polyethersulfone composite ultrafiltration membrane with enhanced antifouling property

Accepted Manuscript Title: Hollow mesoporous silica spheres/polyethersulfone composite ultrafiltration membrane with enhanced antifouling property Author: Qian Li Shunlong Pan Xin Li Chao Liu Jiansheng Li Xiuyun Sun Jinyou Shen Weiqing Han Lianjun Wang PII: DOI: Reference:

S0927-7757(15)30258-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.09.070 COLSUA 20206

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

16-8-2015 27-9-2015 29-9-2015

Please cite this article as: Qian Li, Shunlong Pan, Xin Li, Chao Liu, Jiansheng Li, Xiuyun Sun, Jinyou Shen, Weiqing Han, Lianjun Wang, Hollow mesoporous silica spheres/polyethersulfone composite ultrafiltration membrane with enhanced antifouling property, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.09.070 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.

Hollow mesoporous silica spheres/polyethersulfone composite ultrafiltration membrane with enhanced antifouling property Qian Li, Shunlong Pan, Xin Li, Chao Liu, Jiansheng Li* [email protected], Xiuyun Sun, Jinyou Shen, Weiqing Han, Lianjun Wang* [email protected] Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, School of Environment and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China *Corresponding author. Tel/Fax: +86 25 84315351.

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Graphical Abstract

Water HMSS -OH group Water flow

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Highlights  Hollow mesoporous silica spheres (HMSS) are incorporated in PES ultrafiltration membrane.  The immobilization of HMSS in membrane can improve its hydrophilicity.  The unique hollow structure of HMSS endows the membrane with improved permeability.  HMSS/PES composite membrane exhibits enhanced organic antifouling property.

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Abstract The growing attention to ultrafiltration (UF) processes from drinking water treatment raises the development of restrain strategy for membrane fouling. In this study, a simple method for fabricating fouling-resistant polyethersulfone (PES) UF membranes is described. The hydrophilic hollow mesoporous silica sphere (HMSS) prepared via surfactant-assembly sol-gel route were blended into PES membranes to enhance the filtration property and fouling resistance. The hydrophilicity of the membrane surface was improved by the integration of HMSS, leading to a reduction in membrane contact angle. When incorporating 1.5% HMSS into the casting solution, the pure water flux reached the maximum (195.7 L/m2 h) under 0.1 MPa while still remained the high rejection to BSA (92.6%). Furthermore, the fouling resistance, especially the irreversible fouling resistance was improved for membrane with 1.5% HMSS compared with the pristine PES membrane using BSA as the model foulant. However, as continue to increase the concentration of HMSS up to 2%, no benefit was brought to the PES membrane owing to particle agglomeration and pore blockage. Furthermore, the UF experiment confirmed that the unique hollow structure of HMSS endowed the UF membrane with improved pure water permeability, comparing with the mesoporous silica spheres (MSS) with solid cores. These characteristics provided the further wide application of composite UF membrane in the drinking water treatment area. Keywords: Ultrafiltration; Composite membrane; Hollow mesoporous silica; Antifouling.

