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Exploring an innovative surfactant and phosphate-based draw solution for forward osmosis desalination
Journal of Membrane Science 489 (2015) 212–219
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Exploring an innovative surfactant and phosphate-based draw solution for forward osmosis desalination Hau Thi Nguyen a, Shiao-Shing Chen a,n, Nguyen Cong Nguyen a, Huu Hao Ngo b, Wenshan Guo b, Chi-Wang Li c a Institute of Environmental Engineering and Management, National Taipei University of Technology, No.1, Section 3, Chung–Hsiao E. Rd, Taipei 106, Taiwan, ROC b Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology, Sydney, Broadway, NSW 2007, Australia c Department of Water Resources and Environmental Engineering, TamKang University, 151 Yingzhuan Road, Tamsui District, New Taipei City 25137, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2014 Received in revised form 5 January 2015 Accepted 22 March 2015 Available online 20 April 2015
The reverse salt flux phenomenon of forward osmosis affects the quality of the feed water, reduces water flux, and increases the cost for replenishing lost draw solute. In this study, a novel draw solution comprising a mixture of Triton X100 and Na3PO4 for minimizing the reverse salt flux during forward osmosis (FO) was explored. The results indicated that the reverse salt flux caused by coupling 0.5 mM Triton X100 to 0.55 M Na3PO4 draw solution was only 0.13 g/m2 h, and the specific reverse salt flux was 0.03 g/L using DI water as the feed solution, which are the lowest recorded values among all forward osmosis studies. Hydrophobic attractive interactions between tail groups of Triton X100 with the FO membrane are believed to be the main mechanism for minimizing salt leakage. Results from desalination experiments demonstrated that using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as the draw solution and brackish water and seawater as the feed solution with total dissolved solids of 4090 and 36,800 ppm achieved water fluxes of 4.89 L/m2 h and 1.15 L/m2 h, respectively. Furthermore, using a two-stage ultrafiltration–nanofiltration system for the draw solution recovery enabled 98% recovery of solutes. & 2015 Elsevier B.V. All rights reserved.
Keywords: Forward osmosis Seawater desalination Hydrophobic interaction Surfactant
1. Introduction The increase in the world population accompanied by an increase in agricultural and industrial activities [1], has caused water scarcity to become a concern. Researchers have conducted numerous studies related to desalination of brackish water and seawater to produce potable water. Among the proposed desalination methods, such as thermal desalination, crystallization, ion exchange, and solvent extraction, reverse osmosis (RO) is one of the most promising potential types of technology for seawater desalination [2]. However, several drawbacks are associated with RO, including (1) high energy requirement; (2) low recovery rate; and (3) environmental impact caused by brine discharge [3–6]. To overcome these limitations, researchers in the scientific community must create a more effective desalination technology. Forward osmosis (FO) is a form of technology that has been developed in recent years. It can potentially be applied to seawater desalination
n
Corresponding author. Tel.: þ 886 2 27712171 4142; fax: þ886 2 27214142. E-mail address: [email protected] (S.-S. Chen).
http://dx.doi.org/10.1016/j.memsci.2015.03.085 0376-7388/& 2015 Elsevier B.V. All rights reserved.
[7–10], the food industry [11,12], power generation [13–15], osmotic membrane bioreactors [16–18] and sludge dewatering [19–21]. FO uses natural osmosis as a driving force for separation, and therefore is expected to (1) possess low energy requirements for operation; (2) exhibit less fouling than pressure-driven membrane processes; and (3) achieve a high potential recovery rate. McGinnis et al. [22] demonstrated that energy savings of FO compared to current technologies for seawater desalination on an equivalent work basis are projected to range from 72% to 85%. Altaee et al. [23] found the recovery step in an FO–RO desalination system to use 96–98% of the total power consumption. However, the major challenge of creating marketable FO technology is the lack of an ideal draw solution that can achieve high water flux, low reverse salt flux, and coeffective recovery. Particularly, reverse diffusion of salt from the draw solution to the feed side not only affects the quality of the feed water, but causes water flux decline and increases the cost of replenishing the lost draw solute [24]. Numerous types of draw solutions for FO desalination have been explored in previous studies. For example, monovalent salts (NaCl, KCl, KBr, KNO3, NH4Cl, KHCO3, NaHCO3, NH4HCO3) with
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favorable water solubility are frequently used, but the greatest disadvantage of using these salts as a draw solution for FO is the extremely high salt leakage (reverse salt flux reached 29.2 g/m2 h when using 0.88 M KBr as a draw solution) [25–27]. To overcome this problem, divalent (CaSO4, MgSO4, CuSO4, MgCl2, CaCl2) and trivalent salts (Al2(SO4)3, EDTA–2Na) were proposed as draw solutions [26,28–30]. Compared with monovalent salts, the reverse salt fluxes of divalent and trivalent salts were lower because of the larger hydrated radius and higher electrostatic repulsion. However, the reverse salts still reached 0.9 g/m2 h for 0.62 M MgSO4, 9.5 g/m2 h for 0.56 M CaCl2, 5.6 g/m2 h for 0.50 M MgCl2, and 0.32 g/m2 h for 0.50 M EDTA–2Na. Additionally, Warne and Chung were the first to successfully synthesize magnetic nanoparticles (MNPs) for use as innovative draw solutes for FO. This concept provided the ideal draw solution without salt leakage, but an agglomeration problem was observed for magnetic nanoparticles during the regeneration stage [31–34]. Recently, zwitterions, hexavalent phosphazene, switchable polarity solvents, 2-methylimidazole-based organic compounds, and ferric and cobaltous hydroacid complexes have also been investigated as potential draw solutes for FO [35–39]. Although these draw solutes created high osmotic pressure, synthesis of the solutes was costly and recovery of the diluted draw solution was complex. In this study, a novel draw solution for minimizing the reverse flux of ions during FO desalination by coupling nonionic surfactant (Polyethyleneglycol tert-octylphenyl ether, Triton X100) to a Na3PO4 draw solution was explored. It is hypothesized that the hydrophobic interactions between tail groups of Triton X100 with membrane would form an additional layer on the membrane surface, preventing ions from escaping through membrane pores, thus reducing reverse salt flux. The reasons of using Triton X100 instead of other non-ionic surfactants are due to lower critical micelle concentrations (CMC) of 0.4 mM but larger molecule of Triton X100. Moreover, above the CMC, Triton X100 solution aggregates to form micelles that can couple with the trivalent phosphate to enlarge the molecular size of the draw solute, resulting in enhanced draw solute recovery using a two-stage ultrafiltration–nanofiltration (UF–NF) system. In addition, as compared with RO, UF–NF has been considered as a more energy efficient draw regeneration process using highly charged salts as draw solutions [26,40].Therefore, the objectives of this study were to systematically investigate the effects of coupling Triton X100 to a Na3PO4 draw solution on reverse salt flux and water flux for FO following: (1) effects of various concentrations of Triton X100; (2) effects of various concentrations of Na3PO4; (3) evaluation of desalination efficiency of the proposed draw solution and (4) recovery of the diluted draw solution.
