Interaction of actinides with aluminosilicate colloids in statu nascendi. Part II: spectroscopic speciation of colloid-borne actinides(III)

Abstract

The chemical interaction of trivalent actinides, Am(III) and Cm(III) in the process of hydroxy aluminosilicate (HAS) colloid formation is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) with the assistance of radiometry. A screening experiment is performed at the beginning to ascertain under what conditions HAS colloids are formed favourably and incorporate trivalent actinides in their oxo-bridge structure. While keeping the Si concentration constant at 10(-3) mol l(-1), the Al concentration is varied from 10(-7) to 10(-3) mol l(-1) in the pH range from 4 to 9. The electrolyte medium is made constant with 0.03 M NaCl. A trace amount of Am-241 or (CM)-C-248 is introduced at a concentration of 4.9 x 10(-8) mol l(-1). The favourable condition of HAS colloid formation as ascertained by radiometric experiment is found to be at around I pH 5 with 10(-6) -10(-5) mol l(-1) Al for a marginal amount and at pH greater than or equal to 7 with 10(-6)-10(-4) Mol l(-1) Al for a major amount. Spectroscopic speciation is made on Cm, for its high fluorescence yield, in various experimental solutions for the appraisal of its chemical binding within oxo-bridges of HAS colloids. Speciation made by TRLFS shows two colloid-borne Cm species, Cm-HAS(I) and Cm-HAS(II), which have seven and six hydration water molecules, respectively. These results put forward a conclusion that the former undergoes a bidentate oxo-bridge binding and the latter a tridentate binding. In the absence of Al, Cm undergoes reaction with silanol moieties to form Cm-silicate complexes, which are found to be distributed in three phases: ionic phase, colloid and precipitate. However, the formation of Cm-silicate colloids is marginal due to a low Cm concentration. Applying laser-induced breakdown detection (LIBD), an average size of colloids is found to be 11-13 nm in hard sphere diameter at different pH. (C) 2003 Elsevier B.V. All rights reserved.