Pharmaceutical and personal care products (PPCPs) are one of the largest groups of emerging contaminants. Concerns about PPCPs mainly arise from their potential risks from not only the parent PPCPs, but also their transformation products (TPs). Phototransformation processes, including direct and indirect photodegradation, are one of the important pathways for the removal of PPCPs in sunlit surface waters. Therefore, evaluation of the phototransformation of PPCPs in aquatic environments is essential for assessing the environmental fate and risks associated with PPCPs.

A key step in evaluating the contribution of phototransformation processes is to assess the concentration levels of photoproduction of reactive intermediates (PPRIs) in sunlit surface waters. In this thesis, the PPRIs were characterized in coastal seawater containing chromophoric dissolved organic matter (CDOM) around Hong Kong. CDOM is a crucial photochemical precursor to PPRIs, including excited triplet states of CDOM (³CDOM*), hydroxyl radicals (·OH), and singlet oxygen (¹O₂)), in aquatic systems, all of which drive the photodegradation of contaminants. Higher apparent quantum yields of PPRIs (Φ_PPRIs), and steady-state concentrations of PPRIs ([PPRIs]_ss) with larger spatial variations were observed in samples collected near the Pearl River during the wet season. Significant correlations (Spearman |r| > 0.6, p < 0.05) were found between Φ_PPRIs and the absorbance properties of CDOM, as well as between [PPRIs]_ss and the quantity-reflected properties of CDOM.

The phototransformation of two classes of PPCPs, organic UV filters (OUVFs) and antibiotics, was investigated due to their sensitivity to sunlight. Butyl methoxydibenzoylmethane (BMDBM), which serves as an active ingredient to protect skin from excessive sunlight exposure, is one of the most used OUVFs worldwide. Fluoroquinolones (FQs) are increasingly produced and consumed to treat bacterial infections. BMDBM and FQs are nowadays ubiquitously detected in aquatic environments and are raising concerns about their persistence and ecotoxicity. In this study, direct photodegradation and indirect photodegradation by PPRIs were investigated for BMDBM and three FQs: moxifloxacin (MOX), gatifloxacin (GAT), and sparfloxacin (SPAR). TPs were identified using ultrahigh-performance liquid chromatography-high resolution mass spectrometry (UPLC-HRMS). The direct photolysis was identified as a significant sink for BMDBM compared to ·OH. The TPs were formed through α-cleavage and decarbonylation reactions involving the keto form of BMDBM. For FQs, both direct and indirect photodegradation governed the photochemical fate of FQs, affecting not only the degradation rate but also the TP composition. TPs primarily formed via defluorination, hydroxylation, and oxidation of sidechain. The prediction of aquatic toxicity for BMDBM, FQs, and their TPs using the quantitative structure-activity relationship (QSAR) approach reveals the potential risks of aquatic toxicity and environmental persistence of some TPs.

Overall, this thesis provides information about the photochemical behavior and ecotoxicity of BMDBM, three FQs, and their TPs. The roles of PPRIs were elucidated especially for FQs. These are important for assessing the fate, persistence, accumulation, and adverse impacts of these compounds in aquatic environments. In addition, the modeling results demonstrate the feasibility of predicting the concentrations and quantum yields of RIs in seawater around Hong Kong, offering potential application for the modeling of PPCPs environmental fate on a large scale.