Ultrafast photodegradation of isoxazole and isothiazolinones by UV254 and UV254/H2O2 photolysis in a microcapillary reactor

The photodegradation process of methylisothiazolinone (MIT), benzisothiazolinone (BIT), and isoxazole (ISOX) in ultrapure water and synthetic wastewater by means of UV254 photolysis and by UV254/H2O2 advanced oxidation process were investigated in a microcapillary photoreactor designed for ultrafast photochemical transformation of microcontaminants. For the first time, we estimated key photo-kinetic parameters, i.e. quantum yields (35.4 mmol·ein−1 for MIT, and 13.5 and 55.8 mmol·ein−1 for BIT at pH = 4–6 and 8, respectively) and rate constants of the reaction of photo-generated OH radicals with MIT and BIT (2.09·109 and 5.9·109 L mol−1·s−1 for MIT and BIT). The rate constants of the reaction of photo-generated OH radicals with ISOX in MilliQ water was also estimated (2.15·109 L mol−1·s−1) and it was in good agreement with literature indications obtained in different aqueous matrices. The models were extended and validated to the case of simultaneous degradation of mixtures of these compounds and using synthetic wastewater as an aqueous matrix. High resolution-accurate mass spectrometry analysis enabled identification of the main intermediates (BIT200, B200, saccharin, BIT166) and enabled proposal of a novel degradation pathway for BIT under UV254/H2O2 treatment. This study demonstrates an ultrafast method to determine key photo-kinetic parameters of contaminants of emerging concern in water and wastewater, which are needed for design and validation of photochemical water treatment processes of municipal and industrial wastewaters.