Green Synthesis and Physicochemical Characterization of Biogenic Silver Nanoparticles for Comparative Sunlight and Uv Photocatalytic Degradation of Congo Red

Silver nanoparticles (Ag NPs) were biosynthesized using Bacillus subtilis AVK-21 isolated from water samples, wherein microbial metabolites served as natural reducing and stabilizing agents for the reduction of Ag ⁺ to Ag ⁰ nanoparticles. The formation of Ag NPs was confirmed by a distinct SPR peak observed in UV-Vis spectroscopy at 430 nm, while FTIR analysis revealed functional biomolecules responsible for nanoparticle capping. SAED pattern depicts the crystalline face-centered cubic structure of the particles, and energy-dispersive X-ray spectroscopy (EDS) verified elemental silver with energy peak near 3keV. Transmission electron microscopy (TEM) demonstrated predominantly spherical nanoparticles with an average primary size of 62.4 nm, whereas dynamic light scattering (DLS) indicated a hydrodynamic diameter of 271.3 nm, reflecting the aggregation behavior of the biogenic Ag NPs. The zeta potential value was found to be 0.0 mV which further supported their limited colloidal stability and corresponded with the clustered morphology seen in TEM micrographs. The photocatalytic efficiency of the biosynthesized Ag NPs was evaluated using Congo Red dye under two conditions i.e., sunlight and UV light in which sunlight exposure resulted in significantly greater degradation (67.01%) compared to UV irradiation (37.80%) within 120 min. The enhanced effectiveness under sunlight is attributed to stronger plasmonic excitation and improved ROS generation, which facilitated electron transfer and the subsequent breakdown of the dye’s structure. Overall, this study demonstrates that microbially synthesized Ag NPs possess good photocatalytic potential and can efficiently degrade Congo Red under natural sunlight, highlighting their applicability as sustainable, low-energy catalysts for dye-polluted wastewater treatment.