Investigation of Aquo and Chloro Complexes of UO22+, NpO2+, Np4+, and Pu3+ by X-ray Absorption Fine Structure Spectroscopy

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Abstract
U, Np, and Pu LII,III-edge X-ray absorption fine structure (XAFS) spectra were collected for the UO22+, NpO2+, Np4+, and Pu3+ ions as a function of chloride concentration in aqueous solution. At low chloride concentration, the hydration numbers and corresponding bond lengths for the different ions are as follows: UO22+, N = 5.3, R = 2.41 Å; NpO2+, N = 5.0, R = 2.50 Å; Np4+, N = 11.2, R = 2.40 Å; Pu3+, N = 10.2, R = 2.51 Å. 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) bond lengths. The Pu3+ ion shows only a decrease in hydration number (40%) and no inner-sphere Cl- complexation for [Cl-] < 14 M. For concentrations up to 10−14 M Cl-, the average Cl- coordination numbers and bond lengths are as follows: UO22+, N = 2.6, R = 2.73 Å; NpO2+, N = 1.0, R = 2.84 Å; Np4+, N = 2.0, R = 2.61 Å. Structural changes are observed in the near-edge spectral region as shown by significant changes in the white line intensities upon Cl- complexation. For ions with similar structures, i.e. Pu3+ and Np4+ or the actinyl ions NpO2+ and UO22+, positive energy shifts are observed with increasing oxidation state. The ability to use XAFS speciation results to calculate equilibrium constants and the relationship of these results to previous studies are discussed.