Advanced oxidation of micropollutants in water by photolytic and photocatalytic processes

Photochemical processes and related technologies have been often used in the 20th century for the disinfection of drinking water and wastewater (secondary and tertiary sewage effluents). Recently, direct ultraviolet (UV) photolysis, photocatalysis and advanced oxidation procedures have been widely reported as emerging methods for the removal of organic micropollutants from water. Nowadays, based on the progress made in analytical techniques’ sensitivity, micropollutants such as pharmaceuticals can be determined down to ng L-1 scale in the aquatic environment. There is growing interest in the removal of these contaminants from water, particularly driven by the overall public concern about potential toxic effects they might induce in humans and ecosystems. In this work, the beta-blocker nebivolol has been detected for the first time in effluent samples of 12 wastewater treatment plants (WWTPs) in Germany. The photolytic degradation of nebivolol has been investigated under three different UV sources, namely, UV-C (main emission band at 254 nm), UV-B (main emission band at 312 nm) and UV-A (main emission band at 365 nm) in different matrices: pure water, pure water in the presence of a hydroxyl radical scavenger and in wastewater. During the photodegradation study, no elimination of nebivolol was observed under UV-A radiation. In contrast, nebivolol degradation under UV-B and UV-C radiation followed pseudo first order reaction kinetics, with the highest removal rate under UV-C radiation in pure water (k = 7.8 × 10−4 s−1). Also the degradation mechanism of nebivolol under the UV-B and UV-C radiation has been studied. Three transformation products (TPs) were identified after UV-B and UV-C photolytic degradation using high resolution mass spectrometry. The TPs are formed by the substitution of the fluorine atom from the benzopyran ring with a hydroxyl group. The biologically active part of nebivolol is still preserved in the identified TPs even after two hours of irradiation. The matrices’ pH plays an important role for the elimination mechanism of the micropollutants in the environment. With regard to photolysis, the different species might have various photolytic degradation pathways, transformation products and kinetics of mechanism-based degradation. In order to demonstrate this, the influence of different pH values (3, 5, 7 and 9) on the reaction kinetics and on the degradation mechanism of ciprofloxacin by direct ultraviolet photolysis (UV-C irradiation) and photocatalysis (TiO2/UV-C) has been investigated. During the photolytic and photocatalytic degradation of ciprofloxacin, pseudo-first order kinetics were found with the highest removal rates at pH 9 (kUV and TiO2/UV= 4.0 × 10−4s−1). 18 transformation products have been identified at different pH values (3, 5, 7 and 9). Four transformation products have been detected for the first time, two of the newly proposed structures were supported by the results obtained using deuterated ciprofloxacin. Photolysis was studied for five further common micropollutants. A corrosion inhibitor (1H-benzotriazole) and four pharmaceuticals from different compound classes were included in the study: a -blocker (metoprolol), an antibiotic (sulfamethoxazole), an anti-inflammatory drug (diclofenac) and an anti-epileptic agent (carbamazepine). The photodegradation was affected by the WWTP effluent matrix. Organic and inorganic substances in wastewater or natural water environments played a dual role of sensitizer and quencher in the photodegradation. In this study, photodegradation rate constants of metoprolol and carbamazepine increased in presence of WWTP effluent matrix, probably due to the presence of photosensitizer compounds. In contrast, diclofenac and sulfamethoxazole showed decreased photodegradation rate constants, due to physical or chemical quenching of the photochemical degradation intermediates by competitors. The matrix also posed non-negligible influences on benzotriazole photodegradation process. Overall, direct photolysis was demonstrated to be relevant for micropollutant abatement from the aquatic environment. However, to promote the photolysis application on a broader scale, it is essential to further understand how this process is affected by the UV sources, micropollutant structures and matrix composition.