Selective adsorption of Pd(II) over interfering metal ions (Co(II), Ni(II), Pt(IV)) from acidic aqueous phase by metal-organic frameworks

Metal-organic frameworks (MOFs) have successfully employed as a class of adsorbents for selective separation of CO₂ and some organics from gas and liquid phases; however, little attention has been paid to MOFs applications for selective adsorption of ionic targets from aqueous solutions. Besides, selective separation of precious metals is of great importance from the standpoint of realizing a new sustainable development concept of “urban mining”. While, separation of precious metals is challenging, as they are often present in acidic solutions with many interfering metal ions. Herein, Zr(IV)-cluster-based MOFs were synthesized in this study for the selective adsorption of Pd(II), as a model precious metal ion, from acidic aqueous solutions containing Co(II), Ni(II), Pd(II), and Pt(IV). Among the various Zr-MOFs synthesized (i.e. UiO-66, UiO-66-NH₂ and UiO-66-NHCOCH₃), UiO-66-NH₂ showed the highest Pd(II) selectivity, with the Pd(II) uptake in the mixture-metals solution being around 181 times higher than that of Pt(IV). The Pd(II) selectivity of UiO-66-NH₂ was attributed to higher binding affinity of the protonated amino groups (–NH₃⁺) for PdCl₄²⁻ and more convenient diffusion of the PdCl₄²⁻ through the apertures (pores) of the UiO-66-NH₂ – which have the appropriate inner sizes befitting PdCl₄²⁻ penetration – than for Pt(IV) ions (PtCl₆²⁻).