Effects of the Structure of Benzenesulfonate-Based Draw Solutes on the Forward Osmosis Process

A series of phosphonium-based ionic liquids (ILs) based on benzenesulfonate derivatives (tetrabutylphosphonium benzenesulfonate ([TBP][BS]), tetrabutylphosphonium 4-methylbenzenesulfonate ([TBP][MBS]), tetrabutylphosphonium 2,4-dimethylbenzenesulfonate ([TBP][DMBS]), and tetrabutylphosphonium 2,4,6-trimethylbenzenesulfonate ([TBP][TMBS])) were synthesized via anion exchange with tetrabutylphosphonium bromide ([TBP][Br]). Then, we characterized the ILs and investigated their suitability as draw solutes for forward osmosis (FO), focusing on their thermoresponsive properties, conductivities, and osmotic pressures. We found that aqueous [TBP][BS] was not thermoresponsive, but 20 wt% aqueous [TBP][MBS], [TBP][DMBS], and [TBP][TMBS] had lower critical solution temperatures (LCSTs) of approximately 41, 25, and 21 °C, respectively, enabling their easy recovery using waste heat. Based on these findings, 20 wt% aqueous [TBP][DMBS] was tested for its FO performance, and the water and reverse solute fluxes were found to be approximately 9.29 LMH and 1.37 gMH, respectively, in the active layer facing the draw solution (AL-DS) mode and 4.64 LMH and 0.37 gMH, respectively, in the active layer facing the feed solution (AL-FS) mode. Thus, these tetrabutylphosphonium benzenesulfonate-based LCST-type ILs are suitable for drawing solutes for FO process.


Introduction
The scarcity of clean water and water pollution are two serious global environmental problems that have emerged in recent decades as a result of continued industrial growth and agricultural activities [1][2][3]. To solve these environmental issues, seawater desalination and wastewater recycling are considered good solutions [4,5].
Forward osmosis (FO) is a desalination membrane technology that drives water flow from the feed to permeate the side of a membrane across an osmotic pressure gradient. To achieve the permeation of water, a membrane is placed between the feed and draw solutions, and this creates a high osmotic pressure (higher concentration compared to the feed solution). The FO process has received much attention owing to its advantages, including high water recovery, high energy efficiency, low susceptibility to membrane contamination, and simple operation [6][7][8]. FO is generally an energy-efficient process; however, more research into how to regenerate the draw solute from the diluted draw solution for reuse and to produce purified water is required. In particular, achieving a balance between high osmotic pressure and easy regeneration is a goal but is difficult to achieve in practice [9].
There are two main types of draw solutions: nonresponsive and responsive draw solutes. Nonresponsive draw solutes do not respond to external stimuli, such as pH changes, light, temperature, or electromagnetic fields, and show no significant change in their water affinity after stimulation. To date, inorganic salts, such as NaCl, (NH 4 ) 2 SO 4 , ILs are soluble in water because of the hydrogen-bonding interactions between IL/water molecules. On heating above the T cp , the hydrogen bonds break, and hydrophobic interactions become dominant, making the ILs insoluble and the solution turbid. The advantage of thermoresponsive IL draw solutes, especially the LCST type, is that the hydrated thermoresponsive IL draw solute in the diluted draw solution can be separated from water by inducing a mild temperature change, and most of the draw solute or water can be easily recovered through liquid-liquid phase separation [50,51]. Therefore, FO processes using thermoresponsive draw solutes have lower regeneration energy requirements than other draw solutes because the thermal energy can be obtained from geothermal or low-grade industrial waste heat [52][53][54][55].
In this study, we synthesized several types of tetrabutylphosphonium-based ILs having different anions, including benzenesulfonate (BS), 4-methylbenzenesulfonate (MBS), 2,4dimethylbenzenesulfonate (DMBS), and 2,4,6-trimethylbenzenesulfonate (TMBS). Further, we conducted systematic analyses of their thermoresponsive phase-separation behavior and osmolality generation characteristics. We also investigated the potential of an aqueous solution of the tetrabutylphosphonium benzenesulfonate-based IL as the draw solute for the FO process.

