Chemical Engineering Journal 230 (2013) 260–271

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Chemical Engineering Journal

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Development of phosphorylated silica nanotubes(PSNTs)/polyvinylidene fluoride (PVDF) composite membranes forwastewater treatment

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.06.098

⇑ Corresponding author. Tel.: +86 22 27890470; fax: +86 22 27403389.E-mail address: zhangyuqing@tju.edu.cn (Y. Zhang).

Simeng Zhang b, Rongshu Wang a, Shaofeng Zhang b, Guoling Li b, Yuqing Zhang a,⇑a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR Chinab School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PR China

h i g h l i g h t s

� Phosphorylated silica nanotubes (PSNTs) were successfully prepared.� PSNTs were doped to PVDF to prepare a novel PSNTs/PVDF composite membrane.� The property of anti-fouling and anti-compaction of PVDF membranes are enhanced.� PSNTs/PVDF membranes are desirable in the treatment of wastewater.

a r t i c l e i n f o

Article history:Received 3 April 2013Received in revised form 21 June 2013Accepted 24 June 2013Available online 3 July 2013

Keywords:Phosphorylated silica nanotubesPVDF membranesAnti-foulingAnti-compactionWastewater treatment

a b s t r a c t

PVDF membranes are broadly applied in many fields owing to their good physicochemical stability, resis-tance to oxidation and chlorine. However when treating with wastewater, PVDF membranes are easilycontaminated by pollutant, degrading their properties due to the hydrophobicity and poor anti-compac-tion capabilities. It leads to declining flux and shorting lifespan of PVDF membranes, which further limitstheir large scale applications. In order to enhance the integrative capabilities of PVDF membranes, PSNTswere firstly prepared and then doped to PVDF to form a novel PSNTs/PVDF composite membrane througha phase inversion technique. The preparation conditions of PSNTs/PVDF composite membranes werestudied. And the effect of PSNTs on performance of composite membranes was investigated throughthe Fourier transform infrared (FT-IR), contact angle measurements, tensile strength measurements, fluxof membranes and oil retention ratio experiment. The results show that the contact angle of PSNTs/PVDFmembrane is declined from 68.0� to 43.2� and the tensile strength of PSNTs/PVDF membrane is improvedfrom 1.18 MPa to 2.76 MPa. When treating with wastewater containing oil (45 mg/L), the flux of PSNTs/PVDF composite membrane reaches 251 L/(m2 h) while the flux of PVDF membrane only is 152 L/(m2 h).Meanwhile, the oil retention for PSNTs/PVDF membrane is improved from 86.00% to 95.51%. Therefore,PSNTs/PVDF composite membranes are desirable in the treatment of wastewater containing oil andwastewater.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Membrane technology has been applied extensively in manyindustry sectors including environmental [1], electronic [2], energy[3], chemical and biotechnologies areas [4]. Currently, many effortsare devoted to improve performance of the existing membranes interms of the anti-fouling, anti-compaction and good separationproperty. PVDF membranes have received great attentions for itsthermal stability, high hydrophobicity and resistance to corrosionfrom many chemicals and organic compounds. However, PVDF

membranes have a severe tendency of fouling and permeabilitydecline due to the hydrophobic nature of PVDF, which has becomea great drawback for the application of PVDF membranes inmembrane bioreactor and treating wastewater. Hence, thepreparation of PVDF membranes with excellent properties is stilla challenge.

Recently, many methods such as surface modification, dopinghydrophilic polymers [5,6] and nano-inorganic particles [7–13]into casting solutions have been studied to enhance properties ofPVDF membranes. The strategy of doping inorganic oxide particlesto polymers to fabricate organic–inorganic composite membranesis particularly attractive owing to its simple preparationtechnology and obvious effect for enhancing membranes

S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271 261

capabilities. Inorganic particles such as ZrO2, Al2O3, TiO2, and SiO2

have been incorporated into PVDF membranes. When comparingthe TiO2/PVDF membrane with the PVDF membrane, K.Ebert foundthat TiO2 particles as inorganic fillers make the TiO2/PVDF mem-brane less susceptible to compaction during pressure treatmentat 30 bar [8]. By preparing ZrO2/PVDF composite membrane, A.Bot-tino [10] found that a large mass ratio of ZrO2/PVDF causes an in-crease of the permeate flux and a lowering of the retention fordextran solution. Moreover, many efforts have been taken to inves-tigate the effect of SiO2 particles on PVDF membranes. The hydro-philic properties of PVDF membranes were improved by dopingSiO2 particles into PVDF membranes [11–13], making PVDF mem-branes suitable for the ultrafiltration process of the wastewatercontaining oil. However, some deficiencies of this method restrictthe further improvement of PVDF membranes integrated proper-ties: few hydroxide radicals and Lewis acid sites of these inorganicparticles, inorganic particles dispersing among PVDF membranesonly by physical methods, poor consistent property between inor-ganic particles and polymers as well as agglomeration of dopedparticles [14]. To solve the problems above, Yuqing Zhang firstlyprepared Zr-doped nano-silica particles and sulfated Y-doped zir-conia particles and then doped into polysulfone (PSF) membranesto manufacture composite membranes, respectively. The strategyevidently improve hydrophilic property and anti-fouling abilityof PSF membranes [15–17]. These inorganic particles with oxygendefects show stronger activity and better hydrophilic capabilities.However, they are not rod-shaped or tubal materials with high ra-tio of length to diameter. Thereby, these inorganic particles couldneither be compatible with polymers along the direction of poly-mer chains well nor supply excellent anti-compaction capabilitiesto resist the outer impact forces (the pressure difference betweenthe high-pressure side and the low-pressure side of membrane).Therefore, it is significant to research and develop a sort of materialwith capabilities of being compatible with polymers along thedirection of polymer chains and excellent anti-compaction.

