Sodium Fluoride Transport in Partially Saturated Kaolinite

Beschreibung

To accurately predict the performance of a landfill liner, the transport parameters of the landfill liner must be known. In a number of geographic and geological circumstances the liner is in a state of partial saturation. The transport parameters for saturated conditions no longer apply. Therefore a number of methods exist which allow for the estimation of diffusion coefficients through coarse grained soils. But, no method for the estimation of diffusion coefficients in partially saturated clayey soils exists. This thesis addressed that deficiency. Presented in this thesis is a laboratory based technique which allows for the study of the transport process of sodium fluoride in partially saturated clayey soils. A double reservoir diffusion cell apparatus was used as the basis for the technique. Using osmotic suction, generated by size exclusion, the clay sample was desaturated. That is, polyethylene glycol solution was used in the reservoirs, and excluded from the clay sample by cellulose ester acetate membranes. Each aspect of the system was analysed in detail. The suctions generated by compacted kaolin was measured, and compared to suctions generated by different polyethylene glycol concentrations. The diffusion and partition coefficients of the size exclusion membrane were experimentally determined. Saturated clay experiments were conducted to determine the influence of pH on the sorption coefficient. It was found that increasing the pH to 8 was effective in reducing the sorption coefficient to a minimum. The effect of buffered, and pre-buffered clay samples was analysed, and it was found that pre-buffered samples gave a higher estimated diffusion coefficient. This was due to the change in the effective porosity, which is brought about the increase in the ionic strength of the clay sample. The moisture content of the clay was effectively varied by altering the concentration of the polyethylene glycol. The results in conjunction with the advection dispersion equation were analysed to estimate the effective diffusion coefficients for the contaminants. The results showed that the diffusion coefficients decreased sharply with decreasing moisture content. This result is in agreement with similar results obtained by other researchers for coarse grained soils. The effect of decreased moisture content was more significant in fine grained soils. The method of measurement for the double reservoir diffusion cells was by ion specific electrodes. It was found that this method of analysis was not optimal.

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Christian A Jungnickel

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clay diffusion double reservoir diffusion cell sorption unsaturated

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