Synthesis of LCST-Type ILs
Four thermally responsive draw solutes were investigated in this study. [TBP][BS] was prepared by dissolving [TBP][Br] (4.00 g, 10 mmol) and sodium benzenesulfonate ([Na][BS]) (3.60 g, 20 mmol) in DI water (28 mL) in a flask. After dissolution, the mixture was stirred at room temperature for 24 h. The product was extracted with dichloromethane (80 mL, three times), washed three times with DI water (40 mL), and dried over anhydrous magnesium sulfate. The solvent was removed using a rotary evaporator to yield a product. 1

Characterization
1 H−NMR spectroscopy (MR400 DD2 NMR spectrometer, Agilent Technologies, Inc. (Santa Clara, CA, USA), Fourier transform infrared (FT−IR) spectrometry (NICOLET iS20, Thermo Fisher Scientific Inc., Waltham, MA, USA), and a high−resolution mass spectrometer (HRMS, maXis HD, Bruker Corp., Billerica, MA, USA) were used to confirm the structures of the synthesized ILs. After preparing each draw solution, the conductivity (Mettler Toledo Seven2Go Pro, Zurich, Switzerland) was measured, and the osmotic pressure was determined by measuring the freezing point of the sample using an osmometer (K-7400, Knauer Co., Berlin, Germany). The LCST was determined by measuring the transmittance of aqueous solutions at 650 nm using an ultraviolet-visible (UV-Vis) spectrophotometer (EMC-11D-V, EMCLAB Instruments GmbH Co., Duisburg, Germany). The water flux was determined by comparing the height difference of the draw solution in the tube during FO operation. The reverse solute flux was measured using a conductivity meter by comparing the difference in conductivity before and after the FO test.

FO Tests
The flux was measured using a laboratory-scale FO system connected to a glass tube (L-shaped) [56]. A thin-film porous FO membrane (Hydration Technologies Inc. (Albany, OR, USA), HTI-TFC) was placed between the two glass tubes. One L-shaped tube was filled with DI water, whereas the other contained the IL solution as the draw solution. During the tests, stirring was continued with a magnetic stir bar and the temperature was maintained at room temperature. The water permeation flux was calculated from the volume of the draw solution before and after the FO test, as shown in Equation (1).
here, J V is the water permeation flux in the FO process (L m −2 h −1 or LMH), ∆V (L) is the volume change of the draw solution over time ∆t (h), and A (m 2 ) is the surface area of the FO membrane, which was calculated to be 3325 × 10 −4 m 2 .
The reverse solute flux (J s ) was determined from the amount of draw solute that diffused into the feed solution and was calculated from the TOC value of the feed solution. The reverse solute flux in the FO process (g m −2 h −1 or gMH) was calculated from the difference in the conductivity of the feed solution before and after the FO test using Equation (2).
here, ∆C (mol L −1 ) is the concentration of change in the feed solution after time ∆t, and ∆V (L) is the volume change after time ∆t. The reverse solute flux in the FO process (g m −2 h −1 or gMH) was calculated from the di ference in the conductivity of the feed solution before and after the FO test using Equation (2).

