EXAFS study of aqueous Zr(IV) and Th(IV) complexes in alkaline CaCl(2) solutions: Ca(3)[Zr(OH)(6)](4+) and Ca(4)[Th(OH)(8)](4+)

A hitherto unknown type of aqueous complex, ternary Ca-MIV-OH complexes (M = Zr and Th), causes unexpectedly high solubilities of zirconium(IV) and thorium(IV) hydrous oxides in alkaline CaCl2 solutions (pHc = 10-12, CaCl2] > 0.05 mol.L(-1), and pHc = 11-12, CaCl2] > 0.5 mol.L(-1), respectively). The dominant aqueous species are identified as Ca3Zr(OH)6]4+ and Ca4Th(OH)8]4+ and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The number of OH- ligands in the first coordination sphere detected by EXAFS, NO = 6 (6.6 +/- 1.2) for Zr and NO = 8 (8.6 +/- 1.2) for Th, are consistent with the observed slopes of 2 and 4 in the solubility curves log M]tot vs pHc. The presence of polynuclear hydrolysis species and the formation of chloride complexes can be excluded. EXAFS spectra clearly show a second coordination shell of calcium ions. The Zr(OH)6]2- and Th(OH)8]4- complexes with an unusually large number of OH- ligands are stabilized by the formation of associates or ion pairs with Ca2+ ions. The number of neighboring Ca2+ ions around the Zr(OH)6]2- and Th(OH)8]4- units is determined to be NCa = 3 (2.7 +/- 0.6) at a distance of RZr-Ca = 3.38 +/- 0.02 A and NCa = 4 (3.8 +/- 0.5) at a distance of RTh-Ca = 3.98 +/- 0.02 A. The Ca3Zr(OH)6]4+ and Ca4Th(OH)8]4+ complexes have first (M-O) and second (M-Ca) coordination spheres with the Ca2+ ions bound to coordination polyhedra edges.