In recent years, one-dimensional structure materials especiallythe nano tubular-structured materials have received many atten-tions due to their unique properties and they have been appliedin many aspects such as condensed-matter physics, nanoscale de-vices, biosensors, and energy storage [18–20]. As novel tubularmaterials, silica nanotubes (SNTs) have attracted great interestsfor their hollow core/shell structures, high specific surface areasand high ratio of length to diameter as well as modifiable surfaceproperties. It is reported that SNTs have potential applications inorganic molecular separation, decomposition of pollutants andinorganic ion sensor etc. [21–24]. Therefore, SNTs with high ratioof length to diameter show the desirable potential in improvingproperties of PVDF membranes. Especially in the length directionof SNTs, there are many accessible active sites to be compatiblewith the PVDF polymer along the direction of polymer chains.They would enhance the hydrophilic and anti-fouling abilities ofthe PVDF membrane through hydrogen bonds, also may improvethe anti-compaction capability of PVDF membrane. Moreover, inthe diameter direction of SNTs, there is a unique circular wall withnanometer thickness, which can function as a new energy dissipa-tion approach to resist the outer impact forces. Thus, SNTs couldimprove the anti-deformation properties such as anti-outer impactforces and anti-outer pull of the PVDF membrane when SNTs aredoped to the PVDF polymer matrix. However, it still needs to over-come the agglomeration of SNTs via the chemical modification. Inview of it, we can suggest that SNTs with unique character of highratio of length to diameter after chemical modification will becomethe fitting doped materials for PVDF membranes.

In this paper, SNTs were firstly prepared, followed by thesilylation. Then, SNTs were modified by the phosphoric acid tosynthesize phosphorylated silica nanotubes (PSNTs) with more

functional hydroxyl active sites. Finally, PSNTs were doped to PVDFpolymers to manufacture a novel composite membrane (PSNTs/PVDF) through a phase inversion process. The optimum prepara-tion conditions of PSNTs/PVDF composite membranes are con-firmed and their properties were investigated.

2. Experimental

2.1. Materials

PVDF (1015) was provided by Solvay Company and its densitywas 1.78 g/cm3. PSNTs were prepared in our laboratory. BovineSerum Albumin (BSA) was purchased from Bio Basic Inc. N,N-Dimethylacetamide (DMAC) was purchased from Tianjin DamaoService of Chemical Instruments and (3-aminopropyl)triethoxysi-lane (APTES) was obtained from Chenguang coupling agent Co.Ltd. Polyvinyl pyrrolidone (PVP-K30, PVP-K17) was purchasedfrom Tiantai fine chemistry industry. These reagents were analyti-cal grade and used as received.

2.2. Synthesis of phosphorylated silica nanotubes (PSNTs)

Silica nanotubes (SNTs) were prepared by a template-guidanceroute, which is similar to the literature [25]. The modification pro-cess of SNTs was the following steps [26]: firstly, SNTs were addedto the de-ionized water followed by adjusting the pH to 4.0 whileadding APTES to form a mixing solution. After heating this mixingsolution at 80 �C for 4 h under stirring, SNTs modified by APTESwere obtained. Secondly, SNTs modified by APTES were added intothe mixture of formaldehyde and phosphorous acid. After stirringfor 30 min, they were refluxed at 80 �C for 6 h. Then, the resultingwhite precipitate was collected and washed to a pH of 7.0 usingdeionized water. The obtained gel was dried at 60 �C for 12 h andthen at 100 �C in vacuum for 8 h to finally form PSNTs.

2.3. PSNTs/PVDF composite membrane preparation

SiO2/PVDF, SNTs/PVDF and PSNTs/PVDF composite membraneswere prepared by doping SiO2, SNTs and PSNTs to PVDF castingsolution with the help of an ultrasonicator, respectively [27]. Here,we concretely describe the process of preparation of PSNTs/PVDFcomposite membranes. Initially, PSNTs were dispersed in DMACin a 500 mL reactor equipped with a mechanical stirrer, and a cer-tain amount of PVDF were then added and dispersed with stronglystirring. The solution was heated up to 60 �C in a water bath. Thendifferent kinds of porogens (PVP-K30 (MW 40,000), PVP-K17 (MW11,000), PEG-400 (MW 400) and LiCl (MW 42)) were added to pro-mote the formation of channels and pores of the composite mem-brane in the preparation process. PVP-K30, PVP-K17, PEG-400 andLiCl can be usually selected as porogens for preparing PVDF mem-branes, but then they have the different effect to form channels ofPVDF membranes. Therefore, a fitting porogen will be determinedfor preparing PVDF membranes through observing the effect ofthese porogens on the pure water flux and porosity of membranes.The casting solution was mixed with the help of ultrasonicator andthe reaction was processed at 60 �C constant temperature for 12 huntil a homogenous solution was obtained. The resulting solutionwas transferred into a wide neck flask with stopper and placedfor 24 h to remove bubbles. The solution without bubbles was thenpoured onto a dense glass plate to form a thin membrane. Afterevaporated for certain time in the air, the membranes (the mem-brane thickness approximately is 0.2 mm) were leached from theglass in the coagulation bath. The above similar procedure was fol-lowed for the preparation of SiO2/PVDF and SNTs/PVDF compositemembranes, only via doping SiO2 and SNTs, respectively. For

262 S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271

comparison, PVDF membranes without doping any materials wereprepared and the process was the same with the above procedure.Finally these membranes were stored in deionized water contain-ing 1 wt% formaldehyde to avoid the growth of bacteria.

2.4. Characterization

2.4.1. TEM studiesTEM images of SNTs and PSNTs were recorded on a JEM-100CX

II transmission electron microscope (JEOL Corporation, Japan) withthe operating voltage of 100 kV.

