Fluoride Sorption and Desorption in Soils

Fluoride sorption and desorption reactions in 10 soils were investigated. Fluoride sorption kinetic data were obtained by equilibrating 1.0 mmol/L NaF solutions with four soils for periods from 1 h to 77 d. Fluoride sorption reactions with respect to soil properties were examined by equilibrating 0.0 to 2.5 mmol/L NaF solutions with 10 soils for 24 h. Fluoride sorption was rapid in all soils, with 90% of the sorption occurring within 24 h. Increasing sorption equilibration time decreased the amount of F that could be desorbed from the soil in 24 h. Fluoride sorption conformed to the Langmuir sorption isotherm for all soils over limited concentration ranges. The Freundlich sorption isotherm was applicable over the entire initial F concentration range (0.0–2.5 mmol/L) for all soils. Simple and multiple regression techniques allowed various F sorption parameters to be predicted from selected soil properties. Oxalate‐extractable Fe and Al, dithionite‐extractable Al and soil pH correlated most highly with both Langmuir b and Freundlich K values in a simple regression model. A multiple regression equation including oxalate‐extractable Al, clay, and oxalate‐extractable Fe predicted 90% of the variation in Langmuir b values. Fluoride desorption also conformed to the Freundlich isotherm for all soils. Fluoride desorption was hysteretic in 7 of 10 soils, but hysteresis could not be explained by differences in measured soil properties.