Forward osmosis with electro-responsive P(AMPS-co-AM) hydrogels as draw agents for desalination

Journal of Membrane Science 593 (2020) 117406

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Forward osmosis with electro-responsive P(AMPS-co-AM) hydrogels as draw agents for desalination

T

Huayong Luoa, Kelin Wua, Qin Wangb, Tian C. Zhangc, Hanxing Lua, Hongwei Ronga,∗, Qian Fanga,∗∗ a

School of Civil Engineering, Guangzhou University, Guangzhou, 510006, China School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China c Civil Engineering Dept, University of Nebraska-Linclon, Omaha, NE, 68182, USA b

A R T I C LE I N FO

A B S T R A C T

Keywords: Forward osmosis Electro-responsive hydrogel Draw agent Water flux

We explore the feasibility of utilizing electro-responsive hydrogels as novel forward osmosis (FO) draw agents for desalination. The chemically cross-linked hydrogels were synthesized via free radical copolymerization of common acrylamide (AM) with strong anionic comonomer 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). The morphology, chemical structure, water-adsorbing capacity, and water state of the hydrogels were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), a swelling test, and differential scanning calorimetry (DSC). The water-swollen hydrogels exhibited electroresponsiveness as shrinking and expanding reversibly with the on-off switching of the electric field in an electrode contact system. The magnitude of hydrogels’ deswelling increased with the increase in the degree of swelling and applied voltages. This dehydration phenomenon was induced by the decrease in hydration power of microcounter ions due to the interaction with the electrodes by water electrolysis. The hydrogels were proved to be capable of generating a reasonable water flux of 2.76 L m−2 h−1 (LMH) from brackish water (2000 ppm NaCl solution) due to their high swelling pressure. In addition, the water flux was affected by the amount of hydrogel particles on the membrane surface. Importantly, the prepared hydrogels could effectively release around 71% of the adsorbed water at an applied voltage of 15 V for 40 min, and were able to maintain their water flux and water recovery performances up to three times regeneration. Results indicate that the electric field is an attractive alternative stimulus for extracting desirable water from the hydrogel draw agent in the FO desalination process.

1. Introduction Recently, forward osmosis (FO) is gaining growing attention for desalination [1,2], and one of the focused areas in FO research is to develop appropriate hydrogel-based draw agents [3,4]. Hydrogels are cross-linked and three-dimensional hydrophilic polymer networks, which are able to entrap a large amount of water due to the large swelling pressure, but do not dissolve when placed in contact with water [5,6]. Hydrogels composed of stimuli-responsive polymers can undergo abrupt reversible changes in volume or shape due to their ability to respond to external stimuli (e.g. temperature, solvent, pressure, light, electric and magnetic field, etc.) [7]. These “smart” responsive properties make hydrogels be excellent draw agents for the cost-effective dewatering and regeneration in FO processes [1]. Up to

∗

now, a lot of materials have been used (or proposed to be used) in thermo-responsive hydrogels to enhance the performance of the hydrogel-driven FO process, such as carbon particles [8,9], magnetic nanoparticles [10,11], reduced graphene oxide (rGO) [12], thermoplastic polyurethane (TPU) microfibers [13], commercial polyurethane foam (PUF) [1], poly (sodium 4-styrenesulfonate) (PSSS) chains [14], or hydrophobic elastic polyester (PET) hollow microfiber [15]. Wang's groups [1,7,8,11–13,15–18] have demonstrated that the stimuli-responsive hydrogel draw agents allow easy water recovery and draw agent regeneration. However, the recovered water from these thermoresponsive hydrogels was mostly in vapor state due to evaporation under heat stimuli, even though the heat was smartly generated from solar energy or magnetic induction [19]. Additional condensation unit is then required to recover water in the form of liquid and will increase

Corresponding author. Corresponding author. E-mail addresses: [email protected], [email protected] (H. Rong), [email protected] (Q. Fang).

∗∗

https://doi.org/10.1016/j.memsci.2019.117406 Received 28 February 2019; Received in revised form 28 July 2019; Accepted 22 August 2019 Available online 22 August 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

