Desalination 327 (2013) 24–31

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Desalination

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Effects of ordered mesoporous silica on the performances of compositenanofiltration membrane

Qiang Li ⁎, Zhaokui Li, Hui Yu, Xianhui Pan, Xuliang Wang, Yihua Wang, Jie SongInstitute of Tianjin Seawater Desalination and Multipurpose Utilization, State Oceanic Administration (SOA), Tianjin 300192, China

H I G H L I G H T S

• Mesoporous silica SBA-15 was firstly utilized on the NF-membrane fabrication.• Influences of mesoporous silica on the membrane performances were deeply analyzed.• We investigated the changes on microstructures of the SBA-15 modified NF membrane.• The influential mechanisms on NF-membrane performances were carefully proposed.

⁎ Corresponding author. Tel.: +86 22 8789 9081; fax: +E-mail address: qiangli_chem@hotmail.com (Q. Li).

0011-9164/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.desal.2013.08.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 June 2013Received in revised form 2 August 2013Accepted 3 August 2013Available online 27 August 2013

Keywords:Mesoporous silicaNanofiltration membraneFouling resistanceSurface modification

Surface modified composite nanofiltration (MCNF) membrane was prepared by traditional interfacial polymer-ization approach using trimesoyl chloride and piperazine as reacting monomers, mesoporous silica SBA-15 as asurface modification additive, for investigating the influences of mesoporous-silica introduction on the mem-brane performances. Its surface properties andmicrostructureswere carefully characterized by various analyzinginstruments. Furthermore, the rejection performance was tested using 2000 ppm of MgSO4 aqueous solutionunder 1 MPa working pressure. Its fouling-resistance performance was also evaluated by using bovine serumalbumin as an organic pollutantmodel, based on the changes of permeate flux in the running process. The exper-imental results show that the introduction of SBA-15 material can obviously enable the permeate flux of the NFmembrane to be increased from 32.4 to 45.6 L/h·m2, accompanied with slight decrease of MgSO4 rejection from97.88 to 85.23%. The SBA-15 modified MCNFmembrane exhibits excellent fouling-resistance performance, withthe permeate flux still retaining 72.53% of the initial flux value, after the anti-fouling test running for 270 min.The introduction of SBA-15 significantly improves the hydrophilicity, effective surface area and pore size of themembrane separation layer, which is favorable for the enhancement of permeate flux and fouling-resistanceperformances.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, with the rapid development of industrialization andurbanization, the scarcity of potable water has become an increasinglyserious problem [1,2]. Seawater and/or brackish water desalination aswell as wastewater treatment are generally adopted strategies for solvingthis problem. Therefore, many researchers and users are focusing on thefabrication and application of pressure-derived polymer membranes[3,4], owing to relatively low costs, lower energy consumption, andgood controllability of the membrane separation process. Nanofiltrationmembrane (NF) is very attractive for its extensive applications in manyfields, including water softening, RO-process pretreatment, and theremoval of macromolecules (M.W. N 100 Da) [5,6].

86 22 8789 8169.

ghts reserved.

At present, themain problems hindering the further development ofNFmembrane application aremembrane fouling and relatively lowper-meate flux in separation process [7,8]. Both these factors can signifi-cantly increase the energy consumption and costs of membranerunning process. Membrane fouling usually derives from the chemicaland/or physical interactions between membrane surface and foulantdispersed in feed water. The occurrence of fouling can also decreasethe permeate flux and membrane lifespan. Hydrophilic surface andhigh zeta potential usually favor improving fouling-resistance perfor-mance [9,10], due to the reduction of mutual physical and electrostaticinteractions between the foulants and membrane surface. In addition,permeate flux is closely related with microstructures and surface prop-erties of NF-membrane separation layer, including effective surfacearea, cross-linking degree, hydrophilicity and so on [11]. Hydrophilicsurface, low cross-linking degree and high effective surface area shouldbe advantageous for the improvement of permeate flux. It is becausethat these factors can strengthen the water molecule-membrane

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surface interactions, and improve the water transportation in NFmembrane.

Many researchers have proposed numerous strategies to synthesizenovel NF membranes for improving their permeate flux and foulingresistance, mainly including exploring new raw materials, surfacemodifying agents, and novelmodification approaches [12–14]. Amongstthese strategies, surface modification is the most popular and effective,because of its controllability, flexibility, and low cost. Generally, it can beclassified into two types: physical surface coating by the electrostaticattraction, hydrogen-bond and/or van der Waals interactions [15,16];and chemical grafting ignited by initial surface activations (such as UV,ozone, and plasma), and/or directly chemical reactions (such as redox,electrophilic, and nucleophilic reactions) with membrane surface func-tional groups [17,18]. Until now, different types of organic polymers,monomers, nanoparticles, and zeolites have been extensively used asmodifying agents [19,20], on account of their high hydrophilicity andfavorably potential effect. For example, Seman et al. fabricated theacrylic-acid monomer modified polyethersulfone NF membrane withhigh permeate flux and excellent fouling-resistance performance bymeans of UV-initiated graft polymerization technique [21]. Bai et al. pre-pared the TiO2-nanoparticle modified polyamide NF membrane by insitu interfacial polymerization strategy [22]. In contrast with organicmodifying agents, their counterparts, such asmetal oxide nanoparticlesand zeolites, generally have more advantages on membrane modifica-tion in terms of chemical and physical stabilities.

