Photocatalytic degradation of sulfamethoxazole using TiO2 in simulated seawater: Evidence for direct formation of reactive halogen species and halogenated by-products

Abstract

Nowadays photoactivation mechanism of titanium dioxide nanoparticles (TiO2 NPs) and reactive species involved in saline waters is not sufficiently established. In this study, TiO2 photocatalytic process under simulated solar irradiation was evaluated in synthetic seawater and compared with deionized water, using sulfamethoxazole (SMX) as model organic compound. For a TiO2 concentration of 100 mg L−1, SMX degradation resulted two times slower in seawater than in deionized water by the determination of their pseudo-first order rate constants of 0.020 min−1 and 0.041 min−1, respectively. Selected scavenging experiments revealed no significant contribution of hydroxyl radicals (OH) on the degradation process in seawater, while these radicals contributed to circa 60% on the SMX depletion in deionized water. Instead, the involvement of reactive halogen species (RHS) as main contributors for the SMX degradation in seawater could be established. A mechanism for the RHS generation was proposed, whose initiation reactions involve halides with the TiO2 photogenerated holes, yielding chlorine and bromine radicals (Cl and Br) that may later generate other RHS. Production of RHS was further confirmed by the identification of SMX transformation products (TPs) and their evolution over time, carried out by liquid chromatography-mass spectrometry (LC-MS). SMX transformation was conducted through halogenation, dimerization and oxidation pathways, involving mainly RHS. Most of the detected transformation products accumulated over time (up to 360 min of irradiation). These findings bring concerns about the viability of photocatalytic water treatments using TiO2 NPs in saline waters, as RHS could be yielded resulting in the generation and accumulation of halogenated organic byproducts.