A study of poly (sodium 4-styrenesulfonate) as draw solute in forward osmosis

Desalination 360 (2015) 130–137

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A study of poly (sodium 4-styrenesulfonate) as draw solute in forward osmosis Enling Tian a, Chengbo Hu b, Yan Qin c, Yiwei Ren a,⁎, Xingzu Wang a, Xiao Wang a, Ping Xiao a, Xin Yang b a Center of Membrane Technology and Application Engineering, Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, No. 266 Fangzheng Avenue, Shuitu Hi-tech Industrial Park, Shuitu Town, Beibei District, Chongqing 400714, China b Chongqing Key Laboratory of Environmental Materials & Remediation Technologies, Honghe Campus, Chongqing University of Arts and Sciences, No. 319 Honghe Avenue, Yongchuan District, Chongqing 402160, China c Environmental Monitoring Station of Banan District, Chongqing 401320, China

H I G H L I G H T S • • • • •

Novel draw solutes based on PSS have been studied. The pH, conductivity and viscosity of PSS have also been investigated. 0.24 g·mL− 1 PSS (70,000) draw solute exhibits the best FO performance. The repeatability of FO performance improves with increasing the PSS Mw. The PSS was easily recycled by a low pressure-driven UF system under 2 bar.

a r t i c l e

i n f o

Article history: Received 29 May 2014 Received in revised form 7 December 2014 Accepted 1 January 2015 Available online 24 January 2015 Keywords: Draw solution Forward osmosis Poly (sodium 4-styrenesulfonate) Salt leakage Water flux

a b s t r a c t Draw solution (DS) has a great influence on the forward osmosis (FO) technology. The study of novel draw solute is essential in the development of FO technology. In this paper, poly (sodium 4-styrenesulfonate) (PSS) polyelectrolytes with different molecular weights (Mws) and different concentrations were studied. The physical properties, such as pH, conductivity and viscosity, have also been investigated. The conductivity increases with the increase of PSS concentration, which may lead to higher osmotic pressure. Higher viscosity, lower diffusion coefficient and more severe concentration polarization, which is generated by the polyelectrolyte with higher Mw, result in a lower water flux. Among the PSS polyelectrolytes, 0.24 g·mL−1 PSS (70,000) exhibits the best FO flux. Experiment results demonstrate the advantage of using PSS as draw solute to conventional ionic salt of 0.5 mol·L−1 NaCl. The regeneration of PSS from diluted DSs and the repeatability of the FO performance after recovery have been evaluated. The PSS was easily recycled by a low pressure-driven ultrafiltration (UF) system under 2 bar with low energy consumption. In order to realize a satisfactory regeneration of PSS DS in the FO process, it is necessary to select or prepare an appropriate UF membrane with accurate MWCO. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Forward osmosis (FO) technology, utilizing the natural phenomenon of osmosis, is an emerging and a novel technology for seawater/ brackish desalination [1,2], wastewater treatment [3,4], food processing [5,6], power generation [7,8], protein and pharmaceutical enrichment [9,10]. FO has edge over pressure-driven membrane processes, such as reverse osmosis (RO), nanofiltration (NF) in terms of energy consumption and fouling consistence [11,12]. In the FO unit, the energy required to transport water across the membrane is almost negligible. Moreover, ⁎ Corresponding author. E-mail address: [email protected] (Y. Ren).

http://dx.doi.org/10.1016/j.desal.2015.01.001 0011-9164/© 2015 Elsevier B.V. All rights reserved.

FO exhibits high rejections to many contaminants [1]. By virtue of these unique features, FO has gained attentions of the worldwide researchers. Both the semi-permeable membrane and draw solution (DS) have great effects on the FO process. Lots of efforts reported in the literatures have focused on the development of FO membranes [13–16] and the design or economics of process [17–19]. Less attention has been paid to find potentially appropriate draw solutes and their regeneration methods. The draw solute leakage in FO process and high energy consumption during the recovery of draw solutes from diluted DSs are the main issues, which constrain the development of FO technology. Desirable draw solutes are supposed to possess the following characteristics: (1) high osmotic pressure which may induce a high water flux; (2) minimal reverse draw solute flux; (3) easy recovery from the

