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Antifouling enhancement of PVDF membrane tethered with polyampholyte hydrogel layers.

INTRODUCTION

Poly(vinylidene fluoride) (PVDF) is a polymeric material for fabricating microfiltration (MF) and ultrafiltration (UF) membranes due to its excellent chemical and thermal tolerance, well-controlled porosity, as well as good mechanical property [1, 2]. PVDF membrane is widely used in the water purification, beverage production, and biotechnology etc. [3-5]. Nevertheless, the hydrophobic PVDF membrane is susceptible to membrane fouling caused by nonspecific adsorption or deposition of organic matters (such as proteins). During an aqueous filtration, serious membrane fouling usually leads to substantial energy consumption and high operational cost [6, 7]. Therefore, hydrophilic modification of PVDF membrane is a pressing need to endow the membrane with antifouling property.

Many investigations have reported the covalent immobilization of hydrophilic materials onto PVDF membrane to improve the antifouling ability via surface grafting polymerization and modification of membrane bulk materials [8-15], For example, polyethylene glycol)(PEG)-based monomer was grafted onto PVDF membrane to yield a hydrophilic surface [16, 17]. Although the PEGylated PVDF membranes are effective to resist the membrane fouling during the filtration process, they are often insufficient since the ether bonds are easily oxidized in the presence of heavy-metal ions. Compared with the PEGylated membrane surface, polyzwitterionic membranes can bind water molecules more strongly via electrostatically induced hydration, and then have the long-term antifouling ability [18, 19]. Polyzwitterionic materials can be divided into polybetaine carrying the oppositely charged groups on the same monomer unit and polyampholyte carrying oppositely charged groups on two different monomer units [20, 21]. Recent works have focused on the surface zwitterionization of PVDF membrane with polybetaines [such as poly(sulfobetaine methacrylate) (PSBMA), poly(carboxybetaine methacrylate) (PCBMA)] [22, 23]. However, there is still a shortcoming such as the incompatibility of hydrophobic PVDF membrane with polybetaine, which hinders the direct use of polybetaine in membrane modification [24, 25], Therefore, the incorporation of polyampholyte into PVDF membrane should be an alternative approach to improve the antifouling ability, in which the polyampholyte surface can be constructed via the surface copolymerization of a pair of separate monomers with two oppositely charged groups, respectively.

As is well known, hydrogels can hold large amounts of water yet remaining insoluble in water because of the three-dimensional and crosslinked networks [26], Because of the high capability of absorbing water, recent works have focused on the preparation of hydrogels as the antifouling filtration membrane [27]. It is noteworthy that hydrogels are often weak in mechanical strength and unable to be used as free-standing membranes [28]. Thus, the covalent immobilization of hydrogel onto porous membranes as surface layers is feasible to achieve the fouling resistance of hydrophobic membranes [29, 30]. For example, Chang et al. [31] reported surface modification of PVDF membrane grafted with polyethylene glycol) methacrylate (PEGMA) hydrogel layer via surface-activated ozone treatment and thermally induced graft copolymerization. PVDF membranes functionalized by PEGMA hydrogels could resist the irreversible membrane fouling as compared with the pristine PVDF membranes. However, to our knowledge, so far rare work has been reported on the surface modification of PVDF membrane with polyampholyte hydrogel.

This work aims to improve the fouling resistance of PVDF membrane through the combination with hydrophilic characteristic of polyampholyte materials and high capacity of adsorbing water of hydrogel. The pretreatment of pristine PVDF membrane reported in our previous works was conducted to produce double bonds (C=C) [10, 25]. Subsequently, a polyampholyte hydrogel layer was covalently immobilized onto the PVDF membrane surface via radical copolymerization. The chemical composition, morphology, and fouling resistance of modified membrane were evaluated in detail.

EXPERIMENTAL

Materials

The PVDF powder was purchased from Solvey, (Brussels, Belgium). The [2-(methacryloyloxy)ethyl] trimethylammonium chloride (DMC) and 2-acrylamide-2-methyl propane sulfonic acid (AMPS) were provided by Darui Fine Chemical, (Shanghai, China). N,N'-methylenebisacrylamide (MBAA) and coomassie brilliant blue (G250) were obtained from Sigma-Aldrich chemical. Sodium chloride (NaCl) was purchased from Tianjin Chemical Reagent, (Tianjin, China). N,N-dimcthyl formamide (DMF), polyethylene glycol) (PEG, [M.sub.n] = 10,000), potassium hydroxide (KOH) and ethanol were kindly provided by Kermel Chemical, (Tianjin, China). Ammonium persulphate (APS) was purchased from Tianjin Tingda Rare Chemical Reagents Factory (Tianjin, China). Bovine serum albumin (BSA) was provided by Beijing Aoboxing Bio-tech. (Beijing, China). The pure water (18 M[OMEGA]) used for all experiments was purified with a Milli-Q system (Millipore, America).

