Anion Effect on Forward Osmosis Performance of Tetrabutylphosphonium-Based Draw Solute Having a Lower Critical Solution Temperature

The applicability of ionic liquids (ILs) as the draw solute in a forward osmosis (FO) system was investigated through a study on the effect of the structural change of the anion on the FO performance. This study evaluated ILs composed of tetrabutylphosphonium cation ([P(4444)](+)) and benzenesulfonate anion ([BS](−)), para-position alkyl-substituted benzenesulfonate anions (p-methylbenzenesulfonate ([MBS](−)) and p-ethylbenzenesulfonate ([EBS(−)]), and methanesulfonate anion ([MS](−)). The analysis of the thermo-responsive properties suggested that the [P(4444)][MBS] and [P(4444)][EBS] ILs have lower critical solution temperatures (LCSTs), which play a beneficial role in terms of the reusability of the draw solute from the diluted draw solutions after the water permeation process. At 20 wt% of an aqueous solution, the LCSTs of [P(4444)][MBS] and [P(4444)][EBS] were approximately 36 °C and 25 °C, respectively. The water flux and reverse solute flux of the [P(4444)][MBS] aqueous solution with higher osmolality than [P(4444)][EBS] were 7.36 LMH and 5.89 gMH in the active-layer facing the draw solution (AL-DS) mode at osmotic pressure of 25 atm (20 wt% solution), respectively. These results indicate that the [P(4444)](+)-based ionic structured materials with LCST are practically advantageous for application as draw solutes.