Evaluation of food additive sodium phytate as a novel draw solute for forward osmosis

Desalination 448 (2018) 87–92

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Evaluation of food additive sodium phytate as a novel draw solute for forward osmosis Jiaqi Huanga,b,c,1, Shu Xionga,b,c,1, Qingwu Longd, Liang Shena,b,c, Yan Wanga,b,c,

T

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a

Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Wuhan, 430074, PR China b Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan, 430074, PR China c Shenzhen Institute of Huazhong University of Science and Technology, Shenzhen 518000, PR China d School of Chemistry and Chemical Engineering, Lingnan Normal University, Zhanjiang 524048, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Forward osmosis Draw solution Phytic acid salt Food additive Brackish water desalination

Proper draw solution is a crucial factor to generate a high performance in FO process. In this study, a phytic acid and its salt (PA-Na) were evaluated as novel draw solutes in FO process. The effects of the solution pH and concentration on the physicochemical properties were investigated systematically. Using 0.45 M PA-Na draw solution (pH = 7) and DI water feed solution, a relative high water flux of 19.02 LMH and a low solute flux of 0.51 gMH can be obtained, with HTI-TFC membrane in the PRO mode. An even better performance of a 30.35 LMH water flux and a 0.61 gMH solute flux can be achieved using the self-made HPAN-TFC membrane. The comparison of PA-Na and NaCl draw solutions shows that PA-Na draw solution has a much lower draw solute leakage and a competitive water flux. In addition, the application of PA-Na draw solution for the brackish water desalination was explored. The diluted PA-Na draw solution after FO may be used directly in food production by further dilution. In general, the non-toxicity and satisfactory FO performance of this food additive demonstrate its great potential as a draw solute for FO applications.

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Corresponding author at: Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Wuhan 430074, PR China. E-mail address: [email protected] (Y. Wang). 1 The first two authors contribute equally to this study. https://doi.org/10.1016/j.desal.2018.10.004 Received 8 February 2018; Received in revised form 21 September 2018; Accepted 1 October 2018 0011-9164/ © 2018 Elsevier B.V. All rights reserved.

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O

1. Introduction Forward osmosis (FO) is a membrane-based technology driven by the osmotic pressure between the feed solution and the draw solution [1–5]. Compared to traditional reverse osmosis and nanofiltration driven by external hydraulic pressure, FO has a lower energy requirement and exhibits a reduced membrane-fouling tendency [6–9]. Thus, FO has attracted increasing research interest in recent years and exhibited great potential for various applications in desalination, water treatment, power generation, and so on [10–14]. Draw solutions play a critical role as osmotic pressure suppliers; regardless, research about draw solutions relatively lags behind [15] because of prerequisites that have to be fulfilled, including the following: (1) a high osmotic pressure to generate a high water flux; (2) a suitable molecular size to ensure low reverse draw solute leakage; (3) convenient recovery at a low cost; (4) nontoxicity; and so on [16]. Commonly used draw solutes include inorganic salts such as sodium chloride (NaCl) and magnesium chloride (MgCl2) [17,18]. These salts can be ionized completely and generate high osmotic pressure in the aqueous solution, ensuring considerably high water fluxes in FO; however, salt leakage is also extremely severe and entails high energy consumption in the draw solution recovery. Magnetic nanoparticles, thermosensitive polyelectrolytes, and thermally responsive hydrogels have been developed as potential draw solutes [19,20] with negligible solute leakage and easy recovery. However, they generally exhibit low water fluxes. As draw solutes, thermolytic salts [10,21,22], such as ammonium bicarbonate (NH4HCO3), can produce high water fluxes in FO and be easily recovered by decomposition with industrial waste heat. However, the corrosive and harmful nature of ammonia gas (NH3) may deteriorate the quality of the product water with even a slight residue of the draw solute. Various organic acid salts were subsequently developed as potential draw solutes with superior FO performances because of their ionogenic chemical structures in the aqueous solution and suitable molecular size, which can ensure high water flux, low viscosity, and relative ease in draw solution recovery [23–28]. Draw solutes based on natural compounds present advantages over other compounds because draw solutes cannot be used to obtain potable water directly without a regeneration step. For instance, a “draw water” bag with edible products as the draw solute can obtain water from contaminated water in the wilderness [29]. Nontoxic gluconate salts have also been reported as novel FO draw solutes with good FO performance for the reconcentration of various fruit juices [29]. Similarly, sodium lignin sulfonate and commercial fertilizers reported as FO draw solutes can also be directly applied in crop irrigation after dilution via FO [30,31]. In the present study, phytic acid (PA) and its sodium salt (sodium phytate, PA-Na), which are natural green additives used in the food and pharmaceutical industries [32–35], are explored as draw solutes for applications in FO for the first time. Fig. 1 shows the chemical structure of phytic acid. Numerous phosphate groups are found in the molecule, which can easily ionize and produce high osmotic pressure in the aqueous solution. The molecular weight of PA is 660, which can sufficiently ensure low-solute leakage in FO. In addition, both PA and PANa are environment- and human-friendly [32]. These advantages of PA suggest its potential as a desirable draw solute. In the current study, the effects of both the pH and concentration of the PA solutions on FO performance are systematically evaluated. The PA-Na draw solution is further evaluated for brackish water desalination by FO. This study can potentially contribute to the development of new draw solutes for FO.

