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

• Published in: Journal of colloid and interface science 2004-03, Vol.271 (1), p.28-34
• Main Authors: Pan, Gang, Qin, Yanwen, Li, Xianliang, Hu, Tiandou, Wu, Ziyu, Xie, Yaning
• Format: Article
• Language: English
• Subjects: Adsorption reversibility; Adsorption–desorption; Chemistry; Exact sciences and technology; EXAFS; General and physical chemistry; Manganite; Metastable-equilibrium adsorption; Surface physical chemistry; Zinc
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2003.11.028

Microscopic structures of Zn(II) surface complexes adsorbed at the manganite ( γ-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 Å. EXAFS analysis of the second sphere showed that two typical atomic ZnMn distances of 3.07±0.01 and 3.52±0.02 Å 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.