Accelerated kinetics of Pd(II) solvent extraction using functionalized 1-methyl-2-(nonylthio)-1H-imidazole and its comparison with dialkylsulfide to separate Pd(II) and Pt(IV)

Abstract

Dialkylsulfide is considered as the most promising extractant for separating Pd(II) and Pt(IV) in industrial processes. However, the slow kinetics for Pd(II) extraction limits its extensive application. The development of new extractants with faster kinetics, high extraction efficiency, and exceptional Pd(II) separation selectivity has been a persisting goal over the past few decades. In this study, a new functionalized extractant, 1-methyl-2-(nonylthio)-1H-imidazole (MNI) containing both N and S atoms, was synthesized and employed to separate Pd(II) from mixed hydrochloric acid solutions with different initial Pd(II)/Pt(IV) concentration ratios. Experimental results revealed that MNI not only has a high selectivity for Pd(II) over Pt(IV) but also has faster kinetics for Pd(II) extraction than diisopentylsulfide (S201) under similar extraction conditions. In contrast to traditional dialkylsulfide extractants, MNI coordinates with Pd(II) ions via the N(5) atom in its imidazole ring to form Pd(MNI)2Cl2 complexes, instead of using the S atom as with dialkylsulfide. The extraction mechanism involving the coordination between Pd(II) and MNI was demonstrated using the slope analysis method and ultraviolet (UV), Fourier-transform infrared (FT-IR), 1H NMR, and 13C NMR spectroscopic methods. The fast extraction kinetics between MNI and Pd(II) was possibly attributed to the high charge density of the N(5) atom in the imidazole ring, whereas the outstanding separation selectivity for Pd(II) was due to the strong affinity of the N(5) atom. The facile synthesis, high saturation capacity, excellent Pd(II) selectivity, and remarkable reusability endow MNI with potential applications in Pd(II) recovery from leach solutions of waste automotive catalysts and other secondary resources.