Membrane Preparation and Modification

The pristine PVDF membrane was formed via immersed phase inversion method. Four gram of PVDF and 2 g of PEG were simultaneously dissolved in 19 g of DMF. The solutions were stirred at 60[degrees]C for 4 h and then left for 24 h to allow release of bubbles. After cooling to room temperature, the casting solution was poured onto a glass plate, and spread using a casting knife of a nominal thickness of 300 [micro]m. Subsequently, the glass plate was immediately immersed in a water bath at 25[degrees]C. After the nascent membrane was peeled off from a glass plate, the sample was rinsed with pure water and then kept immersed until use.

The grafting copolymerization of AMPS and DMC monomers on the PVDF membrane was performed via radical copolymerization. The schematic illustration was shown in Fig. 1. Three pieces of pristine PVDF membranes with 5.5 cm of diameter were soaked in ethanol for 5 min, and then were treated with 50 ml of KOH solution (2.5 mol/L) at 60[degrees]C for 15 min. The alkali-treated PVDF membranes were washed with pure water for several times to remove the residue alkali solution. The obtained samples were dried completely in a vacuum oven at 60[degrees]C for 72 h.

Three pieces of alkali-treated PVDF membranes were immersed into a 100 ml of aqueous solution containing an equimolar mixture of DMC and AMPS monomers. The feed mass ratio of DMC monomer to PVDF membrane is about 6:1. Subsequently, the initiator (APS, 0.03 g) and crosslinking agent (MBAA) were added to the reaction solution. The feed mole percentage of MBAA was about 0.1%, 0.3%, and 0.5% of total monomers, respectively. After bubbling of N2 for 15 min, the reactor was sealed and the reaction was processed at 70[degrees] C for 10 h. The samples were alternately washed with pure water and NaCl solution (1 g/1) for several times to remove the residue monomers. The pristine membrane PVDF membrane was marked as M0. The modified membranes were termed MO. 1, M0.3, and M0.5, respectively. The grafting density (GD) was calculated gravimetrically according to the following Eq. I [22]:

GD = [w.sub.1] - [w.sub.0]/S (1)

where [w.sub.0] ([micro]g) is the initial alkali-treated PVDF membrane weight, [w.sub.1] ([micro]g) denotes the weight of PVDF membrane after the grafting, S ([cm.sup.2]) is the total area of both top and bottom membrane surface. Weight measurements were conducted via three independent membranes for each modified membrane, and the average value was reported.

Membrane Characterization

Fourier transform infrared attenuated total reflection spectroscopy (ATR-FTIR) was carried out on a FT1R spectrophotometer (Tensor37, Bruker, Germany). The spectra were collected by cumulating 32 scans at a resolution of 2 [cm.sup.-1].

The X-ray photoelectron spectroscopy (XPS, PHI5000C ESCA system, PHI, USA) was used to investigate the surface chemical change of membranes. The membranes were cut by sharp razor and then mounted on the standard sample studs using the double-side adhesive tapes. The XPS data were obtained with electron spectrometer using 300 W AlK[alpha] radiation. The core-level signals were achieved at the electron takeoff angle of 90[degrees] relative to the sample plane.

The surface morphology of membranes was observed by field emission scanning electron microscopy (FESEM, S-4800, Hitachi). The samples were initially dried in in a vacuum oven at 60[degrees]C for 72 h, and then were sputter-coated with a thin gold layer before microscopic analysis.

Contact angle goniometer (DSA100, Kriiss, Germany) equipped with high-speed video camera was applied to characterize the surface hydrophilicity of the membranes. A membrane sample was attached to a glass slide, and then was mounted in the goniometer. A water drop (5 [micro]l) was dripped onto the membrane surface at room temperature. The change of the contact angles was recorded as a function of the drop age.