E. Tian et al. / Desalination 360 (2015) 130–137

chloride (NaCl, crystalline, ≥99.5%) was provided by the National Medicine Group Chemical Reagent Co. Ltd. (China). DI water was produced by an ultrapure water system (Molecular∑H2O®, China). 2.2. The preparation of PSS solution and NaCl solution The 0.48 g·mL−1 of PSS (70,000) solution was prepared by dissolving 72 g of PSS (70,000) powder into DI water at room temperature. The resultant solution was stirred at room temperature until all was dissolved and finally the volume of the solution was fixed at 150 mL. 0.24 g·mL−1, 0.12 g·mL−1, and 0.04 g·mL−1 of PSS (70,000) solutions were achieved by dilution method. A similar procedure was followed for the preparation of 0.48 g·mL−1, 0.24 g·mL− 1, 0.12 g·mL− 1, and 0.04 g·mL−1 of PSS (200,000) solutions. In addition, as the viscosity of PSS (1,000,000) is higher than that of PSS (70,000) and PSS (200,000), PSS (1,000,000) is not easily soluble in water when the concentration is greater than 0.24 g·mL−1. So only 0.04 g·mL−1, 0.12 g·mL−1, and 0.24 g·mL−1 of PSS (1,000,000) solutions were prepared by the similar method as that of PSS (70,000). As comparison, a 0.5 mol·L− 1 NaCl solution was used as the DS. All solutions were stored in a fridge refrigerator at 4 °C. 2.3. Characterization of PSS At these concentrations, the pH values of PSS (70,000), PSS (200,000) and PSS (1,000,000) were tested by an acidometer (PB-10, Sartorius, Beijing, China). Electrical conductivity of the solutions was measured by Ray Magnetic Conductivity Meter (DDSJ-308A, Rex Electric Chemical, China) to estimate the degree in ionization. The viscosity of PSS (70,000), PSS (200,000) and PSS (1,000,000) of different concentrations is measured by a viscosity meter (DV-II, Brookfield, America) at 25 °C, 30 °C, 35 °C, and 40 °C, respectively. 2.4. FO process FO experiments were conducted on a lab-scale system, as shown in Fig. 5(a). Commercial thin film composite (TFC) FO membranes from Hydration Technologies Inc. (HTI, USA) were used, which involve a dense selective active layer onto a phase inversed polysulfone supporting layer. DSs were prepared from PSS (70,000), PSS (200,000) and PSS (1,000,000). DI water was used as the FS during FO experiments. The test was conducted with the membrane active layer facing the DS at room temperature (23 °C ± 1), and both FS and DS circulated at a fixed volumetric flow rate (184 mL/min). A balance (BSA6202S-CW, Sartorius, Beijing, China) connected to a computer logged the mass of water permeating into the DS from the FS over a selected period of time.

10

8

6

4

2. Materials and methods

2

2.1. Materials

0

Poly (sodium-4-styrenesulfonate) (PSS, Mw = 70,000, 200,000, 1,000,000, Sigma-Aldrich, USA) was used as draw solute. Sodium

PSS (70,000) PSS (200,000) PSS (1,000,000)

