Dual roles of Cu2+ 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 (3DOM⁎) 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 3DOM⁎-induced photodegradation of TOPs has not been systemically explored. Herein, we investigated the effect of environmental Cu2+ on the 3DOM⁎-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), Cu2+ complexation notably inhibited 3DOM⁎-induced photodegradation. This may be ascribed to the decrease of 3DOM⁎ steady-state concentration because Cu2+ complexation reduces its formation rates and enhances scavenging rates tested by sorbic acid isomerization experiment. Meanwhile, it was found that Cu2+ 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 3DOM⁎ 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 3DOM⁎ in the presence/absence of Cu2+ with their 3DOM⁎ reactivity. These findings helped to improve our understanding of the photochemical reactivity of 3DOM⁎ in natural waters, especially the effects of environmentally concomitant metal ions.