Protein Adsorption on the Membranes

The protein fouling resistances of membranes were evaluated via protein adsorption on the membrane. Bovine serum albumin (BSA) was selected as a model protein. The membrane sample with about 12.5 [cm.sup.2] of external surface area was immersed in ethanol for 10 min followed by phosphate buffered solution (PBS) for 30min to prewet the membrane surface. Subsequently, each sample was put into a tube containing 10 ml of BSA solution (pH 7.4 in PBS) at a concentration of 500 mg/1. The vials were vibrated in a shaking bath at a constant temperature of 25[degrees]C for 24 h to reach the BSA adsorption equilibrium. The amount of adsorbed protein (Q) was quantitatively calculated by the following Eq. 2:

Q = [C.sub.0] - [C.sub.1]/S x V (2)

where V(1) is the volume of BSA solution, S ([cm.sup.2]) is the total area of both top and bottom membrane surface, [C.sub.0] (mg/l) and [C.sub.1] (mg/l) denote the initial and equilibrium BSA concentrations, respectively. BSA concentrations were obtained from the Bradford method relying on the binding of the dye Coomassie Blue G250 to protein [32], The absorbance at 595 nm was determined by a UV-vis spectrophotometer (UV-1601, Shimadzu, Japan). The reported data was the mean value of triplicate samples for each membrane.

Filtration Experiments of the Membranes

The permeation properties of the membranes were evaluated via the apparatus described in our previous work [10]. The membrane sample was pressured with pure water at 0.2 MPa for 2 h. The pressure was then lowered to 0.1MPa and the steady pure water flux ([J.sub.w,0]) was obtained following the relationship (3):

J = V/At (3)

where the parameter V(1), is the volume of the pure water, A ([m.sup.2]) and t (h) represent the effective area of membrane top surface and permeation time, respectively.

To investigate the long-term performance of membranes, cycle filtration was performed at 0.1 MPa and at room temperature. The cycle consisted of three stages. The first stage was the pure water filtration and the flux ([J.sub.w,0]) was record. The second step was the filtration of BSA solution (1 g/l) and the flux ([J.sub.p]) was obtained. The third stage was the pure water filtration after hydraulic cleaning at 0.1 MPa for 30 min, and the pure water flux ([J.sub.w,1]) was calculated again. Thus, the flux recovery ratio (FRR) of the membrane can be calculated using the following Eq. 4:

FRR (%) = [J.sub.w,1]/[J.sub.w,0] x 100 (4)

For the further evaluation of the reversible and irreversible fouling occurring on the membranes, three parameters were defined in detail [33], Reversible membrane fouling ([R.sub.r]) could be removed via the hydraulic cleaning. The membrane fouling that could not be eliminated only through hydraulic cleaning was irreversible fouling ([R.sub.ir]). The flux loss was caused by both reversible and irreversible fouling in the cycle ([R.sub.r] and [R.sub.ir]), which were calculated from the Eqs. 5 and 6:

[R.sub.r](%) = [J.sub.w,1] - [J.sub.p]/[J.sub.w,0] x 100 (5)

[R.sub.ir](%) = [J.sub.w,0] - [J.sub.w,1]/[J.sub.w,0] x 100 (6)

The degree of flux loss caused by total fouling in the cycle filtration process ([R.sub.t]), was defined as the Eq. 7:

[R.sub.1](%) = [J.sub.w,0] - [J.sub.p]/[J.sub.w,0] x 100 (7)

RESULTS AND DISCUSSIONS

Chemical Structure and Morphology of Membrane Surfaces

The mechanism of radical copolymerization of functional monomers onto hydrophobic PVDF membrane surface has been stated in our previous works [10, 25]. Briefly, the pretreatment of the pristine PVDF membrane was conducted to produce double bonds (C=C) with an alkali solution. The vinyl monomers were grafted to yield a functional membrane surface. In the present work, a polyampholyte hydrogel layer was designed into the PVDF membrane surface via radical copolymerization. In fabricating these membranes, DMC, AMPS, and MBAA were used as the monomer and crosslinking agent, respectively. The surface chemical structures of membranes were determined by ATR-FTIR spectra and the result was shown in Fig. 2. The membranes exhibit the typical characteristic peaks of PVDF, i.e., --C[F.sub.2] and --C[H.sub.2] deformation and stretching vibration bonds at 1180 and 1407 [cm.sup.-1], and amorphous phase at 880 and 842 [cm.sup.-1]. Compared with the spectrum of pristine PVDF membrane (M0), additional peaks are observed in the spectra of membranes M0.5. The peak at 1038 [cm.sup.-1] is assigned to the O=S=O stretching vibration of sulfonic acid groups. The peaks at 1652 and 1550 [cm.sup.-1] are corresponding to the amide groups in the AMPS units. The existence of DMC units can be ascertained from the C-[N.sup.+] stretching vibration in the quaternary amine groups at 952 [cm.sup.-1]. The surface compositions of membranes were investigated by XPS. As shown in Fig. 3, it is seen that there are two obvious signals at 285.7 eV for C1s and 687.3 eV for F1s. Compared with PVDF membrane (MO), additional emission peaks can be observed on the modified membrane surfaces (M0.1, M0.3, and M0.5), 532.3 eV for O1s, 400.6 eV for N1s and 167.8 eV for S2p [34], respectively. These additional signals were originated from DMC and AMPS monomers. The presence of DMC and AMPS could be further confirmed by the XPS N1s core-level spectra of membrane surfaces, as revealed in Fig. 4. N1s core-level spectra of modified membranes can be cut-fitted with two peak components. Except for peak component (399.7 eV) corresponding to N of the tertiary amine within AMPS units, an additional peak component at 402.5 eV is attributed to the N+ of quaternary amine within DMC units. Table 1 summarized the element content determined by XPS wide-scan spectra of membrane surfaces. The mole percentages of N and S element increase with the increasing amount of crosslinking agent. The grafting density (GD) was calculated gravimetrically according to the Eq. I. The GD value increases from 0 to 369 [micro]g/[cm.sup.2] with the increasing amount of crosslinking agent. Similar experimental result can be obtained from the previous work [34], This result has resulted from the fact that the crosslinking reaction of MBAA with DMC and AMPS is the radical copolymerization. Increasing amount of crosslinking agent can produce more C=C bonds and active radicals, which in turn causes more DMC and AMPS monomers to graft onto the alkali-treated PVDF membrane. These results indicate the covalent immobilization of DMC and AMPS monomers onto the membrane surface.

