Solubility and colloid formation of Th(IV) in concentrated NaCl and MgCl₂solution

Abstract

Tetravalent actinides / Thorium(IV) / Solubility / Chloride solution / Ultracentrifugation / Eigencolloids / PseudocolloidsSummary. The solubility of crystalline ThO 2 (cr) and amorphous hydrated Th(IV) oxyhydroxide ThO n (OH) 4−2n ·xH 2 O(am) has been measured in dilute to concentrated NaCl and MgCl 2 solutions equilibrated with magnesium hydroxide or hydroxychloride at 22 ± 2 • C. The contributions of colloids to the total thorium concentrations observed in both overand undersaturation experiments with amorphous Th(IV) precipitates have been analysed by ultracentrifugation. The solubility increasing effect of long-time stable Th(IV) eigencolloids, previously investigated in 0.5 M NaCl solutions, is also observed in concentrated 5 M NaCl. Ionic strength and chloride concentration have no effect on the stability of these hydrophilic Th(IV) oxyhydroxide eigencolloids, which are the predominant species in solution. They cause relatively high total thorium concentration in neutral to alkaline steady state solutions, independent of ionic strength: log[Th] tot ≈ log[Th] coll = −6.3 ± 0.5. In concentrated MgCl 2 solutions saturated with magnesium hydroxychloride colloids, the formation of pseudocolloids, i.e., Th(IV) sorbed onto Mg 2 (OH) 3 Cl·4H 2 O(coll), leads to a further increase of the total thorium concentration up to 10 −5 M. The present results are discussed with regard to maximum Th(IV) and Pu(IV) concentrations in performance assessment calculations.

Formation of uranium(IV)-silica colloids at near-neutral pH

“…At pH values above the onset of An(IV) hydrolysis, the solid is no longer in equilibrium with An 4+ but with products of An(IV) hydrolysis (the most important of which is An(OH) 4 (aq)), and a surface layer of amorphous actinide oxyhydroxide on the solid particle surface becomes solubility-limiting (Neck and Kim, 2001;Neck et al, 2002;Reiller et al, 2008). Consequently, it has been emphasized in the literature that performance assessment calculations on the long-time behavior of actinides should not take into account the extremely low solubilities resulting from the thermodynamic data of the crystalline actinide dioxides, An(IV)O 2 (cr), but should rather be based on the higher solubility values of amorphous An(IV) oxyhydroxide (Altmaier et al, 2004). According to Altmaier et al (2004), solubilities (saturation concentrations) of log[Th(OH) 4 (aq)] = À8.8 ± 0.6 and log[Pu(OH) 4 (aq)] = À10.4 ± 0.5 are obtained at the nearneutral to alkaline region.…”

“…Consequently, it has been emphasized in the literature that performance assessment calculations on the long-time behavior of actinides should not take into account the extremely low solubilities resulting from the thermodynamic data of the crystalline actinide dioxides, An(IV)O 2 (cr), but should rather be based on the higher solubility values of amorphous An(IV) oxyhydroxide (Altmaier et al, 2004). According to Altmaier et al (2004), solubilities (saturation concentrations) of log[Th(OH) 4 (aq)] = À8.8 ± 0.6 and log[Pu(OH) 4 (aq)] = À10.4 ± 0.5 are obtained at the nearneutral to alkaline region. For U(IV) this value is in the range of log[U(OH) 4 (aq)] = À8.0 (Rai et al, 1990;Cachoir et al, 2003;Warwick et al, 2005).…”

“…If colloid removal (ultrafiltration, ultracentrifugation) is omitted, total actinide concentrations, [An(IV)] tot , higher than those based on true solubility, [An(-OH) 4 (aq)], by a factor of about 100-1000 are found due to the presence of An(IV) colloids. Interestingly, the colloids of amorphous actinide oxyhydroxide responsible for these increased An(IV) concentrations, which are formed via the build-up of An(IV) oligomers and polymers (Walther et al, 2008(Walther et al, , 2009), seem to be in equilibrium with the truly dissolved actinide species Bitea et al, 2003;Altmaier et al, 2004;Rothe et al, 2004). They are hydrophilic and their stability is neither influenced by the ionic strength of the solution nor by the pH (within the pH range in which they do not re-dissolve by chemical dissolution).…”

Radioactivity, Geochemistry, and Health

“…Intrinsic colloids potentially could be produced by direct degradation of the nuclear waste or by remobilization of precipitated actinide compounds (Avogadro and de Marsily, 1984;Bates et al, 1992;Kim, 1994). An(IV) colloids have been shown to considerably increase the total radionuclide concentration in solution relative to dissolved concentrations (Altmaier et al, 2004).…”

Salt disposal of heat-generating nuclear waste.

“…In addition, some of the key radioelements in HLW in their expected oxidation states tend to form colloids (intrinsic colloids) or strongly associate with colloids that may be present because of the presence of clay seams in the salt formation (pseudocolloids). Thorium intrinsic colloids have been shown to form at low temperatures in brine (Altmaier, Neck, and Fanghänel 2004;Altmaier et. al.…”