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2. Materials and methods

the overall costs of FO process [3,20,21]. On the other hand, electro-responsive hydrogels can exhibit swelling, shrinking, or bending behaviors under the influence of electric fields, and thus have gained more attraction due to their potential applications in the fields of sensors and actuators, robotics and artificial muscles, optical systems, drug delivery, space, ocean and energy harvesting applications [22]. When a water-swollen polyelectrolyte hydrogel is inserted between a pair of electrodes and a DC field is applied, the hydrogel undergoes anisotropic contraction and concomitant water extrusion in the air [23,24]. For example, a swollen poly (2-acrylamido2-methyl-1-propanesulfonic acid) (PAMPS) hydrogel (1 g dry PAMPS absorbed 2250 g water), reduced its weight by 70%, due to loss of water, when a 12 V/cm DC current was applied for 20 min, and the dehydration could be repeated after immersing the hydrogel in water [25]. The electrically induced contraction of the hydrogel was caused by the transport of hydrated ions and water in the network, and the observed contractile behaviors are essentially electrochemical phenomena [23,24]. Recently, Zhang et al. employed 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) monomers to prepare a series of electrically responsive AMPS/DMAEMA hydrogels as FO draw agents [26]. They found the hydrogels shrunk and released water when a voltage of 15 V was applied, and proposed the dewatering mechanism of the stimuliresponse to be associated with the migration of different charged ions in the hydrogel network resulting in partial shielding of the charges in the polyionic groups due to the electrostatic attraction. However, from the previous study on the swelling of poly (DMAEMA-co-AMPS) hydrogels, the lower value of swelling was obtained for the copolymer hydrogel due to an inter/intramolecular complex formation between anionic and cationic groups of the hydrogel [27], while the swelling property of hydrogels is a key parameter for achieving high water flux in FO [15]. AMPS is an ionic monomer widely used for the synthesis of polyelectrolyte hydrogels possessing a large water adsorption capacity [28]. It is reported that the linear polymer containing sulfonate groups derived from AMPS exhibits extensive coil expansion even in 5 M NaCl solution, and the expansion of polymer coils due to charge repulsion cannot be totally screened [29]. The chemically cross-linked hydrogels derived from AMPS have received great interest because they exhibit pH independent swelling behavior and good electro-responsive property, making them attractive materials in a wide range of potential applications, such as in soft-biomimetic actuators, superabsorbents, biomaterials, bioengineering, water purification, agriculture, and food industry [28]. However, poly (2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) hydrogels can be relatively fragile [30]. Thus, in our work, acrylamide (AM) was selected to copolymerise with AMPS to form hydrogels with improved mechanical properties. Meanwhile, AM, a non-ionic hydrophilic monomer, was chosen as a comonomer of AMPS because it is rather cheap, common monomer of preparing superabsorbent polymers with a high degree of swelling [31,32]. Herein, strong polyelectrolyte cross-linked hydrogels with high swelling property based on an anionic monomer AMPS and a non-ionic monomer AM were prepared as novel electro-responsive draw agents for the FO process, and to our knowledge there is no adequate literature available regarding the FO desalination performance by this kind of strong polyanion hydrogel. We prepared and characterized poly (2-acrylamido-2-methyl-1- propanesulfonic acid-co-acrylamide) hydrogels (P (AMPS-co-AM)) with different ratios of AMPS and AM to investigate the chemical structure, interior morphology, swelling property and water state of hydrogels as well as electroresponsiveness. Particularly, an inferred mechanism of deswelling behaviors of the hydrogels under electric field in a self-built electrode contact system was proposed. The objectives of this research were to evaluate the performance (e.g., water flux, water recovery and draw agent regeneration) of a bench-scale FO system with its draw agent being the electro-responsive P (AMPS-coAM) hydrogels and to determine the associated dewatering behaviors of these hydrogel draw agents.

