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spherical mesoporous silica nanocomposite reverse osmosis membrane with superior permeability
Accepted Manuscript Title: Fabrication of semi-aromatic polyamide/spherical mesoporous silica nanocomposite reverse osmosis membrane with superior permeability Author: Qiang Li Hui Yu Feiyang Wu Jie Song Xianhui Pan Meng Zhang PII: DOI: Reference:
S0169-4332(15)03022-6 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.028 APSUSC 32010
To appear in:
APSUSC
Received date: Accepted date:
29-10-2015 4-12-2015
Please cite this article as: Q. Li, H. Yu, F. Wu, J. Song, X. Pan, M. Zhang, Fabrication of semi-aromatic polyamide/spherical mesoporous silica nanocomposite reverse osmosis membrane with superior permeability, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Highlights
A semi-aromatic polyamide/spherical mesoporous silica membrane was fabricated. This membrane has better separation performance than other polyamide membranes.
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The separation performance was comprehensively investigated.
Relationship between separation performance and membrane structure was
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proposed.
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*Graphical Abstract (for review)
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Graphical abstract:
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*Manuscript
Fabrication of semi-aromatic polyamide/spherical mesoporous silica nanocomposite reverse osmosis membrane with superior
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Qiang Li*, Hui Yu, Feiyang Wu, Jie Song, Xianhui Pan, Meng Zhang
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permeability
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The institute of seawater desalination and multipurpose utilization, SOA (Tianjin),
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Tianjin 300192, China
*To whom correspondence should be addressed. Phone: +86-22-8789-7017. Fax:
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86-22-8789-8169. E-mail: [email protected] (Q. Li).
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Abstract Semi-aromatic polyamide (SAP)/spherical mesoporous silica nanocomposite reverse osmosis (RO) membrane was successfully fabricated using m-phenylene diamine solution
and
cyclohexane-1,3,5-tricarbonyl
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aqueous
chloride/mesoporous-silica-sphere (MSS) organic solution as main raw materials. The
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experimental suggests that the microstructures and surface features are significantly
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different from those of the contrast samples (the full- and semi-aromatic polyamide membranes), including the surface morphology, polymer framework structure, surface
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charge density, hydrophilicity, and the thickness of barrier layer. It was observed that
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many MSSs with ca. 1.5 nm of pore size are evenly embedded on the surface of the fabricated SAP/MSS RO membrane. Furthermore, the separation performance testing
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results indicate that the permeabilities range from 62.53 to 72.73 L/m2h with the increase of the introduced MSSs from 0.02 to 0.08 w/v % under 1.5 MPa operating
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pressure and 2000 mg/L NaCl solution, which is obviously better than the contrast samples. Simultaneously, their salt rejections can be still maintained at a comparable
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level (94.78%–91.46%). The excellent separation performance of the nanocomposite RO membrane is closely related to the higher-freedom-degree semi-aromatic framework, the incorporation of MSSs, the improved surface hydrophilicity, the thinner barrier layer, and the enhanced surface negative charge density. Keywords Semi-aromatic polyamide; mesoporous silica; permeability; separation performance; nanocomposite membrane 2
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1. Introduction As a fascinating strategy for getting fresh water, membrane-based desalination technology has been extensively employed in many different districts and countries
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lacking drinking water, due to the relatively low energy consumption, low cost, and fine operability [1–3]. Reverse osmosis (RO) membrane, which plays the key role in
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the desalination process, has been attracting broad attentions in all over the world.
separation performance of RO membranes [4–6].
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Recent years, many material scientists and researchers pay great efforts to improve the
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Up to now, RO membrane is usually manufactured by interfacial polymerization
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(IP) approach using trimesoyl chloride (TMC) and m-phenylene diamine (MPDA) as raw materials. This kind of full-aromatic polyamide (FAP) composite RO membranes
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has great deal of advantages including its straightforward pore forming mechanism, flexibility, and relatively low cost compared to inorganic membrane, and so on [7].
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However, the trade-off relationship between permeability and selectivity (also called Robeson upper boundary in membrane gas separation) restrict the further
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improvement of separation performance of pure aromatic polyamide RO membrane. Therefore, how to simultaneously elevate the permeability and selectivity has been becoming an attractive hot spot of the membrane field. Recently, introducing various nanomaterials into the barrier layer takes great promise for solving this problem [8–10]. For example, Deng and coworkers [11] synthesized the aromatic polyamide/porous-silica-nanoparticle nanocomposite membrane by IP method, and the results demonstrate that the permeate flux dramatically increased from 28.5 to 3
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46.6 L/m2h with the incorporation of the silica nanoparticles, and the salt rejection can be stably maintained at a relatively high level. Nevertheless, it has been still unclear that the detailed influential mechanism of nanomaterial introduction on RO membrane
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separation performance [11,12]. Some researchers proposed that the incorporation of nanomaterials (silica, zeolite, carbon nanotube, etc) [13–16] can increase the surface
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hydrophilicity of polyamide RO membrane, optimize the polymer cross-linking
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degree, change the surface charge distribution, and even form some drawbacks on interface between the incorporated nanomaterials and polymer membrane. These
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factors are all favorable for the increase of permeability of aromatic polyamide RO
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membrane.
In comparison with other traditional nanomaterial additives employed in RO
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membrane fabrication, mesoporous silica spheres (MSSs) synthesized using cetyltrimethyl ammonium bromide (CTAB) as soft template not only have fine
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hydrophilicity, but also controllable spherical morphology (90-750 nm), and abundant mesopore channels (1.5–3.6 nm) [17,18]. As a result, the mesoporous silica
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incorporated RO membranes usually possess higher permeate flux and comparable selectivity compared with other nanocomposite RO membranes [11]. However, there are only few reports [11,19] involved with the fabrication of polyamide RO membranes containing mesoporous silica. On the other hand, it was found that semi-aromatic polyamide (SAP) membrane possesses excellent separation performance in comparison with FAP membranes [20,21]. Arthur reported [20] that this membrane exhibited fine permeate flux (51 4
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gallon/ft2-day) and comparable salt rejection (97.2%) under 420 psig and 0.2 % NaCl solution, and its permeability is obviously higher than that (30 gallon/ft2-day) of the FAP membrane tested under the same condition [20]. The excellent separation
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performance of the SAP RO membrane can be attributed to the flexible polymer framework, relatively lower framework cross-linking degree, higher negative-charge
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density, and thinner barrier layer deriving from the quick reaction between CTC and
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MPDA. However, many reports [13–16,22–25] currently mainly focus on the fabrication of FAP/nanomaterial RO membranes, and we have not found any reports
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involving with incorporating nanomaterials into the barrier layer of SAP RO
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membrane until now. Based on the physicochemical properties and microstructures of mesoporous silica spheres and SAP RO membrane, it can be deduced that introducing
ed
MSSs into the barrier layer of SAP RO membrane might combine the advantages of both materials and further improve the separation performance.
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Herein, semi-aromatic polyamide/spherical mesoporous silica RO membrane was firstly successfully fabricated by IP strategy using MPDA aqueous solution and
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cyclohexane-1,3,5-tricarbonyl chloride (CTC)/MSS organic solution as main raw materials. The permeate flux can reach up to ca. 72.73 L/m2h (0.08w/v% mesoporous silica loading), and the salt rejection can be maintained at above 93.57% under 2000 mg/L NaCl solution and 1.5 MPa testing condition. Its permeability is significantly better than those of the reported FAP/MSS (46.6 L/m2h) [11] and pristine semi-aromatic polyamide (46.30 L/m2h) membranes. This fabricated novel RO membrane should have extensive application prospect on brackish water desalination. 5
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2. Experimental 2.1. Materials and agents Cetyltrimethyl ammonium bromide (CTAB, 95%), m-phenylene diamine (MPDA,
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99%), camphor-10-sulfonic acid (CSA, 98%), and Triethylamine (TEA, 99.5%) were all purchased from Aldrich Chemical Inc.. Tetraethyl orthosilicate (TEOS, A.R.) was
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obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium chloride (99.5%) and
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sodium hydroxide (98%) were bought from Guangfu Chemical Co., Ltd. Isopar G as organic solvent was purchased from ExxonMobil Inc.. Thionyl chloride (99%) was
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obtained from Aladdin Industrial Corporation. Cyclohexane-1,3,5-tricarboxylic acid
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(98%) was purchased from Tokyo Industrial Chemical Co., Ltd. All the above chemicals were used as received without any further purification. Polysulfone (PS)
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ultrafiltration membrane coupons were obtained from Pureach Co., Ltd. Millipore water was used in all experiments.
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2.2. Synthesis of cyclohexane-1,3,5-tricarbonyl chloride Cyclohexane-1,3,5-tricarbonyl
chloride
(CTC)
as
the
raw
material
of
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RO-membrane fabrication was synthesized according to the reported method [26]. Typically, 5 g of cyclohexane-1,3,5-tricarboxylic acid was added into a round-bottom flask containing excess thionyl chloride (15 g) under magnetic stirring at room temperature. Then, the mixture solution was gradually heated to 88 ºC for the occurrence of the reaction under reflux, and this reaction was lasted for 24 h for total finishing. In addition, the whole reaction was conducted under a sealed nitrogen environment for avoiding the product (CTC) hydrolysis. After this, the final product 6
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(CTC) was obtained by rotating evaporation under vacuum to remove the residual thionyl chloride. 2.3. Synthesis of mesoporous silica spheres
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Mesoporous silica spheres (MSSs) as additive was synthesized based on a reported approach [17]. Typically, 0.7 mL of 2 M NaOH aqueous solution was added into 96
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mL of deionized water under mechanical stirring at room temperature. After this, 0.2
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g of CTAB was introduced into the above mixture solution. Then the solution was heated to 80 ºC, and 1 mL of TEOS was dropwise added into the obtained solution.
