Synthesis and Characterization of Chitosan-Grafted BPPO

Article pubs.acs.org/IECR

Synthesis and Characterization of Chitosan-Grafted BPPO Ultrafiltration Composite Membranes with Enhanced Antifouling and Antibacterial Properties Yi Feng,† Xiaocheng Lin,† Huazhen Li,† Lizhong He,† Tam Sridhar,† Akkihebbal K Suresh,‡ Jayesh Bellare,‡ and Huanting Wang* †

Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia Department of Chemical Engineering, Indian Institute of Technology Bombay, Bombay, Maharashtra 400076, India

‡

ABSTRACT: In this study, chitosan was successfully grafted onto the top surface of a bromomethylated poly(phenylene oxide) (BPPO) ultrafiltration membrane without pretreating the membrane at harsh conditions and/or using other cross-linkers. Due to the grafting of polar groups of chitosan onto the membrane top surface, the hydrophilicity of the membrane top surface and the polar component of the total surface energy are improved compared to the pristine BPPO membrane. Theoretical calculation shows that the polar component of surface energy is a major contributor to the reduction in interfacial free energy of the membrane surface and the interaction strength between foulants and membrane surface. Therefore, the foulants adsorbed onto the top surface of chitosan/BPPO composite membranes are much easier to desorb during the cleaning process and as a result, a higher flux recovery was obtained compared to the pristine BPPO membrane. Moreover, due to the antibacterial nature of chitosan, such composite membranes show a better antibacterial property and the antibacterial rate was improved by 70% in comparison with the pristine BPPO membrane. serum albumin (BSA) have less conformation change;9,10 therefore, the foulants on hydrophilic surfaces maintain their high antifoulant binding ability and are more easily washed off the membrane surface.2,11 To date, surface modification of hydrophobic polymer membranes has been performed by various physical and chemical surface treatment procedures on polymer membranes. Generally, there are four ways to convert nonpolar surfaces to hydrophilic surfaces: (1) active gas treatment involving corona discharge or gas plasma discharge or deposition;12−15 (2) chemical reactions such as gas phase oxidation or liquid phase oxidation;16,17 (3) surfactant or hydrophilic polymer immobilization;18−23 (4) graft polymerization by radiation or chemical initiation.24 Despite that in those studies, the hydrophilicity of polymer membranes has been improved, the surface chemical reactions were usually carried out under strongly hazardous conditions such as UV treatment and ozone treatment, and the chemicals used for grafting are sometimes not economical and environmentally unfriendly. Moreover, grafting hydrophilic materials by γ-ray and UV radiation, or in the plasma chamber, is not easy to apply on a large industrial scale. Therefore, there is a need to develop a simple, practical and economical way to graft hydrophilic materials onto the top surface of polymer membranes. In this study, bromomethylated poly(phenylene oxide) (BPPO) is chosen as the base membrane. BPPO is an outstanding ultrafiltration material because of its good thermal

1. INTRODUCTION Membrane fouling involving biological foulants such as proteins is a major problem encountered in membrane filtration processes, and becomes a major factor in determining their practical application in water and wastewater treatment. Protein fouling can be initiated from interactions between the membrane and the protein and causes a significant reduction in membrane performance. It is reported that the protein adsorptive fouling could account for up to 90% of permeability losses in a typical ultrafiltration (UF) membrane system.1 Membranes are cleaned or replaced when the flux reaches an unacceptable level and the cleaning and membrane replacement contribute to 50% of the operating costs or 30% of the total costs.2 Therefore, much attention has been paid in the past decades to study the mechanism of protein adsorption to enhance the antifouling property of polymer membranes.3,4 In the separation processes involving proteins, it has been widely accepted that the electrostatic forces and the hydrophobic interactions between protein and membrane surfaces are the main factors in governing the fouling process.5,6 Many researchers have followed the idea of increasing the hydrophilicity of a membrane with the goal of reducing fouling. Among all the methods, the direct and facile approach toward preventing protein fouling is the generation of antifouling surfaces, that is, hydrophilic coatings that repel the adhesion of proteins and cells.7,8 The principle behind this is that the hydrophilic surfaces are less sensitive to protein adsorption than hydrophobic ones because a hydrophilic surface preferentially adsorbs water rather than solutes, thus leaving a membrane surface with good protein resistance. Moreover, on less hydrophobic surfaces, the affinity between the foulants and the surface is weakened and some foulants such as bovine © 2014 American Chemical Society

