Mechanistic insight into multiple effects of copper ion on the photoreactivity of dissolved organic matter

Sunlight irradiation of dissolved organic matter (DOM) in surface water results in the production of photochemically produced reactive intermediates (PPRIs). This process is inevitably influenced by co-existing metal ions in aquatic environments; However, the underlying mechanism remains unclear. In this study, the effect of co-existing copper ion (Cu^2 +) on PPRIs produced by irradiation of DOM was systematically investigated, because Cu²⁺ is a typical redox transient cation and has strong affinity to DOM. The findings demonstrated that Cu²⁺, acting as cation bridge, caused DOM to aggregate, and had impacts on the optical properties and conformation of DOM. The electron shuttle and catalyst effect of Cu²⁺ could accelerate the charge transfer processes for the increasing of quantum yield and steady concentrations of hydroxyl radical (·OH) with the increase of concentrations of e^-_aq, O₂^.-, hydrogen peroxide (H₂O₂) and charge separated states of DOM (DOM^·+ or DOM^·-); On the other hand, Cu²⁺, as excited state quencher, decrease of apparent quantum yield of triplet state of DOM (³DOM*) and singlet oxygen (¹O₂) through static quenching of singlet excited of DOM (¹DOM*) and dynamic quenching of ³DOM*, respectively. The results provide a deeper understanding of the effect mechanism of Cu²⁺ on the DOM photochemistry in real environment and will be useful for assessment the photodegradation of organic contaminants in the presence of both DOM and Cu²⁺.