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After 2 h of stirring, a white slurry mixture formed was centrifuged and washed with
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deionized water, until the pH of washing solution became neutral. The final product was obtained by drying at 60 ºC for 12 h and calcined at 550 ºC in air for 4 h with
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ramping rate of 1 ºC/min to remove the CTAB template. 2.4. Fabrication of semi-aromatic polyamide/ mesoporous-silica-sphereRO membrane
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The semi-aromatic polyamide (SAP)/MSS RO membrane was fabricated by IP approach. Typically, 2.0 g of MPDA, 2.3 g of CSA, and 1.1 g of TEA were
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respectively added into 100 mL of deionized water under magnetic stirring at room temperature to form a mixture aqueous solution. Then, the above solution (aqueous phase) was poured on a PS ultrafiltration membrane at room temperature. After keeping for 2 min, the residual solution was removed from the PS membrane. Then, the organic-phase solution formed by dispersing certain amount of MSSs and 0.2 g of CTC in 100 ml of Isopar G solvent via ultrasonication, was poured on the above wetted PS membrane for the occurrence of IP reaction. After the reaction lasted for 60 7
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s, the residual organic phase solution was removed. Finally, the fabricated fresh SAP/MSS RO membrane was aged in oven at 80 ºC for 5 min for further cross linking, and stored in deionized water for the following performance evaluation. Additionally,
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for clearly indicating the different MSS incorporating amount, the fabricated samples are named as “SAP/MSS-x”. For example, SAP/MSS-2 indicates that 0.02w/v%
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MSSs are introduced in the fabrication process.
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As control experiments, pristine SAP and FAP RO membranes were also respectively fabricated. The fabrication process of SAP RO membrane was same with
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that of SAP/MSS RO membrane, and just no MSS was introduced into the
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organic-phase solution. Furthermore, the FAP RO membrane was also synthesized using the same method mentioned as above, and just the organic-phase solution was
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changed into Isopar G solution containing 0.2 w/v% TMC. 2.5. Surface feature and microstructural characterizations
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Nitrogen sorption isotherms were measured with a Micromeritics Tristar II (USA) analyzer at −196 ºC for measuring the pore sizes and specific surface areas of MSSs.
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Before measurement, the sample was degassed at 180 ºC in vacuum for more than 6 h. Its specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. The pore diameter (D) was determined from the adsorption branch, according to Barrett–Joyner–Halenda (BJH) method. The total pore volume (Vt) was estimated from the amount adsorbed at the relative pressure (p/p0) of 0.99. Transmission electron microscopy (TEM) image was recorded on a FEI G2F20 microscope (USA) operated at 200 kV for observing the MSS mesostructures. Before TEM 8
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characterization, the MSSs were dispersed in ethanol. The suspension of the sample was dropped on a holey copper grid. The IR spectra of the fabricated RO membranes were measured using VERTEX 70 IR spectrometer (Bruker, Germany) with
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wavenumbers ranging from 400 to 4000 cm-1 under the attenuated total reflectance infrared (ATR) mode for analyzing their surface functional groups. The surface charge
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properties of the fabricated RO membranes were measured with a solid surface zeta
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potential instrument (SurPASS, Anton Paar) in 0.001 mol/L of KCl aqueous solution with pH value adjusted in the range of 3–11. The surface morphologies of all the
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materials were characterized using field-emission scanning electronic microscopy
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(FE-SEM) S-4800 (Hitachi, Japan) operated at 5 kV. Moreover, for observing the microstructures of the cross section of the active barrier layers, the RO membranes
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were respectively pretreated by the following method: the nonwoven-fabric layer was carefully peeled off by a scalpel from a RO membrane. The residual part was pasted
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on a piece of glass and the polysulfone supporting layer was on the top. Then, it was washed with chloroform for completely removing the polysulfone layer and the
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isolated barrier layer located on the glass sheet was dried in vacuum oven to remove the residual chloroform. Before morphology measurements, all the SEM samples were coated with a layer of gold nanoparticles by using an ion spraying instrument. The contact angles were measured on a contact angle instrument (POWEREACH, China) for evaluating their hydrophilicity. 2.6. Separation performance tests The separation performance tests were conducted with a 2000 mg/L of NaCl 9
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solution under different operating pressures (0.9–1.9 MPa) at ca. 25 ºC by using a cross-flow type apparatus (Scheme 1). Both permeate and retentate were recycled back to the feed tank during the tests. The permeate water can be taken at the tip of
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the permeate-water pipe. Rectangular membrane samples were placed in the test apparatus with the active skin layer facing the feed water. The area of each membrane
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was ca. 36 cm2. The salt rejection and permeate flux of each membrane were
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respectively tested for two times, and the average value was considered as the final result. The permeating volumes collected for 30 min were used to describe flux in
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terms of liter per square meter per hour. A standardized conductivity meter was used
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to measure the salt (NaCl) concentrations in the feed and product water for determining membrane selectivity as given below:
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Cp salt rejection % = 1- 100% Cf
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in which, Cf and Cp represent feed and permeating concentrations, respectively. 3. Results and discussion
3.1. Structural analysis of mesoporous silica spheres
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The nitrogen sorption measurement (Fig. 1) suggests that the synthesized MSSs exhibit typical type IV isotherm, and it is similar with the pore-channel structure of MCM-41-type material. The hysteresis loop in high relative pressure (0.93–0.99 p/p0) derives from the aggregation of MSSs. The total pore volume recorded at 0.99 p/p0 and specific surface area are 0.92 cm3/g and 1030 m2/g, respectively. In addition, the PSD curve (Fig. 1 inset) shows this material possesses uniform small pore size centered at ca. 1.5 nm. Due to the determination limitation of the measurement 10
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instrument, only a half of the PSD curve can be observed (Fig. 1 inset). The steep adsorption curve in relatively low p/p0 range (0.01–0.37) further shows that this material has abundant micropores and small mesopores.
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The FE-SEM image (Fig. 2a) shows that the synthesized mesoporous silica has regular spherical morphology, and the diameter of the silica spheres is concentrated
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on ca. 115 nm. Moreover, the porous structure and pore channels can be clearly
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observed from the TEM images (Fig. 2b and c). Under high magnification (Fig. 2c), it can be found that the mesopore channels exhibit typical radiate structure from the
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center of the silica sphere, and the pore size is ca. 1.4 nm consistent with the nitrogen
3.2. ATR-IR analysis of RO membrane
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sorption result.
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The typical absorption peaks of amide group of all the fabricated RO membranes can be clearly observed from the ATR-IR spectra (Fig. 3). The peak at 1667 cm-1 is
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assigned to the C=O vibration stretching of amide (amide I). The stretching and in-plane bending vibration (amide II) of N-H bond in amide group can be respectively
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observed at 1610 and 1540 cm-1. Furthermore, the small peak at 1450 cm-1 is attributed to the C=O stretching and O-H stretching vibration of carboxylic group which derives from the hydrolysis of unreacted acyl-chloride groups. On the other hand, it can be found that a small peak at 2971 cm-1 assigned to the =C-H stretching of benzene ring appeared on the spectrum (Fig. 3a) of full aromatic polyamide (FAP) membrane. However, for the fabricated semi-aromatic polyamide (SAP) based RO membranes (Fig. 3b-f), this peak becomes broad and shifts to ca. 2928 cm-1, due to 11
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the absorption-peak part overlap of =C–H of phenyl group and –CH2– of cyclohexyl group. This result further suggests the formation of SAP membrane in some degree. Furthermore, owing to the low MSS incorporation amount (0.02 w/v%-0.08 w/v%),
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the related silica absorption peaks nearly cannot be identified (Fig. 3c-f). For SAP/MSS-6 and SAP/MSS-8 membranes with relatively high incroporation amount
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(Fig. 3e-f), the asymmetric stretching of ≡Si-O-Si≡ was detected at ca.1046 cm-1 [27],
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and the weak peak at 956 cm-1 deriving from the stretching vibration of ≡Si-OH group also can be found. These results illustrate that MSSs were successfully incorporated
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3.3. Zeta potential analysis of RO membrane
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into the barrier layer of SAP/MSS nanocomposite membrane.
Zeta potential analysis results (Fig. 4) show that all the fabricated RO membranes
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possess negative potential values in the pH range of ca. 2-11. With the increase of pH value, the zeta potentials of RO membrane surfaces show nearly same varying trend
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and all potentials gradually decrease, suggesting the increase of surface negative charge density. In the case of FAP membrane (Fig. 4), its zeta potential decreases from
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-1.16 to -28.2 mV, with pH varying from 2.2 to 11.8. Compared with FAP membrane, SAP membrane with semi-aromatic-polymer skeleton has lower potential value in the whole employed pH range. It indicates that the surface charge density of SAP membrane is higher than that of the FAP membrane. Additionally, it was observed that after MSSs were incorporated into the barrier layer of SAP membrane, the zeta potential of the obtained SAP/MSS membranes were further decreased in comparison with pristine FAP and SAP membranes. For example, the zeta potential of 12
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SAP/MSS-2 membrane decreases from -2.30 mV to -32.4 mV with the pH variation in the range of 2.39-11.5. Moreover, it was found that SAP/MSS-8 membrane with higher silica content has lower potential value in contrast with SAP/MSS-2
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membrane. The surface negative charge of FAP membrane is mainly due to the presence of
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pendant carboxylic groups deriving from the hydrolysis of unreacted acyl-chloride
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groups. The higher negative charge density of SAP membrane illustrate that more carboxylic groups were formed on the membrane surface in contrast with the FAP
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membrane, and this result is also supported by the previous report [21]. The formation
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of abundant carboxylic groups for SAP membrane may be related to the conformational mobility of CTC monomer and faster polymerization reaction process.
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Furthermore, the incorporation of MSS can generally influence the IP reaction and decrease the cross-linking degree of the obtained polyamide barrier layer [28,29]. The
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decrease of cross-linking degree is proportional to the incorporation amount. Correspondingly, the amount of pendant carboxylic groups of the MSS incorporated
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SAP/MSS series membranes was further increased. As a result, these membranes exhibit the higher negative charge densities and lower zeta potentials (Fig. 4) compared with the fabricated FAP and SAP membranes. SAP/MSS-8 membrane possesses lower zeta potential than SAP/MSS-2 membrane. 3.4. FE-SEM analysis of RO membrane The top surface FE-SEM images (Fig. 5) show that all the fabricated RO membranes have typical “leaf-like” structures. The FAP and SAP membranes (Fig. 5a 13
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and b) have similar surface morphologies, and the SAP membrane seems having larger surface roughness than FAP membrane. Except the “leaf-like” structures, some spherical bumps also can be observed from the surfaces of both membranes.
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Additionally, it can be found that many MSSs are uniformly embedded on the surface of SAP/MSS-2 membrane (Fig. 5c), after 0.02 w/v% of silica-sphere concentration
cr
was introduced in the fabrication process. Simultaneously, its “leaf-like” structure
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becomes larger than that of the pristine SAP membrane (Fig. 5b). With the increase of the introduced concentration, the embedded mesoporous silica spheres also
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correspondingly gradually increases from SAP/MSS-2 to SAP/MSS-8 membranes
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(Fig. 5c-f). Moreover, owing to the higher introducing concentrations, some agglomerations of MSSs can be found on the surfaces of SAP/MSS-6 and
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SAP/MSS-8 membranes.
