Fluoride sorption by soil components: calcium carbonate, humic acid, manganese dioxide and ailica

Abstract

The ability of individual soil components to adsorb fluoride ions from dilute solutions (1-12 mg L-1 F-), at different pH values, has been investigated. No significant uptake was detected using substrates such as calcite, hydrous manganese(1v) oxide, cryptomelane, �-MnO2, pyrolusite, silica or silica gel, over the pH range 3-8 (for calcite, 6-9). The sorption of F- by calcite and humic acids at higher solution levels (up to 200 mg L-1) was also examined. Uptake by CaCO3 was observed when [F-] was >7 x 10-4 mol L-1, with the moles retained (pH -8.5) being approximately 1.6[F-]2. The amount of F removed increased on lowering the pH or on adding Ca2+, and it is proposed that the overall reaction is described by the equation CaCO3(S) + 2F- <=>CaF2(S) + CO23-. Two humic acid samples adsorbed increasing amounts of F- as the pH was lowered below 6.5, and increasing amounts of A1 and fluorocomplexes were detected in solution. The amount sorbed (mol kg-', at pH 5.5-6) varied with ash content and equalled either 16[F-]0.64 or 60[F-1. The major interaction appears to be HF attack on aluminosilicates in the ash fraction, with lesser contributions from sorption on calcium compounds and interaction with the counter-ions associated with the humic acid functional groups (e.g. Ca2+, Al3+, Fe3+).