Effect of the Structural Changes in a Styrenesulfonate-Based Draw Solute Having a Lower Critical Solution Temperature for the Forward Osmosis Process

Abstract

Thermo-responsive ionic liquids, tributylalkylphosphonium styrenesulfonate ([P444#][SS], # = 4, 6, and 8) and tetrabutylammonium styrenesulfonate ([N4444][SS]), were synthesized to investigate the possibility of using a draw solute in forward osmosis, where # is the number of carbons of the alkyl groups in tributylalkylphosphonium. ILs in an aqueous solution exhibited a lower critical solution temperature (LCST)-type phase transition, which enables the recovery of the draw solute or water from the diluted draw solution. LCSTs of 50 wt % aqueous [P4444][SS] and [N4444][SS] solutions were approximately 33 and 66 °C, respectively. The water and reverse solute fluxes of the [P4444][SS] aqueous solution, which is more efficient in terms of solute recovery, were approximately 8.51 LMH and 1.66 gMH at 50 wt %, respectively, in the active layer facing the draw solution mode. This study demonstrates that an ionic structured material with thermo-responsive properties is a promising draw solute.