P

HO O

P

OH

O

O

HO

P O

O

HO

OH OH

O HO

O O

O

P OH

O

OH

P

HO

OH

P

HO

O Fig. 1. Chemical structure of phytic acid.

hydroxide (NaOH) (≥96%) were acquired from Sino-pharm Chemical Reagent Co. (Shanghai, China). Deionized water (DI water) with a resistivity of 18.25 MΩ·cm was generated from Wuhan Pin Guan Ultrapure Water LAB System (China). 2.2. Preparation of PA-Na draw solution PA-Na draw solutions of different pH values were prepared by diluting the PA aqueous solution with the designed concentration and adjusting the pH with NaOH, where the solution pH was determined with a pH meter (Mettler toledo, FE28). 2.3. Osmotic pressure and relative viscosity of PA-Na solution The osmotic pressures of PA-Na aqueous solutions were measured using a home-made lab-scale setup [23] based on the freezing point depression method and calculated by Eq. (1):

π=

T0 − Tt × 22.66 1.86

(1)

where T0 and Tt are the freezing points of DI water and the PA-Na solution respectively. The determination of relative viscosities (ηr) of PA-Na aqueous solutions was conducted with a commercial Ubbelohde viscometer at 25 ± 1 °C and calculated with Eq. (2):

ηr =

η ρt = η0 ρ0 t 0

(2)

where t0 and t (s) are the outflow time of the DI water and PA-Na solution, respectively, ρ0 and ρ (g/cm3) are the densities of DI water and PA-Na solution detected by a density meter (KEM DA-130 N, Japan). 2.4. FO process

2. Materials and methods FO tests were carried out by a commercial lab-scale FO faclity (Suzhou Faith Hope Membrane Technology) with a fixed effective membrane area of 3.87 cm2. Two kinds of FO membranes were used to evaluate the FO performance of PA-Na draw solution, i.e., the commercial thin film composite (TFC) FO membrane from HTI (Hydration

2.1. Materials Phytic acid (PA) (70% aqueous solution) was supplied by Aladdin (Shanghai, China). Sodium chloride (NaCl, ≥99.5%). Sodium 88

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20

Membrane note

Water flux (LMH)

Salt leakage (gMH)

Membrane orientation

HTI-TFC

7.13 ± 0.22 3.98 ± 0.13 17.16 ± 0.73 9.72 ± 0.82

5.13 2.75 9.29 4.99

PRO mode FO mode PRO mode FO mode

HPAN-TFC

± ± ± ±

0.18 0.15 0.68 0.35

Osmotic pressure (bar)

Table 1 Basic FO performance of HTI-TFC and self-made HPAN-TFC membranes.

Note: DI water and 0.5 M NaCl solution were used as the feed solution and draw solution respectively.