12

pH

diluted DS. In addition, it should be nontoxic, reasonably low cost and required to be compatible with the FO membrane. In recent years, some researchers have been focused on inorganic salt draw solutes, such as ammonium carbonate [13], fertilizer [20–22], and magnetic nanoparticles [23–25]. Ammonium carbonate has been the most promising system with very high osmotic pressure and relative ease of regeneration. However, its recycle method is energy intensive and even a trace level of residuals may deteriorate the taste of the product water. Fertilizer has been also employed as draw solute. The diluted fertilizer after FO process can be directly used for fertigation without recovering draw solutes. However, it's only applicable to agriculture. Magnetic nanoparticles, which can generate high osmotic pressure and multifunctional nanoparticles [26] have been found to be used as new DS recently. The efficient regeneration of the magnetic nanoparticles via heat-facilitated magnetic separation is a distinct advantage. But the particles are prone to agglomerate during recycling process via magnetic or electric separators. And the methods have not been tested on a largescale level. In addition, the synthesis of the magnetic nanoparticles is relatively complicated. Lately, stimuli-responsive polymer hydrogel draw solutes have also been widely investigated due to their requiring less energy in the regenerative process [12,27,28]. However, the water flux was low. Furthermore, switchable polarity solvents [29] have also shown a potential to be used as DS. Wilson and his coworkers [29] demonstrated that switchable polarity solvents can be mechanically separated from the purified water after polar to nonpolar phase shift. Nevertheless, it has degradation effects on the cellulose triacetate (CTA) FO membrane. A range of other possible draw solute candidates including organic ionic salt [30,31] and organic compounds [32,33] have also been investigated, which encounter the trade-off between high osmotic pressure and easy regeneration. So far, almost no suitable draw solute can meet all the aforementioned criteria [34]. Strong polyelectrolytes may be an appropriate option, which meet all the aforementioned requirements. Polyelectrolyte of polyacrylic acid sodium salt (PAA-Na) has been explored as draw solute in the FO process [35]. However, it's relatively energy-intensive for the PAA-Na regeneration by ultrafiltration under pressure of 10 bar. Moreover, the mechanism about the molecular weight (Mw) and concentration of the PAA-Na on the water flux has yet to be elaborated on clearly. The following experimental data indicates that the water flux generated by 0.24 g·mL− 1 PSS (70,000) (18.20 LMH) is higher than that of the 0.24 g·mL−1 PAA-Na (1800) (underneath 12 LMH) [35], where both the tests were conducted in the mode of the membrane active layer facing the DS and deionized (DI) water was used as feed solution (FS). Therefore, the aims of this paper are to explore cost-effective draw solutes which can (1) generate a high water flux with a minimal reverse salt diffusion and (2) be easily regenerated by recovery from the diluted DS using ultrafiltration (UF) system with low energy consumption. Poly (sodium-4-styrenesulfonate) (PSS) is chosen as the draw solute for the following reasons: (1) It is highly water-soluble and high degree of dissociation, therefore it can generate high osmotic pressure, which may induce high water flux; (2) its expanded structure with a chain pendant would be expected to minimize the reverse salt diffusion, easily and efficiently separate from diluted DS by low pressure-driven UF process. In this work, three different Mws of PSS salts have been investigated as draw solutes in the FO process. The water flux and reverse salt leakage of the PSS were estimated in FO process. The regeneration of PSS from diluted DSs and the repeatability of the FO performance after recovery have also been evaluated. It is a promising candidate for the use as DS and shows potential in FO applications.

131

0.04

0.24 0.12 -1 Concentration (g⋅mL )

0.48

Fig. 1. A comparison of pH of PSS (70,000), PSS (200,000) and PSS (1,000,000).

132

E. Tian et al. / Desalination 360 (2015) 130–137 PSS (70,000) PSS (200,000) PSS (1,000,000)

40 35

The diluted PSS (70,000), PSS (200,000) and PSS (1,000,000) DSs are concentrated to 150 mL for further FO tests. Self-made flat sheet TFC–FO membranes with a polyamide selective layer onto the electrospun composite nanofiber substrate [36] were used to evaluate the FO performance of PSS (70,000), PSS (200,000) and PSS (1,000,000) before and after regeneration. The main performances of the self-made flat sheet TFC–FO membranes are exhibited in Table 1. During FO experiments, PSS (70,000), PSS (200,000) and PSS (1,000,000) solutions were used as DSs and DI water as the FS. The test was conducted with the membrane active layer facing the DS at room temperature (23 °C ± 1), and both FS and DS circulated at a fixed volumetric flow rate (184 mL/min).

-1

Conductivity (mS cm )

30 25 20 15 10 5 0

0.04

0.12

0.24

0.48

3. Results and discussion

-1

Concentration (g⋅mL )

3.1. Characterization of PSS Fig. 2. A comparison of conductivity of PSS (70,000), PSS (200,000) and PSS (1,000,000).

−2

−1

The osmosis water flux (Jw, Lm h , LMH) was determined based on the weight change of the DS using Eq. (1). Jw ¼

Δm 1  Δt Am

ð1Þ

where Δm (g) is the weight of water permeated from the FS to the DS over a predetermined time Δt (h) during FO tests; and Am (m2) is the effective membrane area. Since PSS salts dissociate and are conductive in their aqueous solutions, some ions can permeate from the DS to the FS. The reverse salt leakage (Js, gm−2 h−1, gMH) was determined from the conductivity measurement of the FS at certain time points based on a standard concentration–conductivity curve. The Js is calculated by Eq. (2). Js ¼

