Novel reverse osmosis membranes incorporated with a hydrophilic additive for seawater desalination

Journal of Membrane Science 455 (2014) 44–54

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Novel reverse osmosis membranes incorporated with a hydrophilic additive for seawater desalination Lin Zhao, W.S. Winston Ho n William G. Lowrie Department of Chemical and Biomolecular Engineering, Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210-1178, USA

art ic l e i nf o

a b s t r a c t

Article history: Received 1 August 2013 Received in revised form 18 November 2013 Accepted 25 December 2013 Available online 3 January 2014

Novel thin-film-composite reverse osmosis membranes were synthesized successfully for seawater desalination by incorporating the hydrophilic additive, o-aminobenzoic acid–triethylamine (o-ABA–TEA) salt, into the aqueous m-phenylenediamine (MPD) solution to react with trimesoyl chloride (TMC) in the organic solution during the interfacial polymerization on a nanoporous polysulfone support. The membrane synthesis conditions, including o-ABA–TEA salt concentration, isopropanol (IPA) concentration, additional amine drying time, and hydrocarbon removal time, were optimized by characterizing membrane performances using synthetic 3.28 wt% NaCl solution under seawater desalination conditions at 800 psi (5.52 MPa) and 25 1C. The synthesized membranes showed a very high flux of 1.81 m3/m2/day (44.4 gallons/ft2/day (gfd)) and a salt rejection of 99.41%. The improvement of membrane hydrophilicity was confirmed by comparing the contact angles of the membranes synthesized with and without the hydrophilic additive. The high-flux membrane was further tested using seawater from Port Hueneme, CA and exhibited a very good and stable desalination performance for 30 days. The fouling-resistant properties of the membranes synthesized with and without the hydrophilic additive were evaluated by using sodium alginate, a common contaminant derived from seaweed, as the model foulant. The membrane synthesized with hydrophilic additive showed significantly smaller water flux decline. The surface morphologies of the membranes were analyzed using atomic force microscopy (AFM). The results showed a smoother membrane surface for the membrane incorporated with the hydrophilic additive. & 2013 Elsevier B.V. All rights reserved.

Keywords: Reverse osmosis Thin-film-composite membrane Seawater desalination Hydrophilic additive Fouling resistance

1. Introduction Global water crisis has become one of the most serious challenges influencing people all over the world over the past decades [1]. The rapid growths of population and industrialization, poor wastewater management, and global climate change have resulted in a greater demand of drinking water with good quality [1–3]. It has been forecast that the worldwide water requirements would increase from 4500 billion m3 to at least 6900 billion m3 by 2030 [4], which will not be satisfied by the currently available freshwater resources. Desalination, removing salts and other impurities from saline water to produce clean water for human consumption, agriculture, and industrial application, has been developed as one of the most sustainable solutions to meet the exponential growth of water demand [5,6]. Among all the available desalination approaches, RO is the most promising and widely applied technology for water desalination due to its low capital cost, high energy efficiency, and

n

Corresponding author. Tel.: þ 1 614 292 9970; fax: þ 1 614 292 3769. E-mail address: [email protected] (W.S.W. Ho).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.12.066

simple operation [1,7–15], which has contributed up to 59.8% of the total desalination capacity in the world (first quarter of 2012) [16]. Moreover, seawater has accounted for 58.9% of the global desalination source water [16] with the fact that 97% of earth0 s water is in the ocean [17]. Therefore, desalination of seawater using RO technology exhibits great potential to alleviate and eventually solve worldwide water shortage. The polyamide (PA) thin-film-composite (TFC) membrane was first developed by John Cadotte in 1981 [9] and remains the primary choice for the present commercial RO desalination processes [8,9,11,18], which is generally synthesized through interfacial polymerization between m-phenylenediamine (MPD) and trimesoyl chloride (TMC) (Fig. 1). Although this type of TFC membrane has demonstrated good water flux and salt rejection, its hydrophobic aromatic groups and high cross-linking degree have inherently limited water flux, which ultimately increases the desalination cost. As a result, intensive research efforts have been dedicated to enhance the water flux of the existing PA TFC membranes. Kim et al. [19] added poly(m-aminostyrene-co-vinyl alcohol) into the aqueous phase during membrane fabrication process, and the resulting membrane showed a considerable water flux increase due to the hydrophilic and flexible poly(vinyl alcohol).

L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

H2N

NH2 O

O

O

m-Phenylenediamine (MPD)

+

O Cl

45

HN

O

C

C

NH

NH

C

NH 1-n

n Cl O

O

O

NH

O

OH

Cl

Trimesoyl chloride (TMC)

HN

Fig. 1. Chemical structure of PA TFC membrane from MPD and TMC by interfacial polymerization (modified with permission from Zhao [38,39]).

