Nonequilibrium nonaqueous phase liquid mass transfer model for soil vapor extraction systems

Soil vapor extraction is a popular soil remediation technology that is hampered by less than optimal performance in the field due to mass transfer limitations. Therefore, laboratory column venting experiments were completed to quantify mass transfer limitations for the removal of multicomponent nonaqueous phase liquid (NAPL) contaminants from a silt loam soil at three water contents. The observed mass transfer limitations were quantified using a four phase multicomponent, nonequilibrium contaminant transport model based on first-order mass transfer kinetics. The overall mass transfer coefficient K(g)a was treated as a variable and modeled as a linear function of the NAPL volumetric fraction using two adjustable parameters (m, the slope parameter and K(g)a(min), the intercept). Both were back calculated from column venting data. The agreement between the calibrated model and experimental results were favorable for the removal of single and binary contaminants under conditions ranging from near equilibrium to severe mass transfer limitations and extended tailing. A strong dependency of K(g)a on water content was evident by the differences in K(g)a(min) and to a lesser extent, m, at the three water contents investigated. A single expression K(g)a captured the performance of both components in the binary mixture. For the quaternary venting experiments a single expression for K(g)a captured the performance of all four components well under air dry conditions. However, the agreement between the hexane model versus the experimental result deteriorated significantly as the water content increased. This difference is attributed to hexane's lower affinity for the water phase relative to the other three components in the mixture.