Since the discovery of MCM series of mesoporous silicas,mesoporous materials with different compositions have attracted agreat deal of attentions, owing to their extensive applications in manyfields, such as catalysis, adsorption, and separation [23,24]. Amongstthem, mesoporous silica SBA-15 reported by Zhao et al. has a robustframework, highly specific surface area (up to 1000 m2/g), controllablepore size (4.6–30 nm) and ordered two-dimensional (2-D) hexagonalmesostructures [25], compared with commonly nonporous nano-particles. These structural properties suggest that SBA-15 has potentialapplications inmembrane separation process. The reason is that its highsurface area and orderedmesostructure should be favorable for improv-ing permeate flux, and its strong hydrophilicity can enhance the anti-fouling performance of NFmembrane. Deng and co-workers [26] intro-ducedmesoporous carbon into aromatic-polyamide separation layer byinterfacial polymerization (IP) method. The experimental results dem-onstrate that the obtained composite membrane exhibits well foulingresistance to bovine serum albumin (BSA) and high Na2SO4 rejection(90%) performance. However, there have been no reports involvingthe application of SBA-15 on membrane fabrication until now.

Herein, the influences of mesoporous silica SBA-15 on the perfor-mances of NF membrane were firstly explored for improving the anti-fouling property and permeate flux. Themodified polyamide compositeNF membrane (MCNF) was synthesized by IP strategy using TMC andPIP as reacting monomers, mesoporous silica SBA-15 as a modifyingagent, sodium dodecyl sulfonate (SDS) as an additive, and polysulfoneUF membrane as the supporting layer. This research paves the way fordeveloping the utilization of mesoporous silica materials on watertreatment.

2. Experimental

2.1. Chemicals and materials

Tetraethyl orthosilicate (TEOS) and hexane were purchased fromGuangfu Chemical Co. Ltd. Trimesoyl chloride (TMC), triblock copoly-mer poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethyleneoxide) Pluronic P123 (M.W. = 5800), bovine serum albumin (BSA),sodium dodecyl sulfonate (SDS), and piperazine (PIP) were all pur-chased from Aldrich Chemical Inc. Hydrochloric acid was purchasedfrom Beijing Chemical Co. Ltd. Magnesium sulfate was bought fromKejin Fine Chemical Co. Ltd. All the above chemicals were used as

received without any further purification. Polysulfone ultrafiltrationmembrane couponswere provided by Center of HangzhouWater Treat-ment. Millipore water was used in all experiments.

2.2. Synthesis of mesoporous silica SBA-15

The mesoporous silica SBA-15 was synthesized according to the re-ported method [25]. Typically, 4.0 g of Pluronic P123 was dissolved in150 mL of 1.6 M hydrochloric acid solution at around 30 °C. Then thetemperature of the above solution was elevated to 40 °C, followed bythe addition of 10.7 g of TEOS. After being stirred at 40 °C for 24 h, themixture was transferred into an autoclave and aged at 100 °C for 24 h.Then, the white precipitate was collected by centrifugation and driedat 80 °C. The product was obtained by removing the template at550 °C for 5 h.

2.3. Fabrications of pristine and SBA-15 modified piperazine-amidecomposite NF membranes

The pristine piperazine-amide composite NF (PCNF)membranewasfabricated by IP approach. Firstly, 5 mL of 1 g/L SDS aqueous solutionand 0.2 g of PIP were added into 100 mL of Na3PO4 aqueous solutionwith pH value ca.10 under stirring. Then, the above solution was evenlypoured on a polysulfoneUFmembrane used as supporting layer at roomtemperature. After keeping for 2 min, the residual solution wasremoved from the UF membrane. After this, 100 mL of hexane solutioncontaining 0.2% (w/v) TMC was evenly poured on the UF membranewetted with PIP for the occurrence of IP reaction. Then the residualTMC solutionwas removed after the reaction has lasted for 20 s. Finally,the obtained fresh composite NFmembrane was aged in oven at 75 °Cfor ca. 15 min for further cross linking. The obtained PCNF mem-brane was stored in deionized water for the following performanceevaluation.