Yu et al. [20] reported an interfacially polymerized polyamide– urethane membrane which was derived from 5-chloroformloxyisophthaloyl chloride and MPD. The synthesized membrane exhibited a water flux of 42 l/m2/h and a salt rejection of 99.4% using 3.5% NaCl solution at 5.5 MPa. Li et al. [21,22] synthesized PA thin films by substituting TMC with various biphenyl acid chloride monomers during membrane preparation and obtained enhanced water flux. In consideration of preferential water transport through zeolite nanoparticles, Lind et al. [23] incorporated Linde type A nanoparticles of different sizes during the interfacial polymerization to produce thin film nanocomposite (TFN) membranes which showed 37–42 l/m2/h water flux when tested at 5.5 MPa using 3.2% NaCl solution. The economics of seawater RO desalination process is also deteriorated by the inevitable membrane fouling problem during the commercial application. Membrane fouling usually refers to the deposition of various foulants such as rust, scaling, colloids, humics, and bacteria onto the membrane surface and/or into the membrane porous structure [24–27]. It is remarkably influenced by the surface morphologies and properties of RO membranes [3,26,28]. Numerous investigations have been focused on solving this problem, and it is found that the membrane fouling resistance can be improved by smoothing membrane surface [28–32], increasing surface hydrophilicity [33–35], and introducing strong electrostatic repulsion between membrane surface and charged foulants [36]. In this work, the concept of hydrophilic additive has been extended to seawater desalination based on the earlier research for brackish water desalination from our group [37–39]. A hydrophilic additive refers to a particular chemical containing at least one hydrophilic portion and one reactive portion which can react with either MPD or TMC during the interfacial polymerization [37]. o-Aminobenzoic acid–triethylamine (o-ABA–TEA) salt has demonstrated its capability of enhancing water flux of PA TFC membrane for brackish water desalination [37–39]. It was investigated in this work for the effects on seawater desalination performance with different incorporation concentrations in the aqueous amine solution. In addition, other membrane synthesis parameters, including isopropanol (IPA) concentration, additional amine drying time, and hydrocarbon removal time, were studied and optimized under seawater desalination conditions using synthetic 3.28 wt% NaCl solution at 800 psi (5.52 MPa) and 25 1C. The membrane performance was also evaluated using seawater from Port Hueneme, CA. Contact angle measurements were conducted to characterize the change of surface hydrophilicity. Sodium alginate (NaAlg) was selected as the model foulant to investigate the fouling propensities of the membranes synthesized with and without the hydrophilic additive. Atomic force microscopy (AFM) was applied to characterize the surface morphologies of the synthesized membranes.

2. Experimental 2.1. Materials m-Phenylenediamine (MPD, 99% þ), o-aminobenzoic acid (o-ABA, 98% þ), triethylamine (TEA, 99.5% þ), sodium chloride (NaCl, 99% þ), and sodium alginate (NaAlg, powder) were acquired from Sigma-Aldrich. Hydrochloric acid (HCl, Certificated ACS Plus) and isopropanol (IPA, 99.9%) were purchased from Fisher Scientific. Trimesoyl chloride (TMC, 98% þ) was bought from TCI America whereas Isopar Gs (isoparaffinic solvent) was from ExxonMobile Chemical Company. All the chemicals were used as received without further purification. The substrate for the interfacial polymerization was the nanoporous polysulfone support (U100 PSF, flat sheet with a pore size of about 50 nm) purchased from TriSep Corporation. The PSF support was stored in deionized water before usage. The seawater from Port Hueneme, CA after ultrafiltration was kindly provided by Naval Facilities Engineering Service Center (NFESC) with an average turbidity of 0.032 Nephelometric Turbidity Units (NTU) and a silt density index (SDI) (15 min) of 2.2. 2.2. Membrane preparation The PSF support was taped on a glass plate carefully to prevent any possible penetration of liquid into its back from the sides. Then, it was soaked in deionized water for 1 min. At the end, the PSF support on the glass plate was taken out to remove the excess water on its top surface by standing vertically under ambient conditions. The PSF support was first immersed in an aqueous amine solution for 8 s. The basic aqueous amine solution contained 3.0 wt% MPD, 5.2 wt% HCl–TEA salt and 1.0 wt% TEA in deionized water, while different concentrations of o-ABA–TEA salt (0–2.0 wt%) and IPA (0–50 wt%) were employed. 5.2 wt% HCl–TEA was applied to protect the polysulfone substrate from pore fusion or collapse during the heat treatment [11,38]. After the excess solution was removed from the surface by applying a plastic squeegee roller, the PSF support was dried in air vertically for different periods of time. Then, it was dipped into an organic solution containing 0.21 wt% TMC in Isopar Gs for 6 s to carry out the interfacial polymerization for PA TFC membrane formation. The resulting membrane was dried under convection at 90 1C for a pre-determined hydrocarbon removal time to remove the organic solvent. Finally, the membrane was washed and soaked in deionized water for storage before testing.

2.3. Characterization of membrane desalination performance The membrane desalination tests were conducted in a lab-scale RO water permeation apparatus with cross-flow design (Fig. 2).

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L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

obtained by measuring the conductivities of the solutions prepared with different NaCl concentrations in deionized water. The calibration curve for the seawater was determined by diluting the original seawater (its salt concentration determined by removing all the water) to different concentrations with deionized water for conductivity measurement.

The synthesized membrane sample was sealed in a stainless steel cell with an effective desalination area of 28.6 cm2 (4.43 in.2) for transport measurement. Synthetic 3.28 wt% NaCl solution was prepared to simulate seawater composition as feed solution. The applied pressure was controlled at 800 psi (5.52 MPa) and the operation temperature was maintained at 25 1C for all the experiments throughout the entire testing period. The membrane desalination performance was characterized by its water flux and salt rejection at the steady state under seawater desalination conditions (synthetic 3.28 wt% NaCl solution or seawater from Port Hueneme, CA at 800 psi). The water flux was calculated by weighing the permeate water which was accumulated within a certain period of time, and its unit was converted into m3/m2/day or gallon/ft2/day (gfd) (1 gfd ¼0.0407 m3/m2/day). The salt rejection (R) presented in this work was the apparent salt rejection without considering the effects of concentration polarization. It was calculated according to the following equation: R ¼ 1

2.4. Characterization of membrane properties The surface hydrophilicity of the membrane sample was evaluated by contact angle measurements using the sessile drop technique with deionized water as the reference liquid. The synthesized membrane was washed with deionized water and then dried overnight under ambient conditions. About 1 mL deionized water droplet was deposited on the leveled membrane surface to measure the contact angle of each sample. Fourier transform infrared (FTIR) spectrometry was employed to characterize the chemical property of the synthesized membrane. FTIR spectra were created and documented on a Nexus 470 FTIR Spectrometer (Thermo Nicolet), using Smart MIRacle Single Reflection Horizontal ATR (attenuated total reflectance). The membrane surface morphology was characterized by atomic force microscopy (AFM). AFM images were generated by using a Nanoscope IIIa atomic force microscope (Digital Instruments), and average roughness of each sample was calculated.

