Solubility and colloid formation of Th(IV) in concentrated NaCl and MgCl2 solution

The solubility of crystalline ThO2(cr) and amorphous hydrated Th(IV) oxyhydroxide ThOn(OH)(4-2n).xH(2)O(am) has been measured in dilute to concentrated NaCl and MgCl2 solutions equilibrated with magnesium hydroxide or hydroxychloride at 22+/-2degreesC. The contributions of colloids to the total thorium concentrations observed in both over- and 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 oil the stability of these hydrophilic Th(IV) oxyhydroxide eigencolloids. which are the predominant species ill solution. They cause relatively high total thorium concentration ill neutral to alkaline steady state solutions, independent of ionic strength: log[Th](tot) approximate to log[Th](coll) = -6.3+/-0.5. In concentrated MgCl2 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.