The synthesis process of SBA-15 modified piperazine-amide com-posite nanofiltration (MCNF) membrane was similar with that of PCNFmembrane mentioned above. Only mesoporous siliceous SBA-15 wasintroduced into the fabrication process. Typically, 0.2 g of SBA-15 wasfirstly dispersed in 100 mL of Na3PO4 aqueous solution with a pHvalue of 10 by ultrasonic treatment. 5 mL of 1 g/L SDS aqueous solutionand 0.2 g of PIPwere added into the above solution under stirring. Then,themixturewas homogeneously poured on a polysulfoneUFmembrane.After keeping for 2 min, the residual solution was removed from the UFmembrane. After this, 100 mL of hexane solution containing 0.2% (w/v)TMC was evenly poured on the UF membrane wetted with PIP andSBA-15 containing solution. Then the residual TMC solution was re-moved, after the reaction lasted for 20 s. Finally, the fabricated freshSBA-15 modified composite NF membrane was aged in oven at 75 °Cfor ca. 15 min for further cross linking, and stored in deionized waterfor the following performance evaluation.

2.4. XRD measurement

For analyzing the ordering of SBA-15 mesostructure, small-anglepowder X-ray diffraction (SAXRD) pattern was recorded with a BrukerD4 powder X-ray diffractometer (Germany) using Cu Ka radiation(40 kV, 40 mA).

2.5. Nitrogen sorption measurement

Nitrogen sorption isotherm was measured with a MicromeriticsTristar II (USA) analyzer at −196 °C for measuring the pore size andspecific surface area of SBA-15. Before measurement, the sample wasdegassed at 180 °C in vacuum for more than 6 h. The specific surfacearea 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.

Scheme 1. Schematic of NF-membrane performance testing apparatus.

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2.6. Transmission electron microscopy (TEM) measurement

Transmission electronmicroscopy (TEM) imageswere recorded on aJEOL 2011 microscope (Japan) operated at 200 kV for observing themesostructures of SBA-15. Before TEM characterization, the sampleswere dispersed in ethanol. The suspensions of the samples weredropped on a holey carbon coated copper grid.

2.7. Infrared spectroscopy (IR) measurement

IR spectrum of mesoporous silica SBA-15 in the range of 400–4000 cm−1 was collected on a VERTEX 70 FT-IR spectrometer (Bruker,Germany) by using KBr pellet technique. The IR spectra of the fabricatedNF membranes were also measured with wavenumbers rangingfrom 400 to 4000 cm−1 under the attenuated total reflectance(ATR) mode for analyzing their surface functional groups. The ATRaccessory contains a ZnSe crystal at a nominal incident angle of45°, yielding a single reflection on the membrane surface.

2.8. Contact angle measurement

The contact angles of the fabricated NF membranes were measuredon POWEREACH contact angle instrument (China) for evaluating theirhydrophilicity. Briefly, a small droplet of purified water was deliveredonto aNFmembrane, and a static image of the droplet on themembranesurfacewas taken after ca. 2 s. Image analysis and contact angle compu-tationwere performed using the drop shape analysis software assuminga circular profile of the droplet. For ensuring the accuracy of the contactangle, the measurements were performed at 10 different locations, andthen the average value was regarded as the final result.

2.9. Zeta potential measurement

The surface charge properties of the fabricated NF membranes weremeasured with a solid surface zeta potential instrument (SurPASS,Anton Paar). In the measurement process, the NF membranes werefirstly rinsed with purified water, and then they were determined in0.001 mol/L of KCl aqueous solution with pH value adjusted in therange of 3–11 by adding different amounts of HCl and/or KOH aqueoussolution.

2.10. Scanning electron microscopy (SEM) measurement

The surface morphology of polysulfone UF membrane as supportinglayer was characterized using field-emission scanning electronicmicroscopy (FE-SEM) S-4800 (Hitachi, Japan) operated at 15 kV.The fabricated NF membranes were also observed using FE-SEMNANOSEM 430 (FEI, USA) operated at 10 kV. For avoiding the surfacecharging in the measuring process, the membrane surfaces were be-forehand coatedwith a layer of gold nanoparticles by using ion sprayinginstrument.

2.11. Rejection and flux testing

The performance tests were conducted with a 2000 ppm of MgSO4

solution under 1 MPa operating pressures at ca. 20 °C by using across-flow type apparatus (Scheme 1). Both permeate and retentatewere recycled back to the feed tank during the tests. The permeatewater can be taken at the tip of the permeate-water pipe. Rectangularmembrane samples were placed into the test apparatus with the activeskin layer facing the feedwater. The area of eachmembranewas around36 cm2. The salt rejection and permeate flux of each membranewere tested for two times, results of which have been averaged. Thepermeating volumes collected for 30 min were used to describe fluxin terms of liter per square meter per hour. A standardized conductivitymeter was used tomeasure the salt (MgSO4) concentrations in the feed

and product water for determining membrane selectivity as givenbelow:

salt rejection %ð Þ ¼ 1−CpCf

� �� 100%

in which, Cf and Cp represent feed and permeating concentrations,respectively.

