Phytochemical-Mediated Green Synthesis of ZnO/Hydroxyapatite Nanocomposite from Shallot Peel Extract and Seashell Waste for Visible-Light-Assisted Photocatalytic Degradation of Remazol Red RB Textile Dye

Textile wastewater containing persistent synthetic dyes poses significant environmental concerns due to the toxicity and low biodegradability of these dyes. This study developed a sustainable ZnO/hydroxyapatite (ZnO/HA) nanocomposite photocatalyst using shallot peel extract and seashell waste as renewable precursors for the visible-light-assisted degradation of Remazol Red RB (RRRB) dye. ZnO nanoparticles were synthesized through a phytochemical-mediated route using aqueous shallot peel extract, while hydroxyapatite was obtained from seashell waste. The resulting ZnO/HA nanocomposite, prepared at a mass ratio of 1:2.5, was characterized using XRD, FTIR, SEM-EDX, and point-of-zero-charge analysis. The effects of catalyst dosage, solution pH, initial dye concentration, and irradiation time on RRRB degradation were investigated under a 50 W visible-light source. XRD analysis confirmed the coexistence of crystalline ZnO and hydroxyapatite phases, with an average crystallite size of 24.80 nm and crystallinity of 80.20%. The highest degradation efficiency obtained in the investigated conditions was 95.85% for 10 mg/L RRRB in 250 mL solution at pH of 6 using 1.0 g of ZnO/HA after 150 min irradiation. Under identical conditions, pristine ZnO achieved 93.40% degradation. The modest improvement indicates that the contribution of hydroxyapatite was primarily associated with surface adsorption, dispersion of ZnO, and increased pollutant–catalyst interaction rather than a demonstrated modification of the ZnO band structure. Because UV–Vis diffuse reflectance and catalyst-reusability measurements were not performed, changes in optical bandgap and long-term catalyst stability could not be experimentally established. Nevertheless, the results demonstrate the feasibility of utilizing two locally available biowaste resources to prepare a ZnO/HA material for visible-light-assisted dye treatment.