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1. Introduction Polyethersulfone (PES) is a well-known transparent amorphous polymer for its excellent heat deflection temperatures, excellent toughness, dimensional stability, and resistance to steam, boiling water, and mineral acids [1]. These advantages make PES to be one of the most popular polymer for the fabrication of ultrafiltration (UF) membranes which have been widely used in drinking water treatment and wastewater reuse areas [2, 3]. However, the inherent hydrophobic nature of PES leads to the membrane fouling which can result in reduced productivity, additional operating costs, and the need for frequent chemical cleaning that shortens membrane lifespan [4, 5]. Therefore, developing hydrophilic and antifouling membranes are urgently needed to further extend the applications of membrane-based processes for water purification. Several methods have been reported to improve the hydrophilicity of polymeric membranes, including surface coating [6], grafting [7] and blending hydrophilic or functionalized fillers [8]. Among the methods mentioned above, blending inorganic particles has attracted great attention owing to its simplicity and practicability. The inorganic particles can provide specific functionalities to polymeric matrix, while retaining the intrinsic separation performance of the bare membrane. Enhancing the hydrophilic nature of the membrane is assumed to yield a better performance in terms of permeability, antifouling properties and solute rejection [9]. Currently, a variety of inorganic particles have been introduced into polymeric matrix to modify UF membranes, such as Al2O3 [10], SiO2 [11], TiO2 [12], ZrO2 [13], Fe3O4 [14], ZnO [15] and Ag [16]. Among them, SiO2 is the most favorable inorganic additive because of its cost, good size controllability and surface reactivity [17]. Various meaningful works have confirmed that the addition of solid silica particles is an effective way to modify the properties of polymeric membranes, endowing the membranes with good selectivity, thermal ability and mechanical strength [18-21]. Currently, the rapid development of mesoporous material owing to its high specific surface area, tunable pore structures, and well-defined surface property for modification has made it possible to prepare mesoporous silica/polymer membrane. The incorporated mesoporous silica endows the membranes with enhanced permeability without sacrificing their selectivity due to its good compatibility with the polymer matrix. Besides, it can also improve the membrane thermal stability and fouling resistance [11, 22-27]. Compared with solid additive, hollow material with controllable inner core size, wall thickness and low density was considered as a more promising candidate for 5

numerous applications such as chemical/drug delivery, catalyst and membrane separation [17]. The closed space in the lumen of hollow materials allows the creation of a new mini-world where the microenvironment is less affected by the external surroundings. The mass and energy exchange between the internal world and external world can be easily manipulated by tuning the hierarchical structures [28]. Hollow silica spheres were firstly employed into Nafion proton exchanging membranes to enhance water retention [29]. Micron-sized hollow spheres with silicalite-1 shell were also used as inorganic fillers in PDMS-based mixed matrix membranes which indicating enhanced solvent permeability [30]. Recently, the promoting effect of hollow silica material was also found in drinking water treatment and wastewater reuse areas, which its unique structure provides the shortcut for the pass of water molecules. Hollow mesoporous silica spheres (HMSS) prepared via the hard template method were introduced into brominated polyphenylene oxide (BPPO) UF membrane matrix. The composite membrane exhibited improved pure water permeability and water content, while maintaining a high rejection rate to egg albumin [17]. However, the high cost and difficult process molding of BPPO polymer limited its further application in water treatment areas. Therefore, to introduce HMSS into well-established polymeric UF system is of great significance. In this work, a simple method for fabricating HMSS/PES composite UF membranes with enhanced fouling-resistant performance is described. The preformed HMSS fillers were incorporated into PES/PVP/DMF casting solution system and the morphology, surface hydrophilicity, filtration performance of the prepared membranes were evaluated. Moreover, the effect of hollow structure on UF performance of PES membranes was investigated. 2. Experimental 2.1 Materials Polyethersulfone (PES Ultrason E6020P with Mw=58 kDa) was supplied by BASF Company (Germany). Polyvinylpyrrolidone (PVP, K-30), bovine serum albumin (BSA, Mw=67 kDa) protein, anhydrous ethanol, concentrated ammonia aqueous solution (25 wt %) and tetraethoxysilane (TEOS) were purchased from Sinopharm Chemical Reagent Co., Ltd. Cetyltrimethylammonium bromide (CTAB) was purchased from Chengdu Kelong Chemical Co., Ltd. N,N-dimethylformamide (DMF) and isopropanol were obtained from Nanjing Chemical Reagent Co., Ltd. Deionized water purified with a Millipore Elix Water Purification system was used in all the experiment. All the chemicals 6

were of commercially analytical grade and used without further purification. 2.2 Synthesis of silica spheres Mesostructured silica spheres were synthesized via a surfactant-assembly sol-gel process in a solution [31]. Typically, CTAB was dissolved in ethanol aqueous solution containing concentrated ammonia aqueous solution (1 mL, 25 wt %). Then, the mixture was heated to 35 ◦C, and TEOS (1 mL) was rapidly added under vigorous stirring. The molar ratio of the reaction mixture was 1.00 TEOS: 0.0922 CTAB: 2.96 NH3: 621H2O: 115 C2H5OH. After stirring at 35 ◦C

prepare the hollow mesoporo

◦

-

C for 12 h

and then collected by centrifugation and washed three times with ethanol. To remove the pore-generating template (CTAB), the as-synthesized materials were transferred to an ethanol HC

