Supramolecular host-guest complex of methylated β-cyclodextrin with polymerized ionic liquid ([vbim]TFSI)n as highly effective and energy-efficient thermo-regenerable draw solutes in forward osmosis

Abstract

Supramolecular inclusion complexes with lower critical solution temperature (LCST) properties were investigated for the first time as forward osmosis (FO) draw solutes. Randomly methylated-β-cyclodextrin (Rm-β-CD) host molecules accommodate polymerized ionic liquids (([vbim]TFSI)n PILs) through their hydrophobic TFSI− anions as guests. LCST properties were tuned by varying the chain lengths of ([vbim]TFSI)n, from which, short-chain oligo([vbim]TFSI) was found most suitable. Draw solutions (DS) of Rm-β-CD/oligo([vbim]TFSI) complex have highly tunable cloud-point temperatures (Tc), fast LCST kinetics and sufficient osmotic properties for an efficient FO. Under PRO mode, 0.5 M Rm-β-CD/0.078 M oligo([vbim]TFSI) induced competitive FO water flux (Jv ~13.73 L m−2h−1) and negligible reverse solute flux (Js ~4.41 × 10−3 mol m−2h−1) against DI water feed. It successfully processed different saline feeds (0.034 M and 0.6 M NaCl) with reasonable FO performance and superior Js/JV ~0.001 mol m−2h−1, demonstrating its competence for FO desalination. When heated slightly above its Tc = 29 °C (TTP = 30 °C), thermal precipitation (TP) is ensued with the release of TFSI− anions in oligo([vbim]TFSI) from Rm-β-CD. Due to its hydrophobicity, oligo([vbim]TFSI) precipitates while entrapping the suspended Rm-β-CDs between its chains causing flocculation and sedimentation. Thus, with only +5 °C heating above FO temperature (25 °C), 95% of draw solutes are effectively recovered from the spent DS after settling. Residual (~5%) Rm-β-CD in the DS supernatant is subsequently removed via nanofiltration at 99.33% rejection, producing non-toxic water effluent based on in vitro cytotoxicity results. Energy consumption estimates reveal the feasibility of Rm-β-CD/oligo([vbim]TFSI) as it requires minimal heat energy for recovery. This study offers new insights on the potential of host-guest complexes as a new class of energy-efficient draw solutes for FO desalination technology.