Cp Cf

where Cp and Cf are the salt concentrations of the permeate and feed solutions, respectively, which were measured by a conductivity meter. Different calibration curves were prepared to relate the solution conductivity to its salt concentration for the synthetic 3.28 wt% NaCl solution and the seawater from Port Hueneme, CA. The calibration curve for the synthetic 3.28 wt% NaCl solution was

3. Results and discussion

Recycling

3.1. Effects of o-ABA–TEA salt on membrane desalination performance

Feed Tank

Pressure Gauge

Heating/cooling System

o-ABA–TEA salt is considered as a hydrophilic additive since it contains a –COO–(HNEt3) þ group as the hydrophilic portion and a –NH2 group as the reactive portion to react with –COCl group from TMC. Through the same reaction mechanism between MPD and TMC, o-ABA–TEA salt can be chemically bonded onto the PA TFC membrane with the formation of the amide bond, which ensures the stability of its incorporation. Fig. 3 illustrates the chemistry of interfacial polymerization between MPD and TMC in the presence of o-ABA–TEA salt.

Water Permeation Metering Cell Valve

Water Pump Fig. 2. Schematic representation of reverse osmosis water permeation apparatus (modified with permission from Zhao [38]).

H 2N

NH2

-

+

O [HNEt3]

O

Cl

+

O

O

C

C

O

NH2

C

Cl

Cl

o-Aminobenzoic acid-triethylamine (o-ABA-TEA) salt

O HN

O

O HN

NH

NH

NH

NH

O

NH 1-m-n

n

m O

O

O

O

NH

O

O -

O [HNEt3]

OH

+

HN Fig. 3. Interfacial polymerization of PA TFC membrane with incorporation of the hydrophilic additive.

L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

99.0

1.4

98.5

1.2 98.0 1.0 0.8 0.0

97.5 0.5

1.0

1.5

2.0

2.5

o-ABA-TEA Salt Concentration (%) Fig. 4. Effects of o-ABA–TEA salt concentration in aqueous amine solution on membrane desalination performance (synthetic 3.28 wt% NaCl solution at 800 psi).

o-ABA–TEA salt was added into the aqueous amine solution with different concentrations in order to investigate its effects on membrane desalination performance. As shown in Fig. 4 for the membranes with a hydrocarbon removal time of 6.5 min, the water flux was enhanced significantly from 0.99 m3/m2/day (24.2 gfd) to 1.60 m3/m2/day (39.2 gfd) when the concentration of o-ABA–TEA salt increased from 0 wt% to 1.0 wt% in the amine solution, while the salt rejection increased at the same time. However, both the water flux and salt rejection declined when the concentration of o-ABA–TEA salt was further increased. At low concentrations, the incorporation of this hydrophilic additive could create an additional pathway for the transport of water molecules by increasing membrane hydrophilicity and reduce the cross-linking degree of PA TFC membrane by competing with MPD (Fig. 3), which resulted in a great enhancement of water flux. On the other hand, the decrease of the cross-linking degree could also increase salt passage through the membrane. Nevertheless, the incorporation of o-ABA–TEA salt can reduce salt passage by providing electrostatic repulsion via its hydrophilic group, –COO–(HNEt3) þ (presumably converted to –COO  Na þ in the presence of NaCl). As a result, the observed salt rejection increased slightly. However, the reaction between the hydrophilic additive and TMC interfered with the main reaction between MPD and TMC during the interfacial polymerization when the concentration of o-ABA–TEA salt was high enough. This deteriorated the subsequently formed polyamide membrane structure and hence the desalination performance. Therefore, an optimal incorporation concentration of o-ABA–TEA salt was determined to be 1.0 wt% with the consideration of water flux and salt rejection, which was applied in the following experiments. This value was much lower than that observed for brackish water desalination (2.85 wt%) [38], which is reasonable because the desalination conditions were more stringent (3.28 wt% NaCl solution at 800 psi vs. 2000 ppm NaCl solution at 225 psi). 3.2. Effects of IPA concentration on membrane desalination performance The primary purpose of adding isopropanol (IPA) into the aqueous amine solution is to form a uniform liquid layer on the PSF support by reducing its surface tension. This is important for the even distribution of the solutes (MPD and o-ABA–TEA salt) and the simultaneous evaporation of the solvent throughout the PSF surface during the following amine drying process, which could eventually affect the membrane formation during the interfacial polymerization. Surprisingly, the water flux increased by 13.1% and the salt rejection remained almost constant, when 25 wt% IPA was

100.0

2.4 2.2

99.5

2.0 99.0 1.8 98.5

1.6

Salt Rejection (%)

1.6

Flux (m3/m2/day)

99.5

1.8

Salt Rejection (%)

Flux (m3/m2/day)

