The persistence and photostabilizing characteristics of benzotriazole and 5-methyl-1H-benzotriazole reduce the photochemical behavior of common photosensitizers and organic compounds in aqueous environments.

Benzotriazole (BT) and 5-methyl-1H-benzotriazole (5-MeBT) are broadly used in industrial applications, such as anti-icing fluids and dishwashing detergent, and act as the primary building blocks for UV absorbers and photostabilizers. This study examined the occurrence of these two compounds in the environment and their unique photochemical behavior affecting photosensitizers and other micro-organic pollutants in aqueous environments. BT and 5-MeBT were detected in all river water samples from the major rivers in Taipei City in the concentration ranges of 147 to 1560 ng/L and 22 to 235 ng/L, respectively, and both compounds persisted through a conventional wastewater treatment plant. The direct photolysis half-lives of BT and 5-MeBT were 56.9 and 14.0 h, respectively. The half-life of photolysis in river water for BT was 44.2 h, whereas the half-life of 5-MeBT was 24.7 h. The long half-lives in real-water matrices resulted in their prevalence in water bodies, and these compounds were also found to minimize the photosensitizing ability of nitrate and dissolved organic matter (DOM) and increase the persistence of other micro-organic pollutant. With BT present, the production of ·OH in nitrate photolysis was reduced, the degradation of DOM under sunlight was hindered, and the photodegradation of pharmaceutical residues in surface water, such as methotrexate, was completely impeded. This study suggests that in cases in which BT and 5-MeBT are highly concentrated, the effectiveness of natural attenuation process, i.e., photodegradation, in the aqueous environment is diminished, which increases the persistence of the pollutants as well as the risk of exposure.