TRANSIENT MULTICOMPONENT GAS-PHASE TRANSPORT OF VOLATILE ORGANIC-CHEMICALS IN POROUS-MEDIA

Liquid mixtures of volatile organic chemicals (VOCs) contaminating man y soil and groundwater sites give rise to multicomponent mixtures of t heir vapors. Column studies were conducted to characterize multicompon ent diffusive transport of TCE and benzene vapors through air-dry sand . The vapors diffused from their liquid mixture source at one end of t he column to the other end where their concentration was maintained at zero by Rowing clean air across the column. Simulations based on Stef an-Maxwell equations of vapor diffusion predicted that the TCE and ben zene multicomponent steady-state flux densities should be 6.7 and 5.5% higher than the predictions based on Pick's law, respectively. The me asured steady-state multicomponent mass flux densities were higher tha n Pick's law estimates by 4.5 to 10.6 and 3.9 to 10.0% for TCE and ben zene, respectively. These results show that multicomponent equations s hould be used to predict the steady-state flux densities in a multicom ponent VOC vapor mixture. During the transient phase of the experiment s, use of Pick's law led to adequate predictions of flux density and c oncentration. A transient transport model based on Fick's law and a mu lticomponent, nonlinear adsorption model predicted measured TCE and be nzene concentrations along the column in both experiments more accurat ely than a model based on a single-species linear isotherm, which unde rpredicted vapor concentrations for both species. The magnitude of the underestimation was less for benzene, which is less strongly adsorbed at low concentrations than TCE. The linear adsorption isotherm adequa tely predicted concentrations at the early stages of the experiment wh en low concentrations were predominant.