Ultraviolet absorption redshift induced direct photodegradation of halogenated parabens under simulated sunlight

As disinfection by-products of parabens, halogenated parabens are frequently detected in aquatic environments and exhibit higher persistence and toxicity than parabens themselves. An interesting phenomenon was found that UV absorption redshift (∼45 nm) occurs after halogenation of parabens at circumneutral pH, leading to overlap with the spectrum of terrestrial sunlight. This work presents the first evidence on the direct photodegradation of seven chlorinated and brominated parabens under simulated sunlight. These halogenated parabens underwent rapid direct photodegradation, distinguished from the negligible degradation of the parent compounds. The photodegradation rate depended on their forms and substituents. The deprotonation of halogenated parabens facilitated the direct photodegradation. Brominated parabens exhibited higher degradation efficiency than chlorinated parabens, and mono-halogenated parabens had higher degradation than di-halogenated parabens. The pseudo-first-order rate constants (k_obs) for brominated parabens (0.075-0.120 min^-1) were approximately 7-fold higher than those of chlorinated parabens (0.011-0.017 min^-1). A quantitative structure-activity relationship (QSAR) model suggested that the photodegradation was linearly correlated with the C-X bond energies, electronic and steric effects of halogen substituents. The photodegradation products were identified using QTOF-MS analyses and a degradation pathway was proposed. The yeast two-hybrid estrogenicity assay revealed that the estrogenic activities of the photoproducts were negligible. These findings are important for the removal of halogenated parabens and predictions of their fate and potential impacts in surface waters.