Ultra-sensitive CO2 switchable low temperature transition mixture for intensified retrieval of oily waste

[Display omitted] •CO2 trigger the ultra-sensitive switch of Low temperature transition mixture (LTTM)•Time consumed for hydrophobicity transition of LTTM required merely 0.32 min.•Oil content of soil decreased to 0.31% by LTTM washing via tailoring its molar ratio.•Rapid transition capacity of LTTM allowed efficient retrieval of oil and LTTM itself. One of the challenge in the remediation of contaminated soil by solvent washing is the retrieval of total petroleum hydrocarbons (TPH) and solvents. Herein, a CO2 switchable hydrophobic low temperature transition mixture ([DPA][EL]) composed of ethyl lactate (EL) and dipropylamine (DPA) is proposed, accompanied by a proof-of-concept for contaminated soil washing. The path and energy of hydrogen bond interactions in [DPA][EL] were revealed by the quantum chemical calculation using the quantum theory of atoms in molecule. [DPA][EL] achieved a thorough CO2-induced polarity and hydrophobicity transition undergoing an incomplete CO2 absorption that results in the partial formation of carbonate and carbamate. [DPA][EL] was regenerated after the removal of CO2 by heating at 85 °C for 2 min. Thanks to this peculiarity, the time required for hydrophobicity transition was 197 times shorter than that of the typical CO2 switchable hydrophobic solvent, N,N-dimethylcyclohexylamine. The polarity of [DPA][EL], quantified by the normalized polarity energy, increased by 346.69 % after the introduction of CO2 because of the synergistic effect of water. This ultra-sensitive transition behavior intensified the CO2-induced separation of oily compounds, e.g., kerosene and crude oil. The conceptual usage of [DPA][EL] in the contaminated soil washing was demonstrated. The TPH content of contaminated soil decrease from 8.27 % to 0.31 % by [DPA][EL] washing through tailoring the molar ratio of DPA to EL in [DPA][EL], temperature, and [DPA][EL] to contaminated soil ratio (L/S, mL/g). [DPA][EL] whose composition possesses different polarities compensated for the poor solubility of single solvents towards TPH, thereby improving the washing efficiency of contaminated soil. Afterwards, TPH was separated upon triggered by CO2/water in 2 min, and 91.33 % of [DPA][EL] was subsequently regenerated at 85 °C in 2 min.