EXAFS analysis of uranium(IV) and thorium(IV) complexes in concentrated CaCl2 solutions

Abstract The coordination of U4+ and Th4+ in concentrated CaCl2 solutions is studied by U and Th L III edge extended X-ray absorption fine structure (EXAFS) spectroscopy. With the decrease of concentration of CaCl2 from 6.9 to 4 M, the Cl− ion coordination number N Cl in the U4+ coordination sphere decreases from 3.4 to 1.3, while the hydration number N O increases from 4.5 to 6.7. The combined coordination number N O+NCl of U4+ in concentrated Cl− solution (8.0), is lower than that in 1.5 M perchloric acid aqueous solution (9.0). For Th4+, the decrease of concentration from 6.9 to 4 M CaCl2, the coordination number N Cl in the Th4+ coordination sphere decreased slightly from 1.9 to 1.5, while coordination number N O increased from 7.6 to 8.8. The N O+NCl of Th4+ in concentrated Cl− solution (9.0), is similar to that in 1.5 M perchloric acid. The bond distance of U–Cl (2.67 Å), is shorter than that of Th–Cl (2.76 Å), because of low coordination number N O to U4+. By adding HCl into the system, the U4+ and Th4+ coordination sphere is unchanged. The coordination structures of U4+ and Th4+ in concentrated LiCl is also discussed.