2.12. Evaluation of fouling resistance performance

For comparing the fouling-resistance performances of the PCNF andMCNF membranes, fouling experiments were performed on the cross-flow type apparatus (Scheme 1). A typically experimental procedurefor resistance to BSA fouling can be described as: the NF membranefirst run in 12 L of aqueous solution containing 20 mM/L of NaCl at1 MPa, until its initial permeate flux (J0) became stable. Then, 2.4 g ofBSA was added into the purified water to form the synthetic foulant(100 ppm). After running for 30 min, the permeate flux (J) wasrecorded every other 30 min. Herein, NaCl was used as backgroundelectrolyte for stabilizing the formed foulant solution. The J/J0 ratiowas used to indicate the fouling-resistance performance. For improvingthe accuracy of the test results, the fouling-resistance test was repeatedfor two times.

3. Results and discussion

3.1. Structural analysis of synthesized mesoporous silica SBA-15

The structural parameters of the synthesized SBA-15 material, in-cluding mesostructure ordering, surface area, pore volume, and poresize, were comprehensively analyzed by using XRD, nitrogen sorption,and TEM. Three well-resolved diffraction peaks, indexed as 10, 11, and20, can be clearly observed from the small angle XRD (SAXRD) pattern(Fig. 1a). It suggests that the synthesized mesoporous silica SBA-15exhibits highly ordered 2-D hexagonal mesostructure. The nitrogensorption results (Fig. 1b) exhibit typical type IV isotherm with type H1hysteresis loop, suggesting that this material has typical cylindricalpore channels. Its specific surface area and pore volume are 392 cm2/gand 0.66 cm3/g, respectively. In addition, the nitrogen sorption iso-therm (Fig. 1b) in low p/p0 range of 0–0.1 indicates that large amountsof micropores were formed on the silica pore wall. Moreover, the corre-sponding pore size distribution curve (Fig. 1c) indicates that thismaterialhas uniform pore size centered at ca. 9.0 nm. Combined with the SAXRD

Fig. 1. Small angle XRD pattern (a), nitrogen sorption isotherm (b) and correspondingpore size distribution (c) of the synthesized mesoporous silica SBA-15.

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results, it can be concluded that its pore-wall thickness is ca. 3.5 nm.Additionally, TEM images of the obtained SBA-15 material (Fig. 2) alsoconfirm that it has well ordered hexagonal mesostructure, which isconsistent with the corresponding SAXRD result (Fig. 1a). The pore sizeevaluated from theTEMresults is ca.8.2 nm. It is similarwith the nitrogensorption result (Fig. 1b and c). In addition, it was found that the particlesize of the synthesized SBA-15 is not very uniform (0.1–3.0 μm),confirmed by corresponding SEM results.

Based on the above analysis, it can be demonstrated that the synthe-sized mesoporous silica SBA-15 has highly ordered hexagonalmesostructure and specific surface area (392 cm2/g), uniform pore size(9.0 nm). Generally, in the viewpoint of NF membrane fabrication, highsurface area is advantageous for the improvement of permeate flux. Uni-formly cylindrical pore channel can favor the transportation of fluid inthe membrane process. As a result, these structural parameters implythat this material has potential application in water treatment.

3.2. Surface structures of fabricated PCNF and MCNF membranes

The surface functional groups of the fabricated NF membranes andmesoporous silica SBA-15were analyzed (Fig. 3) for confirming the for-mation of piperazine–amide separation layer on the polysulfonesupporting layer and investigating the effect of SBA-15 on themembrane-surface structure. In the case of PCNF membrane (Fig. 3a1),the appearance of a strong peak at ca. 1620 cm−1 assigned to thestretching vibration of C=O double bond of an amide group, suggeststhat the occurrence of IP reaction between PIP and TMC. The peak at ca.1570 cm−1 should be attributed to the C\N stretching vibration of thepolyamide group [27]. The peak at ca. 1720 cm−1 may be related to thepolysulfone supporting layer, due to the relatively strong penetrationeffect (N300 nm) of IR light with wavenumbers around 1700 cm−1

[28]. In addition, the broad peak centered at ca. 3400 cm−1 is related tothe formation of \COOH groups on the membrane surface. They shouldbe caused by the hydrolysis of acyl chloride groups of TMC in the mem-brane fabrication process. These results confirm that piperazine-amideseparation layer has been formed on the support layer by interfacialpolymerization.

