Evaluation of Renewable Gluconate Salts as Draw Solutes in Forward

Research Article pubs.acs.org/journal/ascecg

Evaluation of Renewable Gluconate Salts as Draw Solutes in Forward Osmosis Process Qingwu Long,† Guangxian Qi,† and Yan Wang*,†,‡ †

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Huazhong University of Science and Technology, Wuhan 430074, China ‡ Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, China ABSTRACT: Draw solution is one important factor to determine the separation performance in forward osmosis (FO) process. However, most draw solutions face severe reverse solute leakage and energy-intensive recovery problems, which lead to a significant performance decline. Exploration of renewable natural compounds as draw solutes may effectively break out the predicament of most current draw solution. In this work, a series of renewable, no-toxic gluconate salts are systematically investigated as draw solutes for FO applications. Their physicochemical properties are investigated systematically and related to the FO performance. The result shows that 2 M Glu-K draw solution may generate a comparable water flux (∼23.17 LMH) to that of NaCl solution, but with a significantly lower solute leakage (∼1.09 gMH), with DI water as the feed solution under PRO mode. Glu-K draw solution is further applied for juice reconcentration with a reasonable good performance achieved. Draw solution recovery by nanofiltration (NF) is also performed. This study provides useful information on using natural draw solutes in the FO process and facilitates its practical applications in the food processing field. KEYWORDS: Forward osmosis, Draw solution, Membrane separation, Gluconate salt, Juice reconcentration

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applications.7−10 Therefore, exploration of suitable draw solutions with high osmotic pressure and easy regeneration methods remains to be a big challenge on the road to FO industrialization. In the past few years, various draw solutes have been investigated for FO applications. Inorganic salts (e.g., NaCl, MgCl2) are mostly studied draw solutions, which can create reasonable water flux but render a severe reverse salt flux. However, their large-scale applications are severely constricted due to the high energy cost involved in the draw solution recovery with current technologies.11 Later, the reported thermolytic ammonium carbonate salt shows to be a promising draw solute because of its high osmotic pressure and easy recovery by heating with available industrial waste or geothermal energy.12,13 Afterward, synthetic draw solutes with the controllable molecular size have been investigated in recent years to solve the salt leakage and recovery problems, including poly(acrylic acid) sodium salts (PAA-Na),14 hydroacid complexes (Fe-CA, Fe-OA),15,16 carboxyethyl amine sodium salts (CASSs),17,18 and so forth. Other synthetic draw solutes still include functionalized magnetic nanoparticles (MNPs),19,20

INTRODUCTION With the escalating global water crisis, various water treatment technologies (e.g., multistage flash distillation, dew point evaporation) have been emerging to address water concerns in past decades. Among them, reverse osmosis (RO), an efficient desalination technology, plays a dominant role in relieving water source shortages. Although suitable process design and optimization may reduce the energy consumption of RO process, an energy consumption about 2 kWh/m3 is still required for a controlled pilot-scale system with an equivalent recovery of 50%.1 Besides, RO process generally suffers severe membrane fouling problems.2 Alternatively, membrane-based FO technology shows a promising prospect for water treatment in recent years.3,4 In the FO process, the osmotic pressure difference between two sides of FO membrane is employed as the driving force for water to permeate from the draw solution to the feed solution. Its low fouling tendency and high separation efficiency gain valuable advantages as a promising water treatment technology.5−7 A high-efficient FO system is mainly affected by two essential factors: a high-performance membrane and suitable draw solution, both of which will determine its performance and practical applications. However, the development of the suitable draw solution always makes slower progress than the advance of FO membranes, leading to difficulties for FO © XXXX American Chemical Society