2.4.2. SEM studiesThe PSNTs/PVDF composite membrane pieces were thoroughly

rinsed by deionized water, and then dried in the vacuum freezedrier (Labconco Corporation, America) for SEM analysis. Cross-sec-tion samples were obtained by being freeze-fractured in liquidnitrogen and sputtered with gold. Cross-section of the samplewas observed under scanning electron microscope Nanosem-430(FEI Corporation, America).

2.4.3. FT-IR spectrum studiesThe FTIR spectra of materials and membranes samples were col-

lected on an Avatar 370 spectrometer (Thermo Nicolet Corpora-tion, America), using the potassium bromide pellet technique.Each spectrum was recorded after 32 scans at a resolution of4 cm�1.

2.4.4. Measurement of pure water flux and porosity of membranesIn order to choose a suitable porogen of composite membranes,

PVP-K30 (MW 40,000), PVP-K17 (MW 11,000), PEG-400 (MW 400)and LiCl (MW 42) were used as porogens in the experiment,respectively. It is known that both membrane contamination andmembrane compaction lead to flux changes. Pure water was usedas the feed solution in this paper, so the negative effect of mem-brane contamination on the flux of membrane could be eliminated.Therefore, effect of different porogen on permeation flux of com-posite membranes is evaluated by the pure water flux of thesemembranes, thereby a suitable porogen can be chosen. Here feedsolution is pure water; the pure water flux of membranes is the va-lue after flux achieved a constant under operating pressure of0.20 MPa. The pure water flux of membranes is calculated by thefollowing equation:

J ¼ VS � t ð1Þ

J is the permeation flux (L/(m2 h)), V the permeation volume (L), Sthe effective membrane area (m2) and t the operating time (h).Inaddition, porosity (Pr) of membranes is described by the followingequation:

Pr ¼WT �WD

qAl� 100% ð2Þ

A ¼ 14pd2 ð3Þ

WT is weights of membrane containing water at equilibriumswelling (g); WD the weights of membrane at dry state (g); A thearea of the membrane (cm2); l the thickness of the membrane(cm); d the average diameter of the membrane (cm); q the densityof water (g/cm3).

2.4.5. Measurement of hydrophilic property of membranesContact angles of membrane samples were measured by OCA20

Contact Angle System (Dataphysics Corporation, Germany). Theaccuracy of measurements is ±0.1�. A water droplet (2.0 lL) was

deposited on the membrane with a microliter syringe. Each valuewas obtained in 40 s after dropped water on the membranesurface.

2.4.6. Measurement of membranes retention for BSAThe retention of membrane were measured by the solution of

Bovine Serum Albumin (BSA; MW: 67,000 Da, average diameterof BSA molecule = 11 nm). BSA concentration in the permeationsolution was determined spectrophotometrically at 280 nm withthe SP-752 UV–visible spectrophotometer (Shanghai spectrumCorporation). The prepared BSA solution (0.5 mg/L) was utilizedwithin 6 h to minimize the aggregation or denaturation of BSA dur-ing storage. The rejection coefficient of BSA is evaluated using thefollowing equation:

R ¼ 1� CC0

� �� 100% ð4Þ

where R is the retention rate (%), C is the concentration of BSA in thepermeation (mg/L), C0 is concentration of BSA in the feed (mg/L).

2.4.7. Tensile strength of membranesDogbone-shaped specimen was cut out from a membrane sam-

ple. The specimen was tested with a M350 tensile testing machinesupplied by Testometric Corporation, England. Operating parame-ters were: the length of the specimen in gauge section, 25 mm;the width, 4 mm; the thickness, 0.20 mm; applied stroke speed,15 mm/min; measuring range, 0–3 MPa; Test temperature, 20 �C.

The elongation at break of PVDF membrane is calculated by thefollowing equation:

e ¼ L1

L0� 1

� �� 100% ð5Þ

where e is the elongation at break (%), L1 is the length of the spec-imen in gauge section after the PVDF membrane broken (mm), L0 isthe length of the specimen in gauge section before the PVDF mem-brane broken (mm).

2.4.8. Oil separation from waterIn order to observe and research the properties of membranes

for treating with wastewater containing oil, the model oil-in-wateremulsion was created by machine oil (SJ 10W-30, Tianjin FAW Co.China) and deionized water with vigorous stirring at 3000 rpmspeed over 60 min until a homogenous solution was obtained.The stability of the emulsion was observed visually over 24 h per-iod and the mixture maintained cloudy, turbid, indicating that oilexisted in emulsified and soluble condition. Diameter of the usedmembrane sample was 50 mm, and the thickness of the usedmembrane sample was approximately 0.2 mm. All experimentsof oil separation from water were carried out at 25 �C.

The oil concentration in the permeation was determined by UVspectrophotometer (PE Co. Ltd. America) at 225 nm [28,29] and theretention rate R is calculated by the following equation:

R ¼ 1� C2

C1

� �� 100% ð6Þ

R is the oil retention rate (%), C1 the oil concentration in the feedsolution (mg/L) and C2 the oil concentration in the permeation(mg/L).

3. Results and discussion

3.1. characterization of SNTs and PSNTs

Fig. 1 shows that TEM images of SNTs and scheme of PSNTsformation. TEM images the formation of silica nanotubes with

Fig. 1. TEM images of SNTs and scheme of PSNTs.

S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271 263

interior hollow structure. Based on the TEM characterization, theSNTs have interior hollow structure, the inner diameter of120 nm, wall thickness of 10–20 nm and length of 2–3 lm. TheFT-IR spectrograms of SNTs and PSNTs are shown in Fig. 2. Afterphosphorylation, more hydroxyl groups of SNTs were introduced,which makes the peak of –OH stretching and H–O–H bendingstronger; comparing the silanized SNTs with PSNTs, peak of –NH2

stretching on PSNTs (around 1550 cm�1) disappears because the–NH2 group linked with (CH2PO(OH)2) group after the Kabach-nik–Fields (K–F) reaction; peaks of P–O group stretching and bend-ing is shown around 937 and 532 cm�1 which indicates that thephosphorylated modification of SNTs were successfully completed.