On the other hand, from the cross-section SEM images (Fig. 6), it can be observed
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that the barrier-layer thicknesses of the fabricated RO membranes are all very uniform. For SAP membrane (Fig. 6a), the thickness (marked with white arrow) is ca. 163 nm.
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With the increase of the introduced silica concentration from 0.02 w/v% to 0.06 w/v%, the thicknesses gradually decrease from 142 nm (SAP/MSS-2, Fig. 6b) to 121 nm (SAP/MSS-6, Fig. 6d). Moreover, it can be clearly observed that some MSSs (marked with black arrows) are embedded on the barrier layers for the SAP/MSS series membranes (Fig. 6c and d). The greater conformational mobility of CTC monomers increases its reaction rate with MPDA monomers [21], and the rapid reaction can quickly promote the 14
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cross-linking degree of the formed polyamide barrier layer of the semi-aromatic based membrane in a relatively short time. Therefore, it is blocked that the diffusion of MPDA molecules from the aqueous phase into the oil phase containing CTC. The IP
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reaction was correspondingly terminated. As a result, the obtained barrier layer is very thin and its thickness cannot be further increased. Furthermore, due to the high
cr
specific surface area of MSSs, some MPDA molecules are physically adsorbed on the
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silica pore wall. With the increase of the incorporated MSSs, more MPDA molecules can be adsorbed. Correspondingly, the barrier layer becomes thinner due to less
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MPDA monomers reacting with CTC monomers. On the other hand, because that
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diameters of MSSs (ca.115 nm) are comparable with the barrier-layer thickness (ca. 121-142 nm), many spheres are embedded on the barrier-layer surface.
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3.5. Contact angle analysis of RO membranes
The contact angle analysis (Table 1) shows that SAP membrane has lower contact
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angle (49.5°) compared with FAP membrane (53.3°), suggesting the hydrophilicity of the former is better than that of the latter. After mesoporous silica spheres were
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incorporated, the contact angle of SAP/MSS-2 membrane further decreases to 45.4°. It indicates that the incorporation of mesoporous silica spheres is favorable for the improvement of hydrophilicity. With the increase of the MSS amount, the contact angle of the fabricated nanocomposite RO membrane decreases from 45.4° to 35.6° (Table 1), illustrating that the hydrophilicity is gradually enhanced. Based on zeta potential results mentioned above, in contrast with the FAP membrane, the barrier-layer surface of SAP membrane has more hydrophilic 15
Page 17 of 37
carboxylic groups. As a result, this membrane possesses better hydrophilicity than FAP membrane. After MSSs are introduced into the barrier layer, the cross-linking degree of the obtained SAP/MSS nanocomposite RO membrane is decreased and
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more carboxylic groups are formed. Simultaneously, the surface roughness is also increased. These factors are all advantageous on the hydophilicity improvement.
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Therefore, SAP/MSS nanocomposite RO membrane has better hydrophilicity
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compared with the pristine SAP membrane. 3.6. Separation performance
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The testing results of separation performance (Table 2) show that the permeability
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(46.30 L/m2h) of SAP membrane is obviously better than that (25.62 L/m2h) of the FAP membrane under 1.5 MPa operation pressure and 2000 ppm of NaCl solution.
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The rejection performance (97.54 %) of the former is slightly lower in comparison with the latter (98.33 %) under the same testing condition. However, after the
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introduction of MSS nanomaterial, the permeate flux of the fabricated SAP/MSS-2 nanocomposite membrane is further increased into 62.53 L/m2h, and its salt rejection
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slightly deceased by 3.55% compared with SAP membrane (Table 2) under the same testing condition mentioned above. Moreover, it was observed that the permeate flux of the MSS incorporated RO membranes gradually increased from 62.53 to 72.73 L/m2h. Simultaneously, their salt rejections decrease from 96.82 % to 93.57 %. Furthermore, the variation trends of the separation performances of all the fabricated SAP and SAP/MSS membranes (Fig. 7) were also detailedly investigated by changing the operation pressure from 0.9 to 1.9 MPa. It suggests that the permeate 16
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fluxes (Fig.7a) of these membranes dramatically increase with the operation pressure. Additional, it can be found that SAP membrane still exhibits the lowest permeate flux in the whole of the testing pressure range (0.9–1.9 MPa). After MSS was incorporated
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into the barrier layer, the permeate fluxes of the obtained nanocomposite membranes are obviously improved, and it exhibits gradually increase trend from SAP/MSS-2 to
cr
SAP/MSS-8 RO membranes. These results are consistent with the results (Table 2)
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mentioned above. The permeate flux of SAP/MSS-8 membrane steeply increases from 43.57 to 92.70 L/m2h with the change of operating pressure from 0.9 to 1.9 MPa.
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On the other hand, it was observed that the salt rejections of these RO membranes
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(Fig.7b) are also gradually enhanced under the same testing pressure range (0.9-1.9 MPa). The SAP membrane possesses the highest salt rejection compared with the
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other fabricated RO membranes. Its salt rejection increases from 92.05 to 98.47 % with the operation pressure varying from 0.9 to 1.9 MPa. However, from the
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salt-rejection change curves (Fig.7b), it was found that the salt rejection gradually decreases with the addition of the MSS nanomaterial from 0 to 0.08%. Thereinto,
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SAP/MSS-8 membrane exhibits the lowest salt rejection. 3.7. Desalting mechanism analysis Based on the static repulsion and solution-diffusion mechanisms [11] which are
widely used for explaining the separation processes of pressure-drive desalting membranes, it can be deduced that the surface charge property, surface hydrophilicity, thickness and microstructure of polyamide barrier layer should be closely related to the separation performance. The FAP membrane fabricated with TMC and MPDA 17
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monomers has rigid polyamide framework, and the framework possesses lower freedom degree, which has great contribution to the high salt rejection. However, the low freedom degree of the framework and relatively poor hydrophilicity (Table 1) are
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unfavorable for the transportation of water molecules. As a result, the FAP membrane possesses poorer permeability than the other fabricated RO membranes. In
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comparison with FAP membrane, the SAP membrane made from MPDA and CTC
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with greater conformational mobility has high-freedom-degree polyamide framework, thinner barrier layer (Fig. 6) and better hydropilicity (Table 1). These factors enable
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water molecules easily go through the barrier layer, and the permeate flux is
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dramatically enhanced. Simultaneously, the semi-aromatic framework with high freedom degree and thin barrier layer also make salt ions more easily go through the
ed
membrane, although this membrane has higher surface negative charge density (Fig. 4). Therefore, the salt rejection of SAP membrane was slightly decreased compared to
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the FAP membrane.
On the other hand, MSSs have abundant uniform mesopore channels (ca. 1.5 nm)
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and fine hydrophilicity deriving from the presence of great deal of silanol groups. It can be easily concluded that introducing MSS into the barrier layer of RO membrane can favor improving water permeability. This viewpoint is also consistent with some other reports [11,19]. The embedding of MSS nanomaterials into FAP/MSS membranes provides abundant water-flow paths, and also shortens the diffusion path of water molecules in barrier layer. Moreover, owing to the disturbance of MSS in the IP reaction process, the cross-linking degree of barrier layer becomes lower in 18
Page 20 of 37
contrast with pristine SAP membrane and more hydrophilic uncross-linked carboxylic groups are formed. Therefore, the permeate fluxes of the fabricated FAP/MSS membranes are further enhanced compared with pristine FAP membranes, and their
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salt rejections are correspondingly slightly decreased. Furthermore, many salt ions can be adsorbed on the pore wall of MSS in the desalting process, which gives rise to the
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formation of double electric layers in the mesopores. The electric double layers can
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restrict the entry and motion of metal ions and have no effect on the transportation of neutral water molecules [19,28–30]. Furthermore, the increase of surface negative
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charge density (Fig. 4) of FAP/MSS membranes can strengthen the repulsion
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interaction with salt ions. As a result, the incorporation of MSS do not lead to sharp decrease of selectivity, accompanying with substantial enhancement of permeate flux.
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Moreover, with the increase of incorporated MSS nanomaterials, there are more MSS embedded on the surface of barrier layer. The cross linking degree also become more
ce pt
low and the hydrophilicity of the membrane surface can be further improved. Therefore, the permeate fluxes gradually increase from FAP/MSS-2 to FAP/MSS-8
Ac
membranes, and the salt rejections correspondingly become gradually lower in the same sequence.
4. Conclusions
Semi-aromatic membranes,
polyamide/spherical
fabricated
using
mesoporous
m-phenylene
silica
diamine
nanocomposite
aqueous
solution
RO and
cyclohexane-1, 3, 5-tricarbonyl chloride/mesoporous-silica-sphere organic solution as main raw materials, have excellent permeability (up to 72.73 L/m2h) and comparable 19
Page 21 of 37
salt rejections (93.57–98.33%) in 1.5 MPa of operating pressure and 2000 mg/L of NaCl solution. The separation performances are significantly better than those of traditional full-aromatic polyamide and pristine semi-aromatic polyamide membranes.
ip t
With the change of the incorporated mesoporous silica spheres in the range of 0.02–0.08 w/v%, the permeate fluxes are gradually enhanced from 62.53 to 72.73
cr
L/m2h accompanied with the slight decrease of salt rejection. Additionally, the
us
experimental results suggest that the enhanced operating pressure enables the separation performances of nanocomposite membranes to be dramatically improved.
an
Compared with the contrast samples (SAP and FAP membranes), the enhancement of
M
permeability can be attributed to the high freedom degree of semi-aromatic-polyamide framework, the better hydrophilicity, thinner barrier layer, and the embedding of
ed
mesoporous silica spheres. Therein, the introduction of mesoporous silica spheres provides abundant water-flow paths, and also shortens the diffusion path of water
ce pt
molecules in barrier layer. Furthermore, the increase of surface negative charge density and the formation of electrical double layers in the mesopores make the
Ac
selectivity of the fabricated RO membrane have no significant decay. Acknowledgements The authors thank the financial support of the National Natural Science Foundation of China (NSFC) (No. 21406041), and Basic Scientific Research Founds for the State-level Scientific Research Institute (No. 2013-T06). References [1] L. Zhang, L. Xie, H.-L. Chen, C.-J. Gao, Progress and prospects of seawater 20
Page 22 of 37
desalination in China, Desalination 182 (2005) 13–18. [2] L. Malaeb, G.M. Ayoub, Reverse osmosis technology for water treatment: State of the art review, Desalination 267 (2011) 1–8.
ip t
[3] A.D. Khawaji, I.K. Kutubkhanah, J.-M. Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47–69.
cr
[4] W.J. Lau, A.F. Ismail, N. Misdan, M.A. Kassim, A recent progress in thin film
us
composite membrane: A review, Desalination 287 (2012) 190–199.
