Effective adsorption of Pd(II), Pt(IV) and Au(III) by Zr(IV)-based metal-organic frameworks from strongly acidic solutions

Separation of precious metal ions from acidic solutions is of great importance due to their cumulative supply risk and environmental concern. However, their separation is still challenging as they are often present in extremely low pH solutions. Metal-organic frameworks (MOFs) have attracted extensive curiosity for adsorption owing to their fascinating physicochemical features including tunable pore sizes, active functional sites, and porosity. Here, two Zr-based MOFs of UiO-66 and UiO-66-NH₂ with an 8-ligand-connected Zr₆ node were fabricated and tested for adsorption of Pd(ii), Pt(iv) and Au(iii) anions (PdCl₄^2-, PtCl₆^2- and AuCl₄^-) in strongly acidic solutions. Both MOFs were tested for uptake time and adsorption capacities and showed rapid and high precious metal adsorption performances. Inner-sphere complexation between the precious metal anions and the incompletely coordinated Zr atoms in the MOFs was the key mechanism involved in the precious metal adsorption. In the case of UiO-66-NH₂, additional electrostatic attraction was found between the protonated amine group (-NH₃⁺) and the precious metal anions, as well as partial reduction of the bound precious metal ions. Moreover, the pathway of adsorption-reduction-crystallization-precipitation for the interaction of UiO-66-NH₂ and AuCl₄^- resulted in extremely high recovery efficiency for gold.