Fluoride adsorption onto α-Al₂O₃and its effect on the zeta potential at the alumina–aqueous electrolyte interface

Controlling the transport of cations through permselective mesoporous alumina layers by manipulation of electric field and ionic strength

“…8. As was already mentioned, fluoride is known to adsorb specifically on α-and γ -alumina surfaces by chemisorption [16,18], and maximum uptake of fluoride occurs at pH 5-6 [16]. Under the assumption that F − is incorporated and/or physisorbed only at the interior pore surface, the F content measured by XPS appears sufficiently high to cause a net negative fixed surface charge and explain the observed cation permselectivity.…”

“…This feature cannot be explained by the amphoteric behavior of the native oxide, which predicts a positive surface charge for alumina below pH 9.1 [16]. It therefore seems more likely that preferential anion adsorption occurred inside the mesopores, producing a largely immobile negative charge density.…”

“…Since the concentration of H + ions, which are ∼10 times more mobile than other anions or cations [15], increases with a decrease of pH, an increased H + flux may somehow suppress the flux of Cu 2+ . An alternative explanation is that upon increasing pH the positive surface charge density of α-and γ -alumina decreases via replacement of charged OH + 2 surface groups by chemisorption of F − according to the net reaction Al-OH + 2 + F − = Al-F + H 2 O, which occurs maximally at pH around 6 [16]. The decreased concentration of positive surface charge will lead to an increased local concentration of positive charge in the double layer of the pore, thereby promoting the transport of positively charged species (Cu 2+ ) through the pores.…”

Temperature effect on the zeta potential and fluoride adsorption at the α-Al2O3/aqueous solution interface

“…The parameters that influence the most fluoride adsorption are the pH, fluoride concentration, and temperature. The optimum pH for maximum fluoride adsorption on α-Al 2 O 3 occurs at about pH 5 [10], and at pH 6 for alum sludge [9]. Temperature effects on fluoride adsorption on alum sludge have been reported by Sujana et al [9], who found a decrease in adsorption with increasing temperature.…”

“…Another aspect that should be considered in the interpretation of zeta potential results is the effect of temperature and pH on the solubility of α-Al 2 O 3 . Using the appropriate thermodynamic data, Reyes Bahena et al [10] have calculated that the minimum solubility of this alumina occurs at about pH 7 at 25 • C. It is well known that metal oxide solubility increases with temperature.…”