Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 12253−12260
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Morpholine Derivatives as Thermoresponsive Draw Solutes for Forward Osmosis Desalination Asuka Inada, Tomoki Takahashi, Kazuo Kumagai, and Hideto Matsuyama* Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Graduate School of Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan
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ABSTRACT: To develop a forward osmosis (FO) process, selection of draw solutes (DSs) is a critical factor in determining water permeability of the process. In this search for novel high-performance DSs, various morpholine derivatives were investigated for their thermoresponsive potential. 4-Butylmorpholine (BuMP) showed a preferable minimum lower critical solution temperature for the FO process (31.7 °C). The dilute phase of BuMP after phase separation at 70 °C showed a low concentration (3.3 wt %) and low osmotic pressure (3.16 bar). In the FO flux test, the water permeability and reverse solute flux of BuMP (55.0 wt %, 28 bar) against water were Jw 2.09 L m−2 h−1 and Js 14.0 g m−2 h−1, respectively. Using 0.6 M NaCl (model seawater) as feed solution, BuMP (94.6 wt %) could extract water from this model seawater (Jw 0.56 L m−2 h−1). These results indicate a high potential for MP derivatives as DSs and provide new guidance for their development for FO desalination. water flux due to their low osmotic pressure and/or high viscosity. Moreover, DSs are required to be low leakage from the FO membrane. However, some low-molecular-weight DSs reported previously showed low rejection.32,33 In this study, we conducted a screening of more than 500 of a variety of compounds to identify a novel DS structure, and morpholine (MP) derivatives were found to show the lower critical solution temperature (LCST)-type phase separation behavior in aqueous solution. MP is a typical heterocyclic organic compound which has nitrogen and oxygen atoms in its structure (Table S1, compound 1), and it has both amine and ether properties. A number of MP derivatives are used in a wide range of applications: as sensors,34 in catalysis,35 and as solvents.36 Some MP derivatives have anticancer,37 antiinflammatory,38 antiviral,39 antimicrobial,40 and antibacterial biological properties,41 but to our knowledge, there is no report regarding draw solute. Moreover, MP is biodegradable, and thus, if used on a large scale, it will have a low environmental burden. So far, there has been no report regarding the phase separation properties of MP derivatives nor their application to seawater desalination by the FO process. Herein, we report MP derivatives as new thermoresponsive DSs for the FO desalination process. In this study, the solution behaviors of various types of MP derivatives were investigated. Among
1. INTRODUCTION Due to the rapid increase in the world’s population, the shortage of fresh water has become a serious sustainability problem.1,2 During the past decade, the forward osmosis (FO) process has been proposed and has attracted attention as one solution to this problem.3,4 The driving force behind transferring water molecules in the FO process is an osmotic pressure gradient, in which water molecules are spontaneously transferred from a lower solute concentration solution (feed solution (FS)) with lower osmotic pressure, to a higher solute concentration solution (draw solution (DS)) with higher osmotic pressure through a semipermeable membrane. The FO process has a great advantage compared with pressuredriven processes such as the reverse osmosis (RO) process, because it can be operated at low or no hydraulic pressure conditions. The FO process also has the potential to achieve a higher water recovery ratio, energy efficiency,5−7 and lower fouling.8,9 However, there is still a need to improve the performance of the FO process for practical use in seawater desalination.10 Since there is room for improvement in the process, a number of DSs such as inorganic salts,11−13 polyelectrolytes,14−17 copolymers,18,19 ionic liquids,20,21 hydrogels,22−25 and nanoparticles26−28 have been developed, some of them having responsiveness to external stimuli such as heat, light, and magnetic fields. Among them, the thermoresponsive DSs have been attracting much of the recent interest in this field of study, as they can be repeatedly used following their regeneration using low-grade waste heat. Some thermoresponsive DSs have been developed in the past few decades;29−31 however, these agents generally exhibit low © 2019 American Chemical Society
Received: Revised: Accepted: Published: 12253
March 28, 2019 May 18, 2019 June 3, 2019 June 3, 2019 DOI: 10.1021/acs.iecr.9b01712 Ind. Eng. Chem. Res. 2019, 58, 12253−12260
Article
Industrial & Engineering Chemistry Research
Figure 1. Schematic diagram of the FO system.
