Tetrabutylammonium 2,4,6-trimethylbenzenesulfonate as an effective and regenerable thermo-responsive ionic liquid drawing agent in forward osmosis for seawater desalination

Abstract

Efficient drawing agents are essential in forward osmosis (FO) for clean water production. Monomeric thermo-responsive ionic liquid (IL) [N4444]2,4,6-MeBnSO3 was thoroughly investigated as a drawing agent in FO. The IL can be safely employed due to its thermal stability and low cytotoxicity. It has a van't Hoff factor i = 1.21, with sufficient ionic strength to generate osmotic pressure ~ 58.92 bars (2 M). FO operations especially under PRO mode demonstrate that 2 M [N4444]2,4,6-MeBnSO3 can induce competitive water flux Jv ~ 12.3 L m−2 h−1 with remarkably low reverse solute flux Js < 0.006 mol m−2 h−1 and specific reverse solute flux Js/Jv ~ 4.5 × 10−4 mol L−1. Using 0.6 M NaCl as feed demonstrates its consistency in desalinating seawater (Jv ~ 3.72 L m−2 h−1, Js ~ 0.004 mol m−2 h−1, and Js/Jv ~ 0.91 × 10−3 mol L−1). After FO, [N4444]2,4,6-MeBnSO3 can be effectively retrieved (~98%) through thermal precipitation at 60 °C, above its cloud point temperature (57 °C). Meanwhile, >99% of the remaining 2% can be recovered through reverse osmosis or membrane distillation to produce water effluents with non-toxic IL concentrations (≪100 mg L−1). Results indicate that thermo-responsive [N4444]2,4,6-MeBnSO3 is a promising alternative reusable drawing agent in FO process.