Figure 5 shows the typical surface morphologies of pristine PVDF (MO) and modified membranes (M0.1, M0.3, and M0.5). The pristine PVDF membrane surface is porous, while the pore size of modified membrane surfaces decreases with the increase of grafting density of polyampholyte hydrogel layer. Especially for the modified membrane with a high grafting density, a hydrogel layer can be observed obviously, indicating that the incorporation of polyampholyte hydrogel layer results in the complete coverage of membrane surface pores.

Surface Hydrophilicity of Membranes

An important characteristic of the polyampholyte hydrogel is that it has both positively and negatively charged moieties, which can be strongly hydrated through ionic solvation [20]. Water contact angles of membranes were measured to evaluate the hydrophilization effect of grafted polyampholyte hydrogel layers. As shown in Fig. 6, the pristine PVDF membrane exhibits the highest initial contact angle (88[degrees]), owing to the intrinsic hydrophobicity of PVDF. After grafting polyampholyte hydrogel layers, the initial water contact angle of the modified membrane has a significant decrease with the grafting density growing. In addition, a faster decay rate of contact angle can be observed on the modified membranes surface than pristine PVDF membrane. When a water drop contacts the membrane surface, the instant spread of water drop was driven by the electrostatic interactions between charged groups and water molecules [35]. These results indicate that the polyampholyte hydrogel layers have enhanced the surface hydrophilicity of PVDF membrane.

Protein Adsorption

The antifouling property of the membrane is found to be related to the protein adsorption [22], In this work, the protein adsorption amounts to membranes were determined by immersion into the BSA solution. The results were shown in Fig. 7. It is found that the protein adsorption amounts decrease with the increase of grafting density. When the grafting density is 369 [micro]g/[cm.sup.2] (M0.5), the amount is 22.1 [micro]g/[cm.sup.2], about 84% decrease compared to pristine PVDF membrane. As the polyampholyte hydrogel can bind a significant amount of water molecules, a strongly hydrated membrane surface is formed and then suppresses the nonspecific protein adsorption. On the other hand, the blockage of membrane pores by the grafted layers leads to the difficulty in the protein adsorption to the membrane pore channel surface [10]. These two phenomena trim down the protein adsorption from 139.8 to 22.1 [micro]g/[cm.sup.2], as grafting density increases from 0 to 369 [micro]g/[cm.sup.2]. As a matter of fact, since the membrane structure is porous and the actual adsorption surface area is much larger than the total area of both top and bottom membrane surface, actual adsorption amounts should be substantially lower than the above-reported data. The results demonstrate that the protein adsorption can be significantly reduced after the polyampholyte hydrogel layer is tethered onto PVDF membrane surfaces.

