Soil Cleanup by In-Situ Aeration. XXI. Effects of Desorption Rates and Equilibria on Remediation Rates

A model for soil vapor extraction (SVE) is developed which permits the use of nonlinear adsorption isotherms and takes desorption kinetics into account. The configuration modeled is a single vertical well. The model is used to exhibit the dependence of cleanup rate on the parameters of the nonlinear adsorption isotherm and on a rate constant k f associated with adsorption. Cleanup curves similar to those found in modeling diffusion-limited SVE by the lumped diffusion parameter method are observed for small values of k f. Slow cleanup rates can also result from isotherm parameter values. In particular, severe tailing in the terminal phase of remediation may result from an equilibrium isotherm which approaches the form C gas=K(C sorbed) B , where B > 1 as C sorbed approaches zero. As with diffusion-limited SVE, short-term pilot-scale experiments may well not identify conditions which cause tailing during terminal phase cleanup. One can distinguish between poor SVE performance due to small rate constants and poor SVE performance due to the adsorption isotherm by soil gas volatile organic compound (VOC) concentration rebound curves. Rebound occurs if diffusion and/or desorption rales are limiting. Rebound does not result if adsorption isotherm characteristics are limiting. Increases in the gas flow rate will not be helpful in the former case, while they will result in increased VOC removal rates in the latter.