Composition and speciation of sodium fluoride extracted soil solutions

The effects of F on the chemical components of soils were examined by equilibrating 0.0 to 2.5 mM NaF solutions with ten soils for 24 h. The solutions were then analyzed for total F, Al, Fe, Mn, Ca, Mg, K, Na, Cu, Zn, Ni, Cd, Cr, P, Si, N03, NH3, organic C, SO4, free F, and pH. The computer equilibrium program CHEMSPEC was used to calculate ion speciation in solution. The solution concentration of Al, Fe, organic C, OH, and P increased with initial F concentration. Aluminum existed in solution almost entirely as complexes with F and organic C, while Fe was complexed almost completely by OH and organic C. When Al was present, an average of 72% of the F in solution was complexed with it. Fluoride existed primarily as the uncomplexed ion in the absence of Al. The solution concentration of Ca and Mg remained constant as a function of F concentration when Al was not detected, but decreased when Al was present. Solutions where Al was not detectable were supersaturated with respect to CaF2, and precipitation would probably occur at reaction times long enough for equilibrium to be obtained. In the presence of Al, complexation of Ca and Mg by F decreased, and complexation by organic C increased.