Retrieving Oil and Recycling Surfactant in Surfactant-Enhanced Soil Washing

How to simultaneously realize retrieving oil and recycling surfactant in the remediation of leaked oil-polluted soil by means of surfactant enhanced soil washing is still a significant challenge. Here, we reported for the first time a novel CO2-switchable anionic surfactant, 11-dimethylamino-undecyl sulfate sodium salt (DUSNa), to retrieve leaked oil and recycle surfactant simultaneously in the process of oil-polluted sand remediation by DUSNa-enhanced soil washing. Because a CO2-switchable tertiary amine group had been incorporated into a traditional anionic surfactant of sodium alkyl sulfate to form DUSNa, DUSNa was readily converted into its inactive form of DUS upon CO2 treatment, leading to complete oil/water phase separation from a DUSNa-stabilized oil-in-water emulsion. The formed DUS was insoluble in both the oil and the water, but was suspended or precipitated in the lower aqueous phase, allowing subsequent retrieval of almost surfactant free oil. Recycling of DUSNa was enabled by its recovery from DUS by treatment with N-2 or NaOH. Upon CO2 treatment, both around 92.1% of the oil and around 90.8% of the DUSNa at least could be retrieved and recycled over three cycles of DUSNa-enhanced soil washing.