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Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part II. Correspondence of structural change to irregular variations of partitioning coefficients and tetrad effect variations appearing in interatomic distances

¹ Institute of Geoscience, Geological Survey of Japan, AIST, Tsukuba 305-8567, Japan
² Geochemical Laboratory, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
³ Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan
⁴ Faculty of Education and Human Sciences, Yokohama National University, Kanagawa, 240-8501, Japan
⁵ Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan

Correspondence: ^* E-mail: a.ohta{at}aist.go.jp

Experimental distribution coefficients of rare earth elements (REEs) between Fe oxyhydroxides (FeOOH) and Mn dioxide (-MnO₂) and solutions [K_D(REE)] exhibit anomalous variations: preferential uptake of light REEs by Mn dioxide, a step-like trend in K_D(REE) in the Er-Tm-Yb-Lu region, and fractionation of K_D(Y) from K_D(Ho). Extended X-ray absorption fine structure (EXAFS) spectroscopy was applied to determine coordination states of Er, Tm, Yb, Lu, and Y adsorbed onto FeOOH and -MnO₂ to assess structural changes around the REE site. The structures obtained, combined with previously determined structures of light REEs-sorbed Fe and Mn samples, corresponded to variations found in K_D(REE). The structural parameters in the first coordination sphere suggest that La, Pr, and Nd adsorbed onto -MnO₂ have a distorted tenfold-coordination sphere and differ greatly from La-, Pr-, Nd-, and Sm-sorbed FeOOH, which have a mixture of eightfold- and ninefold-coordination spheres. In contrast, heavy REEs including Y adsorbed onto Fe and Mn samples’ local structures have an eightfold-coordination sphere. The preferential uptakes of light REEs by -MnO₂ are explained by the structural change. The irregular variations of heavy REEs and Y fractionation from Ho in K_D(REE) do not, however, correspond to any change found in the coordination sphere. During characterization of the first coordination sphere, the W-type tetrad effect appears in the series variation of interatomic distances of REE_aq³⁺ and REE-sorbed FeOOH and -MnO₂. The occurrence of a tetrad effect indicates that the interatomic distances relate not only to the electrostatic field but also to a quantum field.

Key Words: X-ray absorption spectra • rare earth elements • iron oxyhydroxide • manganese dioxide • structural change • tetrad effect • Y fractionation from Ho

This article has been cited by other articles:

				A. Ohta, H. Kagi, M. Nomura, H. Tsuno, and I. Kawabe Coordination study of rare earth elements on Fe oxyhydroxide and Mn dioxides: Part I. Influence of a multi-electron excitation on EXAFS analyses of La, Pr, Nd, and Sm American Mineralogist, April 1, 2009; 94(4): 467 - 475. [Abstract] [Full Text] [PDF]