Evaluation of nitrate effects in the aqueous photodegradability of selected phenolic pollutants

The effect of nitrate in the aqueous photodegradation of five phenolic environmental pollutants (ortho-phenylphenol, OPP; methyl paraben, MeP; propyl paraben, PrP; Triclosan, TCS and bisphenol A, BPA) is evaluated. Time-course of precursor compounds and formation of transformation products (TPs) were investigated by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS). Nitrate showed a positive effect in the removal of selected pollutants. Observed TPs resulted from hydroxylation, aromatic nitration (with or without molecule cleavage) and nitro dehalogenation processes. The above reactions involved the participation of ·OH and ·NO₂ radicals arising from photolysis of nitrate. Nitro TPs were produced in a different extent depending on the structure of the precursor pollutant, nitrate concentration, light source (254 nm UV and solar light) and water matrix (ultrapure, surface and urban wastewater). Some of these nitro TPs were also observed during UV irradiation of untreated and unbuffered wastewater, either naturally polluted with parent phenolic compounds or spiked with these species at the sub μg L^-1 level. Nitration reactions were particularly favourable for OPP with the generated nitro TPs displaying a higher stability than the precursor molecule and the hydroxylated TPs of the same compound.