μ

◦

3 %

C for 3 h. The

surfactant extraction step was repeated two times to ensure complete removal of CTAB. The template-removed hollow mesoporous silica spheres were washed with ethanol three times and dried under high vacuum. Mesoporous silica spheres (MSS) were prepared as same as HMSS just without the incubation step at 70 ◦C for 12 h to keep solid cores existent. 2.3 Preparation of PES composite UF membranes The HMSS/PES composite UF membranes were prepared via non-solvent induced phase separation (NIPS) process. The composition of casting solutions are shown in Table 1 and the detailed procedure is listed as follows: Firstly, 4 g PES and 1 g PVP were added in 12.2 mL DMF and the mixture was stirred vigorously at 70 ◦C. At the same time, certain amount of HMSS were added into 9 mL DMF and then supersonicated in bath (KH5200DE, 70 kHz, China) for 1 h to guarantee its well dispersion. Next, the part of DMF containing HMSS was added to the polymeric solution. Subsequently, the casting solution was subjected to vigorous stirring at 70 ◦C for 24 h. The obtained uniform casting solution was supersonicated again for 2 h and then degassed at room temperature. Afterwards, the solution was superadded onto a clear glass plate with an automated film applicator (MRX-TM300, Shenzhen, China)

3

μ

A

s evaporation time in air, the

glass plate was immediately immersed into the coagulation bath with a mixture of 2500 ml water 7

and 250 mL isopropanol at 25 ◦C. Finally, the prepared membranes were taken out from the coagulation bath and kept in deionized water overnight for fully exchanging of solvent and non-solvent. The fabricated membranes were labeled as PES-0.5, PES-1, PES-1.5 and PES-2 respectively. The numbers in membrane ID denote the corresponding weight percentages of HMSS relative to the weight of PES. For comparison, the pure PES membrane (labeled as PES-0) was prepared on the same condition just without the addition of HMSS. Moreover, in order to explore the effect of hollow structure on UF performance of PES membranes, same mole numbers MSS as HMSS in PES-1.5 were incorporated into PES matrix to prepare a MSS/PES composite membrane, which labeled as PES-MSS. 2.4 Characterization 2.4.1 Characterization of silica spheres The morphology and structure of HMSS and MSS were observed by a scanning electron microscope (SEM, JSM-6380, JEOL, Japan) and transmission electron microscope (TEM, JEM-2100, JEOL, Japan). The surface area was calculated by the Brunauer Emmett Teller (BET) method, and the pore size distribution was calculated by the Barrett Joyner Halenda (BJH) method. 2.4.2 Characterization of membranes Viscosities, the rheological characteristic of casting solutions, were measured using a rotating viscometer (Brookfield LVDV-IIþP, USA) with a rotating rate of 20 rpm at a constant temperature of 25 ◦C. To ensure reproducibility of the data, each casting solution was tested three times. Thermo gravimetric analysis (TGA, SDT Q600 Simultaneous DSC-TGA, TA, USA) was applied to manifest the successfully addition of silica particles into the PES matrix and it was implemented from room temperature to 700 ◦C with a heating rate of 10 ◦C per minute under a flow of 100 mL/min of nitrogen. The cross sections of the obtained membranes were observed by scanning electron microscope (SEM, JSM-6380, JEOL, Japan). The samples were dried in air by keeping them between tissue papers. Before SEM analysis, membranes were immersed in liquid nitrogen and then fractured. Moreover, all samples were sputtered with platinum prior to examination. SEM-EDX (energy dispersive X-ray) mapping was used to monitor the presence and distribution of HMSS in the membrane matrix of PES-1.5. PHI Quantera II X-ray photoelectron spectrometer (XPS, PHI, USA) was also used to determine the presence of incorporated HMSS. 8

