Significant Effect of Evaporation Process on the Reaction of Sulfamethoxazole with Manganese Oxide

Soil in the vadose zone is an important sink for antibiotics. However, previous studies have examined only the degradation of antibiotics in soil slurry systems, which were largely different from real-world unsaturated soil environments. Whether the same transformation mechanisms apply to unsaturated soil systems has been a question. Here, the degradation of sulfamethoxazole (SMX) by manganese dioxide (γ-MnO₂) in both suspension systems and evaporation processes were examined. Results show that the slow degradation of SMX in the suspension system can be significantly promoted as the water gradually evaporates. SMX degraded differently in evaporation as compared to suspension systems because of the quenching effect of generated Mn²⁺. Transformation products of SMX in both systems also showed different toxicity toward Escherichia coli because of different evolutions of intermediates. This study has strong implications for the assessment and prediction of the transformation and fate of antibiotics in natural soil environments.