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Performance evaluation of electric-responsive hydrogels as draw agent in forward osmosis desalination
Desalination 426 (2018) 118–126
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Performance evaluation of electric-responsive hydrogels as draw agent in forward osmosis desalination
MARK
Hongtao Cui, Hanmin Zhang⁎, Mingchuan Yu, Fenglin Yang Key Lab of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Forward osmosis AMPS/DMAEMA hydrogels Electric-responsive Dewatering
Forward osmosis (FO) has gained more attentions because it has the potential to be an emerging desalination technology. Stimuli-responsive hydrogels, as a novel class of FO draw agent, can completely avoid reverse solute diffusion and release water easily under external stimuli. In this study, 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and 2-(Dimethylamino)ethyl methacrylate (DMAEMA) monomers are used to prepare a series of electric-responsive AMPS/DMAEMA hydrogels by aqueous solution polymerization. These hydrogels are adopted as draw agent, by using 2000 ppm NaCl solution as the feed, A0.55D0.45 and A0.53D0.47 hydrogel show better initial water fluxes which are 2.09 and 1.63 LMH, separately. And A0.55D0.45 hydrogel produces 16.12 L·m− 2 total water flux and 0.67 average 24 h water flux. These hydrogels are dehydrated under 15 V electric stimuli. After the first regeneration, their corresponding water recovery rates are 67.45% and 39.36%, separately. After the second regeneration, their initial water fluxes are still recorded at 1.54 and 1.08 LMH, respectively. In contrast with other published literatures, the water flux produced by as-prepared hydrogels has increased significantly. Moreover, using an external electric field to achieve regeneration can simplify the operation of forward osmosis process.
1. Introduction Rapid population growth and global warming have caused an inhomogeneous supply distribution for available water resource. And the shortage of fresh water has become an important global issue that seriously affects the survival of mankind. Membrane technology for desalination, such as reverse osmosis (RO) [1], nanofiltration (NF) [2] and membrane distillation (MD) [3] and forward osmosis (FO) [4], had a potential advantage to relieve the crisis of freshwater scarcity. FO depends on the osmotic pressure difference across the semi-permeable membrane as the driving force and operates at no or very low hydraulic pressure [5], which leads to decreasing energy consumption of drawing water [6]. Besides, it had higher water recovery, and may have lower fouling propensity or fouling that was more reversible than in RO processes [7,8]. And this membrane technology requires low influent water quality and simplifies the pretreatment process of wastewater, so it has attracted wide attentions around the world. At present, the most widely used draw solutes are inorganic salts like NaCl, which can obtain
higher water flux [9,10]. However, the reverse draw solute diffusion not only result in contaminating the feed solution and reducing the osmotic pressure difference, but also the separation and regeneration of water need high pressure RO process. Therefore, although the membrane develops quickly in FO desalination, preparing and selecting a suitable draw solute, which has some characteristics including high osmotic pressure, lower reverse solute diffusion, low-cost regeneration, non-toxicity and safety, is still the focus for FO technology. Many researchers have done a lot of researches and attempts on the draw solute. McCutcheon et al. [11] firstly presented ammonium bicarbonate (NH4HCO3) as draw solute in FO desalination due to its high osmotic pressure and concluded that it produced higher water flux and needed thermal decomposition at 60 °C to regenerate. Although the moderate thermal regeneration consumed less energy, the reverse diffusion of NH4HCO3 was much higher than that of NaCl and alkaline NH4HCO3 would affect the service life of FO membrane. Magnetic nanoparticles as draw solute have attracted much attention due to their high osmotic pressures, no reverse solute osmosis and facile recovery,
Abbreviations: AMPS, 2-Acrylamido-2-methyl-1-propanesulfonic acid; DMAEMA, 2-(Dimethylamino)ethyl methacrylate; FT-IR, fourier transform infrared spectroscopy; SEM, scanning electron microscopy; DSC, differential scanning calorimetry; SR, swelling ratio; RO, reverse osmosis; NF, nanofiltration; MD, membrane distillation; FO, forward osmosis; FS, feed solution; DS, draw solution; NaCl, sodium chloride; PAA-Na, polyacrylic acid sodium salts; MBAAm, N,N′-methylenebisacrylamide; APS, ammonium persulfate; NaHSO3, sodium bisulfate; W, water content; R, water recovery rate; GO, graphene oxide; NIPAm, N-isopropylacrylamide; PSA, poly(sodium acrylate); PVA, poly(vinyl alcohol) ⁎ Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.desal.2017.10.045 Received 28 July 2017; Received in revised form 12 October 2017; Accepted 26 October 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.
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were purchased from the Aladdin (Shanghai). N,N′-methylenebisacrylamide (MBAAm, ≥99.0%) was provided by the Tianjin Kermel Chemical Reagent Co. Ltd. (Tianjin). Sodium chloride (NaCl, ≥99.5%) and sodium bisulfate (NaHSO3) were acquired from Tianjin DaMao Chemical Reagent Factory (Tianjin). Ammonium persulfate (APS) was obtained from the Tianjin GuangFu Technology Development Co. Ltd. (Tianjin). APS and NaHSO3 were formulated as 8 wt% and 4 wt% aqueous solution before use. All the chemicals were used as received without further purification. The dialysis bag (MWCO = 14,000) used for swelling study was purchased from Sigma. CTA-ES membrane made from cellulose triacetate (CTA) with an embedded polyester screen mesh was obtained from Hydration Technologies Inc. (Albany, OR).
but the conglomerations of nanoparticles and nanoparticles residues in produced water would still limit the practical application [12]. Ge et al. [13] introduced polyelectrolytes of a series of polyacrylic acid sodium salts as draw solutes for FO process in 2012, and their reverse leakages were much lower. Nevertheless, in order to obtain a higher water flux, these polyelectrolytes needed increasing their concentration, which may exacerbate membrane fouling and concentration polarization. At the end of 2014, Guo et al. [14] proposed that 0.5 g/mL Na+-functionalized carbon quantum dots could produce the osmotic pressure of 153.6 atm, which was appropriate to become the draw solute for FO. But, it required membrane distillation to concentrate when it was diluted, so the problem for complex regeneration process and higher energy consumption still existed. Hydrogel was a three dimensional network of hydrophilic polymer which was crosslinked by either physical or chemical bonds, and could contain a large amount of water when maintaining the structure [15]. In 2011, it was the first time for Li et al. [16] to report that temperatureresponsive polymer hydrogels were explored as draw agent in FO desalination, followed by light-responsive hydrogels [17], pressure-responsive hydrogels [18] and other kinds of stimuli-responsive hydrogels as draw agent. These kinds of draw agent extracts water molecules from the feed solution via the osmotic pressure, then these swollen hydrogels were dehydrated under the light, heat or pressure to obtain the fresh water [19], so these hydrogels can completely avoid the reverse osmosis. It was reported that weakly crosslinked poly(acrylic acid)/poly(sodium acrylate) copolymer hydrogels with the volume fraction between 0.03 and 0.3 exhibited a swelling pressure ranging from 0.20 to 4.23 MPa [20]. Razmjou et al. [17] designed bifunctional polymer hydrogels layers as a water-absorptive layer and dewatering layer to become the FO draw agents in order to realize the continuous production of fresh water. However, compared to other stimuli-responsive polymer hydrogels, electric-responsive polymer hydrogels are more convenient to apply and control [21]. Hydrogels sensitive to electric current are usually made of polyelectrolytes. And electric-responsive hydrogels undergo shrinking or swelling in the presence of an applied electric field. In 1965, it was first reported that the deformation of polyelectrolyte hydrogels could be observed under the stimuli of an electric field [22]. Electric-responsive hydrogels have shown merits in some fields such as micro-mechanics, flexible actuators and artificial muscle tissues when comparing to other rigid materials. So, our research group have done research on preparing electric-responsive hydrogels as FO draw agent, and simplified the regeneration process by applying electric field which is easy to control and operate [23], but the dewatering rate under electric-stimuli is not carried on. In this work, the monomer of 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and 2-(Dimethylamino)ethyl methacrylate (DMAEMA), N,N′-methylenebisacrylamide as a cross-linker, ammonium persulfate and sodium bisulfite as redox initiator were employed to prepare AMPS/DMAEMA hydrogels with a range of different monomer ratios by aqueous solution polymerization. AMPS monomer has sulfonic group with high charge density, which is easy to dissociate in solution to become negatively sulfonate ion. And DMAEMA monomer has alkaline tertiary amine groups which can be protonated in acidic medium to become positive ion. Increasing number of ions can enhance the swelling pressure, and high hydrophilicity and conductivity can be helpful to improve the water flux and regeneration ratio under electric stimuli. More importantly, the research on the electric-responsive hydrogels for FO desalination is quite little.