The surface hydrophilicity of all the membranes was characterized by sessile drop contact angle measurements using a drop shape analysis system (Krüss DSA30, Germany). A water droplet of 3 uL was deposited on the top surface of dry membrane and the contact angles were determined. To minimize the measurement error, a total of five replicates were taken and averaged. 2.5 Evaluation of the filtration and antifouling properties The performances of prepared membranes, including flux (J), BSA retention (R) and resistance to fouling, were measured on a home-made cross-flow filtration apparatus with the effective area of 12.56 cm2 and a cross-flow velocity of 0.35 m/s. The feed pressure on the membrane was initially set to 0.15 MPa. After 30 min pre-compacted with deionized water, the flux was steady and then the pressure was lowered to the operating pressure of 0.1 MPa. The flux of pure water (J0) was calculated by Eq. (1).

J0 

V AΔt

(1)

Where J0 (L/m2 h) is the pure water flux, V (L) is the volume of permeated water, A (m2) is the effective membrane area and Δt (h) is the permeation time. After the filtration of pure water, a 1 g/L BSA solution in phosphate buffer solution (PBS, pH=7.4) as a substitution, was forced to permeate trough membrane at the same condition to water flux measurement. The flux of BSA was recorded as J1. In addition, the concentration of BSA both in feed and permeate solution was measured with an UV-vis spectrophotometer (Lambda25, PerkinElmer, USA) at 280 nm. The rejection of BSA was calculated by Eq. (2).  C  R  1  p   100%  C  f  

(2)

Where Cp and Cf are the concentration of BSA in permeate and feed solution respectively. To minimize experimental error, the measurements of water permeation and rejection were repeated at three different membranes respectively to obtain the average. Afterwards, the membrane contaminated by BSA was back washed with deionized water for 20 min and then the water flux of cleaned membrane (J2) was measured. Generally speaking, the antifouling property of membranes was evaluated by flux recovery ratio (FRR) which can be defined as Eq. (3).

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J  FRR   2   100%  J0 

(3)

The total protein fouling (Rt) of membrane bringing about the flux loss can be divided into two parts: reversible (Rr) and irreversible (Rir) fouling. They can be defined by the following Eq. (4), Eq. (5) and Eq. (6) [12].

 J  Rt  Rr  Rir  1  1   100%  J0 

(4)

 J  J1    100% Rr   2  J0 

(5)

 J  J2    100% Rir   0 J 0  

(6)

3. Results and discussion 3.1 Characterization of synthetic silica particles SEM was used to investigate the morphology of the obtained HMSS and MSS. As shown in Fig. 1, both the two kinds of silica particles represented the spherical morphology with smooth surface and their uniform diameter is about 650 nm. TEM was employed to further investigate the interior construction of the as-synthesized materials and the images corresponding to HMSS and MSS are shown in Fig. 1 (c-d) respectively. It can be observed that both HMSS and MSS have an average diameter of about 650 nm, which is consistent with the results of SEM. In addition, the HMSS have a mean shell thickness of 80 nm while MSS are observed with solid cores. Fig. 2a displays the nitrogen adsorption-desorption isotherms of HMSS and MSS. Both the two isotherms exhibit inflection characteristic of capillary condensation within the pores [32], where the p/p0 position of the inflection points is related to the uniform mesopores, similar to MCM-41 type mesoporous silica nanoparticles [33-35]. The BET surface area and pore volume of HMSS are 860 m2/g and 0.69 cm3/g while that of MSS are 1196 m2/g and 0.93 cm3/g. Besides, pore size distribution derived from desorption branch for the two samples are shown in Fig. 2b. It can be seen that HMSS show a primary mesoporous size peaking at 2.6 nm which is a little larger than that for MSS centered at 2.3 nm. All the results indicated that the silica particles were successfully prepared and they may provide the prolonged effect on the performances of PES membrane, which will be discussed in the following sections. 10