Additionally, ATR-IR spectrum of MCNF membrane (Fig. 3a2)modified with mesoporous silica SBA-15 is very similar with that ofPCNF membrane. Just its intensity of the peak centered at 3400 cm−1

is obviously stronger than that of pristine NF membrane (Fig. 3a1). Itis because that large amounts of SBA-15 mesoporous particles withstrong hydrophilicity were introduced in the IP process. Therefore, thepeak at 3400 cm−1 for MCNF membrane (Fig. 3a2) is not only relatedwith \COOH groups, but also related with hydrophilic \OH groups. Itcan be also confirmed by the presence of silanol (`Si\OH) group relatedpeaks at 3400 and 990 cm−1 in the spectrum of SBA-15 (Fig. 3b). In ad-dition, the characteristic peaks related with introduced SBA-15 cannotbe observed in the ATR-IR spectrum of MCNF membrane (Fig. 3a2). Itmay be due to the lower silica content relative to a great deal ofpiperazine-amidepolymers formedon themembrane surface. As a result,it can be deduced that the MCNFmembrane should be more hydrophilicthan the unmodified one, owing to the abundant hydrophilic \OHgroups of introduced SBA-15.

Based on the SEM measurement results, it can be found that thesurface of polysulfone supporting layer is very flat (Fig. 4a). After TMCand PIP polymerization, the PCNF-membrane surface (Fig. 4b) becomesslightly rough. Lots of “valley-ridge” structures on the membrane sur-face can be observed. After SBA-15 material was introduced, manymesoporous silica particles with ca. 3.0 μm were immobilized on thesurface of MCNF membrane (Fig. 4c). Except these silica particles,some polyamide nanoparticles with ca. 40–50 nm are also found onthe membrane surface. Therefore, it can be concluded that the surfaceof MCNF membrane is rougher than that of PCNF membrane (Fig. 4).Moreover, its effective surface area is correspondingly higher than thatof the pristine NF membrane, because of the modification of SBA-15with highly specific surface area (392 cm2/g).

3.3. Surface properties of fabricated PCNF and MCNF membranes

The contact angle of piperazine-amide NF membrane (PCNF) is 48°.However, after surface modified with hydrophilic SBA-15, its contactangle (MCNF) was obviously decreased to 35° (Fig. 5). It indicates thatthe hydrophilicity of MCNF is better than that of PCNF membrane.

Fig. 2. TEM images obtained along the [001] (a) and [110] (b) directions of SBA-15 material, respectively.

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This result can be also confirmed by comparing the contact angle imagesof PCNF (Fig. 5a) and MCNF (Fig. 5b) membranes.

The decrease of MCNF-membrane contact angle should be ascribedto the following two points: Firstly, mesoporous-silica particles haveabundant \OH groups (Fig. 3b) and correspondingly strong hydrophi-licity. After they were introduced in the membrane fabrication process,many SBA-15 particles were modified on the NF-membrane surface(Fig. 4), due to the occurrence of IP reaction. As a result, the hydrophilicityof piperazine-amide NF membrane (MCNF) was significantly enhanced,and its contact angle was also correspondingly decreased. Secondly, themodification of SBA-15 material simultaneously enhances the surfaceroughness of NF membrane, which is also favorable for the contact-

Fig. 3. Normalized ATR-IR spectra of the fabricated NF membranes (a) and synthesizedmesoporous silica SBA-15 (b). (a1) PCNF membrane; (a2) MCNF membrane.

angle decrease, due to the change of surface tension betweenmembranesurface and water droplet [28].

Furthermore, the zeta potentials of both of the twomembraneswerealso carefully investigated. The results (Fig. 6) show that the potentialvalue of PCNF membrane ranges from 2.9 to −47.5 mV with the pHvalue varying from ca. 2.9 to 11. However, after the modification ofSBA-15, the zeta potential value (10.2 to −42.8 mV) of NF membrane(MCNF) is relatively increased in the pH range of 2.8–11 (Fig. 6). Addi-tionally, the isoelectric point of NF membrane (Fig. 6) is also changedfrom ca. pH 3.0 to 3.3, owing to the introduction of mesoporous silicaSBA-15. Furthermore, the results suggest that the potential value ofMCNF membrane is obviously higher than that of PCNF membrane inthe general pH range (6–8) of seawater and/or brackish water (Fig. 6).Generally, themembrane surface with zeta potential more near to neu-tral can decrease the electrostatic interactions with charged foulants,due to the decrease of the amounts of its surface charges. On the otherhand, the SBA-15 modification caused higher potential should exhibitstrong anti-fouling property to hydrophobic organic foulants, owing tothe weak interactions between hydrophilic \OH groups and organicfoulants. Therefore, it can be concluded that MCNF membrane mayhave stronger anti-fouling property than that of PCNF membrane [29].

