Removal of safranin-O dye from an aqueous solution using a synthesized copper oxide nanoparticles doped by silver: Thermodynamic and kinetic study

Silver-doped copper oxide nanoparticles (Ag@CuO NPs) were synthesized via a UV-assisted photolysis route followed by calcination, and were evaluated as an efficient adsorbent for the removal of the cationic dye Safranin-O from aqueous media. The obtained nanomaterial was characterized by X-ray diffraction and transmission electron microscopy, confirming the formation of a monoclinic CuO phase with nanoscale crystallites and a near-spherical morphology. Batch adsorption experiments were conducted by varying the initial Safranin-O concentration, contact time, solution pH, and temperature. Rapid uptake was observed, and equilibrium was attained within 20 min at an initial concentration of 30 mg/L under natural pH conditions. The equilibrium data were better described by the Freundlich isotherm, indicating adsorption on a heterogeneous surface with non-uniform site energies. Kinetic analysis revealed that the adsorption followed the pseudo-second-order model (k₂ = 2.80 × 10 −3 g·mg −1 ·min −1 at 25 °C). In contrast, diffusion diagnostics indicated that neither intraparticle diffusion nor liquid-film diffusion was the sole rate-limiting step. Thermodynamic parameters indicated an exothermic adsorption process (ΔH < 0), whereas slightly positive ΔG values under the studied conditions implied weak driving forces and limited spontaneity, consistent with predominantly physical interactions. Overall, Ag@CuO NPs demonstrated high adsorption performance for Safranin-O and represent a promising nanosorbent for rapid dye removal from contaminated water.