2.2. Preparation of xerogel particles draw agent AMPS and DMAEMA monomer were polymerized via aqueous solution polymerization to produce the AMPS/DMAEMA hydrogels. First of all, a desired amount of AMPS, DMAEMA and MBAAm were put in a beaker with 30 mL deionized water added. And it was thoroughly dissolved by using a magnetic stirrer for several minutes at room temperature to form a colorless and transparent solution. Secondly, a certain amount of APS and NaHSO3 solutions (molar ratio = 1:1) were added to the above solution with the molar ratio of monomers, crosslinker and initiator fixed at 100:1:1. The concentration of monomers was 1 mol/L. Subsequently, it was mixed completely by a magnetic stirrer. Finally, it was transferred to the sealed centrifuge tube with a cover, which was put in 50 °C water bath for 5 h to finish polymerization. After polymerization, the centrifuge tube was taken out and cooled to room temperature. Then, columnar hydrogel was removed and soaked into deionized water for three days which was changed every day to remove the unreacted monomers and linear polymer of low molecular weight. The molar ratio of AMPS and DMAEMA was 0.5/ 0.5, 0.53/0.47 and 0.55/0.45, and as-prepared hydrogels were denoted as A0.5D0.5, A0.53D0.47 and A0.55D0.45, separately. Swollen hydrogels were dried at 60 °C in a convection oven and samples of hydrogel particle size (1–2 mm) were made by grinding dried hydrogels and sieving them. Consequently, the xerogel particles were treated as draw agent in FO desalination. Only the amount of cross-linker changed, so the molar ratio of monomers, cross-linker and initiator fixed at 100:2:1. According to the above preparation process, the hydrogels fabricated were labeled as A0.5M2, A0.53M2 and A0.55M2, separately. The photos for different hydrogels were shown in Fig. 1a. A0.5D0.5 is white and columnar hydrogel that is very soft and has good elasticity, while A0.53D0.47 and A0.55D0.45 become colorless, transparent and hard with the increasing of hydrophilic sulfonic acid groups. However, A0.5M2 columnar hydrogel is pale white, less soft and much harder than the A0.5D0.5. A0.53M2 and A0.55M2 hydrogel become colorless and transparent because of the more hydrophilic sulfonic acid groups. Overall, the intensity of the latter is better than that of the former, which is attributed to the increasing amount of cross-linker. In the polymerization, ionic complex with two double bonds was formed by proton transferring reaction between AMPS and DMAEMA firstly. This complex bonded mutually because of the electrostatic force between anion and cation in order to produce a similar polymerized monomer of organic salt structure [24]. Then, ionically crosslinked AMPS/DMAEMA hydrogel was fabricated by free radical polymerization, and its protonated process and the structure of AMPS/DMAEMA hydrogel were exhibited in Fig. 1b.
2. Experimental section 2.3. Characterization 2.1. Chemicals and materials Swelling behaviors of different xerogels particles were investigated by the gravimetric method. The water uptake of different xerogels particles was determined as follows. 0.3 g different particles were put
Both monomers 2-(Dimethylamino)ethyl methacrylate (DMAEMA, 99%) and 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS, 98%)
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Fig. 1. Photos of prepared hydrogels after polymerization (a) and a schematic illustration of the protonation and molecular structure of AMPS/DMAEMA hydrogel (b) (1#: A0.5D0.5, 2#: A0.53D0.47, 3#: A0.55D0.45, 4#: A0.5M2, 5#: A0.53M2, 6#: A0.55M2.)
apparatus (Fig. 2), the membrane was immersed in deionized water for > 12 h to maintain hydration before use. The weight of FS was monitored by an electronic scale (JSC-QHC-3 +) and the data was recorded by a computer software. Water flux (F, L/m2·h (LMH)) is determined by measuring the weight change of FS with time and calculated from the Eq. (2):
into a dialysis bag (MWCO = 14,000), which was sealed and placed in 1 L deionized water at room temperature. Deionized water was replaced every day and the weight of swollen polymer hydrogels was measured after three days of immersion in deionized water. The swelling ratio (SR) of polymer hydrogels was calculated by the formula (1) below:
SR =
Ws − Wd Wd
F = V (A × t )
(1)
where Ws is the weight of swollen polymer hydrogel after three days (g); Wd is the weight of initial xerogels particles before test (g). And SR was the average value of triplicate measurements. Fourier transform infrared spectroscopy (FT-IR) (EQUINOX55, Germany) was used to confirm the chemical structure of samples. The morphologies of the as-prepared products were determined at 20 kV by using QUANTA 450 (USA) scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) (NETZSCH DSC 204, Germany) was employed to examine the water state in the swollen hydrogels. Swollen hydrogels were cut into the small pieces. And its surface water was wiped softly. Then, sample sealed in aluminum pan was cooled to − 25 °C and then heated to 25 °C at a heating rate of 3 °C/min under 30 mL/min of nitrogen gas flow.
(2)
where V is the volume of water permeating through the membrane (L) over a given period of time, t (h); A is the effective membrane area in the module (m2). The water flux was measured for three times under the same experimental conditions. And it was the average value of triplicate measurements. Water content (W) is an important parameter for assessing the performance of dewatering [25]. So, after 24 h FO process, water content of xerogel particles is calculated as follows:
W=
Wm − Wd × 100% Wm
(3)
where Wm is the weight of swollen xerogel particles after 24 h FO test; Wd is the weight of initial xerogel particles (2 g) applied in the FO test. In an electric stimuli dewatering process, different swollen hydrogels were placed on the measuring apparatus (Fig. 9) for dewatering. So, water recovery rate (R) is calculated by the formula (4) below:
2.4. FO performances evaluation and electric-stimuli dewatering The water flux for different xerogel particles was evaluated by using cross-flow FO membrane module with an effective membrane area of 12.56 cm2 at room temperature. 2000 ppm NaCl solution was utilized as FS, and 2 g different xerogel particles were studied as DS in FO tests. Namely, 2 g different xerogel particles were placed on the active layer of FO membrane to assess the DS performances. The FS was pumped by a peristaltic pump at a volumetric flow rate of 167.6 mL/min (a cross flow velocity of 5.56 cm/s). Before each test in our customized FO
R = W1 W0 × 100%
(4)
where W1 is the weight of water released during the electric stimuli dewatering test (g); and W0 is the weight of absorbed water in the xerogel particles during 24 h FO process (g).
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Fig. 2. Structure diagrams for homemade FO apparatus.
Fig. 4. FT-IR spectra of AMPS, DMAEMA monomer and three different hydrogels.
Fig. 3. Swelling ratio histograms for different xerogel particles (Error bars represent one standard deviation at triplicate measurements.)
suggesting that SR increases with the increasing amount of hydrophilic sulfonic acid group. The literature [16] reported early that hydrogels were able to entrap large volumes of water attracted by the high concentration of hydrophilic groups, which is consistent with the SR result. The hydrophilicity of prepared hydrogels enhances due to the increment of sulfonic groups.
3. Results and discussion 3.1. Swelling property Gravimetric method was used to examine the swelling property of different xerogel particles, as shown in Fig. 3. Swelling ratio of polymer hydrogels could be considered as a function of their chemical and structure characteristics such as ionic strength, crosslinking density, the thickness of the polymer network and the relaxation rate [26]. Results show that cross-linker of higher content leads to lower SR under the same monomer ratio. The polymer chains would be further cross-linked due to the increasing content of cross-linker, and the polymer networks become more compact. Consequently, it is difficult for water molecules to penetrate into the polymer networks, resulting in low SR. The SR order of the former is A0.5D0.5 < A0.53D0.47 < A0.55D0.45, and the SR order of the latter is A0.5M2 < A0.53M2 < A0.55M2,
3.2. Characterization 3.2.1. FT-IR analysis Swollen hydrogels were dried at 60 °C in a convection oven and were ground into the powder with a morta for characterization. FT-IR analysis is carried out to identify functional groups within prepared hydrogels in order to confirm the successful copolymerization. As shown in Fig. 4, typical peaks corresponding to α and β unsaturated ester in the DMAEMA are at the wavenumber around 1725 and 1165 cm− 1, which are attributed to the carbonyl stretching (υC]O) and
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Fig. 5. SEM images of six different swelling hydrogels after freeze drying method.
the stretching vibration of CeO bond (υCeO) separately. And the band around 1637 cm− 1 is assigned to stretching vibration of unsaturated double bonds (υC]C). Typical peaks at the bands around 1616 and 1675 cm− 1 for AMPS are corresponding to C]O stretching vibration of amide (υC]O) and stretching vibration of unsaturated C]C (υC]C) respectively. For composite hydrogels, the stretching vibration of ester group for DMAEMA (υC]O = 1725 cm− 1) and stretching vibration of amide for AMPS (υC]O = 1675 cm− 1) still exist, but stretching vibration of unsaturated double bond does not exist, which indicates that polymerization reaction occurs. The broad peak at 3455 cm− 1 can be predicted to be intermolecular hydrogen bond formed by the polymer. At the same time, eHO3S group and tertiary amide interacted to cause ionic bonds, and the peak at band around 2990 cm− 1 transfers to broad peak at around 3455 cm− 1. So, it confirms that ionic complex which has similar organic salts structure forms due to the proton transferring reaction between AMPS and DMAEMA. Besides, appearance of the wavenumber at 1043 cm− 1 for the A0.5D0.5, A0.55D0.45, A0.5 M2 spectrums that are assigned to the symmetric stretching of the S]O in the sulfonic acid group verifies the successful synthesis of the prepared hydrogels with sulfonic acid functional group. The appearance of these peaks proves the successful incorporation of comonomer into AMPS/ DMAEMA hydrogel which has complete polymer networks. 3.2.2. SEM measurement Six different hydrogels in a swelling equilibrium state were taken out from the deionized water, and these hydrogels were in rapid quenching cold by using liquid nitrogen, then they were placed in the freeze dryer for 18 h. After freeze drying, these hydrogels showed a white and fluffy solid. Subsequently, these hydrogels were cut, and their surfaces were sputtered by Au to observe the morphology using SEM. So, the SEM images were shown in Fig. 5. As we can see from Fig. 5, six different hydrogels have obviously porous structure. With the increasing content of hydrophilic groups, the porous cavity for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel becomes more neat and uniform. However, in contrast with A0.5D0.5 hydrogel, A0.5M2 hydrogel has smaller porous structure due to an
Fig. 6. DSC thermograms for different swollen hydrogels.
