EXAFS Studies on Adsorption Microscopic Structures of Zn at Manganite-Water Interaface and δ-MnO2-Water Interaface
- Received Date: 2003-11-21
- Accepted Date: 1900-01-01
- Available Online: 2003-01-02
Abstract: Microscopic structures of Zn(Ⅱ) surface complexes adsorbed at the manganite-water interface and δ-MnO2 water interface (in a 0.1M NaNO3 solution at 25°C )were studied using extended X-ray absorption fine structure (EXAFS) spectroscopy.Quantitative analysis of the EXAFS spectra showed that Zn(Ⅱ) was adsorbed onto the solid surface by sharing the oxygen atom in the hydrous Zn2+ ions and in the structural unit MnO6 on the manganite surface at pH7.5. Most of the adsorbed Zn(Ⅱ) was in the form of octahedral Zn(H2O)62+,but part of Zn(Ⅱ) was adsorbed as tetrahedral Zn(OH)2 or Zn(OH)42-.The average Zn-O bond length was 2.00±0.01A.EXAFS analysis of the second sphere indicated that Zn(Ⅱ) adsorbed on the manganite resulted in two Zn-Mn atomic distances of 3.08±0.02A and 3.54±0.02A,corresponding to the edge-linkage(stronger adsorption site) and corner-linkage(weaker adsorption site),respectively.The ZnO polyhedron was linked to the octahedron MnO6 of the manganite in these two modes by sharing two O atoms on the edges of the polyhedral and/or one O atom on the corners of the polyhedral.At pH 5.50,Zn(Ⅱ) was adsorbed onto δ-MnO2 surface in the form of octahedral hydrous Zn2+ ions.The octahedral Zn2+ was linked to the structural unit of octahedral MnO6 of the δ-MnO2 surface by sharing the O atoms.The average bond length of RZn-O was 2.07±0.01A and the Zn-Mn atomic distance was 3.53±0.01A,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 δ-MnO2 was highly reversible and no apparent adsorption hysteresis was observed.EXAFS results indicated that the adsorption reversibility was corresponded to the corner sharing linkage mode or edge sgaring linkage mode between the adsorbate and adsorbent polyhedra.