Synthesis and Characterization of LCST-Type ILs
here, ΔC (mol L −1 ) is the concentration of change in the feed solution after time Δt, and Δ (L) is the volume change after time Δt.    . Therefore, this region can be used to discuss changes in the cations. The absorption at 1600−1500 cm −1 can be assigned to the stretching of the C=C bond in the benzene ring, but the coarse spectrum makes it difficult to track changes in this region [57]. On the other hand, the sulfonate group appeared as bands generated from the asymmetric stretching vibrations of SO3 -at 1350−1200 cm −1 . However, the peaks located at 1100-1010 cm −1 correspond to the symmetric stretching of SO3 − , which makes it possible to follow changes in the anions. The FT−IR peaks of the measured sulfonate groups were observed at wavenumbers similar to those previously reported [58,59]. Consequently, the bands corresponding to the C-H and S=O groups were useful for the interpretation of the IL structures. The molecular weight of ILs was measured by HRMS, data showed in Figure 4, the actual measured molecular weight of both the cation and anion portion of the ILs was found almost consistent with the calculated value, respectively.  . Therefore, this region can be used to discuss changes in the cations. The absorption at 1600-1500 cm −1 can be assigned to the stretching of the C=C bond in the benzene ring, but the coarse spectrum makes it difficult to track changes in this region [57]. On the other hand, the sulfonate group appeared as bands generated from the asymmetric stretching vibrations of SO 3 − at 1350-1200 cm −1 . However, the peaks located at 1100-1010 cm −1 correspond to the symmetric stretching of SO 3 − , which makes it possible to follow changes in the anions. The FT−IR peaks of the measured sulfonate groups were observed at wavenumbers similar to those previously reported [58,59]. Consequently, the bands corresponding to the C-H and S=O groups were useful for the interpretation of the IL structures. The molecular weight of ILs was measured by HRMS, data showed in Figure 4, the actual measured molecular weight of both the cation and anion portion of the ILs was found almost consistent with the calculated value, respectively. . Therefore, this region can be used to discuss changes in the cations. The absorption at 1600−1500 cm −1 can be assigned to the stretching of the C=C bond in the benzene ring, but the coarse spectrum makes it difficult to track changes in this region [57]. On the other hand, the sulfonate group appeared as bands generated from the asymmetric stretching vibrations of SO3 -at 1350−1200 cm −1 . However, the peaks located at 1100-1010 cm −1 correspond to the symmetric stretching of SO3 − , which makes it possible to follow changes in the anions. The FT−IR peaks of the measured sulfonate groups were observed at wavenumbers similar to those previously reported [58,59]. Consequently, the bands corresponding to the C-H and S=O groups were useful for the interpretation of the IL structures. The molecular weight of ILs was measured by HRMS, data showed in Figure 4, the actual measured molecular weight of both the cation and anion portion of the ILs was found almost consistent with the calculated value, respectively.

Conductivity
Conductivity is an indicator of the number of solute ions in the draw solution, as well as the degree of ion mobility. Furthermore, conductivity is affected by the degree of dissociation of ions, which is related to the osmotic pressure. In general, more conductive draw solute ions result in higher osmotic pressures [60]. were approximately 5460, 4508, 4055, and 2406 μS cm −1 , respectively, at 10 wt% and approximately 7037, 5436, 4452, and 3143 μS cm −1 , respectively, at 20 wt%. Thus, the conductivity of the ILs is proportional to their concentration, as reported previously [61][62][63]. This result indicates that the ILs dissociate well in water, resulting in a high ion concentration. [BS] as the draw solution was better than those of the other ILs, as discussed later. This seems to be due to the structural differences in the substituent groups of each IL. Interestingly, the conductivities of the aqueous IL solutions at all solution concentrations decreased with an increase in the number of methyl substituents on the benzene ring. The main factors affecting the conductivity of ILs are ion mobility and the volume of the functional groups [64,65]. In particular, increasing the number of methyl groups in the benzene ring increases ion aggregation and decreases ion mobility, leading to decreased ionic conductivity [66]. The decrease in conductivity can also be explained by the fact that, when a methyl group is attached to the benzene ring, the torsion angle between the conjugated rings increases, which increases the steric strain as the distance between the IL units increases [67,68]. Therefore, as the number of substituents in the aromatic ring increases, the ion mobility decreases and the torsion angle increases compared to the single substituted derivative, resulting in a decrease in the electrical conductivity.

Conductivity
Conductivity is an indicator of the number of solute ions in the draw solution, as well as the degree of ion mobility. Furthermore, conductivity is affected by the degree of dissociation of ions, which is related to the osmotic pressure. In general, more conductive draw solute ions result in higher osmotic pressures [60]. were approximately 5460, 4508, 4055, and 2406 µS cm −1 , respectively, at 10 wt% and approximately 7037, 5436, 4452, and 3143 µS cm −1 , respectively, at 20 wt%. Thus, the conductivity of the ILs is proportional to their concentration, as reported previously [61][62][63]. This result indicates that the ILs dissociate well in water, resulting in a high ion concentration. [BS] as the draw solution was better than those of the other ILs, as discussed later. This seems to be due to the structural differences in the substituent groups of each IL. Interestingly, the conductivities of the aqueous IL solutions at all solution concentrations decreased with an increase in the number of methyl substituents on the benzene ring. The main factors affecting the conductivity of ILs are ion mobility and the volume of the functional groups [64,65]. In particular, increasing the number of methyl groups in the benzene ring increases ion aggregation and decreases ion mobility, leading to decreased ionic conductivity [66]. The decrease in conductivity can also be explained by the fact that, when a methyl group is attached to the benzene ring, the torsion angle between the conjugated rings increases, which increases the steric strain as the distance between the IL units increases [67,68]. Therefore, as the number of substituents in the aromatic ring increases, the ion mobility decreases and the torsion angle increases compared to the single substituted derivative, resulting in a decrease in the electrical conductivity.

