Enhancing Aquatic Persistence Assessments: Effects of Photosensitizers and Surfactants on Photochemical Degradation in the Environment

Photochemical processes substantially influence the environmental fate of chemicals, yet current persistence assessments rarely account for indirect photochemistry or interfacial effects. Here, we systematically investigated the photodegradation of the fungicide carbendazim, which is classified as persistent in aquatic environments according to current regulations. Experiments were conducted in aqueous systems in presence of key environmental photosensitizers (nitrate, chromophoric dissolved organic matter [CDOM], iron) and representative non-ionic surfactants, with a particular focus on the role of the air-water interface. Across 36 controlled irradiation experiments, carbendazim degradation was strongly enhanced by nitrate, CDOM, and Fe-CDOM interactions. Additionally, surfactants induced accelerated degradation after a pronounced lag phase, highlighting the importance of processes at the air/water interface. In contrast to the persistence classification of carbendazim, first-order degradation rates of up to 9.68 × 10-2 d-1 were observed, corresponding to extrapolated environmental half-lives of 75 days even under the low-light conditions of southern Sweden, and of ∼46 days at the sunniest European location defined by the Forum for the Co-ordination of Pesticide Fate Models and their Use (FOCUS). Dedicated experiments indicated that interfacial enrichment of carbendazim, surfactant, and CDOM are a prerequisite for efficient photosensitized transformations. Bulk-phase experiments showed that increasing bicarbonate concentrations inhibited carbendazim degradation, confirming hydroxyl radicals as the dominant oxidants under these conditions. A case study with two natural waters further indicated significant photodegradation in the aquatic environment (environmental half-lives, DT50, of 5.8-106 days for low-light conditions). Collectively, these findings demonstrate that interfacial processes and indirect photochemistry can dominate the environmental transformation of recalcitrant chemicals such as carbendazim. Persistence assessments that rely solely on degradation in the dark may therefore underestimate degradation rates in sunlit surface waters. Incorporating representative irradiation, photosensitizers, and interfacial processes into regulatory higher-tier assessments will enable more realistic evaluations of chemical persistence in aquatic environments.