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

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 241Am or 248Cm is introduced at a concentration of 4.9×10−8 mol l−1. The favourable condition of HAS colloid formation as ascertained by radiometric experiment is found to be at around pH 5 with 10−6–10−5 mol l−1 Al for a marginal amount and at pH≥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.