Metal ion coordination at the water-manganite (γ-MnOOH) interface I.: An EXAFS study of cadmium(II)

The local structure of Cd(II) surface complexes adsorbed onto manganite (gamma-MnOOH) has been investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. Adsorption experiments were carried out within the pH range 7.4-9.8 and with surface coverage from 2.2 to 8.9 mu mol/m(2). Quantitative analysis of the EXAFS spectra shows little difference in the local coordination environment of Cd regardless of the coverage. Analysis of the first shell required a third cumulant in an asymmetric distribution model (i.e., non-Gaussian distribution), indicating distorted CdO6 octahedra with an average Cd-O distance of 2.31 Angstrom. A single second shell of Mn neighbors at a constant distance of 3.33 Angstrom was detected. This comparatively short distance shows that inner-sphere complexes are formed. No Cd neighbor was found which means that cadmium hydroxide precipitation or cluster formation does not occur at the surface at the experimental conditions probed in this study. We assign the Cd-Mn distance to edge sharing between CdO6 and MnO6 octahedra, with a geometry similar to that in the mixed solid Cd2Mn3O8 This adsorption mode is possible on the {110} and {010} cleavage planes of manganite. (C) 2000 Academic Press.