pH-Responsive Natural Deep Eutectic Solvent: An Environmental Alternative for the Sustainable Extraction of Petroleum Hydrocarbons from Oil Sands

The reversible shift in polarity or hydrophilicity/hydrophobicity of switchable solvents greatly simplifies the recovery capacity in extraction applications. However, the environmental and economical advantages of switchable solvents are not significant. In this work, we designed three pH-responsive natural deep eutectic solvents (NADESs) by combining the pH-switchable solvent fatty acids with the nonswitchable solvent ethyl lactate (EL), followed by the exploration of the solubilization and separation performance of these NADESs for petroleum hydrocarbons. EL can be miscible in fatty acids and water; however, when in contact with both at the same time, EL binds to fatty acids through stronger intermolecular hydrogen bonds, whereas when fatty acids are deprotonated to fatty acid salts, EL can bind to water. The deprotonation/protonation of fatty acids could reversibly change the NADES hydrophilicity, and the recovery of NADES HA/EL could exceed 95% after three cycles. Furthermore, after extractive separation of simulated oils of differing complexity, NADES HA/EL was selected as the best extractant. Compared with the extraction of oil sands with a single solvent, NADES provides better wetting of the sand surface, better stripping efficiency of the heavy components that adhere to the surface of oil sands, and better dispersion of the stripped petroleum hydrocarbons. Petroleum hydrocarbons can be separated by NaOH-induced hydrophilic changes in NADES, which can be regenerated upon the addition of HCl. The recovered NADES showed good reusability in the cleaning of oil sands. The oil removal rates were 96.9%, 94.4%, and 91.9% after three cycles of cleaning with NADES at 25 °C. This method is expected to expand the application of nonswitchable solvents in sustainable extraction.