Structural Investigation of the Interaction of Uranium(VI) with Modified and Unmodified Humic Substances by EXAFS and FTIR Spectroscopy

We studied the interaction of uranium(VI) with humic and fulvic acids to obtain information on the binding of uranium(VI) onto functional groups of humic substances. Therefore, various natural and synthetic humic acids (HAs) were chemically modified resulting in HAs with blocked phenolic OH groups. From the original and modified HAs, solid uranyl humate complexes were prepared at pH 2. FTIR spectroscopy and U LIII-edge extended X-ray absorption fine structure (EXAFS) analysis were applied to study the derivatization process of HAs, to study the structure of the uranyl humates and to evaluate the effect of HA functional groups (carboxylic and phenolic OH groups) on the uranyl complexation.
By FTIR spectroscopy it could be shown that the synthesis of modified HAs with blocked phenolic OH groups was successful. These modified HAs are suitable model substances to study the complexation of actinides by humic substances, especially to study the role of phenolic OH groups in dependence on pH.
By EXAFS, identical structural parameters were determined for all uranyl humates. Axial U-O bond distances of 1.78 Å were determined. In the equatorial plane approximately five oxygen atoms were found at a mean distance of 2.39 Å. The blocking of phenolic OH groups of HAs did not change the near-neighbor surrounding of uranium(VI).
The comparison of results obtained by FTIR spectroscopy and EXAFS analyses for uranyl humates prepared from original and modified HAs confirmed that predominantly HA carboxylate groups are responsible for binding of uranyl ions at pH 2. These carboxylate groups are monodentate coordinated to uranyl ions. The influence of phenolic OH groups is insignificant.