MODELING MULTICOMPONENT VOLATILE ORGANIC AND WATER-VAPOR ADSORPTION ON SOILS

Volatile organic chemical (VOC) and water vapors are present simultane ously in the sol gas phase. Any modeling of VOC vapor flow must accoun t for the strong competition they experience from water and any compet ition between themselves. To account for these competitions, the multi component form of the Brunauer Emmett-Teller equation with finite numb er of adsorption layers was tested. Of the three input constants requi red in the multicomponent model, two can be obtained from single speci es adsorption experiments. The third constant was found to be bounded within a fairly narrow range; although its physical meaning is debatab le, its value can be approximated fairly accurately. The data used to check the applicability of this model were for the adsorption of trich loroethylene and toluene on a sand and Yolo silt loam (fine-silty, mix ed, nonacid, thermic Typic Xerorthent). For relative humidities (RHs) corresponding to less than one molecular layer of water coverage and a t low to very low toluene and trichloro ethylene (TCE) vapor pressures , it underpredicted the adsorbed amounts of both species from their bi nary mixtures with water on both soils. At RHs corresponding to betwee n one and two molecular layers of water coverage, predictions compared well with data. At RHs corresponding to about two layers of water, th e 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 mod el was further tested on published adsorption data of para-xylene and water on soil. It was reasonably successful in describing adsorption o f para-xylene from its binary mixture with water above para-xylene rel ative vapor pressure of 0.069 and water relative humidity of 0.084. No competition between TCE and toluene was observed at the low vapor pre ssures in our experiments. Simulations of adsorption in the ternary sy stems of TCE, toluene, and water followed the same pattern as for the binary simulations.