The relatively lower zeta potential of PCNF membrane mainlyderives from the presence of many\COOH groups on the surface con-firmed by the corresponding ATR-IR results (Fig. 3a). These \COOHgroups can dissociate into negatively carboxylate ions in aqueous solu-tion, which makes the membrane surface exhibits relatively highnegative-charge density. Only in higher acidity of aqueous solution,the dissociation process of \COOH groups can be resisted and themembrane-surface charge turns into neutral. Therefore, the isoelectricpoint of PCNF membrane is located at ca. 3.0 of pH (Fig. 6). The modifi-cation of mesoporous silica enables the membrane surface to haveabundant neutral\OH groups, and these groups decrease the negativecharge density ofmembrane surface in aqueous solution in someextent.As a result, the zeta potential of MCNF membrane was obviouslyenhanced. Correspondingly, its isoelectric point was also shifted to 3.3of pH, due to the decrease of the amounts of carboxylate ions.

3.4. Effects of mesoporous silica SBA-15 on separation performance

The separation-performance testing suggests that the rejection ofPCNF membrane to 2000 ppm of MgSO4 aqueous solution is ca.97.88% at 20 °C and 1 MPa working pressure. Its corresponding perme-ate flux is ca. 32.4 L/h·m2. However, after modifying mesoporous silicaon the pristine NF membrane, the permeate flux (MCNF membrane) isobviously enhanced to 45.6 L/h·m2 under the same testing conditionsas above. Simultaneously, its salt rejection is decreased from 97.88 to85.23%.

Fig. 4. FE-SEM images of the UF supporting layer (a), fabricated PCNF (b) andmesoporoussilica modified MCNF membranes (c).

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These results indicate that the pristine piperazine-amide NF (PCNF)membrane has relatively high salt rejection and low permeate flux, be-cause of the high compactness and relatively low effective surface areaof piperazine-amide separation layer. The introduction of mesoporoussilica SBA-15 makes the pore size of separation layer partly enlarged.Moreover, the high specific surface area of SBA-15 (392 cm2/g) andlarge membrane-surface roughness endow the obtained MCNF withhigh effective surface area. These structural changes make its salt rejec-tion decrease to ca. 85.23% and permeate flux increase to 45.6 L/h·m2.In addition, the increasing hydrophilicity of MCNF membrane con-firmed by contact-angle results is also favorable for the improvementof permeate flux, due to the strongly hydrogen-bond interactionsbetween the water molecules and \OH groups.

3.5. Effects of mesoporous silica SBA-15 on the fouling-resistanceperformance

The fouling-resistance testing results (Fig. 7) suggest that the per-meate flux of PCNFmembrane obviously decreases with the elongationof running time in 100 ppm of BSA aqueous solution under 1.0 MPaworking pressure and 20 °C. Especially in the initial period (30 min)of the anti-fouling experiment, its permeate flux drastically decreasesto 75.86% (J/J0) of the initial value, suggesting that many BSAmoleculesadsorbed on the PCNF membrane surface. Then, the decay rate of per-meate flux becomes relatively slow, and its J/J0 ratio is finally decreasedto 63.81% after running for 270 min. However, in the case of MCNFmembrane modified with mesoporous silica SBA-15, its permeate fluxdecreased to 80.80% of the initial value after lasting for 30 min(Fig. 7). It is higher than that (75.86%) of the PCNF membrane. Withthe further elongation of running time, the permeate-flux decreasingrate becomes very slow comparing with PCNF membrane. Its J/J0 ratiois finally decreased to 72.53% (Fig. 7) after running for 270 min, whichis also higher than that of the unmodified one. These results suggestthat the introduction of SBA-15 can obviously improve the fouling resis-tance performance of piperazine-amide NF membrane.

Generally, the NF membrane fouling mainly derives from the π–πstacking and van der Waals interactions between hydrophobic func-tional groups of BSA and piperazine-amide polymer, such as phenyland alkyl groups. In addition, another important factor is the electrostat-ic attractions between BSA molecules and membrane surface. The im-provement on the fouling resistance performance of MCNF membranemodified with SBA-15 should be mainly attributed to the changes ofsurface properties, including the improvement of surface hydrophilicityand enhancement of zeta potential. Its excellent surface hydrophilicityconfirmed by the contact angle results can weaken the interactions be-tween BSA andNFmembrane surface. It is because that the introductionof mesoporous silica SBA-15 increases the amounts of hydrophilic\OHgroups and decreases the amounts of hydrophobic groups on themem-brane surface. On the other hand, the introduced SBA-15 materialenhances the zeta-potential value of MCNF membrane (Fig. 6) and de-creases the amounts of negative charges from the dissociation ofmembrane-surface functional groups, which maybe weaken the elec-trostatic attractions between BSA foulants and membrane surface.

3.6. Formation process of SBA-15 modified MCNF membrane

In the fabrication process of MCNF membrane, PIP molecules andSBA-15 particles are first uniformly dispersed on the supporting layer(polysulfone UF membrane). Simultaneously, many PIP molecules arealso adsorbed on the surfaces of SBA-15 particles via the hydrogen-bond interactions between \NH\ groups of the former and \OHgroups of the latter. After TMC containing hexane solution was intro-duced onto the supporting layer wetted by PIP and SBA-15, thepiperazine-amide separation layer is formed by IP reaction betweenTMC and PIP molecules. In the same time, the polymerization reactionalso occurs around the SBA-15 particles adsorbed with PIP molecules.