Osmotic Pressure
In the FO process, osmosis drives the spontaneous diffusion of water from a feed solution having a lower osmotic potential to a draw solution having a higher osmotic potential. Therefore, the draw solute must have high diffusion in aqueous solution to achieve the active diffusion of water molecules from the influent solution towards the draw solution. The osmotic pressure is a function of solution concentration and is an indicator of FO performance and can be described by the Van 't Hoff equation (Equation (3)).

Π = CiRT
here, Π is the osmotic pressure, Ci is the molar concentration of solute i in the dilute solution, R is the gas constant, and T is the absolute temperature. The osmotic pressure was measured using the freezing point depression method to study the possible applications of the ILs as draw solutes because the difference in osmotic pressure between the draw and feed solutions is the driving force behind the FO process. The osmotic pressure of the ILs as a function of their concentration in water is shown in Figure 6.

Osmotic Pressure
In the FO process, osmosis drives the spontaneous diffusion of water from a feed solution having a lower osmotic potential to a draw solution having a higher osmotic potential. Therefore, the draw solute must have high diffusion in aqueous solution to achieve the active diffusion of water molecules from the influent solution towards the draw solution. The osmotic pressure is a function of solution concentration and is an indicator of FO performance and can be described by the Van 't Hoff equation (Equation (3)).
here, Π is the osmotic pressure, C i is the molar concentration of solute i in the dilute solution, R is the gas constant, and T is the absolute temperature. The osmotic pressure was measured using the freezing point depression method to study the possible applications of the ILs as draw solutes because the difference in osmotic pressure between the draw and feed solutions is the driving force behind the FO process. The osmotic pressure of the ILs as a function of their concentration in water is shown in Figure 6.

Thermoresponsive Behavior
To enable the reuse of the draw solution, the diluted solution must undergo a regeneration process involving separation from the water after the FO process. The LCST phenomenon can be used as a recovery method for thermoresponsive draw solutes and is the critical temperature at which the water and draw solute change from a homogeneous to a heterogeneous state on an increase in temperature. [TMBS] were 48, 33, and 30 °C, respectively, but, at 20 wt%, the LCSTs decreased to 41, 25, and 21 °C, respectively. It has been reported that an LCST near room temperature is ideal for the recovery of draw solutes, suggesting that these ILs are promising draw solutes for FO applications [70]. Therefore, the tetrabutylphosphonium benzenesulfonate-based draw solutes can be separated from water by varying the temperature, and the energy required for this could be obtained from waste heat from power plants or geothermal heat.

Thermoresponsive Behavior
To enable the reuse of the draw solution, the diluted solution must undergo a regeneration process involving separation from the water after the FO process. The LCST phenomenon can be used as a recovery method for thermoresponsive draw solutes and is the critical temperature at which the water and draw solute change from a homogeneous to a heterogeneous state on an increase in temperature. The critical temperature at which the aqueous were 48, 33, and 30 • C, respectively, but, at 20 wt%, the LCSTs decreased to 41, 25, and 21 • C, respectively. It has been reported that an LCST near room temperature is ideal for the recovery of draw solutes, suggesting that these ILs are promising draw solutes for FO applications [70]. Therefore, the tetrabutylphosphonium benzenesulfonate-based draw solutes can be separated from water by varying the temperature, and the energy required for this could be obtained from waste heat from power plants or geothermal heat.