Table 1 Characteristics of ultrafiltration and nanofiltration membranes. Membrane name
Manufacture Material
UF-GE
GE osmonics
NF-TS80
TriSep
Molecular weight cut-off 25 1C pH (MWCO) range
TF (Thin 1000 Film) Polyamide 150
1–11 2–11
213
2. Materials and methods 2.1. FO membranes The flat-sheet cellulose triacetate (CTA) FO membranes used in this study were supplied by Hydration Technology Innovations (HTIs OsMem™ CTA Membrane 130806, Albany, OR, USA) with size of 15 22 cm2 for each piece. The FO membrane possessed a water permeation coefficient of 3.07 10 12 m s 1 Pa 1 and salt rejection of approximately 95–99% [8,40]. The overall thickness of the membrane was approximately 50 mm, and the FO membrane was negatively charged at pH 44.5 [41]. The contact angle of the CTA FO membrane was determined to be 60–801, indicating that the membrane was also moderately hydrophobic [42,43]. Table 1 shows the characteristics of the UF and NF membranes used for draw solution recovery. 2.2. Feed solution and draw solution In FO experiments, deionized (DI) water, synthetic brackish water, and seawater were used as feed solutions. The synthetic brackish water and seawater were prepared with total dissolved solid (TDS) from 4900 to 36,800 ppm by adding NaCl salt to DI water as shown in Table 2. The draw solution was prepared using laboratory-grade Na3PO4 12H2O (Merck Co., Ltd., Germany) mixed with Triton X100 (Sigma-Aldrich, USA) as shown in Table 3 at mole ratios of 1000:1, 500:1, 200:1, 100:1, and 20:1 at room temperature for 60 min. These mixtures of phosphate sodium and Triton X100 were then maintained at pH 8 by adding phosphoric acid (H3PO4, Merck, 85% purity) and were continually stirred for 24 h before performing FO tests. 2.3. Experimental setup and product water recovery FO experiments were conducted using a dual-channel crossflow FO membrane cell, as shown in Fig. 1. The FO test cell (FO Sterlitech, USA) was designed with symmetric channels on both sides for the feed and draw solutions, and each channel was 4.5 cm in width, 9.2 cm in length, and 0.2 cm in height. The total effective FO membrane area for mass transfer was 41.40 cm2. Two peristaltic pumps (Masterflex L S Drive, Model 7518-00) with a flow rate of 500 mL/min were used to circulate the feed and draw solutions on both sides of the FO membrane. The temperature during experiments was maintained at 25 7 0.5 1C using two water baths. Conductivity and pH sensors were installed in the containers for the feed solution and the draw solution to monitor any changes. The volume of the draw solution was 4 L; the feed solution tank (2 L) was placed on a digital weighing scale (BW12KH, Shimadzu, Japan) connected to a computer data logging system to monitor the weight and volume changes at specified time intervals. All data were obtained from three repeated tests. The experimental water flux (Jw, L/m2 h) was calculated by measuring the volume change in the feed container based on time as follows: Jw ¼
ΔV A Δt
ð1Þ
Table 2 Synthetic brackish water and seawater as feed solutions at various TDS concentrations. Feed solution
Brackish water 1 (BW1) Brackish water 2(BW2) Brackish water 3 (BW3) Brackish water 4 (BW4) Brackish water 5 (BW5) Seawater
TDS, ppm 4090 Osmotic pressure, bar 2.43
5020 3.10
6450 4.06
12,270 7.85
22,900 15.34
36,800 26.61
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where ΔV is the total increase in volume of permeate water (L) collected over a predetermined amount of time Δt (h), and A is the effective FO membrane area (m2). The reverse solute flux (Js, g/m2 h) of the draw solution was determined by the amount of salt accumulated in the feed tank, as indicated in the following equation: Js ¼
V t :C t V 0 :C 0 A:t
ð2Þ
where Ct and Vt are the concentration and volume of feed solution measured at time t, respectively, and C0 and V0 are the initial concentration and volume of feed solution. Specific reverse salt flux (Js/Jw, g/L) is defined here as the ratio of salt (Js, g/m2 h) in the reverse direction and water flux (Jw, L/m2 h) in the forward direction to estimate the amount of draw solute lost per liter of water produced during FO. The amount of reverse salt was determined by converting the electrical conductivity, measured using a calibrated conductivity meter (Oakton Instruments, USA) because the large Triton-X 100 (647 Da) was completely removed by the FO membrane. After FO tests, the diluted draw solution was recovered using a two-stage UF–NF system with a crossflow module (CF042 Crossflow Membrane Test Cell, Delrin Acetal, USA) under operating hydraulic pressures of 4 and 8 bars , respectively. The filtration experiments were repeated at least three times using fresh membranes. 2.4. Analytical methods The concentrations of Na þ , Cl , and PO34 -P were analyzed using ion chromatography (a Dionex ICS-90) and an Ultraviolet– visible spectrophotometer (HACH Model DR-4000, Japan). The osmolality of solutions was measured using an osmometer (Model 3320, Advanced Instruments, Inc., USA), based on the freezingpoint depression method [44]. The viscosity was determined by a Table 3 Properties of the surfactant used in this study. Surfactant name
Type
Mol. wt. Abbreviation CMC (g/mol) (mM)
Polyethyleneglycol tertoctylphenyl ether
Nonionic 647
Triton X100
0.4
vibro viscometer (AD Company, Japan). The particle size of the draw solution was measured using a nanoparticle analyzer SZ-100 (Horiba, Japan). In addition, Mineqlþ software will be used to determine complex and charged formations of the draw solution at different pH values based on the chemical equilibrium model from the thermodynamic database.