ðCt Vt Þ−ðC0 V0 Þ Am Δt

ð2Þ

where C0 (mol·L− 1) and V0 (L) are the initial salt concentration and feed volume, respectively, while Ct (mol·L− 1) and Vt (L) are the salt concentration and feed volume over a predetermined time △t (h), respectively, during the test. Am (m2) is the effective membrane area. 2.5. Regeneration of PSS draw solute via ultrafiltration and the repeatability of the FO performance

3.1.1. pH of PSS (70,000), PSS (200,000) and PSS (1,000,000) The comparison of pH of PSS (70,000), PSS (200,000) and PSS (1,000,000) is illustrated in Fig. 1. As shown in Fig. 1, there is little impact of concentration on pH value of PSS with different Mws. The PSS (200,000) exhibits faintly acidic characteristics (pH ~ 6), the PSS (1,000,000) presents alkalescence (pH ~ 9) and the PSS (70,000) is almost neutral (pH ~ 7). According to the supplier, TFC–FO membrane made of aromatic polyamide is available in the range of 2.0–12.0 pH environments. Therefore, the membrane will not undergo hydrolysis and the structure does not alter when it is tested in the PSS DS. In all, PSS (70,000), PSS (200,000) and PSS (1,000,000) are suitable for the FO membrane to act as the draw solutes with pH values in the range of 5.95–9.27. 3.1.2. Electrical conductivity of PSS (70,000), PSS (200,000) and PSS (1,000,000) The conductivity of PSS (70,000), PSS (200,000) and PSS (1,000,000) at different concentrations is recorded in Fig. 2. It is observed that the conductivity increases dramatically with the polymer concentration ranging from 0.04 g·mL− 1 to 0.48 g·mL − 1 . At low concentration (0.04 g·mL− 1), there is little impact of Mw on conductivity. However, the conductivity increases with the increase of PSS Mw at higher concentrations (0.12 g·mL− 1 –0.48 g·mL − 1). The numbers of charged ions increase with the increase of the concentration, which leads to an increase in electrical conductivity. However,

ΔV 1  : Jw ¼ Δt Am

ð3Þ

When using DI water as the feed, the pure water flux of the UF membrane was 132.69 LMH under 2 bar, calculated by Eq. (3). The salt rejection of the system indicates the percentage of the draw solute that is retained by the membrane and is calculated by Eq. (4).   Cp R ¼ 1−  100% Cf

Viscosity (cp)

After the FO tests, the dilute draw solutions of PSS (70,000), PSS (200,000) and PSS (1,000,000) were reconcentrated by a lab-scale pressure-driven setup as depicted in Fig. 5(b). UF membrane (Batch No. PT8273856, GE Water & Process Technologies, USA) with MWCO of 5 kDa was used in the PSS regeneration under a pressure of 2 bar. Jw was calculated by dividing the volumetric permeate rate (L·h−1) by the membrane area (m2),

PSS (70,000) PSS (200,000) PSS (1,000,000)

16000 14000 12000 10000 8000 6000 4000 2000

where R is the salt rejection, Cf (g·mL−1) and Cp (g·mL−1) represent the salt concentration in the feed and permeate solution, respectively.

1014

0.24

0.48

140 120 100 80 60 40 20 0

0.04

ð4Þ

537.9

0.12

-1

Concentration (g⋅mL ) Fig. 3. The viscosity of PSS (70,000), PSS (200,000) and PSS (1,000,000) as a function of concentration.

E. Tian et al. / Desalination 360 (2015) 130–137 -1

0.04 (g⋅mL )

a)

-1

0.12 (g⋅mL )

1000

-1

0.24 (g⋅mL ) -1

600

Viscosity (cp)

conductivity of 39.7 mS/cm. The higher electrical conductivity may lead to higher osmotic pressure [20], thus 0.48 g·mL − 1 PSS (200,000) is expected to have better FO performance as DS.

