Green-Synthesized Ag/Zn Nanocomposites from Chlorella vulgaris Polar Extract: Sustainable Photocatalytic Water Remediation and Kinetic Modeling

The growing demand for sustainable water treatment technologies requires photocatalysts that combine low environmental impact, energy efficiency, and mechanistic robustness. In this work, Ag/Zn nanocomposites were green-synthesized using Chlorella vulgaris polar extract as a bio-mediated reducing and stabilizing agent, eliminating hazardous reagents and high-energy processing steps. Structural characterization (XRD, FTIR, SEM, UV–Vis) confirmed the coexistence of crystalline wurtzite ZnO with metallic Ag and Ag2O phases. Photocatalytic activity was evaluated through Congo Red degradation under a sequential dark–light protocol, enabling clear separation of adsorption and photoactivated pathways. During the 60 min dark stage, removal remained limited (~9–11%), consistent with adsorption–desorption equilibration. Upon UV irradiation, a distinct kinetic transition occurred, leading to final removal efficiencies of 44–49% after 180 min. Notably, performance remained stable across the investigated photon flux range, indicating operation beyond a strictly photon-limited regime and highlighting an intrinsically energy-resilient catalytic response. A mechanistic kinetic model integrating reversible adsorption with light-dependent degradation accurately reproduced all experimental profiles (NRMSE = 3.14%) and successfully predicted an independent dark-control experiment without additional fitting. By coupling green synthesis with quantitative kinetic validation, this study proposes a sustainability-oriented framework for designing photocatalysts that align low-impact fabrication with energy-conscious water remediation.