J.I. Kim

Interaction of actinides with aluminosilicate colloids in statu nascendi: Part I: generation and characterization of actinide(III)–pseudocolloids

Abstract A study is presented to elucidate how the trivalent actinide ions become incorporated into the formation of aluminosilicate colloids. The acidic Al solution of varying concentration from 10 −5 to 10 −3 mol l −1 , containing the Am 3+ or Cm 3+ ion at 4.9×10 −8 mol l −1 , is titrated with the alkaline Si solution of different concentrations from 10 −5 to 10 −3 mol l −1 to arrive at each prefixed pH from 4 to 9. The aluminosilicate precipitate is separated by filtration at 450 nm pore size. The filtrate containing ionic and colloidal species is subsequently put into filtration at 10-kDa-pore size (ca. 1.5 nm) to separate aluminosilicate colloids from ionic species in solution. The distribution of Am or Cm in the three different phases: precipitate, colloids and ionic species is analysed by radiometric measurement and the optimal conditions are ascertained for the formation of colloid-borne Am or Cm species. The particle size of the aluminosilicate colloids observed by atomic force microscope (AFM) appears to be 5–10 nm height and 10–50 nm length for most particles, whereas the average hard sphere diameter of preponderant particles evaluated by laser-induced breakdown detection (LIBD) appears to be 10–50 nm with an approximate mass concentration of 10–50 ppb. According to these data, the colloid number density may range from 10 11 to 10 14 particles per litre solution. XPS and EDX analyses on colloids result in an atomic ratio of Al/Si to an average value of 0.7, suggesting that it may vary from 0.5 to 1.0. The speciation of colloids by time-resolved laser fluorescence spectroscopy (TRLFS) shows the formation of two different colloid-borne Cm species as coordinated with bidentate and tridentate bindings within the aluminosilicate structure. The latter species becomes predominant at pH≥6. A desorption experiment reveals that Am incorporated into aluminosilicate colloids is not dissociable at pH 7 and 9, indicating the formation of a stable colloid-borne Am species.

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 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.