Rh(III) Aqueous Speciation with Chloride as a Driver for Its Extraction by Phosphonium Based Ionic Liquids.

Lenka Svecova,Isabelle Billard,Nicolas Papaïconomou

doi:10.3390/molecules24071391

Lenka Svecova, Isabelle Billard + Show 1 more

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https://doi.org/10.3390/molecules24071391

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Abstract

In this work, the aqueous speciation of Rh(III) in chloride medium was investigated by UV-vis spectroscopy for ligand to metal ratios R = (Cl−)/(Rh) ranging from 300 to 5000, at fixed Rh concentration (2.4 × 10−3 M). Under the chemical conditions of this work, no time evolution was observed, which allows for the fitting of the UV-vis data by Principal Component Analysis (PCA) and Multi-Curve Resolution (MCR). From this, and by comparison with literature data, the three independent species [RhCl4]−, [RhCl5]2− and [RhCl6]3− were identified, their individual absorption spectra derived and their respective contribution to the collected experimental UV-vis spectra calculated. Then, extraction of Rh(III) towards the ionic liquid trihexyltetradecylphosphonium chloride was performed. Comparison with the speciation data gives insight into the extraction mechanism and the extracted species.

Highlights

In recent years, there has been a considerable renewal of interest in liquid-liquid extraction of platinum group metals (PGM), owing to their scarcity on earth and their increased use in automotive catalysts or other technological objects, that lead to no other choice than efficient recycling.Among PGM, Rh appears as the trickiest element to separate from a mixture of Pt, Pd and Ru dissolved in chloride media
Under conditions that prevent any kinetic effect, the speciation of Rh(III) in H2 O/HCl medium was investigated up to very high chloride-metal ratios by collecting UV-vis spectra. These data were liable to a Principal Component Analysis (PCA) and a Multi-Curve Resolution (MCR) analysis, allowing the speciation diagram to be drawn, thermodynamic constants to be derived and the stoichiometry of the various species to be given
Comparison with the Rh(III) extraction profile towards a pure phosphonium-based ionic liquids (ILs) evidences the interest of such speciation studies in order to decipher the extraction mechanism

Summary

Introduction

Among PGM, Rh appears as the trickiest element to separate from a mixture of Pt, Pd and Ru dissolved in chloride media With this aim, several extracting systems were investigated, such as molecular solvents combined to various extracting agents [1,2,3] or mixtures of imidazolium-based [2], ammonium-based [3] or phosphonium-based ionic liquids (ILs). 20 years ago, a review paper pointed out the difficulties in Rh extraction related to ageing of the Rh samples, complex speciation and slow extraction kinetics in aqueous chloride medium [9]. Among the numerous chloro-complexes of Rh(III), which range from the cationic hexaaquo-rhodate [Rh(H2 O)6 ]3+ , to the anionic hexachloro-rhodate [RhCl6 ]3− , the latter, together with the [RhCl5 (H2 O)]2− , are claimed to dominate the speciation at high

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