membranes. Ultrapure water was produced by a milli-Q ultrapure water system (Millipore, Bedford, MA, USA). [N4444][CF3COO] was prepared as described before,43 and this ionic liquid was also used as a conventional thermoresponsive draw solute. 2.2. Characterization of the Physical Properties of MP Derivatives for an FO Application. The phase diagram, osmotic pressure, and viscosity of MP derivatives were measured to characterize their behaviors in aqueous solution. For the measurement of the phase diagram, aqueous solutions of MP derivatives of different concentrations (40−50 wt %) were put into an incubator for 24 h at different temperatures (0−80 °C) to observe the occurrence of phase separation. If phase-separated, the dense and dilute phases were collected using a syringe. The water content of the dense phase was determined by a Karl Fischer titrator (MKH-700, Kyoto Electronics Manufacturing Co., Ltd., Kyoto, Japan), and the total organic carbon (TOC) content was determined by a TOC analyzer (TOC-VCSH, Shimadzu Co., Ltd., Kyoto, Japan) as reported previously.43,44 The osmotic pressures π of both FS and DS were calculated from the water activity by using the van Laar equation45,46 (eq 1)
them, MP derivatives that showed thermoresponsivity were evaluated in terms of their physical properties such as phase diagram, osmotic pressure, and viscosity. Moreover, the performance of those MP derivatives as DSs in the FO process was evaluated in an FO water permeability test. Like morpholine derivatives used in this study, polypropylene glycol with a molecular weight of 400 (PPG400) is a lowmolecular-weight compound, and it is known to show LCST phase separation.42 Moreover, its osmotic pressure is high enough to use as a DS in FO. Thus, the authors used PPG400 as a benchmark draw solute to compare the performance of MP derivatives.
2. EXPERIMENTAL SECTION 2.1. Materials. The chemical structures of MP derivatives used in this work are summarized in Table S1. Morpholine, (1, abbreviated as MP), 4-methylmorpholine (2), 4-ethylmorpholine (3), 4-isobutylmorpholine (7, abbreviated as iBuMP), 4(2-hydroxyethyl)morpholine (10), 4-(3-hydroxypropyl)morpholine (11), N-(2-hydroxypropyl)morpholine (12), bis(2-morpholinoethyl) ether (17), cis-2,6-dimethylmorpholine (15), 2-morpholinoaniline (13), and 4-morpholinoaniline (14) were purchased from Tokyo Chemical Industry Co. (Tokyo, Japan). 4-Cyclohexylmorpholine (9, abbreviated as CHMP) was purchased from Arctom Chemicals LLC (Newton, MA, USA). Ethyl morpholine-4-carboxylate (16) was purchased from Sigma-Aldrich Japan Co. (Tokyo, Japan). 4-Propylmorpholine (4, abbreviated as PrMP), 4-isopropylmorpholine (5), 4-butylmorpholine (6, abbreviated as BuMP), and 4-cyclopentylmorpholine (8, abbreviated as CPMP) were kindly provided by Nippon Shokubai Co. (Osaka, Japan) and Nippon Nyukazai Co. (Tokyo, Japan). These MP derivatives were used as the candidates of draw solutes. Polypropylene glycol (dioltype, 400, abbreviated as PPG400) was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and was used as a conventional thermoresponsive draw solute to compare water-drawing ability with those of MP derivatives. 10Camphorsulfonic acid (CSA), 1,3,5-benzenetricarbonyl trichloride (TMC), and hexamethylphosphoric triamide (HMPTA) were purchased from Tokyo Chemical Industry Co. (Tokyo, Japan). Resorcinol, acetone, methanol, hexane, 1,3-phenylenediamine (MPD), sodium dodecyl sulfate (SDS), and triethylamine (TEA) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). These reagents were used for preparation of the active layer of the FO membrane. Polyketone (PK) (Mw: 200 000; Asahi Kasei Fibers Co., Japan) was provided as the polymer for the support layer of the FO