2.0

incorporated. Therefore, the concentration of IPA in the amine solution was gradually increased to systematically study its effects on membrane desalination performance. As depicted in Fig. 5 for the membranes with a hydrocarbon removal time of 6.5 min, the water flux continuously increased and the salt rejection reduced with the increase of IPA concentration. As reported previously [38], the hydrophilic additives containing a –OH group such as 2-(2-hydroxyethyl) pyridine and 4-(2-hydroxyethyl) morpholine, could also be incorporated onto the PA TFC membrane through the reaction between –OH and –COCl groups. Therefore, IPA could also be chemically bonded onto the PA TFC membrane through its –OH group during the interfacial polymerization, in addition to its function of reducing surface tension of the aqueous solution. Although its reactivity is lower than those of MPD and o-ABA– TEA salt, this reaction could still take place when the concentration was sufficiently high (i.e., above 25 wt%). In order to verify the incorporation of IPA, the ATR-FTIR spectra of the membrane synthesized without IPA and the membrane synthesized with 40 wt% IPA in the aqueous MPD solution were taken for comparison. Although the reaction between –OH and –COCl groups should result in the formation of ester groups with C–O stretches at 1300–1000 cm  1, the same groups could also be generated from the hydrolysis of unreacted –COCl groups (Fig. 1). Therefore, the incorporation of IPA could not be confirmed by analyzing the ATR-FTIR spectra difference at the characteristic absorption peaks for C–O stretches. However, as shown in Fig. 6, the membrane synthesized without IPA only showed the characteristic absorption of aromatic C–H stretch at 3100–3060 cm  1, which should come from the aromatic structure of the two reactants. In comparison, the membrane synthesized with IPA showed an additional

98.0

1.4 0

10

20

30

40

50

60

IPA Concentration (%) Fig. 5. Effects of IPA concentration in aqueous amine solution on membrane desalination performance (synthetic 3.28 wt% NaCl solution at 800 psi).

0.18 0.15 Absorbance

100.0

2.2

47

0.12 0.09

C-H stretch (aromatic) 3100-3060

Membrane synthesized without IPA

0.06 0.03 0.00 3500

C-H stretch (alkyl) 2960

3000

Membrane synthesized with IPA

2500

2000

1500

1000

Wave Number (cm-1) Fig. 6. ATR-FTIR spectra of the membranes synthesized without and with IPA.

48

L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

characteristic absorption of alkyl C–H stretch at 2960 cm  1, which should be attributed to the pendent –CH3 group from IPA. The presence of alkyl C–H stretch suggested that IPA was indeed incorporated onto the PA TFC membrane. The incorporation of IPA reduced the cross-linking degree of PA TFC membrane and increased its hydrophilicity by introducing more –O– groups, while the free rotation of the hydrophilic pendant group created more free volume for both water and salt molecules to pass through. At low IPA concentrations, the increment of water flux was comparable to that of salt flux and the salt rejection did not reduce too much. Beyond a particular concentration (i.e., 50 wt%), the effects of IPA on increasing salt transport became dominant, and the observed salt rejection started to decline significantly. The same phenomena were observed for the incorporation of 2-(2-hydroxyethyl) pyridine and 4-(2-hydroxyethyl) morpholine in the previous research [38], where the pendent groups were larger with more pronounced effects on creating free volumes and hence the reduction of salt rejection occurred at much lower incorporation concentrations. Considering the critical salt rejection requirement of seawater desalination, 40 wt% was determined as the maximum concentration of IPA in the aqueous amine solution because it further increased the water flux from 1.60 m3/m2/day (39.2 gfd) to 1.81 m3/m2/day (44.4 gfd) while maintained the salt rejection at 99.41%. 3.3. Effects of additional amine drying time on membrane desalination performance During membrane preparation, the amine solution-containing PSF support has to be dried to some extent before dipping in the organic solution for the interfacial polymerization. In this research, it typically took about 75 s for the solvent on the surface of PSF support to evaporate completely. Immediately after this initial amine drying, the duration before the interfacial polymerization is called the additional amine drying time, which could affect the PA TFC membrane formation and thereafter its desalination performance. To evaluate its effects on water flux and salt rejection, membrane samples were prepared with different additional amine drying times while all the other preparation conditions were maintained the same. The results are summarized in Fig. 7. Within the time range that was investigated, the water flux declined and the salt rejection increased as the additional amine drying time increased. This could be explained by the mechanism of the interfacial polymerization for the PA TFC membrane preparation. Although both MPD and TMC could partition through the interface between two phases and react with each other, the polymerization

2.4

occurs mainly in the organic phase, because the solubility of TMC in the aqueous phase is relatively low [40–42]. With a longer additional amine drying time, the MPD concentration became higher and the level of aqueous phase became deeper within the nanoporous PSF support. Consequently, a denser membrane film was formed more inside the PSF support. As a result, the mass transfer resistances for both water and salt molecules increased, leading to lower water flux and higher salt rejection. In consideration of good salt rejection and high water flux, the optimal additional amine drying time was found to be 45 s.