2.1. Materials 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS, 98%), acrylamide (AM, 99%), N,N′-methylene bisacrylamide (MBA, 99%) were purchased from Aladdin (Shanghai, China). Ammonium persulfate (APS), sodium bisulfite (NaHSO3), and sodium chloride (NaCl) were analytical grade and were purchased from Shanghai Experiment Reagent Co. (Shanghai, China). All the chemicals were used as received. The deionized (DI) water (resistivity = 18 MΩ cm) used in the experiments was produced by a water purification system (TST-P, Teste, Shijiazhuang, China). The commercially available aquaporin-embedded FO flat-sheet membranes were supplied by Aquaporin A/S, Lyngby, Denmark. The details of membrane's properties and structure images could be found in the literature [33]. 2.2. Preparation of hydrogels A series of cross-linked P (AMPS-co-AM) hydrogels with different ratios of AMPS and AM were synthesized via the free-radical polymerization using MBA as a cross-linker, and APS/NaHSO3 as the redox initiator (see Fig. 1). In all these hydrogels, the total repeat concentration of monomers was 1 M. For example, to prepare S0.65M0.35, the concentrations of AMPS and AM were 0.65 and 0.35 M respectively. The molar ratio of monomers, cross-linker, and initiator was fixed at 100:1:1 [26,34]. In actual synthesis, the predetermined amounts of AMPS, AM, and MBA were firstly dissolved in 30-ml DI water at room temperature under constant stirring until a homogeneous solution was made. Then, the solution was bubbled with nitrogen for 20 min, which was followed by the addition of initiator (APS/NaHSO3). After complete dissolution, the polymerization was carried out at 60 °C in an oven for 5 h. The as-prepared hydrogels were denoted as S0.65M0.35, S0.55M0.45, S0.45M0.55 with various compositions (the molar ratio of AMPS and AM = 0.65/0.35, 0.55/0.45, 0.45/0.55, respectively). To remove the unreacted monomers and other low-molecular-weight polymer, the hydrogels were cut into small pieces and immersed into DI water at room temperature for 5 days with a change of water every 8 h. The hydrogel was subsequently dried to a constant weight at 70 °C in an oven, and the powder form (50–200 μm) was obtained by grinding dried hydrogels with a mortar and pestle. 2.3. Characterization 2.3.1. Scanning electron microscope (SEM) The morphological structures of dry and swollen hydrogel particles were observed by a scanning electron microscope (SEM) (JSM-7001F, JEOL, Japan) operating at 15 kV. To observe the morphology of swollen hydrogels, they were freeze-dried before sputter-coating with platinum. 2.3.2. Fourier transform infrared spectroscopy (FTIR) The chemical structures of hydrogel samples were analyzed by a Fourier transform infrared spectroscopy (FTIR) (Tensor 27, Bruke, Germany). 2.3.3. Equilibrium swelling ratio (ESR) The equilibrium swelling ratio (ESR) of the prepared hydrogel was measured by a weighed method [34]. The pre-weighed dry hydrogel pieces were put in a custom-made nylon mesh bag, and then immersed in DI water (at 20 °C) for 72 h until no increment in swelling was observed in hydrogels [34]. The whole swelling process was carried out with the sealed nylon mesh bag immersed in a 1000-mL breaker. After given periods, the nylon mesh bag was hung up for several minutes and the excess surface water was removed by a filter paper, and then weighed quickly until a constant weight [35]. The ESR of hydrogels was calculated by Eq. (1): 2

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Fig. 1. Schematic representation of the synthesis of P (AMPS-co-AM) hydrogels.

ESR =

Ws − Wd Wd

plate electrodes, and an electric field was applied by using a potentiostat (KXN-645D, ZHAOXIN, China) as a DC power supply. Contraction was followed by measuring the weight change of the P (AMPS-co-AM) hydrogel with time. The exuded water was automatically in situ pumped out by using a slightly evacuated flask [24]. The self-built dewatering setup for measuring the hydrogel contraction under electric stimuli was shown in Fig. S1 (details in Supplementary Material). The hydrogels after the deswelling measurement under an electric field stimulus were dried in an oven at 60 °C until a constant weight (Wd) was obtained, based on which the swelling ratio (Q) and water retention (WR) during the course of deswelling could be calculated. The swelling ratio (Q) and water retention (WR) at a certain time were determined as follows [38,39]:

(1)

where Wd (g) is the weight of the initial dry hydrogel, and Ws (g) is the weight of the swollen hydrogel after immersion. The equilibrium water content (EWC) used to calculate the state of water in the hydrogels was calculated from Eq. (2) [36]:

EWC (%) =

We − Wd × 100 We

(2)

where We (g) denotes the weight of the swollen state hydrogel at equilibrium, and Wd (g) is the initial weight of the dry hydrogel. 2.3.4. Differential scanning calorimetry (DSC) The state of water present in the swollen P (AMPS-co-AM) hydrogel was examined using a differential scanning calorimetry (DSC) (DSC8000, PerkinElmer, USA). The hydrogel samples equilibrated in DI water were cooled to −30 °C and then heated to 30 °C using a heating rate of 3 °C/min under N2 flow of 20 mL/min. The fractions of free water (free and frozen-bound water) and bound water (non-frozen water) were calculated from the melting enthalpies using Eq. (3), which assumes that the heat of fusion of free water in the hydrogel is the same as that of ice [36,37].

Q=

(4)

W − Wd ⎞ WR = ⎛ t × 100% ⎝ We ⎠ ⎜

⎟

(5)

where Wt (g) is the weight of the hydrogel at regular time intervals during deswelling; Wd (g) is the weight for the completely dry state; and We (g) is the weight for the swollen state at equilibrium. For reswelling test, all the hydrogel samples after electrically induced deswelling were again soaked in DI water to achieve the swelling equilibrium, and the swelling ratios were calculated using Eq. (4) to evaluate the reswelling property of the hydrogel.

Wb (%) = Wc − (Wf + Wfb) =Wc − (Qendo/ Qf ) × 100

Wt − Wd Wd

(3)

where, Wc (%) is the equilibrium water content (EWC); Wb (%) is the amount of the bound water; Wf (%) and Wfb (%) are the amounts of free and frozen-bound water, respectively. Qendo (J/g) and Qf (J/g) denote the heats of fusion of free water in the P (AMPS-co-AM) hydrogel and of pure water (334 J/g), respectively [36].