Received: Revised: Accepted: Published: 14974

June 30, 2014 September 7, 2014 September 10, 2014 September 10, 2014 dx.doi.org/10.1021/ie502599p | Ind. Eng. Chem. Res. 2014, 53, 14974−14981

Industrial & Engineering Chemistry Research

Article

Figure 1. Illustration of reaction between chitosan and BPPO.

and chemical stability, good flexibility and film forming properties.25 Moreover, compared to other UF polymers, such as poly(ether sulfone) (PES), polysulfone (PS) and polyvinylidene fluoride (PVDF), BPPO contains abundant reactive −CH2Br functional groups, which can easily react with amine or imidazole groups at room temperature without pretreating base membranes and the use of cross-linkers.26−28 Therefore, the hydrophobic surface of BPPO membranes can be easily modified by grafting hydrophilic materials containing specific functional groups, such as amine, to obtain a more hydrophilic surface. For the coating materials, chitosan is chosen in this study on the basis of the following considerations: (1) chitosan (CS) is an economically attractive and environmental friendly material; (2) chitosan possesses good hydrophilicity and broad space for modification due to the abundant reactive amine (−NH2) groups; (3) chitosan has a good antibacterial property.29−31 Therefore, on the basis of what has been discussed above, chitosan can be introduced to membrane top surface via chemical grafting under mild conditions, as Figure 1 shows. It is hypothesized that due to the introduction of polar groups of chitosan onto the top surface of hydrophobic BPPO membrane and the antibacterial nature of chitosan itself, an improved antifouling and antibacterial ability of chitosan-grafting BPPO membranes is expected. In summary, this study offers a simple and facile method to modify the hydrophobic surface of polymer membranes by chitosan grafting, and the effect of chitosan on the UF properties of the membrane and the mechanism of the improved antifouling property are fully investigated.

surface of chitosan/acetic acid/DI water solution (the weight ratio of chitosan:acetic acid:DI water is 1:2:9734) with the active layer facing the solution. Due to the hydrophobic nature of the BPPO membrane, it would float on the top of chitosan/ acetic/DI water solution and this avoided the reaction between chitosan and the bottom layer of BPPO membrane. For convenience, the reaction temperature was fixed at room temperature while the reaction time was varied from 2 to 6 h. And then the composite membranes were denoted as chitosan/ BPPO-2h, chitosan/BPPO-4h and chitosan/BPPO-6h membrane. After that, the composite membranes were washed by DI water and then immersed into a 2 wt % NaOH/DI water solution for 20 min for neutralization followed by a thoroughly wash with DI water again. The final membranes were stored in DI water for at least 1 day before any further use. 2.3. Characterization of Membranes. 2.3.1. FTIR, SEM and Contact Angle. The composition of the membrane top surface was characterized by Fourier transform infrared spectroscopy (FTIR spectrometer, PerkinElmer, Australia). The morphology of the top surface was examined by scanning electron microscopy (SEM) (Magellan SEM, FEI Company, America) and the hydrophilicity of membrane top surface was characterized by contact angle measurement (video based optical contact angle measuring instrument, OCA-15EC, Dataphysics, Germany). The static contact angle of all membranes was recorded at 30 s after a water drop was added onto the top surface of the membranes to get a steady reading. The advancing and receding contact angles were autorecorded by increasing and decreasing the volume of water drop on the membrane top surface and the surface energy was calculated from the results of contact angle of different solvents (DI water, ethylene glycol and diiodomethane). 2.3.2. Permeation and Molecular Weight Cutoff. The flux of the membranes was measured using a dead end cell (HP4750 Stirred Cell, Sterlitech, USA). The double deionized (DDI) water was held in a filtration cell and the feed pressure was controlled by compressed nitrogen gas. The water was collected using a beaker sitting on an electronic balance. The mass change on the balance was automatically recorded. During the flux test, 120 kPa was used to precompact membrane for at least 60 min until the flux became stable. Water flux was recorded at a pressure of 100 kPa. For each batch, at least five membrane samples were tested. A separation test was performed by using poly(ethylene oxide) (PEG) with molecular weights of 35, 100 and 200 kDa. The rejection rate was determined by measuring the PEG concentration in the permeate water and feedwater by a total organic carbon analyzer (TOC, Shimadzu, Japan). 2.3.3. Antifouling Test. The fouling resistance of the membranes was characterized by constant flux fouling test.35 The membrane was precompacted by DDI water at 120 kPa for

