Mechanistic Insight into the Effect of Metal Ions on Photogeneration of Reactive Species from Dissolved Organic Matter.

The photogeneration of reactive species (RS) from dissolved organic matter (DOM) exhibits a great impact on the attenuation of pollutants in natural waters. However, the effect of metal ions on the photogeneration of excited triplet-state DOM (DOM*), singlet oxygen (O), and hydroxyl radical (OH) by effluent organic matter (EfOM), fulvic acid (FA), and humic acid (HA) is poorly understood. Here, we provided the first evidence that the quenching of DOM* was positively correlated with the complexation capacity of metal ions with DOM. Generally, the paramagnetic metal ions (Cr, Mn, Fe, and Cu) exhibited higher conditional stability constants (log K) with DOM and stronger inhibition for RS than the others (Mg, Ca, Al, and Zn). For DOM of different sources, the metal binding capacity increased in the order of EfOM < HA < FA and the humic substances were more susceptible to metal ions. The inhibition was attributed to both static and dynamic quenching of DOM* by metal ions. The dynamic quenching rate constants of metal ions for DOM* were estimated as ∼10 M s, which was positively related to the corresponding log K. These findings highlight crucial links between metal-DOM complexation and DOM* quenching and, consequently, the inhibition of RS.