2.3.6. FO process The water fluxes of the prepared hydrogels as draw agents were tested in a bench-scale FO system (see Fig. 2). The permeation cell was designed in a plate and frame configuration with a circular channel (6 cm in diameter, 0.3 cm in lower height, 6 cm in upper height) on either side of the membrane. To begin each test, the FO membrane soaked in DI water for 1 h was installed in the permeation cell. 2000 ppm NaCl solution with an initial volume of ~300 mL was used as feed solution (FS), which was pumped by a peristaltic pump (Chuangrui

2.3.5. Deswelling and reswelling properties The P (AMPS-co-AM) hydrogels were swollen to an equilibrium condition in DI water at room temperature (24 ± 1 °C). A piece of swollen hydrogel was inserted between a pair of 25 × 25-mm platinum 3

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Fig. 2. Schematic representation of the hydrogel-driven forward osmosis desalination process.

coupling peak of N–H and O–H stretching at around 3000–3500 cm−1, peaks of C–H stretching of CH and CH2 at around 2900–3000 cm−1, a sharp peak of C]O stretching of amide-I at around 1654 cm−1, and a strong peak of the N–H bending of amide II at 1545 cm−1. The peak located at 1453 cm−1 along with that at 1654 cm−1 is attributed to the symmetrical and asymmetrical C (=O)2 stretching. Similar results can be observed in the spectrum of poly (AM-co-AMPS-H+) superabsorbent polymers [32]. The characteristic peak of AMPS unit is shown at 1042 cm−1 due to the SO3 group [42]. The peak at around 628 cm−1 originates from the C–S stretching of the AMPS unit [43]. Importantly, the spectra show a broad and strong peak at 3438 cm−1 due to the NH or the OH stretching of P (AMPS-co-AM) hydrogels, indicating the OH group in the free acid superimposed the NH stretching of acrylamide moiety [32]. Therefore, the peaks observed in the FTIR spectra confirm the presence of AM and AMPS units in the hydrogels.

Pump, BT100 M, Baoding, China) at a volumetric flow rate of 20 mL/ min. A desired amount of hydrogel particles used as draw agents were placed on the active layer of the FO membrane with an effective membrane area of ~22.05 cm2. The FS tank was mounted on a digital balance (FX-3000i, AND, Japan), and the water flux, JV (Lm−2h−1, or LMH) was determined from Eq. (6) by measuring the weight change (ΔW, g) of the FS with time during the tests.

JV =

ΔW (A × Δt ) × ρ

(6)

where ΔW (g) is the FS decreasing water weight due to the water permeating through the FO membrane over a predetermined interval Δt (h) during the FO process; A (m2) is the effective membrane area used in the FO cell; and ρ (g/L) is the FS density (usually assumed as 1000 g/ L, the density of water) [34,40]. In the dewatering process, the swollen hydrogels after 24-h FO operation were placed in the same apparatus (Fig. S1) to recover the liquid water released during the electrically induced dewatering. The released water from the P (AMPS-co-AM) hydrogels was collected by turning on a vacuum pump, and the weight change of the swollen hydrogels was checked periodically under an electric field. The water recovery fraction (R, %) was calculated as follows:

R=

W1 × 100% W0

3.1.3. Swelling properties The swelling properties of hydrogels represent their abilities to adsorb water, which is used to achieve high water fluxed in FO. According to the Flory-Rehner theory, the swelling pressure (osmotic pressure) within the hydrogels is related to three major interactions: the polymer-solvent interaction πmix; the entropy-elastic forces of the network πel; and the contribution of the charges πion [5,15]. The swelling pressure π could be expressed as the sum of these contributions in Eq. (8).

(7)

where W1 (g) is the weight of water released during the electrically induced dewatering process; and W0 (g) is the weight of absorbed water in the hydrogel particles during 24-h FO operation.

π = πmix + πel + πion

(8)

Therefore, it is well-known that the π of hydrogel increases with the incorporation of increasing ionic component, and thus the ESR of the hydrogel will be increased. From Fig. 3, the ESRs of the prepared hydrogels indeed increased with increasing AMPS content. Note that the incorporation of AMPS has a significant influence on the ESR of the hydrogels. This is because AMPS is a strong ionic electrolyte with hydrophilic acrylamide and sulfonic groups, which produce a high swelling pressure, and thus leading to adsorbing large quantities of water.

3. Results and discussion 3.1. Characterization of hydrogels 3.1.1. SEM analysis The SEM image of dry P (AMPS-co-AM) hydrogel particles shows that they possess smooth surfaces and exhibit sizes ranging from 50 μm to 200 μm, despite some small, fine particles being present (Fig. S2a). A similar trend could be observed in the previous studies [8,17,34]. On the other hand, macro-structural morphology was noticeably apparent after the P (AMPS-co-AM) hydrogels were swollen and freeze-dried (Fig. S2b), which was benefited for the formation of ice crystals during the freeze-drying process and eventually, the production of pores [41].