0.48 (g⋅mL )

800

400

30

20

10

0

133

25

30

35

40

o

Temperature ( C)

3.1.3. Viscosity of PSS (70,000), PSS (200,000) and PSS (1,000,000) The viscosity of PSS (70,000), PSS (200,000) and PSS (1,000,000) at different concentrations is obtained in Fig. 3. As for PSS (70,000) and PSS (200,000), it can be seen that there is only a small viscosity increment when increasing PSS concentration from 0.04 to 0.24 g·mL−1. However, a significant increase in viscosity was noted from 0.24 g·mL− 1 to 0.48 g·mL−1. It is also observed that there is a fast growth in viscosity as the concentrations of PSS (1,000,000) increase from 0.04 to 0.24 g·mL−1. At the same mass concentration, the PSS with a larger Mw exhibits a larger value of viscosity. The viscosity variation of PSS (70,000), PSS (200,000) and PSS (1,000,000) as a function of temperature is shown in Fig. 4(a), (b) and (c), respectively. As shown in Fig. 4(a), when the concentration

-1

b) 16000

0.04 (g⋅mL )

14000

0.24 (g⋅mL )

12000

0.48 (g⋅mL )

-1

0.12 (g⋅mL )

a)

-1

2

-1

2

10000

Viscosity (cp)

8000 6000

40

3

30 20 10 0

25

35

30

40

o

Temperature ( C) -1 -1

0.12 (g⋅mL )

c)

6

1

0.04 (g⋅mL )

4

-1

0.24 (g⋅mL )

5

500

b)

Viscosity (cp)

400

7

8

300

200

100

9

0 25

30

35

40

10

o

Temperature ( C) Fig. 4. (a) The viscosity of PSS (70,000) as a function of temperature. (b) The viscosity of PSS (200,000) as a function of temperature. (c) The viscosity of PSS (1,000,000) as a function of temperature.

the increment in conductivity is not directly proportional to that of the concentration. The reason may be that the dissociation degree of polyelectrolyte decreases with the increase of the concentration. The PSS (200,000) at 0.48 g·mL− 1 concentration exhibits the highest

4

11

Fig. 5. (a) Schematic diagram of the lab-scale FO process and (b) the recovery of draw solute via ultrafiltration process. 1 Feed solution; 2 Peristaltic pump; 3 FO membrane unit; 4 Draw solution; 5 Balance; 6 Computer; 7 Diaphragm pump; 8 Pressure gauge; 9 UF membrane unit; 10 Valve; 11 Product water.

E. Tian et al. / Desalination 360 (2015) 130–137

Table 1 Properties of self-made flat sheet TFC–FO membranes. Thickness (μm)

Water flux (LMH)

Reverse salt flux (gMH)

Reference

55

21.9

7.0

[36]

Note: The water flux and reverse salt flux were evaluated on a lab-scale FO system; where DI water (FS) and 0.5 mol·L−1 NaCl (DS) were used.

increases from 0.04 g·mL− 1 to 0.24 g·mL−1, the viscosity of PSS (70,000) presents a slow decline. At 0.48 g·mL−1 concentration, the viscosity of PSS (70,000) exhibits a steeper drop off indicating that the viscosity is greatly influenced by temperature at higher concentration. Similar trends are noted in Fig. 4(b) and (c); the viscosity of PSS (200,000) and PSS (1,000,000) decreases slightly at low concentrations (0.04 g·mL− 1–0.24 g·mL−1). Therefore, it's not necessary to reduce the viscosity of the PSS DS at lower concentrations (0.04 g·mL− 1– 0.24 g·mL−1) by increasing the temperature in the FO process. At 0.48 g·mL−1 concentration, the viscosity of PSS (200,000) decreases dramatically with the temperature increasing from 25 °C to 35 °C. Increasing the temperature up to 40 °C leads to an increase in the viscosity of PSS (200,000) (Fig. 4(b)). The reason may be that as the temperature increases, the chain segment becomes more flexible, which enhances the diffusion rate of molecular. As temperature continues to rise, the molecular configuration expands fully and the space for molecular motion gets smaller, especially for the polyelectrolyte with a large Mw at high concentration. Therefore, the viscosity of PSS (200,000) at the concentration of 0.48 g·mL−1 increases after declines.

leakage (under 7.00 gMH). Based on the above analysis, increasing the DS concentration will produce more charged ions of the PSS, which leads to an increase in osmotic pressure and thus, higher water flux and salt leakage. Overall, the salt leakage can be controlled in a low level when PSS is used as DS. This may be attributed to its expanded structure with a chain pendant and its strong charge characteristics [37].

