PHOTOLYTIC TRANSFORMATION OF CONTAMINANTS OF EMERGING CONCERN IN THE AQUATIC ENVIRONMENT

Abstract

The reuse of secondary effluents from wastewater treatment plants (WWTPs) is an important alternative water source to meet increasing demand, particularly in semi-arid and arid regions. However, contaminants of emerging concern persist in treated effluents at concentrations ranging from ng L⁻¹ to μg L⁻¹, where they may remain biologically active and pose risks to aquatic ecosystems and human health. This study investigates the role of photolysis as a key natural attenuation process governing the fate of anthropogenic trace organic compounds (TOrCs) in aquatic environments. Field investigations were conducted in an effluent-dependent reach of the Santa Cruz River in Tucson, Arizona, and in surface and tap water sources in El Salvador to quantify TOrC occurrence. Complementary laboratory experiments were performed to elucidate transformation mechanisms, including direct and indirect photolysis pathways. Results demonstrate that direct photolysis is responsible for the attenuation of certain TOrCs in the environment. In particular, hydrochlorothiazide and ibuprofen were susceptible to this mechanism in field and laboratory studies. In contrast, other compounds (e.g. trimethoprim) showed significant attenuation by indirect photolysis mediated by triplet-excited chromophoric dissolved organic matter. Experiments with water samples from El Salvador further indicate that singlet oxygen (1O2) is a major contributor to TOrC transformation The stability and photolytic behavior of N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), a common antioxidant identified as an important contaminant derived from tire wear particles, was studied under various conditions. Results show that the deprotonated form of the compound, present at pH > pKa, is highly unstable in water matrices but not susceptible to photodegradation, while the protonated form is stable, but it undergoes direct photolysis. Overall, the results demonstrate that 6PPD degrades rapidly under environmentally relevant conditions, particularly at elevated temperature and dissolved oxygen levels. These findings highlight the compound-specific nature of photochemical attenuation processes and underscore the importance of both direct and indirect photolysis in controlling the environmental fate of contaminants of emerging concern in natural waters.