Day-night alternation and effect of sulfate ions on photodegradation of triclosan in water

Photodegradation is an essential process for the in-stream elimination of triclosan (TCS). There are still some knowledge gaps concerning reaction kinetics, pathways, and transformation products, which are important for understanding the environmental fate of TCS. For example, the effects of ionic strength and day-night alternation on TCS photodegradation have been barely considered in previous studies. To fill these gaps, this study assesses the effect of sulfate ions (SO₄²⁻) on the photodegradation of TCS under day-night shifts. Compound-specific carbon and chlorine isotope analysis were applied to characterize different bond-cleavage pathways. Moreover, variations in TCS reaction products distribution were analyzed by using Fourier transform ion cyclotron resonance mass spectrometry. A numerical model was developed to describe concentration and stable carbon isotopic evolution of TCS under different conditions. Based on the observed experimental data, we show that SO₄²⁻ can affect the kinetics and product distribution during the photodegradation of TCS. In particular, sulfate ions (100 mM) inhibited the yield of the toxic product, 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD). Furthermore, day-night alternations lead to different predominant bond-cleavage pathways depending on sulfate concentration. By so doing, we propose a realistic framework for the direct photodegradation of TCS in sunlit natural surface waters.