-1

0.04 g⋅mL

a) 30

-1

0.12 g⋅mL

-1

0.24 g⋅mL

-1

0.48 g⋅mL

25

Water Flux (LMH)

134

20

15

10

5

0 5

10

15

20

25

30

35

T(min) 0.04 g⋅mL-1 0.12 g⋅mL-1 0.24 g⋅mL-1 0.48 g⋅mL-1

b) 30

3.2. FO performance of PSS

Water Flux (LMH)

25

20

15

10

5

0 5

10

15

20

25

30

35

T(min)

c) 30 -1

0.04 g⋅mL

-1

0.12 g⋅mL

25

-1

0.24 g⋅mL

Water Flux (LMH)

Fig. 6 presents the water fluxes using PSS (70,000, 200,000, 1,000,000) with different concentrations as draw solutes during a 30 min FO process. The first 10 min of the data was disregarded in the full calculation to allow for transport equilibration. Successive blocks of 30 min of data were averaged to yield the data shown in Fig. 6(a), (b) and (c). For all the concentration range, the variations of fluxes are steady during the initial 30 min FO process because of the constant driving force. Generally, Fig. 6 also demonstrates that the water flux is greater at high concentrations than that at low concentrations. According to Fig. 2, the conductivity increases as the mass concentration increases of PSS. The higher electrical conductivity derives from the increased numbers of charged ions of the PSS, which leads to higher osmotic pressure. And the osmosis pressure is the driving force of FO process. However, the PSS at higher concentration exhibits a larger value of viscosity, as shown in Fig. 3. Based on the two aspects, the PSS polyelectrolyte at an appropriate concentration may exhibit better FO performance if used as the draw solute. A special phenomenon is found in Fig. 6(a) that the water flux of 0.24 g·mL− 1 PSS (70,000) is greater than that of 0.48 g·mL− 1 PSS (70,000), which is due to the larger value of viscosity for 0.48 g·mL−1 PSS (70,000). It also can be known that the increase of water flux is not directly proportional to that of the concentration. This phenomenon may arise from the decrease in the dissociation and increase in the viscosity with increasing DS concentration. As aforementioned, 0.48 g·mL−1 PSS (200,000) exhibits the greatest electrical conductivity (Fig. 2), yet it didn't generate the highest water flux (Fig. 6(a) and (b)). This may be due to the fact that the viscosity of 0.48 g·mL− 1 PSS (200,000) is much higher than that of 0.24 g·mL−1 PSS (70,000). The viscosity of the former is amounted to approximately 750 times as high as that of the latter (Fig. 3). Fig. 7 illustrates the salt leakage of PSS (70,000, 200,000, 1,000,000) with different concentrations during a 30 min FO process. It can be seen in Fig. 7(a), (b) and (c) that increasing the concentration of DS results in an increase in salt leakage. According to Fig. 7(a), the 0.48 g·mL−1 PSS (70,000) has the highest salt leakage, while the 0.04 g·mL−1, 0.12 g·mL− 1 and 0.24 g·mL− 1 PSS (70,000) exhibit much lower salt

20

15

10

5

0 5

10

15

20

25

30

35

T(min) Fig. 6. (a) Effect of concentration on the water flux in a 30 min FO process using PSS (70,000) as the draw solute. (b) Effect of concentration on the water flux in a 30 min FO process using PSS (200,000) as the draw solute. (c) Effect of concentration on the water flux in a 30 min FO process using PSS (1,000,000) as the draw solute.

E. Tian et al. / Desalination 360 (2015) 130–137

a) 100 90

0.04(g/ml) 0.12(g/ml) 0.24(g/ml) 0.48(g/ml)

80

60 50 40 30 20 10 0 5

10

15

20

25

30

T(min)

b)

0.04 (g/ml) 0.12 (g/ml) 0.24 (g/ml) 0.48 (g/ml)

60

Salt Leakage (gMH)

50

3.4. Regeneration of PSS from diluted DSs via ultrafiltration and the repeatability of the FO performance

40

30

20

10

0 5

10

15

20

25

30

T(min)

c)

20

0.04(g/ml) 0.12(g/ml) 0.24(g/ml)

