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 exchan ge capacity, % PEC, loaded with uranyl ions and synthetic humic acid l oaded with 14% PEC were prepared either from solution or from suspensi on. The interaction of uranium with the humic acids was studied using U L-m-edge extended X-ray absorption fine structure, EXAFS, and infrar ed, IR, spectroscopy. IR results indicate complexation of the uranyl i ons onto the humic acid carboxylic groups. The uranium-oxygen (U-O) bo nd distances determined from the EXAFS analysis are the same, within t he 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 st udied, axial U-O distances of 1.77-1.78 Angstrom and five equatorial o xygen atoms at distances of 2.37-2.39 Angstrom were found. Comparison of the equatorial bond distances to those for Various uranyl carboxyla tes reported in the literature indicates that the humic acid carboxyla te groups act predominantly as monodentate ligands when bound to the u ranyl unit. Additional, neutral ligands must also be coordinated to th e uranyl ion in order to satisfy the uranyl cation coordination number determined as five.