Permeation and Antifouling Properties of Membranes

The permeation and antifouling properties of membranes were evaluated in the present work. The cycle filtrations of pure water and BSA solution were performed, which was divided into three steps. The first step was the pure water filtration and the steady initial flux ([J.sub.w,0]) was obtained. As shown in Fig. 8, the value of the membrane grafted with 186 [micro]g/[cm.sup.2] of polyampholyte hydrogel layer is actually higher than that of the virgin PVDF membrane. It is also observed that the [J.sub.w,0] values decrease as the grafting density of polyampholyte hydrogel layer is further increased. It is well known that the surface hydrophilicity and membrane pore affect the pure water flux oppositely [22]. The hydrophilic membrane surface will increase the water flux to some extent, while small membrane pore will lead to a low water flux. For the modified membrane with an overhigh grafting density of polyampholyte hydrogel layer, although the surface hydrophilicity is accordingly good, the crowded layers will result in the decrease of membrane effective pore size and the reduction of the water flux.

The second stage of the cycle filtration was the permeation of BSA solution ([J.sub.p]) and the third was the steady permeation ([J.sub.w,1]) of pure water after simple membrane flushing with pure water. The flux recovery (FRR) was introduced to evaluate the protein-fouling resistance of membranes. As summarized in Table 2, higher FRR was recovered for the modified membranes as compared to the pristine PVDF membrane. Moreover, FRR of the modified membrane is in the ascending order as the grafting degree of polyampholyte hydrogel layer increases, especially for M0.5 reached 96.7%. This characteristic is ascribed to that the proteins adsorbed to modified membranes are easily removed via the simple hydraulic cleaning [36].

To quantitatively estimate the antifouling performance of membranes in the filtration process, the total fouling ratio ([R.sub.t]), reversible fouling ratio ([R.sub.r]), and irreversible fouling ratio ([R.sub.ir]) was calculated by the Eqs. 5-7, respectively. The reversible fouling which can be easily removed by hydraulic cleaning is contributed by loosely adsorbed proteins. Nevertheless, the irreversible fouling is mainly induced by firmly adsorbed proteins or organic macromolecules, which is difficult to be eliminated only by hydraulic cleaning. The total membrane fouling is actually established by both the loosely attached and firmly adsorbed proteins. From Table 2, the [R.sub.t] value decreases from 53.2% (M0) to 10.2% (M0.5). The lower [R.sub.t] represent a lower permanent flux loss, which is corresponded to less protein adsorption [10]. Importantly, both the [R.sub.r] and [R.sub.ir] decreases with the increase of grafting density, indicating the inhibition of loose and firm adsorption of proteins to the modified membrane surface. These results further confirm that protein-fouling resistance of PVDF membrane has been significantly elevated by the tethering of polyampholyte hydrogel layer.

CONCLUSIONS

In this work, a polyampholyte hydrogel layer was successfully tethered onto PVDF membrane via radical copolymerization. The grafting density of polyampholyte hydrogel layer increases with the increase amount of crosslinking agent. The membrane surface pore was completely covered with grafting layers. Especially for the membrane with grafting density of 369 [micro]g/[cm.sup.2]. a hydrogel layer could be obviously observed on the membrane surface. The protein adsorption to the modified membranes was much lower than that to the pristine PVDF membrane, which was ascribed to the enhanced hydrophilicity. Cycle filtration tests confirmed that both the reversible and irreversible membrane fouling was alleviated after the incorporation of polyampholyte hydrogel layer into the PVDF membrane. This work provided an effective pathway to improve the fouling resistance of hydrophobic membrane materials.

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Xiang Shen, (1) Xuebin Yin, (2) Yiping Zhao, (2) Li Chen (2)

(1) College of Chemistry and Chemical Engineering, Qujing Normal University, Qujing 655011, China

(2) State key Laboratory of Hollow Fiber Membrane Materials and Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China

Correspondence to: Li Chen; e-mail: tjpuchenlis@163.com

Contract gram sponsor. National Nature Science Foundation of China; contract grant numbers: 21174103: 21374078.

DOI 10.1002/pen.24077

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Chemical compositions determined by XPS wide-scan spectra
of membrane surfaces.

                                                  GD ([micro]g/
Sample   C (%)   F (%)   O (%)   N (%)   S (9b)    [cm.sup.2])

M0       51.49   45.63    2.88    --       --          --
M0.1     60.54   22.88   10.49    3.8     1.3          186
M0.3     63.72   19.3    11.37   4.07     1.54         277
M0.5     63.69   17.31   12.62   4.46     1.9          369

TABLE 2. The [R.sub.r], [R.sub.ir], [R.sub.t] and FRR values
of membranes in the cycle filtration process.

Sample   [R.sub.r] (%)   [R.sub.ir] (%)   [R.sub.t] (%)   FRR (%)

M0           11.7             41.5            53.2         58.5
M0.1         13.2              6.7            19.9         93.3
M0.3         12.5              5.6            18.1         94.4
M0.5          6.9              3.3            10.2         96.7
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Date:Jun 1, 2015
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