Soil Clean Up byin-situAeration. XIII. Effects of Solution Rates and Diffusion in Mass-Transport-Limited Operation

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Abstract
A model for soil vapor extraction (SVE) in laboratory columns is developed which includes the effects of mass transport kinetics of volatile organic compounds (VOCs) between nonaqueous phase liquid (NAPL) droplets and the aqueous phase, and between the aqueous and vapor phases. The model provides a detailed treatment of diffusion of VOCs through a stagnant aqueous boundary layer, and permits time-dependent gas flow rates in the vapor extraction column. Runs made with the model exhibit high initial effluent soil gas VOC concentrations typically followed by a fairly rapid decrease in concentration which in turn is followed by a prolonged tailing region in which the effluent soil gas VOC concentrations decrease quite slowly until nearly all of the VOC has been stripped from the column. The model demonstrates the futility of trying to predict SVE clean-up times on the basis of pilot scale experiments carried out for only a few days, in that these give no idea whatsoever as to the rate of VOC removal which can be expected late in the remediation. The model permits the gas flow to be varied with time; shutting off the gas flow after partial clean up results in rebounds in the soil gas VOC concentrations which can be quite large, particularly if some NAPL is still present.