(376l) Thermo-Responsive Ionic Liquids with LCST-Type Phase Transition Property As Draw Solutes in Forward Osmosis for Seawater Desalination

Desalination technologies have attracted significant attention owing to their immense potential to solve water crisis. Forward osmosis (FO) has emerged as one of the most promising technology in desalination. FO is an innovative membrane separation technology that relies solely on concentration gradient differences between two solutions separated by a semi-permeable membrane. However, the full implementation of FO remains highly constrained by the lack of ideal draw solutes imparting substantial water permeation, low reverse solute leakage, and convenient recyclability for long-term use.

Ionic liquids (ILs) are coined as green substances due to their low volatility and high thermal stability. In addition, ILs have high ionicity, moderate to high molecular weight, and moderate diffusivity, potentially permitting them to be suitable drawing agents in FO system. ILs having suitable hydrophobicity can undergo low critical solution temperature (LCST)-type phase transition in water. Herein, ILs exhibiting LCST-type phase transition in IL/water mixtures were synthesized. The synthesized ILs were thoroughly characterized by FT-NMR, FTIR, and TGA. The LCST property of the ILs was tested by UV-Vis absorption spectrum. Osmotic pressure, ionic strength, and diffusion coefficient measurement were conducted and later correlated with the ILs FO performance. Moreover, the capability of LCST-type ILs as osmotic pressure generators were compared with NaCl as a control in FO system using commercially available HTI-CTA membranes. The performance of the LCST-type ILs was evaluated in terms of water flux (J_v), reverse solute flux (J_s) and specific solute selectivity (J_s/J_v) during FO runs against deionized (DI) water and simulated sea water (0.6M) in both pressure retarded osmosis (PRO) and FO modes. The effect of membrane orientation, concentration of draws solutes and the respective FO performances were thoroughly investigated.

The investigated ILs generated considerable water flux with negligible solute leakage. This was attributed to their ionic strength, low diffusivity, and low affinity towards the FO membrane. All draw solutes had performed better in PRO mode attributed to lower concentration polarization effects relative to the FO mode. Over all, results demonstrated that the performance of LCST-type ILs is superior over the extensively studied NaCl attributed to their low specific solute selectivity during FO runs. These ILs can undergo phase transition upon mild heating leading to an IL-rich phase that can be recycled and re-used. The ability of these LCST-type ILs to phase separate from water would be beneficial for its convenient recovery and recyclability for long-term use in FO desalination technology.

This research was supported by the National Research Foundation of Korea (NRF) under the Ministry of Science and ICT (No. 2016R1A2B1009221) and the Ministry of Education (No. 22A20130012051(BK21Plus)).