Photodegradation of triclosan in SiO₂-water heterogeneous systems: Mechanisms coupled with adsorption effects

Triclosan (TCS) is a hydrophobic and endocrine-disrupting contaminant that poses significant ecological risks. Suspended particulate matter (SPM) is ubiquitous in natural waters and can simultaneously influence both photolysis and adsorption of TCS. However, the mechanistic understanding of this dual modulation remains limited. In this study, we systematically investigated the phototransformation of TCS in SiO₂-water heterogeneous systems. Results demonstrated that SiO₂ particles generated reactive oxygen species (ROS) under irradiation, leading to accelerated photodegradation kinetics and an enhanced contribution of indirect photolysis. Adsorbed TCS exhibited slower degradation kinetics and markedly reduced pH sensitivity relative to dissolved TCS, suggesting that the adsorbed species existed in a form distinct from the ionized aqueous species. Dissolved organic matter (DOM) influenced TCS photolysis via light shielding, ROS quenching, and enhanced adsorption onto SiO₂; however, the effect of DOM was attenuated in the heterogeneous system, likely due to the increased dominance of ROS-mediated indirect photolysis, which diminished the relative importance of DOM's light-shielding effect. Notably, the solid-phase yield of 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) was lower than its aqueous-phase yield, confirming the modulating effect of SiO₂ on dioxin formation. Collectively, these findings suggested that SPM and its adsorptive interactions critically regulate TCS persistence, byproduct production, and phase-dependent transport.