Sustainable Treatment\nof Oil-Based Drilling Cuttings\nUsing a Switchable Fatty-Acids-Based Hydrophobic Deep Eutectic Solvent

Oil-based drilling cuttings (OBDCs)\nare typical hazardous\nsolid\nwastes due to their strong mutagenic, carcinogenic, and teratogenic\nproperties. Herein, a pH-switchable hydrophobic deep eutectic solvent\n(HDES) based on branched-chain fatty acids was developed, using 2-ethylhexanoic\nacid (EHA) as the hydrogen bond donor (HBD) and valproic acid (PPA)\nas the hydrogen bond acceptor (HBA), along with a feasible approach\nto the efficient treatment of OBDC. It was found that NaOH/HCl solutions\ntriggered the deprotonation and protonation of the −COOH groups\nin HDES, enabling a reversible transformation of HDES from hydrophobicity\nto hydrophilicity. Interestingly, HDES featured a remarkable antiemulsifying\nproperty attributed to its short-branched-chain structure, which intensified\nthe separation capacity of simulated oils (heptane, kerosene, and\nwhite oil). The pH-induced separation efficiency of the simulated\noils reached 99%. Due to these unique properties, the usage of HDES\nfor cleaning OBDC was demonstrated. The first and fourth oil removal\nrates of HDES respectively reached 98.41 and 87.04% by tailoring the\nPPA to EHA molar ratio of HDES to 1:2. Correspondingly, after being\nalternately triggered by NaOH/HCl solutions, 88.89 and 72.13% of HDES\nwere recovered in the first and fourth cleaning processes. HDES is\nexpected to be a promising alternative for the resource utilization\nof OBDC due to its lower volatility and toxicity compared with traditional\nsolvents. We postulate that the proposed HDES has great promise for\nthe reduction, resource utilization, and harmless treatment of oily\nsolid waste.