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 NaNO3 solution of pH 7.5, Zn(II) was adsorbed as a mixture of tetrahedral and octahedral structure (ZnO4,6 polyhedra) and the average Zn-O distance was 2.00 +/- 0.01 Angstrom. EXAFS analysis of the second sphere showed that two typical atomic Zn-Mn distances of 3.07 +/- 0.01 and 3.52 +/- 0.02 Angstrom 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 ZnO4,6 polyhedra and the MnO6 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. (C) 2003 Elsevier Inc. All rights reserved.