Recovery of platinum by solvent extraction and direct electrodeposition using ionic liquid

Abstract

The diluent characteristics and the wide electrochemical window of the ionic liquid (IL), triethyl-n-hexyl phosphonium bis(trifluoromethyl-sulfonyl)amide; [P2225][NTf2], has been exploited for the extraction of Pt(IV) using tri-n-octylamine (TOA, R3N, R = (CH2)7CH3) hydrochloride, followed by direct electrodeposition as Pt metal from organic phase. The extraction mechanism of Pt(IV) with [R3N·HCl]/[P2225][NTf2] has been investigated from the slope analysis. As a result, it was revealed that the extraction mechanism of Pt(IV) was based on the following anion exchange extraction;[PtCl62−]aq + 2[R3N·HCl]org ⇔ [R3NH]2[PtCl6]org + 2[Cl−]aq (R = (CH2)7CH3)The viscosity and the ionic conductivity for the [R3NH]2[PtCl6] were satisfied by Vogel-Fulcher-Tamman (VFT) equation and each best-fit parameter can be estimated in this study. The electrochemical behavior for [R3NH]2[PtCl6] in IL was investigated by Electrochemical Quartz Crystal Microbalance (EQCM) at 373 K. It was revealed that the cathodic reaction: Pt(IV) + 2e− → Pt(II) was estimated from 0 V to −0.87 V and Pt(II) + 2e− → Pt(0) was proceeded from −0.87 V to −1.13 V considering from Mapp = 178.5 evaluated by CV/EQCM. Moreover, the alternation of Δηρ (product of viscosity and density) was corresponded to the locally decrease of the viscosity of IL near the electrode. Finally, the potentiostatic electrodeposition of [R3NH]2[PtCl6] allowed us to recover the blackish electrodeposits, which were identified as most of Pt metal by EDX and XRD analyses.