3.2 Viscosity measurement of casting solutions As an important rheological parameter during the formation process of the membrane, viscosity of the casting solution was measured at 25 ◦C with a rotating viscometer. Viscosity determines the exchanging rate of solvent and non-solvent, which plays an important role in the microstructure and performances of the membrane [36]. As shown in Table 1, the viscosity of the casting solution increased with the addition of HMSS content and climbed to 760.8 cp when the HMSS content was 2%. It can be attributed to the addition of HMSS in the PES solution increased the concentration of casting solution, consequently intensifying the interaction force among PES macromolecules. Similarly, incorporation same mole numbers MSS into PES as HMSS in PES-1.5 can lead to higher concentration and then higher viscosity of the casting solution for PES-MSS than that for PES-1.5. In addition, the hydroxyl groups of silica spheres may have a strong adsorption on polymeric trains, thereby resulting in the increase of viscosity for casting solution system [36]. The increased viscosity could have a profound effect on the morphology and filtration performance, as will be discussed in the following section. 3.3 Thermal behavior of the prepared membranes TGA measurement was applied to manifest the presence of SiO2. From the TGA curves shown in Fig.3, it can be found that there was a small mass loss between 25-400 ◦C for all the membranes. This is caused by the loss of the adsorbed water and/or the residual DMF solvent (its boiling point is 153 ◦C) within the membranes [11]. Then the sample masses decreased sharply when temperature ranged from 400 to 700 ◦C owing to the polymer combustion [11]. The residual masses after TGA measurement were 36.7%, 37.6%, 38.8%, 39.2% and 39.6% for membranes PES-0, PES-0.5, PES-1, PES-1.5 and PES-2 respectively. The increase of the residual weight should confirm to the amount of HMSS added and this can further prove the successfully incorporation of HMSS in the PES membrane. In addition, the residual mass of PES-MSS is 39.5% which was almost as same as that of PES-1.5, indicating the leaching of silica particles in PES-MSS membrane was more than that in PES-1.5 membrane during the NIPS process. 3.4 Morphology of PES composite membranes The SEM images of cross sections for the virgin and silica spheres blended membranes are shown in Fig.4. I ’

k

ed a typical structure of UF

membrane with a dense skin layer and a porous support layer. However, with different HMSS 11

loadings, the cross sections of membranes presented significant changes on microstructure. The formation of macrovoids was promoted in the membrane matrix when increasing the HMSS content up to 1.5% and then obviously suppressed at higher concentration of HMSS. This can be interpreted that the exchanging rate between solvent and non-solvent was accelerated when adding hydrophilic HMSS into the casting solution, resulting in the formation of macrovoids [8]. Nonetheless, with more HMSS, high viscosity slowed down the demixing rate during NIPS process leading to the suppression of macrovoids. A similar phenomenon was found when adding hydrophilic carbon nanotubes into polymer membranes [37-40]. Moreover, the macrovoids in the PES-MSS membrane matrix was also suppressed as compared with PES-1.5. Since that same mole numbers MSS can lead to higher casting solution viscosity than HMSS, the demixing rate during NIPS process for PES-MSS was slower which was similar with PES-2 as discussed above. Looking at both the top and bottom areas of HMSS/PES membranes taken at higher magnification, it can be seen that HMSS were embedded in the PES membrane matrix and with lower HMSS loading, the particles were dispersed well. However, when incorporating more HMSS up to 2%, some agglomerates of HMSS can be found in the membrane matrix which may have a negative effect on the membrane properties. SEM-EDX analysis of PES-1.5 membrane at cross section is shown in Fig. 5b. The characteristic Si peak was observed at 1.8 keV [41], indicating that SiO2 was successfully immobilized into the PES matrix. In addition, corresponding EDX mapping scanning spectra of carbon (C), oxygen (O), I’

sulfur (S), silicon (Si) are shown in Fig. 5 (c-

HM

well dispersed in the PES-1.5 membrane matrix from Fig. 5f. The well dispersion of HMSS in the membrane matrix can be attributed to the spherical shape of HMSS which can limit the contact between silica particles and the sphere size providing a relative low external surface area to volume ratio [42]. It has been recognized that finely dispersed particles are effective additives for membrane modification, and synergism properties between the polymeric materials and particles can usually be generated [43, 44]. XPS technique was used to investigate the chemical structures of pure and composite membrane and the results are shown in Fig.6. The mutual elements of two kinds of membranes are C, O, S, N PE

