A novel zinc oxide/schwertmannite composite for efficient remediation of oxytetracycline contamination

Addressing oxytetracycline (OTC) contamination is essential for controlling agricultural area-source pollution. Developing green remediation materials that can efficiently remove pollutants in a short period holds ecological sig-nificance and research value. This study synthesised a novel zinc oxide/schwertmannite (ZnO/SCH) composite by integrating photocatalytic and adsorptive properties to efficiently remove OTC. Initially, the performance of the ZnO/SCH was evaluated in aqueous systems, which showed excellent OTC removal efficiency. 30 mg/L of OTC was removed by more than 90% under solar light and more than 70% under visible light after irradiation for 4 h at a ZnO/SCH addition of 0.2 g/L, primarily driven by hole (h⁺) and superoxide anion (·O₂^-) radicals. Remarkably, the composite maintained over 95% OTC removal across a wide pH range (4-10). Building on these promising results, the study further investigated the application of ZnO/SCH in OTC-contaminated calcareous soil. Environmental factors, including the degree of contamination, temperature, soil moisture content, soil pH, calcium carbonate (CaCO₃) content, and organic matter (SOM) levels, significantly influenced OTC adsorption and fixation. Soil OTC fixation and removal was favoured by higher temperatures (30 ℃), lower OTC concentrations (30 mg/kg), and a suitable soil moisture content (18%). Alkaline conditions and high calcium carbonate content reduced electrostatic adsorption and cation exchange, while low organic matter content weakened soil adsorption of OTC. Notably, ZnO/SCH acted as a secondary adsorbent, capturing OTC released from soil under changing conditions and mitigating secondary pollution risks. This study provides a high-performance, cost-effective solution for the remediation of OTC-contaminated water and even soil, offering significant ecological and practical value.