3.4. Effects of hydrocarbon removal time on membrane desalination performance Heat treatment is usually required to expedite the complete removal of the organic solvent from the membrane film. Meanwhile, it might also promote the cross-linking between unreacted functional groups to some extent. The duration of this heat treatment is called the hydrocarbon removal time, which depends on the evaporation rate of the particular organic solvent. The impacts of hydrocarbon removal time (at 90 1C under convection with an air flow rate of 45 L/min) on membrane desalination performance were investigated, and the trends of water flux and salt rejection are plotted in Fig. 8, respectively. With the increase of hydrocarbon removal time from 4 min to 6 min, the salt rejection declined while the water flux increased. In addition, strong hydrocarbon smells were noticed for the membranes heated for 4 and 5 min, even after soaking in deionized water for 2 h, which suggested that the evaporation of Isopar Gs (with a boiling point of 166 1C) was not complete. As a result, a significant amount of the hydrophobic organic solvent remained within the membrane matrix, which could remarkably increase the mass transfer resistances to both water and salt. When the hydrocarbon removal time was around 6–7 min, all the organic solvent was removed and the optimal water flux was obtained. After 7 min, the water flux started to decrease while the salt rejection did not change much. The similar phenomenon of water flux reduction was reported in [43] when hexane was used as the organic solvent and the heat treatment time was increased to more than 2 min at 90 1C, which might result from the pore collapse or shrinkage of the polysulfone support due to the excessive heat [11,43]. Compared to [43,44], the effects of overheating were more pronounced (observed in 1 min) after the organic solvent was completely removed in this work, because the air flow was applied for convection to promote the evaporation of Isopar Gs. As a result, 7 min was determined as the optimal hydrocarbon removal time.

2.4

100.0

1.8

99.0

1.6 98.5 1.4

Flux (m3/m2/day)

2.0

Salt Rejection (%)

99.5

99.5 2.0 1.8

99.0

1.6 98.5

Salt Rejection (%)

2.2

2.2

Flux (m3/m2/day)

100.0

1.4

1.2

98.0 20

30

40

50

60

70

80

90

Additional Amine Drying Time (sec) Fig. 7. Effects of additional amine drying time on membrane desalination performance (synthetic 3.28 wt% NaCl solution at 800 psi).

1.2

98.0 3

4 5 6 7 8 Hydrocarbon Removal Time (min)

9

Fig. 8. Effects of hydrocarbon removal time on membrane desalination performance (synthetic 3.28 wt% NaCl solution at 800 psi).

L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

The desalination performances of the membrane synthesized without the hydrophilic additive, referred to the basic membrane, and the high-flux membrane synthesized with 1.0 wt% o-ABA–TEA salt and 40 wt% IPA were compared under seawater desalination conditions using synthetic 3.28 wt% NaCl solution at 800 psi (5.52 MPa) and 25 1C. All the other synthesis conditions were kept identical as determined in the previous sections. The comparative results between the basic membrane and the high-flux membrane are summarized in Fig. 8 in terms of water flux and salt rejection. As shown in the figure, the high-flux membrane surpassed the basic membrane with 83.5% increase in the water flux (1.81 m3/ m2/day or 44.4 1gfd vs. 0.99 m3/m2/day or 24.2 1gfd) and a small increase in the salt rejection (99.41% vs. 99.32%). Moreover, the contact angle reduced significantly from about 741 for the basic membrane to about 581 for the high-flux membrane (Fig. 9), which could be interpreted as the result of the increase of surface hydrophilicity due to the incorporation of the hydrophilic additive. This confirmed the effects of the hydrophilic additive in addition to the comparison of desalination performances. It should be noted that the incorporation of the hydrophilic additive was during the entire period of the interfacial polymerization for PA TFC membrane formation. Therefore, the improvement of hydrophilicity should be throughout the membrane film, which not only provided good affinity to water on the surface but also facilitated its transport inside the thin film.

and then with the seawater at 800 psi (5.5 MPa) for comparison. As shown in Fig. 10, although the salt rejection remained at around 99.4%, the average water flux for the seawater was 1.70 m3/m2/day (41.6 gfd), which was consistently lower than that for the 3.28 wt% NaCl solution (1.81 m3/m2/day or 44.4 gfd). This 0.11 m3/m2/day (2.8 gfd) water flux difference could not be explained by the experimental error because the fluctuations of water flux were much smaller for either the synthetic 3.28 wt% NaCl solution or the seawater. One possible explanation is that the seawater from Port Hueneme, CA had a higher total solid concentration which could result in a higher osmotic pressure during the desalination process. In order to verify this hypothesis, the total solid concentration of seawater was determined by two approaches for comparison: (1) calculated by employing the concentration polarization model based on the water flux measurements and (2) measured by removing all the water in the vacuum oven at 80 1C overnight. Fig. 11 illustrates the schematic to calculate the total solid concentration of the seawater with the equations described in the following paragraphs. In the reverse osmosis desalination process, the difference between the applied pressure and the osmotic pressure of the saline water is the driving force for water to permeate through the membrane. Therefore, the water flux could be described by [8] jw ¼ Aw ðΔp  ΔπÞ 2.4

200 175 150

Flux (m3/m2/day)

In the previous sections, all the membrane samples were tested with the synthetic 3.28 wt% NaCl solution at 800 psi (5.52 MPa) to simulate the commercial seawater desalination conditions for membrane characterization because NaCl is the major component in seawater. However, the seawater after pre-treatments for the realistic reverse osmosis desalination could possess different total solid concentrations and compositions. Therefore, the desalination performance of the synthesized high-flux membrane was evaluated by using the seawater from Port Hueneme, CA after ultrafiltration as the feed solution, which had an average turbidity of 0.032 Nephelometric Turbidity Units (NTU) and a silt density index (SDI) (15 min) of 2.2. Six samples were prepared following the same synthesis procedure for the high-flux membrane (Section 3.5). Each membrane was first tested with the synthetic 3.28 wt% NaCl solution

99.5 2.0 1.8

99.0

1.6 98.5 1.4 1.2

98.0 0

1

2

3 4 5 Sample Number

6

7

Fig. 10. Membrane desalination performances at 800 psi with synthetic 3.28 wt% NaCl solution and seawater as feed solution.

of pure water =0

of synthetic 3.28% NaCl solution

Flux (10-2 m3/m2/day) Salt rejection (%) Contact angle (o)

100.0

Synthetic 3.28% NaCl solution Seawater

2.2

3.6. Membrane desalination performance with seawater as feed solution

ð1Þ

Salt Rejection (%)

3.5. Comparisons of membrane desalination performance and contact angle

49

of seawater

125 100 75 50 25 0 Basic Membrane

High-FluxMembrane

Fig. 9. Comparison of desalination performances and contact angles between the basic membrane and the high-flux membrane.