2. EXPERIMENTAL SECTION 2.1. Materials. Bromomethylated poly(phenylene oxide) (BPPO) (Tianwei Membrane Corporation Ltd., Shandong, China); NMP (purity ≥99%, Sigma-Aldrich, Australia); chitosan (Sigma-Aldrich, Australia); NaOH (purity ≥99%, Merck KGaA Company, German); acetic acid (purity ≥99.7%, Ajax Finechem Pty Ltd., Australia); ethylene glycol (purity ≥99%, Sigma-Aldrich, Australia); poly(ethylene oxide) (PEG) (35 kDa, 100 kDa, 200 kDa, Sigma-Aldrich, Australia); bovine serum albumin (BSA) (agarose gel electrophoresis, SigmaAldrich, Australia); Escherichia coli (strain K12, Lyophilized cells, Sigma-Aldrich, Australia); Lysogeny broth (LB) (Luvia broth base, Invitrogen, Australia). 2.2. Membrane Preparation. The BPPO UF membrane was prepared by the phase inversion method: a 20 wt % BPPO/ NMP solution was first cast onto a precleaned glass using a casting knife with a 150 μm air gap and then kept in deionized (DI) water for further use.32,33 To get the chitosan/BPPO composite membrane, the BPPO membrane was put onto the 14975

dx.doi.org/10.1021/ie502599p | Ind. Eng. Chem. Res. 2014, 53, 14974−14981


Article

60 min and then was put into the cell filled with a 0.5 mg·mL−1 BSA solution. During the fouling process, the flux was fixed at approximately 40 L·m−2·h−1 (LMH) by using a pump after dead-end cell. Each fouling test lasted 1 h and the transmembrane pressure (TMP) was recorded per 30 s. After the fouling test, the membrane was physically cleaned by using DDI water and chemically cleaned by NaOH and DDI water. After cleaning, flux was tested and the flux recovery was obtained by comparing the flux after fouling and cleaning and the original flux before fouling. Fouling behavior can be determined by the estimation of resistances of membrane as shown below:35 (1) Intrinsic membrane resistance (Rm) R m = TMP/(μJbf )

The strength of the interactions between a protein and the membrane surface can be evaluated via the Hamaler constant (Apws):37 A pws = −16πd02ΔFpws(d0)

(6)

ΔFpws(d0) = rs(w)p(w) − rp(w)w − rs(w)w

(7)

d d rs(w)p(w) = {(rs(w) )1/2 − (rp(w) )1/2 }2 p p + {(rs(w) )1/2 − (rp(w) )1/2 }2

(8)

p d rp(w)w = {(rp(w) )1/2 − (rwd )1/2 }2 + {(rp(w) )1/2 − (rwp )1/2 }2

(1)