Technologies Inc., OR, USA) (HTI-TFC) and the homemade TFC membrane prepared by interfacial polymerization of m-phenylenediamine and trimesoyl chloride on the surface of the hydrolyzed polyacrylonitrile substrate (HPAN-TFC), as characterized in our earlier work [36]. The details about the membrane fabrication can be found in the Supporting information. FO performance with HTI-TFC and self-made HPAN-TFC FO membranes was evaluated and shown in Table 1, using 0.5 M NaCl solution as the draw solution and DI water as the feed solution. During the FO test, both feed and draw solutions were looped at the same volumetric flow rate (300 mL/min) at room temperature (25 ± 1 °C). FO performance in both FO mode (feed solution facing the selective layer) and PRO mode (draw solution facing the selective layer) was tested. The water flux (Jw, L/m2·h, referenced to as LMH) was calculated by Eq. (3):

Jw =

∆m A × ∆t × ρ0

(Ct V) t − (C 0 V0 ) A × ∆t

2.00

18 17

1.75

16 15 6

7

8

9

1.50

Fig. 2. The osmotic pressure and relative viscosity of PA-Na solutions with different pH (0.15 M).

draw solution was shown in Fig. S2 in the Supporting Information.

3. Results and discussion 3.1. PH optimization of PA-Na solution

(3)

PH optimization of PA-Na solution was conducted to achieve the desired FO performance. To prevent the impairment to the TFC membrane, a relative neutral pH range (6–9) for the draw solution was used in this study. As shown in Fig. 2, the osmotic pressure of the PA-Na solution increases with an increase in solution pH. Such rise in osmotic pressure could be attributed to the generation of more ions in the aqueous solution when phosphoric acid groups in phytic acid are converted to sodium phosphate groups, which is similar with results obtained in previous reports [24]. This hypothesis is verified by the conductivity measurements of the four draw solutions, which are listed in Table 2. The conductivities obey the order of osmotic pressures [18]. With an increase in pH by adding NaOH, neutralization occurs to convert eH2PO3 groups to eHPO3−Na or ePO32Na groups, which are dissociated more easily in the aqueous solution. A higher conductivity indicates the generation of a larger number of free ions in PA-Na solution and enhanced dissociation of PA-Na with the increase in pH, and consequently, a higher osmotic pressure. Fig. 2 also shows that relative viscosity increases with an increase in solution pH because of increased mass concentration with the addition of NaOH into the solution. Consequently, the FO flux increases when the pH of the PA-Na solution increases, as shown in Fig. 3, which is attributed to the increased osmotic pressure. However, the increase in water flux does not exhibit the same linear trend as that of the osmotic pressure. The difference could be attributed to the increased severity of the internal concentration polarization (ICP) arising from the higher viscosity of the PANa solution at a higher pH [37]. Simultaneously, the solute leakage of the PA-Na solution also increases with an increase in pH because of a rise in the number of ions (Na+) generated in the solution. Because Na+ ions are easier to leak through the membrane owing to its much smaller size than PA anions and the electrolytic attraction with the negativelycharged membrane surfaces [38,39], more Na+ ions in the draw solution could result in a higher reverse draw solute leakage. Accordingly,

(4)

2.5. Application of PA-Na in FO process The potential of PA-Na draw solution for the brackish water treatment was further investigated with 0.15 M NaCl aqueous solution as brackish water feed solution and 0.45 M PA-Na solution as the draw solution for a 30-min FO test. The test conditions were the same as those described in Section 2.4. 2.6. NF recovery of draw solution The draw solution recovery after the FO test was also studied by a pressure-driven NF process (Cross-flow NF setup, Suzhou Faith Hope Membrane Technology) under a 2 bar upstream pressure with a commercial flat-sheet NFW membrane (Snyder Filtration, with an effective membrane area of 7.065 cm2 and MWCO of 300–500 Da). The water permeability and the salt rejection (R) were evaluated by Eqs. (3) and (5) respectively. ⎜

2.25

pH

where C0 and V0 are the initial concentration and feed volume of the feed solution, while Ct and Vt are the concentration and the feed volume after a predetermined time Δt. The feed concentration can be measured using a calibrated conductivity meter (Mettler toledo, FE30). Calibration curves of the solution conductivity vs PA-Na concentration with different solution pH were shown in Fig. S1 in the Supporting Information.

