Retention mechanisms and microstructure of Eu(III) on manganese dioxide studied by batch and high resolution EXAFS technique

Abstract

The batch experiment and high resolution EXAFS technique with bent crystal analyzer were combined to study Eu(III) retention mechanisms on β-MnO₂ as a function of various environmental factors. The results indicated that the sorption of Eu(III) onto β-MnO₂ was obviously dependent on pH but independent of ionic strength, suggesting the formation of inner-sphere surface complexes. Results of high resolution EXAFS analysis showed that Eu was surrounded by 8 ± 1 O atoms in the first shell at R_Eu−O ≈ 2.40 Å for all sorption samples. A second shell of Mn atoms at R_Eu−Mn ≈ 3.78 Å was observed for all sorption samples, which was associated with the formation of a bidendate inner-sphere complex with chemical bonding via edge sharing to MnO₆-octahedron. This study shows the utility of high resolution EXAFS technique with bent crystal analyzer to discriminate the fluorescence peak of Eu(III) from that of manganese hydroxides, whose fluorescence peaks are too close in energy to be discriminated by the conventional EXAFS technique with a low resolving solid-state detector. Both the macroscopic interaction data and the molecular level evidence of Eu(III) surface speciation at the oxide-water interface should be factored into better evaluation of Eu(III) and related radionuclide mobility in the natural environment.