(9)

where TMP is the transmembrane pressure (100 kPa) and μ is the permeate viscosity. Jbf is the pure water flux of membrane before the fouling. (2) Irreversible resistance (Rir)

where d0 is the equilibrium distance between protein and surface (which is taken to be approximately 0.16 nm); ΔFpws(d0) is the change in free energy when the protein and membrane surface in water are brought from infinite to d p equilibrium; rp(w) and rp(w) are the dispersive and polar component of the BSA in its hydrated state (which is taken to be 30.8 and 40.8 mN·m−1, respectively38). 2.3.4. Antibacterial Test. The plate counting method was used to evaluate the antibacterial property of the membranes.39 Escherichia coli was inoculated in 10 mL of LB liquid nutrient medium and incubated overnight at 37 °C under shaking in an orbital shaker incubator (Thermoline Scientific Equipment Pty Ltd., Sydney, Australia). Membranes were cut into 3 × 3 cm samples and sterilized by 80% (v/v) ethanol solution before being placed into E. coli/LB solution (optical density at 600 nm OD600 = 0.1). After 12 h of incubation under shaking, the membranes retrieved from E. coli cultures were soaked in phosphate-buffered saline (PBS) buffer solution (pH = 7.4) for 20 min, and then the washing suspension solution (7.5 mL in total) was collected (labeled as the washing solution). Then the washed membranes were transferred into 7.5 mL of PBS buffer solution and bath-sonicated in a Brason B1510 ultrasonic cleaner (maximum 80 w) for 20 min, and the PBS cell solution was retrieved (labeled as the ultrasonication solution). The optical density (OD) value of the washing solution and the ultrasonication solution were measured. The solutions were diluted to a final OD600 value of 0.0001 by PBS buffer and then spread onto LB agar plates. Plates were incubated at 37 °C overnight, followed by manual counting of colonies.

R ir = TMP/(μJaf ) − R m

(2)

where Jaf is the water flux after physical and chemical cleaning (TMP = 100 kPa). (3) Reversible resistance (Rr) R r = TMP′/(μJf ) − R m − R ir

(3)

where Jf is the BSA filtration flux which is set at 40 LMH in this paper (TMP′ was considered after 1 h of filtration). (4) Total resistance (Rt) R t = R m + R r + R ir

(4)

The Rr, Rir and Rt values shown in this paper are the average values for three fouling tests. The static BSA adsorption experiments were carried out by standard batch equilibrium adsorption studies at room temperature.1 Specifically, membranes were put into the stirring cells and kept static with only the active layer facing 0.5 mg· mL−1 BSA/Tris−HCl buffer solution (pH = 7.0) for 24 h to reach an adsorption−desorption equilibrium. The amount of protein adsorbed on the top surface membranes was calculated from the decreased concentration of BSA solution. The concentration of BSA solution was determined based on the absorbance at 280 nm using a UV spectroscope (UV mini-1240 spectrophotometer, Shimadzu, Japan). After static BSA adsorption, the flux of fouled membranes was tested and the flux ratio was calculated by comparing the flux after and before static BSA adsorption. The same cleaning method as described above was used to clean the fouled membranes and then the flux was tested again. The flux recovery was obtained by comparing the flux after fouling and cleaning and the initial flux before fouling. The interfacial free energy (rs(w)w) was calculated using the following equation:36,37

3. RESULTS AND DISCUSSION To confirm the reaction between BPPO and chitosan and the chemical composition of the top surface of membranes, the pristine chitosan and the top surfaces of all membranes were characterized by FTIR. As shown in Figure 2, pristine chitosan shows the characteristic peak at 1558 cm−1 (R−NH2) whereas the pristine BPPO membrane shows the characteristic peak at 1469 cm−1 (CC of benzene). As their composite, the top surfaces of all chitosan/BPPO membranes show the characteristic peaks of chitosan and BPPO at 1558 and 1469 cm−1. Moreover, a newly formed peak at 1389 cm−1 (R2−NH) is observed in chitosan/BPPO membranes and it becomes more obvious with increasing chitosan coating time, indicating the successful grafting of chitosan onto the membrane top surface instead of simply adsorbing onto the membrane top surface. Static contact angle measurements are commonly used to estimate the hydrophilicity and wettability of polymer surfaces. In general, the smaller the contact angle, the better the

p d rs(w)w = {(rs(w) )1/2 − (rwd )1/2 }2 + {(rs(w) )1/2 − (rwp )1/2 }2

(5)

rds(w)

rps(w)

where and are the dispersive component and polar component of surface energy of membrane top surface respectively; rdw and rpw are the dispersive component and polar component of surface energy of water, which is taken to be 21.8 mN·m−1 and 51 mN·m−1. 14976