3. Results and discussions 3.1. Effects of various Triton X100 concentrations on reverse salt and water flux Fig. 2 shows the reverse salt fluxes and water fluxes for six draw solutions with various Triton X100 concentrations coupled with a fixed Na3PO4 concentration of 0.1 M. The FO experiments were conducted in membrane; the orientation of the active layer facing the feed solution, which was DI water. It was observed that the reverse flux was reduced significantly when coupling Triton X100 with concentrations ranging from 0.1 to 5 mM into the Na3PO4 draw solution. Compared with pure Na3PO4 (Js ¼0.62 g/ m2 h), 0.1 M Na3PO4 draw solution coupled with 0.5 mM Triton X100 exhibited less reverse salt flux (Js ¼ 0.08 g/m2 h). In particular, Fig. 2 indicates that coupling higher concentrations of Triton X100 to the Na3PO4 draw solution achieved lower reverse salt flux. For example, Js was reduced from 0.11 to 0.07 g/m2 h when 0.1 to 5 mM Triton X100 was coupled with 0.1 M Na3PO4 draw solution. When the pH value of the draw solution was adjusted to pH 8, the main ion composition was Na þ , NaHPO4 , and HPO2 as shown in 4 Figure S1 and the FO membrane (pore radius of 0.37 nm) exhibited a negative charge [45]. Therefore, positively charged Na þ ions (hydrated radius of 0.36 nm) may easily pass through the FO membrane because of the electrostatic attraction and pore-size effect. When Triton X100 was coupled to Na3PO4 draw solution, adsorption of Triton X100 on the membrane caused by the hydrophobic interaction between the tails of Triton X100 and the membrane constricted membrane pores, significantly reducing reverse salt diffusion of Na þ , NaHPO4 and HPO2 (Fig. 3). This phenomenon is in agreement with Kiso and Jin 4 et al. who observed that the hydrophobic interactions between selected pharmaceuticals and CTA FO membranes were the dominant removal mechanism, and the hydrophobicity of the pharmaceuticals exhibited a strong influence on their rejection; an increase in rejection
Fig. 1. Experimental setup of the lab-scale FO and UF–NF hybrid system using highly charged phosphate coupled with Triton X100 as a draw solution.
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Fig. 2. Comparison of reverse salt flux and water flux with addition of Triton X100 into Na3PO4 draw solution (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 25 7 0.5 1C, pH of 8, using DI water as feed solution).
Fig. 3. Schematic illustration of reduced back diffusion of anions and cations with presence of Triton X100 during FO.
with increasing their hydrophobicity was observed [43,46]. However, water flux decreased slightly when increasing the concentration of Triton X100 because of the increased internal concentration polarization and viscosity of the draw solution, thus changing the diffusivity of water through the FO membrane, as shown in Fig. 4. The optimal Triton X100 concentration was observed at 0.5 mM, which simultaneously achieved high water flux (2.72 L/m2 h), low reverse flux (0.08 g/m2 h), and low specific reverse salt flux (0.03 g/L).
3.2. Effect of various Na3PO4 concentrations on water flux and reverse salt flux To evaluate the effect of Na3PO4 concentration on the FO performance, Na3PO4 concentrations from 0.01 to 0.55 M with a fixed Triton X100 concentration of 0.5 mM and a pH value of 8 were investigated. Fig. 5 presents the water flux and reverse flux as a function of the draw solution concentration during FO. High water flux was observed at high Na3PO4 concentrations because of an increase in driving force across the membrane. Water flux increased gradually but nonlinearly from 1.03 to 5.68 L/m2 h when increasing the Na3PO4 concentration from 0.01 to 0.55 M because
of the external concentration polarization and internal concentration polarization effects of the FO membrane. Zhao and Zou [47] showed that when the solution against the membrane support layer had a lower aqueous diffusivity but higher viscosity, the ICP phenomenon will be more severe, resulting in lower water flux. According to Alnaizy et al. [29], the experimental water flux was observed to be 7 times lower than the ideal water flux. Similarly, reverse salt flux increased slightly when the draw solution concentration was increased because of the increased amount of free Na þ ions in the draw solution. Fig. 6 indicates that specific reverse salt flux (Js/Jw) decreased slightly when the draw solution concentration increased during FO. At 0.55 M Na3PO4 coupled with 0.5 mM Triton X100, the ratio of Js/Jw was 0.03 g/L (i.e., for each liter of water recovered via the FO membrane, approximately 0.03 g of Na3PO4 draw solute diffuses to the feed side). The reason for decrease in Js/Jw was due to rapid increase in water flux (Jw) but slow increase in reverse salt flux (Js). Compared with the current most widely used draw solutes for FO, such as MgCl2, NaCl, CaCl2, Ca(NO3)2, and NH4HCO3, the draw solute of Na3PO4 coupled with Triton X100 in this study exhibited less loss (approximately 0.03 g) than did the aforementioned ionic salts (0.22–2.48 g) [25] and PAA-2Na (0.08 g) [48] per liter of water recovered (Table 4).
