Retention and Movement of Fluoride in Soils

Displacement and retention of fluoride in soils were studied in laboratory experiments. Fluoride adsorption and desorption isotherms were determined in the range of 0 to 5,000 µ g mL −1 of dissolved F by shaking 1:10 soil/NaF solution suspensions. The observed relationship was nonlinear. Neither Langmuir nor Freundlich equations were unbiased predictors of the measured relationship. The experimental errors propagated in the computational analysis and made such adsorption data quite error‐prone. For soils with a low F retention the confidence interval was wide, and vice‐versa. In an acid clay soil an apparent adsorption equilibrium was attained within 3 h, but several days were required for the equilibration of the calcareous soils with the NaF solution. Adsorption and desorption of the calcareous soils exhibited a hysteretic behavior, but in the case of the acid soil no hysteresis was observed. The transport of F was studied by percolating soil columns at a constant flow rate. The F breakthrough depended primarily upon the F concentration in the infiltrating solution and soil type, whereas the ionic composition of the infiltrating solution had a less pronounced effect on the breakthrough behavior. The F retention within the soil columns and the adsorption estimates obtained from the isotherms agreed well in the case of the acid soil, but the calcareous soils retained one‐third to one‐half as much F during the percolation experiment.