Extraction mechanism, behavior and stripping of Pd(II) by pyridinium-based ionic liquid from hydrochloric acid medium

• The [Hpy]Cl/chloroform system shows high extractability and selectivity for Pd(II).
• The mechanism of Pd(II) extraction by [Hpy]Cl was studied.
• Thermodynamic parameters of the Pd(II) extraction reaction were obtained.
• The Pd stripping and the [Hpy]Cl regeneration can be achieved using oxalate.

The hexadecylpyridinium chloride/chloroform system was employed for the Pd(II) extraction from hydrochloric acid medium. The effects of extraction time and material concentrations were examined for Pd(II) extraction. The anion-exchange mechanism of Pd(II) extraction by [Hpy]Cl was confirmed by Job's method, UV–Vis, infrared spectrum and 1H NMR analysis. Thermodynamic parameters of the Pd(II) extraction reaction were obtained from the thermodynamics analysis, which illustrates that higher temperatures show a negative effect on the Pd(II) extraction. When the mole ratio of [Hpy]Cl to Pd(II) is above 4, the Pd(II) extraction yield (E%) is almost 100%. It also has high selectivity over some metals (Cu(II), Co(II), Ni(II), Fe(III), Al(III) and Sn(IV)). Moreover, oxalate is explored as a reducing agent for [Hpy]+2[PdCl4]2 − complexes, by which Pd(II) is stripped in the form of palladium powders and [Hpy]Cl also can be recycled. Therefore, the method is an efficient, effective and highly selective approach to extract Pd(II) and recover metal palladium.