Exploring the performance-affecting factors of monocationic and dicationic phosphonium-based thermoresponsive ionic liquid draw solutes in forward osmosis

Because of easy recovery, thermoresponsive phosphonium-based ionic liquids (ILs) can be used as a potential draw solute in forward osmosis (FO). Its phase separation behaviors and water-drawing potential are two main factors affecting FO performance. In this study, these two factors were investigated for the lower critical solution temperature (LCST) type mono-cationic and di-cationic phosphonium-based IL draw solutes with different anions, including p-toluenesulfonate (TSO), hydrogen maleate (Mal), and trimethylbenzenesulfonate (TMBS). In phase separation behaviors, the structure chemistry of cation/anion can affect the LCST cloud temperature point (Tcloud). High hydrophilicity of the anion leads to high LCST Tcloud of phosphonium-based IL aqueous solution. For the same anion, the LCST Tcloud of di-cationic phosphonium-based IL is lower than that of mono-cationic phosphonium-based IL. In water-drawing potential, it is related to the osmolality and the viscosity of IL aqueous solution. Mono-cationic phosphonium-based IL aqueous solution with high osmolality and low viscosity has high water-drawing potential and water flux in FO test. After FO process, diluted IL draw solution is phase separated into IL-rich layer and water-rich layer above LCST. Phosphonium-based IL with low osmolality can lead to low residual IL content in water-rich layer.