PVP H

’

new signal attributed to the Si element appearing in the composite membrane which confirmed SiO2 12

is successfully introduced in the PES membrane [45]. Furthermore, the membrane morphology and filtration performance of obtained membranes can be affected. 3.5 Hydrophilicity of prepared membranes Surface hydrophilicity is one of the most important factors to the liquid filtration membrane and much attention has been paid to it [46, 47]. Water contact angle (CA), as a regular method, is usually used to characterize the hydrophilicity and wetting property of solid surface [48]. As depicted in Fig.7, the CA of pure PES membrane was 76.8◦, indicating the strong intrinsic hydrophobicity of pure PES membrane. With the increase of HMSS loading (up to 1.5%), the CA of PES composite membrane first decreased to 63.8◦, confirming the improvement of hydrophilicity for PES composite membrane. During the synthesis of HMSS, hydrophilic hydroxyl group formed around the HMSS and part of the hydrophilic silica particles were prone to segregate to membrane surface via the NIPS process, which endowed the membranes with improved hydrophilicity [49-51]. However, with the further addition of HMSS to 2%, the CA of HMSS/PES membrane presented the ascending tendency. The possible reason is that the amount of hydroxyl groups decreased owing to the aggregation between HMSS particles. Besides, the CA of PES-MSS was smaller than that of PES-1.5. This may be caused by the higher viscosity of casting solution for PES-MSS which hindered the migration of MSS to the membrane surface. Overall, the best hydrophilicity of PES membrane with 1.5% HMSS is expected to result in high antifouling performance. 3.6 Membrane permeability and BSA rejection The pure water flux and BSA rejection for the pristine PES membrane and HMSS/PES composite membranes were all carried out with a home-made cross-flow filtration apparatus under 0.1 MPa. As show in Fig.8, the pure water flux firstly increased with fraction of HMSS in the membrane matrix up to 1.5% and then decreased at a fraction of 2%. A maximum value of 195.7 L/m2 h presented for PES-1.5 was almost increased by five times compared with the minimum value of 38.0 L/m2 h presented for PES-0. Three factors listed as follows may be responsible for changing in the water permeability. Firstly, rich hydroxyl groups on the surface of HMSS contribute to the enhanced hydrophilicity of HMSS/PES composite membranes. Secondly, compared with polymer or solid particles, HMSS can provide the unique inner cavity for the fast flow of pure water, which endowed the membrane with enhanced permeability. Finally, the incorporation of HMSS may disrupt and/or damage polymer chain packing and linking, and it leads to the formation of polymer 13

free volume, which is in favor of the water permeability [17] I

’

the BSA rejection of HMSS/PES composite membranes was almost stable at around 93%. In summary

HM

PE

UF

’

permeability while still remaining relative high BSA rejection. 3.7 Antifouling performance against BSA Membrane fouling is a major problem in protein separation because it can bring about the deterioration of permeability and selectivity. Therefore, antifouling performance is an important index for UF membranes. In this study, BSA was selected as the model foulant to evaluate the antifouling performance of the neat and composite membranes. The relevant time dependent flux curves are shown in Fig.9 and the calculated flux recovery data are summarized in Table 2. The pure water fluxes for all membranes in the first 50 min filtration stage were relatively stable. However, when the pure water was substituted by 1 g/L BSA solution, the fluxes decreased sharply caused by adsorbed and deposited BSA on the pores and the surfaces of the membranes [52]. After that, the membranes were washed and the pure water fluxes recovered in different degrees. Based on the three steps, the values of FRR corresponding to all the membranes were calculated according Eq. (3). In the first cycle, the FRR of pure PES membrane was only 62.2% meaning serious membrane fouling. With incorporating HMSS, the FRR value increased. The highest value of 82.4% was achieved as the HMSS loading was 1.5%. However, when adding more HMSS up to 2%, the FRR value decreased to 66.0% due to the pore lockage as can be seen in Fig. 4. Furthermore, it can be found that the FRR value of PES-MSS is 79.0% which is little lower than that of PES-1.5. The trend for all the membranes can also be found in the second cycle. FRR, which is always used to evaluate the fouling resistance of a membrane, is mainly associated with the hydrophilicity of the membrane. More hydrophilic the membrane surface is, more difficult the protein molecules can I’

