Enhancing the photodegradation of tetracycline in aquaculture wastewater via iron(III)-alginate: Degradation pathway transformation and toxicity reduction

Tetracycline's (TC) incomplete self-photolysis by light irradiation generally produces toxic intermediate products, which posing serious harm to the aqueous environment. In order to diminish the environmental risks of TC self-photolysis, an iron(III)-alginate (Fe-SA) hydrogel assisted photocatalytic method was developed and the underlying mechanisms was also analyzed in this work. Under simulated sunlight, the photo-degradation efficiency of TC was 61.1% at pH 7.0 within 2 h. Importantly, four of the seven intermediate products that identified during the self-photolysis of TC were found toxic based on QSAR analysis. In contrast, the removal efficiency of TC could be improved to 87.4% by adding Fe-SA under the same conditions. Moreover, only two relatively weakly toxic intermediate products were detected after exposing to the Fe-SA photocatalytic system, indicating a significant reduction of the potential ecological risks caused by TC self-photolysis. Furthermore, the determination of reactive oxidation species (ROS) demonstrated that the addition of Fe-SA primarily facilitated the degradation of TC and the related toxic intermediate products through assisting the free radical (∙OH and ∙O2-) photocatalytic degradation pathway. Additionally, the photocatalytic application under actual sunlight conditions and the reusability experiments of Fe-SA further confirmed its effectiveness and low cost in removing TC. This study revealed the photodegradation mechanisms of TC from the perspective of the self-photolysis process, and also offering new insights into the removal of TC pollution in the environment.