Interaction of actinides(III) with aluminosilicate colloids: Part IV. Influence of humic acid

Abstract

The present investigation deals with how aquatic hydroxy aluminosilicate (HAS) colloids are generated in nature via co-nucleation of Si and Al in the presence of humic acid and moreover how trivalent actinides are integrated into the process. For the experiment, 241Am (5 × 10 −8 M) is introduced into a colloid forming mother solution where competition takes place for the complexation of two trivalent metal ions, Al and Am, each with two different ligands: 14C-labelled humic acid and silicic acid. Colloids thus formed are separated from precipitate and solution by sequential filtrations at 450 and 1.5 nm pore size. Formation of the colloid-borne Al and Am species in the neutral pH range is ascertained radiometrically by determining the phase distribution of 14C and Am, respectively. Varying the pH as well as the concentration of reaction components involved, a broad screening experiment is further carried out. Replacing Am by Cm, the speciation of colloid-borne Cm is made by time-resolved laser fluorescence spectroscopy (TRLFS), exciting at two different wavelengths. A direct excitation is made at 382 nm, whereas an indirect excitation is followed at 370 nm to confirm the sensitized Cm excitation via Cm-borne humic acid. The result substantiates the formation of colloid-borne Cm in a mixed structure of HAS and humic acid aggregated. Affinity of Am and Al binding to silicic and humic acid, leading to incorporation into the colloidal phase, can be correlated with their different hydrolysis behaviour. The Am 3+ ion, less hydrolyzing than the Al 3+ ion, is favoured for complexation with humic acid and thus discriminated in the co-nucleation with silicic acid. In the pH range (≥6.6) where the Am 3+ ion becomes hydrolyzed, a synergic bond coupling of hydrolyzed Am species yields a mixed hybrid of HAS-humic colloid-borne Am. As measured by laser-induced breakdown detection (LIBD), colloids thus formed reveal an average particle size of about 15 ± 5 nm hard sphere diameter. The value is comparable to that of Am-HAS colloids and remains consistent for a 2-month period of observation. The present study shows that aquatic colloids composed of HAS and humic acid combine actinides via different but synergic mechanisms and hence enhance the stability of colloid-borne actinides.