Water and Reverse Solute Fluxes
To evaluate the effect of the draw solutes on FO performance, the water and reverse solute fluxes must be considered [71,72]. Based on the obtained results, which suggest good FO performance and easy recovery, [TBP][DMBS] was selected as a representative IL, and its water and reverse solute fluxes were measured at respective concentrations (5,10,15 improved with an increase in concentration because a higher concentration of the draw solution induces a higher osmotic pressure, thus improving the water flux in the FO system [73]. For example, in AL-DS mode, the water fluxes were approximately 1.58, 2.11, 4.64, and 9.29 LMH at concentrations of 5, 10, 15, and 20 wt%, respectively. Furthermore, in AL-FS mode, the water fluxes were approximately 0.58, 1.35, 2.32, and 4.64 LMH at concentrations of 5, 10, 15, and 20 wt%, respectively. In addition, depending on the orientation of the membrane, the water flux changes [74]. In AL-FS mode, water molecules permeate the active layer from the feed solution, thus diluting the draw solution in the porous layer. This phenomenon is known as dilution internal concentration polarization (ICP). In contrast, in AL-DS mode, the ICP effect is negligible when the feed solution is pure because it is in the porous layer [75,76]. Therefore, at all concentrations, the water flux values of aqueous [TBP][DMBS] in the AL-DS mode were larger than those in the AL-FS mode.

Water and Reverse Solute Fluxes
To evaluate the effect of the draw solutes on FO performance, the water and reverse solute fluxes must be considered [71,72]. Based on the obtained results, which suggest good FO performance and easy recovery, [TBP][DMBS] was selected as a representative IL, and its water and reverse solute fluxes were measured at respective concentrations improved with an increase in concentration because a higher concentration of the draw solution induces a higher osmotic pressure, thus improving the water flux in the FO system [73]. For example, in AL-DS mode, the water fluxes were approximately 1.58, 2.11, 4.64, and 9.29 LMH at concentrations of 5, 10, 15, and 20 wt%, respectively. Furthermore, in AL-FS mode, the water fluxes were approximately 0.58, 1.35, 2.32, and 4.64 LMH at concentrations of 5, 10, 15, and 20 wt%, respectively. In addition, depending on the orientation of the membrane, the water flux changes [74]. In AL-FS mode, water molecules permeate the active layer from the feed solution, thus diluting the draw solution in the porous layer. This phenomenon is known as dilution internal concentration polarization (ICP). In contrast, in AL-DS mode, the ICP effect is negligible when the feed solution is pure because it is in the porous layer [75,76]. Therefore, at all concentrations, the water flux values of aqueous [TBP][DMBS] in the AL-DS mode were larger than those in the AL-FS mode.

Recyclability Study of [TBP][DMBS]
To confirm the recyclability of [TBP][DMBS] in the water treatment field, the FO process was repeated four times using a 20 wt% solution of [TBP][DMBS] as the draw solution and DI water as the feed solution. The recycling FO system is illustrated in Figure 9a. When the temperature rises above the critical temperature after the permeation process, [TBP][DMBS] is precipitated in the solution, and pure water can be easily separated by a simple filtration process. As shown in Figure 9b-

Recyclability Study of [TBP][DMBS]
To confirm the recyclability of [TBP][DMBS] in the water treatment field, the FO process was repeated four times using a 20 wt% solution of [TBP][DMBS] as the draw solution and DI water as the feed solution. The recycling FO system is illustrated in Figure 9a. When the temperature rises above the critical temperature after the permeation process, [TBP][DMBS] is precipitated in the solution, and pure water can be easily separated by a simple filtration process. As shown in Figure 9b

Recyclability Study of [TBP][DMBS]
To confirm the recyclability of [TBP][DMBS] in the water treatment field, the FO process was repeated four times using a 20 wt% solution of [TBP][DMBS] as the draw solution and DI water as the feed solution. The recycling FO system is illustrated in Figure 9a. When the temperature rises above the critical temperature after the permeation process, [TBP][DMBS] is precipitated in the solution, and pure water can be easily separated by a simple filtration process. As shown in Figure 9b    [TMBS] solutions were found to have LCSTs of approximately 41, 25, and 21 • C, respectively, which is useful for their recovery. Furthermore, the water and reverse solute fluxes of 20 wt% aqueous [TBP][DMBS] were measured to be approximately 9.29 LMH and 1.37 gMH, respectively, in AL-DS mode and 4.64 LMH and 0.37 gMH, respectively, in AL-FS mode. Based on the above results, tetrabutylphosphonium benzenesulfonate-based draw solutes are promising candidates for the draw solute owing to their excellent FO performance and easy recovery.