Article

consequently better hydrophilicity of the membrane top surfaces. Besides the static contact angle measurement, the dynamic contact angle measurements were also performed because of the great value of characterization for the surface hydrophilicity and dynamic behavior. Specifically, the advancing contact angle usually presents the original surface state of a polymer in air while the receding contact angle presents the condition of migration of chains carrying functional groups of a polymer in water.24 Compared with pristine BPPO membranes, all the chitosan/BPPO composite membranes show a significant decrease in both the advancing (θA) and receding contact angle (θR) (Figure 3), indicating the better hydrophilicity and the increased amount of removable chains on the top surface of the composite membranes. As mentioned above, a fair amount of chitosan was grafted onto the top surface of BPPO membrane after chitosan treatment, so this improvement from BPPO to chitosan/BPPO composite membranes should be attributed to hydrophilic groups and aliphatic chains of chitosan grafted onto the top of composite membranes, which would readjust structure of membrane top surface to exhibit exerted hydrodynamics in water. The surface pore density and the average surface pore size of the pristine BPPO membrane and chitosan/BPPO composite membranes were calculated based on the SEM images (Figure 4). From Figures 4 and 5, it can be seen that both pore density

Figure 2. FTIR spectra of chitosan and the top surface of BPPO membrane and chitosan/BPPO composite membranes.

hydrophilicity of membrane surface. As can been seen in Figure 3, the contact angle of the pristine BPPO membrane is 70.3°.

Figure 4. SEM images of top surfaces of (a) the pristine BPPO membrane, (b) chitosan/BPPO-2h, (c) chitosan/BPPO-4h and (d) chitosan/BPPO-6h membrane.

Figure 3. Static contact angle (top) and dynamic contact angle (bottom) measurement of the pristine BPPO and chitosan/BPPO composite membranes.

After chitosan treatment, it can be seen that the contact angle of all composite membranes decreases to 37.9° from the chitosan/BPPO-2h to chitosan/BPPO-6h membrane with the increasing chitosan treatment. It seems that longer treatment time would result in an increasing amount of chitosan being introduced onto the top surface of the BPPO membrane and

Figure 5. Surface pore density and average surface pore size of the pristine BPPO and chitosan/BPPO composite membranes. 14977



Article

and the average pore size decrease with increasing chitosan treatment time from 2 to 6 h in comparison with the pristine membrane. This might result from the enhanced cross-linking effect of chitosan, which would cover and block some large pores; therefore lower flux with improved rejection rate for such chitosan-grafted BPPO composite membranes is expected. Figure 6 and Table 1 show the flux and molecular weight cutoff (MWCO) of pristine BPPO membranes and chitosan/

Table 2. Fouling Resistance and Flux Recovery (FR) of BPPO and Chitosan/BPPO Composite Membranes sample

pristine

chitosan/ BPPO-2h

chitosan/ BPPO-4h

chitosan/ BPPO-6h

Rm, ×109 m−1 Rir, ×109 m−1 Rr, ×109 m−1 Rt, ×109 m−1 Rr/(Rr + Rir), % FR, %

8.02 9.88 15.07 32.97 60.39 44.80

8.63 9.28 22.53 40.44 70.82 48.18

14.35 2.50 41.40 58.25 94.30 85.14

11.68 3.99 31.66 47.33 88.81 74.55

smaller surface pore size on the modified membrane surface compared to the pristine BPPO membrane. However, with the grafting of chitosan onto the top surface of BPPO membrane, the irreversible resistance greatly decreased while the reversible resistance increased, and consequently, the ratio of Rr/(Rr+Rir) increased compared to the pristine membrane. This indicates that the BSA adsorbed onto the top surface of composite membranes was more easily to be washed off during the cleaning process. Therefore, improved flux recovery can be obtained in composite membranes (Table 2). Specifically, the flux recovery of chitosan/BPPO-4h membrane (85.1%) is almost twice that of the pristine membrane (44.8%). In this study, due to the grafting of hydrophilic chitosan onto the top surface of the BPPO membrane, a more hydrophilic surface was obtained (Figure 3). However, the static BSA adsorption test shows that the amount of BSA adsorbed onto the top surface of composite membranes is similar to that of the pristine BPPO membrane, as Figure 7 shows (the BSA

Figure 6. Flux of the pristine BPPO and chitosan/BPPO composite membranes.