Received: July 29, 2015 Revised: November 30, 2015

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DOI: 10.1021/acssuschemeng.5b00784 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering thermosensitive polyelectrolytes,21,22 responsive hydrogels,22 which also hold out good prospect of the FO applications because of the relative low-energy recovery by magnetic field, solar energy or ultrafiltration. However, even though most synthetic draw solutions exhibit a high water flux and a low salt leakage in FO process, their practical applications in FO remain an uncertainty due to their nonrenewable property. Alternatively, draw solution of natural compounds without regeneration needed could be a desirable choice. One representative application is the “draw water” bag using sugar or nutrient products as draw solute to acquire drinking water in a harsh environment. So far, various natural compounds with good stability, low price, and nontoxicity have been reported for FO applications, such as sugar,23 amino acid,24 and sodium lignin sulfonate.25 Among them, natural glucose with abundant resources and cheap price has shown special advantages. However, its mediocre osmotic pressure and poor biostability limit its widespread applications. In this work, renewable and nontoxic gluconate salts are explored as novel draw solutes for FO applications. Compared with glucose, the gluconate acid salt can release ions in the aqueous solution, resulting in a superior osmotic pressure, whereas its larger molecular size may also render a lower solute leakage in FO. A series of novel gluconate acid salts, e.g., potassium gluconate salt (Glu-K), sodium gluconate salt (GluNa), zinc gluconate salt (Glu-Zn), and iron(II) gluconate salt (Glu-Fe(II)) are investigated as draw solutes for FO application. Their physiochemical properties are studied and analyzed. Glu-K draw solution is further evaluated for juice reconcentration via FO. This work is believed to make useful contributions for the application of natural compounds as FO draw solutes and to pave the way for FO industrialization.

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where tgluc and twater (s) are the respective outflow time of the draw solution and DI water, determined using a commercial Ubbelohde viscometer with temperature maintained by a water bath; their densities, ρgluc and ρwater (g/cm3) were measured by a portable density meter (KEM DA-130N, Japan). FO Test. FO tests were conducted using a commercial bench-scale FO system (Suzhou Faith Hope Membrane Technology Co., Ltd.) as schematically shown in Figure 1. Two peristaltic pumps (Kejian BT00-

Figure 1. Schematic diagram of the lab-scale FO setup.

600M, Changzhou) drive the feed and draw solutions to flow in a loop at each side of cell channels with cocurrent flow rates of 0.3 L/min monitored by two flowmeters.26 The fresh CTA or TFC membrane with an effective membrane area of 18.9 cm2 was immersed in DI water for 30 min before use to remove fully glycerol. Both feed and draw solutions with initial volumes of 150 mL were maintained at 25 ± 1 °C during all FO tests. FO tests under both FO mode (selection layer toward feed solution) and PRO mode (selection layer toward draw solution) were conducted. After the FO membrane was preconditioned in the setup for 1 h, then the feed solution concentration and the draw solution weight were monitored with sample collected each half an hour for at least three times. The water flux, Jw (L/m2 h, referenced to as LMH) in FO process was measured by the weight increase in the draw solution monitored by a balance (AND-EK4100i, Japan) that linked with a computer to export the data automatically at a 30 s interval, and calculated by eq 3.

EXPERIMENTAL SECTION

Materials and System. Four gluconate salts (purity ≥ 98%), i.e., sodium gluconate, potassium gluconate, iron(II) gluconate, and zinc(II) gluconate were all purchased from Aladdin (China), and abbreviated as Glu-Na, Glu-K, Glu-Fe(II), and Glu-Zn, respectivcely. Sodium chloride (NaCl, ≥99.5%) and glucose (Glu, ≥98%) were acquired from Sino-pharm Chemical Reagent Co., Ltd. Four kinds of 100% juice produced by Huiyuan Juice group (China) were obtained in the local supermarket. All above compounds were used as received. Deionized water (DI water) with a resistivity of 18.25 MΩ cm was produced in lab by a Wuhan Pin Guan Ultrapure LAB purification system and utilized in this work. Cellulose triacetate (CTA) and thin film composite (TFC) flat-sheet FO membranes were employed (Pouch-type, Hydration Technology Innovations (USA)). Osmotic Pressure and Relative Viscosity. The osmotic pressure of gluconate salt draw solutions with various concentrations (0.4−2.0 M) were determined according to the freezing point depression method by eq 1, using a self-assembled lab-scale system as described detailedly in our previous work.17 Osmotic pressures of NaCl and glucose solutions were also tested as controls π=

ΔT × 22.66 (bar) 1.86

Jw =

ηgluc ηwater

=

JS =

Δ(CV ) AΔt

(4)

where C (mg/L) and V (L) are the total dissolved solid concentration in the feed and the feed volume, before and after a predetermined operation time Δt (h). C (mg/L) could be accurately detected by a calibrated conductivity meter (Mettler Toledo, FE30). The leakage of Glu-Zn, Glu-Fe(II) and glucose were detected by total organic carbon analyzer (Elementar vario TOC, Germany). NF Recovery of Draw solution. In this study, NF flat-sheet membrane (NFX) with a MWCO of 150−300 Da provided by Snyder Filtration (USA) was employed for the draw solution recovery. Its maximum pure water permeability is 10.79 LMH/bar under a 3.5 bar upstream pressure. The recovery of diluted draw solution was conducted by a lab-scale NF setup where a flat sheet NF module with an effective membrane area of 17.35 cm2 was equipped. More operation details can also be referred to our previous work.17 The pure water permeability (L/m2 h bar or LMH/bar) and solute rejection (R) in the NF process were measured with an upstream pressure of 3.5 bar and calculated by eqs 5 and 6, respectively.