FRR

HM

/PE

composite membranes were all higher than that of the neat PES membrane which implied blending silica particles into PES matrix an effective way to improve membrane fouling resistance. In order to further investigate the antifouling properties of the membranes, total fouling (Rt), reversible fouling (Rr) and irreversible fouling (Rir) were calculated according Eq. (4) ~ Eq. (6) respectively. As shown in Table 2, the membrane PES-1.5 had the lowest Rir value and highest Rr value no matter in the first cycle or the second cycle. This phenomenon indicated that the reversible fouling 14

played a predominant role in the fouling process which can be removed by simple water washing and it was beneficial for a more efficient UF process. To sum up, all the results suggest that the membrane PES-1.5 has the best antifouling performance compared to other membranes. 3.8 Effect of hollow structure on the UF performances of PES membrane In order to investigate the effect of hollow structure on the performances of PES membrane, a UF contrast experiment was conducted. The pure water flux and BSA rejection for PES-MSS and PES-1.5 were measured and the results are shown in Fig. 10. It can be seen that the pure water flux of PES-1.5 (195.7 L/m2 h) is almost as twice much as that of PES-MSS (98.7 L/m2 h). This result could be attributed to the unique hollow structure of HMSS, which provided a reservoir for water molecules and a shortcut for the pass of pure water. During the filtration operation, the pure water flow directly passes through the lumen of HMSS rather than permeating the MSS whose more resistance existed in the mesopores structure. Other than pure water flux, there is a little difference considering BSA rejection between two membranes. In summary, HMSS have an advantage over MSS in improving the UF properties of PES membrane. 4. Conclusions A novel composite UF membrane was successfully fabricated by blending hollow mesoporous silica spheres (HMSS) into polyethersulfone (PES) backbone material using non-solvent induced phase inversion (NIPS) process. Surfactant-assembly sol-gel method was applied to prepare HMSS, which endowed the HMSS with abundant hydroxyl groups round the particle surface, leading to the improvement of membrane hydrophilicity. I ’

% HM

the pure water flux of the modified membrane was up to 195.7 L/m2 h which was almost increased by 5 times in comparison with neat PES membrane, while the BSA rejection still remained at 92.6%. In addition, the incorporation of HMSS improved the antifouling performance of the composite membrane, including higher FRR values after water flushing. To sum up, an appropriate amount of HMSS significantly improved filtration and antifouling properties of neat PES membrane, and among the composite membranes with different contents of HMSS, PES-1.5 exhibited most excellently. Furthermore, the comparison experiment between PES-1.5 and PES-MSS indicated that HMSS have an advantage over MSS in improving UF properties of PES membrane. Acknowledgments 15

This work was financially supported by the National Natural Science Foundation of China (Grantno.51278247) and the priority academic program development of Jiangsu higher education institutions.

