EXAFS investigations of the interaction of humic acids and model compounds with uranyl cations in solid complexes

Natural humic acids, HA's, having Varying amounts of their proton exchange capacities, PEC's, loaded with uranyl ions and synthetic HA loaded with 14% PEC were prepared either from solution or from suspension. The interaction of uranium with the humates was studied using U L-III-edge extended X-ray absorption fine structure, EXAFS, and infrared, IR, spectroscopy. IR results indicate a direct complexation of the uranyl ions onto the HA's. The spectral positions of the asymmetric and symmetric IR stretching frequencies for COO- in the complex suggest monodentate coordination of HA carboxylate groups onto the uranyl cation. In all samples studied, the EXAFS analysis yielded axial uranium-oxygen distances of 1.77-1.78 Angstrom and five oxygen atoms in the plane equatorial to the uranyl unit at distances of 2.37-2.39 Angstrom. The bond distances determined from the EXAFS are the same, within the experimental error, for both synthetic and two different natural uranyl humates, for samples with large loadings and samples with relatively low uranyl loadings, as well as for dry and wet paste samples; Comparison of the EXAFS from the uranyl humates with that observed for two crystalline uranyl carboxylate complexes indicates that the HA carboxylate groups act predominantly as monodentate ligands when bound to the uranyl unit. Additional, neutral ligands must also be coordinated to the uranyl ion in order to satisfy the uranyl cation coordination number determined as 5 +/- 0.7.