(1)

tglucρgluc twaterρwater

(3)

where Δm (g) is the weight difference of the draw solution before and after a predetermined test time Δt (h), A is the effective membrane area (m2), and ρ is the water density (0.996 g/cm3 at 25 °C). The reverse salt flux, JS (g/m2 h, referenced to as gMH) was calculated from the concentration and volume change of the feed solution using eq 4.

where ΔT is the temperature difference between freezing points of pure water (T0) and the draw solution (Tt). Relative viscosities (ηR) of Glu-Na and Glu-Na draw solutions with various concentrations (0.4−2.0 M) or at various temperatures (20− 50 °C) against that of DI water (at the same temperature) were determined and calculated using eq 2:

ηR =

Δm ρAΔt

(2) B


Research Article

ACS Sustainable Chemistry & Engineering Table 1. Fundamental Physicochemical Properties of Draw Solutes Used in This Work

Jw =

Δm ρAΔtP

⎛ Cp ⎞ R = ⎜1 − ⎟ × 100% Cdraw ⎠ ⎝

other gluconate salts and glucose; whereas Glu-Zn and GluFe(II) have unexceptional low solubility of 0.11 and 0.13 g/g water, respectively, indicating that the two draw solutions may generate a negligible water flux in FO, which is confirmed in the next section. Because an ion-type draw solute with a high water solubility could generate more ions than the solute with a poor water solubility under the same conditions, a higher osmotic pressure may be resulted. In addition, Glu-K and Glu-Na solutions (0.4 M) have a neutral pH values of 7.09 and 6.69, which are suitable for commercial CTA and TFC membranes with pH tolerant ranges of 3−8 and 2−11, respectively. However, Glu-Zn and GluFe(II) have relative lower pH values of 3.62 and 5.52, because Fe(II) and Zn ions may partially hydrolyze in the aqueous solution and generate some hydrogen ions, leading to a low pH value.27 The reaction processes can be described by the following chemical equilibrium formulas.

(5)

(6)

where Cp (mg/L) and Cdraw (mg/L) are the solute concentrations in the permeate and the diluted draw solutions, respectively. Juice Reconcentrated by FO. The potential application of gluconate salt as draw solute in FO process for juice reconcentration was investigated with 1.5 M Glu-K aqueous solution as the draw solution and four different juices (tomato, orange, apple, and grape juice) as the feed solution at 25 °C.

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RESULTS AND DISCUSSION Physicochemical Property of Draw solution. The physicochemical properties of four gluconate salts used as draw solutes in this study are listed in Table 1, including the molecular weight, water solubility, and the pH value of the solution. They are all important properties of the draw solute to determine its final FO performance. Basically, higher molecular weight and higher water solubility of the draw solute are more desirable, because the former generally corresponds to a lower solute leakage, whereas the latter brings out a higher water flux in the FO process. With regards to the pH value of the draw solution, it needs to be in the range of the membrane tolerance. Table 1 shows that all gluconate salts have much larger molecular size than those of the glucose and NaCl, which indicates the reverse salt flux using gluconate salts as draw soutes may be significantly lower because big molecules can be largely retained by the FO membrane. It can also be found that the solubility of Glu-K (1.08 g/g water) is higher than those of

Fe2 + + 4H 2O ⇌ Fe(OH)2 + 2H3O+ Zn 2 + + 4H 2O ⇌ Zn(OH)2 + 2H3O+

Relative Viscosity and Osmotic Pressure. To consider the potential of the four gluconate salts as draw solutions comprehensively, the osmotic pressure and the viscosity of their solutions should be investigated, which both have significant impacts on FO performance. Generally, a high osmotic pressure of the draw solution can generate a high water flux in FO process, whereas the high viscosity not only leads to high energy consumption for fluid pumping but also causes severe internal concentration polarization (ICP) (in FO mode) and C


