Enhanced removal of trapped non-aqueous phase liquids from saturated soil using surfactant solutions

Column experiments were performed under water-saturated conditions to evaluate the effectiveness of low-concentration surfactant solution at removing a trapped light non-aqueous phase liquid (LNAPL) o-xylene and a dense non-aqueous phase liquid (DNAPL) o-dichlorobenzene from soil. Sorption of the ethoxylated alcohol (Witconol SN90) surfactant on the porous material was evaluated using both batch and column flow experiments. The batch sorption results showed that the sorption of the surfactant on sand reached a minimum value near the surfactant concentration selected for the experiments. Negligible sorption of the surfactant was observed in the column sorption experiment at the 90 cm day−1 flow rate used in our study, implying that the residence time was too short for equilibrium sorption to be reached. The NAPLs were removed from the columns by solubilization, emulsification and mobilization. Macroemulsions were formed in both batch and column experiments, suggesting that the NAPL was mostly removed by immiscible displacement. The direction of flow was an important parameter in LNAPL removal, influencing both the location of the chemical in the column and the removal efficiency. The percentage of DNAPL removed from the column was successfully described using a dissolving sphere model with increased apparent solubility. However, the model failed to describe the NAPL distribution in the column adequately, suggesting that a more complex process than the solubility enhancement accounted for by the model is occurring.