Role of molecular weight-dependent spectral properties in regulating Cu(II) binding by dissolved organic matter from different sources

The complexation of metals with dissolved organic matter (DOM) under different compositions and molecular weights (MWs) will result in different environmental fate and toxicity, but the specific role and impact of DOM MWs remain less well understood. This study explored the metal binding characteristics by DOM with different MWs from different sources, including sea, river, and wetland waters. The results of fluorescence characterization showed that the >1 kDa high-molecular-weight (HMW)-DOM were mainly from terrestrial sources while the low-molecular-weight (LMW)-DOM fractions were mostly from microbial sources. Based on UV-Vis spectroscopic characterization, the LMW-DOM contained more unsaturated bonds than its HMW counterpart, and the substituents are generally dominated by polar functional groups. Summer DOM had more unsaturated bonds and a higher metal binding capacity than winter DOM. Furthermore, DOM with different MWs had significantly different Cu binding properties. In addition, Cu binding with microbially derived LMW-DOM mainly caused the change in the peak at 280 nm, while binding with terrigenous HMW-DOM resulted in the change of the 210 nm peak. Compared with the HMW-DOM, most of the LMW-DOM had stronger Cu-binding ability. Correlation analysis indicates that metal binding ability of DOM mainly depends on its concentration, number of unsaturated bonds and benzene rings, and types of substituents during interactions. This work provides an improved understanding of the metal-DOM binding mechanism, the role of composition- and MW-dependent DOM from different sources, and thus the transformation and environmental/ecological role of metals in aquatic systems.