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Published on: August 7, 2018
Metal Ion Coordination at the Water-Manganite (gamma-MnOOH) Interface
1Department of Chemistry, Inorganic Chemistry, Umeå University, Umeå, SE-901 87, Sweden
Journal of Colloid and Interface Science
|September 14, 2000
Summary
Zinc(II) coordination at the water-manganite interface changes with pH, shifting from six to four. This suggests Zn(II) forms multinuclear complexes or a zinc hydroxide phase on the manganite surface.
Area of Science:
- Environmental Science
- Geochemistry
- Materials Science
Background:
- Understanding metal ion adsorption at mineral-water interfaces is crucial for environmental remediation and geochemistry.
- Manganite (gamma-MnOOH) is an important manganese oxide with significant sorption capacity for various metal ions.
- Previous studies on cadmium adsorption indicated mononuclear complex formation.
Purpose of the Study:
- To investigate the local atomic structure of zinc(II) adsorbed onto the manganite (gamma-MnOOH) surface.
- To determine how pH and surface coverage influence zinc coordination and speciation at the interface.
- To compare zinc adsorption behavior with previously studied cadmium adsorption.
Main Methods:
- Extended X-ray absorption fine structure (EXAFS) spectroscopy was employed to probe the local atomic environment of adsorbed Zn(II).
- Adsorption experiments were conducted across a pH range of 6.17-9.87 and surface coverages of 0.9-9.7 µmol/m².
- Analysis of EXAFS data involved determining Zn-O bond distances, coordination numbers, and distances to neighboring atoms.
Main Results:
- Zn(II) coordination number decreased from six to four as pH increased, indicated by a change in Zn-O distance from 2.04 to 1.96 Å.
- Two distinct higher shells of backscatterers were observed at approximately 3.08 Å (assigned to Mn neighbors) and 3.33 Å (assigned to Zn neighbors).
- The observed coordination changes and presence of Zn-Zn interactions suggest the formation of multinuclear species or a zinc hydroxide phase, rather than simple mononuclear complexes.
Conclusions:
- Zn(II) adsorption on manganite is not limited to mononuclear inner-sphere complexes, unlike Cd(II).
- The formation of multinuclear hydroxo-complexes or a zinc hydroxide phase is favored at the manganite surface.
- These findings highlight the complexation behavior of Zn(II) at mineral-water interfaces and its dependence on environmental conditions.
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