Diffusive transport of low molecular weight organic solutes through soil-bentonite containment barriers.

Abstract

Effective diffusion coefficients for low-molecular weight solutes (1,4-dichlorobenzene and 4-chlorophenol) in soil-bentonite barriers were determined using both quasi-steady-state and transient experiments. Measured values were found to be reduced approximately three factors from those in free aqueous solution. The relationship between free aqueous and effective diffusion coefficients was found to conform to an exponential function of porosity. Reduced free area for diffusion and increased transport path length are considered to be the primary causes for the observed reductions in magnitude of the diffusion coefficients. Measured values of the hydraulic conductivity of the soil-bentonite mixtures were on the order of 10$\\sp{-7}$ cm/sec, several orders of magnitude lower than would be expected for otherwise identical soil mixtures that do not contain bentonite. Analysis of these data using the Kozeny-Carmen equation supports the concept that hydrated bentonite within soil-bentonite mixtures forms a fractured gel which immobilizes water and reduces the effective porosity of the medium for hydraulic transport. The sorption capacity of three media (silica s and , kaolinite and bentonite) whose mineral surfaces approximate those common in soil-bentonite mixtures for 1,4-dichlorobenzene and lindane was found to be extremely low. Conversely, sorption of these solutes plus five others (carbon tetrachloride, trichloroethene, tetrachloroethene, 1,2,4-trichlorobenzene and 4-chlorophenol) by three high-carbon fly ashes was found to be significant. The sorption capacities of these ashes, normalized to the carbon content, were found to be: (1) at least equivalent to those of organic carbon associated with soils and sediments; (2) related to the octanol/water partition coefficient and aqueous solubility; and (3) related to factors such as surface area and functionality associated with the volatile fractions of the fly ashes tested. Simulations of molecular diffusion through a hypothetical soil-bentonite barrier which is devoid of significant sorption capacity suggest that solute breakthrough can occur within as little as two years, and that near-steady-state flux rates can develop in as little as twelve years. Conversely, addition of a sorbent phase such as high-carbon fly ash to the barrier mixture can increase breakthrough time to nearly 60 years and delay the development of near-steady-state flux rates for 220 years.