15

Salt Leakage (gMH)

of DS among PSS (70,000), PSS (200,000) and PSS (1,000,000), 0.5 mol·L−1 NaCl was also used as a DS in FO tests in the mode of active layer facing the DS. As illustrated in Fig. 8, the average water flux of PSS at the concentration of 0.24 g·mL−1 decreases with the increase of Mw. This phenomenon may arise from the higher viscosity, lower diffusion coefficient and concentration polarization of PSS with higher Mw. As the diffusion coefficient of a draw solute is inversely proportional to solution viscosity [38], a draw solute with large Mw, which exhibits high viscosity, would have a low diffusion coefficient. Therefore, the draw solute with small Mw is favorable. It can be seen in Fig. 8 that the water flux generated by 0.5 mol·L−1 NaCl is 17.14 LMH, while those produced by PSS (200,000) and PSS (70,000) at the concentration of 0.24 g·mL− 1 are 13.04 LMH and 18.20 LMH. Furthermore, PSS (70,000) has lower salt leakage than 0.5 mol·L−1 NaCl. It is commonly believed that the size of polystyrene sulfonate anions in the PSS (70,000) DS is much larger than that of chloride ion in the NaCl DS. So a lower reverse diffusion of draw solutes was observed in the case of PSS (70,000). This is the prominent advantage of using polyelectrolytes as draw solutes.

10

The variations of the water flux and the rejection of PSS (70,000), PSS (200,000) and PSS (1,000,000) with the concentration in the regenerative UF process are shown in Fig. 9(a), (b) and (c). It can be seen that the water flux and the rejection decrease with increasing the concentration of PSS. Fig. 9(a) illustrates that the water flux of PSS (70,000) decreases from 66.35 to 6.18 LMH when the concentration increases from 0.01 to 0.07 g·mL− 1. It can be seen in Fig. 9(b) that the water flux of PSS (200,000) reduced from 50.43 to 6.38 LMH when the concentration increases from 0.01 to 0.09 g·mL−1. Fig. 9(c) showed the water flux of PSS (1,000,000) decreases from 42.02 to 11.6 LMH with the increase of concentration from 0.01 to 0.11 g·mL−1. This may be due to the fact that concentration polarization and/or membrane fouling contribute to the reduced water flux. Moreover, a higher concentration of PSS would result in more solutes permeating through the UF membrane, which leads to the reduction in rejection. At 0.12 g·mL− 1 concentration, the repeatability of the FO performance after regeneration was also evaluated. Fig. 10(a), (b) and (c) exhibits the FO performances of the PSS (70,000), PSS (200,000) and PSS (1,000,000) before and after recovery. As can be seen in Fig. 10(a), the drop in FO flux reaches approximately 40% from the initial value of 16.24 to 9.83 LMH in the mode of the active layer facing the DS. Meanwhile, the salt leakage decreases from 4.72 to 1.23 gMH. It is also

5 22

22 Water flux Salt leakage

20

0

20

18

18

16

16

14

14

12

12

10

10

8

8

6

6

4

4

5

10

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T(min) Fig. 7. (a) Salt leakage of the PSS (70,000) draw solute with different concentrations for 30 min FO process. (b) Salt leakage of the PSS (200,000) draw solute with different concentrations for 30 min FO process. (c) Salt leakage of the PSS (1,000,000) draw solute with different concentrations for 30 min FO process.

Water Flux(LMH)

-5

2

3.3. FO performance comparison of PSS with different molecular weights as draw solutes As aforementioned, PSS with the concentration of 0.24 g·mL−1 exhibits better FO performance. In order to find the best FO performance

0

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0.24g⋅mL PSS (Mw=200,000) -1

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0 -1

0.5mol⋅L NaCl

Fig. 8. FO performance comparison of PSS (70,000), PSS (200,000), PSS (1,000,000) and 0.5 mol·L−1 NaCl as draw solutes using commercial TFC–FO membrane.

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a) 80

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Fig. 9. (a) The recycle of PSS (70,000) with water production by an UF membrane (MWCO: 5 kDa). (b) The recycle of PSS (200,000) with water production by an UF membrane (MWCO: 5 kDa). (c) The recycle of PSS (1,000,000) with water production by an UF membrane (MWCO: 5 kDa).

