Study of the generation and stability of thorium(IV) colloids by LIBD combined with ultrafiltration

The generation of Th(IV) colloids is investigated by laser-induced breakdown detection (LIBD) combined with ultrafiltration in the concentration range from 10^-5 to 10^-2 M at pH 3-5 in 0.5 M NaCl. The amount of colloids generated by coulometric titration of initially ionic thorium solutions depends on the total thorium concentration and the degree of over-saturation with regard to the thermodynamic solubility limit of solid Th(IV) hydroxide at given pH. These generated thorium colloids are found to be small in size and remain stable up to 400 days of investigation, without a tendency towards agglomeration or precipitation. A dilution of the suspension at constant pH downgrades colloids to ionic species, while a dilution with 0.5 M NaCl of neutral pH causes an increased over-saturation and colloid generation, leading to an equilibrium between colloids and ionic species. This equilibrium is comparable to the thermodynamic solid-liquid equilibrium of amorphous Th(IV) hydroxide. The present results allow a precise estimation of colloid generation in the dissolution of a Th(IV) hydroxide solid phase. Such a possibility may facilitate the practical solubility assessment of metal hydroxides in natural aquifer systems.