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The solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides is investigated at 22±2 °C in alkaline 0.1−4.5 M CaCl 2  solutions. Further studies are performed with Zr(IV) over the entire pH range in NaCl and CaCl 2  media, and with Zr(IV) and Th(IV) in alkaline Ca(ClO 4 ) 2  solutions. The comparison of Zr(IV) data in different ionic media (NaCl, NaClO 4 , CaCl 2  and Ca(ClO 4 ) 2 ) of similar ionic strength shows that the solubility in the acidic and neutral pH range is not affected by strong interactions between the aqueous M(IV) species and the medium ions. However, in alkaline CaCl 2  and Ca(ClO 4 ) 2  solutions the formation of ternary Ca-M(IV)-OH complexes causes unexpectedly high solubilities of Zr(IV) at pH c =10−12 and [Ca 2+ ]&gt;0.05 M and of Th(IV) at pH c =11−12 and [Ca 2+ ]&gt;0.5 M. The dependence of the Zr(IV) and Th(IV) solubilities on the H +  and CaCl 2  concentrations shows that the complexes Zr(OH) 6  2−  and Th(OH) 8  4−  with an unusual large number of OH −  ligands are stabilized by the formation of associates or ion pairs with Ca 2+  ions. The SIT is used to derive equilibrium constants at zero ionic strength for the complexes Zr(OH) 6  2−  (in calcium-free solutions), Ca 2 [Zr(OH) 6 ] 2+ , Ca 3 [Zr(OH) 6 ] 4+  and Ca 4 [Th(OH) 8 ] 4+ . In analogous studies with Pu(IV) hydrous oxide, the solubility increasing effect of ternary complex formation with Ca 2+  ions is only observed at CaCl 2  concentrations above 2 M.

Solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides in CaCl 2 solutions and the formation of ternary Ca-M(IV)-OH complexes