Experiments and Molecular Dynamics Simulations on Nature Differences of Humic Acids From Different Origin for Binding Characteristics of U(Ⅵ)

Three humic acids(AHA, YHA, and EHA) from different origin were characterized by elementary analysis, UV-Vis and FTIR spectra, and the functional groups were determined by chemical titration. The effects of pH, time, temperature, and ionic strength on the complexation of three HAs with U(Ⅵ) were investigated systematically. Meanwhile, the interaction of U(Ⅵ) with three HA models in aqueous phase was studied by means of molecular dynamics(MD) simulations, and the dynamics, coordination structures, and interaction mechanism were further explored. The experimental results show that the elemental composition and functional group types of three HAs from different origin are basically similar, with strong aromaticity and conjugated double bonds, but slight differences can be also observed between them. The low H/C atom ratio, high acidity, and high content of functional groups indicate that YHA has high humification degree, high conjugation or aromatic composition, and strong metal complexation ability. The complexation behavior of three HAs with U(Ⅵ) are significantly different, which is substantially affected by pH, time, temperature, and ionic strength. MD simulations demonstrate that the complexation of single HA with U(Ⅵ) can be completed in a short time, and the main binding sites are carboxyl groups. The coordination structures formed by HAs with U(Ⅵ) in aqueous solution have quite differences, and the main driving force is electrostatic interaction. The results reported here are beneficial to further understanding the chemical behavior of uranium in the presence of HA, and provide data and reference for the geological disposal and safety evaluation of radioactive wastes.