INVESTIGATION OF AQUO AND CHLORO COMPLEXES OF UO22-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY(, NPO2+, NP4+, AND PU3+ BY X)

U, NP, and PU L-II.III-edge X-ray absorption fine structure (XAFS) spe ctra were collected for the UO22+, NpO2+, Np4+, and PU3+ ions as a fun ction of chloride concentration in aqueous solution. At low chloride c oncentration, the hydration numbers and corresponding bond lengths for the different ions are as follows: UO22+, N = 5.3, R = 2.41 Angstrom; NpO2+, N = 5.0, R = 2.50 Angstrom; Np4+, N = 11.2, R = 2.40 Angstrom; Pu3+, N = 10.2, R = 2.51 Angstrom. As the Cl- concentration increases , inner-sphere Cl- complexation occurs, resulting in a decrease in the hydration numbers and an expansion of the actinide-oxygen (water) bon d lengths. The Pu3+ ion showing only a decrease in hydration number (4 0%) and no inner-sphere Cl- complexation for [Cl-] < 14 M. For concent rations up to 10-14 M Cl- the average Cl- coordination numbers and bon d lengths are as follows: UO22+, N = 2.6, R = 2.73 Angstrom; NpO2+, N = 1.0, R = 2.84 Angstrom; Np4+, N = 2.O, R = 2.61 Angstrom. Structural changes are observed in the near-edge spectral region as shown by sig nificant changes in the White line intensities upon Cl- complexation. For ions with similar structures, i.e. Pu3+ and Np4+ or the actinyl io ns NpO2+ and UO22+, positive energy shifts are observed with increasin g oxidation state. The ability to use XAFS speciation results to calcu late equilibrium constants and the relationship gf these results to pr evious Studies are discussed.