Effect of temperature on the osmotic behavior of LCST type ionic liquid solutions as draw solutions in the forward osmosis process

In this work, several types of ionic liquid (IL)-based thermosensitive draw solutions (DSs) were prepared for application in the forward osmosis (FO) process. The temperature dependence of the osmotic pressure of IL-based DS was investigated to examine the advantages of high-temperature membrane operation for regeneration and water recovery from the DS lean phase after liquid–liquid phase separation of DS with a lower critical solution temperature (LCST). From surface tension and dynamic light scattering measurements, it was found that the association of IL molecules and aggregation of IL associates occurred in hot water, which led to a decrease in the osmotic pressure of the IL-DS at elevated temperatures. The DS regeneration and water recovery using an RO membrane were examined using tetrabutylammonium 2,4,6-trimethylbenzenesulfonate with the largest temperature dependence on osmotic pressure. At elevated temperatures, the water flux and DS concentration in the concentrated solution increased effectively. The investigation demonstrated that high-temperature membrane operation is effective for the regeneration of LCST-type thermosensitive DS and water recovery.