Solubility, hydrolysis and colloid formation of plutonium(IV)

The solubility and hydrolysis behaviour of Pu(IV) is critically reviewed taking into account the effect of colloid formation. Literature data on mononuclear Pu(TV) hydrolysis constants, determined from a solvent extraction study with Pu(IV) trace concentrations, are used to calculate the solubility product of amorphous Pu(OH)(4)(am) from the available experimental solubility data. Applying the SIT approach, the thermodynamic constant is calculated to be log K(sp)degrees = -58.7 +/- 0.9. The evaluated solubility product agrees well with the literature value determined recently by an indirect method, independent of Pu(IV) hydrolysis reactions. The generation of Pu(IV) colloids is investigated by chemical reduction of Pu(VI) as a function of the Pu concentration in 0.1 M HClO4. Ultrafiltration and Laser induced breakdown detection (LIBD) measurements demonstrate that colloid formation is the predominant reaction, when the solutions are oversaturated with respect to the thermodynamic solubility of Pu(OH)(4)(am). In undersaturated solutions, the contribution of Pu(IV) colloids is negligible. The thermodynamic calculation of the solubility is consistent with the present results on colloid formation.