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 N2 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.