Solvent extraction of Pt(IV), Pd(II), and Rh(III) with the ionic liquid trioctyl(dodecyl) phosphonium chloride

BACKGROUND: Recycling of spent platinum group metals (PGMs) has attracted much attention in order to overcome the problems associated with the low natural abundance of these resources. To recycle such metals, efficient hydrometallurgical processes are required. To improve the efficiency of these processes, we have designed a new phosphonium-based ionic liquid as an extraction solvent. This paper reports on the potential use of the ionic liquid for extraction and separation of PGMs, namely Pt(IV), Pd(II), and Rh(III). RESULTS: An ionic liquid, trioctyl(dodecyl)phosphonium chloride, P₈₈₈₁₂Cl, which was newly synthesized, showed highly efficient extraction for Pt(IV), Pd(II), and Rh(III). P₈₈₈₁₂Cl features several advantages as an extraction solvent, such as high hydrophobicity and low viscosity compared with those features of a commercial analogue, trihexyl(tetradecyl)phosphonium chloride (P₆₆₆₁₄Cl). The favorable features of our novel ionic liquid were reflected by its high extraction efficiency and low release of phosphorus from the ionic liquid into the aqueous feed solution. Stripping operations were possible, and the high reusability of the ionic liquid was confirmed. CONCLUSION: The novel ionic liquid P₈₈₈₁₂Cl is potentially useful as an extraction solvent for PGMs. Results highlight the strengths of ionic liquids and the tuneability of their properties through design of their molecular structure.