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ACS Sustainable Chemistry & Engineering

draw solution with a lower viscosity is favorable to enhance the FO performance, this result indicates that Glu-K draw solution have better FO performance than that of Glu-Na draw solution with the same concentration. Osmotic pressures of glucose, NaCl, and gluconate salt solutions with different concentrations are shown in Table 3. It

external concentration polarization (ECP) (in PRO mode) phenomena.28−31 Gluconate salt draw solutions exhibit comparable relative viscosities at a low concentration of 0.4 M, which follow an order of Glu-Na (1.20) > Glu-Fe(II) (1.15) > Glu-K (1.12) > NaCl (1.07) > Glu-Zn (1.03). Figure 2 also gives relative

Table 3. Osmotic Pressure of Gluconate Salts, Glucose, and NaCl Solutions of Different Concentrations osmotic pressure (bar) conc. (M)

Glu-Na

Glu-XK

Glu-Zna

Glu-Fe(II)a

Glu

NaCl

0.4 0.8 1.0 1.5 2.0

15.75 27.1 36.62 50.29 85.54

15.26 27.22 37.59 53.58 94.56

11.58

14.62

10.97 25.83 34.24 49.11 58.61

17.21 32.47 41.86 64.93 100.04

a

No result due to their limit solubility.

can be observed that, for all solutions of 0.4 M, osmotic pressures of four gluconate salt solutions (Glu-Na, Glu-K, GluFe(II), and Glu-Zn) are close to that of NaCl and about 1.5 times that of glucose, because gluconate salts and NaCl are both electrolytes and can partially or fully dissociate into free ions in the aqueous solution, whereas the glucose (nonelectrolyte) cannot. Among them, the Glu-Zn draw solution exhibits a relative low osmotic pressure (11.58 bar), which is similar to that of glucose solution, because of the existent stronger coordination interactions than dissociation interactions.37 In addition, as the concentration increases, Glu-Zn and Glu-Fe(II) cannot be fully dissolved in water due to their poor solubility as mentioned in the previous section. And the osmotic pressure of Glu-Na, Glu-K draw solutions shows a slower increasing trend than that of NaCl, probably due to the mitigated dissociation ability caused by the hydrophobic interaction among gluconate salt molecules at higher concentration. In general, the draw solution with desirable properties are preferred for FO applications, including high water solubility, appropriate molecular size, high osmotic pressure, low viscosity, etc. However, to consider the potential of any novel draw solutes developed, comprehensive consideration should be taken. A classic example is NaCl, which owns a considerable

Figure 2. Effects of concentration and temperature on the relative viscosities of Glu-Na, Glu-K, and NaCl draw solutions (0.4 M) at 25 °C.

viscosities of Glu-Na, Glu-K, and NaCl solutions as a function of the solution concentration and temperature. Because of the poor solubility of Glu-Fe(II) and Glu-Zn, their viscosities at high concentration cannot be further studied. The result displays that relative viscosities of three solutions all increase remarkably with the concentration increase, whereas decrease with the temperature increase. These trends are consistent with those reported in our previous work.17 We also find that the differences among relative viscosities of Glu-Na, Glu-K, and NaCl solutions are relatively small with a low concentration (0.4 M) or at a low temperature (20 °C), and those of Glu-Na/ K solutions are slightly higher than that of NaCl solution. But the difference becomes magnified with the concentration or temperature increase. Whatever, it should be noticed that all viscosities of Glu-Na and Glu-K solutions with concentration less than 2 M still remain very low ( Glu-K > Glu-Na > glucose > Glu-Fe(II) > Glu-Zn; and the revese salt flux with both membranes follows the order of NaCl > glucose > Glu-K > Glu-Na > Glu-Fe(II) > Glu-Zn. It can be seen that, among all draw solutes, the water flux with Glu-K draw solution is comparable with that with NaCl and superior E


Research Article


Figure 5. Concentration effect on the FO performances using CTA and TFC membranes (DI water as the feed solution).

