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

摘要

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.