Fig. 5. Contact-angle images of the fabricated PCNF (a) and mesoporous silica modified MCNF membranes (b).

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As a result, the SBA-15 particles were immobilized by the formedpiperazine-amide polymers and uniformly embedded in the separa-tion layer. This structure is advantageous for improving the stabilityof SBA-15 and preventing its loss in the membrane running process.Subsequently, the aging process at 75 °C can further prompt thepolymerization of TMC and PIP, and enhance the cross-linking degreeof piperazine-amide separation layer. It is favorable for the improve-ment of salt rejection. Finally, the mesoporous silica modified NF mem-brane is successfully fabricated by the above process.

Fig. 6. Zeta potential curves of the fabricated membranes under different pH values.

Fig. 7. Fouling-resistance performances of the fabricated PCNF and MCNF membranes to100 ppm of BSA solution at 1.0 MPa working pressure.

4. Conclusions

The introduction of orderedmesoporous silica SBA-15 can obviouslyimprove the permeate flux of piperazine-amide NF membrane, accom-panied with the decrease of salt rejection. It is because that the largerpore sizes (ca. 9 nm) and excellent hydrophilicity of the introducedSBA-15 can improve the transportation of water molecules in separa-tion layer and strengthen the interactions between water moleculesand membrane surface, respectively. Moreover, the introduction ofSBA-15 with high specific surface area (392 cm2/g) increases theeffective membrane surface area, which is also favorable for theenhancement of permeate flux. On the other hand, the addition ofSBA-15 partly enlarges the pore size of NF membrane, which leads tothe decrease of salt rejection.

Additionally, mesoporous silica SBA-15 can also significantly im-prove the NF-membrane fouling resistance to BSA. It is because thatSBA-15 improves the membrane hydrophilicity, which can weakenthe chemical and/or physical interactions between BSA molecules andpiperazine-amide polymers. Furthermore, the introduction of SBA-15increases the zeta-potential value and decreases the amounts of nega-tive charges of NF membrane surface, which can also decrease the elec-trostatic attractions between BSA molecules and membrane surface.

Acknowledgments

The authors thank the financial support of the Young Oceanic ScienceFoundation of State Oceanic Administration (SOA) (No. 2012448), andthe Basic Scientific Research Funds for the State-level Scientific ResearchInstitute (Nos. 2013-G4, 2012G01).

References

[1] G. Kang, Y. Cao, Development of antifouling reverse osmosis membranes for watertreatment: a review, Water Res. 46 (2012) 584–600.

[2] L.F. Greenlee, D.F. Lawler, B.D. Freeman, B. Marrot, P. Moulin, Reverse osmosisdesalination: water sources, technology, and today's challenges, Water Res. 43(2009) 2317–2348.

[3] M. Ulbricht, Advanced functional polymer membranes, Polymer 47 (2006)2217–2262.

[4] Y.K. Kim, S.Y. Lee, D.H. Kim, B.S. Lee, S.Y. Nam, J.W. Rhim, Preparation and character-ization of thermally crosslinked chlorine resistant thin film composite polyamidemembranes for reverse osmosis, Desalination 250 (2010) 865–867.

[5] J.-H. Kim, P.-K. Park, C.-H. Lee, H.-H. Kwon, Surface modification of nanofiltrationmembranes to improve the removal of organic micro-pollutants (EDCs andPhACs) in drinking water treatment: graft polymerization and cross-linking followedby functional group substitution, J. Membr. Sci. 321 (2008) 190–198.

[6] C. Ba, D.A. Ladner, J. Economy, Using polyelectrolyte coatings to improve fouling re-sistance of a positively charged nanofiltration membrane, J. Membr. Sci. 347 (2010)250–259.

[7] Y.-C. Chiang, Y. Chang, C.-J. Chuang, R.-C. Ruaan, A facile zwitterionization in theinterfacial modification of low bio-fouling nanofiltration membranes, J. Membr.Sci. 389 (2012) 76–82.

[8] S. Cheng, D.L. Oatley, P.M. Williams, C.J. Wright, Positively charged nanofiltrationmembranes: review of current fabrication methods and introduction of a novelapproach, Adv. Colloid Interf. 164 (2011) 12–20.

31Q. Li et al. / Desalination 327 (2013) 24–31

[9] I. Sawada, R. Fachrul, T. Ito, Y. Ohmukai, T. Maruyama, H. Matsuyama, Developmentof a hydrophilic polymer membrane containing silver nanoparticles with bothorganic antifouling and antibacterial properties, J. Membr. Sci. 387–388 (2012) 1–6.

