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

The local structure of Zn(II) adsorbed at the water-manganite (gamma-MnOOH) interface has been investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy. Adsorption experiments were carried out within the pH range 6.17-9.87 and surface coverages of 0.9 to 9.7 mu mol/m(2).Zn(II) coordination was observed to change from six to four as pH was increased. This was indicated by a change in Zn-O distance from 2.04 to 1.96 Angstrom and by a decrease in the obtained coordination numbers. Two higher shells were detected at about 3.08 and 3.33 Angstrom, at all pH values and surface coverage investigated. As the backscattering phase and amplitude functions of Mn and Zn are similar, we used structural and chemical considerations to assign the backscattering at 3.08 Angstrom to Mn neighbors, and the one at 3.33 Angstrom to Zn atoms. Indeed the size of the Zn polyhedra, especially of ZnO4, does not quite match the structure of the manganite surface. We conclude that Zn(II) forms multinuclear hydroxo-complexes or a zinc hydroxide phase at the surface, as it might be easier for an additional Zn(II) to bond to an already sorbed Zn. These results were compared to our previous EXAFS study of Cd(II) adsorption onto manganite, where mononuclear inner-sphere complexes bound to the surface via edges were found, (C) 2000 Academic Press.