Interaction of uranium(VI) with various modified and unmodified natural and synthetic humic substances studied by EXAFS and FTIR spectroscopy


Interaction of uranium(VI) with various modified and unmodified natural and synthetic humic substances studied by EXAFS and FTIR spectroscopy

Schmeide, K.; Sachs, S.; Bubner, M.; Reich, T.; Heise, K. H.; Bernhard, G.

The complexation of uranium(VI) by humic and fulvic acids was studied to obtain information on the binding of uranium(VI) onto functional groups of humic substances. For this, various natural and synthetic humic acids were chemically modified resulting in humic acids with blocked phenolic OH groups. Both from the original and from the modified humic substances, solid uranyl humate complexes were prepared at pH 2. FTIR and extended X-ray absorption fine structure (EXAFS) spectroscopy were applied to study the chemical modification process of humic substances, to study the structure of uranyl humate complexes and to evaluate the effect of individual functional groups of humic substances (carboxylic and phenolic OH groups) on the complexation of uranyl ions. The results confirmed the predominant blocking of phenolic OH groups in the modified humic acids. These modified humic acids are suitable model substances to study the role of phenolic OH groups of humic acids in dependence on pH. By EXAFS spectroscopy, 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 humic acids did not change the near-neighbor surrounding of uranium(VI) in uranyl humate complexes. Thus, the results confirmed that predominantly humic acid carboxylate groups are responsible for binding of uranyl ions and that the influence of phenolic OH groups is insignificant under the applied experimental conditions. The carboxylate groups are monodentate coordinated to uranyl ions.

Keywords: Uranium; Humic Acid; Fulvic Acid; Complexation; EXAFS; FTIR

  • Inorganica Chimica Acta 351, 133-140 (2003)

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Publ.-Id: 4975