π=
−RT ln a w vw
(1)
where R is the ideal gas constant, T (K) is the absolute temperature, aw is the water activity at temperature T, and vw is the molar volume of water (=18.018 × 10−6 m3 mol−1).47 The water activity was measured by a water activity meter (AquaLab Series4TDL, METER Group, Inc., Pullman, WA, USA). All the measurements were conducted at 20 ± 1 °C. The viscosities of MP derivative solutions were measured by an electromagnetically spinning viscometer (EMS-1000, Kyoto Electronics Manufacturing Co., Ltd., Kyoto, Japan) at 20 ± 1 °C. 2.3. Evaluation of the FO Water Flux and the Reverse DS Flux. The FO water flux and the reverse solute flux were evaluated by a lab-scale cross-flow FO cell (Kokugo Co., Ltd., Japan) and an FO membrane (33.7 mm × 53.3 mm), with an effective membrane area of 17.7 cm2.48 The polyketone (PK) thin film composite (TFC) FO membrane used in this study for evaluation of DS performance was prepared by interfacial polymerization of MPD and TMC on the surface of a porous PK support layer as described previously.49 Briefly, PK, resorcinol, and water were mixed in the ratio 10:58.5:31.5 (w/w/w) and dissolved by stirring at 80 °C for 3 h. The PK 12254
DOI: 10.1021/acs.iecr.9b01712 Ind. Eng. Chem. Res. 2019, 58, 12253−12260
Article
Industrial & Engineering Chemistry Research
Figure 2. Phase diagrams and osmotic pressures of aqueous solutions of (A) PrMP (4), (B) BuMP (6), (C) CPMP (8), (D) CHMP (9) (osmotic pressure of CHMP at concentrations of 10−70 wt % was measured as a suspension of the dilute phase and the dense phase), (E) iBuMP (7) (osmotic pressure could not be determined due to the lack of a homogeneous phase at any concentration), and (F) PPG400 as functions of their concentrations. Osmotic pressures were measured at 20 °C.
change of amount of substance of DS diffused into FS (g). The tests were performed for 1 h, and the decrease of FS weight was recorded by logging it onto a computer to obtain Jw data. Jw was obtained as an average for 30 min in the time range from 30 to 60 min after starting the test, because the time for the completion of the internal concentration polarization (ICP) was estimated to be more than 15 min. Js was determined by measuring the change of TOC content of FS after a 1 h operation. In addition, an FO test using BuMP as the DS and 0.6 M NaCl (model seawater) as the FS was carried out in the AL-FS mode to evaluate the ability of BuMP to extract water from seawater in order to demonstrate the feasibility of seawater desalination. 2.4. Evaluation of the Stability of BuMP for Repeated Use as a DS. The stability test of BuMP for repeated use as a DS was conducted. Aqueous 50 wt % BuMP solution was kept at 80 °C for 8 h, and after that, it was cooled down and kept at 25 °C for 16 h. This heating and cooling cycle was performed repeatedly on each weekday, and the cycle was continued for a month (total of 20 cycles). During the test, samples were collected at a certain interval and their phase diagrams were measured.
