Advances in the synthesis of bismuth-based heterojunction photocatalysts and visible light photodegradation of ciprofloxacin and levofloxacin in aqueous solutions: Degradation efficiency, degradation products, and their toxicity

Antibiotic pollution in water systems is a critical global challenge that worsens the global water scarcity crisis by contaminating vital freshwater reserves and creating dangerous antimicrobial resistance. Photocatalysis uses solar energy to power green catalytic treatments that clean antibiotic-contaminated water by breaking down persistent pharmaceuticals without using any harsh chemicals. Bismuth-based heterojunction photocatalysts leverage heterojunction engineering that drastically improves charge carrier separation and minimizes electron-hole recombination while enhancing redox potential for the destruction of environmental pollutants. In this article, we reviewed the removal of levofloxacin (LEV) and ciprofloxacin (CIP), the two most detected antibiotics in wastewater and aquatic environments globally, by various bismuth-based heterojunctions. Their photodegradation performance has been compiled and compared, and possible degradation routes, photodegradation mechanisms, and ecotoxicity of the degraded products are discussed. A true comparison of the degradation performance of CIP and LEV by various heterojunctions is difficult because various light sources, radiation intensity, pollutant concentration, and catalyst dose were used in various published studies. Of the heterojunctions so far developed, Fe3O4/Bi2WO6 and Bi2O3/(BiO)2CO3 showed excellent CIP removal, and their photodegradation rate constants were 0.5571 and 0.476 min−1, respectively. Conversely, for the removal of LEV, Bi2WO6/CuS and BiOCl/Bi2Fe4O9 provided excellent photodegradation, and the rate constants were 0.2639 and 0.0934 min−1, respectively. Toxicity analyses show that many degradation intermediates of CIP and LEV are toxic and mutagenic if the photodegradation is incomplete. The development of low-cost photocatalytic drug removal technology based on bismuth-based heterojunctions holds significant potential to make drinking water safer for human and animal consumption.