Fig. 11. Schematic for calculating mass transfer coefficient and bulk concentration of seawater based on the concentration polarization model.

L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

 jw ¼ k ln

Cw  Cp Cb  Cp

 ð2Þ

Since the desalination performances of the same membrane for the synthetic 3.28 wt% NaCl solution and the seawater were very similar, the mass transfer coefficient k was assumed to be the same in these two situations. Therefore, the following expression could be obtained from Eq. (2): jw;sea lnððC w;sea  C p;sea Þ=ðC b;sea  C p;sea ÞÞ ¼ jw;NaCl lnððC w;NaCl  C p;NaCl Þ=ðC b;NaCl C p;NaCl ÞÞ

ð3Þ

At first, Aw for pure water was calculated from Eq. (1) as 0.691 m3/m2/day/MPa by measuring the water flux of the highflux membrane at 800 psi (5.52 MPa) and 25 1C with pure water where Δπ was 0. Although the osmotic de-swelling resulted from the increase of salt concentration in the external solution [48] was usually observed for hydrogels [48,49], its effects on the swelling property of the aromatic polyamide membrane was assumed to be negligible in the case under consideration. In addition, the highflux membrane exhibited a very high salt rejection, allowing a very small amount of ions to get into the membrane and pass through it. Therefore, the values of Aw for the synthetic 3.28 wt% NaCl solution and the seawater were considered the same as that for pure water, which were applied in Eq. (1) to calculate the osmotic pressues for the synthetic 3.28 wt% NaCl solution and the seawater, respectively. The calculated osmotic pressure was then related to the concentration near membrane surface based on the data reported in [50] for both the synthetic 3.28 wt% NaCl solution and the seawater to determine Cw,NaCl and Cw,sea. As a result, Cb,sea, the bulk concentration of the seawater, was calculated from Eq. (3), since the bulk concentration of the synthetic NaCl solution was prepared as 3.28 wt% and all the other variables in this equation had been either determined by measuring permeate water fluxes (jw,sea and jw,NaCl) and salt concentrations (Cp,sea and Cp,NaCl) during membrane testing, or calculated by following the aforementioned procedure (Cw,NaCl and Cw,sea) shown in Fig. 11. On the other hand, the important parameter, the mass transfer coefficient k for the synthetic 3.28 wt% NaCl solution, was also obtained via Eq. (2). As reported in Table 1, the average bulk concentration of the seawater calculated from Eq. (3), 3.47 wt%, agreed very well with the value measured by evaporating all the water, 3.45 wt%, which revealed that the water flux reduction mainly resulted from the higher solid concentration in the seawater from Port Hueneme, CA.

3.7. Membrane stability with seawater as feed solution In order to investigate the stability of the improved desalination performance, the high-flux membrane (Section 3.5) was continuously tested for 30 days under seawater desalination conditions at 800 psi (5.52 MPa) using the seawater from Port Hueneme, CA. As shown in Fig. 12, although there were small fluctuations, the desalination performance of the tested membrane was very stable during the entire testing period, in terms of water flux (average 1.67 m3/m2/day or 41.1 gfd) and salt rejection (average 99.49%). This excellent stability is reasonable because the hydrophilic additive was chemically bonded onto the membrane matrix during its formation. Therefore, its effects on increasing membrane hydrophilicity did not weaken over time. Furthermore, the ATR-FTIR spectra of the same membrane sample were taken before and after the 30-day stability test (Fig. 13). As shown in this figure, the chemical structures of the synthesized polyamide thin film, especially, the characteristic absorption peaks of the amide group at 1540 cm  1 and 1660 cm  1, were essentially identical. This demonstrated that the high-flux membrane was chemically stable under seawater desalination conditions (without chlorine) for at least 30 days. 3.8. Membrane fouling by sodium alginate Sodium alginate (NaAlg), a common organic foulant in seawater derived from seaweed, was selected as the model foulant in

2.4

100.0

2.2 99.0

2.0 1.8

98.0

1.6 97.0

1.4

1.2

96.0

1.0 0.8

95.0 0

k for NaCl solution (m3/m2/day)

Calculated bulk concentration of seawater (wt%)

Measured bulk concentration of seawater (wt%)

1 2 3 4 5 6 Ave

8.97 9.03 8.94 8.99 9.05 9.01 9.00

3.45 3.48 3.44 3.46 3.49 3.47 3.47

3.45

8

12 16 20 24 Testing Time (day)

28

32

0.30 Amide

Absorbance

Sample no.

4

Fig. 12. Membrane desalination performances during the 30-day stability test at 800 psi with seawater as feed solution.

0.25

Table 1 Comparison of calculated and measured bulk concentration of seawater.

