Sorption of Eu on montmorillonite studied by time-resolved laser fluorescence spectroscopy and surface complexation modeling

Sasaki, Takayuki*; Ueda, Kenyo*; Saito, Takumi*; Aoyagi, Noboru   ; Kobayashi, Taishi*; Takagi, Ikuji*; Kimura, Takaumi ; Tachi, Yukio  

The influence of pH, ionic strength, and the concentrations of Eu and nitrate salts on the sorption of Eu on Na-montmorillonite was investigated by batch sorption measurements. The distribution coefficient (Kd) indicated that the cation exchange reaction is significant in pH 4-7 at low ionic strength, and the formation of surface complexation is related to the pH dependence of Kd at high ionic strength. The Eu surface species were investigated using time-resolved laser fluorescence spectroscopy (TRLFS) with parallel factor analysis (PARAFAC). The PARAFAC modeling indicated two surface species, the outer-sphere (factor A) and inner-sphere (factor B) Eu complexes. Factor B was dominant at high pH and ionic strength, which was determined to be a Eu colloidal species formed at high Eu concentrations. A cation exchange model combined with a one-site non-electrostatic surface complexation model was applied to the measured Kd data, using the results of the TRLFS-PARAFAC analyses.