Fluoride interactions with hydrous aluminum oxides and alumina

Abstract

Fluoride ion interaction with amorphous Al(OH),, gibbsite or alumina (Al2O3) has been investigated over a range of pH values (3-8) and F- concentrations (0.1-1 mM) . At pH <6 and total F- : Al ratios >2.5, most of the amorphous AI(OH)3 gel dissolved through formation of AlF, complexes, with the distribution of fluoro ions being determined by the equilibrium F- value. At lower F : Al ratios, some solid persisted in the pH 4-7 region and strongly sorbed F- from solution. Maximum uptake occurred in the pH 5.5-6.5 region (up to 9 mol kg-1). At lower pH, uptake declined due to preferential formation of AlFx soluble species; at higher pH, OH- displaced F- from the solid and the amount of F- sorbed or converted to complexes declined rapidly towards zero between pH 6 and 8. Similar pH trends were observed using á Al2O3, except that both substrate solubility and the adsorptive capacity was less. At a iixed pH (between 5 and 7.5) uptake varied in accordance with a Langmuir equation (maximum capacity approx. 1 mol kg-1). The amount of substrate converted into AlFx complexes in acid media increased with decreasing pH and increasing initial fluoride concentration. Gibbsite displayed the same dissolution trends, but reacted at a much slower rate. Little F was sorbed by this substrate (10-20 mmol kg-1). The study has confirmed the ability of F- to mobilize A1 and the capacity of Al(OH)3 to scavenge F- from aqueous phases in the pH 4-7 region.