The optimum pH for the removal of excess fluoride by activated alumina is _____.

The optimum pH for the removal of excess fluoride by activated alumina is _____.

• A. 5.5
• B. 6.5
• C. 7.0
• D. 7.5

Answer: (A)

Fluoride removal with activated alumina works best when the alumina surface is positively charged and fluoride is primarily present as F-. At mildly acidic pH around 5.5, the surface of activated alumina carries a favorable positive charge, which attracts the negatively charged fluoride ions and maximizes adsorption. As the pH moves toward neutral and basic, the surface becomes less positively charged or even negatively charged, reducing attraction and thus lowering removal efficiency. Also, if the pH is too low, some fluoride can shift toward forms that are less readily adsorbed, but around 5.5 the balance is optimal: fluoride remains mostly as F- and the surface is most receptive to adsorption. That’s why about 5.5 is the best pH for removing excess fluoride with activated alumina.

Fluoride removal with activated alumina works best when the alumina surface is positively charged and fluoride is primarily present as F-. At mildly acidic pH around 5.5, the surface of activated alumina carries a favorable positive charge, which attracts the negatively charged fluoride ions and maximizes adsorption. As the pH moves toward neutral and basic, the surface becomes less positively charged or even negatively charged, reducing attraction and thus lowering removal efficiency. Also, if the pH is too low, some fluoride can shift toward forms that are less readily adsorbed, but around 5.5 the balance is optimal: fluoride remains mostly as F- and the surface is most receptive to adsorption. That’s why about 5.5 is the best pH for removing excess fluoride with activated alumina.