A sensitive CO2/NaOH switchable hydrophilic solvent for sustainable treatment of contaminated sands: Analysis of extraction concept and mechanism

One of the challenges of CO 2 switchable hydrophilic solvents in the sustainable recovery of total petroleum hydrocarbons (TPH) from contaminated sands was the long recovery cycle. Herein, a CO 2 /NaOH switchable hydrophilic solvent, thymol was developed while specific reaction modes were investigated by quantum chemical calculations. The hydrophilic transition of thymol was triggered by NaOH solution at 25 ℃ and 1 atm, followed by regeneration through CO 2 bubbling. The time required for regeneration of thymol was reduced by 81.20 % compared to the conventional CO 2 switchable hydrophilic solvent N, N-dimethylcyclohexylamine, because of the high CO 2 reactivity of thymol revealed by the analysis of LUMO-HOMO and electrostatic potential. Thus, the concept of sustainable treatment of contaminated sands using thymol was proposed, achieving a TPH recovery of 87.63 %. We found that the solution of hydrophilic deep eutectic solvents (DESs/H 2 O) attributes to enhance the TPH recovery of thymol. Such synergy mechanism of DESs/H 2 O and thymol was revealed by molecular dynamics simulations. The optimal TPH recovery efficiency was 94.52 % after optimization by single variable experiment, Plackett-Burman design, the path of steepest ascent method and response surface methodology. The reusability of thymol and DES/H 2 O achieved a TPH recovery of 91.26 % after three cycles.