Sustainable Treatment of Oil-Based Drilling Cuttings Using a Switchable Fatty-Acids-Based Hydrophobic Deep Eutectic Solvent

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