Concentration of ionic liquids by nanofiltration for recycling: Filtration behavior and modeling

• The order of rejection followed [BMIM]Cl > [BMIM]BF4 > [AMIM]Cl.
• The organic cation in IL had an important effect on the IL rejection.
• The solution-diffusion transport model can be used to predict the fluxes of ILs.
• [BMIM]Cl in aqueous can be concentrated to 18.85 wt% using NF90 membrane.
• NF is promising for concentrating IL.

The study of filtration behavior in the concentration of ionic liquids (ILs) using nanofiltration (NF) membranes is of significance to recycle IL from the solution obtained by the dissolution of natural polymer and raw biomass. Firstly, the transmembrane pressure for the binary 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) - H2O, 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) - H2O and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) - H2O systems over the flux range from 6.64 to 132.74 L m−2 h−1 was examined using the dead-end filtration cell. Then, the solution-diffusion transport model was used to calculate the permeate flux of IL on the basis of radii of anion and cation in IL taken from literature. The real IL retention with NF270 was lower than that with NF90 which attained around 96% for [BMIM]Cl IL at a permeate flux of 53.10 L m−2 h−1. For the two membranes, the retention of [BMIM]Cl was higher than that of [BMIM]BF4 even if the radius of [BMIM]BF4 was slightly larger. Finally, under the optimal experimental parameters, [BMIM]Cl in the binary system of [BMIM]Cl-H2O can be concentrated to approximately 20 weight percent (wt%). All of these will provide necessary fundamental data for the application of NF technology to concentrate IL in the IL-H2O system.