Fluoride adsorption by a saline sodic soil irrigated with a high F water

Summary

Langmuir isotherm data for F adsorption were obtained from 1:10 soil:water extracts of soil samples from a lysimeter study. A sodic silt loam surface soil with a saline sodic subsoil was irrigated with a high sodium chloride, high fluoride (0.38 mMF) geothermal well water. A previous study showed that fluorite (CaF₂) was precipitated from solution in the upper portion of this profile while another mechanism removed F solution in the lower part of the profile to below 0.02 mMF. The Langmuir isotherm data indi cate that one kind of surfaces or sites remove fluoride from solution over the 0 to 1.1 to 1.2 mMF range. The adsorption capacity for this F removal is about 4.4 to 5.8 mmol F/kg of soil and the equilibrium constant is between 0.54 to 1.001/mmol F. Once these surfaces or sites were saturated, a second kind of sites removed F from solution, and had an adsorption capacity of 9.2 to 11.4 mmol/kg and an equilibrium constant of 0.16 to 0.271/mmol. Both data sets fit the Langmuir equation. At some point before or after this second set of sites or surfaces was saturated, the fluorite ion activity product was exceeded and fluoride was then removed from solution via fluorite precipitation. The two adsorption mechanisms lowered the soil solution F concentration sufficiently to prevent ground water contamination, but once the adsorption sites were saturated, fluorite precipitation does not decrease F concentration sufficiently to meet drinking water standards.