Adsorption and Precipitation of Aqueous Zn(II) on Hematite Nano- and Microparticles
- Surface and Aqueous Geochemistry Group, Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California, 94305-2115 (United States)
As part of a study of the effect of particle size on reactivity of hematite to aqueous metal ions, the sorption of Zn(II) on hematite nanoparticles and microparticles was examined over a wide range of Zn(II) concentrations using Zn K-edge EXAFS. When reacted with nanoparticles at pH 5.5 and low Zn(II) sorption densities (0.04 {<=} {gamma} < 2.76 {mu}mol/m2), Zn(II) formed five-coordinated or a mixture of four- and six-coordinated surface complexes with an average Zn-O distance of 2.04({+-}0.02)A. At pH 5.5 and high Zn(II) sorption densities (2.76 {<=} {gamma} {<=} 3.70 {mu}mol/m2), formation of surface precipitates is suggested based on the presence of second-shell Zn and multiple scattering features in the Fourier transform (FT) of the EXAFS spectra. EXAFS fitting of these high {gamma} samples yielded an average first-shell Zn-O distance of 2.10({+-}0.02)A, with second-shell Zn-Fe and Zn-Zn distances of 3.23({+-}0.03)A and 3.31({+-}0.03)A, respectively. Qualitative comparison between the EXAFS spectra of these sorption samples and that of amorphous zinc hydroxide and Zn-bearing hydrotalcite indicates the development of surface precipitates with increasing {gamma}. EXAFS spectra of Zn(II) sorbed on hematite microparticles under similar experimental conditions showed no evidence for surface precipitates even at the highest Zn surface coverage ({gamma} = 4 {mu}mol/m2). These results indicate that reactivities of hematite nanoparticles and macroparticles differ with respect to Zn(II)aq, depending on Zn(II) sorption density. We suggest that the degree of hematite crystallinity affects the reactivity of hematite surfaces toward Zn(II)aq and the formation of the Zn(II) surface complexes.
- OSTI ID:
- 21054603
- Journal Information:
- AIP Conference Proceedings, Vol. 882, Issue 1; Conference: XAFS13: 13. international conference on X-ray absorption fine structure, Stanford, CA (United States), 9-14 Jul 2006; Other Information: DOI: 10.1063/1.2644487; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-243X
- Country of Publication:
- United States
- Language:
- English
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