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active layer of CTA membrane, with the support layer of CTA membrane facing the feed solution. 2000 ppm NaCl solution was chosen as the feed solution. In hydrogel-driven FO process, CTA membrane was initially wetted because it was soaked in deionized water for a period time. And xerogel particles were in good contact with the membrane surface, and swelled with water, thus producing a swelling pressure. Water molecule was transported from one particle to another particle by the swelling force, resulting in a continuous flow of water through the membrane. And this was very similar to the mechanism of the commercial hydration bag where sugar or beverage powder was used as the draw agent [28]. Fig. 7 presents the water fluxes in a 24 h FO process using different xerogel particles as draw agent. As shown in Fig. 7, the initial water flux for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel are 0.81, 1.63 and 2.09 LMH, separately. And the initial water flux for A0.5M2, A0.53M2 and A0.55M2 hydrogel are 0.80, 0.96 and 1.11 LMH respectively. The initial water flux decreases when the amount of cross-linker becomes higher under the same monomer ratio, which is consistent with the result of swelling ratio. So, A0.55D0.45 hydrogel shows best performance among the hydrogels in FO desalination process. In the first 9 h, the water flux decreases quickly, and then it changed slowly. It is inferred that the swelling pressure of polymer hydrogels reduces with the increasing their degree of swelling when the FO process proceeds, resulting in the observed decrease in water flux. Li et al. [25] explored composite hydrogels (poly(sodium acrylate)‑carbon) as draw agent in FO desalination process, the water flux for poly(sodium acrylate)‑carbon was 1.32 LMH in the first 0.5 h when 2000 ppm NaCl was applied as the feed solution. The initial water flux for A0.53D0.47 and A0.55D0.45 hydrogel is 23.48% and 58.33% higher than that of poly(sodium acrylate)‑carbon draw agent. Besides, thermal-responsive semi-interpenetrating hydrogels of NIPAm-0.5PVA was synthesized by polymerization of N-isopropylacrylamide (NIPAm) in the presence of poly(vinyl alcohol) (PVA), which was considered as draw agent toward temperature driven forward osmosis desalination. But, the initial water fluxes were < 0.30 LMH [29]. Although the dewatering flux of bifunctional hydrogels could reach 10 LMH under solar intensity of 0.5 kW/m2, its initial water flux was only 0.32 LMH when the membrane area was 1.77 cm2 and the feed solution was 2000 ppm NaCl solution [17]. Xerogel particles have higher degree of interstitial volume, which can hold more water, resulting in a high degree of swelling. Increasing the swelling pressure of prepared hydrogels is helpful to improve the water flux. Besides, the good hydrophilicity of prepared hydrogels increases the water flux.
increase content of cross-linker. And the porous structure for A0.53M2 and A0.55M2 hydrogel has collapsed and tangled compared to the former hydrogels, which decreases the regularity of porous structure. The increasing cross-linker content leads to enhancing the cross-linked dot, which makes the network structure become more compact, resulting in the difficult of the water molecules into the porous structures. Larger porous structure provides easy access to internal hydrogels for water molecules. More porous structures are helpful to increase absorbing capacity, which agrees well with the result of swelling ratio. 3.2.3. DSC analysis Water state in the swollen hydrogels can be divided into two kinds, free water and bound water. To further elucidate the swelling behavior of different hydrogels, water states are investigated by using DSC. DSC curves for different hydrogels are shown in Fig. 6. Since the presence of water in the hydrogel is different, Fig. 6 starts to show a significant enthalpy change near 0 °C. However, the temperature of enthalpy becomes larger due to the increasing content of AMPS. As shown in Fig. 6, six different hydrogels have an obvious endothermic peak, which is similar to the DSC curve of ice melting in the vicinity of 0 °C. Namely, these endothermic peaks can be the melting peak of ice. It is concluded that the phase transition absorption peak from ice to liquid water comes from the bound water of six hydrogels. Hydrophilic group like eHSO3 on the polymer and water molecules mutually combine to obtain bound water through the solvation, coordination bonds and hydrogen bonds in internal and external surface, which is similar to the chemical adsorption water. The increasing ratio of AMPS monomer is beneficial to the chemical adsorption water. The literature [27] pointed out that the higher the temperature corresponding to the endothermic peak was, the stronger the interaction force between polymer and water molecules was. The order of the temperature corresponding to endothermic peak for six different swollen hydrogels is A0.55D0.45 > A0.55M2 > A0.53M2 > A0.53D0.47 > A0.5M2 > A0.5D0.5, and interaction force of A0.55D0.45 hydrogel is the strongest among them. The temperature rises with the increasing content of sulfonic acid group, indicating that the introduction of sulfonic acid group enhances the ability of binding with water molecule for hydrogels. 3.3. Water flux in the FO process Different xerogel particles were further used as draw agent due to their porous structure and hydrophilic sulfonic group. The setup for the FO test was shown in Fig. 2. Different xerogel particles were put on the
Fig. 7. Water flux as a function of time for different xerogel particles in FO desalination (FS: 2000 ppm NaCl solution; DS: 2 g xerogel particles.)
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and got the results that the average 24 h water flux obtained was 0.55 LMH for poly(ethylene glycol-[DL-lactic acid-co-glycolic acid]-bethylene glycol)/GO-0.09 wt% draw agent by using 2000 ppm NaCl as feed solutions. The average 24 h water flux acquired in our study is 0.67, 0.60 and 0.59 LMH for A0.55D0.45, A0.53D0.47 and A0.55 M2 hydrogel separately when 2000 ppm NaCl was used as FS, which were higher than that of poly(ethylene glycol-[DL-lactic acid-co-glycolic acid]-b-ethylene glycol)/GO-0.09 wt% draw agent. And our result increased by 21.82%, 9.09% and 7.27% respectively for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel. Based on the above results, A0.55D0.45, A053D0.47 and A0.55M2 hydrogel are selected for the electric-responsive dewatering. 3.4. Electric-stimuli dewatering One important characteristic of hydrogels draw agent is that it can totally avoid the reverse solute diffusion; another important characteristic of them is that it can be easily regenerated. And the regeneration for three different hydrogels is carried out under electric stimuli. The mass maintenance rate of poly(AMPS-co-DMAEMA) hydrogel was measured at different voltages by Liao et al., and the maximum amount of water was released under 15 V voltage [31]. Therefore, the voltage of 15 V is chosen as electric-stimuli for the regeneration. The electric-stimuli effect on the dewatering of electricresponsive hydrogels used as the draw agent in the FO process is shown in Fig. 9. After 24 h FO desalination process is finished, the swollen hydrogels are weighed and their water contents are calculated by Eq. (3), then these hydrogels are dehydrated for 30 and 60 min under 15 V electric field respectively, dehydrated hydrogels are weighed and their water recovery rates are calculated by Eq. (4), as shown in Table 1. And the water recovery rate after first regeneration is 39.36%, 67.45% and 45.74% respectively for A0.53D047, A0.55D0.45 and A0.55M2 hydrogel under 60 min electric-stimuli. By two electric-stimuli dewatering tests, the water recovery rate of high content sulfonic acid hydrogel is high. The reason was that the more content of sulfonic acid group was, the more sensitive was subjected to the electrical stimulation [31]. When the electric field was applied, the different charged ions would
Fig. 8. Histograms of total water flux and average 24 h water flux for different xerogel particles (FS: 2000 ppm NaCl solution; DS: 2 g xerogel particles.)