3.1.4. DSC analysis The state of water in the swollen hydrogels with different molar ratios of AMPS and AM was evaluated from DSC analysis. Free water and bound water contents were measured from the DSC melting thermograms of swollen hydrogels. Fig. 4 shows that the endothermic peaks appeared between 0 and 7 °C, which were assigned to free water associated with the P (AMPS-co-AM) hydrogels and were used to calculate the amount of free water present in the hydrogels. The temperature corresponding to the endothermic peak for three swollen hydrogels followed a sequence of S0.65M0.35 > S0.55M0.45 > S0.45M0.55,

3.1.2. FTIR analysis Fig. S3 presents the FTIR spectra of P (AMPS-co-AM) hydrogels with different compositions. As seen in Fig. S3, all the spectra show a broad 4

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Fig. 5. Deswelling behaviors of P (AMPS-co-AM) hydrogels with different compositions at an applied voltage of 5 V.

Fig. 3. Equilibrium swelling ratios of the prepared P (AMPS-co-AM) hydrogels with different compositions.

ionic groups showed more pronounced deswelling rates and greater amounts of water seepage. Particularly, the S0.65M0.35 hydrogels exhibited a 79% reduction of its original water content within 40 min under the electric stimuli. Generally, the swelling ratios of the P (AMPSco-AM) hydrogels were enhanced with increasing content of strong ionic AMPS. This result indicates that the deswelling efficiency increases with an increase in the degree of swelling. Osada et al. [24] obtained the similar contractile phenomena of polyelectrolyte gels, and identified that the contraction efficiency was almost linearly enhanced with the increase in the degree of swelling, regardless of the value of the applied electric field. The deswelling behaviors of an electrolyte hydrogel in an electrode contact system were considered to be attributed to the electric osmosis in the gel, combined with local pH changes around the electrodes resulting from electrochemical reactions [45,46]. To further support this explanation, the deswelling behavior of the P (AMPS-co-AM) hydrogel as a function of various applied voltages was investigated. Fig. 6 shows that the inappreciable deswelling of the hydrogel occurred when the applied voltage was less than or equal to 1 V, which may be due to the pressure stimuli caused by the slightly evacuated system. However, the slopes of the curves became steeper as the magnitude of the applied voltages exceeded the threshold potential (2 V), which is almost the same as the value of overpotential of water electrolysis [47]. The rate of shrinkage changes increased in proportion to the applied voltages. These facts indicate that the hydrogel deswelling may be induced by electrochemical reactions, particularly water electrolysis. From an application perspective, it is also important to evaluate the reversibility of the deswelling processes. In order to explore the reswelling capability of P (AMPS-co-AM) hydrogels, the phenomena of the relative swelling by switching the electric field on and off were also examined. When exposed to an applied voltage of 15 V, the sample S0.65M0.55 hydrogel with more ionic groups exhibited more

Fig. 4. DSC thermograms of swollen P (AMPS-co-AM) hydrogels with different compositions.

Table 1 Water state of P (AMPS-co-AM) hydrogels estimated with DSC analysis. Sample

Total water (%)

Free water (%)

Bound water (%)

S0.45M0.55 S0.55M0.45 S0.65M0.35

99.82 99.88 99.93

96.92 97.64 97.94

2.90 2.24 1.99

indicating that the incorporation of sulfonic groups enhances the interaction force between polymer and water molecules [26]. The fraction of free water in the total water could be approximately calculated from the ratio of the endothermic peak area of a water-swollen hydrogel to that of the melting endothermic heat of fusion for pure water [44]. Bound water due to hydrogen bonding with P (AMPS-co-AM) chains was expressed as the difference between the total water and the free water. The EWC values, free water contents, and bound water contents were calculated and listed in Table 1. It could be seen that all the hydrogels exhibited a high EWC ranging from 99.82 to 99.93%, and the fraction of free water in the P (AMPS-co-AM) hydrogels increased with an increase in AMPS content. This result indicates that the increase in the swelling ratio was mainly attributed to the free water content of P (AMPS-co-AM) hydrogels [44].

3.1.5. Deswelling and reswelling properties The deswelling capability of hydrogels is an important factor for the availability of clean water by the FO desalination technology. Fig. 5 shows the remaining water contents of different P (AMPS-co-AM) hydrogels as a function time depending on the applied voltage of 5 V. The deswelling of the hydrogels with various compositions could be observed by the naked eye when they were in contact with a pair of electrodes. The P (AMPS-co-AM) hydrogels with increasing contents of

Fig. 6. Deswelling kinetics of the swollen S0.55M0.45 hydrogel at different applied voltages. 5

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Fig. 7. Reswelling capabilities of the P (AMPS-co-AM) hydrogels when the applied voltage of 15 V was switched off.

