New transformation products from ozonation and photolysis of diclofenac in the aqueous phase

Ozone and UV photolysis can be effective for the treatment of pharmaceuticals in wastewater, however, these processes can result in the formation of potentially toxic transformation products (TPs). This study investigates the structural elucidation of the TPs formed during ozone and UV treatments of diclofenac (DCF), a ubiquitous pharmaceutical. Using liquid chromatography - electrospray ionisation - mass spectrometer (LC-ESI-MS) and a Waters Xevo (TM) Quadrupole Time-of-Flight (Qtof) mass spectrometer, the chemical structures of DCF TPs were identified, including the fragmentation ion patterns and exact mass of the detected molecular ions. TPs formed during ozonation showed an addition of one or more hydroxyl groups to the DCF molecule including changes to the double bond equivalent for some products. The other identified DCF ozonation products were formed by removal of the (-CH2COOH) group in two steps and resulted in the formation of a diverse range of TPs. By contrast, UV photolysis formed DCF TPs via removal of its chlorine atoms and ring closure followed by dimerization. In comparison, all ozonation TPs retained the two chlorine atoms of DCF whereas UV TPs did not possess any chlorine atom except for one compound that retained one chlorine atom. Possible reaction pathways for the identified DCF TPs are also proposed. (C) 2021 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.