Freeze-thaw cycles mediate the divergent impacts of iron-manganese oxides on the degradation of veterinary antibiotics in cold-region soils.

The persistence of veterinary antibiotics in soils, particularly in alpine regions, poses environmental risks. A critical knowledge gap exists regarding the degradation mechanisms under freeze-thaw cycles (FTCs), specifically the interplay between iron-manganese oxide-mediated abiotic processes and microbial processes. To address this, we employed loess as the experimental medium and selected oxytetracycline (OTC) and ciprofloxacin (CIP) as representative antibiotics, conducting laboratory experiments to systematically evaluate the effects of sterilization, varying iron-manganese oxide concentrations (0-5.0%), and FTCs on antibiotic degradation kinetics. The results showed that microbial degradation was the primary removal pathway. Sterilization inhibited degradation, reducing the OTC and CIP degradation rates by 66.7% and 43.0%, respectively. The influence of iron-manganese oxides varied depending on the antibiotic. They promoted OTC degradation, with maximum enhancement (70.8%) at 1.25% addition of the catalyst. For CIP, only 1.25% addition promoted degradation, whereas 2.5% and 5.0% additions suppressed the degradation rates by 2.3% and 5.2%, respectively. FTCs delayed antibiotic degradation by inhibiting microbial activity and affected non-biotic degradation by altering soil organic matter and the microenvironment. With iron-manganese oxides present, freeze-thaw impacts varied based on the antibiotic molecular structure and mineral surface properties. This study makes a substantive contribution by elucidating the key factors governing OTC and CIP degradation in alpine loess regions and, importantly, revealing the contrasting regulatory effects of iron-manganese oxides on different antibiotics under FTCs. These findings provide critical parameters for assessing the environmental behavior and risks of antibiotics, thereby informing the development of targeted pollution mitigation strategies.