Surfactant enhanced perchloroethylene dissolution in porous media: The effect on mass transfer rate coefficients

The effect of a surfactant on the mass transfer rate coefficient, K, during the dissolution of a nonaqueous phase liquid (NAPL) was investigated using batch and column experiments. Batch experiments were performed at several surfactant concentrations holding the interfacial area between perchloroethylene (PCE) and the surfactant solution constant. Porous media experiments were also conducted at the same surfactant concentrations and an unknown interfacial area to evaluate the effect of the surfactant on the mass transfer rate coefficient, K, during dissolution of PCE from residual saturation to zero saturation. The experimental results show the following: (1) The addition of surfactant at concentrations below its critical micelle concentration (CMC) yields almost no difference in mass transfer rate coefficient despite a 3-fold decline in surface tension. (2) Above surfactant CMC, the presence of surfactant reduces the mass transfer rate coefficient. (3) Though a decrease in mass transfer rate coefficient is found, there is an overall increase in mass transfer due to the higher driving force. (4) The amount of the change in mass transfer rate coefficient, above CMC, is roughly proportional to the normalized mass transfer rate coefficients found from batch systems with constant interfacial area. The similarity between the results from batch experiments and the porous media experiments yield a technique for prediction of the mass transfer rate coefficient in porous media without having to repeat a dissolution experiment in porous media for each surfactant concentration. Results from modeled simulations demonstrate that the effect of the surfactant on the mass transfer rate coefficient is an important factor. Predictions regarding surfactant enhanced dissolution need to include the effect of the surfactant on the mass transfer rate coefficient. Without the appropriate correction in the mass transfer rate coefficient, simulations will under predict the amount of time and effort required to remove nonaqueous phase liquids from the subsurface using this technology.