solution was cast onto a glass plate using an applicator with a slit height of 200 μm. The porous support membrane was immersed in a methanol/water mixture (30:70 (w/w)) at room temperature for 20 min. The prepared PK membrane was washed by acetone and hexane, followed by air-drying. The active layer was formed on top of the PK support membrane by interfacial polymerization. The membrane was immersed in an aqueous solution consisting of 1.1 wt % TEA, 2 wt % MPD, 2.3 wt % CSA, 0.15 wt % SDS, and 3.0 wt % HMPTA for 5 min and pulled up, followed by it being covered with a hexane solution containing 0.15 wt % TMC for 2 min. After the interfacial polymerization, the membrane was heated at 90 °C for 10 min and was rinsed by milli-Q water immediately. The membrane was prepared immediately before performing the FO test. Figure 1 shows a schematic diagram of the system used in the FO experiment. The DS (300 g) and the FS (500 g, milli-Q water) were circulated in a cocurrent using peristaltic pumps at a constant flow rate to avoid a pressure difference. Both DS and FS temperatures were adjusted at 20 ± 1 °C using magnetic stirrers with temperature regulators. The FO water flux and reverse DS flux of MP derivatives as the DS were measured against the FS (milli-Q water) in the active layer facing FS (AL-FS) mode at atmospheric pressure. The water flux, Jw (L m−2 h−1, abbreviated as LMH), and reverse DS flux, Js (mmol m−2 h−1, abbreviated as gMH), were calculated by eqs 2 and 3, respectively Jw =
Δw A m ρΔt
(2)
Js =
Δ(CV ) A m Δt
(3)
3. RESULTS AND DISCUSSION 3.1. Screening for MP Derivatives Showing Thermoresponsivity. It is important to find a suitable material for use as a DS in an FO process which has both high osmotic pressure and a phase separation property, because these factors directly influence the FO water flux and water/DS recovery performance. Materials with LCST-type phase separation have several advantages as a DS. They are miscible in water even at high concentrations and below the phase separation temperature, and therefore, they can exhibit high osmotic pressure and draw water from the FS efficiently. At temperatures high above the phase separation temperature, they exhibit liquid− liquid phase separation, giving a dilute phase and a dense
where Δw (g) indicates the mass change of FS over time Δt (h), Am shows the effective membrane area (m2), ρ is the density of FS (g/cm3), C shows the diffused DS concentration in FS (g/L), and V shows the volume of FS (L). Δ(CV) is the 12255
DOI: 10.1021/acs.iecr.9b01712 Ind. Eng. Chem. Res. 2019, 58, 12253−12260
Article
Industrial & Engineering Chemistry Research
temperatures of LCST of PrMP and CHMP seem to be too high and too low, respectively, to use as thermoregenerable DSs in the FO process, they are not suitable for use as DSs in the FO desalination. BuMP and CPMP showed good LCST separation profiles: the concentrations of the dense phases of BuMP and CPMP were 94.7 and 87.4 wt %, respectively, and those of the dilute phase were 3.3 and 5.7 wt %, respectively, after phase separation at 70 °C. Compared with the data for PPG400, the concentrations of the dense and dilute phases of BuMP and CPMP are much higher and lower, respectively. This means that the dense phases of BuMP and CPMP are much more suitable to draw water from the FS due to the relatively high osmotic pressures. Furthermore, the lower osmotic pressures (than PPG400) of the dilute phases of BuMP and CPMP can lead to low-pressure operations in the postprocessing, a low-pressure RO process to produce fresh water from the dilute phase. Therefore, with regard to thermoresponsivity, BuMP and CPMP were thought to be good candidates for DSs for the FO process. 3.3. Osmolalities and Viscosities of MP Derivatives. The osmotic pressures and viscosities of MP derivatives were analyzed in detail to characterize their behaviors. Figure 3 shows the osmolalities of the aqueous homogeneous solutions of PrMP, BuMP, and CPMP with LCST-type phase separation as a function of their molalities at 20 °C. The osmolalities of the aqueous solutions of CHMP and iBuMP could not be determined due to the occurrence of their phase separation at 20 °C. The osmolalities of three MP derivatives showed a linear increase with the increase of their molalities and obeyed the van’t Hoff equation in the low-concentration region. The osmolalities of PrMP, BuMP, and CPMP showed a slope of less than 1 at concentrations above 0.9, 0.5, and 1.6 mol/kg, respectively. In general, amphiphilic molecules such as surfactants are known to form molecular aggregates such as a micelle or vesicle above the critical