Adsorption Characteristics of Fluoride onto Hydrous Alumina

Abstract

Fluoride removal by activated alumina, Type F‐1, depends primarily on the pH and surface loading. The optimal pH for fluoride removal is approximately 5. However, at $\mathrm{pH}<6.0\text{,}$ the aluminum (III) dissolved from activated alumina reacts with fluoride ions and forms alumino‐fluoro complexes. These complexes are unstable in neutral or alkaline pH regions. Consequently, the role of pH in minimizing alumina dissolution as opposed to maximizing fluoride removal must be considered. An alkalimetric titration method was used to determine the surface acidity of the activated alumina. The intrinsic constants, ${\mathrm{pK}}_{\mathrm{a1}}^{\mathrm{int}}$ and ${\mathrm{pK}}_{\mathrm{a2}}^{\mathrm{int}}\text{,}$ are 4.2 and 10.5, respectively, in 0.05 M ${\mathrm{NaClO}}_4\text{,}$ and the density of Brönsted acid sites is approximately $5.6\times {10}^{13}/{\mathrm{cm}}^2\text{.}$ A surface complex formation model was used to describe the removal of fluoride by activated alumina. These reaction mechanisms can be identified as a surface complex formation involving $\underset{\text{¯}}{\mathrm{Al}}\mathrm{OH}$ and $\underset{\text{¯}}{\mathrm{Al}}{\mathrm{OH}}_2^{+}\text{.}$ Polynuclear surface complex may be formed at high surface loading.