Fig. 8. Water fluxes in the 24-h FO process by using 1.0 g hydrogel particles with different compositions as draw agents and 2000 ppm NaCl as the feed.

pronounced deswelling and a greater amount of water exudation, which agrees with the data shown in Fig. 7. After the removal of the electric stimuli, a volume expansion occurred, and the swelling ratios of the hydrogels were restored rapidly, which may be due to the relaxation phenomenon of the hydrogels [36,48]. It can also be seen that the reswelling rate of the S0.65M0.55 hydrogel with more ionic groups was higher than those of other samples. This result indicates that the incorporation of ionic groups not only enhance the deswelling efficiency, but also improve the reswelling capability of the hydrogels. Taken together, the P (AMPS-co-AM) hydrogels showed electroresponsiveness as shrinking and expanding reversibly with the on–off switching of the electric field, and thus, have the potential as recyclable water-adsorbing materials.

decreased due to the interaction with the electrode, which makes the water molecules migrating together with microcounter ions exit the hydrogel near the cathode. The schematic diagram of the electrically induced deswelling mechanism of the P (AMPS-co-AM) hydrogel in air was presented in Fig. S5. Note that the overall amounts of ions might not be significantly changed during the deswelling, as the amounts of H+ generation and consumption should be balanced at both electrodes [24]. In addition, as the PAMPS groups in the hydrogels were fullyionized, the observable change in volume associated with the local pH change near the electrodes might be not significant. To summarize, it can be inferred that the contraction near anode is associated with the electroosmosis and electrophoresis inside the hydrogel, and the concomitant water exudation near cathode in the air is attributed to the decrease in hydration power of microcounter ions due to the interaction with the electrodes by electrochemical reactions, particularly water electrolysis.

3.1.6. Electrically-induced deswelling mechanism To gain an in-depth understanding of the electrically-induced deswelling mechanism, the volume changes of P (AMPS-co-AM) hydrogels during the contraction process were monitored and presented in Fig. S4. When a high electric potential (15 V) was applied with a pair of electrodes passing through the sample, the rapid deswelling of P (AMPS-coAM) hydrogel in the air was observed by the naked eyes. For the first 25 s (Fig. S4), some parts of electrode surfaces were frequently covered with bubbles, which continuously detached themselves from the electrode surface. Besides, no contraction was observed in the cathode region but water droplets preferentially seep from the cathode side. After 5 min, the hydrogel near the anode made a pronounced contraction (Fig. S4), allowing us to push and display the electrode continuously to keep it contact with the hydrogel. This deswelling was propagated towards the cathode with time. It can be seen that the direct electric contact caused a extensive but anisotropic deswelling of P (AMPS-coAM) hydrogel in air. Similar results could be observed for other polyanion hydrogels such as poly (methacrylic acid) (PMAA), poly (acrylic acid) (PAA), and PAMPS [47]. In the case of the P (AMPS-co-AM) hydrogels, when a high external electric field was applied, the hydrated H+ ions (more exactly H3O+) migrated to the cathode along the electric field dragging water molecules with them while the macroions were stationary since they were chemically fixed to the polymer work [23,24]. The transport of water (electricosmosis) and the charged ions (electrophoresis) from anode to cathode in the crosslinked anionic polymer network should be accounted for the observation that the hydrogel shows significant contraction at the anode but almost no contraction at the cathode [47]. The hydrated H+ ions later were reduced at the electrode, releasing hydrogen gas. The electrochemical reactions may take place as follows [24].

3.2. FO water flux of hydrogels Fig. 8 shows the water fluxes of the hydrogels with different compositions induced by the swelling of dry hydrogel particles as a function of time in 24-h FO process. As the FO process proceeded, the water fluxes for all the hydrogels decreased gradually because the swelling pressure of the hydrogels decreased with an increase in the degree of swelling. The water flux increased with the increase in concentration of incorporated ionic groups in the hydrogels. For example, in the first 1 h, the water flux for S0.65M0.35 hydrogel with the highest charge density is 2.76 LMH, which is higher than those for S0.55M0.45 (2.30 LMH), and S0.45M0.55 (1.54 LMH). The higher water flux in the FO process by using S0.65M0.35 hydrogel can be attributed to the enhancement in swelling ratios (pressures) resulting from the incorporation of increasing ionic groups. This phenomenon can be also demonstrated by the equilibrium swelling ratios of different P (AMPS-co-AM) hydrogels as shown in Fig. 3. It is reported that the swelling of hydrogels is driven by the osmotic pressure originating from the dissociation of the ionic groups, and the solvation force generated by the hydrogen bonding interaction between the hydrogel network and H2O molecule [12]. In our case, the enhanced swelling pressure of the S0.65M0.35 hydrogel can be explained as follows: 1) the increase in AMPS groups will increase the ionization degree of the hydrogel, resulting in the improved osmotic pressure; 2) the AMPS groups are strongly negatively charged, and the presence of AMPS would expand the polymer chains due to the strong electrostatic repulsion [34]; and 3) more AMPS incorporated in the hydrogel makes the hydrogel more hydrophilic, which can contribute to water adsorption, thus increasing the swelling ratio of the hydrogel [12]. Thus, the strong ionic AMPS group plays a crucial role in improving the water flux of the hydrogel draw agent. However, as the mechanical properties of hydrogels are strongly affected by their degree of swelling [49,50], we choose the S0.55M0.45 hydrogel as draw agent

