Abstract

Coordination states of rare earth elements (REEs) adsorbed by iron oxyhydroxide (FeOOH) and manganese dioxide (δ-MnO2) (REE = La, Pr, Nd, and Sm) were determined using extended X-ray absorption fine structure (EXAFS) spectroscopy. Multi-electron excitation, resulting in double-electron excitation (2p, 4d → 5d, 5d) for REE-LIII edge EXAFS spectra, possibly causes a considerable error in EXAFS analyses for light REEs. To obtain reliable structural parameters this study elucidates the effects of double-electron excitation on the local structure determination of light REEs on poorly crystallized FeOOH and δ-MnO2. For this study, anomalous features attributable to excitation are superimposed on EXAFS signals of La, Pr, Nd, and Sm samples in the 5.0–7.0 Å−1k range. The relative intensity of the double-electron excitation to the LIII adsorption edge is <1.1%. Consequently, the double-electron excitation engenders a smaller error than those estimated in fitting for Pr, Nd, and Sm samples. However, significant correction is necessary for the determination of local coordination states of La samples: interatomic distances are 0.007–0.036 Å shorter after correction. The EXAFS analyses of REE-sorbed Fe samples show that adsorbed La, Pr, Nd, and Sm have a mixture of eightfold and ninefold coordination structures and form inner sphere complexes at the FeOOH surface. The determined structural parameters of light REE-sorbed δ-MnO2 suggest that adsorbed light REEs have distorted tenfold-coordination spheres consisting of six short and four long REE–O bonds and form inner sphere complexes. Their coordination structures are more disordered than those of aquo ions and FeOOH samples.

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