Green-Synthesized ZnO Nanoparticles on Bio-Activated Carbon from Robusta Coffee Husk for Efficient Photodegradation of Chlorophenoxy Herbicide

ZnO's photocatalytic properties make it a widely used semiconductor, while bio-activated carbon (BAC) from agricultural waste is emerging as a sustainable and cost-effective photocatalyst substrate. This study explored the creation and characterization of a nano ZnO/bio-activated carbon (ZnO/BAC) composite from coffee husks, aimed at efficiently degrading Rhodamine B (RhB) under simulated sunlight. The bio-activated carbon (BAC) was produced by carbonizing coffee husks at 900°C in an argon atmosphere and activating with hydrochloric acid (HCl), resulting in a BET surface area of 842.48 m²/g and a pore volume of 0.403 cm³/g after 24 hours. ZnO nanoparticles, sized between 20 to 100 nm, were deposited on the BAC, confirmed by XRD, FTIR, and EDX, which showed a hexagonal wurtzite structure with crystallite size around 10 nm. UV-Vis DRS analysis indicated strong ZnO absorption below 400 nm, enhancing photocatalytic performance. 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) degradation was most effective at 10 mg/L, with an optimal ZnO/BAC dosage of 1.00 to 1.25 g/L. The process followed pseudo-first-order kinetics (k₁ = 0.02881 min⁻¹, R² = 0.9791), achieving 98.94% removal in 240 minutes, particularly rapid in the first 30 minutes. The composite also showed good reusability, retaining over 86% efficiency after seven cycles. These results highlight ZnO/BAC as a promising sustainable photocatalyst for dye removal in wastewater treatment.