[5] T. Humplik, J. Lee, S. CO’Hern, B.A. Fellman, M.A. Baig, S.F. Hassan, M.A.
an
Atieh, F. Rahman, T. Laoui, R. Karnik, E.N. Wang, Nanotechnology 22(2011) 1–19.
Nano Lett. 12 (2012) 3602–3608.
M
[6] D. Cohen-Tanugi, J.C. Grossman, Water desalination across nanoporous graphene,
ed
[7] L.Y. Ng, A.W. Mohammad, C.P. Leo, N. Hilal, Polymeric membranes incorporated with metal/metal oxide nanoparticles: a comprehensive review, Desalination 308
ce pt
(2013) 15–33.
[8] M. Fathizadeh, A. Aroujalian, A. Raisi, Effect of added NaX nano-zeolite into
Ac
polyamide as a top thin layer of membrane on water flux and salt rejection in a reverse osmosis process, J. Membr. Sci. 375 (2011) 88–95. [9] H. Zhao, S. Qiu, L. Wu, L. Zhang, H. Chen, C. Gao, Improving the performance of polyamide reverse osmosis membrane by incorporation of modified multi-walled carbon nanotubes, J. Membr. Sci. 450 (2014) 249–256. [10] E.-S. Kim, B. Deng, Fabrication of polyamide thin-film nano-composite (PA-TFN) membrane with hydrophilized ordered mesoporous carbon (H-OMC) for 21
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water purifications, J. Membr. Sci. 375 (2011) 46–54. [11] J. Yin, E.-S. Kim, J. Yang, B. Deng, Fabrication of a novel thin-film nanocomposite (TFN) membrane containing MCM-41 silica nanoparticles (NPs) for
ip t
water purification, J. Membr. Sci. 423–424 (2012) 238–246. [12] J. Yin, B. Deng, Polymer-matrix nanocomposite membranes for water treatment,
cr
J. Membr. Sci. 479 (2015) 256–275.
us
[13] G.N.B. Baroña, J. Lim, M. Choi, B. Jung, Interfacial polymerization of polyamide-aluminosilicate SWNT nanocomposite membranes for reverse osmosis,
an
Desalination 325 (2013) 138–147.
M
[14] B. Rajaeian, A. Rahimpour, M.O. Tade, S. Liu, Fabrication and characterization of polyamide thin film nanocomposite (TFN) nanofiltration membrane impregnated
ed
with TiO2 nanoparticles, Desalination 313 (2013) 176–188. [15] E.-S. Kim, G. Hwang, M.G. El-Din, Y. Liu, Development of nanosilver and
ce pt
multi-walled carbon nanotubes thin-film nanocomposite membrane for enhanced water treatment, J. Membr. Sci. 394–395 (2012) 37–48.
Ac
[16] M. Ben-Sasson, K.R. Zodrow, Q. Genggeng, Y. Kang, E.P. Giannelis, M. Elimelech, Surface functionalization of thin-film composite membranes with copper nanoparticles for antimicrobial surface properties, Environ. Sci. Technol. 48 (2014) 384–393. [17] Q. Cai, Z.S. Luo, W.Q. Pang, Y.W. Fan, X.H. Chen, F.Z. Cui, Dilute solution routes to various controllable morphologies of MCM-41silica with a basic medium, Chem. Mater. 13 (2001) 258–263. 22
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[18] M. Mizutani, Y. Yamada, K. Yano, Pore-expansion of monodisperse mesoporous silica spheres by a novel surfactant exchange method, Chem. Commun. (2007) 1172–1174.
ip t
[19] M. Bao, G. Zhu, L. Wang, M. Wang, C. Gao, Preparation of monodispersed spherical mesoporous nanosilica–polyamide thin film composite reverse osmosis
cr
membranes via interfacial polymerization, Desalination 309 (2013) 261–266.
us
[20] S.D. Arthur, Structure-property relationship in a thin film composite reverse osmosis membrane, J. Membr. Sci. 46 (1989) 243–260.
an
[21] S. Yu, M. Liu, Z. Lü, Y. Zhou, C. Gao, Aromatic-cycloaliphatic polyamide
M
thin-film composite membrane with improved chlorine resistance prepared from m-phenylenediamine-4-methyl and cyclohexane-1,3,5-tricarbonyl chloride, J. Membr.
ed
Sci. 344 (2009) 155–164.
[22] M.M. Pendergast, A.K. Ghosh, E.M.V. Hoek, Separation performance and
ce pt
interfacial properties of nanocomposite reverse osmosis membranes, Desalination 308 (2013) 180–185.
Ac
[23] W.F. Chan, H.Y. Chen, A. Surapathi, M.G. Taylor, X. Shao, E. Marand J.K. Johnson, Zwitterion functionalized carbon nanotube/polyamide nanocomposite membranes for water desalination, ACS Nano 7 (2013) 5308–5319. [24] M.L. Lind, B.H. Jeong, A. Subramani, X. Huang, E.M.V.Hoek, Effect of mobile cation on zeolite-polyamide thin film nanocomposite membranes, J. Mater. Res. 24 (2009) 1624–1631. [25] D. Rana, Y. Kim, T. Matsuura, H.A. Arafat, Development of antifouling thin23
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film-composite membranes for seawater desalination, J. Membr. Sci. 367 (2011) 110–118. [26] H. Audorff, R. Walker, L. Kador, H.-W. Schmidt, Holographic investigations of
ip t
azobenzene-containing low-molecular-weight compounds in pure materials and binary blends with polystyrene, Chem. Eur. J. 17 (2011) 12722–12728.
cr
[27] G.L. Jadav, P.S. Singh, Synthesis of novel silica-polyamide nanocomposite
us
membrane with enhanced properties, J. Membr. Sci. 328 (2009) 257–267.
[28] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, first ed., John
an
Wiley, New York, 1984.
M
[29] L.X. Li, J.H. Dong, T.M. Nenoff, R. Lee, Desalination by reverse osmosis using MFI zeolite membranes, J. Membr. Sci. 243 (2004) 401–404.
ed
[30] L.X. Li, J.H. Dong, T.M. Nenoff, R. Lee, Reverse osmosis of ionic aqueous
Ac
ce pt
solutions on a MFI zeolite membrane, Desalination 170 (2004) 309–306.
24
Page 26 of 37
Figure captions Scheme 1. Schematic of RO-membrane performance testing apparatus. Fig. 1. (a) nitrogen sorption isotherm and (b) pore size distribution (PSD) of the
ip t
synthesized mesoporous silica spheres. Fig. 2. (a) SEM and (b and c) TEM images of the synthesized mesoporous silica
cr
spheres.
us
Fig. 3. ATR-IR spectra of the fabricated RO membranes. (a) full-aromatic polyamide (FAP) RO membrane; (b) pristine semi-aromatic polyamide (SAP) RO membrane; (c)
an
SAP/MSS-2 membrane; (d) SAP/MSS-4 membrane; (e) SAP/MSS-6 membrane; and
M
(f) SAP/MSS-8 membrane.
Fig. 4. Zeta potentials of the fabricated RO membranes with pH ranging from ca. 3 to
ed
11.
Fig. 5. Top-surface SEM images of the fabricated RO membranes. (a) full-aromatic
ce pt
polyamide (FAP) RO membrane; (b) pristine semi-aromatic polyamide (SAP) RO membrane; (c) SAP/MSS-2 membrane; (d) SAP/MSS-4 membrane; (e) SAP/MSS-6
Ac
membrane; and (f) SAP/MSS-8 membrane. Fig. 6. Cross-section SEM images of the barrier layers of the fabricated RO membranes. (a) pristine semi-aromatic polyamide (SAP) RO membrane; (b) SAP/MSS-2 membrane; (c) SAP/MSS-4 membrane; and (d) SAP/MSS-6 membrane. Fig. 7. (a) permeate fluxes and (b) salt rejections of the fabricated RO membranes with operating pressure ranging from 0.9 MPa to 1.9 MPa.
25
Page 27 of 37
ip t cr us an M Ac
ce pt
ed
Scheme 1.
26
Page 28 of 37
ip t cr us an
Ac
ce pt
ed
M
Fig. 1.
27
Page 29 of 37
ip t cr us an M ed ce pt Ac Fig. 2.
28
Page 30 of 37
ip t cr us an M ed Ac
ce pt
Fig. 3.
29
Page 31 of 37
ip t cr us an M Ac
ce pt
ed
Fig. 4.
30
Page 32 of 37
ip t cr us an M ed ce pt Ac
Fig. 5.
31
Page 33 of 37
ip t cr us an
Ac
ce pt
ed
M
Fig. 6.
32
Page 34 of 37
ip t cr us
Ac
ce pt
ed
M
an
Fig. 7.
33
Page 35 of 37
Table 1 Contact angles of the fabricated RO membranes. FAP
Contact angle (°)
53.3
SAP 49.5
SAP/MSS-2
SAP/MSS-4
45.4
SAP/MSS-6
43.4
SAP/MSS-8
38.5
35.6
Ac
ce pt
ed
M
an
us
cr
ip t
RO membrane
34
Page 36 of 37
Table 2 The separation performances of the fabricated RO membranes tested under ca. 2000 mg/L NaCl aqueous solution with 25 ºC, and 1.5 MPa operating pressure. FAP
SAP
SAP/MSS-2
SAP/MSS-6
SAP/MSS-8
Salt rejection (%)
98.33
97.54
96.82
95.50
93.93
93.57
Permeate flux (L/m2h)
25.62
46.30
62.53
65.09
68.91
72.73
Ac
ce pt
ed
M
an
us
cr
SAP/MSS-4
ip t
RO membrane
35
Page 37 of 37
S0169-4332(15)03022-6 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.028 APSUSC 32010
To appear in:
APSUSC
Received date: Accepted date:
29-10-2015 4-12-2015
Please cite this article as: Q. Li, H. Yu, F. Wu, J. Song, X. Pan, M. Zhang, Fabrication of semi-aromatic polyamide/spherical mesoporous silica nanocomposite reverse osmosis membrane with superior permeability, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Highlights
A semi-aromatic polyamide/spherical mesoporous silica membrane was fabricated. This membrane has better separation performance than other polyamide membranes.
ip t
The separation performance was comprehensively investigated.
