Mass transfer processes in the soil vapour extraction of gasoline from unsaturated soils.

摘要

Three mass transfer resistances in soil vapour extraction were studied: NAPL to air, aqueous to air and sorbed to aqueous. An automated laboratory-scale one-dimensional soil vapour extraction apparatus featuring air and soil temperature control was constructed for this purpose. Laboratory techniques were developed to isolate each of the resistances. NAPL-air mass transfer was assessed by using a non-organic soil in the presence of single component or binary component NAPL. Aqueous-air and sorbed-aqueous mass transfer was assessed by contaminating wetted non-organic or organic soil, respectively, via the vapour phase. A laboratory-scale one-dimensional dispersion apparatus was also developed in order to yield dispersion coefficients.; Values of the mass transfer coefficients were obtained through calibration of FRACMAT, a soil vapour extraction model. Fitting was performed on a logged concentration basis to give balance to low tail-level concentrations. Quality of fit was assessed through evaluation of the sum of squared residual deviations between laboratory and modelling data. Mass closure was also used as a fitting criteria.; Empirical correlations with a physical basis were developed to successfully describe the complex mass transfer behaviour observed. NAPL to air mass transfer was found to decrease with the residual NAPL content of the soil. For binary experiments, NAPL-air mass transfer was also dependent on the component mass fraction in the residual. Aqueous to air mass transfer decreased with the aqueous-phase concentration of the contaminant. Sorbed to aqueous mass transfer was relatively unimportant for the conditions studied and was adequately represented by a constant first-order rate expression.; Extrapolation of the mass transfer data developed for simple soils at simple conditions demonstrated limited success at describing mass transfer for more complex conditions. This was attributed to the effect of soil type and the complex liquid distribution created where NAPL and water were both present in the soil.