C R = ⎛1 − P ⎞ × 100% Cf ⎠ ⎝

19

14

where Δm (g) is the weight increase of the draw solution during a predetermined time Δt (h), A (m2) is the effective membrane area. The reverse solute flux Js (g/m2·h, noted as gMH) was obtained according to Eq. (4):

Js =

Osmotic pressure Relative viscosity

Relative viscosity

J. Huang et al.

Table 2 The conductivity of various draw solutions (0.15 M) at 25 °C.

⎟

(5)

where Cp and Cf (mg/L) are the concentrations of the permeate and feed solutions, respectively. The recovery performance of diluted PA-Na 89

pH

6

7

8

9

Conductivity (ms/cm)

41.7

42.5

43.2

44.1

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Fig. 3. The effect of draw solution pH on the FO performance (feed solution: DI water, PRO mode).

the specific reverse solute flux (Js/Jw) fluctuates with an increase in pH, with the lowest value (0.037) obtained at pH = 7. The Js/Jw value, which is generally used to reflect the loss of the draw solute per unit volume of the water permeation in FO, is a significant index reflecting the efficiency of the draw solute [40]. Notably, the Js/Jw value of the PA-Na draw solutes ranges from 0.037 g/L to 0.056 g/L, which is considerably lower than that of a typical NaCl draw solution (0.4–1.14 g/ L), indicating the high potential of this proposed PA-Na draw solute. 3.2. Effect of PA-Na concentration The concentration of the draw solution is an important factor influencing FO performance. Draw solutions of a higher concentration can generally produce a higher osmotic pressure and a corresponding higher water flux in FO. However, the higher solution concentration may lead to a higher viscosity of the draw solution, increasing the severity of internal concentration polarization (ICP) and ultimately leading to a water flux decline [41,42]. The osmotic pressures of PA-Na solutions with different concentrations are shown in Fig. 4. The osmotic pressure of the PA-Na solution increases linearly from 15.56 bar at 0.15 M to 76.38 bar at 0.75 M because a larger number of free ions are ionized in the PA-Na solution with a higher concentration. Benchmarking in Table S1 also shows that, the osmotic pressure of the proposed novel PA-Na draw solution is considerably higher [43–45] compared to those of most other polyelectrolyte draw solutions with a similar concentration, indicating its great potential for the application in FO. The FO performance of the PA-Na draw solutions with different

Fig. 5. FO performance with PA-Na draw solutions in (a) PRO mode, and (b) FO mode (pH = 7) (feed solution: DI water).

concentrations in both PRO and FO modes is presented in Fig. 5. The water fluxes in both modes increase with an increase in draw solution concentration, attributed to increased osmotic pressure. In addition, the water flux in the PRO mode is about three times higher than that in the FO mode with the same draw solution concentration because of the reduced ICP effect. The PA-Na draw solution with a concentration of 0.75 M obtains the highest water fluxes of 11.67 and 24.55 LMH in the FO and PRO modes, respectively. The low reverse salt flux also ranges from 0.18 gMH to 0.39 gMH in the FO mode and 0.43 gMH to 0.82 gMH in the PRO mode. Fig. 5 also shows that the Js/Jw ratio slightly declines with an increase in the draw solution concentration from 0.15 M to 0.45 M and then slightly increases with a further increase in concentration to 0.75 M in both modes. Regardless, all Js/Jw ratios remain below 0.04 g/L, attributed to the high water flux and low salt flux. These results suggest the great potential of PA-Na as a draw solute for FO. Owing to the relatively large molecular size of the PA-Na draw solutes used in this study, the FO performance of the PA-Na draw solution was further evaluated using self-made HPAN-TFC membranes. As shown in Table 1, both the water flux and reverse draw solute flux of HPAN-TFC membrane are higher than that of HTI-TFC membrane, indicating that HPAN-TFC membrane is more permeable to both water and NaCl molecules. It can be seen from Table 3 that with the PA-NA draw solution (0.45 M), the HPAN-TFC membrane can produce a much higher water flux than the HTI-TFC membrane while producing only a slightly higher reverse salt flux, resulting in a much lower Js/Jw ratio. This result indicates that the performance of PA-Na draw solution with a relatively large molecular size can be optimized using a more permeable FO membrane.

