Modeling multicomponent volatile organic and water vapor adsorption on soils

Abstract

Volatile organic chemical (VOC) and water vapors are present simultaneously in the soil gas phase. Any modeling of VOC vapor flow must account for the strong competition they experience from water and any competition between themselves. To account for these competitions, the multicomponent form of the Brunauer—Emmett—Teller equation with finite number of adsorption layers was tested. Of the three input constants required in the multicomponent model, two can be obtained from single species adsorption experiments. The third constant was found to be bounded within a fairly narrow range; although its physical meaning is debatable, its value can be approximated fairly accurately. The data used to check the applicabi- lity of this model were for the adsorption of trichloroethylene and toluene on a sand and Yolo silt loam (fine-silty, mixed, nonacid, thermic Typic Xerorthent). For relative humidities (RHs) corresponding to less than one molecular layer of water coverage and at low to very low toluene and trichloro ethylene (TCE) vapor pressures, it underpredicted the adsorbed amounts of both species from their binary mixtures with water on both soils. At RHs corresponding to between one and two molecular layers of water coverage, predictions compared well with data. At RHs corres- ponding to about two layers of water, the model overpredicted the adsorbed amounts. At toluene relative vapor pressures above 0.1 the model reasonably described the adsorbed amount on sand at two RHs corresponding to one to two water layers. This model was further tested on published adsorption data of para-xylene and water on soil. It was reasonably successful in describing adsorption of para-xylene from its binary mixture with water above para-xylene relative vapor pressure of 0.069 and water relative humidity of 0.084. No competition between TCE and toluene was observed at the low vapor pressures in our experiments. Simulations of adsorption in the ternary systems of TCE, toluene, and water followed the same pattern as the binary simulations.