The total water flux (L·m− 2) means the total water mass extracted by hydrogels in per unit area at test time (24 h). The total water flux and average 24 h water flux are calculated for different hydrogels, as shown in Fig. 8. The total water flux for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel is 10.25, 14.30 and 16.12 L·m− 2, separately. And the total water flux for A0.5M2, A0.53M2 and A0.55M2 is 9.50, 12.83 and 14.12 L·m− 2, respectively. The total water flux reduces when the amount of cross-linker is higher under the same monomer ratio. This agrees well with the result of swelling ratio. Average 24 h water flux obtained is 0.43, 0.60 and 0.67 LMH separately for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel. And average 24 h water flux for A0.5 M2, A0.53 M2 and A0.55 M2 hydrogel is 0.4, 0.53 and 0.59 LMH, respectively. Nakka et al. [30] studied triblock copolymer hydrogels with graphene oxide (GO) incorporation as draw agents in FO process,
Fig. 9. Schematic diagram of the electric-stimuli effect on the dewatering of electric-responsive hydrogels used as draw agent in the FO process.
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the initial water fluxes for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel decrease. After first regeneration, the initial water fluxes for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel are recorded at 1.76, 1.19 and 0.95 LMH, which reduce by 15.79%, 26.99% and 14.41% separately comparing with xerogel particles as draw agent in FO desalination. And after second regeneration, the initial water fluxes for them are 1.54, 1.08 and 0.86 LMH, which are 26.32%, 33.74% and 22.52% lower than that of xerogel particles as draw agent. The initial water flux for A0.53D0.47 hydrogel decreases more, which is related to the low water recovery under electric-stimuli dewatering test. However, the initial water flux for A0.55D0.45 is still higher than that of poly(sodium acrylate)‑carbon [25]. It is attributed to higher content hydrophilic sulfonic groups and existing porous structure in A0.55D0.45 hydrogel. In brief, using electric-responsive hydrogels as draw agent can eliminate reverse solute leakage and reduce the complexity of the operation in FO process. There is great potential for application in the food and pharmaceutical field which are not allowed to be contaminated for the feed solutions because it can completely avoid the reverse solute diffusion.
Table 1 Water content (W) and water recovery rate (R) for different hydrogels in FO process.
W (%) ⁎ R30 min (%, first regeneration) ⁎ R60 min (%) ⁎ R30 min (%, second regeneration) ⁎ R60 min (%)
A0.53D0.47
A0.55D0.45
A0.55M2
57.85% ± 1.06 20.72% ± 1.93
73.48% ± 3.46 30.25% ± 2.41
50% ± 2.19 22.84% ± 3.02
39.36 ± 2.08 17.06% ± 2.57
67.45% ± 3.29 26.5% ± 3.14
45.74% ± 2.97 20.18% ± 3.36
32.19% ± 2.86
59.06% ± 3.79
40.73% ± 2.92
⁎ R30 min, ⁎R60 min represented the water recovery rate (R) for 30 min and 60 min in an electric-stimuli dewatering process, separately.
move in the hydrogel networks, the charges in the polyionic groups would be shielded due to the electrostatic attraction. The number of polyions reduced, which would decrease the ability of holding water for hydrogel, so the water could seep from the hydrogel networks. It can be observed from Table 1 that, the longer time for dehydration is, the more water releases. So, the water recovery rate of A0.55D0.45 hydrogel exhibits much better than that of the other two hydrogels when the electric field is applied at longer time. Compared to heating dewatering process for 1.5 h [32], applying an external electric field is easy to control and the regeneration time is slightly short. As AMPS/DMAEMA hydrogels are electric-responsive hydrogels, they will shrink and release some water when electric field is applied. Kim et al. [33] reported that electric field caused the migration of the positively charged ion in the hydrogel toward the cathode, which resulted in partial shielding the carboxylate groups reducing the extent of hydration of the gel. The counterions of the polyions in the hydrogel and the free ions in the solution moved toward their counter-electrodes, which resulted in an ionic gradient along the direction of the electric field. So, this difference in ionic concentration induced an osmotic pressure difference that caused volume change of hydrogels in an electric field [34]. Due to the competing between ions and free counterions, ionic groups in the hydrogels were helpful to induce the osmotic pressure which could draw more water. When deswollen or dehydrated hydrogels absorbed water again, the chain of polymer networks could stretch, thus producing the osmotic pressure. The water flux as a function of time is measured after two regenerations under 15 V for three hydrogels, and it is exhibited in Fig. 10. By using 2000 ppm NaCl as the feed,
4. Conclusions Aqueous solution polymerization was used to fabricate a range of electric-responsive AMPS/DMAEMA hydrogels. For these hydrogels, their properties and structural conformations were obtained from swelling ratio, FT-IR analysis, SEM measurement and DSC analysis. Due to hydrophilic groups and porous structure, different AMPS/DMAEMA hydrogels were employed as the draw agent in FO process when 2000 ppm NaCl solution was used as FS. And their dewatering process was evaluated. The major conclusions drawn from this study are as follows.
• The swelling ratios for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel are 20.07, 19.78 and 16.91, respectively. • By using 2000 ppm NaCl solution as the feed, the initial water fluxes •
for A0.55D0.45 and A0.53D0.47 hydrogel are recorded at 2.09 and 1.63 LMH, separately. After the second regeneration, water recovery rates for A0.55D0.45 and A0.53D0.47 hydrogel are 59.06% and 32.19%, respectively. And their initial water fluxes are still recorded at 1.54 and 1.08 LMH. Electric field used in this work represents a much easier and more effective method to achieve dewatering of the particles.
Fig. 10. Water flux as a function of time for different hydrogels after 60 min dewatering under electric-stimuli.
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energy, Environ. Sci. Technol. 47 (2013) 13160–13166. [18] A. Razmjou, G.P. Simon, H. Wang, Effect of particle size on the performance of forward osmosis desalination by stimuli-responsive polymer hydrogels as a draw agent, Chem. Eng. J. 215 (2013) 913–920. [19] H. Wang, J. Wei, G.P. Simon, Response to osmotic pressure versus swelling pressure: comment on “bifunctional polymer hydrogel layers as forward osmosis draw agents for continuous production of fresh water using solar energy”, Environ. Sci. Technol. 48 (2014) 4214–4215. [20] H. Wack, M. Ulbricht, Effect of synthesis composition on the swelling pressure of polymeric hydrogels, Polymer 50 (2009) 2075–2080. [21] S.Y. Kim, H.S. Shin, Y.M. Lee, C.N. Jeong, Properties of electroresponsive poly (vinyl alcohol)/poly (acrylic acid) IPN hydrogels under an electric stimulus, J. Appl. Polym. Sci. 73 (1999) 1675–1683. [22] R.P. Hamlen, C.E. Kent, S.N. Shafer, Electrolytically activated contractile polymer, Nature 206 (1965) 1149–1150. [23] H. Zhang, J. Li, H. Cui, H. Li, F. Yang, Forward osmosis using electric-responsive polymer hydrogels as draw agents: influence of freezing–thawing cycles, voltage, feed solutions on process performance, Chem. Eng. J. 259 (2015) 814–819. [24] Y. Zhao, W. Chen, J. Xia, W. Liu, Y. Yang, H. Xu, X. Yang, Erosion behavior of ionically crosslinked polyampholyte gels under DC electric fields, Macromol. Chem. Phys. 207 (2006) 1674–1679. [25] D. Li, X. Zhang, J. Yao, Y. Zeng, G.P. Simon, H. Wang, Composite polymer hydrogels as draw agents in forward osmosis and solar dewatering, Soft Matter 7 (2011) 10048–10056. [26] A. Razmjou, M.R. Barati, G.P. Simon, K. Suzuki, H. Wang, Fast deswelling of nanocomposite polymer hydrogels via magnetic field-induced heating for emerging FO desalination, Environ. Sci. Technol. 47 (2013) 6297–6305. [27] H. Feil, H.B. You, J. Feijen, S.W. Kim, Effect of comonomer hydrophilicity and ionization on the lower critical solution temperature of N-isopropylacrylamide copolymers, Macromolecules 26 (1993) 2496–2500. [28] T.Y. Cath, A.E. Childress, M. Elimelech, Forward osmosis: principles, applications, and recent developments, J. Membr. Sci. 281 (2006) 70–87. [29] Y. Cai, W. Shen, S.L. Loo, W.B. Krantz, R. Wang, A.G. Fane, X. Hu, Towards temperature driven forward osmosis desalination using semi-IPN hydrogels as reversible draw agents, Water Res. 47 (2013) 3773–3781. [30] R. Nakka, A.A. Mungray, Biodegradable and biocompatible temperature sensitive triblock copolymer hydrogels as draw agents for forward osmosis, Sep. Purif. Technol. 168 (2016) 83–92. [31] L. Liao, Z. Liu, H. Yue, Y. Cui, Study on the sensitive property of poly(AMPS-coDMAEMA) hydrogel under electric stimulation, Mod. Appl. Sci. (2009) 55–59. [32] J. Wei, Z.X. Low, R. Ou, G.P. Simon, H. Wang, Hydrogel-polyurethane interpenetrating network material as an advanced draw agent for forward osmosis process, Water Res. 96 (2016) 292–298. [33] S.J. Kim, K.L. Chang, Y.M. Lee, I.Y. Kim, I.K. Sun, Electrical/pH-sensitive swelling behavior of polyelectrolyte hydrogels prepared with hyaluronic acid–poly(vinyl alcohol) interpenetrating polymer networks, React. Funct. Polym. 55 (2003) 291–298. [34] J. Shang, Z. Shao, X. Chen, Electrical behavior of a natural polyelectrolyte hydrogel: chitosan/carboxymethylcellulose hydrogel, Biomacromolecules 9 (2008) 1208–1213.
Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 51278079). References [1] D. Li, H. Wang, Recent developments in reverse osmosis desalination membranes, J. Mater. Chem. 20 (2010) 4551–4566. [2] B.V.D. Bruggen, M. Mänttäri, M. Nyström, Drawbacks of applying nanofiltration and how to avoid them: a review, Sep. Purif. Technol. 63 (2008) 251–263. [3] A. Razmjou, E. Arifin, G. Dong, J. Mansouri, V. Chen, Superhydrophobic modification of TiO2 nanocomposite PVDF membranes for applications in membrane distillation, J. Membr. Sci. 415–416 (2012) 850–863. [4] S. Zhao, L. Zou, C.Y. Tang, D. Mulcahy, Recent developments in forward osmosis: opportunities and challenges, J. Membr. Sci. 396 (2012) 1–21. [5] A. Achilli, T.Y. Cath, E.A. Marchand, A.E. Childress, The forward osmosis membrane bioreactor: a low fouling alternative to MBR processes, Desalination 239 (2009) 10–21. [6] C. Klaysom, T.Y. Cath, T. Depuydt, I.F. Vankelecom, Forward and pressure retarded osmosis: potential solutions for global challenges in energy and water supply, Chem. Soc. Rev. 42 (2013) 6959–6989. [7] R.W. Holloway, A.E. Childress, K.E. Dennett, T.Y. Cath, Forward osmosis for concentration of anaerobic digester centrate, Water Res. 41 (2007) 4005–4014. [8] B. Mi, M. Elimelech, Organic fouling of forward osmosis membranes: fouling reversibility and cleaning without chemical reagents, J. Membr. Sci. 348 (2010) 337–345. [9] Q. Ge, M. Ling, T.S. Chung, Draw solutions for forward osmosis processes: developments, challenges, and prospects for the future, J. Membr. Sci. 442 (2013) 225–237. [10] A. Achilli, T.Y. Cath, A.E. Childress, Selection of inorganic-based draw solutions for forward osmosis applications, J. Membr. Sci. 364 (2010) 233–241. [11] J.R. Mccutcheon, R.L. Mcginnis, M. Elimelech, A novel ammonia—carbon dioxide forward (direct) osmosis desalination process, Desalination 174 (2005) 1–11. [12] M.M. Ling, K.Y. Wang, T.S. Chung, Highly water-soluble magnetic nanoparticles as novel draw solutes in forward osmosis for water reuse, Ind. Eng. Chem. Res. 49 (2010) 5869–5876. [13] Q. Ge, J. Su, G.L. Amy, T.S. Chung, Exploration of polyelectrolytes as draw solutes in forward osmosis processes, Water Res. 46 (2012) 1318–1326. [14] C.X. Guo, D. Zhao, Q. Zhao, P. Wang, X. Lu, Na+-functionalized carbon quantum dots: a new draw solute in forward osmosis for seawater desalination, Chem. Commun. 50 (2014) 7318–7321. [15] Y. Qiu, K. Park, Environment-sensitive hydrogels for drug delivery, Adv. Drug Deliv. Rev. 64 (2012) 49–60. [16] D. Li, X. Zhang, J. Yao, G.P. Simon, H. Wang, Stimuli-responsive polymer hydrogels as a new class of draw agent for forward osmosis desalination, Chem. Commun. 47 (2011) 1710–1712. [17] A. Razmjou, Q. Liu, G.P. Simon, H. Wang, Bifunctional polymer hydrogel layers as forward osmosis draw agents for continuous production of fresh water using solar
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Desalination journal homepage: www.elsevier.com/locate/desal
Performance evaluation of electric-responsive hydrogels as draw agent in forward osmosis desalination
MARK
Hongtao Cui, Hanmin Zhang⁎, Mingchuan Yu, Fenglin Yang Key Lab of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Forward osmosis AMPS/DMAEMA hydrogels Electric-responsive Dewatering
Forward osmosis (FO) has gained more attentions because it has the potential to be an emerging desalination technology. Stimuli-responsive hydrogels, as a novel class of FO draw agent, can completely avoid reverse solute diffusion and release water easily under external stimuli. In this study, 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and 2-(Dimethylamino)ethyl methacrylate (DMAEMA) monomers are used to prepare a series of electric-responsive AMPS/DMAEMA hydrogels by aqueous solution polymerization. These hydrogels are adopted as draw agent, by using 2000 ppm NaCl solution as the feed, A0.55D0.45 and A0.53D0.47 hydrogel show better initial water fluxes which are 2.09 and 1.63 LMH, separately. And A0.55D0.45 hydrogel produces 16.12 L·m− 2 total water flux and 0.67 average 24 h water flux. These hydrogels are dehydrated under 15 V electric stimuli. After the first regeneration, their corresponding water recovery rates are 67.45% and 39.36%, separately. After the second regeneration, their initial water fluxes are still recorded at 1.54 and 1.08 LMH, respectively. In contrast with other published literatures, the water flux produced by as-prepared hydrogels has increased significantly. Moreover, using an external electric field to achieve regeneration can simplify the operation of forward osmosis process.
1. Introduction Rapid population growth and global warming have caused an inhomogeneous supply distribution for available water resource. And the shortage of fresh water has become an important global issue that seriously affects the survival of mankind. Membrane technology for desalination, such as reverse osmosis (RO) [1], nanofiltration (NF) [2] and membrane distillation (MD) [3] and forward osmosis (FO) [4], had a potential advantage to relieve the crisis of freshwater scarcity. FO depends on the osmotic pressure difference across the semi-permeable membrane as the driving force and operates at no or very low hydraulic pressure [5], which leads to decreasing energy consumption of drawing water [6]. Besides, it had higher water recovery, and may have lower fouling propensity or fouling that was more reversible than in RO processes [7,8]. And this membrane technology requires low influent water quality and simplifies the pretreatment process of wastewater, so it has attracted wide attentions around the world. At present, the most widely used draw solutes are inorganic salts like NaCl, which can obtain
higher water flux [9,10]. However, the reverse draw solute diffusion not only result in contaminating the feed solution and reducing the osmotic pressure difference, but also the separation and regeneration of water need high pressure RO process. Therefore, although the membrane develops quickly in FO desalination, preparing and selecting a suitable draw solute, which has some characteristics including high osmotic pressure, lower reverse solute diffusion, low-cost regeneration, non-toxicity and safety, is still the focus for FO technology. Many researchers have done a lot of researches and attempts on the draw solute. McCutcheon et al. [11] firstly presented ammonium bicarbonate (NH4HCO3) as draw solute in FO desalination due to its high osmotic pressure and concluded that it produced higher water flux and needed thermal decomposition at 60 °C to regenerate. Although the moderate thermal regeneration consumed less energy, the reverse diffusion of NH4HCO3 was much higher than that of NaCl and alkaline NH4HCO3 would affect the service life of FO membrane. Magnetic nanoparticles as draw solute have attracted much attention due to their high osmotic pressures, no reverse solute osmosis and facile recovery,
Abbreviations: AMPS, 2-Acrylamido-2-methyl-1-propanesulfonic acid; DMAEMA, 2-(Dimethylamino)ethyl methacrylate; FT-IR, fourier transform infrared spectroscopy; SEM, scanning electron microscopy; DSC, differential scanning calorimetry; SR, swelling ratio; RO, reverse osmosis; NF, nanofiltration; MD, membrane distillation; FO, forward osmosis; FS, feed solution; DS, draw solution; NaCl, sodium chloride; PAA-Na, polyacrylic acid sodium salts; MBAAm, N,N′-methylenebisacrylamide; APS, ammonium persulfate; NaHSO3, sodium bisulfate; W, water content; R, water recovery rate; GO, graphene oxide; NIPAm, N-isopropylacrylamide; PSA, poly(sodium acrylate); PVA, poly(vinyl alcohol) ⁎ Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.desal.2017.10.045 Received 28 July 2017; Received in revised form 12 October 2017; Accepted 26 October 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.
