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

다국어 초록 (Multilingual Abstract)

<P><B>Abstract</B></P> <P>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 (<I>J</I> <SUB> <I>v</I> </SUB>), reverse solute (<I>J</I> <SUB> <I>s</I> </SUB>), and specific reverse solute (<I>J</I> <SUB> <I>s</I> </SUB>/<I>J</I> <SUB> <I>v</I> </SUB>) fluxes were determined and correlated with IL properties: Van't Hoff factor (<I>i</I>), ionic strength, hydrated ionic radius (<I>r</I> <SUB> <I>H</I> </SUB>), diffusivity and membrane affinity. Most of the ILs have comparable <I>J</I> <SUB> <I>v</I> </SUB> with the benchmark draw solute NaCl but their <I>J</I> <SUB> <I>s</I> </SUB> were significantly lower, particularly under PRO mode. Their remarkably lower <I>J</I> <SUB> <I>s</I> </SUB>/<I>J</I> <SUB> <I>v</I> </SUB> (i.e. <0.010 ± 7.45 × 10<SUP>−4</SUP> mol L<SUP>−1</SUP>) than NaCl (0.021 ± 0.003 mol L<SUP>−1</SUP>) validates their potential use as FO DS. Tetraethylammonium bromide ([N2222]Br) is the most suitable IL DS due to its high <I>π</I>, high ionic strength, small <I>r</I> <SUB> <I>H</I> </SUB>, least membrane permeability (<I>B</I> = 0.14 L m<SUP>−2</SUP> h<SUP>−1</SUP>) and lowest <I>J</I> <SUB> <I>s</I> </SUB>/<I>J</I> <SUB> <I>v</I> </SUB> = 0.004 ± 5.53 × 10<SUP>−4</SUP> mol L<SUP>−1</SUP>. 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 <I>in vitro</I> toxicity tests. These results demonstrate that certain ILs like [N2222]Br can be identified as suitable draw solutes for FO desalination process.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Ionic liquids (ILs) were effectively employed as draw solutes for forward osmosis (FO). </LI> <LI> IL dissociation in water, conductivity and molecular size was important for <I>J</I> <SUB> <I>v</I> </SUB>. </LI> <LI> Hydrated radii and membrane sorption properties of ILs were critical to <I>J</I> <SUB> <I>s</I> </SUB>. </LI> <LI> All ILs especially [N2222]Br performed better than NaCl in terms of <I>J</I> <SUB> <I>s</I> </SUB>/<I>J</I> <SUB> <I>v</I> </SUB>. </LI> <LI> [N2222]Br is regenerable, has low toxicity, and has high potential as FO draw solute. </LI> </UL> </P>