Fate of oxytetracycline mediated by iron redox in simulated river-groundwater interactions

River - groundwater （RW - GW） interaction creates numerous challenges related to water quality. A central concern to this interplay is the redox process-driven transformation of iron and its degradation of organic micropollutants， however， the fundamental mechanisms and variability under different interaction intensities remain unclear. This study aims to elucidate the bidirectional reaction dynamics between Fe（Ⅱ） and oxytetracycline （OTC） under diverse conditions of RW and GW contact， including varying oxygen contents and interaction ratios. Our investigation reveals that Fe（Ⅱ） efficiently degrades OTC through a distinctive cyclic redox process， affected by varying levels of Fe（Ⅱ） and dissolved oxygen. Meanwhile， OTC inhibited the conversion of Fe（Ⅱ） to lepidocrocite by primarily forming complexes. Mechanistic studies indicate that Fe（Ⅱ） complexes with OTC through the tricarbonyl amide group in its A ring， while Fe（Ⅲ） associates with the phenolic diketone moieties on the B and C rings of OTC. The results showed that Ca²⁺ exerted a more pronounced impact on OTC elimination and Fe（Ⅱ） transformation than NO₃^-， with the inhibitory effect increasing proportionally to Ca²⁺ concentration. In contrast， the impact of humic acid （HA） on OTC degradation was found to be concentration-dependent， lower concentrations of HA were conductive to OTC degradation， whereas higher concentrations inhibited it. Given the ubiquity of Fe（Ⅱ） and OTC in RW-GW interactions， this study provides a novel perspective on the environmental fate of Fe（Ⅱ） and tetracycline contaminants in redox-sensitive regions.