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

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 Eu³⁺, 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 correspond to different Eu³⁺ species. These factors resemble those observed for Eu³⁺ binding to HSs from surface environments; however, detailed comparison shows that there are some particularities in Eu³⁺ binding to the deep groundwater HSs. The distribution coefficients (K_d) of Eu³⁺ binding to the HSs calculated from the PARAFAC modeling exhibits a rather strong salt effect. At 0.01 M NaClO₄, the K_d values are relatively large and comparable to those to the surface HSs; they are decreaed at 0.1 M NaClO₄ by more than an order of the magnitude. The K_d values are larger for the humic acid fraction of the deep underground HSs than the fulvic acid fraction over the entire range of pH and salt concentration investigated in this study.