permeation of Na+ and Cl− ions through the TFC membrane into feed solution, and then the decline in the net osmotic pressure difference across the membrane. Therefore, a lower water flux is resulted using NaCl draw solution than using gluconate salt draw solution with the TFC membrane under the same operation conditions. The concentration effect on the FO performance of Glu-K and Glu-Na draw solutions is further studied, and the results are shown in Figure 5. It shows that both water flux and salt flux increase with the increase in the draw solution concentration (0.4−2.0 M), which is attributed to the higher osmotic pressure. In most cases, the water flux with weak electrolyte draw solutions (Glu-K and Glu-Na) are superior to that with nonelectrolyte glucose draw solution, whereas slightly lower than that of strong electrolyte NaCl draw solution. Generally, the draw solute with higher dissociation in water will generate greater flux because it can form more ions and generate higher osmotic pressure. In this work, the dissociation degrees of studied draw solutes follow an order of NaCl (strong electrolyte) > gluconate salts (weak electrolyte) > glucose (nonelectrolyte); and among the gluconate salts, the order is Glu-K ∼ Glu-Na > Glu-Fe > Glu-Zn. The dissociation degree of the draw solutes is closely related to their osmotic pressures, and finally determines the corresponding water flux achieved. In addition, the superior water flux of Glu-K solution to that of Glu-Na solution is ascribed to its lower viscosity as demonstrated in previous section. The highest water fluxes of 17.98 and 23.17 LMH could be obtained with 2 M Glu-K draw solution and DI water feed solution in PRO mode using CTA and TFC membranes, respectively. The very low reverse salt flux in a range of 0.25−0.41 gMH in FO mode (Figure 5a,c) and 0.29−0.48 gMH in PRO mode (Figure 5b,d) for Glu-Na draw solutions can also be observed. In a word, some general trends are observed from the FO performance results, i.e., (1) the FO performance using TFC membrane is superior to that of CTA membrane (both higher water flux and lower salt leakage); (2) both water flux and salt flux in PRO mode are higher than those in FO mode with the same operation conditions; (3) both water flux and salt flux show slight increasing trends with the increase in the draw

solution concentration; (4) the draw solute with a higher osmotic pressure may have a higher water flux, and that with a bigger molecule size may result in a smaller salt flux. Clearly, the proposed novel gluconate salts with relative big molecular size and rich hydrophilic groups that can be easily ionized in aqueous solutions have a great potential as draw solutes for FO applications. In addition, the FO performance with the gluconate salts as draw solutes are also benchmarked with some other reported draw solutes under the comparable conditions. Table 2 shows that the gluconate salt draw solutes exhibit superior performance to most other reported draw solutes in FO application. With 2 M Glu-K draw solution, a high water flux of 23.17 LMH and a negligible salt flux of 1.09 gMH can be obtained because of its high osmotic pressure and low viscosity, which is only slightly inferior to those hydroacid complexes.15,16 The potential of renewable gluconate salts draw solutes can be further investigated for the practical FO applications. Fruit Juice Reconcentration by FO. The fresh fruit juice usually contains a mixture of abundant volatile aroma organic compounds, including esters, alcohols, aldehydes, ketons, hydrocarbons, etc.39 In common, the concentration of each individual aroma substance ranges from 1 to 20 ppm depending on the fruit type. Conventional juice reconcentration processes, such as thermal concentration may damage aroma compounds via chemical changes, thermal degradation or oxidation at high temperature, finally deteriorating the fruit juice quality. Various advanced separation techniques have been developed to minimize the aroma loss, concluding distillation, partial condensation, pervaporation, etc.40−44 Among them, the membrane-based FO separation technology has shown great potential for juice concentration in recent years, because it does not require heating and extensive energy and can thus maximally retain the heat-sensitive aroma compounds in aqueous solutions.45,46 In this work, 1.5 M Glu-K draw solution is used for fruit juice concentration via the FO process with TFC membrane. Four fruit juices (tomato, orange, apple, and grape) to be recovered are employed as the feed solution, and their basic properties are listed in Table 4. The pH values of all the four fruit juices are in F


Research Article


(NF) as a mature separation technology has been extensively used for draw solution recovery in many previous works because of its high efficiency and easy scale-up.47 It is commonly integrated with FO to form a FO−NF hybrid system, where the diluted draw solution produced by FO process can be reconcentrated by NF process continuously, resulting in a stable FO performance. Figure 7 shows the rejection rate and the water flux in NF process under a 3.5 bar upstream pressure with diluted Glu-K

Table 4. Basic Properties of the Studied Fruit Juices pH osmotic pressure (bar) solid net content (g/100 g fruit juice)