Salt Rejection (%)

The concentration polarization during the reverse osmosis desalination process was described by the well-developed film theory, which was commonly employed in the literature [8,45–47]. Therefore, the water flux could also be expressed as

Flux (m3/m2/day)

50

Before 30-day stability test

Amide (1540) (1660)

0.20 0.15 After 30-day stability test

0.10 0.05 0.00 1850

1650

1450

1250

1050

850

650

Wave Number (cm-1) Fig. 13. ATR-FTIR spectra of the high-flux membrane before and after the 30-day stability test.

L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

51

2.5 2.5

Basic membrane High-flux membrane

2.0 Flux (m 3/m 2/day)

Flux (m3/m2/day)

2.0 1.5 1.0 0.5

1.5 1.0 0.5

0.0 0

5

10 15 Testing Time (h)

20

0.0

25

Basic Membrane

Fig. 14. Water flux of the basic membrane and the high-flux membranes under seawater desalination conditions in the presence of 25 ppm NaAlg.

140

2.5

Salt Rejection (%)

120

Basic membrane High-flux membrane

2.0 Flux (m3/m 2/day)

3.28% NaCl 3.28% NaCl + 25 ppm NaAlg 3.28% NaCl + 50 ppm NaAlg

1.5

High-Flux Membrane

3.28% NaCl 3.28% NaCl + 25 ppm NaAlg 3.28% NaCl + 50 ppm NaAlg

100 80 60 40 20

1.0

0 0.5 0.0 0

5

10 15 Testing Time (h)

20

25

Fig. 15. Water flux of the basic membrane and the high-flux membranes under seawater desalination conditions in the presence of 50 ppm NaAlg.

the membrane fouling tests. In addition, it has been used as the model foulant to represent the polysaccharides, the major contaminant in wastewater effluent, for membrane fouling studies [51–53]. The membrane fouling resistance was characterized and analyzed by testing the same membrane sample under seawater desalination conditions at 800 psi (5.52 MPa) without and with NaAlg (25 ppm and 50 ppm) in the synthetic 3.28 wt% NaCl solution. The basic membrane and the high-flux membrane were prepared under the conditions described in Section 3.5, and tested for comparison to study the effects of hydrophilic additive incorporation on membrane fouling propensity. Figs. 14 and 15 depict the tendencies of water flux of one basic membrane and two high-flux membrane samples with respect to time for 24 h in the presence of 25 ppm and 50 ppm NaAlg in the synthetic 3.28 wt% NaCl solution, respectively. Membrane fouling was observed instantly after the addition of NaAlg and the water flux became stable after about 2 h for all the experiments. As summarized in Fig. 16(a), the water flux of the high-flux membrane reduced from 1.81 m3/m2/day (44.4 gfd) to 1.61 m3/m2/day (39.5 gfd) by 11.0% in the presence of 25 ppm NaAlg after 24 h. It further declined to 1.45 m3/m2/day (35.6 gfd) by 19.8% when the concentration of NaAlg was increased to 50 ppm. In comparison, the basic membrane showed much larger water flux reductions of 18.2% and 31.8% for these two NaAlg concentrations, respectively. In addition to the percentage of water flux reduction, the membrane fouling propensity was also characterized by comparing the resistances created by the foulant layer for the basic membrane and the high-flux membrane. The following expression

Basic Membrane

High-Flux Membrane

Fig. 16. Membrane fouling comparison between the basic membrane and the highflux membrane: (a) water flux and (b) salt rejection in the presence of NaAlg after 24 h.

of water flux was employed for this constant-pressure RO process considering the resistance-in-series model [54–56]: jw ¼

Δp Δπ ηðRm þ Rf Þ

ð4Þ

Rm can be determined from the initial water flux. By assuming that the driving forces were identical for the basic membrane and the high-flux membrane in the desalination processes with and without NaAlg in the synthetic 3.28 wt% NaCl solution, the ratio between Rf and Rm for the same membrane could be obtained from Eq. (4): Rf j0  j0 ¼ w0 w Rm jw

ð5Þ

Based on Eq. (4), the ratio between the intrinsic membrane resistance of the high-flux membrane, Rm,HF, and that of the basic membrane, Rm,BA, could be expressed as 0 Rm;HF jw;BA ¼ Rm;BA j0w;HF

ð6Þ

From Eq. (5) with the use of Eq. (6), the ratio between the foulantlayer resistances for the high-flux membrane and the basic membrane could be derived as 0 0 0 0 Rf ;HF jw;HF  jw;HF jw;BA jw;BA ¼ 0   0 Rf ;BA jw;BA  j0w;BA jw;HF j0w;HF

ð7Þ

The values of this ratio were 0.30 for the membrane fouling test with 25 ppm NaAlg and 0.29 for that with 50 ppm NaAlg. These values showed that the foulant-layer resistance for the high-flux membrane was much smaller than that for the basic membrane

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L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

the average surface roughness reduced from 106 nm to 77 nm with the incorporation of the hydrophilic additive. This smoother membrane surface reduced the opportunity for NaAlg to attach and hence its subsequent deposition. The rough peak-and-valley surface morphology mainly resulted from the intensive reaction between MPD and TMC. When o-ABA–TEA salt was incorporated, it competed with MPD to react with TMC during the interfacial polymerization, which might slow down the rapid reaction between MPD and TMC to some extent. Therefore, the membrane surface became smoother.

