FLUORIDE RETENTION BY KAOLIN CLAY

To evaluate the potential effectiveness of kaolin clay liners in stora ge of fluoride contaminated waste, an experimental study of the sorpti on and desorption behaviour of fluoride in kaolin clay was conducted. The degree of fluoride sorption by kaolin was found to depend on solut ion pH and available fluoride concentration with equilibrium being ach ieved within 24 h. A site activation process involving the uptake of f luoride was also observed at the initial stages of sorption. This beha viour was attributed to a layer expansion process of the clay during s orption. The maximum fluoride sorption capacity was found to be 18.3 m eq/100 g at pH 6 and 8.6 meq/100 g at pH 7. A competitive Langmuir sor ption isotherm where sorption is dependant on both pH and fluoride con centration is employed to characterise the experimental sorption and d esorption data. The sorption and desorption isotherms revealed pronoun ced hysteretic behaviour. The model fitting experimental results sugge sts that 75% of fluoride sorbed is irreversibly bound. (C) 1997 Elsevi er Science B.V.