Fig. 4. The osmotic pressure and relative viscosities of PA-Na solutions with different concentrations (0.15–0.75 M, pH = 7). 90

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Table 3 FO performance of different membranes.

Table 4 Relative viscosity of PA-Na and NaCl solutions with different ion molarity.

Membrane

Water flux (LMH)

Reverse salt flux (gMH)

Js/Jw (g/L)

Ion molarity (M)

PA-Na

NaCl

HTI-TFC HPAN-TFC

19.02 ± 1.63 30.35 ± 1.12

0.51 ± 0.03 0.61 ± 0.05

0.0273 ± 0.0004 0.0201 ± 0.0007

2.10 2.88

5.60 ± 0.13 13.22 ± 0.27

1.17 ± 0.01 1.39 ± 0.01

Note: DI water and 0.45 M PA-Na solution were used as the feed solution and draw solution respectively; PRO mode.

Fig. 6(b) shows the FO performance of the PA-Na and NaCl draw solutions with the same ion molarity. To produce the same ion molarity of 0.45 and 0.6 M PA-Na solution, 1.05 and 1.44 M NaCl solutions are required. As shown in Fig. 6(b), the water flux for PA-Na draw solution is lower than that for NaCl solution with the same ion molarity, which is mainly attributed to their different viscosity. As presented in Table 4, the viscosity of PA-Na draw solution is around 5–10 times higher than that of NaCl solution. The significantly higher viscosity of PA-Na solution could lead to the much more severe concentration polarization and the considerable reduction in the effective osmotic driven force, and therefore the resultant lower flux of PA-Na draw solution. However, the salt leakage of the PA-Na draw solution is significantly lower than that of the NaCl solution with the same ion molarity because of the higher molecular weight of the former. 3.4. Application of PA-Na for brackish water desalination In this study, the 0.45 M PA-Na solution was further explored as the draw solution for the desalination application with synthetic brackish water (0.15 M NaCl) as the feed solution. Fig. 7 shows the variation in the water flux in a test running for 30 min, with DI water and 0.15 M NaCl solution as the feed solutions. With DI water as the feed solution, the water flux achieved with the 0.45 M draw solution is about 5.22 LMH, which is much higher than that achieved with the 0.5 M NaCl draw solution (2.75 LMH, as shown in Table 1). In addition, when the feed solution is replaced with the synthetic brackish water, the water flux decreases to about 2.87 LMH because of the reduced difference in transmembrane osmotic pressure. Despite the reduction, this water flux is still considered acceptable and suggests the feasibility of the PA-Na solution for desalination. Compared with NaCl draw solution, PA-Na draw solution holds a greater potential for desalination in terms of the much lower draw solute leakage and the competitive water flux in FO process as shown in Fig. 6. In addition, since a 0.2–1 wt% PA-Na solution can be used as a food additive [33–35], the dilute draw solution after desalination

Fig. 6. FO performance of PA-Na and NaCl draw solutions with the same (a) solute molarity and (b) ion molarity.

3.3. Comparison of PA-Na and NaCl draw solutions The FO performances of PA-Na and NaCl are compared with the same solute molarity and the same ion molarity, respectively. Fig. 6 (a) presents Jw and Js generated with the NaCl and PA-Na draw solutions of the same solute molarity (0.45 or 0.6 M). The PA-Na draw solution has a much higher Jw and a lower Js than the NaCl draw solution, which could be attributed to a large number of free ions dissociated in the PA-Na solution and the larger molecular weight of PA, respectively. According to the determined dissociation degree of PA-Na in Table S2, 0.45 and 0.6 M PA can produce 2.10 M (=0.45 ∗ 13 ∗ 0.36) and 2.88 M (=0.6 ∗ 13 ∗ 0.37) free ions, which are much higher than those generated by the NaCl solution with the same molarity (0.9 and 1.2 M free ions generated). A larger number of free ions indicate higher osmotic pressure and consequently, a higher Jw.

