Separation and concentration of ionic liquid aqueous solution by vacuum membrane distillation

• VMD was proposed to concentrate ILs aqueous solution with high concentration.
• Homemade PAN-based membrane could be used for concentrating ILs by VMD.
• Membrane fouling and wetting mechanism was discussed.
• Timely cleaning could effectively prevent membrane from wetting.
• A 65.5% of [Bmim]Cl solution was obtained with a total recovery of over 99.5%.

Due to the high cost of ionic liquids (ILs), how to economically recycle them from aqueous solutions is one of the critical issues for their large-scale applications. In this work, vacuum membrane distillation (VMD) technique was proposed to separate and concentrate the 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) aqueous solutions at high concentration (e.g. ≥20 wt%), where a polyacrylonitrile-based (PAN-based) hydrophobic membrane was prepared and used. The chemical resistance and VMD separation performance of the PAN-based membrane were estimated. The results showed that the homemade membrane possessed good chemical resistance to several kinds of media and favorable VMD flux and rejection. Four different groups of VMD concentration processes with intermittent operation mode were then carried out to separately concentrate a 20 wt% [Bmim]Cl solution. The concentration process, membrane fouling and wetting, as well as cleaning strategy were compared and discussed. It was found that for the hydrophobic PAN-based membrane, surface hydrophilization resulted from surface chemistry rearrangement happened especially at high feed temperature, which might aggravate the adsorption of [Bmim]Cl on membrane surface. Membrane top layer wetting would occur due to the synergistic effect of surface hydrophilization and deposition of [Bmim]Cl, which not only induced the reductions of the membrane flux and rejection, but also further caused internal pore wetting. Therefore, the fouling mechanism during the VMD concentration might be divided into three stages, surface deposition, top layer wetting and internal pore wetting. Timely cleaning (i.e. before surface wetting) could effectively prevent the fouling deterioration (i.e. from surface deposition to surface wetting) and improve the VMD concentration process, resulting in a high flux recovery over 95%. Under the optimal conditions, a final [Bmim]Cl concentration as high as 65.5 wt% with a total [Bmim]Cl recovery over 99.5% was achieved, indicating that VMD was a promising candidate for concentrating ILs aqueous solutions.

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