Characterization of copper binding to different molecular weight fractions of dissolved organic matter in surface water

Dissolved organic matter (DOM) is a kind of substance with complex compositions and wide molecular weight distribution, which can strongly combine with various pollutants. Therefore, the binding characteristics of DOM and heavy metal pollutants can be studied specifically according to the binding characteristics of DOM and pollutants. In this study, DOM in surface water bodies was divided into three levels (MW < 1 kDa, 1 kDa < MW < 5 kDa, MW > 5 kDa) according to different molecular weights (MW). The binding properties were investigated by fluorescence spectrum analysis and complex model. Four components (C1-C4) were identified by PARAFAC. Among them, the contribution rate of protein-like components C1, C2 and C4 to the total fluorescence intensity reached more than 78%, and the log K_a values of low molecular weight components were the highest, which were 3.28, 3.14 and 3.47, respectively, indicating higher binding ability with Cu²⁺.With the decrease of molecular weight, the log K_b value increases, indicating that the complexation is more stable. The humic component C3 in high molecular weight has stronger binding stability with Cu²⁺, but the number of binding sites for C3 is 0.36, while that for C2 is 1.51, indicating that its binding sites and binding ability are relatively low. The results showed that the DOM ligand of Cu²⁺ in surface water showed a certain molecular weight dependence. In addition, different MW DOM lead to different pollution forms. Different properties of DOM ligand combined with Cu²⁺ were studied in order to control the migration, transformation, bioavailability, morphology and stability of heavy metal pollutants, and to provide theoretical support for the practical application management of surface water pollution control.