Enhanced photo-oxidation of hormones in water using biomass waste-derived hydrochar composites as photocatalysts

This study introduces a green chemistry approach to enhance the catalytic efficiency of metal oxides in photocatalytic reactions using agro-industrial waste. Specifically, rice husks and spent coffee grounds were utilized as carbon sources to produce hydrochars, which served as sustainable supports for ZnO and ZnFe₂O₄ oxides. This method promotes waste minimization and circularity by repurposing agricultural byproducts, reducing the environmental impact of traditional catalyst production. The hydrochars were synthesized through mild hydrothermal carbonization (250 °C for 4 h) and characterized by various physicochemical analyses. Their photocatalytic performance was evaluated using a 20 mg L⁻¹ solution of conjugated estrogens under ultraviolet light (120 W cm⁻²) for 30 min. Composites with ZnO exhibited a significant increase in oxidation reaction rates due to enhanced porosity and an oxygen-rich structure. Reactive oxygen species (ROS) analysis revealed hydroxyl radicals (OH) as the primary agents, with photocatalytic efficiencies exceeding 83 % after eleven cycles. In toxicity tests, 4 mL of treated solution was applied to 20 Lactuca sativa (lettuce) seeds, kept in the dark at 25 °C for 7 days. Seeds exposed to treated solutions showed improved radicle elongation and germination rates compared to controls, except for those with coffee hydrochar and ZnO. This indicates effective degradation of toxic estrogens without harmful by-products. Overall, this work highlights the advantages of integrating green chemistry principles and circular economy practices, transforming waste into valuable materials to enhance environmental sustainability and catalytic performance.