Comparative study on the photocatalytic activity of green-synthesized silver nanoparticles from Hypericum scabrum toward monoazo and diazo dyes

Hypericum scabrum extract, which is rich in phytochemicals such as flavonoids, phenolics, and terpenoids, acts as a natural reducing and stabilizing agent and was used to synthesize silver nanoparticles. The crystalline nature, spherical morphology, and narrow size distribution of nanoparticles were characterized by UV–vis, FTIR, XRD, SEM, TEM, and DLS analysis. Their photocatalytic activity was evaluated for the degradation of azo dyes, Reactive Red 195 and Reactive Black 5, under UV irradiation. The effects of dye concentration, nanoparticle dosage, pH, and irradiation time on degradation efficiency were optimized using the Response Surface Methodology. The optimized conditions for maximum degradation were achieved at 99.78 mg/L dye concentration, 0.994 g/L AgNP dosage, pH 5.33, and 60 min irradiation with 97.1 % degradation for Reactive Red 195, and 96.35 mg/L, 0.915 g/L, pH 4.64, and 75 min with 89.3 % degradation for Reactive Black 5, differing by less than 1 % from the model predictions. Results confirming the strong reliability of the developed models and demonstrate that synthesized nanoparticles are efficient photocatalysts for the degradation of persistent azo dyes. The relationship between dye molecular structure and degradation efficiency was investigated, revealing that photocatalytic degradation performance was significantly affected by structural complexity and the number of azo bonds. Equilibrium isotherm and thermodynamic analyses were performed, and the results showed that the adsorption followed a complex multilayer pattern consistent with the Freundlich isotherm, indicating the presence of diverse active sites on the AgNP surface.