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were purchased from the Aladdin (Shanghai). N,N′-methylenebisacrylamide (MBAAm, ≥99.0%) was provided by the Tianjin Kermel Chemical Reagent Co. Ltd. (Tianjin). Sodium chloride (NaCl, ≥99.5%) and sodium bisulfate (NaHSO3) were acquired from Tianjin DaMao Chemical Reagent Factory (Tianjin). Ammonium persulfate (APS) was obtained from the Tianjin GuangFu Technology Development Co. Ltd. (Tianjin). APS and NaHSO3 were formulated as 8 wt% and 4 wt% aqueous solution before use. All the chemicals were used as received without further purification. The dialysis bag (MWCO = 14,000) used for swelling study was purchased from Sigma. CTA-ES membrane made from cellulose triacetate (CTA) with an embedded polyester screen mesh was obtained from Hydration Technologies Inc. (Albany, OR).
but the conglomerations of nanoparticles and nanoparticles residues in produced water would still limit the practical application [12]. Ge et al. [13] introduced polyelectrolytes of a series of polyacrylic acid sodium salts as draw solutes for FO process in 2012, and their reverse leakages were much lower. Nevertheless, in order to obtain a higher water flux, these polyelectrolytes needed increasing their concentration, which may exacerbate membrane fouling and concentration polarization. At the end of 2014, Guo et al. [14] proposed that 0.5 g/mL Na+-functionalized carbon quantum dots could produce the osmotic pressure of 153.6 atm, which was appropriate to become the draw solute for FO. But, it required membrane distillation to concentrate when it was diluted, so the problem for complex regeneration process and higher energy consumption still existed. Hydrogel was a three dimensional network of hydrophilic polymer which was crosslinked by either physical or chemical bonds, and could contain a large amount of water when maintaining the structure [15]. In 2011, it was the first time for Li et al. [16] to report that temperatureresponsive polymer hydrogels were explored as draw agent in FO desalination, followed by light-responsive hydrogels [17], pressure-responsive hydrogels [18] and other kinds of stimuli-responsive hydrogels as draw agent. These kinds of draw agent extracts water molecules from the feed solution via the osmotic pressure, then these swollen hydrogels were dehydrated under the light, heat or pressure to obtain the fresh water [19], so these hydrogels can completely avoid the reverse osmosis. It was reported that weakly crosslinked poly(acrylic acid)/poly(sodium acrylate) copolymer hydrogels with the volume fraction between 0.03 and 0.3 exhibited a swelling pressure ranging from 0.20 to 4.23 MPa [20]. Razmjou et al. [17] designed bifunctional polymer hydrogels layers as a water-absorptive layer and dewatering layer to become the FO draw agents in order to realize the continuous production of fresh water. However, compared to other stimuli-responsive polymer hydrogels, electric-responsive polymer hydrogels are more convenient to apply and control [21]. Hydrogels sensitive to electric current are usually made of polyelectrolytes. And electric-responsive hydrogels undergo shrinking or swelling in the presence of an applied electric field. In 1965, it was first reported that the deformation of polyelectrolyte hydrogels could be observed under the stimuli of an electric field [22]. Electric-responsive hydrogels have shown merits in some fields such as micro-mechanics, flexible actuators and artificial muscle tissues when comparing to other rigid materials. So, our research group have done research on preparing electric-responsive hydrogels as FO draw agent, and simplified the regeneration process by applying electric field which is easy to control and operate [23], but the dewatering rate under electric-stimuli is not carried on. In this work, the monomer of 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and 2-(Dimethylamino)ethyl methacrylate (DMAEMA), N,N′-methylenebisacrylamide as a cross-linker, ammonium persulfate and sodium bisulfite as redox initiator were employed to prepare AMPS/DMAEMA hydrogels with a range of different monomer ratios by aqueous solution polymerization. AMPS monomer has sulfonic group with high charge density, which is easy to dissociate in solution to become negatively sulfonate ion. And DMAEMA monomer has alkaline tertiary amine groups which can be protonated in acidic medium to become positive ion. Increasing number of ions can enhance the swelling pressure, and high hydrophilicity and conductivity can be helpful to improve the water flux and regeneration ratio under electric stimuli. More importantly, the research on the electric-responsive hydrogels for FO desalination is quite little.
2.2. Preparation of xerogel particles draw agent AMPS and DMAEMA monomer were polymerized via aqueous solution polymerization to produce the AMPS/DMAEMA hydrogels. First of all, a desired amount of AMPS, DMAEMA and MBAAm were put in a beaker with 30 mL deionized water added. And it was thoroughly dissolved by using a magnetic stirrer for several minutes at room temperature to form a colorless and transparent solution. Secondly, a certain amount of APS and NaHSO3 solutions (molar ratio = 1:1) were added to the above solution with the molar ratio of monomers, crosslinker and initiator fixed at 100:1:1. The concentration of monomers was 1 mol/L. Subsequently, it was mixed completely by a magnetic stirrer. Finally, it was transferred to the sealed centrifuge tube with a cover, which was put in 50 °C water bath for 5 h to finish polymerization. After polymerization, the centrifuge tube was taken out and cooled to room temperature. Then, columnar hydrogel was removed and soaked into deionized water for three days which was changed every day to remove the unreacted monomers and linear polymer of low molecular weight. The molar ratio of AMPS and DMAEMA was 0.5/ 0.5, 0.53/0.47 and 0.55/0.45, and as-prepared hydrogels were denoted as A0.5D0.5, A0.53D0.47 and A0.55D0.45, separately. Swollen hydrogels were dried at 60 °C in a convection oven and samples of hydrogel particle size (1–2 mm) were made by grinding dried hydrogels and sieving them. Consequently, the xerogel particles were treated as draw agent in FO desalination. Only the amount of cross-linker changed, so the molar ratio of monomers, cross-linker and initiator fixed at 100:2:1. According to the above preparation process, the hydrogels fabricated were labeled as A0.5M2, A0.53M2 and A0.55M2, separately. The photos for different hydrogels were shown in Fig. 1a. A0.5D0.5 is white and columnar hydrogel that is very soft and has good elasticity, while A0.53D0.47 and A0.55D0.45 become colorless, transparent and hard with the increasing of hydrophilic sulfonic acid groups. However, A0.5M2 columnar hydrogel is pale white, less soft and much harder than the A0.5D0.5. A0.53M2 and A0.55M2 hydrogel become colorless and transparent because of the more hydrophilic sulfonic acid groups. Overall, the intensity of the latter is better than that of the former, which is attributed to the increasing amount of cross-linker. In the polymerization, ionic complex with two double bonds was formed by proton transferring reaction between AMPS and DMAEMA firstly. This complex bonded mutually because of the electrostatic force between anion and cation in order to produce a similar polymerized monomer of organic salt structure [24]. Then, ionically crosslinked AMPS/DMAEMA hydrogel was fabricated by free radical polymerization, and its protonated process and the structure of AMPS/DMAEMA hydrogel were exhibited in Fig. 1b.
2. Experimental section 2.3. Characterization 2.1. Chemicals and materials Swelling behaviors of different xerogels particles were investigated by the gravimetric method. The water uptake of different xerogels particles was determined as follows. 0.3 g different particles were put
Both monomers 2-(Dimethylamino)ethyl methacrylate (DMAEMA, 99%) and 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS, 98%)
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Fig. 1. Photos of prepared hydrogels after polymerization (a) and a schematic illustration of the protonation and molecular structure of AMPS/DMAEMA hydrogel (b) (1#: A0.5D0.5, 2#: A0.53D0.47, 3#: A0.55D0.45, 4#: A0.5M2, 5#: A0.53M2, 6#: A0.55M2.)
apparatus (Fig. 2), the membrane was immersed in deionized water for > 12 h to maintain hydration before use. The weight of FS was monitored by an electronic scale (JSC-QHC-3 +) and the data was recorded by a computer software. Water flux (F, L/m2·h (LMH)) is determined by measuring the weight change of FS with time and calculated from the Eq. (2):
into a dialysis bag (MWCO = 14,000), which was sealed and placed in 1 L deionized water at room temperature. Deionized water was replaced every day and the weight of swollen polymer hydrogels was measured after three days of immersion in deionized water. The swelling ratio (SR) of polymer hydrogels was calculated by the formula (1) below:
SR =
Ws − Wd Wd
F = V (A × t )
(1)
where Ws is the weight of swollen polymer hydrogel after three days (g); Wd is the weight of initial xerogels particles before test (g). And SR was the average value of triplicate measurements. Fourier transform infrared spectroscopy (FT-IR) (EQUINOX55, Germany) was used to confirm the chemical structure of samples. The morphologies of the as-prepared products were determined at 20 kV by using QUANTA 450 (USA) scanning electron microscopy (SEM). Differential scanning calorimetry (DSC) (NETZSCH DSC 204, Germany) was employed to examine the water state in the swollen hydrogels. Swollen hydrogels were cut into the small pieces. And its surface water was wiped softly. Then, sample sealed in aluminum pan was cooled to − 25 °C and then heated to 25 °C at a heating rate of 3 °C/min under 30 mL/min of nitrogen gas flow.
(2)
where V is the volume of water permeating through the membrane (L) over a given period of time, t (h); A is the effective membrane area in the module (m2). The water flux was measured for three times under the same experimental conditions. And it was the average value of triplicate measurements. Water content (W) is an important parameter for assessing the performance of dewatering [25]. So, after 24 h FO process, water content of xerogel particles is calculated as follows:
W=
Wm − Wd × 100% Wm
(3)
where Wm is the weight of swollen xerogel particles after 24 h FO test; Wd is the weight of initial xerogel particles (2 g) applied in the FO test. In an electric stimuli dewatering process, different swollen hydrogels were placed on the measuring apparatus (Fig. 9) for dewatering. So, water recovery rate (R) is calculated by the formula (4) below:
2.4. FO performances evaluation and electric-stimuli dewatering The water flux for different xerogel particles was evaluated by using cross-flow FO membrane module with an effective membrane area of 12.56 cm2 at room temperature. 2000 ppm NaCl solution was utilized as FS, and 2 g different xerogel particles were studied as DS in FO tests. Namely, 2 g different xerogel particles were placed on the active layer of FO membrane to assess the DS performances. The FS was pumped by a peristaltic pump at a volumetric flow rate of 167.6 mL/min (a cross flow velocity of 5.56 cm/s). Before each test in our customized FO
R = W1 W0 × 100%
(4)
where W1 is the weight of water released during the electric stimuli dewatering test (g); and W0 is the weight of absorbed water in the xerogel particles during 24 h FO process (g).
