Separation of Pd(II) and Pt(IV) in concentrated hydrochloric acid using thin-layer oil membrane and comparison with conventional extraction

Abstract

This study proposes a new thin-layer oil membrane (TOM) extraction strategy for enhanced mutual separation of Pd(II) and Pt(IV) chlorocomplex anions in concentrated aqueous hydrochloric acid solutions using di-n-butyl sulfoxide (DBSO) as an extractant. It was discovered that Pd(II) could be preferentially extracted even in concentrated hydrochloric acid solutions. The separation coefficient between Pd(II) and Pt(IV) considerably increased than that between them during conventional extraction using a separating funnel. A new extractor was designed to perform TOM extraction by spreading a thin layer of organic extractant liquid membrane on the surface of aqueous feed solutions under laminar flow. A simplified TOM extraction model and controlled experiments were used to investigate the effect of H+ and Cl− ion concentrations on the mutual separation between Pd(II) and Pt(IV). Molecular dynamics (MD) simulations were used to understand the microscopic separation mechanisms at the molecular level. Experimental results revealed that the competitive distribution of H+ and Cl− ions in the boundary layer along the normal vertical direction of the organic-aqueous interface are crucial in enhancing the separation between Pd(II) and Pt(IV). The competitive adsorption and enrichment of Cl− ions at the interface prevent DBSO molecules from being protonated by H+ ions, which is beneficial for the substituted coordination of DBSO molecules with PdCl42−. However, the ionic association of protonated DBSO molecules with PtCl62− remains dominant at the interface. As a result, even in concentrated hydrochloric acid solutions, the difference in mass transfer between PdCl42− and PtCl62− toward the interface was intensified.