Fig. 7. FO water flux with 0.45 M PA-Na draw solution for the brackish water desalination (HTI-TFC membrane, FO mode). 91

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process can be used in the food industry after being diluted to a required range.

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4. Conclusions In this study, PA-Na as a draw solute for FO is evaluated. The effects of solution pH and concentration on the osmotic pressure, the relative viscosity, and the resulting FO performance are systematically studied. With increases in solution pH and concentration, both the osmotic pressure and relative viscosity increase with more free ions generated in the draw solution, contributing to a higher water flux but more severe salt leakage in FO. With DI water as the feed solution, a high water flux of 24.55 LMH and a relatively low salt flux of 0.39 gMH can be achieved using a 0.75 M PA-Na draw solution (pH of 7) in the PRO mode. Negligible Js/Jw ratios (≤0.04 g/L) can be achieved in both FO and PRO modes. Meanwhile, the FO performance of the PA-Na draw solutions can be improved using a self-made HPAN-TFC FO membrane. Owing to the much lower draw solute leakage and the competitive water flux in FO process, PA-Na draw solution is further explored for brackish water desalination with a relatively high water flux, indicating its great potential in this aspect. The diluted PA-Na draw solution after FO may be used directly in food production by further dilution. In general, the non-toxicity and satisfactory FO performance of this food additive demonstrate its great potential as a draw solute for applications in FO. Acknowledgement We would like to acknowledge the financial supports from National Natural Science Foundation of China (no. 21306058), and the Free Exploring Fundamental Research Project from Shenzhen Research Council, China (no. JCYJ20160408173516757). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2018.10.004. References [1] T.-S. Chung, S. Zhang, K.Y. Wang, J. Su, M.M. Ling, Forward osmosis processes: yesterday, today and tomorrow, Desalination 287 (2012) 78–81. [2] S. Qi, W. Li, Y. Zhao, N. Ma, J. Wei, T.W. Chin, C.Y. Tang, Influence of the properties of layer-by-layer active layers on forward osmosis performance, J. Membr. Sci. 423-424 (2012) 536–542. [3] P. Sukitpaneenit, T.-S. Chung, High performance thin-film composite forward osmosis hollow fiber membranes with macrovoid-free and highly porous structure for sustainable water production, Environ. Sci. Technol. 46 (2012) 7358–7365. [4] Y. Wang, R. Ou, H. Wang, T. Xu, Graphene oxide modified graphitic carbon nitride as a modifier for thin film composite forward osmosis membrane, J. Membr. Sci. 475 (2015) 281–289. [5] L. Shen, S. Xiong, Y. Wang, Graphene oxide incorporated thin-film composite membranes for forward osmosis applications, Chem. Eng. Sci. 143 (2016) 194–205. [6] E.R. Cornelissen, D. Harmsen, K.F. de Korte, C.J. Ruiken, J.-J. Qin, H. Oo, L.P. Wessels, Membrane fouling and process performance of forward osmosis membranes on activated sludge, J. Membr. Sci. 319 (2008) 158–168. [7] B. Mi, M. Elimelech, Organic fouling of forward osmosis membranes: fouling reversibility and cleaning without chemical reagents, J. Membr. Sci. 348 (2010) 337–345. [8] R.L. Mcginnis, M. Elimelech, Energy requirements of ammonia–carbon dioxide forward osmosis desalination, Desalination 207 (2007) 370–382. [9] L. Shen, J. Zuo, Y. Wang, Tris(2-aminoethyl)amine in-situ modified thin-film composite membranes for forward osmosis applications, J. Membr. Sci. 537 (2017) 186–201. [10] J.R. McCutcheon, R.L. McGinnis, M. Elimelech, A novel ammonia—carbon dioxide forward (direct) osmosis desalination process, Desalination 174 (2005) 1–11. [11] 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. [12] 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. [13] R.W. Holloway, A.E. Childress, K.E. Dennett, T.Y. Cath, Forward osmosis for

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