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Fig. 2. Structure diagrams for homemade FO apparatus.
Fig. 4. FT-IR spectra of AMPS, DMAEMA monomer and three different hydrogels.
Fig. 3. Swelling ratio histograms for different xerogel particles (Error bars represent one standard deviation at triplicate measurements.)
suggesting that SR increases with the increasing amount of hydrophilic sulfonic acid group. The literature [16] reported early that hydrogels were able to entrap large volumes of water attracted by the high concentration of hydrophilic groups, which is consistent with the SR result. The hydrophilicity of prepared hydrogels enhances due to the increment of sulfonic groups.
3. Results and discussion 3.1. Swelling property Gravimetric method was used to examine the swelling property of different xerogel particles, as shown in Fig. 3. Swelling ratio of polymer hydrogels could be considered as a function of their chemical and structure characteristics such as ionic strength, crosslinking density, the thickness of the polymer network and the relaxation rate [26]. Results show that cross-linker of higher content leads to lower SR under the same monomer ratio. The polymer chains would be further cross-linked due to the increasing content of cross-linker, and the polymer networks become more compact. Consequently, it is difficult for water molecules to penetrate into the polymer networks, resulting in low SR. The SR order of the former is A0.5D0.5 < A0.53D0.47 < A0.55D0.45, and the SR order of the latter is A0.5M2 < A0.53M2 < A0.55M2,
3.2. Characterization 3.2.1. FT-IR analysis Swollen hydrogels were dried at 60 °C in a convection oven and were ground into the powder with a morta for characterization. FT-IR analysis is carried out to identify functional groups within prepared hydrogels in order to confirm the successful copolymerization. As shown in Fig. 4, typical peaks corresponding to α and β unsaturated ester in the DMAEMA are at the wavenumber around 1725 and 1165 cm− 1, which are attributed to the carbonyl stretching (υC]O) and
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Fig. 5. SEM images of six different swelling hydrogels after freeze drying method.
the stretching vibration of CeO bond (υCeO) separately. And the band around 1637 cm− 1 is assigned to stretching vibration of unsaturated double bonds (υC]C). Typical peaks at the bands around 1616 and 1675 cm− 1 for AMPS are corresponding to C]O stretching vibration of amide (υC]O) and stretching vibration of unsaturated C]C (υC]C) respectively. For composite hydrogels, the stretching vibration of ester group for DMAEMA (υC]O = 1725 cm− 1) and stretching vibration of amide for AMPS (υC]O = 1675 cm− 1) still exist, but stretching vibration of unsaturated double bond does not exist, which indicates that polymerization reaction occurs. The broad peak at 3455 cm− 1 can be predicted to be intermolecular hydrogen bond formed by the polymer. At the same time, eHO3S group and tertiary amide interacted to cause ionic bonds, and the peak at band around 2990 cm− 1 transfers to broad peak at around 3455 cm− 1. So, it confirms that ionic complex which has similar organic salts structure forms due to the proton transferring reaction between AMPS and DMAEMA. Besides, appearance of the wavenumber at 1043 cm− 1 for the A0.5D0.5, A0.55D0.45, A0.5 M2 spectrums that are assigned to the symmetric stretching of the S]O in the sulfonic acid group verifies the successful synthesis of the prepared hydrogels with sulfonic acid functional group. The appearance of these peaks proves the successful incorporation of comonomer into AMPS/ DMAEMA hydrogel which has complete polymer networks. 3.2.2. SEM measurement Six different hydrogels in a swelling equilibrium state were taken out from the deionized water, and these hydrogels were in rapid quenching cold by using liquid nitrogen, then they were placed in the freeze dryer for 18 h. After freeze drying, these hydrogels showed a white and fluffy solid. Subsequently, these hydrogels were cut, and their surfaces were sputtered by Au to observe the morphology using SEM. So, the SEM images were shown in Fig. 5. As we can see from Fig. 5, six different hydrogels have obviously porous structure. With the increasing content of hydrophilic groups, the porous cavity for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel becomes more neat and uniform. However, in contrast with A0.5D0.5 hydrogel, A0.5M2 hydrogel has smaller porous structure due to an
Fig. 6. DSC thermograms for different swollen hydrogels.
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active layer of CTA membrane, with the support layer of CTA membrane facing the feed solution. 2000 ppm NaCl solution was chosen as the feed solution. In hydrogel-driven FO process, CTA membrane was initially wetted because it was soaked in deionized water for a period time. And xerogel particles were in good contact with the membrane surface, and swelled with water, thus producing a swelling pressure. Water molecule was transported from one particle to another particle by the swelling force, resulting in a continuous flow of water through the membrane. And this was very similar to the mechanism of the commercial hydration bag where sugar or beverage powder was used as the draw agent [28]. Fig. 7 presents the water fluxes in a 24 h FO process using different xerogel particles as draw agent. As shown in Fig. 7, the initial water flux for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel are 0.81, 1.63 and 2.09 LMH, separately. And the initial water flux for A0.5M2, A0.53M2 and A0.55M2 hydrogel are 0.80, 0.96 and 1.11 LMH respectively. The initial water flux decreases when the amount of cross-linker becomes higher under the same monomer ratio, which is consistent with the result of swelling ratio. So, A0.55D0.45 hydrogel shows best performance among the hydrogels in FO desalination process. In the first 9 h, the water flux decreases quickly, and then it changed slowly. It is inferred that the swelling pressure of polymer hydrogels reduces with the increasing their degree of swelling when the FO process proceeds, resulting in the observed decrease in water flux. Li et al. [25] explored composite hydrogels (poly(sodium acrylate)‑carbon) as draw agent in FO desalination process, the water flux for poly(sodium acrylate)‑carbon was 1.32 LMH in the first 0.5 h when 2000 ppm NaCl was applied as the feed solution. The initial water flux for A0.53D0.47 and A0.55D0.45 hydrogel is 23.48% and 58.33% higher than that of poly(sodium acrylate)‑carbon draw agent. Besides, thermal-responsive semi-interpenetrating hydrogels of NIPAm-0.5PVA was synthesized by polymerization of N-isopropylacrylamide (NIPAm) in the presence of poly(vinyl alcohol) (PVA), which was considered as draw agent toward temperature driven forward osmosis desalination. But, the initial water fluxes were < 0.30 LMH [29]. Although the dewatering flux of bifunctional hydrogels could reach 10 LMH under solar intensity of 0.5 kW/m2, its initial water flux was only 0.32 LMH when the membrane area was 1.77 cm2 and the feed solution was 2000 ppm NaCl solution [17]. Xerogel particles have higher degree of interstitial volume, which can hold more water, resulting in a high degree of swelling. Increasing the swelling pressure of prepared hydrogels is helpful to improve the water flux. Besides, the good hydrophilicity of prepared hydrogels increases the water flux.
increase content of cross-linker. And the porous structure for A0.53M2 and A0.55M2 hydrogel has collapsed and tangled compared to the former hydrogels, which decreases the regularity of porous structure. The increasing cross-linker content leads to enhancing the cross-linked dot, which makes the network structure become more compact, resulting in the difficult of the water molecules into the porous structures. Larger porous structure provides easy access to internal hydrogels for water molecules. More porous structures are helpful to increase absorbing capacity, which agrees well with the result of swelling ratio. 3.2.3. DSC analysis Water state in the swollen hydrogels can be divided into two kinds, free water and bound water. To further elucidate the swelling behavior of different hydrogels, water states are investigated by using DSC. DSC curves for different hydrogels are shown in Fig. 6. Since the presence of water in the hydrogel is different, Fig. 6 starts to show a significant enthalpy change near 0 °C. However, the temperature of enthalpy becomes larger due to the increasing content of AMPS. As shown in Fig. 6, six different hydrogels have an obvious endothermic peak, which is similar to the DSC curve of ice melting in the vicinity of 0 °C. Namely, these endothermic peaks can be the melting peak of ice. It is concluded that the phase transition absorption peak from ice to liquid water comes from the bound water of six hydrogels. Hydrophilic group like eHSO3 on the polymer and water molecules mutually combine to obtain bound water through the solvation, coordination bonds and hydrogen bonds in internal and external surface, which is similar to the chemical adsorption water. The increasing ratio of AMPS monomer is beneficial to the chemical adsorption water. The literature [27] pointed out that the higher the temperature corresponding to the endothermic peak was, the stronger the interaction force between polymer and water molecules was. The order of the temperature corresponding to endothermic peak for six different swollen hydrogels is A0.55D0.45 > A0.55M2 > A0.53M2 > A0.53D0.47 > A0.5M2 > A0.5D0.5, and interaction force of A0.55D0.45 hydrogel is the strongest among them. The temperature rises with the increasing content of sulfonic acid group, indicating that the introduction of sulfonic acid group enhances the ability of binding with water molecule for hydrogels. 3.3. Water flux in the FO process Different xerogel particles were further used as draw agent due to their porous structure and hydrophilic sulfonic group. The setup for the FO test was shown in Fig. 2. Different xerogel particles were put on the
Fig. 7. Water flux as a function of time for different xerogel particles in FO desalination (FS: 2000 ppm NaCl solution; DS: 2 g xerogel particles.)
