Graduate Thesis Or Dissertation
 

Fluoride sorption and dissolution reactions in soils

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  • The reaction of F, often a component of industrial wastes and phosphatic fertilizers, with soil was studied with regard to sorption reactions and resultant changes in solution composition. Sodium fluoride solutions (0.0 to 2.5 mmol/L F) were equilibrated with ten soils for 24h. The soil extract solutions were analyzed for total F, free F, Al, Fe, Mn, Ca, Mg, K, Na, Cu, Zn, Ni, Cd, Cr, P, Si, NO₃, NH₃, organic C, S0₄, and pH. The computer equilibrium program CHEMSPEC was used to calculate ion speciation in solution. Fluoride sorption was rapid in all soils, with 90% of F sorption occuring within 24h. Increasing equilibration time decreased the amount of F that could be desorbed from the soil by NaClO₄. in 24h. Fluoride sorption conformed to the Langmuir sorption isotherm for all soils over limited concentrations (0.0 to 0.8 mmmol/L), while the Freundlich sorption isotherm was applicable over the entire initial F concentration range (0.0 to 2.5 mmol/L) for all soils. Simple and mutiple 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 rearession equation including oxalate-extractable Al, clay, amd oxalateextractable Fe predicted 90% of the variation in Lanomuir b values. Fluoride desorption could also be expressed by the Freundlich sorption isotherm for all soils. Fluoride desorption was hysteretic in 7 of 10 soils. The tendency of desorption to be hysteretic could not be explained by differences in measured soil properties. Solution concentration of Al, Fe, oraanic C, OH, and P increased, and Mn concentration decreased, with initial F concentration. Aluminum existed in solution almost entirely in complexes with F and organic C, while Fe was comolexed almost completely by OH and organic C. Solution concentration of Ca and Ma 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. Complexation of Ca and Mg by F decreased, and complexation by organic C increased, if Al was present. Fluoride existed primarily as the uncomplexed ion in the absence of Al. When Al was present it complexed an averaae of 72% of the F in solution.
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