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Comparison of isoelectric points of single-crystal and polycrystalline α-Al2O3 and α-Fe2O3 surfaces

Wang, Yingge; Persson, Per LU ; Marc Michel, F. and Brown, Gorden E. (2016) In American Mineralogist 101(10). p.2248-2259
Abstract

The surface charging behavior as a function of pH and isoelectric points (IEPs) of single-crystal α-Al2O3 (0001) and (1102) and α-Fe2O3 (0001) was determined by streaming potential measurements using an electrokinetic analyzer. The IEPs of α-Al2O3 (0001) and (11-02)$(1\overline1 02)$ and α-Fe2O3 (0001) were found to be 4.5, 5.1, and 6.5, respectively. These IEP values for oriented single crystals of α-Al2O3 are in good agreement with literature values, whereas the new IEP value for α-Fe2O3 (0001) is significantly lower than four reported values (IEP = 8-8.5) for single-crystal α-Fe2O3... (More)

The surface charging behavior as a function of pH and isoelectric points (IEPs) of single-crystal α-Al2O3 (0001) and (1102) and α-Fe2O3 (0001) was determined by streaming potential measurements using an electrokinetic analyzer. The IEPs of α-Al2O3 (0001) and (11-02)$(1\overline1 02)$ and α-Fe2O3 (0001) were found to be 4.5, 5.1, and 6.5, respectively. These IEP values for oriented single crystals of α-Al2O3 are in good agreement with literature values, whereas the new IEP value for α-Fe2O3 (0001) is significantly lower than four reported values (IEP = 8-8.5) for single-crystal α-Fe2O3 (0001) (Eggleston and Jordan 1998; Zarzycki et al. 2011; Chatman et al. 2013; Lützenkirchen et al. 2013) and significantly higher than one (IEP = 4) recently measured by Lützenkirchen et al. (2015) on a fresh α-Fe2O3 (0001) surface. Most of the single-crystal IEP values measured recently are lower than IEP values reported for polycrystalline α-Al2O3 and α-Fe2O3, which are generally in the pH range of 8 to 10. Calculations of the IEP values based on estimated Ka values of α-Fe2O3 and α-Al2O3 surfaces in contact with water as a function of defect type and concentration suggest that highly reactive surface defect sites (primarily singly coordinated aquo groups) on the Fe-and Al-oxide powders are possibly a major source of the surface charge differences between polycrystalline samples and their oriented single-crystal counterparts studied here. The results of this study provide a better understanding of the surface charging behavior of Fe and Al-oxides, which is essential for predicting complex processes such as metal-ion sorption occurring at mineral/water interfaces.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
defects, Fe-and Al-oxides, Isoelectric point (IEP), pH point of zero charge (pH), polycrystalline, single crystal, surface
in
American Mineralogist
volume
101
issue
10
pages
12 pages
publisher
Mineralogical Society of America
external identifiers
  • scopus:84979023208
  • wos:000385605100028
ISSN
0003-004X
DOI
10.2138/am-2016-5531
language
English
LU publication?
yes
id
89096800-5bde-4781-bcb0-fa4f5750793a
date added to LUP
2016-11-02 08:50:15
date last changed
2017-09-10 05:11:11
@article{89096800-5bde-4781-bcb0-fa4f5750793a,
  abstract     = {<p>The surface charging behavior as a function of pH and isoelectric points (IEPs) of single-crystal α-Al<sub>2</sub>O<sub>3</sub> (0001) and (1102) and α-Fe<sub>2</sub>O<sub>3</sub> (0001) was determined by streaming potential measurements using an electrokinetic analyzer. The IEPs of α-Al<sub>2</sub>O<sub>3</sub> (0001) and (11-02)$(1\overline1 02)$ and α-Fe<sub>2</sub>O<sub>3</sub> (0001) were found to be 4.5, 5.1, and 6.5, respectively. These IEP values for oriented single crystals of α-Al<sub>2</sub>O<sub>3</sub> are in good agreement with literature values, whereas the new IEP value for α-Fe<sub>2</sub>O<sub>3</sub> (0001) is significantly lower than four reported values (IEP = 8-8.5) for single-crystal α-Fe<sub>2</sub>O<sub>3</sub> (0001) (Eggleston and Jordan 1998; Zarzycki et al. 2011; Chatman et al. 2013; Lützenkirchen et al. 2013) and significantly higher than one (IEP = 4) recently measured by Lützenkirchen et al. (2015) on a fresh α-Fe<sub>2</sub>O<sub>3</sub> (0001) surface. Most of the single-crystal IEP values measured recently are lower than IEP values reported for polycrystalline α-Al<sub>2</sub>O<sub>3</sub> and α-Fe<sub>2</sub>O<sub>3</sub>, which are generally in the pH range of 8 to 10. Calculations of the IEP values based on estimated K<sub>a</sub> values of α-Fe<sub>2</sub>O<sub>3</sub> and α-Al<sub>2</sub>O<sub>3</sub> surfaces in contact with water as a function of defect type and concentration suggest that highly reactive surface defect sites (primarily singly coordinated aquo groups) on the Fe-and Al-oxide powders are possibly a major source of the surface charge differences between polycrystalline samples and their oriented single-crystal counterparts studied here. The results of this study provide a better understanding of the surface charging behavior of Fe and Al-oxides, which is essential for predicting complex processes such as metal-ion sorption occurring at mineral/water interfaces.</p>},
  author       = {Wang, Yingge and Persson, Per and Marc Michel, F. and Brown, Gorden E.},
  issn         = {0003-004X},
  keyword      = {defects,Fe-and Al-oxides,Isoelectric point (IEP),pH point of zero charge (pH),polycrystalline,single crystal,surface},
  language     = {eng},
  month        = {10},
  number       = {10},
  pages        = {2248--2259},
  publisher    = {Mineralogical Society of America},
  series       = {American Mineralogist},
  title        = {Comparison of isoelectric points of single-crystal and polycrystalline α-Al<sub>2</sub>O<sub>3</sub> and α-Fe<sub>2</sub>O<sub>3</sub> surfaces},
  url          = {http://dx.doi.org/10.2138/am-2016-5531},
  volume       = {101},
  year         = {2016},
}