Provenance and Characterization of Aquatic Actinide Colloids : Interaction of Actinides with Aluminosilicate and Humate Colloids

Abstract

The present work investigates the behaviour of actinides (An) of different oxidation states in the process of aquatic colloid formation, e.g. at the generation of aluminosilicates (HAS) by co-nucleation of Si and Al in absence or in presence of natural humic acid (HA). The study comprises three parts. The first part of the work concentrates on the interaction of tri-, tetra-, penta- and hexavalent actinide ions with HAS colloids. The HAS colloids are synthesized through heterogeneous nucleation of Si and Al in the pH region 4 to 9 at room temperature and atmospheric pressure. Si concentration is maintained either below or above the saturation concentration of amorphous silica (2x10-3 M). In the former case 10-3 M Si is mixed with 10-5 M Al and Si is present in solution as monosilicic acid. In the latter case, 10-2 M Si is mixed with 10-4 M Al and polysilicic acid prevails in solution. The following tracer nuclides are introduced in the co-nucleation process: 241Am(III), 234Th(IV), 237Np(V) and 233U(VI). Colloids are separated from solution and precipitate by sequential filtration and ultrafiltration. Definition of the optimum conditions for the formation of colloid-borne actinide species is ascertained by radiometric assay, determining the actinide fraction in the colloidal phase as a function of pH, conditioning time, concentration and concentration ratio of the involved components. Several methods are further applied for the appraisal of the chemical binding state of actinides in the HAS colloids as well as of Al in aluminosilicate solutions: TRLFS (time resolved laser fluorescence spectroscopy), EXAFS (extended X-ray absorption fine structure) spectroscopy and ligand displacement method (using EDTA as competing ligand). The affinity for conucleation with HAS appears to follow the element tendency toward hydrolysis. The second part of the work further concerns the behaviour of An(III, IV, VI) at the interaction with humate colloids. In addition to actinide activity measurements, 14C-labelled natural humic acid is used in order to facilitate the tracing of humic colloids behaviour. The conditions of colloid formation are investigated by radiometric assay as a function of pH (6.6-7.8), time, concentration of HA (0.6-8 mg/L), Al (1x10-5-1x10-4 M) and An (5x10-8-1x10-5 M). The generation of humate-colloid-borne species is distinctively favored for the non-hydrolyzed ionic species. Generation of humate-colloid-borne An(IV, VI) is enhanced by increasing HA concentration or decreasing the An/HA concentration ratio. The third part of the work follows the conditions for formation of colloid-borne An(III, IV, VI) in the mixed system containing the competing HAS and humic colloids. The simultaneous presence of HAS and HA generally enhances the stability region (in respect with pH and Si/Al concentration) of the colloid-borne An species. The formation of mixed HAS-humic colloids with synergic binding appears to be responsible for the stable incorporation of An into the colloidal phase, as ascertained for Cm(III) by TRLFS. Such a synergic effect is assumed to be related to the two different and complementary mechanisms exhibited by the HAS and HA colloids in binding the actinides.