Optical property of dissolved organic matters (DOMs) and its link to the presence of metal ions in surface freshwaters in China

•The deprotonation property of DOM in wide range of surface waters in China was tracked.•Binding capacities of DOMs could be quantified by spectral parameter and NICA Model.•The presence of dissolved Zn, Hg and Ni correlate with total binding sites in DOMs.•Absorbance titration can help understand the speciation and bioavailability of metals.

It is believed that dissolved organic matter (DOM) plays an important role in carrying metal ions through global cycle and distribution, while this point has not yet been elucidated clearly as DOM is extremely heterogeneous and site-specific. In this study, optical properties of DOM in 53 surface freshwater sites all over China were recorded by UV-Vis absorbance spectroscopy in pH range 3-11. The subtle processes of protonation-deprotonation of DOM were quantified by tracking the changes of DOM spectra. The binding capacities of DOMs-binding sites and affinity constants-were interpreted by introducing a spectral parameter, differential log-transformed differential absorbance at 400 nm (DlnA400) in combination with Non-Ideal Competitive Adsorption (NICA) Model. It is found that the presence of dissolved Zn, Hg and Ni in the examined waters show strong correlation with total binding sites in DOMs. However, the presence of some other metals with high affinities to DOM, e.g. Cu, Fe and so on, have not demonstrated strong correlation with the total binding sites in DOMs. This indicates the presence of these metals may be seriously influenced by other factors besides DOM. This study demonstrates that the spectroscopic titration approach could potentially provide more structure-specific in situ information about DOM and help to understand the role of DOM in the speciation and bioavailability of toxic metal ions.