[EXAFS studies on adsorption-desorption mechanism of Zn at delta-MnO2-water interface]

Microscopic structures of Zn(II) surface complexes adsorbed at a delta-MnO2-water interface were studied using extended X-ray absorption fine structure (EXAFS) spectroscopy. In a 0.1 mol/L NaNO3 solution of pH 5.50, Zn(II) was adsorbed onto the solid surface in the form of octahedral hydrous Zn(II) ions. The octahedral Zn(II) was linked to the structural unit of octahedral MnO6 of the delta-MnO2 surface by sharing the O atoms. The average bond length of RZn-O was (2.071 +/- 0.007)A (n = 3) and the Zn-Mn atomic distance was (3.528 +/- 0.006) A (n = 3), which corresponded to a corner-sharing linkage adsorption mode (weaker adsorption). Macroscopic adsorption-desorption isotherm experiments showed that, in contrast to that of Zn-manganite, adsorption of Zn(II) on delta-MnO2 was highly reversible and no apparent adsorption hysteresis was observed. EXAFS results indicated that the microscopic mechanism for the high adsorption reversibility was corresponded to the weak adsorption sites of corner sharing linkage between the adsorbate and adsorbent polyhedra.