4. Conclusions

Fig. 17. AFM images for the surfaces of (a) basic membrane and (b) high-flux membrane. The average roughness is (a) 106 nm and (b) 77 nm.

for both fouling tests. These considerably smaller foulant-layer resistances for the high-flux membrane clearly demonstrated that the membrane fouling resistance was improved with the incorporation of the hydrophilic additive. Moreover, Fig. 16(b) illustrates a smaller salt rejection increase for the high-flux membrane (from 99.41% to 99.50%), compared to that for the basic membrane (from 99.32% to 99.70%). This could also be considered as an indication of a better membrane fouling resistance because the formation of a foulant layer usually increases the salt rejection. The remarkable improvement of the membrane fouling resistance should be attributed to the effects from the incorporation of the hydrophilic additive on membrane surface properties. First of all, the surface of the high-flux membrane with the hydrophilic additive possessed more negatively charged –COO  groups (Fig. 3), which provided an enhanced electrostatic repulsion against negatively charged alginate. Moreover, the surface hydrophilicity of the high-flux membrane was significantly increased, which resulted in a greater affinity to water. As a result, a water layer could be formed at the interface and performed as a barrier to inhibit the adhesion of NaAlg onto the membrane surface [57–59]. Therefore, the deposition of NaAlg was mitigated by the change of membrane surface properties with the incorporation of the hydrophilic additive. As described earlier, the membrane surface morphology also plays an important role in membrane fouling because the PA TFC membrane possesses a rough peak-and-valley surface pattern [3,26,28,39]. As a quantitative technique to analyze the surface roughness, atomic force microscopy (AFM) was applied to these two types of membranes. As shown in Fig. 17, the surface of the high-flux membrane was considerably smoother than that of the basic membrane. The analysis of these AFM images revealed that

The hydrophilic additive, o-ABA–TEA salt, was successfully incorporated onto the PA TFC membrane during the interfacial polymerization to improve membrane desalination performance under seawater conditions. The effects of o-ABA–TEA salt concentration in the amine solution were investigated and optimized for seawater desalination using the synthetic 3.28 wt% NaCl solution at 800 psi (5.52 MPa), resulting in the optimal additive concentration of 1.0 wt%. This hydrophilic additive could create an additional pathway to enhance water transport and provide charge repulsion to increase slat rejection. It was found that the addition of IPA in the amine solution could further increase the water flux. The membrane preparation conditions such as additional amine drying time and hydrocarbon removal time were also studied and optimized. Finally, the resulting high-flux membrane showed a water flux of 1.81 m3/m2/day (44.4 gfd) (99.41% salt rejection) under seawater desalination conditions using the synthetic 3.28 wt% NaCl at 800 psi (5.52 MPa), which was 83.5% higher than the basic membrane (0.99 m3/m2/day or 24.2 gfd). In addition, the contact angle of the synthesized membrane reduced from 741 to 581 with the incorporation of the hydrophilic additive, which confirmed the increase of surface hydrophilicity. This high-flux membrane exhibited a slightly lower water flux when it was tested using the seawater from Hueneme, CA as the feed solution at 800 psi (5.52 MPa), which was interpreted as the result of a higher solid content (3.45 wt%) in this seawater. Moreover, its desalination performance was stable during a 30-day stability test with this seawater as the feed solution. NaAlg was used as the model foulant in membrane fouling tests for the basic and highflux membranes with different concentrations (25 ppm and 50 ppm). In either case, the percentage of water flux decline of the high-flux membrane was considerably less than that of the basic membrane, demonstrating a better fouling resistance. AFM analysis indicated that the membrane surface was smoother with the incorporation of the hydrophilic additive.

Acknowledgments The authors would like to thank Mengzi Zhang for conducting AFM imaging and Bill Varnava, Naval Facilities Engineering Service Center (NFESC), for providing us the seawater sample from Port Hueneme, CA. The authors would also like to thank J. Paul Armistead of the Office of Naval Research, Michelle Chapman of the US Office of Reclamation, David Nordham of Naval Surface Warfare Center, Carderock Division, and Mark Miller of US Army for their helpful discussion. We would like to gratefully acknowledge the Office of Naval Research (N00014-03-1-0994, N0001405-1-00800 and N00014-10-1-00147) for the financial support of this work.

L. Zhao, W.S.W. Ho / Journal of Membrane Science 455 (2014) 44–54

Nomenclature Aw Cb Cb,NaCl Cb,sea Cf Cp Cp,NaCl Cp,sea Cw Cw,NaCl Cw,sea jw j0w j0w;BA j0w;HF j0w j0w;BA j0w;HF k Δp R Rf Rf,BA Rf,HF Rm Rm,BA Rm,HF

apparent water permeation constant salt concentration in bulk feed solution salt concentration in the bulk of synthetic 3.28 wt% NaCl solution salt concentration in the bulk of seawater salt concentration of feed solution salt concentration of permeate solution salt concentration of permeate solution using 3.28 wt% NaCl solution as feed solution salt concentration of permeate solution using seawater as feed solution salt concentration of feed solution at the membrane surface salt concentration of synthetic 3.28 wt% NaCl solution at the membrane surface salt concentration of seawater at the membrane surface water flux initial water flux in the fouling test initial water flux of the basic membrane in the fouling test initial water flux of the high-flux membrane in the fouling test steady-state water flux in the fouling test steady-state water flux of the basic membrane in the fouling test steady-state water flux of the high-flux membrane in the fouling test mass transfer coefficient applied pressure salt rejection foulant-layer resistance foulant-layer resistance for the basic membrane foulant-layer resistance for the high-flux membrane intrinsic membrane resistance intrinsic resistance of the basic membrane intrinsic resistance of the high-flux membrane

Greek letters η Δπ

aqueous solution viscosity osmotic pressure difference

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