Anode H2 O→ 2H+ + 2e− + 1/2O2 Cathode 2H+ + 2e− → H2 Thus, the hydration power of microcounter ions (H3O+) is 6

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Table 2 Water contents in the hydrogles after 24-h FO process and corresponding water recovery fractions after 20 min and 40 min exposure to the voltage of 15 V. Characteristics

1st cycle

2nd cycle

3rd cycle

WR *R20 *R40

96.90 53.30 70.88

96.40 50.03 67.60

96.38 47.52 65.55

*R20 and *R40 represented the water recovery (R) for 20 min and 40 min in an electrically-induced dewatering process respectively.

dewatering study. After 24-h water adsorption, the water-swollen hydrogels were then exposed to the electric stimuli for 20 min and 40 min respectively, leading to a hydrogel shrinkage and consequent water release. The mass of the swollen hydrogels before and after the dewatering test were measured to calculate the water recovery using Eq. (7). As can be seen in Table 2, the amount of released water increased with the prolonging time of the electric stimuli. After 40 min exposure to electric stimuli for the first regeneration, the water recovery fraction of the P (AMPS-co-AM) hydrogels was round 71%, which appeared to be a little lower than that of swollen hydrogel/PUF composites (79%) heated under the simulated sunlight with an intensity of 2.0 kW m−2 for 90 min [1]. However, the recovered water from the hydrogels under the stimuli of heat was mostly in the form of water vapor and a condensation unit was then required to recover liquid water [3]. In addition, the regeneration time by using the convenient electric field for the recycling was relatively short. The water recovery fraction decreased slightly from 71% to 66% during the three recycling FO processes, demonstrating their promising water recovery performances. The similar trend can be observed elsewhere when the swollen AMPS/ DMAEMA hydrogels after FO were dewatered under 15 V electric field [26]. The decrease in the water recovery fraction may be ascribed the following reasons: (i) some NaCl salt may permeate through membrane and enter into hydrogel during the FO process, which may decrease the swelling performance of hydrogel resulting from the weakened or screened electrostatic repulsion in the hydrogels as a consequence of the attraction between salt ions and the groups with opposite charges; and (ii) there may be a little loss of the hydrogel draw agents during the transfer process of swollen hydrogels in a sticky state, since the FO process and regeneration process were operated separately for the production of clean water. The water flux tests were also carried out to confirm the reusability of the electro-responsive P (AMPS-co-AM) hydrogels. After each dewatering process, the dehydrated hydrogels were directly reused as the draw agent for the next FO cycle, employing the same 2000 ppm NaCl as the feed. Fig. 10 shows the time courses of the water fluxes for three cycles of water adsorbing-electric dewatering. The water fluxes at the

Fig. 9. Effect of hydrogel mass (S0.55M0.45) on time courses of the water flux in the 10-h FO process fed with 2000 ppm NaCl.

for the remaining FO experiments due to its balanced water absorbing capacity and mechanical toughness. To investigate the effect of mass of hydrogel draw agents on the water flux, P (AMPS-co-AM) hydrogels with different absolute mass were examined in the 10-h FO process. In Fig. 9, it is clear that the mass of the hydrogel (S0.55M0.45) significantly affected the water flux. Increasing the mass of the hydrogel resulted in an increase in the value of the water flux for the initial 1 h from 3.04 LMH for 1.5 g of the dry hydrogel particles to 1.73 LMH for 0.5 g of the sample hydrogel. This is likely because the addition of more hydrogel particles on the membrane surface will increase the degree of the membrane coverage. The similar tendency has been reported in the FO process using hydrogel copolymers of sodium acrylate and N-isopropylacrylamide [18]. However, further addition of the hydrogel particles (2.0 g) did not increase the water flux significantly. This may be due to the difficult water transport from the first layer of hydrogel particles contacted with the FO membrane to the other layers [1]. When the dry hydrogel particles were used as draw agent, the hydrogels contacted with the membrane were diluted immediately resulting in the external concentration polarization, as water became trapped in the hydrogel particles and was difficult to transport between different hydrogel particles [1,10]. Therefore, only the first layer of hydrogel particles contacted with the FO membrane could be effectively utilized, and the water flux would not be indefinitely enhanced with more hydrogel draw agent. In this work, a comparison on the performance of different polymer hydrogels as FO draw agents is also presented in Table S1. The water flux generated by electro-responsive P (AMPS-co-AM) hydrogels is relatively superior compared to most of the other reported hydrogel draw agents with few exceptions due to their unique properties as shown in Table S1. In addition, our hydrogel here produced a water flux of 2.76 LMH, higher than the recent result of electrically responsive AMPS/DMAEMA hydrogels [26]. A direct quantitative comparison between the water fluxes reported in different hydrogel draw agents is difficult because of the differences in the experimental protocols, such as initial swelling ratio of the hydrogel or hydrogel-membrane contact conditions. Nevertheless, its ability to produce high water flux from water sources with a medium salt concentration still reveals that the P (AMPS-co-AM) hydrogel developed in this study has a potential to be a viable alternative draw agent for low-energy FO desalination.