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Fig. 4. Effect of various Triton X100 concentrations on osmotic pressure and viscosity of the draw solution.
Fig. 5. Effect of draw solution concentration on water flux and reverse salt flux during FO (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 257 0.5 1C, pH of 8, using DI water as feed solution).
Fig. 6. Effect of various Na3PO4 concentrations on viscosity and specific reverse salt flux (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 257 0.5 1C, using DI water as feed solution).
3.3. Forward osmosis desalination process Fig. 7 shows the water flux of the FO process for desalinating brackish water of various TDS concentrations from 4090 to
22,900 ppm using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as a draw solution. It is observed that water flux decreased quickly when the osmotic pressure gradient between the draw and feed solutions decreased. The feed solution with the
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Table 4 Comparison of reverse salt flux and specific reverse salt from various draw solutions for CTA FO membranes. Draw solution
Concentration (M)
Osmotic pressure (Mpa)
Js (g/m2 h)
Js/Jw (g/L)
References
MgSO4 Na2SO4 MgCl2 NaCl CaCl2 KCl NH4HCO3 KBr PAA-Na (1800) Na3PO4 þTriton
0.62 0.60 0.50 0.60 0.56 0.63 0.69 0.60 0.48 g/mL 0.55 M Na3PO4 þ0.5 mM Triton X100
1.4 2.8 4.2 2.8 4.2 2.8 2.8 2.8 3.6 3.8
0.90 1.90 5.60 7.20 9.50 12.30 18.20 22.00 0.60 0.13
0.22 0.36 0.58 0.74 0.82 1.13 2.48 2.15 0.08 0.03
[25] [25] [25] [25] [25] [25] [25] [25] [48] Present study
Fig. 7. Water fluxes for FO desalination of brackish water and seawater using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as the draw solution (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 25 7 0.5 1C, pH of 8). Error bars represent the standard deviation of experiments performed in triplicate.
lowest TDS (4090 ppm) achieved the highest water flux (Jw ¼4.89 L/m2 h), followed by feed solutions with a TDS of 5020 ppm (Jw ¼ 4.71 L/m2 h), 6450 ppm (Jw ¼4.26 L/m2 h), 12, ), ppm (Jw ¼ 3.04 L/m2 h), and 22, 22 ppm (Jw ¼2.15 L/m2 h). Similarly, the FO experiment conducted using seawater (36,800 ppm) as a feed solution achieved an average flux of 1.15 L/m2 h. 3.4. Recovery process using a continuous two-stage ultrafiltration– nanofiltration system A continuous pressure-driven UF–NF process was used to reconcentrate the diluted draw solution after FO tests. Fig. 8 shows size distribution of 0.5 mM Triton X100 coupled with 0.1 M Na3PO4 at pH 8. In the first stage of the recovery process, UF was used to retain the draw solution for large particles with mean size of 15.2 nm, as specified in Fig. 8. The first-stage permeate stream was continuously re-concentrated using the NF membrane at the second stage. The first- and second-stage concentrate streams were returned to the draw solution tank for reuse. Table 5 shows the variation in reconcentration efficiency of the draw solution using UF and NF membranes. It is recorded that the first-stage UF achieved a solute rejection of 36% because the membrane pore of UF was large and individual ions could easily pass through membrane. However, the efficiency of solute rejection of the NF membrane reached 96% at the second stage because of electrostatic repulsion and a steric-hindrance effect. At pH 8, Na3PO4 was converted to 25.9% divalent of HPO24 , 4.1% of H2PO4 and 70%
complex formation of NaHPO4 (simulated from Mineql þbased on the chemical equilibrium model from the thermodynamic database [49,50]), and NF-TS80 membrane was negatively charged [51–54]. Thus, the negatively charged membrane repulsed the negatively charged HPO24 and NaHPO4 , increasing the efficiency of solute rejection. Furthermore, the secondary layer of Triton X100 that formed on the membrane surface also enhanced solute rejection. Although a total solute rejection of 98% was achieved when using a two-stage UF–NF system, a higher hydraulic pressure or other membrane process such as membrane distillation should be used to further enhance the reconcentration efficiency.
Fig. 8. Size distribution of 0.5 mM Triton X100 coupled with 0.1 M Na3PO4 at pH 8.
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Table 5 Recovery efficiency of draw solution using a two-stage UF–NF system. Feed solute (TDS, ppm)
12,6407 36
First stage—UF
Second stage—NF
Total rejection (%)
TDS (ppm)
Rejection (%)
TDS (ppm)
Rejection (%)
8040 7 17
367 0.15
2757 9
967 0.16
4. Conclusion The results obtained from the laboratory scale FO tests suggest that coupling Triton X100 to the Na3PO4 draw solution could reduce reverse salt flux successfully for FO. Compared with traditional draw solutes for FO (0.22–2.48 g), the draw solute of Na3PO4 coupled with Triton X100 in this work exhibited less loss (approximately 0.03 g) per liter of water recovered. The results indicated that using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as a draw solution could achieve water fluxes of 4.89 L/m2 h and 1.15 L/m2 h, corresponding to feed solutions with TDS of 4090 and 36,800 ppm. In addition, Triton X100 coupled with a highly charged phosphate could be easily recovered using a two-stage UF–NF system with solute rejection of 98%.
