UV photolysis kinetics of sulfonamides in aqueous solution based on optimized fluence quantification

The ultraviolet (UV) photolysis kinetics of eight sulfonamide (SA) antibiotics was investigated in a quasi-collimated beam apparatus. By using a micro fluorescent silica detector to monitor online the dynamic irradiance fluctuation, the accuracy in fluence quantification could be increased by up to 15%. Solution pH governed the speciation of selected SAs, thus impacting significantly their molar absorption coefficients (ε), fluence-based photolysis rate constants (k′), and quantum yields. An increasing pH induced a hyperchromic effect and a blue shift of the UV–Vis absorption spectra of selected SAs, thus causing the anionic species to show a relatively higher specific ε value than the neutral species at 254 nm. On UV exposure, the photodegradation of selected SAs all followed pseudo-first order reaction kinetics. The SAs with a penta-heterocycle, because of their higher electron densities, exhibited an obviously higher photodegradation rate than those with a hexa-heterocycle. The specific k′ values of the neutral and anionic species were in the ranges of (0.30–14.49) × 10−3 and (0.61–20.90) × 10−3 cm2 mJ−1, respectively. With the specific k′ values obtained, it is estimated that only part of SAs can be photodegraded during UV disinfection of water and wastewater, so an advanced oxidation process is necessary if a higher removal of selected SAs is to be achieved.