3.3. Electrically-induced dewatering of hydrogels after FO An electrically-induced dewatering process was carried out to recover clean water from the water-swollen hydrogels after FO, which were placed on the measuring apparatus (Fig. S1) and subjected to the electric stimuli. By considering the deswelling performances of P (AMPS-co-AM) hydrogels at different applied voltages as shown in Fig. 6, we have chosen the voltage of 15 V as the electric stimuli for the

Fig. 10. Water flux as a function of time produced by the fresh and recycled (1st, 2nd, and 3rd) hydrogels (S0.55M0.45). The initial mass of draw agent was 0.5 g, and 2000 ppm NaCl was used as the feed. 7

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doi.org/10.1016/j.memsci.2019.117406.

initial 1 h produced by the fresh and recycled (1st, 2nd, and 3rd) hydrogels were measured at 1.73, 1.18, 1.03 and 0.85 LMH, respectively. The reduction of water flux after the first regeneration was mainly due to the weight increase of the hydrogels in comparison to the water flux generated by fresh samples from completely dry state, since the water content of the swollen hydrogel after 40 min electric dewatering could not fully removed. Generally, the initial water flux at 1 h was decreased slightly throughout the subsequent cycles, which was associated with the little decrease of water recovery during the repeated electric dewatering tests. The cycles of water flux and water recovery illustrated in Table 2 and Fig. 10 confirm that the electro-responsive hydrogels are able to maintain their water flux and water recovery performances. This result indicated that the hydrogel draw agent in this work has the potential to be used in commercial and industrial applications for desalination.

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4. Conclusions In this study, an alternative method for the desalination of saline water by swelling and electrically deswelling hydrogels in FO processes is presented. We have successfully synthesized a series of polymeric hydrogels based on strong anions AMPS and common AM using the free radical copolymerization technique. The swelling ratios of hydrogels were found to be significantly dependent on the incorporation of strong anionic AMPS groups. From DSC experiments, the free water contents within the hydrogels were calculated, and were proved to have a dominant effect on the swelling ratio. The electric field applied in a contact system to the hydrogels caused the anisotropic shrinking at the anode and water seepage at the cathode, which may be induced by the electric osmosis in the hydrogels resulting from water electrolysis. The hydrogels’ desalination performances in terms of water flux and water recovery ability were evaluated in a bench-scale FO system. Results shows that a reasonable water flux of 2.76 LMH was achieved due to the high swelling pressure of hydrogels ascribed by the introduction of strong charged AMPS groups, when 1.0 g S0.65M0.35 (with a molar ratio of AMPS and AM = 0.65/0.35) was used as draw agent with 2000 ppm NaCl solution as a feed. The water flux could be improved by increasing the amount of hydrogel particles on the membrane surface but would not be significantly enhanced above a certain level. The electric field used in this work represents a much more convenient and effective method to achieve the dewatering of the hydrogel draw agent after FO; the water recovery fraction reached around 71% for 40 min. The water flux and water recovery rates after the first regeneration were not as high as those of the fresh hydrogels, because these hydrogels would not start from their dried state to adsorb water from saline water. However, the hydrogels remained functional after the third regeneration. This work warrants further studies and development focusing on the practical application of electro-responsive hydrogels to desalination in the FO process. Acknowledgements We would like to thank the National Natural Science Foundation of China (51608133, 51778155), the Natural Science Foundation of Guangdong Province (2017A030313310), the Guangzhou University's Training Program for Excellent New-recruited Doctors (YB201718), the Program for Guangzhou University Graduate Innovative Research (2018GDJC-M47), the Undergraduate Innovation Program of Guangzhou University (CX2017117), and the Foundation for Fostering the Scientific and Technical Innovation of Guangzhou University (LHY2-2608) for the financial support. We also thank the Analytical and Testing Center of Guangzhou University for related analysis. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// 8

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