Relationship between separation performance and membrane structure was
us
cr
Ac
ce pt
ed
M
an
proposed.
Page 1 of 37
*Graphical Abstract (for review)
Ac
ce pt
ed
M
an
us
cr
ip t
Graphical abstract:
Page 2 of 37
*Manuscript
Fabrication of semi-aromatic polyamide/spherical mesoporous silica nanocomposite reverse osmosis membrane with superior
cr
Qiang Li*, Hui Yu, Feiyang Wu, Jie Song, Xianhui Pan, Meng Zhang
ip t
permeability
us
The institute of seawater desalination and multipurpose utilization, SOA (Tianjin),
M
an
Tianjin 300192, China
*To whom correspondence should be addressed. Phone: +86-22-8789-7017. Fax:
Ac
ce pt
ed
86-22-8789-8169. E-mail: [email protected] (Q. Li).
1
Page 3 of 37
Abstract Semi-aromatic polyamide (SAP)/spherical mesoporous silica nanocomposite reverse osmosis (RO) membrane was successfully fabricated using m-phenylene diamine solution
and
cyclohexane-1,3,5-tricarbonyl
ip t
aqueous
chloride/mesoporous-silica-sphere (MSS) organic solution as main raw materials. The
cr
experimental suggests that the microstructures and surface features are significantly
us
different from those of the contrast samples (the full- and semi-aromatic polyamide membranes), including the surface morphology, polymer framework structure, surface
an
charge density, hydrophilicity, and the thickness of barrier layer. It was observed that
M
many MSSs with ca. 1.5 nm of pore size are evenly embedded on the surface of the fabricated SAP/MSS RO membrane. Furthermore, the separation performance testing
ed
results indicate that the permeabilities range from 62.53 to 72.73 L/m2h with the increase of the introduced MSSs from 0.02 to 0.08 w/v % under 1.5 MPa operating
ce pt
pressure and 2000 mg/L NaCl solution, which is obviously better than the contrast samples. Simultaneously, their salt rejections can be still maintained at a comparable
Ac
level (94.78%–91.46%). The excellent separation performance of the nanocomposite RO membrane is closely related to the higher-freedom-degree semi-aromatic framework, the incorporation of MSSs, the improved surface hydrophilicity, the thinner barrier layer, and the enhanced surface negative charge density. Keywords Semi-aromatic polyamide; mesoporous silica; permeability; separation performance; nanocomposite membrane 2
Page 4 of 37
1. Introduction As a fascinating strategy for getting fresh water, membrane-based desalination technology has been extensively employed in many different districts and countries
ip t
lacking drinking water, due to the relatively low energy consumption, low cost, and fine operability [1–3]. Reverse osmosis (RO) membrane, which plays the key role in
cr
the desalination process, has been attracting broad attentions in all over the world.
separation performance of RO membranes [4–6].
us
Recent years, many material scientists and researchers pay great efforts to improve the
an
Up to now, RO membrane is usually manufactured by interfacial polymerization
M
(IP) approach using trimesoyl chloride (TMC) and m-phenylene diamine (MPDA) as raw materials. This kind of full-aromatic polyamide (FAP) composite RO membranes
ed
has great deal of advantages including its straightforward pore forming mechanism, flexibility, and relatively low cost compared to inorganic membrane, and so on [7].
ce pt
However, the trade-off relationship between permeability and selectivity (also called Robeson upper boundary in membrane gas separation) restrict the further
Ac
improvement of separation performance of pure aromatic polyamide RO membrane. Therefore, how to simultaneously elevate the permeability and selectivity has been becoming an attractive hot spot of the membrane field. Recently, introducing various nanomaterials into the barrier layer takes great promise for solving this problem [8–10]. For example, Deng and coworkers [11] synthesized the aromatic polyamide/porous-silica-nanoparticle nanocomposite membrane by IP method, and the results demonstrate that the permeate flux dramatically increased from 28.5 to 3
Page 5 of 37
46.6 L/m2h with the incorporation of the silica nanoparticles, and the salt rejection can be stably maintained at a relatively high level. Nevertheless, it has been still unclear that the detailed influential mechanism of nanomaterial introduction on RO membrane
ip t
separation performance [11,12]. Some researchers proposed that the incorporation of nanomaterials (silica, zeolite, carbon nanotube, etc) [13–16] can increase the surface
cr
hydrophilicity of polyamide RO membrane, optimize the polymer cross-linking
us
degree, change the surface charge distribution, and even form some drawbacks on interface between the incorporated nanomaterials and polymer membrane. These
an
factors are all favorable for the increase of permeability of aromatic polyamide RO
M
membrane.
In comparison with other traditional nanomaterial additives employed in RO
ed
membrane fabrication, mesoporous silica spheres (MSSs) synthesized using cetyltrimethyl ammonium bromide (CTAB) as soft template not only have fine
ce pt
hydrophilicity, but also controllable spherical morphology (90-750 nm), and abundant mesopore channels (1.5–3.6 nm) [17,18]. As a result, the mesoporous silica
Ac
incorporated RO membranes usually possess higher permeate flux and comparable selectivity compared with other nanocomposite RO membranes [11]. However, there are only few reports [11,19] involved with the fabrication of polyamide RO membranes containing mesoporous silica. On the other hand, it was found that semi-aromatic polyamide (SAP) membrane possesses excellent separation performance in comparison with FAP membranes [20,21]. Arthur reported [20] that this membrane exhibited fine permeate flux (51 4
Page 6 of 37
gallon/ft2-day) and comparable salt rejection (97.2%) under 420 psig and 0.2 % NaCl solution, and its permeability is obviously higher than that (30 gallon/ft2-day) of the FAP membrane tested under the same condition [20]. The excellent separation
ip t
performance of the SAP RO membrane can be attributed to the flexible polymer framework, relatively lower framework cross-linking degree, higher negative-charge
cr
density, and thinner barrier layer deriving from the quick reaction between CTC and
us
MPDA. However, many reports [13–16,22–25] currently mainly focus on the fabrication of FAP/nanomaterial RO membranes, and we have not found any reports
an
involving with incorporating nanomaterials into the barrier layer of SAP RO
M
membrane until now. Based on the physicochemical properties and microstructures of mesoporous silica spheres and SAP RO membrane, it can be deduced that introducing
ed
MSSs into the barrier layer of SAP RO membrane might combine the advantages of both materials and further improve the separation performance.
ce pt
Herein, semi-aromatic polyamide/spherical mesoporous silica RO membrane was firstly successfully fabricated by IP strategy using MPDA aqueous solution and
Ac
cyclohexane-1,3,5-tricarbonyl chloride (CTC)/MSS organic solution as main raw materials. The permeate flux can reach up to ca. 72.73 L/m2h (0.08w/v% mesoporous silica loading), and the salt rejection can be maintained at above 93.57% under 2000 mg/L NaCl solution and 1.5 MPa testing condition. Its permeability is significantly better than those of the reported FAP/MSS (46.6 L/m2h) [11] and pristine semi-aromatic polyamide (46.30 L/m2h) membranes. This fabricated novel RO membrane should have extensive application prospect on brackish water desalination. 5
Page 7 of 37
2. Experimental 2.1. Materials and agents Cetyltrimethyl ammonium bromide (CTAB, 95%), m-phenylene diamine (MPDA,
ip t
99%), camphor-10-sulfonic acid (CSA, 98%), and Triethylamine (TEA, 99.5%) were all purchased from Aldrich Chemical Inc.. Tetraethyl orthosilicate (TEOS, A.R.) was
cr
obtained from Sinopharm Chemical Reagent Co., Ltd. Sodium chloride (99.5%) and
us
sodium hydroxide (98%) were bought from Guangfu Chemical Co., Ltd. Isopar G as organic solvent was purchased from ExxonMobil Inc.. Thionyl chloride (99%) was
an
obtained from Aladdin Industrial Corporation. Cyclohexane-1,3,5-tricarboxylic acid
M
(98%) was purchased from Tokyo Industrial Chemical Co., Ltd. All the above chemicals were used as received without any further purification. Polysulfone (PS)
ed
ultrafiltration membrane coupons were obtained from Pureach Co., Ltd. Millipore water was used in all experiments.
ce pt
2.2. Synthesis of cyclohexane-1,3,5-tricarbonyl chloride Cyclohexane-1,3,5-tricarbonyl
chloride
(CTC)
as
the
raw
material
of
Ac
RO-membrane fabrication was synthesized according to the reported method [26]. Typically, 5 g of cyclohexane-1,3,5-tricarboxylic acid was added into a round-bottom flask containing excess thionyl chloride (15 g) under magnetic stirring at room temperature. Then, the mixture solution was gradually heated to 88 ºC for the occurrence of the reaction under reflux, and this reaction was lasted for 24 h for total finishing. In addition, the whole reaction was conducted under a sealed nitrogen environment for avoiding the product (CTC) hydrolysis. After this, the final product 6
Page 8 of 37
(CTC) was obtained by rotating evaporation under vacuum to remove the residual thionyl chloride. 2.3. Synthesis of mesoporous silica spheres
ip t
Mesoporous silica spheres (MSSs) as additive was synthesized based on a reported approach [17]. Typically, 0.7 mL of 2 M NaOH aqueous solution was added into 96
cr
mL of deionized water under mechanical stirring at room temperature. After this, 0.2
us
g of CTAB was introduced into the above mixture solution. Then the solution was heated to 80 ºC, and 1 mL of TEOS was dropwise added into the obtained solution.
an
After 2 h of stirring, a white slurry mixture formed was centrifuged and washed with
M
deionized water, until the pH of washing solution became neutral. The final product was obtained by drying at 60 ºC for 12 h and calcined at 550 ºC in air for 4 h with
ed
ramping rate of 1 ºC/min to remove the CTAB template. 2.4. Fabrication of semi-aromatic polyamide/ mesoporous-silica-sphereRO membrane
ce pt
The semi-aromatic polyamide (SAP)/MSS RO membrane was fabricated by IP approach. Typically, 2.0 g of MPDA, 2.3 g of CSA, and 1.1 g of TEA were
Ac
respectively added into 100 mL of deionized water under magnetic stirring at room temperature to form a mixture aqueous solution. Then, the above solution (aqueous phase) was poured on a PS ultrafiltration membrane at room temperature. After keeping for 2 min, the residual solution was removed from the PS membrane. Then, the organic-phase solution formed by dispersing certain amount of MSSs and 0.2 g of CTC in 100 ml of Isopar G solvent via ultrasonication, was poured on the above wetted PS membrane for the occurrence of IP reaction. After the reaction lasted for 60 7
Page 9 of 37
s, the residual organic phase solution was removed. Finally, the fabricated fresh SAP/MSS RO membrane was aged in oven at 80 ºC for 5 min for further cross linking, and stored in deionized water for the following performance evaluation. Additionally,
ip t
for clearly indicating the different MSS incorporating amount, the fabricated samples are named as “SAP/MSS-x”. For example, SAP/MSS-2 indicates that 0.02w/v%
cr
MSSs are introduced in the fabrication process.
us
As control experiments, pristine SAP and FAP RO membranes were also respectively fabricated. The fabrication process of SAP RO membrane was same with
an
that of SAP/MSS RO membrane, and just no MSS was introduced into the
M
organic-phase solution. Furthermore, the FAP RO membrane was also synthesized using the same method mentioned as above, and just the organic-phase solution was
ed
changed into Isopar G solution containing 0.2 w/v% TMC. 2.5. Surface feature and microstructural characterizations
ce pt
Nitrogen sorption isotherms were measured with a Micromeritics Tristar II (USA) analyzer at −196 ºC for measuring the pore sizes and specific surface areas of MSSs.
