Recent advances in photocatalytic removal of sulfonamide pollutants from waste water by semiconductor heterojunctions: a review

The continuous proliferation of antibiotics in the environment has led to tremendous growth of multiresistant bacteria. This has increased the interest in sustainable solutions for their degradation and mineralization. Historically, the sulfonamide antibiotics have been employed for longest for treatment of bacterial and protozoan infections and are considered as the first systemically used antibiotics. These are poorly biodegradable which leads to bioaccumulation, biomagnification and increasing the antibiotic resistance of bacteria in marine ecosystems. The advanced oxidation processes (AOPs) have received highest attention for degradation of antibiotics owing to strong oxidation capability and mineralization capacity. Among photocatalysts, semiconductor heterojunctions as Z-scheme and S-scheme heterojunctions have been most effective owing to high charge carrier separation, better charge transfer and superior photo-redox potential. This review focuses on high performance photocatalytic degradation and mineralization of various sulfonamide antibiotics using heterojunction photocatalysts. The heterojunction properties, band structures and thermodynamic feasibilities of reactive oxygen species are also explained. The review also presents the degradation routes, intermediates and most probable reactions involved during their mineralization of sulfonamide antibiotics. Very few review reports on sulfonamide antibiotics removal have been summarized with more focus on biodegradation and adsorption only. However, this review presents in-depth discussion on relationship of photocatalytic heterojunctions and sulfonamides degradation mechanism and pathways. This review also critically analyzes the insights of heterojunctions band structure and band match transformation for achieving maximum and hydroxyl and superoxide radicals. The ecotoxicity or biotoxicity of degradation intermediates is also discussed in detail. Finally, future outlook viewpoints and proposed research directions to provide deep insights into improved mineralization and biotoxicity by innovations in designing of novel heterojunctions and reaction engineering are critically discussed.