Only limited data are available on the diffusion of volatile organic solvents through flexible membrane liners (FMLS) used for lining impoundments and landfills. To expand this database, a rapid, inexpensive method is needed to measure the diffusion coefficients of volatile organic solvents through FML materials. An absorption method has been developed to determine the diffusion coefficients of volatile organic solvents through FML materials. The method is based on the depletion of an organic compound from an aqueous solution due to absorption by a submerged sample of FML. A numerical solution of Fick's second law of diffusion was used to develop a graph which can be used to determine the diffusion coefficient from the time dependent concentration data. The diffusion coefficients obtained from the absorption tests were validated by comparing them with coefficients determined using a two chamber diffusion cell. The diffusion coefficients determined for toluene and xylene in high density polyethylene (HDPE) were 5.1 × 10-9 cm2s-1 and 1.0 x 10-9 cm2s-1 by the two methods, respectively. The data indicate that the coefficient of distribution (Kd) between the FML and the organic solution, a value which is needed to calculate the diffusion coefficient from the data, can be estimated from the log of the octanol-water partition coefficient (Kow), a commonly measured and reported value for many chemicals.
The diffusion of organic contaminants through plastic Flexible Membrane Liners (FMLs) used in landfills and surface impoundments is an important criteria for evaluating synthetic liner performance. An experimental method has been developed and used to measure diffusivities of organic contaminants commonly found in landfill leachate from an aqucous solution through High Density Polyethylene (HDPE). These data were utilized to evaluate the applicability of the Permachor Method developed by Salame (1961) to predict the diffusion of other organics through FMLs. The Permachor equation can be used in combination with the relationship between the partition coefficients of organics in aqueous organic solution and the octanol-water partition coefficients to predict permeability factors which in turn can be used to calculate the diffusion coefficients of a range of organic chemicals in a plastic FML. Once the relationship between permeability and Permachor are determined for a given plastic FML for a few organic chemicals, the method can be used to predict the permeability of many other organic chemicals. The properties of the plastics and the additives used in its formulation including carbon black appear to influence the diffusion rates.
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