Efficient and recyclable UiO-66 metal-organic framework for selective molybdenum (VI) removal from aqueous solutions: kinetic and thermodynamic insights

The removal of molybdenum (VI) from contaminated water is challenging due to its high solubility and environmental mobility. This study explores the zirconium-based metal–organic framework UiO-66 as an efficient adsorbent for Mo(VI) removal and elucidates its adsorption mechanism through systematic physicochemical and kinetic analyses. UiO-66 was synthesized via a solvothermal method, and its crystalline structure, functional groups, morphology, and porosity were confirmed by XRD, FTIR, SEM, TEM, and N2 adsorption–desorption analyses, indicating the formation of a stable and highly porous framework. Batch adsorption experiments were conducted to investigate the effects of pH, temperature, contact time, initial concentration, and competing ions. The adsorption process exhibited strong pH dependence, with optimal performance at pH 3. Under these conditions, UiO-66 achieved a maximum adsorption capacity of 275.9 ± 0.9 mg g−1 at 45 °C, outperforming many conventional adsorbents. Kinetic data followed the pseudo-second-order model (R2 = 0.998), suggesting chemisorption as the rate-limiting step, while equilibrium data were best described by the Langmuir isotherm, indicating monolayer adsorption on homogeneous sites. Selectivity studies confirmed preferential Mo(VI) uptake in the presence of competing ions. Thermodynamic analysis revealed that the adsorption is spontaneous (ΔG < 0) and endothermic (ΔH > 0). Additionally, UiO-66 demonstrated excellent reusability over five adsorption–desorption cycles, maintaining high efficiency and structural stability. The adsorption mechanism is mainly attributed to electrostatic attraction and coordination interactions between molybdate species and Zr–O clusters. These findings highlight UiO-66 as a promising material for efficient water purification applications.