Ac
Before measurement, the sample was degassed at 180 ºC in vacuum for more than 6 h. Its specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. The pore diameter (D) was determined from the adsorption branch, according to Barrett–Joyner–Halenda (BJH) method. The total pore volume (Vt) was estimated from the amount adsorbed at the relative pressure (p/p0) of 0.99. Transmission electron microscopy (TEM) image was recorded on a FEI G2F20 microscope (USA) operated at 200 kV for observing the MSS mesostructures. Before TEM 8
Page 10 of 37
characterization, the MSSs were dispersed in ethanol. The suspension of the sample was dropped on a holey copper grid. The IR spectra of the fabricated RO membranes were measured using VERTEX 70 IR spectrometer (Bruker, Germany) with
ip t
wavenumbers ranging from 400 to 4000 cm-1 under the attenuated total reflectance infrared (ATR) mode for analyzing their surface functional groups. The surface charge
cr
properties of the fabricated RO membranes were measured with a solid surface zeta
us
potential instrument (SurPASS, Anton Paar) in 0.001 mol/L of KCl aqueous solution with pH value adjusted in the range of 3–11. The surface morphologies of all the
an
materials were characterized using field-emission scanning electronic microscopy
M
(FE-SEM) S-4800 (Hitachi, Japan) operated at 5 kV. Moreover, for observing the microstructures of the cross section of the active barrier layers, the RO membranes
ed
were respectively pretreated by the following method: the nonwoven-fabric layer was carefully peeled off by a scalpel from a RO membrane. The residual part was pasted
ce pt
on a piece of glass and the polysulfone supporting layer was on the top. Then, it was washed with chloroform for completely removing the polysulfone layer and the
Ac
isolated barrier layer located on the glass sheet was dried in vacuum oven to remove the residual chloroform. Before morphology measurements, all the SEM samples were coated with a layer of gold nanoparticles by using an ion spraying instrument. The contact angles were measured on a contact angle instrument (POWEREACH, China) for evaluating their hydrophilicity. 2.6. Separation performance tests The separation performance tests were conducted with a 2000 mg/L of NaCl 9
Page 11 of 37
solution under different operating pressures (0.9–1.9 MPa) at ca. 25 ºC by using a cross-flow type apparatus (Scheme 1). Both permeate and retentate were recycled back to the feed tank during the tests. The permeate water can be taken at the tip of
ip t
the permeate-water pipe. Rectangular membrane samples were placed in the test apparatus with the active skin layer facing the feed water. The area of each membrane
cr
was ca. 36 cm2. The salt rejection and permeate flux of each membrane were
us
respectively tested for two times, and the average value was considered as the final result. The permeating volumes collected for 30 min were used to describe flux in
an
terms of liter per square meter per hour. A standardized conductivity meter was used
M
to measure the salt (NaCl) concentrations in the feed and product water for determining membrane selectivity as given below:
ed
Cp salt rejection % = 1- 100% Cf
ce pt
in which, Cf and Cp represent feed and permeating concentrations, respectively. 3. Results and discussion
3.1. Structural analysis of mesoporous silica spheres
Ac
The nitrogen sorption measurement (Fig. 1) suggests that the synthesized MSSs exhibit typical type IV isotherm, and it is similar with the pore-channel structure of MCM-41-type material. The hysteresis loop in high relative pressure (0.93–0.99 p/p0) derives from the aggregation of MSSs. The total pore volume recorded at 0.99 p/p0 and specific surface area are 0.92 cm3/g and 1030 m2/g, respectively. In addition, the PSD curve (Fig. 1 inset) shows this material possesses uniform small pore size centered at ca. 1.5 nm. Due to the determination limitation of the measurement 10
Page 12 of 37
instrument, only a half of the PSD curve can be observed (Fig. 1 inset). The steep adsorption curve in relatively low p/p0 range (0.01–0.37) further shows that this material has abundant micropores and small mesopores.
ip t
The FE-SEM image (Fig. 2a) shows that the synthesized mesoporous silica has regular spherical morphology, and the diameter of the silica spheres is concentrated
cr
on ca. 115 nm. Moreover, the porous structure and pore channels can be clearly
us
observed from the TEM images (Fig. 2b and c). Under high magnification (Fig. 2c), it can be found that the mesopore channels exhibit typical radiate structure from the
an
center of the silica sphere, and the pore size is ca. 1.4 nm consistent with the nitrogen
3.2. ATR-IR analysis of RO membrane
M
sorption result.
ed
The typical absorption peaks of amide group of all the fabricated RO membranes can be clearly observed from the ATR-IR spectra (Fig. 3). The peak at 1667 cm-1 is
ce pt
assigned to the C=O vibration stretching of amide (amide I). The stretching and in-plane bending vibration (amide II) of N-H bond in amide group can be respectively
Ac
observed at 1610 and 1540 cm-1. Furthermore, the small peak at 1450 cm-1 is attributed to the C=O stretching and O-H stretching vibration of carboxylic group which derives from the hydrolysis of unreacted acyl-chloride groups. On the other hand, it can be found that a small peak at 2971 cm-1 assigned to the =C-H stretching of benzene ring appeared on the spectrum (Fig. 3a) of full aromatic polyamide (FAP) membrane. However, for the fabricated semi-aromatic polyamide (SAP) based RO membranes (Fig. 3b-f), this peak becomes broad and shifts to ca. 2928 cm-1, due to 11
Page 13 of 37
the absorption-peak part overlap of =C–H of phenyl group and –CH2– of cyclohexyl group. This result further suggests the formation of SAP membrane in some degree. Furthermore, owing to the low MSS incorporation amount (0.02 w/v%-0.08 w/v%),
ip t
the related silica absorption peaks nearly cannot be identified (Fig. 3c-f). For SAP/MSS-6 and SAP/MSS-8 membranes with relatively high incroporation amount
cr
(Fig. 3e-f), the asymmetric stretching of ≡Si-O-Si≡ was detected at ca.1046 cm-1 [27],
us
and the weak peak at 956 cm-1 deriving from the stretching vibration of ≡Si-OH group also can be found. These results illustrate that MSSs were successfully incorporated
M
3.3. Zeta potential analysis of RO membrane
an
into the barrier layer of SAP/MSS nanocomposite membrane.
Zeta potential analysis results (Fig. 4) show that all the fabricated RO membranes
ed
possess negative potential values in the pH range of ca. 2-11. With the increase of pH value, the zeta potentials of RO membrane surfaces show nearly same varying trend
ce pt
and all potentials gradually decrease, suggesting the increase of surface negative charge density. In the case of FAP membrane (Fig. 4), its zeta potential decreases from
Ac
-1.16 to -28.2 mV, with pH varying from 2.2 to 11.8. Compared with FAP membrane, SAP membrane with semi-aromatic-polymer skeleton has lower potential value in the whole employed pH range. It indicates that the surface charge density of SAP membrane is higher than that of the FAP membrane. Additionally, it was observed that after MSSs were incorporated into the barrier layer of SAP membrane, the zeta potential of the obtained SAP/MSS membranes were further decreased in comparison with pristine FAP and SAP membranes. For example, the zeta potential of 12
Page 14 of 37
SAP/MSS-2 membrane decreases from -2.30 mV to -32.4 mV with the pH variation in the range of 2.39-11.5. Moreover, it was found that SAP/MSS-8 membrane with higher silica content has lower potential value in contrast with SAP/MSS-2
ip t
membrane. The surface negative charge of FAP membrane is mainly due to the presence of
cr
pendant carboxylic groups deriving from the hydrolysis of unreacted acyl-chloride
us
groups. The higher negative charge density of SAP membrane illustrate that more carboxylic groups were formed on the membrane surface in contrast with the FAP
an
membrane, and this result is also supported by the previous report [21]. The formation
M
of abundant carboxylic groups for SAP membrane may be related to the conformational mobility of CTC monomer and faster polymerization reaction process.
ed
Furthermore, the incorporation of MSS can generally influence the IP reaction and decrease the cross-linking degree of the obtained polyamide barrier layer [28,29]. The
ce pt
decrease of cross-linking degree is proportional to the incorporation amount. Correspondingly, the amount of pendant carboxylic groups of the MSS incorporated
Ac
SAP/MSS series membranes was further increased. As a result, these membranes exhibit the higher negative charge densities and lower zeta potentials (Fig. 4) compared with the fabricated FAP and SAP membranes. SAP/MSS-8 membrane possesses lower zeta potential than SAP/MSS-2 membrane. 3.4. FE-SEM analysis of RO membrane The top surface FE-SEM images (Fig. 5) show that all the fabricated RO membranes have typical “leaf-like” structures. The FAP and SAP membranes (Fig. 5a 13
Page 15 of 37
and b) have similar surface morphologies, and the SAP membrane seems having larger surface roughness than FAP membrane. Except the “leaf-like” structures, some spherical bumps also can be observed from the surfaces of both membranes.
ip t
Additionally, it can be found that many MSSs are uniformly embedded on the surface of SAP/MSS-2 membrane (Fig. 5c), after 0.02 w/v% of silica-sphere concentration
cr
was introduced in the fabrication process. Simultaneously, its “leaf-like” structure
us
becomes larger than that of the pristine SAP membrane (Fig. 5b). With the increase of the introduced concentration, the embedded mesoporous silica spheres also
an
correspondingly gradually increases from SAP/MSS-2 to SAP/MSS-8 membranes
M
(Fig. 5c-f). Moreover, owing to the higher introducing concentrations, some agglomerations of MSSs can be found on the surfaces of SAP/MSS-6 and
ed
SAP/MSS-8 membranes.
