EXAFS studies on adsorption-desorption reversibility at manganese oxides-water interfaces. I. Irreversible adsorption of zinc onto manganite (gamma-MnOOH).

Microscopic structures of Zn(II) surface complexes adsorbed at the manganite (gamma-MnOOH)-water interface were studied using extended X-ray absorption fine structure (EXAFS) spectroscopy. Quantitative analysis of the first sphere showed that, in a 0.1 M NaNO(3) solution of pH 7.5, Zn(II) was adsorbed as a mixture of tetrahedral and octahedral structure (ZnO(4,6) polyhedra) and the average Zn-O distance was 2.00+/-0.01 A. EXAFS analysis of the second sphere showed that two typical atomic Zn-Mn distances of 3.07+/-0.01 and 3.52+/-0.02 A existed in the surface complexes, indicating that there were two types of linkage, i.e., the edge-linkage of high affinity and the corner-linkage of low affinity, between the ZnO(4,6) polyhedra and the MnO(6) octahedra of the manganite. Macroscopic adsorption-desorption experiments showed that adsorption of Zn(II) onto manganite was largely irreversible and the stronger edge-linkage mode was found to be responsible for the adsorption irreversibility. This result provided direct evidence from the molecular level for the basic hypothesis of the metastable-equilibrium adsorption (MEA) theory that adsorption density is not a thermodynamic state variable because a given value of adsorption density could have different values of chemical potential, depending on the proportion between the edge and corner linkage modes.