Switchable hydrophilicity solvents incorporating hydrophilic deep eutectic solvents: A sustainable alternative for oil-based drilling cuttings washing

Oil-based drilling cuttings (OBDC) contaminated by total petroleum hydrocarbons (TPH) causes detrimental environmental consequences. Solvents washing is capable of separating TPH from OBDC efficiently, but their high volatility and toxicity limit the large-scale application. Here a sustainable OBDC washing strategy is developed by using a heterogeneous mixture (SHSs&HDESs) which is consisted of switchable hydrophilicity solvents (SHSs) and hydrophilic deep eutectic solvents (HDESs). N, N-Dimethylcyclohexylamine (DMCHA) and CHEG (choline chloride/ethylene glycol) is respectively selected as the optimum SHSs and HDESs, considering the miscibility with TPH and recovery capacity. The advantage of the sustainable washing strategy originates from the synergistic effect for enhancing forward the TPH removal efficiency, where DMCHA plays the role on TPH extraction and dilution, and CHEG promotes the detachment of TPH from sands surfaces. Based on the Box-Behnken experimental design of response surface method, the TPH removal efficiency reaches a maximum value of 96.02 % at the derived optimal washing conditions, including 32 °C, 19.81 mL DMCHA, and 9.62 mL CHEG. TPH can be separated from DMCHA by CO2-induced hydrophobicity transformation, DMCHA is recovered upon N2/65 °C afterwards, and CHEG is recovered by liquid separation. The recovered SHSs&HDESs heterogeneous mixture shows good reusability in subsequent OBDC washing processes. Our findings offer a sustainable approach for efficiently OBDC washing, which is expected to overcome the inherent defects of solvents washing methods.