3.2. Preparation of PSNTs/PVDF composite membrane

3.2.1. Effect of porogens on the performance of composite membranesPVP-K30 (MW 40,000), PVP-K17 (MW 11,000), PEG-400(MW

400) and LiCl(MW 42) were used as porogens in the experiment,

respectively. Porogens concentration in the casting solution is5 wt%, PVDF concentration in the casting solution 20 wt% andDMAC in the casting solution 75%; moreover, the mass ratio ofPSNTs/PVDF is 1:10. It can be seen from Fig. 3 that different poro-gens have different effects on the pure water flux of compositemembranes: namely PVP-K30 > PVP-K17 > PEG-400 > LiCl. WithPVP-K30 as the porogen, the flux and porosity of PSNTs/PVDF com-posite membranes are 407 L/(m2 h) and 48%, respectively. As a sortof water-soluble polymer, PVP-K30 could change the dissolvedstate of PVDF polymer in the casting solution and accelerate thephase-invasion speed of composite membrane during the processof membrane formation. Meanwhile, PVP with different molecularweights show varied impact on the performance of compositemembranes. When the molecular weight of PVP is bigger, namelythe PVP chains are longer, making the hindrance between PVPchains and PVDF polymer stronger. This result leads to more PVPleft inside PVDF membrane after the formation of PVDF compositemembranes, which is also confirmed in the literature [30,31]. Thus,

4000 3500 3000 2500 2000 1500 1000 500

Tran

smitt

ance

/ (%

)

Wavenumber /(cm-1)

3440

550

1100 800

465

532

937

2930

1640

1470

1470

1.silanized SNTs

2.SNTs

3.PSNTs

15503260

Fig. 2. FT-IR spectrograms of SNTs and PSNTs.

PVP-K30 PVP-K17 PEG-400 LiCl150

200

250

300

350

400

450Fluxporosity

Different kinds of porogens

Flu

x / L

/(m

2 h)

20

25

30

35

40

45

50 Porosity of com

posite mem

branes / %

Fig. 3. Effect of various porogens on the flux and porosity of PVDF compositemembrane. (Concentration of Porogens, PVDF and DMAC in the casting solution is5 wt%, 20 wt% and 75%, respectively; the mass ratio of PSNTs/PVDF is 1:10.)

14 16 18 20 22250

300

350

400

450

500

550

600

Flux

/ L/

(m2h)

Pure water permeation flux Porosity BSA retention

Concentration of PVDF (wt. %)

30

40

50

60

70

80

90

100

110

BSA retention and porosity / (%)

ig. 5. Effect of PVDF concentration on membrane flux, porosity of membranes andSA retention. (The mass ratio of PSNTs/PVDF is 1:10, the pre-evaporation time is0 s and the concentration of PVP-K30 in the casting solution is 5 wt%.)

250

300

350

400

450

500

550

Flux

of w

ater

per

mea

tion

(L/m

2 h)

Concentration of PVP-K30 / (wt%)

Flux of water permeation BSA retention

1 3 5 7 1070

80

90

100

BSA retention / (%)

Fig. 4. Effect of PVP-K30 concentration on the flux and BSA retention of PVDFcomposite membranes. (Concentration of PVDF and DMAC in the casting solution is20 wt% and 75%, respectively; the mass ratio of PSNTs/PVDF is 1:10.)

264 S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271

we selected PVP-K30 as the fitting porogen for preparing thecomposite PVDF membrane.

The PVDF concentration in the casting solution is 20 wt% andthe mass ratio of PSNTs/PVDF is 1:10. It can be indicated fromFig. 4 that the PVP-K30 concentration in the casting solution has

a bigger effect on the permeation flux and BSA retention rate ofPSNTs/PVDF membranes. It is observed that with the porogen con-centration changes from 1 wt% to 5 wt%, the flux of compositemembrane increases while the BSA retention changes a little be-tween 99.3% and 98.7%. When the porogen concentration increasesfrom 5 wt% to 10 wt%, the flux of the PSNTs/PVDF membrane alsoincreases, but the BSA retention of the PSNTs/PVDF membrane in-deed declines from 98.7% to 94.4%. This is because when the poro-gen concentration is smaller (from 1 wt% to 5 wt%) the betterchannels in PVDF membranes can be formed, namely with theporogen concentration changes from 1 wt% to 5 wt%, the channelsamount in composite membranes increases. So that the flux ofcomposite membrane increases whiles the BSA retention changesa little between 99.3% and 98.7%. In contrast, when the porogenconcentration is bigger (from 5 wt% to 10 wt%) the better channelsin PVDF membranes cannot be formed while many cavities andholes are formed, making the flux of the PSNTs/PVDF membraneincrease, but the BSA retention of the PSNTs/PVDF membrane in-deed decline from 98.7% to 94.4%. Thereby, it can be determinedfrom Fig. 4 that the fitting concentration of porogen PVP-K30should be 5 wt%.

