Ecotoxicity mitigation and biodegradability enhancement during sulfamethazine removal by a pilot-scale rotating advanced oxidation contactor equipped with TiO2–zeolite composite sheets

This study investigated the ecotoxicity mitigation, biodegradability enhancement performance, and underlying mechanisms in treating sulfamethazine (SMT) using a pilot-scale rotating advanced oxidation contactor (RAOC) equipped with TiO₂-zeolite composite sheets, which adsorbed and photocatalytically decomposed SMT and hydrophobic degradation intermediates. For the ecotoxicity evaluation, the green microalga Scenedesmus obliquus was cultured with treated SMT solution. The 96-h growth inhibition ratio was initially 53.3% but then dropped to nearly 0 after 96 h of RAOC treatment under ultraviolet irradiation, which showed that RAOC treatment effectively eliminated ecotoxicity. Both theoretical predictions using ECOSAR and experimental validation showed that 4-amino-2,6-dimethylpyrimidine, an intermediate with higher toxicity than SMT, contributed to the increased toxic effect as SMT removal neared completion. We also found that the specific content of chlorophyll a (0.55%-0.90% g-chlorophyll/g-biomass) was sensitive to toxic stress caused by SMT and the intermediates, whereas that of chlorophyll b (0.42%-0.58%) did not show a substantial correlation. Comprehensive analyses of the 5-day biochemical oxygen demand (BOD₅) and total organic carbon (TOC) in the treated solution showed consistent improvement in biodegradability throughout the treatment. This also suggested efficient degradation and mitigation of the toxic effects of SMT and its degradation intermediates on microbes. Furthermore, comparison with TiO₂ sheets showed that TiO₂-zeolite composite sheets were superior in terms of both SMT removal and ecotoxicity mitigation.