Recyclable dynamic imine-based nonionic surfactants and potential use in postprocessing of eluted emulsions

Recovery of surfactants and retrieval of hydrocarbons from eluted emulsions in surfactant-enhanced washing are essential to save useful resources and avoid secondary contamination. A novel nonionic surfactant of C6-DIB-PEG has been constructed by using a dynamic imine bond (DIB) as the linker between hexyl amine (C6-NH2) and an aromatic aldehyde modified polyethylene glycol (PEG-CHO). The results presented in this study reveal that an oil-in-water emulsion of tetradecane-H2O stabilized by C6-DIB-PEG is pH-switched. Upon reaction with HCl, C6-DIB-PEG readily converts to its inactive forms C6-NH3+Cl- and PEG-CHO when the pH is changed from 12.0 to 4.0, resulting in a complete tetradecane-water two-phase separation. Both C6-NH3+Cl- and PEG-CHO are insoluble in tetradecane but soluble in water, allowing subsequent retrieval of nearly surfactant-free tetradecane. Recovery of C6-DIB-PEG was enabled by treating aqueous solutions of C6-NH3+Cl- and PEG-CHO with NaOH after pH was increased from 4.0 to 12.0. The pH-switched C6-DIB-PEG renders a potential use for oil retrieval and surfactant recovery in the postprocessing of eluted emulsions, as demonstrated by the C6-DIB-PEG-enhanced washing of tetradecane-contaminated sand. Around 95.2% of the tetradecane and about 99.7% of the C6-DIB-PEG were retrieved and recycled over 10 cycles of washing. In addition, the total organic carbon (TOC) of the residual wastewater was remarkably reduced from around 7065.3 to similar to 54.1 mg L-1 after NaHSO3 treatment followed by ion-exchange resin and active carbon adsorption.