Prediction of Sorption by Soils of Volatile Hydrocarbon Mixtures

Mixtures of hydrocarbon vapors evaporating from petroleum spills are often transported through the unsaturated zone. To predict the course of evaporation the sorption isotherm of each component, and how this is affected by the other components, must be known. The sorption of m-xylene and n-dodecane vapors sorbed on air- and ovendry soil was studied. Mixtures of m-xylene and n-dodecane containing 0, 20, 40, 60, 80, and 100% n-dodecane by volume were prepared. The pure compounds and each mixture were diluted with nonvolatile paraffin to contain 5, 10, 20, 50, 80, and 100% of the pure compound or mixture of m-xylene and n-dodecane, thus providing a wide range of vapor composition. Samples of Evesham clay (Orthic Grumaquert) were suspended over each of the vapors, and the amounts sorbed by the soil were determined by gas chromatography after extraction of the soil with CCl₄ . On oven-dry soil at low relative vapor pressures the two gases competed for sorption sites. At higher relative vapor pressures the increased sorption due to formation of multilayers more than compensated for competition by the other gas, so that addition of the second component increased sorption of the first. On air-dry soil the addition of a second component always increased sorption of the first at all relative vapor pressures. Empirical isotherms of the pure components give no information about their influences on each other's sorption. Hill's theory of multilayer multicomponent sorption allows one to predict such influences from parameters derived solely from the isotherms of the pure components. The Hill equation is shown to be more successful in predicting the sorption from mixtures of m-xylene and n-dodecane vapors than an empirical equation based on the isotherms of the pure components.