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21

Figure Captions Fig. 1. (a) SEM image of HMSS, (b) SEM image of MSS, (c) TEM image of HMSS, (d) TEM image of MSS. Fig. 2. (a) Nitrogen adsorption-desorption isotherms of HMSS and MSS, (b) Pore size distribution of HMSS and MSS. Fig. 3. TGA curves of PES-0, PES-0.5, PES-1, PES-1.5, PES-2 and PES-MSS. Fig. 4. SEM images of cross sections for membranes (A) PES-0, (B) PES-0.5, (C) PES-1, (D) PES-1.5, (E) PES-2 and (F) PES-MSS. Fig. 5. (a) SEM image of PES-1.5, (b) corresponding EDX spectrum of Si element on the surface and the EDX mapping scanning spectra of (c) C, (d) O, (e) S, and (f) Si. Fig. 6. XPS wide-scan spectra of PES-0 and PES-1.5. Fig. 7. Contact angles of PES composite membranes. Fig. 8. Effect of HMSS addition on the pure water flux and BSA rejection of PES composite membranes under 0.1 MPa. Fig. 9. Antifouling property evaluation of PES composite membranes via three times ultrafiltration of pure water and two times of 1 g/L BSA solution under 0.1 MPa. Two cleaning processes were carried out after every time BSA solution ultrafiltration, and then the pure fluxes of cleaned membrane were measured. Fig. 10. Comparison results of filtration performance between PES-1.5 and PES-MSS.

22

Fig.1

a

b

c

d

Fig.2 2.0

600

b

Volume Adsorbed (cm3/g.STP)

a 500

dV/dD (cm /g.nm)

1.5

1.0

3

400

300

HMSS MSS

200

0.5

HMSS MSS

0.0

100 0.0

0.2

0.4

0.6 0.8 Relative Pressure (P/P0)

1.0

0

2

4

6

Pore Size (nm)

8

10

23

Fig.3 100 90

Weight (%)

80 70

PES-0 PES-0.5 PES-1 PES-1.5 PES-2 PES-MSS

60 50 40 30 0

100

200

300

400 o

500

600

700

Temperature ( C)

24

Fig.4 Top area

Cross section

Bottom area

A

B

C

D

E

25

F

Fig.5 a Counts (cps/eV)

b

S

C

Pt O

0

Si

1

2

3

4

5

Energy (KeV)

c

e

d

f

26

Fig.6 C1s PES-1.5 PES-0

O1s

N1s S2s

S2p

Si2p

600 550 500 450 400 350 300 250 200 150 100

50

Binding Energy (eV)

Fig.7

27

Fig.8 100

pure water flux BSA rejection

Pure water flux (L/m2h)

250

95

200

90

150

85

100

80

50

75

0

BSA rejection (%)

300

70 0.0

0.5

1.0

1.5

2.0

The content of HMSS relative to PES (wt %)

Fig.9 200 PES-0 PES-0.5 PES-1

180 160

PES-1.5 PES-2 PES-MSS

2

Flux (L/m h)

140 120 100 80 60 40 20 0 0

50

100

150

200

250

300

350

Time (min)

28

Fig.10 100

Pure water flux BSA rejection

95

2

Pure water flux (L/m h)

200

150 90 100 85

50

0

BSA rejection (%)

250

80 HMSS

MSS

29

Tables Table 1 Composition and viscosity of casting solutions. Dope solution composition (wt %) Membrane

Viscosity

PES

PVP

DMF

(cp)

PES-0

16

4

80

479.9

PES-0.5

16

4

80

629.9

PES-1

16

4

80

674.5

PES-1.5

16

4

80

715.3

PES-2

16

4

80

760.8

PES-MSS

16

4

80

755.2

30

Table 2 Flux recovery ratio and resistances of neat and composite membranes. First cycle

Second cycle

FRR (%)

Rt (%)

Rr (%)

Rir (%)

FRR (%)

Rt (%)

Rr (%)

Rir (%)

PES-0

62.2

65.0

27.2

37.8

54.6

74.6

29.2

45.4

PES-0.5

69.6

61.2

30.8

30.4

62.6

69.3

31.9

37.4

PES-1

72.2

57.6

29.8

27.8

65.9

65.8

31.7

34.1

PES-1.5

82.4

60.5

42.9

17.6

76.7

67.4

44.1

23.3

PES-2

66.0

61.9

27.9

34.0

59.7

67.0

26.7

40.3

PES-MSS

79.0

62.8

41.8

21.0

72.3

68.8

41.1

27.7

31