Measurements of the structural parameters for the interaction of uranium(VI) with natural and synthetic humic acids using EXAFS

Fluka humic acid having 87, 35, 10, and 8 percent of its proton exchange capacity, % PEC, loaded with uranyl ions and synthetic humic acid loaded with 14% PEC were prepared either from solution or from suspension. The interaction of uranium with the humic acids was studied using U L-m-edge extended X-ray absorption fine structure, EXAFS, and infrared, IR, spectroscopy. IR results indicate complexation of the uranyl ions onto the humic acid carboxylic groups. The uranium-oxygen (U-O) bond distances determined from the EXAFS analysis are the same, within the experimental error, for both synthetic and natural uranyl humates, for samples with large loadings and samples with relatively low uranyl loadings, as well as for dry and wet paste samples. In all samples studied, axial U-O distances of 1.77-1.78 Angstrom and five equatorial oxygen atoms at distances of 2.37-2.39 Angstrom were found. Comparison of the equatorial bond distances to those for Various uranyl carboxylates reported in the literature indicates that the humic acid 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 five.