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and got the results that the average 24 h water flux obtained was 0.55 LMH for poly(ethylene glycol-[DL-lactic acid-co-glycolic acid]-bethylene glycol)/GO-0.09 wt% draw agent by using 2000 ppm NaCl as feed solutions. The average 24 h water flux acquired in our study is 0.67, 0.60 and 0.59 LMH for A0.55D0.45, A0.53D0.47 and A0.55 M2 hydrogel separately when 2000 ppm NaCl was used as FS, which were higher than that of poly(ethylene glycol-[DL-lactic acid-co-glycolic acid]-b-ethylene glycol)/GO-0.09 wt% draw agent. And our result increased by 21.82%, 9.09% and 7.27% respectively for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel. Based on the above results, A0.55D0.45, A053D0.47 and A0.55M2 hydrogel are selected for the electric-responsive dewatering. 3.4. Electric-stimuli dewatering One important characteristic of hydrogels draw agent is that it can totally avoid the reverse solute diffusion; another important characteristic of them is that it can be easily regenerated. And the regeneration for three different hydrogels is carried out under electric stimuli. The mass maintenance rate of poly(AMPS-co-DMAEMA) hydrogel was measured at different voltages by Liao et al., and the maximum amount of water was released under 15 V voltage [31]. Therefore, the voltage of 15 V is chosen as electric-stimuli for the regeneration. The electric-stimuli effect on the dewatering of electricresponsive hydrogels used as the draw agent in the FO process is shown in Fig. 9. After 24 h FO desalination process is finished, the swollen hydrogels are weighed and their water contents are calculated by Eq. (3), then these hydrogels are dehydrated for 30 and 60 min under 15 V electric field respectively, dehydrated hydrogels are weighed and their water recovery rates are calculated by Eq. (4), as shown in Table 1. And the water recovery rate after first regeneration is 39.36%, 67.45% and 45.74% respectively for A0.53D047, A0.55D0.45 and A0.55M2 hydrogel under 60 min electric-stimuli. By two electric-stimuli dewatering tests, the water recovery rate of high content sulfonic acid hydrogel is high. The reason was that the more content of sulfonic acid group was, the more sensitive was subjected to the electrical stimulation [31]. When the electric field was applied, the different charged ions would
Fig. 8. Histograms of total water flux and average 24 h water flux for different xerogel particles (FS: 2000 ppm NaCl solution; DS: 2 g xerogel particles.)
The total water flux (L·m− 2) means the total water mass extracted by hydrogels in per unit area at test time (24 h). The total water flux and average 24 h water flux are calculated for different hydrogels, as shown in Fig. 8. The total water flux for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel is 10.25, 14.30 and 16.12 L·m− 2, separately. And the total water flux for A0.5M2, A0.53M2 and A0.55M2 is 9.50, 12.83 and 14.12 L·m− 2, respectively. The total water flux reduces when the amount of cross-linker is higher under the same monomer ratio. This agrees well with the result of swelling ratio. Average 24 h water flux obtained is 0.43, 0.60 and 0.67 LMH separately for A0.5D0.5, A0.53D0.47 and A0.55D0.45 hydrogel. And average 24 h water flux for A0.5 M2, A0.53 M2 and A0.55 M2 hydrogel is 0.4, 0.53 and 0.59 LMH, respectively. Nakka et al. [30] studied triblock copolymer hydrogels with graphene oxide (GO) incorporation as draw agents in FO process,
Fig. 9. Schematic diagram of the electric-stimuli effect on the dewatering of electric-responsive hydrogels used as draw agent in the FO process.
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the initial water fluxes for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel decrease. After first regeneration, the initial water fluxes for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel are recorded at 1.76, 1.19 and 0.95 LMH, which reduce by 15.79%, 26.99% and 14.41% separately comparing with xerogel particles as draw agent in FO desalination. And after second regeneration, the initial water fluxes for them are 1.54, 1.08 and 0.86 LMH, which are 26.32%, 33.74% and 22.52% lower than that of xerogel particles as draw agent. The initial water flux for A0.53D0.47 hydrogel decreases more, which is related to the low water recovery under electric-stimuli dewatering test. However, the initial water flux for A0.55D0.45 is still higher than that of poly(sodium acrylate)‑carbon [25]. It is attributed to higher content hydrophilic sulfonic groups and existing porous structure in A0.55D0.45 hydrogel. In brief, using electric-responsive hydrogels as draw agent can eliminate reverse solute leakage and reduce the complexity of the operation in FO process. There is great potential for application in the food and pharmaceutical field which are not allowed to be contaminated for the feed solutions because it can completely avoid the reverse solute diffusion.
Table 1 Water content (W) and water recovery rate (R) for different hydrogels in FO process.
W (%) ⁎ R30 min (%, first regeneration) ⁎ R60 min (%) ⁎ R30 min (%, second regeneration) ⁎ R60 min (%)
A0.53D0.47
A0.55D0.45
A0.55M2
57.85% ± 1.06 20.72% ± 1.93
73.48% ± 3.46 30.25% ± 2.41
50% ± 2.19 22.84% ± 3.02
39.36 ± 2.08 17.06% ± 2.57
67.45% ± 3.29 26.5% ± 3.14
45.74% ± 2.97 20.18% ± 3.36
32.19% ± 2.86
59.06% ± 3.79
40.73% ± 2.92
⁎ R30 min, ⁎R60 min represented the water recovery rate (R) for 30 min and 60 min in an electric-stimuli dewatering process, separately.
move in the hydrogel networks, the charges in the polyionic groups would be shielded due to the electrostatic attraction. The number of polyions reduced, which would decrease the ability of holding water for hydrogel, so the water could seep from the hydrogel networks. It can be observed from Table 1 that, the longer time for dehydration is, the more water releases. So, the water recovery rate of A0.55D0.45 hydrogel exhibits much better than that of the other two hydrogels when the electric field is applied at longer time. Compared to heating dewatering process for 1.5 h [32], applying an external electric field is easy to control and the regeneration time is slightly short. As AMPS/DMAEMA hydrogels are electric-responsive hydrogels, they will shrink and release some water when electric field is applied. Kim et al. [33] reported that electric field caused the migration of the positively charged ion in the hydrogel toward the cathode, which resulted in partial shielding the carboxylate groups reducing the extent of hydration of the gel. The counterions of the polyions in the hydrogel and the free ions in the solution moved toward their counter-electrodes, which resulted in an ionic gradient along the direction of the electric field. So, this difference in ionic concentration induced an osmotic pressure difference that caused volume change of hydrogels in an electric field [34]. Due to the competing between ions and free counterions, ionic groups in the hydrogels were helpful to induce the osmotic pressure which could draw more water. When deswollen or dehydrated hydrogels absorbed water again, the chain of polymer networks could stretch, thus producing the osmotic pressure. The water flux as a function of time is measured after two regenerations under 15 V for three hydrogels, and it is exhibited in Fig. 10. By using 2000 ppm NaCl as the feed,
4. Conclusions Aqueous solution polymerization was used to fabricate a range of electric-responsive AMPS/DMAEMA hydrogels. For these hydrogels, their properties and structural conformations were obtained from swelling ratio, FT-IR analysis, SEM measurement and DSC analysis. Due to hydrophilic groups and porous structure, different AMPS/DMAEMA hydrogels were employed as the draw agent in FO process when 2000 ppm NaCl solution was used as FS. And their dewatering process was evaluated. The major conclusions drawn from this study are as follows.
• The swelling ratios for A0.55D0.45, A0.53D0.47 and A0.55M2 hydrogel are 20.07, 19.78 and 16.91, respectively. • By using 2000 ppm NaCl solution as the feed, the initial water fluxes •
for A0.55D0.45 and A0.53D0.47 hydrogel are recorded at 2.09 and 1.63 LMH, separately. After the second regeneration, water recovery rates for A0.55D0.45 and A0.53D0.47 hydrogel are 59.06% and 32.19%, respectively. And their initial water fluxes are still recorded at 1.54 and 1.08 LMH. Electric field used in this work represents a much easier and more effective method to achieve dewatering of the particles.
Fig. 10. Water flux as a function of time for different hydrogels after 60 min dewatering under electric-stimuli.
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