On the other hand, from the cross-section SEM images (Fig. 6), it can be observed
ce pt
that the barrier-layer thicknesses of the fabricated RO membranes are all very uniform. For SAP membrane (Fig. 6a), the thickness (marked with white arrow) is ca. 163 nm.
Ac
With the increase of the introduced silica concentration from 0.02 w/v% to 0.06 w/v%, the thicknesses gradually decrease from 142 nm (SAP/MSS-2, Fig. 6b) to 121 nm (SAP/MSS-6, Fig. 6d). Moreover, it can be clearly observed that some MSSs (marked with black arrows) are embedded on the barrier layers for the SAP/MSS series membranes (Fig. 6c and d). The greater conformational mobility of CTC monomers increases its reaction rate with MPDA monomers [21], and the rapid reaction can quickly promote the 14
Page 16 of 37
cross-linking degree of the formed polyamide barrier layer of the semi-aromatic based membrane in a relatively short time. Therefore, it is blocked that the diffusion of MPDA molecules from the aqueous phase into the oil phase containing CTC. The IP
ip t
reaction was correspondingly terminated. As a result, the obtained barrier layer is very thin and its thickness cannot be further increased. Furthermore, due to the high
cr
specific surface area of MSSs, some MPDA molecules are physically adsorbed on the
us
silica pore wall. With the increase of the incorporated MSSs, more MPDA molecules can be adsorbed. Correspondingly, the barrier layer becomes thinner due to less
an
MPDA monomers reacting with CTC monomers. On the other hand, because that
M
diameters of MSSs (ca.115 nm) are comparable with the barrier-layer thickness (ca. 121-142 nm), many spheres are embedded on the barrier-layer surface.
ed
3.5. Contact angle analysis of RO membranes
The contact angle analysis (Table 1) shows that SAP membrane has lower contact
ce pt
angle (49.5°) compared with FAP membrane (53.3°), suggesting the hydrophilicity of the former is better than that of the latter. After mesoporous silica spheres were
Ac
incorporated, the contact angle of SAP/MSS-2 membrane further decreases to 45.4°. It indicates that the incorporation of mesoporous silica spheres is favorable for the improvement of hydrophilicity. With the increase of the MSS amount, the contact angle of the fabricated nanocomposite RO membrane decreases from 45.4° to 35.6° (Table 1), illustrating that the hydrophilicity is gradually enhanced. Based on zeta potential results mentioned above, in contrast with the FAP membrane, the barrier-layer surface of SAP membrane has more hydrophilic 15
Page 17 of 37
carboxylic groups. As a result, this membrane possesses better hydrophilicity than FAP membrane. After MSSs are introduced into the barrier layer, the cross-linking degree of the obtained SAP/MSS nanocomposite RO membrane is decreased and
ip t
more carboxylic groups are formed. Simultaneously, the surface roughness is also increased. These factors are all advantageous on the hydophilicity improvement.
cr
Therefore, SAP/MSS nanocomposite RO membrane has better hydrophilicity
us
compared with the pristine SAP membrane. 3.6. Separation performance
an
The testing results of separation performance (Table 2) show that the permeability
M
(46.30 L/m2h) of SAP membrane is obviously better than that (25.62 L/m2h) of the FAP membrane under 1.5 MPa operation pressure and 2000 ppm of NaCl solution.
ed
The rejection performance (97.54 %) of the former is slightly lower in comparison with the latter (98.33 %) under the same testing condition. However, after the
ce pt
introduction of MSS nanomaterial, the permeate flux of the fabricated SAP/MSS-2 nanocomposite membrane is further increased into 62.53 L/m2h, and its salt rejection
Ac
slightly deceased by 3.55% compared with SAP membrane (Table 2) under the same testing condition mentioned above. Moreover, it was observed that the permeate flux of the MSS incorporated RO membranes gradually increased from 62.53 to 72.73 L/m2h. Simultaneously, their salt rejections decrease from 96.82 % to 93.57 %. Furthermore, the variation trends of the separation performances of all the fabricated SAP and SAP/MSS membranes (Fig. 7) were also detailedly investigated by changing the operation pressure from 0.9 to 1.9 MPa. It suggests that the permeate 16
Page 18 of 37
fluxes (Fig.7a) of these membranes dramatically increase with the operation pressure. Additional, it can be found that SAP membrane still exhibits the lowest permeate flux in the whole of the testing pressure range (0.9–1.9 MPa). After MSS was incorporated
ip t
into the barrier layer, the permeate fluxes of the obtained nanocomposite membranes are obviously improved, and it exhibits gradually increase trend from SAP/MSS-2 to
cr
SAP/MSS-8 RO membranes. These results are consistent with the results (Table 2)
us
mentioned above. The permeate flux of SAP/MSS-8 membrane steeply increases from 43.57 to 92.70 L/m2h with the change of operating pressure from 0.9 to 1.9 MPa.
an
On the other hand, it was observed that the salt rejections of these RO membranes
M
(Fig.7b) are also gradually enhanced under the same testing pressure range (0.9-1.9 MPa). The SAP membrane possesses the highest salt rejection compared with the
ed
other fabricated RO membranes. Its salt rejection increases from 92.05 to 98.47 % with the operation pressure varying from 0.9 to 1.9 MPa. However, from the
ce pt
salt-rejection change curves (Fig.7b), it was found that the salt rejection gradually decreases with the addition of the MSS nanomaterial from 0 to 0.08%. Thereinto,
Ac
SAP/MSS-8 membrane exhibits the lowest salt rejection. 3.7. Desalting mechanism analysis Based on the static repulsion and solution-diffusion mechanisms [11] which are
widely used for explaining the separation processes of pressure-drive desalting membranes, it can be deduced that the surface charge property, surface hydrophilicity, thickness and microstructure of polyamide barrier layer should be closely related to the separation performance. The FAP membrane fabricated with TMC and MPDA 17
Page 19 of 37
monomers has rigid polyamide framework, and the framework possesses lower freedom degree, which has great contribution to the high salt rejection. However, the low freedom degree of the framework and relatively poor hydrophilicity (Table 1) are
ip t
unfavorable for the transportation of water molecules. As a result, the FAP membrane possesses poorer permeability than the other fabricated RO membranes. In
cr
comparison with FAP membrane, the SAP membrane made from MPDA and CTC
us
with greater conformational mobility has high-freedom-degree polyamide framework, thinner barrier layer (Fig. 6) and better hydropilicity (Table 1). These factors enable
an
water molecules easily go through the barrier layer, and the permeate flux is
M
dramatically enhanced. Simultaneously, the semi-aromatic framework with high freedom degree and thin barrier layer also make salt ions more easily go through the
ed
membrane, although this membrane has higher surface negative charge density (Fig. 4). Therefore, the salt rejection of SAP membrane was slightly decreased compared to
ce pt
the FAP membrane.
On the other hand, MSSs have abundant uniform mesopore channels (ca. 1.5 nm)
Ac
and fine hydrophilicity deriving from the presence of great deal of silanol groups. It can be easily concluded that introducing MSS into the barrier layer of RO membrane can favor improving water permeability. This viewpoint is also consistent with some other reports [11,19]. The embedding of MSS nanomaterials into FAP/MSS membranes provides abundant water-flow paths, and also shortens the diffusion path of water molecules in barrier layer. Moreover, owing to the disturbance of MSS in the IP reaction process, the cross-linking degree of barrier layer becomes lower in 18
Page 20 of 37
contrast with pristine SAP membrane and more hydrophilic uncross-linked carboxylic groups are formed. Therefore, the permeate fluxes of the fabricated FAP/MSS membranes are further enhanced compared with pristine FAP membranes, and their
ip t
salt rejections are correspondingly slightly decreased. Furthermore, many salt ions can be adsorbed on the pore wall of MSS in the desalting process, which gives rise to the
cr
formation of double electric layers in the mesopores. The electric double layers can
us
restrict the entry and motion of metal ions and have no effect on the transportation of neutral water molecules [19,28–30]. Furthermore, the increase of surface negative
an
charge density (Fig. 4) of FAP/MSS membranes can strengthen the repulsion
M
interaction with salt ions. As a result, the incorporation of MSS do not lead to sharp decrease of selectivity, accompanying with substantial enhancement of permeate flux.
ed
Moreover, with the increase of incorporated MSS nanomaterials, there are more MSS embedded on the surface of barrier layer. The cross linking degree also become more
ce pt
low and the hydrophilicity of the membrane surface can be further improved. Therefore, the permeate fluxes gradually increase from FAP/MSS-2 to FAP/MSS-8
Ac
membranes, and the salt rejections correspondingly become gradually lower in the same sequence.
4. Conclusions
Semi-aromatic membranes,
polyamide/spherical
fabricated
using
mesoporous
m-phenylene
silica
diamine
nanocomposite
aqueous
solution
RO and
cyclohexane-1, 3, 5-tricarbonyl chloride/mesoporous-silica-sphere organic solution as main raw materials, have excellent permeability (up to 72.73 L/m2h) and comparable 19
Page 21 of 37
salt rejections (93.57–98.33%) in 1.5 MPa of operating pressure and 2000 mg/L of NaCl solution. The separation performances are significantly better than those of traditional full-aromatic polyamide and pristine semi-aromatic polyamide membranes.
ip t
With the change of the incorporated mesoporous silica spheres in the range of 0.02–0.08 w/v%, the permeate fluxes are gradually enhanced from 62.53 to 72.73
cr
L/m2h accompanied with the slight decrease of salt rejection. Additionally, the
us
experimental results suggest that the enhanced operating pressure enables the separation performances of nanocomposite membranes to be dramatically improved.
an
Compared with the contrast samples (SAP and FAP membranes), the enhancement of
M
permeability can be attributed to the high freedom degree of semi-aromatic-polyamide framework, the better hydrophilicity, thinner barrier layer, and the embedding of
ed
mesoporous silica spheres. Therein, the introduction of mesoporous silica spheres provides abundant water-flow paths, and also shortens the diffusion path of water
ce pt
molecules in barrier layer. Furthermore, the increase of surface negative charge density and the formation of electrical double layers in the mesopores make the
Ac
selectivity of the fabricated RO membrane have no significant decay. Acknowledgements The authors thank the financial support of the National Natural Science Foundation of China (NSFC) (No. 21406041), and Basic Scientific Research Founds for the State-level Scientific Research Institute (No. 2013-T06). References [1] L. Zhang, L. Xie, H.-L. Chen, C.-J. Gao, Progress and prospects of seawater 20
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desalination in China, Desalination 182 (2005) 13–18. [2] L. Malaeb, G.M. Ayoub, Reverse osmosis technology for water treatment: State of the art review, Desalination 267 (2011) 1–8.
ip t
[3] A.D. Khawaji, I.K. Kutubkhanah, J.-M. Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47–69.
cr
[4] W.J. Lau, A.F. Ismail, N. Misdan, M.A. Kassim, A recent progress in thin film
us
composite membrane: A review, Desalination 287 (2012) 190–199.