Acknowledgment The authors would like to acknowledge the sponsor of the Ministry of Science and Technology of the Republic of China under the Grant number of 101-2221-E-027-061-MY3.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Exploring an innovative surfactant and phosphate-based draw solution for forward osmosis desalination Hau Thi Nguyen a, Shiao-Shing Chen a,n, Nguyen Cong Nguyen a, Huu Hao Ngo b, Wenshan Guo b, Chi-Wang Li c a Institute of Environmental Engineering and Management, National Taipei University of Technology, No.1, Section 3, Chung–Hsiao E. Rd, Taipei 106, Taiwan, ROC b Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology, Sydney, Broadway, NSW 2007, Australia c Department of Water Resources and Environmental Engineering, TamKang University, 151 Yingzhuan Road, Tamsui District, New Taipei City 25137, Taiwan, ROC
art ic l e i nf o
a b s t r a c t
Article history: Received 5 November 2014 Received in revised form 5 January 2015 Accepted 22 March 2015 Available online 20 April 2015
The reverse salt flux phenomenon of forward osmosis affects the quality of the feed water, reduces water flux, and increases the cost for replenishing lost draw solute. In this study, a novel draw solution comprising a mixture of Triton X100 and Na3PO4 for minimizing the reverse salt flux during forward osmosis (FO) was explored. The results indicated that the reverse salt flux caused by coupling 0.5 mM Triton X100 to 0.55 M Na3PO4 draw solution was only 0.13 g/m2 h, and the specific reverse salt flux was 0.03 g/L using DI water as the feed solution, which are the lowest recorded values among all forward osmosis studies. Hydrophobic attractive interactions between tail groups of Triton X100 with the FO membrane are believed to be the main mechanism for minimizing salt leakage. Results from desalination experiments demonstrated that using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as the draw solution and brackish water and seawater as the feed solution with total dissolved solids of 4090 and 36,800 ppm achieved water fluxes of 4.89 L/m2 h and 1.15 L/m2 h, respectively. Furthermore, using a two-stage ultrafiltration–nanofiltration system for the draw solution recovery enabled 98% recovery of solutes. & 2015 Elsevier B.V. All rights reserved.
Keywords: Forward osmosis Seawater desalination Hydrophobic interaction Surfactant
1. Introduction The increase in the world population accompanied by an increase in agricultural and industrial activities [1], has caused water scarcity to become a concern. Researchers have conducted numerous studies related to desalination of brackish water and seawater to produce potable water. Among the proposed desalination methods, such as thermal desalination, crystallization, ion exchange, and solvent extraction, reverse osmosis (RO) is one of the most promising potential types of technology for seawater desalination [2]. However, several drawbacks are associated with RO, including (1) high energy requirement; (2) low recovery rate; and (3) environmental impact caused by brine discharge [3–6]. To overcome these limitations, researchers in the scientific community must create a more effective desalination technology. Forward osmosis (FO) is a form of technology that has been developed in recent years. It can potentially be applied to seawater desalination
n
Corresponding author. Tel.: þ 886 2 27712171 4142; fax: þ886 2 27214142. E-mail address: [email protected] (S.-S. Chen).
http://dx.doi.org/10.1016/j.memsci.2015.03.085 0376-7388/& 2015 Elsevier B.V. All rights reserved.
[7–10], the food industry [11,12], power generation [13–15], osmotic membrane bioreactors [16–18] and sludge dewatering [19–21]. FO uses natural osmosis as a driving force for separation, and therefore is expected to (1) possess low energy requirements for operation; (2) exhibit less fouling than pressure-driven membrane processes; and (3) achieve a high potential recovery rate. McGinnis et al. [22] demonstrated that energy savings of FO compared to current technologies for seawater desalination on an equivalent work basis are projected to range from 72% to 85%. Altaee et al. [23] found the recovery step in an FO–RO desalination system to use 96–98% of the total power consumption. However, the major challenge of creating marketable FO technology is the lack of an ideal draw solution that can achieve high water flux, low reverse salt flux, and coeffective recovery. Particularly, reverse diffusion of salt from the draw solution to the feed side not only affects the quality of the feed water, but causes water flux decline and increases the cost of replenishing the lost draw solute [24]. Numerous types of draw solutions for FO desalination have been explored in previous studies. For example, monovalent salts (NaCl, KCl, KBr, KNO3, NH4Cl, KHCO3, NaHCO3, NH4HCO3) with
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favorable water solubility are frequently used, but the greatest disadvantage of using these salts as a draw solution for FO is the extremely high salt leakage (reverse salt flux reached 29.2 g/m2 h when using 0.88 M KBr as a draw solution) [25–27]. To overcome this problem, divalent (CaSO4, MgSO4, CuSO4, MgCl2, CaCl2) and trivalent salts (Al2(SO4)3, EDTA–2Na) were proposed as draw solutions [26,28–30]. Compared with monovalent salts, the reverse salt fluxes of divalent and trivalent salts were lower because of the larger hydrated radius and higher electrostatic repulsion. However, the reverse salts still reached 0.9 g/m2 h for 0.62 M MgSO4, 9.5 g/m2 h for 0.56 M CaCl2, 5.6 g/m2 h for 0.50 M MgCl2, and 0.32 g/m2 h for 0.50 M EDTA–2Na. Additionally, Warne and Chung were the first to successfully synthesize magnetic nanoparticles (MNPs) for use as innovative draw solutes for FO. This concept provided the ideal draw solution without salt leakage, but an agglomeration problem was observed for magnetic nanoparticles during the regeneration stage [31–34]. Recently, zwitterions, hexavalent phosphazene, switchable polarity solvents, 2-methylimidazole-based organic compounds, and ferric and cobaltous hydroacid complexes have also been investigated as potential draw solutes for FO [35–39]. Although these draw solutes created high osmotic pressure, synthesis of the solutes was costly and recovery of the diluted draw solution was complex. In this study, a novel draw solution for minimizing the reverse flux of ions during FO desalination by coupling nonionic surfactant (Polyethyleneglycol tert-octylphenyl ether, Triton X100) to a Na3PO4 draw solution was explored. It is hypothesized that the hydrophobic interactions between tail groups of Triton X100 with membrane would form an additional layer on the membrane surface, preventing ions from escaping through membrane pores, thus reducing reverse salt flux. The reasons of using Triton X100 instead of other non-ionic surfactants are due to lower critical micelle concentrations (CMC) of 0.4 mM but larger molecule of Triton X100. Moreover, above the CMC, Triton X100 solution aggregates to form micelles that can couple with the trivalent phosphate to enlarge the molecular size of the draw solute, resulting in enhanced draw solute recovery using a two-stage ultrafiltration–nanofiltration (UF–NF) system. In addition, as compared with RO, UF–NF has been considered as a more energy efficient draw regeneration process using highly charged salts as draw solutions [26,40].Therefore, the objectives of this study were to systematically investigate the effects of coupling Triton X100 to a Na3PO4 draw solution on reverse salt flux and water flux for FO following: (1) effects of various concentrations of Triton X100; (2) effects of various concentrations of Na3PO4; (3) evaluation of desalination efficiency of the proposed draw solution and (4) recovery of the diluted draw solution.