3.2.2. Effect of PVDF concentration on the performance of compositemembranes

When the mass ratio of PSNTs/PVDF is 1:10, the pre-evapora-tion time of the composite membrane 10 s and the concentrationof porogens PVP-K30 in the casting solution 5 wt%, the effects ofPVDF concentration on performance of composite membraneswere investigated. It is shown from Fig. 5 that when the PVDFconcentration increases from 14 wt% to 22 wt%, the pure waterpermeation flux of the PSNTs/PVDF membrane declines from496 L/(m2 h) to 370 L/(m2 h). At the same time, the BSA retentionof the composite membrane firstly increases from 89.7% to 98.0%then slightly declines to 96.5%. This is mainly because the porosityof the PSNTs/PVDF membrane is affected by different materialsdoped into PVDF and the PVDF concentration. When the PVDFconcentration increases, the formed polymer matrix is very tightwhich leads to forming pores of small sizes. It can be seen fromTable 1 that these doped materials improve the porosity ofcomposite membranes, which is in accordance with descriptionsin the literature [32]. Moreover when the mass ratio of PSNTs/PVDF is 3:20, the porosity of the PSNTs/PVDF composite membraneis higher than that of the SNTs/PVDF membrane as well as that ofSiO2/PVDF membrane. It can be explained from these aspects be-low. It is reported that PVDF membranes prepared with PVP as

FB1

Table 1Porosity of different composite PVDF membranes.

Membrane Mass ratio of PSNTs/PVDF Porosity (%)

PVDF 0:100 37.6SiO2/PVDF-15 3:20 47.9SNTs/PVDF-15 3:20 60.8PSNTs/PVDF-10 1:10 50.3PSNTs/PVDF-15 3:20 67.2PSNTs/PVDF-20 1:5 57.2

Remarks:(1) PSNTs/PVDF-10: the mass ratio of PSNTs/PVDF is 1:10.(2) PSNTs/PVDF-15: the mass ratio of PSNTs/PVDF is 3:20.(3) PSNTs/PVDF-20: the mass ratio of PSNTs/PVDF is 1:5.(PVP-K30 concentration in the casting solution is 5 wt%, PVDF concentration is20 wt%, the pre-evaporation time 15 s, the operating pressure is 0.10 MPa, theoperating temperature is 25 �C.).

S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271 265

porogen and DMAC as solvent have macrovoids attributing to theporosity of membrane [33]. Comparing with the tubular SNTs,the porosity of the SiO2/PVDF membrane is less than that of theSNTs/PVDF membrane. This result suggests that the sphericalSiO2 particles are easily filled up into the macrovoids inside thePVDF membrane or the different interdiffusion ratio between sol-vent and nonsolvent. On the other hand, compatibility betweenPSNTs and PVDF polymer chains can be enhanced by modifyingPSNTs with APTES. Thus, PSNTs as effective doped materials signif-icantly improve the porosity of the PSNTs/PVDF composite mem-brane and the fitting PVDF concentration is 20 wt%.

3.2.3. Effect of PSNTs concentration on the performance of compositemembranes

When the pre-evaporation time of composite membranes is10 s, the PVDF concentration in the casting solution 20 wt% andPVP-K30 concentration 5 wt%, the effect of PSNT concentrationon the performance of PVDF composite membranes was investi-gated. For comparison, SiO2 particles and SNTs were added intoPVDF to fabricate SiO2/PVDF and SNTs/PVDF composite mem-branes, respectively. In Fig. 6, the effect of different materials onimproving the flux of PVDF membrane is PSNTs > SNTs > SiO2

particles. The flux of these composite membranes is bigger thanthat of PVDF membrane and the pure water flux of PSNTs/PVDFcomposite membrane is the biggest, indicating that PSNTs/PVDFcomposite membrane has the excellent porosity and water

150

200

250

300

350

400

450

500

550

600

Flux

/ L/

m2 h

Concentration of doped materials

SiO2/PVDFSNTs/PVDFPSNTs/PVDFPVDF

0:100 5:100 10:100 15:100 20:100

Fig. 6. Effect of SiO2 particles, SNTs and PSNTs on the flux of composite membranes.(The pre-evaporation time is 10 s, the PVDF concentration in the casting solution is20 wt%, PVP-K30 concentration in the casting solution is 5 wt%.)

permeate flux. The result can be interpreted as follows: PVDF is asort of hydrophobic polymer and its hydrophilicity can be im-proved significantly by doping SNTs with favorable characteristics,such as hydrophilicity, high specific surface areas and high ratio oflength to diameter. Moreover, it can be also seen from Fig. 6, thewater permeation flux of SiO2/PVDF and SNTs/PVDF compositemembranes increase to a peak value when the mass ratio of dopedmaterials/PVDF reaches 1:10; in contrast, it is shown from Fig. 6and Table 1, the water permeation flux and the porosity ofPSNTs/PVDF composite membrane just increase to a peak valuewhen the mass ratio of PSNTs/PVDF reaches 3:20. It is becausecomparing PSNTs with SiO2 and SNTs, PSNTs have more activegroups to be compatible with the PVDF polymer along the direc-tion of polymer chains. Meanwhile aggregation of PSNTs in PVDFpolymers can be indeed alleviated after modifying PSNTs withAPTES. Thus, the fitting mass ratio of PSTNs/PVDF is 3:20.

3.2.4. Effect of pre-evaporation time on the performance of compositemembranes

When the PVDF concentration in the casting solution is 20 wt%,PVP-K30 concentration 5 wt%, the effect of pre-evaporation timeon the performance of PSNTs/PVDF-15 composite membranes(the mass ratio of PSNTS/PVDF 3:20) was investigated. Table 2shows that with the pre-evaporation time extends, the pure waterflux of the PSNTs/PVDF membrane increases at first then declines.This is because when the pre-evaporation time is shorter, a densefilm layer on the membrane has not formed yet and the evapora-tion of the solvent helps to form more channels in the membrane.Thereby, the permeation flux of pure water increases at first as thepre-evaporation time extends within 15 s. However, after the pre-evaporation time exceeds 15 s, the dense film layer on the mem-brane has been formed while the surface becomes denser, stoppingthe diffusion of solvent and porogen from the inside of membraneto the coagulation bath. Thus it will be more difficult to form morechannels in the membrane, leading to the decline of the pure waterpermeation flux. Therefore, when the pre-evaporation time is 15 s,the flux of PSNTs/PVDF membrane reaches the biggest value of526 L/(m2 h) and the porosity is 66.9%, satisfying the separationrequirement. Hence, the fitting pre-evaporation time is 15 s.

