Development of highly efficient bimetallic metal organic frameworks for the extraction of Pd(ii) from aqueous solutions

Abstract

The safe handling of nuclear waste as well as expanding needs due to global industrialization necessitates the recovery of palladium from high level liquid waste. However, effective extraction of palladium remains challenging, and the expanding area of metal organic frameworks with adjustable functionality may offer a practical solution. The Zr-based UiO-66 metal organic frameworks are well known for their high stability due to the strong electrostatic interaction between Zr⁴⁺ and dicarboxylic acid linkers. Herein, we report the post-synthetic modification of UiO-66-NH₂ with glutaric anhydride and pyridine dicarboxylic acid, to synthesize UiO-66-GA and UiO-66-PDCA, which were employed for sorption/extraction of palladium. In addition, we report the synthesis of novel Zr/Zn based bimetallic MOFs which were also examined for the sorption of Pd(II) from aqueous streams. UiO-66 MOFs were doped with various ratios of zinc to obtain bimetallic MOFs via a one-step method for the efficient recovery of Pd(II) from aqueous solutions. The synthesized MOFs were characterized using FT-IR, powder-XRD, TGA, SEM-EDX and BET surface area analysis. The particle sizes of the various MOFs were also measured using the dynamic light scattering (DLS) technique since size plays a critical role in tuning the properties of the MOFs. Sorption studies were performed at various pH conditions, and it was found that the bimetallic MOF Zr–Zn-MOF-1 exhibited the maximum sorption of 77.4% within 3 hours at pH 5. The effect of pH on the surface charge of the various MOFs was also explored to get further insight into the correlation between pH and sorption behavior. Furthermore, the contact time, temperature and concentration studies explained the monolayer adsorption of Pd(II) onto the MOFs following the pseudo-second order kinetics. Following this, recovery of the adsorbed Pd(II) from the MOFs was performed using different eluting agents in which 1 M Na₂CO₃ exhibited better desorption. Furthermore, the adsorbent was regenerated and employed for a total of three cycles, demonstrating remarkable stability with no discernible alterations in its adsorption capacity throughout the successive usage. In addition, density functional theory (DFT) calculations were carried out to provide insight into the nature of the interactions that exist between Pd(II) and the MOFs.