Zinc oxide nanoparticles modified by empty fruit bunch derived carbon dots for enhanced photocatalytic tetracycline removal

The widespread occurrence of antibiotic residues in aquatic environments, particularly tetracycline (TC), presents a significant environmental concern due to their persistence and contribution to antibiotic resistance. Despite extensive research on photocatalytic nanomaterials, many reported systems remain limited by poor visible light utilization, rapid electron hole recombination, and reliance on non-sustainable or chemically derived modifiers, which constrain their practical application. In addition, the role of biomass derived carbon-based modifier in photocatalysis in photocatalytic electrochemical charge transfer mechanisms remains insufficiently understood, particularly regarding their influence on charge carrier generation, interfacial charge separation, electron transport pathways, and recombination suppression. To address these gaps, this study develops a zinc oxide/carbon dots (ZnO/CDs) nanocomposite synthesized via a hydrothermal method, where carbon dots are sustainably derived from oil palm empty fruit bunches, offering a green and cost-effective strategy. A series of ZnO/CDs nanocomposites with mass ratios of 2:1, 1:1, and 1:2 was systematically evaluated under varying tetracycline concentrations, catalyst dosages, and light sources. The ZnO/CDs (1:2) nanocomposite exhibited superior photocatalytic performance, achieving 96.17% degradation of 5 ppm TC under UV irradiation at a catalyst dosage of 1 g L⁻¹, where the variation is attributed to differences in pollutant concentration and irradiation energy. Kinetic analysis followed pseudo first order behaviour with a rate constant of 0.017 min⁻¹ under optimal conditions. Structural characterization confirmed effective integration of spherical CDs (0.32 nm lattice spacing) with ZnO nanoparticles (0.26 nm), resulting in enhanced light absorption and interfacial charge transfer. Electrochemical impedance spectroscopy showed a reduction in charge transfer resistance from 1.8 kΩ to 0.6 kΩ, while cyclic voltammetry and transient photocurrent measurements confirmed improved charge mobility and separation efficiency. Mott–Schottky analysis indicated n-type semiconductor behaviour and suitable band alignment supporting a Z scheme charge transfer mechanism. Reactive species trapping experiments identified hydroxyl and superoxide radicals as the dominant active species. The ZnO/CDs nanocomposite demonstrated good stability, retaining 88% efficiency after five cycles. Overall, this study provides mechanistic insight into biomass derived carbon dots in photocatalytic systems and offers a sustainable approach for antibiotic removal while supporting waste valorization.