Solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides in CaCl₂solutions and the formation of ternary Ca-M(IV)-OH complexes

Abstract

Zirconium / Thorium / Plutonium / Ternary Ca-M(IV)-OH complexes / Solubility / Thermodynamics / SIT Summary. 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+ ] > 0.05 M and of Th(IV) at pH c = 11-12 and [Ca 2+ ] > 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 and hydrolysis of Np(V) in dilute to concentrated alkaline NaCl solutions: formation of Na–Np(V)–OH solid phases at 22 °C

“…Sparingly soluble ternary Ca-Np(V)-OH and quaternary Ca-Np(V)-OH-Cl solid phases were found to control the solubility of Np within the investigated pH m -range (8)(9)(10)(11)(12) and CaCl 2 concentrations (0.25-4.5 M). The authors observed also the formation of very stable ternary Ca x NpO 2 (OH) y 1+2x-y species in the aqueous phase (x = 1 and 3; y = 2 and 5), in line with previous studies available for An(III) and An(IV) in alkaline CaCl 2 solutions [40][41][42]. No further experimental studies dedicated to shed light on the relevant topic of Np(V) solubility and hydrolysis in NaCl systems have been reported after the discussion between Neck and Rao.…”

Thermodynamics of trivalent actinides and neodymium in NaCl, MgCl2, and CaCl2 solutions: Solubility, hydrolysis, and ternary Ca-M(III)-OH complexes

“…It clearly showed the known XRD pattern of Nd(OH) 3 (cr) (JCPDSFile 70-0214). The experimental and analytical procedures and the chemicals used to prepare the matrix solutions were the same as in previous studies and described therein [10,11]. The samples for the solubility measurements were prepared and stored at 22 ± 2 °C in an Ar glove box.…”

“…Moreover, our recent solubility measurements and spectroscopic studies with Zr(IV), Th(IV), Pu(IV), Nd(III), and Cm(III) in alkaline CaCl 2 solutions [9][10][11][12] revealed a hitherto unknown phenomenon, the formation of ternary Ca-M-OH complexes with unusually high numbers of OH -ligands. The solubilities of ZrO 2 ؒxH 2 O(s) at pH = 10-12 in 0.1-2.0 M CaCl 2 and of ThO 2 ؒxH 2 O(s) at pH = 11-12 in 0.5-4.5 M CaCl 2 are raised to unexpectedly high values by the formation of the complexes Ca 3 [Zr(OH) 6 ] 4+ and Ca 4 [Th(OH) 8 ] 4+ , respectively, which could be identified and characterized by extended X-ray absorption fine structure spectroscopy (EXAFS) [9,10]. The equilibrium constants for these complexes were calculated in [10] with the specific ion interaction theory (SIT) [13] recommended in the NEA-TDB for ionic strength corrections.…”

“…The solubilities of ZrO 2 ؒxH 2 O(s) at pH = 10-12 in 0.1-2.0 M CaCl 2 and of ThO 2 ؒxH 2 O(s) at pH = 11-12 in 0.5-4.5 M CaCl 2 are raised to unexpectedly high values by the formation of the complexes Ca 3 [Zr(OH) 6 ] 4+ and Ca 4 [Th(OH) 8 ] 4+ , respectively, which could be identified and characterized by extended X-ray absorption fine structure spectroscopy (EXAFS) [9,10]. The equilibrium constants for these complexes were calculated in [10] with the specific ion interaction theory (SIT) [13] recommended in the NEA-TDB for ionic strength corrections. However, highly saline MgCl 2 or CaCl 2 solutions with ionic strengths up to 16 mol kg -1 are far beyond the validity range of the SIT (I < 4 mol kg -1 ).…”

Radioactivity, Geochemistry, and Health

“…In some potential repository settings, carbonate-poor and Ca-rich alkaline plumes may form owing to corrosion of cementitious waste forms. In these waters, tetravalent actinides form stable An-Ca-OH complexes (Altmaier et al, 2008;Fellhauer et al, 2010).…”