[10] A. Asatekin, A. Menniti, S. Kang, M. Elimelech, E. Morgenroth, A.M. Mayes, Antifoulingnanofiltration membranes for membrane bioreactors from self-assembling graftcopolymers, J. Membr. Sci. 285 (2006) 81–89.

[11] H.S. Lee, S.J. Im, J.H. Kim, H.J. Kim, J.P. Kim, Polyamide thin-film nanofiltration mem-branes containing TiO2 nanoparticles, Desalination 219 (2008) 48–56.

[12] S.Y. Lee, H.J. Kim, R. Patel, S.J. Im, J.H. Kim, B.R. Min, Silver nanoparticles immobilizedon thin film composite polyamide membrane: characterization, nanofiltration, anti-fouling properties, Polym. Adv. Technol. 18 (2007) 562–568.

[13] M. Khayet, M.N. Abu Seman, N. Hilal, Response surface modeling and optimizationof composite nanofiltration modified membranes, J. Membr. Sci. 349 (2010)113–122.

[14] L. Li, S. Zhang, X. Zhang, Preparation and characterization of poly(piperazineamide)composite nanofiltrationmembrane by interfacial polymerization of 3,3′,5,5′-biphenyltetraacyl chloride and piperazine, J. Membr. Sci. 335 (2009) 133–139.

[15] M.L. Bruening, D.M. Dotzauer, P. Jain, L. Ouyang, G.L. Baker, Creation of functionalmembranes using polyelectrolyte multilayers and polymer brushes, Langmuir 24(2008) 7663–7673.

[16] E. Kharlampieva, S.A. Sukhishvili, Hydrogen-bonded layer-by-layer polymer films,Polym. Rev. 46 (2006) 377–395.

[17] A.A. Abuhabib, A.W. Mohammad, N. Hilal, R.A. Rahman, A.H. Shafie, Nanofiltrationmembrane modification by UV grafting for salt rejection and fouling resistance im-provement for brackish water desalination, Desalination 295 (2012) 16–25.

[18] X. Li, P. Vandezande, I.F.J. Vankelecom, Polypyrrole modified solvent resistantnanofiltration membranes, J. Membr. Sci. 320 (2008) 143–150.

[19] J.E. Kilduff, S. Mattaraj, J.P. Pieracci, G. Belfort, Photochemical modification ofpoly(ether sulfone) and sulfonated poly(sulfone) nanofiltration membranes forcontrol of fouling by natural organic matter, Desalination 132 (2000) 133–142.

[20] P. Mukherjee, K.L. Jones, J.O. Abitoye, Surface modification of nanofiltration mem-branes by ion implantation, J. Membr. Sci. 254 (2005) 303–310.

[21] M.N.A. Seman, M. Khayet, Z.I.B. Ali, N. Hilal, Reduction of nanofiltration membranefouling by UV-initiated graft polymerization technique, J. Membr. Sci. 355 (2010)133–141.

[22] X. Bai, Y. Zhang, H. Wang, H. Zhang, J. Liu, Study on the modification of positivelycharged composite nanofiltration membrane by TiO2 nanoparticles, Desalination313 (2013) 57–65.

[23] C. Li, Y. Wang, Y. Guo, X. Liu, Y. Guo, Z. Zhang, Y. Wang, G. Lu, Synthesis of highlyordered, extremely hydrothermal stable SBA-15/Al-SBA-15 under the assistance ofsodium chloride, Chem. Mater. 19 (2007) 173–178.

[24] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Ordered mesoporousmolecular sieves synthesized by a liquid-crystal template mechanism, Nature 359(1992) 710–712.

[25] D.Y. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredickson, B.F. Chmelka, G.D. Stucky,Triblock with periodic 50 to 300 angstrom pores, Science 279 (1998) 548–552.

[26] E.-S. Kim, B. Deng, Fabrication of polyamide thin-film nano-composite (PA-TFN)membrane with hydrophilized ordered mesoporous carbon (H-OMC) for waterpurifications, J. Membr. Sci. 375 (2011) 46–54.

[27] Y.Mansourpanah, S.S.Madaeni, A. Rahimpour, Preparation and investigation of separa-tion properties of polyethersulfone supported poly(piperazineamide) nanofiltrationmembrane using microwave-assisted polymerization, Sep. Purif. Technol. 69 (2009)234–242.

[28] Q. Li, X. Pan, C. Hou, Y. Jin, H. Dai, H.Wang, X. Zhao, X. Liu, Exploring the dependenceof bulk properties on surface chemistries and microstructures of commercially com-posite RO membranes by novel characterization approaches, Desalination 292(2012) 9–18.

[29] H. Zou, Y. Jin, J. Yang, H. Dai, X. Yu, J. Xu, Synthesis and characterization of thin filmcomposite reverse osmosis membranes via novel interfacial polymerizationapproach, Sep. Purif. Technol. 72 (2010) 256–262.