Fluoride adsorption onto α-Al2O3 and its effect on the zeta potential at the alumina-aqueous electrolyte interface
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Fluoride removal from aqueous solutions using α-Al2O3 as an adsorbent was investigated through adsorption and electrokinetic studies, using two different initial fluoride concentrations relevant to fluoride contents in drinking water, namely 3 and 10mg/L. Maximum fluoride removal was achieved between pH 5 and 6 at 25°C. Adsorption at both pH 5 and 9 correlates to the Langmuir isotherm. The point of zero charge (pzc) of α-Al2O3 at 20°C was determined using three different concentrations of NaNO3 as an indifferent electrolyte. It was found to occur at pH 9.2. Through mineral/solution equilibria, this pzc was determined to be close to pH 9.5 of minimum solubility of amorphous aluminum hydroxide, suggesting the formation of this species onto the adsorbent%27s surface. Zeta potential of α-Al2O3 in the presence of fluoride reversed sign towards more acidic pH values in comparison to the pzc, indicating specific adsorption of fluoride. Fluoride adsorption onto alumina under mildly acidic conditions seemed to involve mainly a replacement of surface OH- by F-. Adsorption is small at pH 9 and up to about 10 and it seems to take place through hydrogen bonding.
Fluoride removal from aqueous solutions using α-Al2O3 as an adsorbent was investigated through adsorption and electrokinetic studies, using two different initial fluoride concentrations relevant to fluoride contents in drinking water, namely 3 and 10mg/L. Maximum fluoride removal was achieved between pH 5 and 6 at 25°C. Adsorption at both pH 5 and 9 correlates to the Langmuir isotherm. The point of zero charge (pzc) of α-Al2O3 at 20°C was determined using three different concentrations of NaNO3 as an indifferent electrolyte. It was found to occur at pH 9.2. Through mineral/solution equilibria, this pzc was determined to be close to pH 9.5 of minimum solubility of amorphous aluminum hydroxide, suggesting the formation of this species onto the adsorbent's surface. Zeta potential of α-Al2O3 in the presence of fluoride reversed sign towards more acidic pH values in comparison to the pzc, indicating specific adsorption of fluoride. Fluoride adsorption onto alumina under mildly acidic conditions seemed to involve mainly a replacement of surface OH- by F-. Adsorption is small at pH 9 and up to about 10 and it seems to take place through hydrogen bonding.
