Photodegradation of polychlorinated diphenyl sulfides mediated by reactive oxygen species on silica gel

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Polychlorinated diphenyl sulfides (PCDPSs) are a group of dioxin-like compounds that have been widely used in agricultural and industrial productions. Here, we systematically investigated the photochemical behaviors of 2,2′,3′,4,5-pentachlorodiphenyl sulfide (2,2′,3′,4,5-PCDPS) on the surface of silica gel (SG) in an aqueous environment. Under the simulated sunlight irradiation, 2,2′,3′,4,5-PCDPS adsorbed on SG (0.11 mg/g) was found to be degraded with time, giving a removal rate of 68.5% within 16 h at pH 7.0. Environmental factors like pH and humic acid can also affect the removal rate. Results showed that the removal rate of 2,2′,3′,4,5-PCDPS in alkaline conditions (75.6% at pH 11) was higher than that in acidic conditions (46.7% at pH 3). Moreover, the addition of (1–5 mg/L) humic acid can promote the degradation rate compared to control group. In our study, it was found that SG can act as photocatalyst to generate reactive oxygen species (ROS) for the degradation of PCDPSs, and UV light contributed much more than visible light (>420 nm). According to electron paramagnetic resonance (EPR) technology and radicals quenching experiments, hydroxyl radical (OH), single oxygen (1O2), and superoxide radical (O2−) participated in the removal of the contaminant. Based on the identified intermediate products via LC–MS, two main pathways, i.e., the hydroxyl-substituted reaction of the benzene ring and the oxidation of the sulfur atom, were proposed for the photodegradation of 2,2′,3′,4,5-PCDPS. Then, the density functional theory (DFT) was employed to confirm these reaction pathways. This study would enhance the understanding of the photochemical transformation and environmental fate of PCDPSs on the surface of solids in natural waters.