Removal of fluoride ions from aqueous solutions by multivalent metal compounds

Abstract

The removal of fluoride ions by Al(III) ion, Ca(II) ion and solid compounds of multivalent metal elements such as Al(III), Y(III), La(III), Ce(III), Nd(III), Sm(III), Gd(III), Si(IV), Ti(IV), Zr(IV) and Ce(IV) in the form of oxides, hydrous oxides and basic carbonates is studied. With an excess amount (20 mM) of Ca or Al ion, fluoride was removed from 5.0 to < 0.02 mM by precipitation (pH 4‐) and coprecipitation (pH 5.5–7.5), respectively. With a small dose, the solid trivalent metal compounds were effective in decreasing the fluoride concentration to < 0.02 mM below a certain pH. The dissolution of the material became appreciable at a pH where the lowest fluoride concentration was attained. Among the tetravalent metal compounds, only hydrous cerium(IV) oxide was effective in removing fluoride ions without a significant dissolution even at pH 2. It is suggested that fluoride ions are removed mainly by an ion exchange process with surface hydroxide groups in a neutral pH range, but by both ion exchange and precipitation processes in the lower pH range. These solid multivalent metal compounds can be used for the treatment of fluoride‐containing wastewater from industries, such as those manufacturing semiconductors, and fluoride can be reduced to the permissible level.