Analysis of diffusion and sorption of organic solutes in soil-bentonite barrier materials

A series of laboratory studies was conducted to assess the transport of chloride, trichloroethylene (TCE), and aniline in soil-bentonite (SB) slurry wall materials. Column transport experiments were conducted, with a primary emphasis on the measurement of spatial contaminant mass profiles within columns after 25-50 days of transport under diffusion-dominated conditions. The advective-dispersive-reactive (ADR) equation was found to provide good predictions of the results of the column experiments with calibrated diffusion parameters that were within the range observed by other researchers for earthen barrier materials. However, values of the porosity-corrected hindrance factor were higher than those observed in other studies of diffusive transport in SB, and sorption coefficients calibrated from the column experiments were significantly lower than values measured in batch isotherm tests conducted with unconsolidated SB. A number of factors could contribute to variability across experiments and investigators, including differences between batch and column conditions, differences in experimental apparatus and design used to study diffusive transport in SB, correlation between parameters in the calibration process, and the potential influence of nonequilibrium sorption in column experiments. For the design of conventional slurry walls, it is likely that conservative predictions of organic contaminant penetration can be obtained with the ADR equation using established correlations for effective diffusion coefficients and neglecting sorption to barrier materials.