Valorization of Agro-Industrial Waste Streams for Green Synthesis of Zinc Oxide Nanoparticles

The synthesis of zinc oxide nanoparticles (ZnO NPs) using agro-industrial biowaste extracts represents a paradigm shift toward circular economy and green nanotechnology. Conventional chemical synthesis of ZnO NPs employs hazardous reagents, generates toxic by-products and demands considerable energy inputs, rendering it environmentally unsustainable at scale. Biowaste-derived green synthesis leverages the abundant phytochemical content of agricultural residues—including polyphenols, flavonoids, tannins, saponins and organic acids—to serve as reducing, capping and stabilizing agents in the formation of ZnO NPs with controlled morphology and desirable physicochemical properties. This comprehensive review systematically examines the role of diverse agro-industrial waste streams—encompassing fruit peels (banana, orange, pomegranate), processing residues (sugarcane bagasse, coffee grounds), seed wastes (date seeds, açaí berry seeds, longan seeds) and vegetable peels (onion, tomato)—as biogenic precursors for ZnO NP synthesis. The paper critically evaluates synthesis protocols, key process parameters (pH, temperature, calcination conditions, precursor concentration) and characterization findings obtained through UV-Vis spectroscopy, X-ray diffraction (XRD), FTIR, SEM, TEM, DLS and zeta potential analyses. Resulting ZnO NPs typically exhibit hexagonal wurtzite crystal structures with particle sizes ranging from 10 to 100 nm. Documented applications including antimicrobial activity, photocatalytic dye degradation, anticancer effects and agricultural use are critically reviewed. The paper further addresses challenges related to reproducibility, scalability and nanotoxicology and proposes future research directions to advance the industrial valorization of biowaste-derived ZnO NPs within sustainable development frameworks.