Removal of parabens from water by UV-driven advanced oxidation processes

This study analyzed the degradation by direct photolysis with UV radiation of three common-use parabens (methylparaben, MetP, ethylparaben, EtP, and butylparaben, ButP) that have been detected in different types of water. The effect of different operating variables on their degradation by UV radiation was determined. A correlation between the degradation rate constant and the energy difference between the LUMO and HOMO orbitals of the parabens was established. A further study objective was to establish the effectiveness of different advanced oxidation processes in EtP removal from water. Rate constant values increased in the order MetP < EtP < ButP and the highest photodegradation (96.3%) was obtained for ButP, which has the longest alkyl chain. The absence of organic and inorganic compounds in ultrapure water had a positive effect on the photodegradation rate and percentage EtP removal, which ranged from 0.005 min(-1) and 37.9%, respectively, in wastewater, to 0.024 min(-1) and 88.0% in ultrapure water at 90 min of irradiation. A better performance was observed with the UV/S2O82- system (95.4% removal and 59% mineralization of EtP) than the UV/H2O2 system (83% removal and 48% mineralization of EtP). We identified the degradation by-products of the different parabens and proposed various reaction mechanisms based on these, including esterification, radical hydroxylation, and decomposition processes. Finally, we established that the identified degradation by-products are not cytotoxic.