micelle concentration (CMC) or critical aggregation concentration (CAC).50 Materials with LCST-type phase separation also tend to form micelles or aggregations in aqueous media as the concentration or temperature rises, regardless of their chemical structure.51,52 Kamio et al. reported that the slope change point of osmolality of thermoresponsive ionic liquids was correlated to CAC.43 Similar to these previous studies, it may be suggested that PrMP, BuMP, and CPMP with LCST-type phase separations also form aggregates above the slope change point (≈CAC) and exist as a single molecule below the CAC in aqueous media. The order of the osmolality of PrMP, BuMP, and CPMP is PrMP > CPMP > BuMP for both dense phase over LCST and uniform solution phase under LCST (Figure 2). This order is partly similar to that of the minimum temperature of LCST as a function of log P (Figure S1) and may be explained by the hydrophobicity or the intermolecular forces that induce the aggregation of the molecules. The viscosity of DS is another important factor that affects the water-drawing ability in an FO process. Figure 4 shows the viscosities of aqueous homogeneous solutions of PrMP, BuMP, and CPMP as functions of their concentrations, respectively, at 20 °C. The viscosities of the aqueous solutions of CHMP and iBuMP could not be measured due to the occurrence of their phase separation at 20 °C. The viscosities of PrMP, BuMP, and CPMP were relatively low compared to those of other thermoresponsive DSs such as PPG400 (117.2 mPa s at 90 wt %) and [N4444][CF3COO] (61.4 mPa s at 90 wt %) as shown in Figure S2. In this study, BuMP in particular showed
phase. Pure water can be easily produced from the dilute phase by using the postprocessing low-pressure RO process. The dense phase can be used as a regenerated DS and can be recycled in the FO process. In this study, we conducted a screening of more than 500 of a variety of compounds to identify a novel DS structure, and morpholine (MP) derivatives were found to show the lower critical solution temperature (LCST)-type phase separation behavior in aqueous solution. Therefore, in this study, we focused on MP derivatives with LCST-type phase separation as candidates for DSs. Table S2 shows the detailed result of screening for MP derivatives that show phase separation. Four MP derivatives (4, 6, 8, and 9) showed LCST-type phase separation behavior in the temperature range 25−80 °C. Compounds 13 and 14 were not soluble in water at a concentration of 50 wt % at 25−80 °C. Compound 7 retained two aqueous phases even at the lowest temperature (25 °C) tested in this study, suggesting that the LCST of compound 7 is lower than 25 °C. The other MP derivatives were completely miscible in water, presumably due to their high hydrophilicity. 3.2. Phase Separation Properties and Osmotic Pressures of LCST-Type MP Derivatives. To evaluate the phase separation properties of the five LCST-type MP derivatives (4, 6, 7, 8, and 9) in detail, their phase diagrams were investigated. Additionally, their osmotic pressures were also measured, because it is known that some thermoresponsive DSs show nonlinear concentration-dependent osmotic pressure curves.43,44 The phase diagrams and osmotic pressures as functions of their concentrations of the five MP derivatives, PrMP (4), BuMP (6), CPMP (8), CHMP (9), and iBuMP (7), are shown in Figure 2A−E. It was reported that polypropylene glycol with a molecular weight of 400 (PPG400) showed LCST-type phase separation and its DS property was investigated.42 The properties of PPG400 were measured in this work and have been included in Figure 2F. Aqueous solutions of PrMP, BuMP, CPMP, and CHMP showed LCST-type phase separation at 10−80 °C and nonlinear osmotic pressure curves. iBuMP showed no homogeneous solution at 10−80 °C, and thus, the osmotic pressure could not be determined. The minimum temperature of LCST, the concentrations of the dense and dilute phase after phase separation at 70 °C, and osmotic pressures of each phase measured after cooling at 20 °C are summarized in Table 1. As shown in Table 1, the minimum temperature of LCST of PrMP, BuMP, CPMP, and CHMP were 65.1, 31.7, 44.9, and 16.7 °C, respectively. Because the minimum Table 1. Summary of the Minimum Temperatures of LCST, Concentrations of the Dense and Dilute Phases after Phase Separation at 70 °C, and Osmotic Pressures of Each Phase Measured at 20 °C concentration [wt %]
osmotic pressure [bar]
phase-separated MP derivatives
minimum temp. of LCST [°C]
dense phase
dilute phase
dense phase
dilute phase
PrMP (4) BuMP (6) CPMP (8) CHMP (9) iBuMP (7) PPG400
65.1 31.7 44.9 16.7