Table 1 Characteristics of ultrafiltration and nanofiltration membranes. Membrane name
Manufacture Material
UF-GE
GE osmonics
NF-TS80
TriSep
Molecular weight cut-off 25 1C pH (MWCO) range
TF (Thin 1000 Film) Polyamide 150
1–11 2–11
213
2. Materials and methods 2.1. FO membranes The flat-sheet cellulose triacetate (CTA) FO membranes used in this study were supplied by Hydration Technology Innovations (HTIs OsMem™ CTA Membrane 130806, Albany, OR, USA) with size of 15 22 cm2 for each piece. The FO membrane possessed a water permeation coefficient of 3.07 10 12 m s 1 Pa 1 and salt rejection of approximately 95–99% [8,40]. The overall thickness of the membrane was approximately 50 mm, and the FO membrane was negatively charged at pH 44.5 [41]. The contact angle of the CTA FO membrane was determined to be 60–801, indicating that the membrane was also moderately hydrophobic [42,43]. Table 1 shows the characteristics of the UF and NF membranes used for draw solution recovery. 2.2. Feed solution and draw solution In FO experiments, deionized (DI) water, synthetic brackish water, and seawater were used as feed solutions. The synthetic brackish water and seawater were prepared with total dissolved solid (TDS) from 4900 to 36,800 ppm by adding NaCl salt to DI water as shown in Table 2. The draw solution was prepared using laboratory-grade Na3PO4 12H2O (Merck Co., Ltd., Germany) mixed with Triton X100 (Sigma-Aldrich, USA) as shown in Table 3 at mole ratios of 1000:1, 500:1, 200:1, 100:1, and 20:1 at room temperature for 60 min. These mixtures of phosphate sodium and Triton X100 were then maintained at pH 8 by adding phosphoric acid (H3PO4, Merck, 85% purity) and were continually stirred for 24 h before performing FO tests. 2.3. Experimental setup and product water recovery FO experiments were conducted using a dual-channel crossflow FO membrane cell, as shown in Fig. 1. The FO test cell (FO Sterlitech, USA) was designed with symmetric channels on both sides for the feed and draw solutions, and each channel was 4.5 cm in width, 9.2 cm in length, and 0.2 cm in height. The total effective FO membrane area for mass transfer was 41.40 cm2. Two peristaltic pumps (Masterflex L S Drive, Model 7518-00) with a flow rate of 500 mL/min were used to circulate the feed and draw solutions on both sides of the FO membrane. The temperature during experiments was maintained at 25 7 0.5 1C using two water baths. Conductivity and pH sensors were installed in the containers for the feed solution and the draw solution to monitor any changes. The volume of the draw solution was 4 L; the feed solution tank (2 L) was placed on a digital weighing scale (BW12KH, Shimadzu, Japan) connected to a computer data logging system to monitor the weight and volume changes at specified time intervals. All data were obtained from three repeated tests. The experimental water flux (Jw, L/m2 h) was calculated by measuring the volume change in the feed container based on time as follows: Jw ¼
ΔV A Δt
ð1Þ
Table 2 Synthetic brackish water and seawater as feed solutions at various TDS concentrations. Feed solution
Brackish water 1 (BW1) Brackish water 2(BW2) Brackish water 3 (BW3) Brackish water 4 (BW4) Brackish water 5 (BW5) Seawater
TDS, ppm 4090 Osmotic pressure, bar 2.43
5020 3.10
6450 4.06
12,270 7.85
22,900 15.34
36,800 26.61
214
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where ΔV is the total increase in volume of permeate water (L) collected over a predetermined amount of time Δt (h), and A is the effective FO membrane area (m2). The reverse solute flux (Js, g/m2 h) of the draw solution was determined by the amount of salt accumulated in the feed tank, as indicated in the following equation: Js ¼
V t :C t V 0 :C 0 A:t
ð2Þ
where Ct and Vt are the concentration and volume of feed solution measured at time t, respectively, and C0 and V0 are the initial concentration and volume of feed solution. Specific reverse salt flux (Js/Jw, g/L) is defined here as the ratio of salt (Js, g/m2 h) in the reverse direction and water flux (Jw, L/m2 h) in the forward direction to estimate the amount of draw solute lost per liter of water produced during FO. The amount of reverse salt was determined by converting the electrical conductivity, measured using a calibrated conductivity meter (Oakton Instruments, USA) because the large Triton-X 100 (647 Da) was completely removed by the FO membrane. After FO tests, the diluted draw solution was recovered using a two-stage UF–NF system with a crossflow module (CF042 Crossflow Membrane Test Cell, Delrin Acetal, USA) under operating hydraulic pressures of 4 and 8 bars , respectively. The filtration experiments were repeated at least three times using fresh membranes. 2.4. Analytical methods The concentrations of Na þ , Cl , and PO34 -P were analyzed using ion chromatography (a Dionex ICS-90) and an Ultraviolet– visible spectrophotometer (HACH Model DR-4000, Japan). The osmolality of solutions was measured using an osmometer (Model 3320, Advanced Instruments, Inc., USA), based on the freezingpoint depression method [44]. The viscosity was determined by a Table 3 Properties of the surfactant used in this study. Surfactant name
Type
Mol. wt. Abbreviation CMC (g/mol) (mM)
Polyethyleneglycol tertoctylphenyl ether
Nonionic 647
Triton X100
0.4
vibro viscometer (AD Company, Japan). The particle size of the draw solution was measured using a nanoparticle analyzer SZ-100 (Horiba, Japan). In addition, Mineqlþ software will be used to determine complex and charged formations of the draw solution at different pH values based on the chemical equilibrium model from the thermodynamic database.
