Sustainable synthesis of Ag/UiO-66 composites using recycled silver for efficient adsorption of methylene blue and crystal violet.

Synthetic dyes are persistent and toxic water pollutants that require efficient removal technologies, and metal-organic frameworks (MOFs) have emerged as promising adsorbents for this purpose. In this work, a defect-engineered zirconium-based UiO-66 framework was synthesized by an HCl-modulated solvothermal route. UiO-66 was then functionalized with silver nanoparticles through a post-synthetic impregnation-reduction method to form an Ag/UiO-66 composite for the removal of the cationic dyes methylene blue (MB) and crystal violet (CV). The silver was recovered from electrical contact waste, thereby adding a resource-recovery dimension to the synthesis. Powder X-ray diffraction, XPS, FTIR, SEM, and TEM supported the incorporation of metallic Ag⁰ nanoparticles (about 3.3 nm; 3.11 wt%), uniformly dispersed on the crystals, with full preservation of the UiO-66 framework. Silver loading decreased the BET surface area from 870 to 642 m2 g-1, indicating partial pore occupation. Nevertheless, Ag/UiO-66 exhibited higher Langmuir capacities than pristine UiO-66, reaching 113.49 and 46.72 mg g-1 for MB and CV, respectively, compared with 104.98 and 43.72 mg g-1 for the pristine framework. Removal efficiencies above 98% were achieved at neutral pH, with equilibrium attained within 45 min. The adsorption was favored above the point of zero charge (pHpzc = 5.88) through electrostatic attraction, followed the pseudo-second-order kinetic model, and was best described by the Langmuir isotherm (R2 > 0.98), indicating monolayer adsorption. Thermodynamic analysis indicated a spontaneous, exothermic process, with ΔH° values ranging from - 4.91 to - 9.31 kJ mol-1. The combined isotherm, kinetic, and spectroscopic results indicated that the uptake occurs through both physical and chemical interactions, including electrostatic attraction, π-π stacking, hydrogen bonding, and binding at the silver and defect sites. The composite retained over 90% removal after five adsorption-desorption cycles and removed 95.3% of MB and 93.6% of CV from Nile River water. These results demonstrate that silver recovered from electronic waste can be used to prepare effective, reusable Ag/UiO-66 adsorbents for the removal of cationic dyes.