CO ₂ Switchable Solvent System Comprising Hydrophilic Deep Eutectic Solvent and <i>N</i> -Ethylbutylamine for Enhanced Treatment of Oily Solid Waste

Solvent extraction that relies on the dissolution-based “like dissolves like” principle proves inadequate for achieving efficient oil-solid separation in oily solid waste, as oily components are inclined to be tightly adsorbed to solid surfaces. Enhancing the detachment of oily components from solid surfaces is critical for improving the cleaning efficiency. Herein, we propose a sustainable strategy for enhanced cleaning of oily solid waste using a CO2 switchable solvent system (DES/EBA) comprising deep eutectic solvent (DES) and N-ethylbutylamine (EBA). In this DES/EBA system, DES weakens the oil-solid adhesion, while EBA plays the role of extracting oily components. The system changes from low to high polarity (π*, 0.27–0.67) upon CO2 introduction, facilitating a biphasic-to-monophasic transition and achieving a >99% recovery of oily components. The residual oil rate in oily solid waste remained below 2.00% after four cleaning cycles, with both DES and EBA recovery rates exceeding 90%. This CO2 DES/EBA system offers a promising approach for sustainable oily solid waste treatment.