tomato

orange

apple

grape

4.97 10.96 7.64

4.77 14.62 19.28

4.45 19.49 12.36

4.05 23.15 14.82

the range of 4−5, compatible to the used TFC membrane. Their solid contents are also found to be different in a range of 7.64 to 19.28 g in 100 g of fruit juice. The osmotic pressure follows an order of grape > orange > apple > tomato with the highest osmotic pressure being grape juice (23 bar). Figure 6a shows the water flux for the reconcentration of various fruit juices, ranging from 1.6−2.6 LMH with an order of tomato > apple > grape > orange. It is to our surprise that the order of water fluxes is not consistent with that of their osmotic pressures, probably because of the different fouling and concentrated ICP effects in PRO mode. Basically, the fouling effect by the fruit juice may be less severe in FO mode than that in PRO mode. But in which mode the flux in FO process is lower is depended on the combined effect of fouling and concentrated ICP effects in both FO and PRO modes. To investigate that, FO tests using 1.5 M Glu-K as the draw solution and apple juice as the feed solution were conducted under both PRO and FO modes. However, the results in Figure 6b show that the water flux values obtained in two modes are about the same. The membrane morphology was further observed by SEM (Figure 6c) to investigate the fouling extent after the 2 h FO test, but no severe fouling phenomenon can be observed on both surfaces of the membrane. It is probably because that the filtered fruit juice mainly contains a mixture of water-soluble organic compounds, i.e., alcohols, aldehydes, ketons, hydrocarbons, which will not lead to severe membrane fouling. The 2 h operation time is also considered not long enough in lab-scale FO setup to cause significant fouling phenomenon. In addition, the water flux may be further improved if a FO membrane with higher performance could be employed. Draw Solution Recovery. Draw solution recovery is always a critical challenge in the FO process. Nanofiltration

Figure 7. NF performance for the recovery of diluted draw solution.

as the feed solution of various concentrations (0.01−0.1 M). A high rejection rate of about 91% against Glu-K can be achieved and exhibits no obvious change with the concentration change of the Glu-K solution. Besides, the water flux shows a sharp decline from 8 to 2 LMH/bar when the concentration increases from 0.01 to 0.1 M, which should be attributed to the higher osmotic pressure of the feed solution at a higher concentration. In the NF process, the generated water flux is dependent on the osmotic pressure of diluted draw solution to be recovered. In this work, under the applied pressure about 3.5 bar, the maximum treatment concentration extrapolated from the NF flux is about 0.12 M. According to the osmotic pressure of GluK solution in function of the concentration (C) (π = 42.728C, derived from the data in Table 3), when 1.5 M Glu-K solution was diluted to below 0.4 M concentration, the NF process for its recovery had to be operated with an external pressure more than 10 bar. This result, however, also indicates that the

Figure 6. Water flux for the fruit juice reconcentration with 1.5 M Glu-K draw solution via FO process (TFC membrane, PRO mode). G


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pressure-driven NF process is actually an energy-consumptive process for the recovery of concentrated draw solution. Exploration of other energy-efficient techniques should be devoted for the practical application of draw solutes in the FO process.

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CONCLUSION In this study, four gluconate salts (Glu-Na, Glu-K, Glu-Zn, and Glu-Fe(II)) are employed as novel draw solutes for the FO application. Their physicochemical properties (solubility, molecular size) and solution properties (osmotic pressure, viscosity, and pH value) are studied systematically. FO performances with different membrane (TFC vs CTA membranes), different operation mode (PRO vs FO modes), and various draw solutions of different concentrations are significantly different. Among the four draw solutions, Glu-K draw solution with desirable big molecule size, highest water solubility, and neutral pH value, low relative viscosity, and highest osmotic pressure exhibits a comparable water flux to that of NaCl draw solution, which is superior to most other gluconate salt draw solutions, as well as a much lower revese salt flux than that of NaCl solution. A highest water flux of 23.17 LMH and an acceptable salt flux of 1.09 gMH can be achieved with 2 M Glu-K as draw solution in PRO mode, which is superior to most other reported draw solutes in FO application. Besides, the reconcentration of various fruit juices with 1.5 M Glu-K draw solution via the FO process is performed and shows a reasonable good water flux, suggesting the applicability of Glu-K draw solution for juice reconcentration. The diluted draw solution after FO can be effectively recovered by NF. Further efforts may be focused on the development of high-performance FO membrane and more energy-efficient techniques for draw solution recovery. This study is believed to provide useful insights to use renewable natural compounds as next-generation draw solutes in the FO process to resolve potential concerns in the food industry.

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AUTHOR INFORMATION

Corresponding Author

*Yan Wang. E-mail: [email protected]. Tel.: 86 02787793436. Fax: 86 027-87543632. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from National Natural Science Foundation of China (no. 21306058), Huazhong University of Science and Technology, China (nos. 0124013041 and 2014YQ012), and “Thousand Youth Talent Plan”. Special thanks are also due to the Analysis and Testing Center, Huazhong University of Science and Technology for their help with material characterizations.

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REFERENCES

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