Interaction of dilute fluoride solutions with hydrous iron oxides

Abstract

The amount of F- retained by hydrous iron oxide suspensions was found to be a function of F- concentration (2-100 mg L-1), pH, and structural form of the iron oxide. Uptake was minimal at pH >7, and increased with decreasing pH (e.g, with 10 mg L-1 NaF in solution, the amount sorbed by freshly precipitated hydrous iron oxide varied between 7 mg/g at pH 6 and 35.5 mg/g at pH 4.5). With fixed pH (between 4.5 and 6) the amount sorbed varied with concentration in accordance with a Langmuir isotherm relationship. The pH effect was countered at lower values by solid dissolution, hence each F- concentration studied exhibited a pH (between 3.5 and 4.5) where uptake was a maximum. Solid dissolution was accompanied by the formation of soluble iron(III) fluoro complexes. In acid media, the amount of F sorbed by hydrous iron oxide >> goethite > limonite >> hematite. In environmental systems, F interaction and retention should thus be most pronounced in acid media, containing recently precipitated hydrous oxides.