Oil removal from solid surface by using surfactant-free microemulsion regulated by CO2: A sustainable approach for treating oily waste

Facing the issues of residual surfactant by using surfactant-based microemulsion, the surfactant-free microemulsion (SFME) are chosen as the alternate for extraction process. Especially, the sustainable separation for oil by using SFME has been not completed. Herein, the CO2-responsive SFME were formed and perform excellent behavior on oil removal and separation, providing the sustainable approach for oil removal. Here, this SFME were simply formed with hydrophobic tertiary amine as oil phase, ethanol as "amphisolvent" and water. This SFME exhibit reversible transformation of microstructure controlled by CO2, and also perform thermal-stable microstructure. Moreover, the SFME have good oil solubilization behavior that can be improved by the oil phase content (N,N-Dimethylbenzylamine) of SFME and the reduced interfacial tension between SFME and oil. After introducing CO2, the oil solubilized by SFME is separated. Furthermore, due to the reduced free energy for oil detachment by using this SFME, the coated oil was removed from solid surface. Finally, the application for washing oil sands was completed, where above 90% oil was removed. Moreover, the oil is separated upon CO2 and the SFME was reused after treating N2 at 55 °C, implying the sustainable process for oil removal from oily waste.