3. Results and discussions 3.1. Effects of various Triton X100 concentrations on reverse salt and water flux Fig. 2 shows the reverse salt fluxes and water fluxes for six draw solutions with various Triton X100 concentrations coupled with a fixed Na3PO4 concentration of 0.1 M. The FO experiments were conducted in membrane; the orientation of the active layer facing the feed solution, which was DI water. It was observed that the reverse flux was reduced significantly when coupling Triton X100 with concentrations ranging from 0.1 to 5 mM into the Na3PO4 draw solution. Compared with pure Na3PO4 (Js ¼0.62 g/ m2 h), 0.1 M Na3PO4 draw solution coupled with 0.5 mM Triton X100 exhibited less reverse salt flux (Js ¼ 0.08 g/m2 h). In particular, Fig. 2 indicates that coupling higher concentrations of Triton X100 to the Na3PO4 draw solution achieved lower reverse salt flux. For example, Js was reduced from 0.11 to 0.07 g/m2 h when 0.1 to 5 mM Triton X100 was coupled with 0.1 M Na3PO4 draw solution. When the pH value of the draw solution was adjusted to pH 8, the main ion composition was Na þ , NaHPO4 , and HPO2 as shown in 4 Figure S1 and the FO membrane (pore radius of 0.37 nm) exhibited a negative charge [45]. Therefore, positively charged Na þ ions (hydrated radius of 0.36 nm) may easily pass through the FO membrane because of the electrostatic attraction and pore-size effect. When Triton X100 was coupled to Na3PO4 draw solution, adsorption of Triton X100 on the membrane caused by the hydrophobic interaction between the tails of Triton X100 and the membrane constricted membrane pores, significantly reducing reverse salt diffusion of Na þ , NaHPO4 and HPO2 (Fig. 3). This phenomenon is in agreement with Kiso and Jin 4 et al. who observed that the hydrophobic interactions between selected pharmaceuticals and CTA FO membranes were the dominant removal mechanism, and the hydrophobicity of the pharmaceuticals exhibited a strong influence on their rejection; an increase in rejection
Fig. 1. Experimental setup of the lab-scale FO and UF–NF hybrid system using highly charged phosphate coupled with Triton X100 as a draw solution.
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215
Fig. 2. Comparison of reverse salt flux and water flux with addition of Triton X100 into Na3PO4 draw solution (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 25 7 0.5 1C, pH of 8, using DI water as feed solution).
Fig. 3. Schematic illustration of reduced back diffusion of anions and cations with presence of Triton X100 during FO.
with increasing their hydrophobicity was observed [43,46]. However, water flux decreased slightly when increasing the concentration of Triton X100 because of the increased internal concentration polarization and viscosity of the draw solution, thus changing the diffusivity of water through the FO membrane, as shown in Fig. 4. The optimal Triton X100 concentration was observed at 0.5 mM, which simultaneously achieved high water flux (2.72 L/m2 h), low reverse flux (0.08 g/m2 h), and low specific reverse salt flux (0.03 g/L).
3.2. Effect of various Na3PO4 concentrations on water flux and reverse salt flux To evaluate the effect of Na3PO4 concentration on the FO performance, Na3PO4 concentrations from 0.01 to 0.55 M with a fixed Triton X100 concentration of 0.5 mM and a pH value of 8 were investigated. Fig. 5 presents the water flux and reverse flux as a function of the draw solution concentration during FO. High water flux was observed at high Na3PO4 concentrations because of an increase in driving force across the membrane. Water flux increased gradually but nonlinearly from 1.03 to 5.68 L/m2 h when increasing the Na3PO4 concentration from 0.01 to 0.55 M because
of the external concentration polarization and internal concentration polarization effects of the FO membrane. Zhao and Zou [47] showed that when the solution against the membrane support layer had a lower aqueous diffusivity but higher viscosity, the ICP phenomenon will be more severe, resulting in lower water flux. According to Alnaizy et al. [29], the experimental water flux was observed to be 7 times lower than the ideal water flux. Similarly, reverse salt flux increased slightly when the draw solution concentration was increased because of the increased amount of free Na þ ions in the draw solution. Fig. 6 indicates that specific reverse salt flux (Js/Jw) decreased slightly when the draw solution concentration increased during FO. At 0.55 M Na3PO4 coupled with 0.5 mM Triton X100, the ratio of Js/Jw was 0.03 g/L (i.e., for each liter of water recovered via the FO membrane, approximately 0.03 g of Na3PO4 draw solute diffuses to the feed side). The reason for decrease in Js/Jw was due to rapid increase in water flux (Jw) but slow increase in reverse salt flux (Js). Compared with the current most widely used draw solutes for FO, such as MgCl2, NaCl, CaCl2, Ca(NO3)2, and NH4HCO3, the draw solute of Na3PO4 coupled with Triton X100 in this study exhibited less loss (approximately 0.03 g) than did the aforementioned ionic salts (0.22–2.48 g) [25] and PAA-2Na (0.08 g) [48] per liter of water recovered (Table 4).
