Fluoride Speciation in Aqueous Suspensions of Montmorillonite and Kaolinite

Abstract

The speciation of fluoride (F− ), a naturally occurring water pollutant, externally added in aqueous suspensions of montmorillonite and kaolinite has been investigated as a function of period of agitation, pH and F− amount. Utilizing the appropriate TISAB (total ionic strength adjusting buffer) solution for F− analysis, experiments were conducted to quantitatively determine the equilibrium partitioning of added F− in sorbed, complexed and free ionic fluoride phases. Results suggest the disruption of montmorillonite lattice in contact with water, particularly at high acidic pH, resulting in the release of Fe 3+ , Al 3+ , and silica in significant amounts, which, along with the pH of the medium, effectively control F− speciation. In case of kaolinite, although disruption of lattice is not that prominent, but variations in pH do significantly alter F− speciation. The findings of this study, are particularly important for environmentalists and public health agencies, working in F− affected rural parts of most Asian and African countries, who are still trying to develop a cost effective and user friendly water defluoridation technique.