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 perchloroeth ylene (PCE) and the surfactant solution constant. Porous media experiments were also conducted at the same surfactant concentrations and an unknown in terfacial area to evaluate the effect of the surfactant on the mass transfe r rate coefficient, K,during dissolution of PCE from residual saturation to zero saturation. The experimental results show the following: (1) The addi tion of surfactant at concentrations below its critical micelle concentrati on (CMC) yields almost no difference in mass transfer rate coefficient desp ite a 3-fold decline in surface tension. (2) Above surfactant CMC, the pres ence of surfactant reduces the mass transfer rate coefficient. (3) Though a decrease in mass transfer rate coefficient is found, there is an overall i ncrease in mass transfer due to the higher driving force. (4) The amount of the change in mass transfer rate coefficient, above CMC, is roughly propor tional to the normalized mass transfer rate coefficients found from batch s ystems with constant interfacial area. The similarity between the results f rom batch experiments and the porous media experiments yield a technique fo r prediction of the mass transfer rate coefficient in porous media without having to repeat a dissolution experiment in porous media for each surfacta nt concentration. Results from modeled simulations demonstrate that the eff ect of the surfactant on the mass transfer rate coefficient is an important factor. Predictions regarding surfactant enhanced dissolution need to incl ude the effect of the surfactant on the mass transfer rate coefficient. Wit hout the appropriate correction in the mass transfer rate coefficient, simu lations will under predict the amount of time and effort required to remove nonaqueous phase liquids from the subsurface using this technology.