Fig. 10. (a) The FO performance comparison of PSS (70,000) before and after regeneration. (b) The FO performance comparison of PSS (200,000) before and after regeneration. (c) The FO performance comparison of PSS (1,000,000) before and after regeneration.

observed that both FO flux and salt leakage of PSS (200,000) decrease after recovery from 11.22 to 7.36 LMH and 4.36 gMH to 0.17 gMH (Fig. 10(b)). As for PSS (1,000,000), the reduction of FO flux is less than 30% after regeneration from 9.36 to 6.74 LMH (Fig. 10(c)). On the other hand, the salt leakage decreases from 1.36 to 0.097 gMH. The repeatability of FO performance improves with increasing the PSS Mw.

On the whole, the decrease in the FO flux may be due to the loss of PSS during the UF recovery process. Nevertheless, the ease regeneration of PSS from the diluted draw solution via UF under pressure of 2 bar needs low energy consumption. In order to realize better regeneration of PSS DS in the FO process, it is necessary to select or prepare an appropriate UF membrane with accurate MWCO.

E. Tian et al. / Desalination 360 (2015) 130–137

4. Conclusions Polyelectrolytes of PSS with different Mws have been explored as draw solutes in FO process. The following conclusions can be drawn from this work: (1) Both the conductivity and viscosity increase with an increase in PSS concentration. At the same mass concentration, the PSS with a larger Mw exhibits a larger value of viscosity. At low concentration (0.04 g·mL− 1), there is little impact of Mw on conductivity. However, the conductivity increases with the increase of PSS Mw at higher concentrations (0.12 g·mL−1– 0.48 g·mL−1). (2) Among PSS (70,000), PSS (200,000) and PSS (1,000,000) at different mass concentration ranges, 0.24 g·mL−1 PSS (70,000) exhibits the best FO flux. In general, the FO flux increases with an increase in PSS concentration. On one hand, the conductivity increases as the increase of PSS concentration, which leads to higher osmotic pressure. On the other hand, the PSS at higher concentration exhibits a larger value of viscosity. Based on the two aspects, the PSS DS at an appropriate concentration may exhibit better FO performance. (3) FO experiments demonstrate that the superiority of using PSS polyelectrolytes as draw solutes to conventional ionic salt. Since the former not only can produce comparable water flux, but also the larger size of molecular structural configuration results in lower reverse leakage. (4) The PSS was easily recycled by a low pressure-driven UF system under 2 bar with low energy consumption. The repeatability of FO performance improves with increasing the PSS Mw. In order to realize a satisfactory regeneration of PSS DS in the FO process, it is necessary to select or prepare an appropriate UF membrane with accurate MWCO.

On the whole, the concept of using PSS as draw solute in FO process opens up a new way of exploring novel draw solute. Acknowledgments The authors would like to thank Chongqing Commission of Science and Technology for funding this research project with a Grant Number of cstc2012ggC20001 entitled “The development and application of high-throughput nanofibers forward osmosis membranes and membrane modules”. The research was also funded by the National Natural Science Foundation of China for the project entitled, “Research on the anti-corrosion, anti-heat mechanism and air filtration performance of Graphene oxide enhanced aramid nanofibers” (No. 51478452). References [1] J.R. McCutcheon, R.L. McGinnis, M. Elimelech, A novel ammonia–carbon dioxide forward (direct) osmosis desalination process, Desalination 174 (2005) 1–11. [2] J.R. McCutcheon, R.L. McGinnis, M. Elimelech, Desalination by ammonia–carbon dioxide forward osmosis: influence of draw and feed solution concentrations on process performance, J. Membr. Sci. 278 (2006) 114–123. [3] 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. [4] 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. [5] E.M. Garcia-Castello, J.R. McCutcheon, M. Elimelech, Performance evaluation of sucrose concentration using forward osmosis, J. Membr. Sci. 338 (2009) 61–66. [6] K.B. Petrotos, H.N. Lazarides, Osmotic concentration of liquid foods, J. Food Eng. 49 (2001) 201–206. [7] A. Seppälä, M.J. Lampinen, Thermodynamic optimizing of pressure-retarded osmosis power generation systems, J. Membr. Sci. 161 (1999) 115–138. [8] A. Achilli, T.Y. Cath, A.E. Childress, Power generation with pressure retarded osmosis: an experimental and theoretical investigation, J. Membr. Sci. 343 (2009) 42–52.

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