Application of EXAFS with a bent crystal analyzer to study the pH-dependent microstructure of Eu(III) onto birnessite

Herein, Eu(III) retention mechanism and microstructure on δ-MnO₂ as a function of solution pH were studied using both the conditional macroscopic method and molecular-level high resolution EXAFS technique with a bent crystal analyzer. The results suggested that the sorption Eu(III) onto δ-MnO₂ was strongly dependent on pH but not on ionic strength, implying the formation of inner-sphere surface complexation for Eu(III) on δ-MnO₂ surface. Molecular-level results from EXAFS methods showed that Eu was surrounded by ∼8O atoms in first coordination shell at R_Eu-O ≈ 2.42 Å, and second shell of Mn atoms at R_Eu-Mn ≈ 3.91 Å was found in four sorption samples from different solution pH. All this results reflected the formation of a bidentate surface complex with Eu(III) bonding by corner sharing to MnO₆-octahedron on δ-MnO₂ surface. Both the macroscopic findings and microstructure of Eu(III) at the δ-MnO₂-water interface enhance our understanding for the reaction dynamics and states of Eu(III) and related radionuclides in natural environment.