Abstract Details
| Zinc Surface Speciation on Biogenic Manganese (IV) Oxides: Influence of pH and Surface Coverage | |
|---|---|
| Abstract ID | ENV-13 |
| Presenter | Jasquelin Pena |
| Presentation Type | Poster |
| Full Author List | J. Pena (1) , J. R. Bargar (2) , G. Sposito (1) |
| Affiliations | (1) Dept. of Civil and Environmental Engineering, UC Berkeley (2) SSRL |
| Category | Environmental Science |
| Abstract | The biogeochemical cycling of zinc, an important trace nutrient and toxicant in terrestrial and aquatic ecosystems, is strongly influenced by adsorption to Mn oxide minerals. Studies to date indicate that natural Mn minerals are often nanocrystalline hexagonal birnessite, a layer type Mn(IV) oxide precipitated by microorganisms (Tebo et al. 2004). The presence of structural Mn vacancy defects, along with high surface area, is generally thought to account for the remarkable sorptive capacity of hexagonal birnessites for Zn. Vacant Mn sites create significant negative charge that is compensated by the binding of metal cations and protons. However, the precise chemical conditions controlling the formation of Zn surface complexes at these reactive sites, and hence of a primary environmental mobility control, have not yet been established.
We have extended the study of Toner et al. (2006) by measuring K-edge EXAFS spectra for Zn sorbed to biogenic birnessite samples at pH 6 and 8 with various initial Zn concentrations. The biogenic birnessite samples used in this study were produced in culture by the well-characterized model bacterium, Pseudomonas putida GB-1 (Villalobos et al. 2006). Spectral analysis showed that Zn was bound to vacancy sites as inner-sphere surface complexes having Zn in either tetrahedral or octahedral coordination with three surface O atoms and one or three water molecules, respectively. Studies of Zn sorption to biogenic manganese oxide at pH 7 indicate that tetrahedral complexes dominate at low surface coverage, with an increasing contribution from octahedral complexes as Zn loading increases. At pH 6 we found that the adsorbed Zn was primarily bound as a tetrahedral triple corner sharing complex, which is in contrast to the findings of Toner et al. at pH 7, despite similar surface coverage. At pH 8 and low surface coverage, however, we observed a significant contribution from octahedral Zn complexes. The results from this study show that, in addition to surface coverage, pH is an important variable controlling the surface speciation of Zn on Mn oxides and thus the mechanism by which Zn may be attenuated in natural and polluted environments. |
| Footnotes | Tebo B. M., Bargar, J. R., Clement B. G., Dick G. J., Murray K. J., Parker D., Verity R.,
Webb S. M. (2004) Biogenic manganese oxides: Properties and mechanisms of formation. Annu. Rev. Earth Planet. Sci. 32, 287-328.
Toner B., Manceau A., Webb S. M., and Sposito G. (2006) Zinc sorption to biogenic hexagonal-birnessite particles within a hydrated bacterial biofilm. Geochim. Cosmochim. Acta 70, 27-43. Villalobos M., Lanson B., Manceau A., Toner B., and Sposito G. (2006) Structural model for the biogenic Mn oxide produced by Pseudomonas putida. Am. Mineral. 91, 489-502. |
| Funding Acknowledgement | This project was supported by the UC Toxics Program and the Switzer Foundation. J. Pena is funded by a UC Dissertation-Year Fellowship. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. |