Experimental analysis of fluoride diffusion and sorption in clays

Double reservoir diffusion cell tests were employed to estimate parameters for fluoride sorption and diffusion through porous clay media. Tests were conducted on both commercial kaolin and bentonite clays. The bentonite was found to be a better fluoride barrier than kaolin, especially when volumetric changes from swelling were minimised by clamping. Modelled values of effective diffusion in kaolin were typically around 0.015 m2/yr, which are nearly three times lower than the molecular diffusion in free solution and 10 times greater than the value obtained for clamped bentonite (constant volume and constant confining pressure). Optimised sorption parameters obtained by fitting both linear and non-linear sorption models to diffusion cell breakthrough data were compared with equivalent parameters from batch sorption tests. Both models were equally capable of accurately fitting all experimental data. However, in cases of highly non-linear sorption, care must be taken with the linear model as the sorption parameter depends on the concentration range. Sorption parameters from fluoride batch tests and clay tortuosities from chloride diffusion tests were found to adequately predict fluoride diffusion results. It was found that the modelled parameters for diffusion and partition did not depend significantly on the initial moisture content at compaction of the clay plug. Clay plugs formed through consolidation of a slurry, however, appear to provide more tortuous barriers than compacted clay plugs. Some decline in fluoride retention was observed with increasing ionic strength of solution.