Interaction of actinides(III) with aluminosilicate colloids in “statu nascendi”: Part III. Colloid formation from monosilanol and polysilanol

Abstract

The chemical interaction of trivalent actinides, Am(III) and Cm(III), during the formation of hydroxy-aluminosilicate (HAS) colloids is investigated starting with polysilanol (polysilicic acid) in the near neutral pH range of 4–9. Polymerisation and depolymerisation of silanol (silicic acid) are also examined to ascertain the optimal condition of HAS colloid formation with polysilanol. The formation of colloid-borne Am(III) is investigated radiometrically by varying the Al concentration from 10 −7 to 10 −3 M, while keeping the Si concentration constant at ≥10 −2 M. Spectroscopic speciation is performed by time-resolved laser fluorescence spectroscopy (TRLFS) to characterise how trivalent actinides become chemically incorporated in HAS colloids. For this purpose optically sensitive Cm is used. An average size of HAS colloids, as determined by laser-induced breakdown detection (LIBD), is found to be in the range of 10–20 nm in diameter, increasing with lowering pH. The SEM-EDXS analysis results in an atomic Si/Al ratio of 1.23 ± 0.03. The TRLFS speciation shows the formation of three different colloid-borne Cm species: Cm-HAS(I), Cm-HAS(II) and Cm-HAS(III). These results are different from the HAS colloid formation with monosilanol, in which only two species – Cm-HAS(I) and Cm-HAS(II) – are observed. The number of hydration water molecules in the three colloid-borne Cm species, as confirmed by measuring the fluorescence lifetime with TRLFS, varies from 7 for Cm-HAS(I), to 6 for Cm-HAS(II) and to 0–1 for Cm-HAS(III), which infers bi-dentate and tri-dentate oxo-bridging and structural incorporation of Cm, respectively. The prevailing Cm-HAS(III) species at pH 9 converts by decreasing pH first to Cm-HAS(II) and then to Cm-HAS(I), revealing a gradual dissociation of Cm from HAS colloids upon acidification.