Adsorption, epimerization and self-sustaining degradation: Muti-step detoxification of tetracycline via chitosan-supported mixed-phase FeOOH

• α/δ-FeOOH@CS efficiently removes diluted TC and ETC. • TC and ETC show different removal behaviors and degradation mechanisms. • A self-sustaining Fe(III)/Fe(II) redox cycle for ROS generation is achieved. • Microalgae tests confirm the minimal ecological impact of degradation products . Tetracycline (TC) antibiotics are common trace pollutants in aquatic environments, where they can accumulate and transform into epimers such as 4- epi -tetracycline (ETC), potentially increasing ecotoxicity. Therefore, developing effective and safe in-situ remediation technologies for the removal of TCs from natural waters is of critical importance. This study presents a fabrication method for magnetic chitosan-supported mixed-phase FeOOH particles ( α / δ -FeOOH@CS), which are enriched with oxygen vacancies, for the adsorption and degradation of TCs. Significant epimerization of TC into ETC was observed during its removal. Batch experiments showed that 96% of TC and 97.6% of ETC could be removed by α / δ -FeOOH@CS. The main mechanism involves the chitosan-induced accumulation, Fe(III)-mediated complexation and oxidation, coupled with reactive oxygen species (ROS) generation initiated by Fe(II). These processes establish a self-sustaining Fe(III)/Fe(II) redox cycle for efficient in-situ remediation of TCs. The differing removal behaviors of TC and ETC are likely attributed to their distinct interactions with Fe(III), resulting from spatial changes in the C4 dimethylamino group. Additionally, algal toxicity tests conducted throughout the degradation process confirmed the near non-toxic nature of the removal steps, indicating that no significant new environmental toxic effects were introduced.