Europium binding to humic substances extracted from deep underground sedimentary groundwater studied by time-resolved laser fluorescence spectroscopy

Saito, Takumi*; Aoyagi, Noboru   ; Terashima, Motoki  

Humic substances (HSs) are ubiquitous in various environments including deep underground and play an important role in the speciation and mobility of radionuclides. The binding of Eu, a chemical homologue of trivalent actinide ions, to HSs isolated from sedimentary groundwater at -250 m below the surface was studied by time-resolved laser fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) as a function of pH and salt concentration. PARAFAC modeling reveals the presence of multiple factors that corresponds to different Eu species. These factors resemble those observed for Eu binding to HSs from surface environments; however, detailed comparison shows that there are some particularities in Eu binding to the deep groundwater HSs. The distribution coefficients () of Eu binding to the HSs calculated from the PARAFAC modeling exhibits a rather strong salt effect. At 0.01 M NaClO the values are relatively large and comparable to those to the surface HSs; they are decreaed at 0.1 M NaClO by more than an order of the magnitude. The values are larger for humic acid fraction of the deep underground HSs than fulvic acid over the entire range of pH and salt concentration investigated in this study.