Development of carriers with controlled concentration of charged surface groups in aqueous solutions: I. Modification of γ-Al 2O 3 with various amounts of sodium ions

Abstract

The point of zero charge (ZPC), the surface acidity constants of γ-Al 2O 3, and the concentrations of charged surface groups AlOH 2 + and AlO − over a wide pH range were determined potentiometrically for a series of carriers prepared by modification of γ-Al 2O 3 with various amounts of sodium ions. It was shown that 0.083 mmol Na +/g of γ-Al 2O 3 is sufficient to increase drastically the ZPC value (from 5.3 to 9.7) and the concentration of AlOH 2 + over the pH range 1–9. Moreover, this amount is sufficient to decrease the surface acidity constants and minimize the concentration of AlO − in the pH range mentioned. Thus the pH range in which γ-Al 2O 3 can adsorb negatively charged active species is extended, due to doping, from 1–5 to 1–9. The above effect was attributed to the tendency of sodium ions to replace the H + of the AlOH , forming δ(−)δ(+) Al-O-Na which serve as adsorption sites for H s +, thus facilitating protonation of the adjacent surface hydroxyls. An additional increase in the sodium content changes the concentrations of the positive and negative surface groups markedly, but it does not affect the surface acidity constants and ZPC values: It was found that the concentration of AlOH 2 + increases linearly with Na + content at pH values lower than the ZPC. This was explained assuming that the small amount of Na + dissolved during equilibration is adsorbed specifically on the shear plane of the double layer, forming “ion pairs” with AlOH and releasing hydrogen ions on the surface. On the contrary, there was no correlation between sodium content and concentration of AlO - at pH values higher than the ZPC. The practical consequences of the sodium effects from the preparative point of view are discussed with respect to the adsorption on γ-Al 2O 3 of Mo, V, W, and Cr species.