3.3. Characterization of the PSNTs/PVDF composite membrane

In this section, characterizations of the PSNTs/PVDF compositemembrane (PSNTs/PVDF-15) fabricated under the fitting prepara-tion condition (PVP-K30 as the porogens and its concentration inthe casting solution 5 wt%, PVDF concentration 20 wt%, the massratio of PSTNs/PVDF 3:20 and the fitting pre-evaporation time15 s) were investigated and analyzed.

3.3.1. SEM characterization of the PSNTs/PVDF composite membraneFig. 7 shows the SEM images of the cross section of the compos-

ite membrane (the mass ratio of PSNTs/PVDF is 3:20). It can beobserved from the (a), (b) and (c) that PSNTs exist in the PVDFmembrane and the dispersion of PSNTs also is more uniform. In

Table 2Properties of composite membranes with different pre-evaporation times.

PSNTs/PVDF -15 membranes Pre-evaporation time (s)

0 10 15 20

Water permeation flux (L/(m2 h)) 350 439 526 463Porosity (%) 60.3 66.3 66.9 66.6

(PVP-K30 concentration in the casting solution is 5 wt%, PVDF concentration is20 wt%, the mass ratio of doped materials/PVDF is 3:20, the operating pressure is0.10 MPa, the operating temperature is 25 �C.).

Fig. 7. SEM images of the cross section of the membrane. (a–c) PSNTs/PVDF composite membranes; (d) SiO2/PVDF composite membranes; (e) SNTs/PVDF compositemembranes. (The mass ratio of doped materials/PVDF is 3:20, pre-evaporation time is 10 s, the PVDF concentration in the casting solution is 20 wt%, PVP-K30 concentration inthe casting solution is 5 wt%.)

266 S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271

addition, compared with (d) and (e), (c) also presents that theagglomeration of PSNTs is not obvious while many filamentaryconnections and bonds between PSNTs and PVDF polymers areformed. The phenomena can improve the compatibility betweenPSNTs and PVDF polymer chains, enhancing the integrativeproperties of PSNTs/PVDF-15 membranes.

3.3.2. FT-IR spectrum of membranesFig. 8 shows that the FT-IR spectrum of PVDF composite mem-

branes (the mass ratio of doped materials/PVDF is 3:20). In Fig. 8the intensity of O–H stretching peak and bending peak are en-hanced, indicating that more O–H bonds are formed by addingSNTs and PSNTs to PVDF membrane. It is reported that PVDF is a

Fig. 8. FTIR spectra of PVDF, SNTs/PVDF-15 and PSNTs/PVDF-15 compositemembranes.

Fig. 9. The interaction between SiO2 particles and PVDF chains.

S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271 267

sort of semi-crystal polymer, which is commonly formed in a non-polar a-phase with TGTG’ type of molecular chains, no in the polarb-phase with TT type of molecular chains [34]. And it is interestingto observe characteristic peaks of a-phase PVDF and b-phase PVDFcoexist in the spectrum of SNTs/PVDF-15 and PSNTs/PVDF-15 sam-ples. The reason for the appearance of b-phase PVDF suggests thatduring the formation of casting solution, SNT/TGTG’ and PSNT/TGTG’ microparticles are formed, which can act as a ‘‘seed’’ in thecrystallization process from a-phase to b-phase with the energyfrom sonication condition. Moreover, character peaks of PSNTsare not observed from the FT-IR spectrum because the mass ratioof PSNTs/PVDF-15 is only 3:20.

3.3.3. Tensile strength of membranesAs indicated in Table 3, the tensile strength of SiO2/PVDF-15,

SNTs/PVDF-15 and PSNTs/PVDF-15 composite membranes isbigger than that of PVDF membrane. The tensile strength of thePSNTs/PVDF-15 composite membrane is biggest, reaching2.76 MPa while the elongation at break is 22.4%. It is because underouter force effects (pull forces) crazes appear on the surface ofPVDF membrane without doping inorganic materials, making PVDFmembrane finally broken. For SiO2/PVDF composite membranes,SiO2 spherical particles would fill up into the macrovoids in thePVDF membrane, which decelerates the membrane developmentfrom crazes to cracks. Namely, with increasing tensile strengththe elongation rate of the membrane also increases. This is becauseunder outer force effects (pull forces) when tensile strength ofmembranes is bigger the elongation rate of the membrane also willbe bigger which can make membrane have the bigger elongationrate without being broken. It can be seen from Fig. 9 that OHgroups on the surface of SiO2 particles can interact with F atoms

Table 3Tensile strengths of different PVDF composite membranes.

PVDF membranes Mass ratio ofdoped materials/PVDF

Tensilestrength (MPa)

Elongationat break (%)

PVDF / 1.18 10.6SiO2/PVDF-15 3:20 1.52 12.0SNTs/PVDF-15 3:20 2.03 15.7PSNTs/PVDF-15 3:20 2.76 22.4

(PVP-K30 concentration in the casting solution is 5 wt%, PVDF concentration is20 wt%, the mass ratio of doped materials/PVDF is 3:20, the pre-evaporation time15 s the operating temperature is 25 �C.).

