposition of Pt metal in Alamine336/IL system were performed at 10 cycles. High extraction percentage (E>95.1%) and good current efficiency (ε >85.8%) were maintained in the first to sixth cycles.The XPS Pt-4f7/2 spectrum confirmed that all the electrodeposits were in the metallic state.
It is essential to develop solvent extraction (SX), and electrodeposition (ED) processes for palladium to reduce the volume of acid and organic media. In this study, the extraction reaction of Pd(II) in chloride media using of N, N, N', N', N'', N''-hexaoctyl-nitrilotriacetamide (NTAamide(C8)) as a novel extractant has been demonstrated. Three diluents with high dielectric constants (acetophenone (AP), 1,2-dichloroethane (DCE), and 1-octanol (OC)) were used for the solvent extraction reaction. The slope analysis indicated that the anion exchange extraction reaction of Pd(II) was consistent with an approximately 2:1 stoichiometry for the NTAamide(C8)/AP and NTAamide(C8)/DCE systems, and 1:1 for NTAamide(C8)/OC. Moreover, the ED behavior of the extracted Pd(II) complex was investigated using cyclic voltammetry (CV). Palladium reduction was found to be an irreversible process based on analysis of the standard rate constant. A semi-integral analysis of the voltammogram determined the diffusion coefficients of the extracted Pd(II) complex in the AP, DCE, and OC systems to be 3.7±0.1 × 10 −10 , 2.8±0.1 × 10 −10 and 1.5±0.2 × 10 −10 m 2 s −1 , respectively. Furthermore, consecutive extraction-electrodeposition processes using the NTAamide(C8)/AP system were carried out for five cycles. High extraction percentage (E>91%) and current efficiency (ε>83.1%) were attained in all cycles. The electrodeposits recovered from the extraction-electrodeposition process were identified as Pd metal through X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analyses.
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