[5] T. Humplik, J. Lee, S. CO’Hern, B.A. Fellman, M.A. Baig, S.F. Hassan, M.A.
an
Atieh, F. Rahman, T. Laoui, R. Karnik, E.N. Wang, Nanotechnology 22(2011) 1–19.
Nano Lett. 12 (2012) 3602–3608.
M
[6] D. Cohen-Tanugi, J.C. Grossman, Water desalination across nanoporous graphene,
ed
[7] L.Y. Ng, A.W. Mohammad, C.P. Leo, N. Hilal, Polymeric membranes incorporated with metal/metal oxide nanoparticles: a comprehensive review, Desalination 308
ce pt
(2013) 15–33.
[8] M. Fathizadeh, A. Aroujalian, A. Raisi, Effect of added NaX nano-zeolite into
Ac
polyamide as a top thin layer of membrane on water flux and salt rejection in a reverse osmosis process, J. Membr. Sci. 375 (2011) 88–95. [9] H. Zhao, S. Qiu, L. Wu, L. Zhang, H. Chen, C. Gao, Improving the performance of polyamide reverse osmosis membrane by incorporation of modified multi-walled carbon nanotubes, J. Membr. Sci. 450 (2014) 249–256. [10] E.-S. Kim, B. Deng, Fabrication of polyamide thin-film nano-composite (PA-TFN) membrane with hydrophilized ordered mesoporous carbon (H-OMC) for 21
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water purifications, J. Membr. Sci. 375 (2011) 46–54. [11] J. Yin, E.-S. Kim, J. Yang, B. Deng, Fabrication of a novel thin-film nanocomposite (TFN) membrane containing MCM-41 silica nanoparticles (NPs) for
ip t
water purification, J. Membr. Sci. 423–424 (2012) 238–246. [12] J. Yin, B. Deng, Polymer-matrix nanocomposite membranes for water treatment,
cr
J. Membr. Sci. 479 (2015) 256–275.
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[13] G.N.B. Baroña, J. Lim, M. Choi, B. Jung, Interfacial polymerization of polyamide-aluminosilicate SWNT nanocomposite membranes for reverse osmosis,
an
Desalination 325 (2013) 138–147.
M
[14] B. Rajaeian, A. Rahimpour, M.O. Tade, S. Liu, Fabrication and characterization of polyamide thin film nanocomposite (TFN) nanofiltration membrane impregnated
ed
with TiO2 nanoparticles, Desalination 313 (2013) 176–188. [15] E.-S. Kim, G. Hwang, M.G. El-Din, Y. Liu, Development of nanosilver and
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multi-walled carbon nanotubes thin-film nanocomposite membrane for enhanced water treatment, J. Membr. Sci. 394–395 (2012) 37–48.
Ac
[16] M. Ben-Sasson, K.R. Zodrow, Q. Genggeng, Y. Kang, E.P. Giannelis, M. Elimelech, Surface functionalization of thin-film composite membranes with copper nanoparticles for antimicrobial surface properties, Environ. Sci. Technol. 48 (2014) 384–393. [17] Q. Cai, Z.S. Luo, W.Q. Pang, Y.W. Fan, X.H. Chen, F.Z. Cui, Dilute solution routes to various controllable morphologies of MCM-41silica with a basic medium, Chem. Mater. 13 (2001) 258–263. 22
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[18] M. Mizutani, Y. Yamada, K. Yano, Pore-expansion of monodisperse mesoporous silica spheres by a novel surfactant exchange method, Chem. Commun. (2007) 1172–1174.
ip t
[19] M. Bao, G. Zhu, L. Wang, M. Wang, C. Gao, Preparation of monodispersed spherical mesoporous nanosilica–polyamide thin film composite reverse osmosis
cr
membranes via interfacial polymerization, Desalination 309 (2013) 261–266.
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[20] S.D. Arthur, Structure-property relationship in a thin film composite reverse osmosis membrane, J. Membr. Sci. 46 (1989) 243–260.
an
[21] S. Yu, M. Liu, Z. Lü, Y. Zhou, C. Gao, Aromatic-cycloaliphatic polyamide
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thin-film composite membrane with improved chlorine resistance prepared from m-phenylenediamine-4-methyl and cyclohexane-1,3,5-tricarbonyl chloride, J. Membr.
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[22] M.M. Pendergast, A.K. Ghosh, E.M.V. Hoek, Separation performance and
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interfacial properties of nanocomposite reverse osmosis membranes, Desalination 308 (2013) 180–185.
Ac
[23] W.F. Chan, H.Y. Chen, A. Surapathi, M.G. Taylor, X. Shao, E. Marand J.K. Johnson, Zwitterion functionalized carbon nanotube/polyamide nanocomposite membranes for water desalination, ACS Nano 7 (2013) 5308–5319. [24] M.L. Lind, B.H. Jeong, A. Subramani, X. Huang, E.M.V.Hoek, Effect of mobile cation on zeolite-polyamide thin film nanocomposite membranes, J. Mater. Res. 24 (2009) 1624–1631. [25] D. Rana, Y. Kim, T. Matsuura, H.A. Arafat, Development of antifouling thin23
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film-composite membranes for seawater desalination, J. Membr. Sci. 367 (2011) 110–118. [26] H. Audorff, R. Walker, L. Kador, H.-W. Schmidt, Holographic investigations of
ip t
azobenzene-containing low-molecular-weight compounds in pure materials and binary blends with polystyrene, Chem. Eur. J. 17 (2011) 12722–12728.
cr
[27] G.L. Jadav, P.S. Singh, Synthesis of novel silica-polyamide nanocomposite
us
membrane with enhanced properties, J. Membr. Sci. 328 (2009) 257–267.
[28] D.M. Ruthven, Principles of Adsorption and Adsorption Processes, first ed., John
an
Wiley, New York, 1984.
M
[29] L.X. Li, J.H. Dong, T.M. Nenoff, R. Lee, Desalination by reverse osmosis using MFI zeolite membranes, J. Membr. Sci. 243 (2004) 401–404.
ed
[30] L.X. Li, J.H. Dong, T.M. Nenoff, R. Lee, Reverse osmosis of ionic aqueous
Ac
ce pt
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24
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Figure captions Scheme 1. Schematic of RO-membrane performance testing apparatus. Fig. 1. (a) nitrogen sorption isotherm and (b) pore size distribution (PSD) of the
ip t
synthesized mesoporous silica spheres. Fig. 2. (a) SEM and (b and c) TEM images of the synthesized mesoporous silica
cr
spheres.
us
Fig. 3. ATR-IR spectra of the fabricated RO membranes. (a) full-aromatic polyamide (FAP) RO membrane; (b) pristine semi-aromatic polyamide (SAP) RO membrane; (c)
an
SAP/MSS-2 membrane; (d) SAP/MSS-4 membrane; (e) SAP/MSS-6 membrane; and
M
(f) SAP/MSS-8 membrane.
Fig. 4. Zeta potentials of the fabricated RO membranes with pH ranging from ca. 3 to
ed
11.
Fig. 5. Top-surface SEM images of the fabricated RO membranes. (a) full-aromatic
ce pt
polyamide (FAP) RO membrane; (b) pristine semi-aromatic polyamide (SAP) RO membrane; (c) SAP/MSS-2 membrane; (d) SAP/MSS-4 membrane; (e) SAP/MSS-6
Ac
membrane; and (f) SAP/MSS-8 membrane. Fig. 6. Cross-section SEM images of the barrier layers of the fabricated RO membranes. (a) pristine semi-aromatic polyamide (SAP) RO membrane; (b) SAP/MSS-2 membrane; (c) SAP/MSS-4 membrane; and (d) SAP/MSS-6 membrane. Fig. 7. (a) permeate fluxes and (b) salt rejections of the fabricated RO membranes with operating pressure ranging from 0.9 MPa to 1.9 MPa.
25
Page 27 of 37
ip t cr us an M Ac
ce pt
ed
Scheme 1.
26
Page 28 of 37
ip t cr us an
Ac
ce pt
ed
M
Fig. 1.
27
Page 29 of 37
ip t cr us an M ed ce pt Ac Fig. 2.
28
Page 30 of 37
ip t cr us an M ed Ac
ce pt
Fig. 3.
29
Page 31 of 37
ip t cr us an M Ac
ce pt
ed
Fig. 4.
30
Page 32 of 37
ip t cr us an M ed ce pt Ac
Fig. 5.
31
Page 33 of 37
ip t cr us an
Ac
ce pt
ed
M
Fig. 6.
32
Page 34 of 37
ip t cr us
Ac
ce pt
ed
M
an
Fig. 7.
33
Page 35 of 37
Table 1 Contact angles of the fabricated RO membranes. FAP
Contact angle (°)
53.3
SAP 49.5
SAP/MSS-2
SAP/MSS-4
45.4
SAP/MSS-6
43.4
SAP/MSS-8
38.5
35.6
Ac
ce pt
ed
M
an
us
cr
ip t
RO membrane
34
Page 36 of 37
Table 2 The separation performances of the fabricated RO membranes tested under ca. 2000 mg/L NaCl aqueous solution with 25 ºC, and 1.5 MPa operating pressure. FAP
SAP
SAP/MSS-2
SAP/MSS-6
SAP/MSS-8
Salt rejection (%)
98.33
97.54
96.82
95.50
93.93
93.57
Permeate flux (L/m2h)
25.62
46.30
62.53
65.09
68.91
72.73
Ac
ce pt
ed
M
an
us
cr
SAP/MSS-4
ip t
RO membrane
35
Page 37 of 37