Photodegradation of anthelmintic drugs under natural sunlight and simulated irradiation: kinetics, mechanisms, transformation products, and toxicity

Albendazole (ALB) and bithionol (BIT) are two anthelmintic drugs (ADs) with high consumption from benzimidazole group and diphenylsulfide group, respectively. However, information on the transformation of the two anthelmintics under environmental condition is scare. Therefore, in the present study, we investigated the natural attenuation of the two ADs in the aquatic environment, including biodegradation, hydrolysis, and direct and indirect photodegradation. The direct photodegradation occupied a vast portion among other degradation pathways of the two ADs in natural water, with near-surface summer half-lives of 0.272–0.387 h and 0.110–0.520 h for ALB and BIT, respectively. Suspended particles in water were found to facilitate the photodegradation of the two ADs. Study on the indirect photodegradation demonstrated the positive roles of singlet oxygen (1O2) and excited triplet dissolved organic matter (3DOM*) in the photolysis of the two ADs, whereas the hydroxyl radical (•OH) affected little on the overall photodegradation procedures of ALB due to the scavenging effect of HCO3−. Dual effects of DO, DOM, HCO3−, NO3−, and NO2− on the photodegradation of ALB and BIT were perceived. Transformation intermediates (TIs) of the two ADs during photodegradation were analyzed by UHPLC-QTOF-MS. Six TIs of ALB were identified, including a broad-spectrum fungicide carbendazim and another common AD ricobendazole. Two TIs of BIT yielded from dechlorination were also detected. Probable transformation mechanism and predicted aquatic ecotoxicity based on the identified TIs were unveiled.