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Surface speciation of Eu$$^{3+}$$ adsorbed on kaolinite by time-resolved laser fluorescence spectroscopy (TRLFS) and parallel factor analysis (PARAFAC)

Ishida, Keisuke*; Saito, Takumi*; Aoyagi, Noboru   ; Kimura, Takaumi ; Nagaishi, Ryuji ; Nagasaki, Shinya*; Tanaka, Satoru*

Time-resolved laser fluorescence spectroscopy is an effective speciation technique for fluorescent metal ions, which can be further extended by the parallel factor analysis. The adsorption of Eu$$^{3+}$$ on kaolinite as well as gibbsite as a reference mineral was investigated by TRLFS together with batch adsorption measurements. The PAFAFAC modeling provided the fluorescence spectra, decay lifetimes and relative intensity profiles of three Eu$$^{3+}$$ surface complexes with kaolinite; an outer-sphere complex and two inner-sphere complexes. Their intensity profiles qualitatively explained the measured adsorption of Eu$$^{3+}$$. Based on the TRLFS results in varied H$$_{2}$$O/D$$_{2}$$O media, it was shown that the outer-sphere complex exhibited more rapid fluorescence decay than Eu$$^{3+}$$ aquo ion, because of the energy transfer to the surface. Factor B was an inner-sphere complex, which became dominant at relatively high pH, high salt concentration and low Eu$$^{3+}$$ concentration. Its spectrum and lifetime were similar to those of Eu$$^{3+}$$ adsorbed on gibbsite, suggesting its occurrence on the edge face of the gibbsite layer of kaolinite. From the comparison with the spectra and lifetimes of crystalline or aqueous Eu(OH)$$_{3}$$, factor C was considered as a poly-nuclear surface complex of Eu$$^{3+}$$ formed at relatively high Eu$$^{3+}$$ concentration.

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Category:Chemistry, Physical

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