of PVDF chains through hydrogen bonds, which is stronger thanintermolecular forces between PVDF molecular chains. It isbecause SiO2 particle with stronger rigidity characteristic can resistthe effect from pull force; in contrast PVDF chains without rigiditycharacteristic can indeed not resist the effect from pull force. So thepull force resulting in the fracture of SiO2/PVDF-15 compositemembrane is bigger than the pull force resulting in the fractureof pure PVDF membrane, namely the fracture of SiO2/PVDF-15composite membrane is harder than that of pure PVDF membrane.Therefore, the tensile strength of SiO2/PVDF-15 composite mem-brane is stronger than that of pure PVDF membrane. On the otherhand, when the pull forces reach a critical value which can causepure PVDF membrane fracture, SiO2/PVDF-15 composite mem-brane is not fractured yet. But then, at this moment the elongationrate of pure PVDF membrane is equal to that of SiO2/PVDF-15 com-posite membrane. Afterwards, when the pull force is unceasinglyincreased to make SiO2/PVDF-15 composite membrane fracture,SiO2/PVDF-15 composite membrane will be stretched again,namely its elongation rate increases and tensile strength increasestoo. So that the elongation rate of SiO2/PVDF-15 composite mem-brane is bigger than that of pure PVDF membrane. Thereby, theelongation rate of the membrane increases along with increasingtensile strength. But owing to their spherical shape, SiO2 sphericalparticles disperse in the PVDF membrane as dots, not along withthe chain direction of PVDF polymers. Therefore, SiO2 particlesare not compatible with PVDF well, which limits the tensilestrength improvement of SiO2/PVDF-15 membranes. It can be alsoshown from Fig. 10 that in the diameter direction of SNTs, there is aunique circular wall with nanometer thickness, which can functionas a new energy dissipation approach to resist the outer forceeffects. It results in the higher tensile strength of SNTs/PVDF-15membrane than that of SiO2/PVDF-15 membrane. After modifica-tion by APTES, there are fewer agglomerations among PSNTs thanSNTs. Furthermore in the length direction of PSNTs, there are moreaccessible active sites to be compatible with the PVDF along thechain direction of PVDF polymers. It can enhance the compatibilitybetween PSNTs and PVDF polymer. Moreover, it can be seen fromSEM images that filamentous connections and bonds are formedbetween PSNTs and PVDF chains, making the PSNTs/PVDF-15 com-posite membrane has higher tensile strength and higher elongationat break.

3.3.4. Hydrophilic property of membranesContact angle measurement is based on the three-phase

equilibrium that occurs at the contact point of solid/liquid/vaporor solid/liquid/liquid interface [35]. In our experiment, the phases

Fig. 10. the sketch of force analysis for PSNTs/PVDF composite membranes.

268 S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271

are membrane/water/air and the contact angle is a measurementof wettability adsorbed from PVDF composite membranes. Thecontact angle can be interpreted as the hydrophobicity or

hydrophilicity of membranes, namely the smaller the contact angleis, the better hydrophilic property of the membrane is [17]. Thecontact angles between the water and PVDF composite membrane

Fig. 11. Contact angles of different PVDF composite membranes.

40 60 80 100 120 140 16060

80

100

120

140

160

180

200

220

240

260

Flux

/ (L

/(m2 h)

)

Oil concentration of the wastewater / (mg/L)

PVDF SiO2/PVDF-15 SNTs/PVDF-15 PSNTs/PVDF-15

Fig. 12. Effect of doped materials on the membrane flux for wastewater containingoil. (PVP-K30 concentration in the casting solution is 5 wt%, PVDF concentration is20 wt%, the mass ratio of doped materials/PVDF is 3:20, the pre-evaporation time15 s, the operating pressure is 0.10 MPa, the operating temperature is 25 �C.)

S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271 269

surface were tested and results were shown in Fig. 11. It can beobserved that the contact angle of PSNTs/PVDF-15 membranereaches 43.2�, smaller than that of SiO2/PVDF-15 membrane,SNTs/PVDF-15 membrane and PVDF membrane. It is indicated thatthe hydrophilic property of PSNTs/PVDF-15 composite membraneis better than that of SNTs/PVDF-15 membrane, as well as SiO2/PVDF-15 and PVDF membrane. This is because there are more ex-posed hydroxide radicals and phosphate groups on the surfacestructure of PSNTs than SNTs and SiO2 particles. Especially, in thelength direction of PSNTs, more accessible active sites are compat-ible with the PVDF along the chain direction of PVDF polymers asmuch as possible. It would enhance the hydrophilicity of the PVDFmembrane through hydrogen bonds between PSNTs and PVDFpolymers. Thus when PSNTs were doped in the PVDF membrane,hydrophilic property of the PSNTs/PVDF-15 composite membranewas improved evidently. In addition, according to the analysis re-sult of FT-IR spectrum, there is polar b-phase PVDF coexist withnon-polar a-phase PVDF in the PSNTs/PVDF-15 membranes, whichwould affect the polarity of composite membranes and improvethe hydrophilicity of PVDF composite membranes.

3.3.5. Separation properties of membranes for wastewater containingoil

In order to observe and research the properties of various mem-branes (PVDF membrane and three types of composite membranes,there into the mass ratio of doped materials/PVDF is 3:20), mem-branes are used to treat wastewater containing oil, determiningthe optimum operating conditions. Fig. 12 shows the comparisonof permeation flux among the PVDF membrane, SiO2/PVDF-15composite membrane, SNTs/PVDF-15 composite membrane andPSNTs/PVDF-15 composite membrane when treating with waste-water containing oil of different concentrations. The permeationflux of the PSNTs/PVDF-15 composite membrane is the biggestamong these membranes. It is attributed to the following reasons:(1) PSNTs have good compatibility with PVDF polymers, enhancingthe hydrophilicty of the PSNTs/PVDF-15 composite membrane. (2)

The PSNTs/PVDF-15 composite membrane has good anti-foulingcapability: a pure water layer could be formed on the surface ofcomposite membrane owing to plenty of hydroxyl groups on thesurface of the composite membrane. Oil droplets can be blockedoff and then easily washed off by the running water. Therefore,the PSNTs/PVDF-15 composite membrane has a higher permeationflux than other membranes.

