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

Abstract

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 (³DOM*), singlet oxygen (¹O₂), 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 ³DOM* was positively correlated with the complexation capacity of metal ions with DOM. Generally, the paramagnetic metal ions (Cr³⁺, Mn²⁺, Fe³⁺, and Cu²⁺) exhibited higher conditional stability constants (log K_ML) with DOM and stronger inhibition for RS than the others (Mg²⁺, Ca²⁺, Al³⁺, and Zn²⁺). 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 ³DOM* by metal ions. The dynamic quenching rate constants of metal ions for ³DOM* were estimated as ∼10⁹ M^-1 s^-1, which was positively related to the corresponding log K_ML. These findings highlight crucial links between metal-DOM complexation and ³DOM* quenching and, consequently, the inhibition of RS.