The diffusion and sorption of volatile organic compounds through kaolinitic clayey soils

Abstract

Laboratory experiments to estimate the effective molecular diffusion coefficient ( D e) and sorption coefficient ( K d) for volatile organic compounds through natural clayey soils were conducted using diffusion testing apparatus. The compounds tested were methyl ethyl ketone (MEK), toluene and trichloroethylene (TCE). The D e and K d values were determined by a curve fitting procedure. The compound losses, and the effects of porous disks used in the apparatus were significant. The transport of MEK was faster than that of TCE and toluene because of the lower sorption to the soils. The D e values of all the compounds were of the order of 10 −10 m 2/s and smaller than the diffusion coefficient in pure aqueous solution at infinite dilution ( D 0), due to the tortuosity of the samples. The effects of the sample thickness on the parameter determination were not significant. Comparison to the K d values estimated from batch sorption tests and from organic carbon content ( f oc)-based predictions showed that the diffusion test results were intermediate between those from the other two methods. The diffusion tests use compacted soil samples and should be more relevant to in situ conditions, but the reliability of the tests is affected by large compound losses that cause uncertainties in their interpretation. It is recommended that more than one method be used to assess K d values.