The potential of monocationic imidazolium-, phosphonium-, and ammonium-based hydrophilic ionic liquids as draw solutes for forward osmosis

Abstract

The widespread implementation of forward osmosis (FO) is highly constrained by the limited availability of suitable draw solutes (DS). Herein, monocationic hydrophilic ionic liquids (ILs) were probed as FO DS. Water (Jv), reverse solute (Js), and specific reverse solute (Js/Jv) fluxes were determined and correlated with IL properties: Van't Hoff factor (i), ionic strength, hydrated ionic radius (rH), diffusivity and membrane affinity. Most of the ILs have comparable Jv with the benchmark draw solute NaCl but their Js were significantly lower, particularly under PRO mode. Their remarkably lower Js/Jv (i.e. <0.010 ± 7.45 × 10−4 mol L−1) than NaCl (0.021 ± 0.003 mol L−1) validates their potential use as FO DS. Tetraethylammonium bromide ([N2222]Br) is the most suitable IL DS due to its high π, high ionic strength, small rH, least membrane permeability (B = 0.14 L m−2 h−1) and lowest Js/Jv = 0.004 ± 5.53 × 10−4 mol L−1. Moreover, [N2222]Br effectively desalinated seawater (0.6 M NaCl). It is thermally stable and can be effectively regenerated through direct contact membrane distillation. The final permeated water had only trace [N2222]Br, which is safe for consumption as confirmed by in vitro toxicity tests. These results demonstrate that certain ILs like [N2222]Br can be identified as suitable draw solutes for FO desalination process.