Dual-responsive imidazolium-based ionic liquid with different alkyl chains as draw solutes for forward osmosis

Abstract

Imidazolium-based magnetic ionic liquids (MILs) with dual responsiveness, a series of 1-alkyl-3-methylimidazolium tetrachloroferrates (Cn-FeCl4, n = 4, 8, and 12, where n denotes the number of carbon atoms in the alkyl group of 1-alkyl-3-methylimidazolium), were synthesized to assess their ability as smart draw solutes for forward osmosis (FO). The recovery properties and FO performance of Cn-FeCl4 were investigated extensively. This series of Cn-FeCl4 exhibits lower critical solution temperature (LCST) characteristics and paramagnetic behavior in the magnetization curve as a function of the magnetic field, which minimizes the energy consumption for recovering draw solutes from spent draw solutions. The LCSTs of the C4-FeCl4, C8-FeCl4, and C12-FeCl4 draw solutions at a concentration of 20 wt % were approximately 76, 43, and 55 °C, respectively. The respective calculated magnetic susceptibilities of MILs were 5.92 × 10−5, 4.53 × 10−5, and 4.12 × 10−5 emu cm−3 at 300 K. Moreover, when the concentration of the draw solution was 20 wt %, the water flux of C8-FeCl4 was approximately 17.30 LMH and 10.04 LMH in active layer facing draw solution and active layer facing feed solution modes, respectively. Subsequently, in desalination applications, C8-FeCl4 exhibited feasibility in saline water with 2000 and 30,000 ppm NaCl. Therefore, for the first time, we studied the effect of the structural transformation of MILs on draw solute properties and examined the applicability of magnetic and thermal dual-responsive MILs as draw solutes for FO processes.