Photochemical fate of synthetic progestins in sunlit surface waters: experimental kinetics, modeling, and ecotoxicological implications.

This study combines experimental and theoretical methods to explore the photochemical stability of progestins in a climate-sensitive model system for urban reservoirs worldwide, with a focus on the Guarapiranga Reservoir in São Paulo, Brazil. The target contaminants were levonorgestrel (LNG) and gestodene (GES), synthetic progestins used in contraceptive methods. Photoinduced reaction kinetics showed that LNG's indirect photolysis is affected by triplet excited states of chromophoric dissolved organic matter (3CDOM*), hydroxyl radicals (HO·), and singlet oxygen (1O2), in the order 3CDOM* > HO· > 1O2. Conversely, for GES, the sequence was HO· ≈ 3CDOM* > 1O2. Water quality data from 2022 to 2024, along with kinetic modeling, estimated half-lives of 6.9 ± 1.2 days for LNG and 25.8 ± 9.0 days for GES. Density Functional Theory (DFT) analysis identified key reactive sites and degradation pathways, aligning well with experimental results. An ecotoxicological assessment of the photo intermediates using the ECOSAR model showed notable toxicity to daphnia, fish, and green algae, especially after the addition of HO· radicals. These findings offer valuable insights into the environmental fate of LNG and GES in urban surface waters and contribute scientific evidence to decision-making on monitoring and removing emerging contaminants through advanced wastewater treatment technologies.