Effects of Cu(II)-DOM complexation on DOM degradation: Insights from spectroscopic evidence

The fate of dissolved organic matter (DOM) is primarily governed by its sources, degradation, and transformation processes within the environment. However, the influence of metal-DOM complexation on DOM degradation remains ambiguous. In this study, controlled laboratory experiments were conducted using Cu(II) and natural water from the Duliujian River and the Beidagang Wetland to examine the effects of metal-DOM binding on the degradation pathway of DOM. Our results showed that Cu(II)-DOM complexation affected the distribution of DOM molecular weight with elevated Mw after complexed with Cu(II). Nevertheless, the concentration of DOM decreased over the incubation period due to degradation. In the absence of Cu(II) binding, both wetland and river DOM followed similar degradation pathways, transforming from high to low molecular weight with changes predominantly in the 1-10 kDa size-fraction during DOM degradation. In contrast, in the presence of Cu(II) and thus Cu(II)-DOM binding, the degradation of DOM was enhanced, resulting in higher kinetic rate constants for both wetland and river DOM. The results of differential spectra further confirmed the degradation of DOM with a decrease in bulk spectroscopic properties and an increase in the degree of DOM-Cu(II) complexation. These findings imply a mutually reinforcing relationship between metal-DOM complexation and the degradation of DOM in aquatic environments, providing new insights into the biogeochemical behavior and environmental fate of DOM.