Enhanced Persistence at Trace Levels: Unraveling the Concentration-Dependent UV Photodegradation and Environmental Hazard Amplification of 20α-Dihydroprogesterone

Steroid metabolite 20α-dihydroprogesterone (20α-DHP) is increasingly detected in aquatic environments, yet its photochemical transformation during UV-based water treatment remains unclear. This study investigated 20α-DHP UV photodegradation across environmentally relevant concentrations (100 ng/L to 500 μg/L), focusing on kinetics, degradation mechanisms, and product hazards. The UV photodegradation exhibited concentration-dependent kinetics, following a bell-shaped curve with a maximum degradation rate at 50 μg/L. Critically, the slowest degradation occurred at 100 ng/L, indicating that conventional high-concentration experiments substantially underestimate the environmental persistence of 20α-DHP. Under extended irradiation to 12-48 h, 20α-DHP (initial concentration of 50 μg/L) stabilized at residual concentrations (1.15-1.23 μg/L), confirming incomplete mineralization. UV photodegradation was insensitive to pH, temperature, salinity, or natural water constituents. 20α-DHP photodegradation in estuarine water was initiated by direct photolysis and radical-mediated self-sensitized oxidation. Using high-resolution mass spectrometry combined with density functional theory calculations, 10 products were identified, and three primary pathways involving oxygenation, dehydrogenation, and side-chain cleavage were proposed. Persistence, bioaccumulation, and toxicity (PBT) assessment revealed that all products exhibit at least one or more PBT properties, with androstenone and progesterone representing particularly highly hazardous products. These findings challenge the conventional view of UV irradiation simply as a decontamination process and highlight that it may amplify environmental risks through the formation of persistent, hazardous transformation products.