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Fig. 4. Effect of various Triton X100 concentrations on osmotic pressure and viscosity of the draw solution.
Fig. 5. Effect of draw solution concentration on water flux and reverse salt flux during FO (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 257 0.5 1C, pH of 8, using DI water as feed solution).
Fig. 6. Effect of various Na3PO4 concentrations on viscosity and specific reverse salt flux (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 257 0.5 1C, using DI water as feed solution).
3.3. Forward osmosis desalination process Fig. 7 shows the water flux of the FO process for desalinating brackish water of various TDS concentrations from 4090 to
22,900 ppm using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as a draw solution. It is observed that water flux decreased quickly when the osmotic pressure gradient between the draw and feed solutions decreased. The feed solution with the
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217
Table 4 Comparison of reverse salt flux and specific reverse salt from various draw solutions for CTA FO membranes. Draw solution
Concentration (M)
Osmotic pressure (Mpa)
Js (g/m2 h)
Js/Jw (g/L)
References
MgSO4 Na2SO4 MgCl2 NaCl CaCl2 KCl NH4HCO3 KBr PAA-Na (1800) Na3PO4 þTriton
0.62 0.60 0.50 0.60 0.56 0.63 0.69 0.60 0.48 g/mL 0.55 M Na3PO4 þ0.5 mM Triton X100
1.4 2.8 4.2 2.8 4.2 2.8 2.8 2.8 3.6 3.8
0.90 1.90 5.60 7.20 9.50 12.30 18.20 22.00 0.60 0.13
0.22 0.36 0.58 0.74 0.82 1.13 2.48 2.15 0.08 0.03
[25] [25] [25] [25] [25] [25] [25] [25] [48] Present study
Fig. 7. Water fluxes for FO desalination of brackish water and seawater using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as the draw solution (active layer facing the feed solution, flow rate of 500 mL/min, temperature of 25 7 0.5 1C, pH of 8). Error bars represent the standard deviation of experiments performed in triplicate.
lowest TDS (4090 ppm) achieved the highest water flux (Jw ¼4.89 L/m2 h), followed by feed solutions with a TDS of 5020 ppm (Jw ¼ 4.71 L/m2 h), 6450 ppm (Jw ¼4.26 L/m2 h), 12, ), ppm (Jw ¼ 3.04 L/m2 h), and 22, 22 ppm (Jw ¼2.15 L/m2 h). Similarly, the FO experiment conducted using seawater (36,800 ppm) as a feed solution achieved an average flux of 1.15 L/m2 h. 3.4. Recovery process using a continuous two-stage ultrafiltration– nanofiltration system A continuous pressure-driven UF–NF process was used to reconcentrate the diluted draw solution after FO tests. Fig. 8 shows size distribution of 0.5 mM Triton X100 coupled with 0.1 M Na3PO4 at pH 8. In the first stage of the recovery process, UF was used to retain the draw solution for large particles with mean size of 15.2 nm, as specified in Fig. 8. The first-stage permeate stream was continuously re-concentrated using the NF membrane at the second stage. The first- and second-stage concentrate streams were returned to the draw solution tank for reuse. Table 5 shows the variation in reconcentration efficiency of the draw solution using UF and NF membranes. It is recorded that the first-stage UF achieved a solute rejection of 36% because the membrane pore of UF was large and individual ions could easily pass through membrane. However, the efficiency of solute rejection of the NF membrane reached 96% at the second stage because of electrostatic repulsion and a steric-hindrance effect. At pH 8, Na3PO4 was converted to 25.9% divalent of HPO24 , 4.1% of H2PO4 and 70%
complex formation of NaHPO4 (simulated from Mineql þbased on the chemical equilibrium model from the thermodynamic database [49,50]), and NF-TS80 membrane was negatively charged [51–54]. Thus, the negatively charged membrane repulsed the negatively charged HPO24 and NaHPO4 , increasing the efficiency of solute rejection. Furthermore, the secondary layer of Triton X100 that formed on the membrane surface also enhanced solute rejection. Although a total solute rejection of 98% was achieved when using a two-stage UF–NF system, a higher hydraulic pressure or other membrane process such as membrane distillation should be used to further enhance the reconcentration efficiency.
Fig. 8. Size distribution of 0.5 mM Triton X100 coupled with 0.1 M Na3PO4 at pH 8.
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Table 5 Recovery efficiency of draw solution using a two-stage UF–NF system. Feed solute (TDS, ppm)
12,6407 36
First stage—UF
Second stage—NF
Total rejection (%)
TDS (ppm)
Rejection (%)
TDS (ppm)
Rejection (%)
8040 7 17
367 0.15
2757 9
967 0.16
4. Conclusion The results obtained from the laboratory scale FO tests suggest that coupling Triton X100 to the Na3PO4 draw solution could reduce reverse salt flux successfully for FO. Compared with traditional draw solutes for FO (0.22–2.48 g), the draw solute of Na3PO4 coupled with Triton X100 in this work exhibited less loss (approximately 0.03 g) per liter of water recovered. The results indicated that using 0.55 M Na3PO4 coupled with 0.5 mM Triton X100 as a draw solution could achieve water fluxes of 4.89 L/m2 h and 1.15 L/m2 h, corresponding to feed solutions with TDS of 4090 and 36,800 ppm. In addition, Triton X100 coupled with a highly charged phosphate could be easily recovered using a two-stage UF–NF system with solute rejection of 98%.
Acknowledgment The authors would like to acknowledge the sponsor of the Ministry of Science and Technology of the Republic of China under the Grant number of 101-2221-E-027-061-MY3.
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