Concentration and Recovery of Ionic Liquids from Aqueous Solutions By Nanofiltration

Ionic liquids (ILs) have been considered as a class of promising solvents for dissolution of natural polymer such as cellulose, silk, and wool chitin as well as raw biomass like lignocellulose and crustacean shells due to their  high dissolving capacity of biopolymer. Given the fact that ILs are much more expensive than conventional solvents, it is of significance to recycle ILs during the dissolution of natural polymer and raw biomass.

In this work, the filtration behavior of three ILs ([AMIM]Cl, [BMIM]Cl and [BMIM]BF₄) in aqueous solutions by two nanofiltration (NF) membranes (NF90 and NF270) were investigated. The experimental results showed that the permeate flux increased linearly with the pressured applied at a constant IL concentration and decreased with the IL concentration at a constant pressure. Compared to NF270 membrane, NF90 showed the higher rejection because of its smaller pore size. For both membranes examined, the retention of IL was in the order of [BMIM]Cl > [BMIM]BF₄ > [AMIM]Cl, and was inversely proportion to the IL diffusion coefficients in water. The experimental results indicated that the organic cation in IL had an important effect on the IL rejection because of its larger size compared to anion. The permeate fluxes of ILs were calculated using the solution-diffusion transport model coupled with film theory. The results showed that this approach was able to calculate the permeate fluxes of ILs for IL/H₂O mixtures studied. Under the optimal experimental conditions determined in the present work, [BMIM]Cl in the aqueous solution could be concentrated to 18.85 wt% from its initial concentration of 5 wt% using the NF90 membrane, and a recovery of around 96% was obtained. The outcome of the present work will provide necessary guide for the application of concentrating ILs in aqueous solutions by nanofiltration.