Volatilization of Binary Nonaqueous Phase Liquid Mixtures in Unsaturated Porous Media

Abstract

This study examines the volatilization behavior of binary nonaqueous phase liquid (NAPL) mixtures consisting of styrene, and toluene or tetrachloroethylene (PCE). Residual NAPL saturations were emplaced in unsaturated (residual water saturation) soil columns packed with Wagner 50-80 sand. Initial column effluent concentrations were measured for the NAPL mixtures at several pore gas phase velocities. Rate-limited volatilization occurred at higher gas phase pore velocities, and mass transfer coefficients could be reasonably predicted with a correlation developed from single component NAPL volatilization data. Long-term volatilization studies for the binary NAPL mixtures were also conducted. The effluent concentrations for both NAPL components were observed to be initially proportional to their mole fractions. After the more volatile component became depleted, a rapid drop in the effluent concentration of this component was accompanied by an increase in the mole fraction and effluent concentration of the remaining constituent to near saturated values until the free phase NAPL was volatilized. The final stage of removal was associated with a dramatic decrease in effluent concentration, attributed to reduction in the gas-NAPL interfacial area, followed by low concentration tailing. The tailing and subsequent flow interruption behavior are likely a consequence of rate-limited desorption. Equilibrium and rate-limited simulations of the long-term volatilization experiments did not provide a satisfactory description of the data. A simulation that included a fixed concentration gradient and fitted activity coefficients provided a better characterization of the volatilization data. Various potential explanations for this “fixed gradient” volatilization behavior were considered, but additional research is needed to test these hypotheses.