Dual roles of Cu complexation with dissolved organic matter on the photodegradation of trace organic pollutants: Triplet- and OH-induced reactions.

The triplet excited state of dissolved organic matter (DOM) is highly effective in the photodegradation of a broad spectrum of trace organic pollutants (TOPs), and its photoactivity is affected by concomitant metal ions in surface waters. However, the impact of environmental metal ions on the DOM-induced photodegradation of TOPs has not been systemically explored. Herein, we investigated the effect of environmental Cu on the DOM-induced photodegradation kinetics of 16 TOPs. A fluorescence quenching experiment showed that a Cu(II)-DOM complex was formed. For the TOPs with stronger electron-donating groups (triplet-labile moieties, e.g., phenols and anilines), Cu complexation notably inhibited DOM-induced photodegradation. This may be ascribed to the decrease of DOM steady-state concentration because Cu complexation reduces its formation rates and enhances scavenging rates tested by sorbic acid isomerization experiment. Meanwhile, it was found that Cu complexation facilitated the photolysis of refractory TOPs (lower triplet reactivity) because of enhanced electron transfer between DOM and Cu(II), causing photoinduced OH formation. These findings implied that DOM reactivity differences in TOPs could affect the photodegradation rates in the complex system, which was confirmed via a linear correlation of photodegradation rate ratios for 16 TOPs induced by DOM in the presence/absence of Cu with their DOM reactivity. These findings helped to improve our understanding of the photochemical reactivity of DOM in natural waters, especially the effects of environmentally concomitant metal ions.