The oil retention of membranes can be applied to judge whetherPSNTs/PVDF composite membranes are effective materials toremove oil from water. PVDF membrane, SiO2/PVDF-15 compositemembrane, SNTs/PVDF-15 composite membrane and PSNTs/PVDF-15 composite membrane were used to process wastewatercontaining oil of different concentrations at the operating pressureof 0.10 MPa and at 25 �C. Fig. 13 and Table 4 show results of vari-

0

2

4

6

8

10

12

14

16

Oil

conc

entra

tion

in th

e pe

rmea

tion/

(mg/

L)

120 16075

Oil concentration in the feed solution/ mg/L45

PVDF SiO2/PVDF-15 SNTs/PVDF-15 PSNTs/PVDF-15

Fig. 13. Results about different PVDF composite membranes for oil separation.(PVP-K30 concentration in the casting solution is 5 wt%, PVDF concentration is20 wt%, the mass ratio of doped materials/PVDF is 3:20, the pre-evaporation time15 s, the operating pressure is 0.10 MPa, the operating temperature is 25 �C.)

Fig. 14. Effect of operating pressure on permeation flux of PSNTs/PVDF-15composite membranes. (PVP-K30 concentration in the casting solution is 5 wt%,PVDF concentration is 20 wt%, the mass ratio of doped materials/PVDF is 3:20, thepre-evaporation time 15 s, the operating temperature is 25 �C.)

Table 4Oil retention rate of different PVDF composite membranes.

Coil of thefeed(mg/L)

R for PVDFmembrane(%)

R for SiO2/PVDF-15membrane (%)

R for SNTs/PVDF-15membrane (%)

R for PSNTs/PVDF-15membrane (%)

45 86.0 91.2 92.6 95.575 87.0 93.5 94.4 97.1

120 90.3 94.7 95.8 98.1160 91.5 95.2 96.1 98.4

Remarks: Coil, the concentration of oil; R, the retention; the mass ratio of SiO2/PVDF3:20; the mass ratio of SNTs/PVDF 3:20; the mass ratio of PSNTs/PVDF 3:20(PVP-K30 concentration in the casting solution is 5 wt%, PVDF concentration is20 wt%, the mass ratio of doped materials/PVDF is 3:20, the pre-evaporation time15 s, the operating pressure is 0.10 MPa, the operating temperature is 25 �C.).

270 S. Zhang et al. / Chemical Engineering Journal 230 (2013) 260–271

ous membranes for separation of oil from wastewater. When theoil concentration of the feed solution increases from 45 mg/L to160 mg/L, the oil concentration in the permeation of PSNTs/PVDF-15 composite membrane change a little and the oil retentionrate for PSNTs/PVDF-15 composite membrane is improved from95.5% to 98.4%. Oil droplets hard pass through the channels ofthe membrane due to hydrophilic property and character of easilyforming hydrogen bonds of PSNTs/PVDF-15 composite membrane,thereby reaching a better separation result.

Based on above analysis and discussion the effect of the operat-ing pressure and time on the flux of PSNTs/PVDF-15 compositemembranes was investigated. Fig. 14 shows that the effect of oper-ating pressure on permeation flux of PSNTs/PVDF-15 compositemembranes and variation relationship of permeation flux withtime (oil concentration of the feed solution is 45 mg/L, temperature25 �C). As can be seen from Fig. 14, the permeation flux declinesrapidly at initial stage, and the rate of declination slows down withoperating time increases, when the time reaches 1.2 h the perme-ation flux can be considerate as a constant, namely a steady flux.When operating pressure changes from 0.05 MPa to 0.10 MPa,the steady flux reaches 225 L/(m2 h), a maximum; however, thesteady flux appears downtrend after operating pressure exceeds0.10 MPa. This is because before operating pressure reaches0.10 MPa, the increase of operating pressure has a positive effecton the increase of permeation flux due to the increase of drive.But when operating pressure exceeds 0.10 MPa, it is suggested thatchannels are blocked by oil droplets for the compressible propertyof oil droplets, making the permeation flux declines. But then the

quality of permeation of PSNTs/PVDF-15 composite membranessatisfies the A1 recycle standard of the Chinese oil-field (SY/T5329-94, oil concentration <5 mg/L), thereby PSNTs/PVDF-15 com-posite membranes have potential applications in removing oil fromwastewater.

4. Conclusions

TEM images and FT-IR spectrums show that phosphorylated sil-ica nanotubes were successfully synthesized. The fitting prepara-tion conditions of PSNTs/PVDF composite membrane are:porogens PVP-K30 and its concentration in the casting solution5 wt%; PVDF concentration in the casting solution 20 wt%; themass ratio of PSNTs/PVDF 3:20; the pre-evaporation time of com-posite membranes 15 s. There are many accessible active sites inthe length direction of PSNTs, which makes PSNTs compatible withthe PVDF polymer along with the direction of polymer chains asmuch as possible. The hydrophilicity of PSNTs/PVDF-15 compositemembranes is enhanced. Moreover, there is a unique circular wallwith nanometer thickness in the diameter direction of PSNTs,which provides a new energy dissipation approach to resist theouter force. According to the result analysis of the hydrophilicity,tensile strength, the quality of permeation and oil retention rate,PSNTs/PVDF-15 composite membranes have potential applicationsin the treatment of wastewater containing oil and wastewater.

Acknowledgements

This project is supported by National Natural Science Founda-tion of China (No. 21076143), by RAEng Research Exchanges withChina and India award supported by UK Royal Academy of Engi-neering (No. 2012-5502), by The project funded by the Key Labora-tory of Inorganic film materials, Chinese Academy of Sciences (No.KLICM-2011-07), by the Basic Research of Tianjin Municipal Sci-ence and Technology Commission (13JCYBJC20100), by the Pro-gram of Introducing Talents of Discipline to Universities (No.B06006).

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