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

The surface charging behavior as a function of pH and isoelectric points (IEPs) of single-crystal alpha-Al2O3 (0001) and (1 (1) over bar 02) and alpha-Fe2O3 (0001) was determined by streaming potential measurements using an electrokinetic analyzer. The IEPs of alpha-Al2O3 (0001) and (1 (1) over bar 02) and alpha-Fe2O3 (0001) were found to be 4.5, 5.1, and 6.5, respectively. These IEP values for oriented single crystals of alpha-Al2O3 are in good agreement with literature values, whereas the new IEP value for alpha-Fe2O3 (0001) is significantly lower than four reported values (IEP = 8-8.5) for single-crystal alpha-Fe2O3 (0001) (Eggleston and Jordan 1998; Zarzycki et al. 2011; Chatman et al. 2013; Ltitzenkirchen et al. 2013) and significantly higher than one (IEP = 4) recently measured by Lfltzenkirchen et al. (2015) on a fresh alpha-Fe2O3 (0001) surface. Most of the single-crystal IEP values measured recently are lower than IEP values reported for polycrystalline alpha-Al2O3 and alpha-Fe2O3, which are generally in the pH range of 8 to 10. Calculations of the IEP values based on estimated K-a values of alpha-Fe2O3 and alpha-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.