Systematic change in relative stabilities of REE-humic complexes at various metal loading levels

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  • YAMAMOTO YUHEI
    Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University
  • TAKAHASHI YOSHIO
    Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University
  • SHIMIZU HIROSHI
    Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University

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Complexation of rare earth elements (REEs) with humic substances (HSs) at various REE loading levels was studied by solvent extraction and equilibrium dialysis methods coupled with inductively coupled plasma-mass spectrometry (ICP-MS). Apparent stability constants (βM-HS) of REE ions with standard Suwannee River humic acid (SRHA) were determined simultaneously for all REEs except for Pm. βM-HS depends on the REE loading level to HSs, where βM-HS decreases with an increase in the REE loading level at a lower REE loading region, whereas βM-HS is almost constant beyond a certain REE loading level. The variation in βM-HS with the REE loading level suggests the existence of several types of binding sites of REEs in HSs. Next, βM-HS was converted into KMA (which is the REE-HS interaction parameter for Tipping's “Humic Ion-Binding Model VI”) to be compared with the data by previous studies on REE-HS complexation at various REE loading levels in terms of relative differences among REEs (i.e., REE pattern). In the case of solvent extraction, the REE pattern of logKMA changes with variation in the REE loading level; a peak around Sm and Eu was observed in the REE pattern of logKMA at a higher REE loading region, whereas logKMA increases with an increase in the atomic number of REEs at a lower REE loading region. On the other hand, such variation in the REE pattern of logKMA was not observed in the results of equilibrium dialysis. This difference in the REE pattern of logKMA can be explained by the total metal loading level, including the contributions of Fe3+ and Al3+ originally contained in HS. The main binding sites of REEs in HSs were determined by comparing the REE pattern of logKMA experimentally obtained for HSs with those estimated based on the linear free-energy relationship (LFER) for carboxylates. Similarity of the REE pattern suggested that simple carboxylates are the main binding sites of REEs at a higher metal loading region, whereas chelating sites play a key role at a lower metal loading region. Our results show that relative stabilities of HS complexes among various REEs can change as a function of the metal loading level.

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