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 � 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.

Review of fluoride removal from drinking water

“…However, less stringent limits for effluent that is not to be released in sensitive locations are readily reached. For example in New South Wales a fluoride concentration of 20 mg/L is the maximum concentration permissible before the waste becomes a controlled liquid waste requiring a licence for transport and disposal (EPA, 1999 Farrah et al (1987) investigated the interaction of fluoride ion with amorphous aluminium hydroxide (Al(OH) 3 ), gibbsite (naturally occurring aluminium hydroxide) and aluminium oxide (Al 2 O 3 ) over a range of pH values from 3 to 8 and F À concentrations from 0.1-1 mM (w1.9-19 mg/L). At pH < 6 and total F:Al ratios >2.5, most of the amorphous Al(OH) 3 gel dissolved through formation of Al-F, complexes, with the distribution of fluorine being determined by the equilibrium F À value.…”

Batch defluoridation appraisal of aluminium oxide infused diatomaceous earth

“…8d showed that, at the equilibrium pH of the defluoridation system (pH = 6.4-6.6), three different species of aluminium fluoride were feasible (AlF 4 À AlF 5 2À and AlF 6 3À ) in the system. The F À interaction with different species of aluminium rich materials have been studied [35][36][37][38] and it has been reported that the maximum uptake of F À occur in the pH range of 5.5-6.5 [39]. At lower pH, fluoride uptake decreased due to the tendency of Al 3+ to dissolve and form positively charged AlF complexes (AlF 2+ , AlF 2 + ) which are repelled by the positively charged surfaces of Al oxides at these conditions (because the PZC of Al oxides > pH 7) [36].…”

Effect of strong acids on red mud structural and fluoride adsorption properties

“…Phosphate and sulphate (inner-sphere forming species) have been shown to have negative effects on the loading capacity of fluoride, while outer-sphere forming species (chloride and nitrate) improved fluoride removal slightly [22]. At pH values less than 3.7, fluoride removal generally decreases due to the formation of AlF x soluble species, while in alkaline solutions OH À displaced F À [31]. A study by Cengeloglu et al [24] investigated the adsorption capacity of untreated and hydrochloric treated red mud.…”