Table 1. Molecular Weight Cutoff of the Pristine BPPO and Chitosan/BPPO Composite Membranes rejection, % sample BPPO chitosan/ BPPO-2h chitosan/ BPPO-4h chitosan/ BPPO-6h

35 kDa PEG

100 kDa PEG

200 kDa PEG

molecular weight cutoff, kDa

21.9 27.5

96.5 97.7

97.0 97.8

92 87

32.9

98.3

98.3

86

47.8

98.1

98.5

80

BPPO composite membranes, respectively. With the chitosan treatment, the flux decreases from 400 to 300 LMH from BPPO to chitosan/BPPO-6h membrane while MWCO decreases from 92 to 80 kDa. The average pore size of the pristine BPPO membrane, chitosan/BPPO-2h, chitosan/ BPPO-4h and chitosan/BPPO-6h sample calculated from the MWCO40 is 15.84, 15.40, 15.31 and 14.75 nm, respectively. It means that composite membrane would possess an obvious improvement in rejection without much permeation loss. That should be due to the above-mentioned cross-linking effect of chitosan, which would decrease the number and the size of the pores in the top surface of the composite membranes (Figures 4 and 5). To quantitatively investigate the membrane antifouling property, flux recovery (FR), intrinsic membrane resistance, which is related to membrane properties, reversible resistance due to the loose attachment of foulants on the top surface of membranes, irreversible resistance because of adsorption of foulants and total resistance were calculated via dynamic BSA fouling test (see the Experimental Section). It can be seen from Table 2 that the modified membranes suffered more severe reversible fouling due to the fact that higher transmembrane pressure was needed during the fouling process due to the

Figure 7. Effect of static BSA adsorptive fouling and cleaning on the flux changes and the static BSA adsorption on the pristine and chitosan/BPPO-4h membrane.

adsorbed onto the top membrane surface of the pristine and the chitosan/BPPO-4h membrane is 17.0 μg·cm−2 and 16.4 μg· cm−2, respectively). The effect of static BSA adsorption on the permeation performances of the pristine and the composite membranes was also measured (Figure 7). After the static BSA adsorption on both the pristine and chitosan/BPPO-4h membrane for 24 h, the flux ratio of both membranes is almost the same (the flux ratio is the flux changes after and before static BSA adsorption). However, the most significant difference is the reversibility of flux loss after cleaning. It can be seen from Figure 7 that, after physical and chemical cleaning, the flux recovery of the chitosan/BPPO-4h membrane is almost doubled compared to the flux ratio before cleaning whereas the flux ratio of the pristine membrane does not change much before and after cleaning. This means that though the mount of BSA absorbed onto the membrane surface is similar, the BSA adsorbed onto the surface of composite membranes are loosely 14978



Article

Table 3. Surface Energy, Interfacial Free Energy and Interaction Strength between BSA and Membrane Surface of the Pristine BPPO and Chitosan/BPPO Membranes sample

rs(w), mN·m−1

rPs(w), mN·m−1

rds(w), mN·m−1

rs(w)w, mN·m−1

ΔFpws, ergs·cm−2

Apws, ergs

pristine chitosan/BPPO-2h chitosan/BPPO-4h chitosan/BPPO-6h

38.68 48.31 59.23 56.73

12.22 33.43 50.59 46.45

26.46 14.88 8.60 10.28

13.52 2.51 3.02 2.25

−6.33 −0.62 3.01 2.09

814.60 79.79 −387.83 −268.19

rs(w), the total surface energy of membrane top surface; rPs(w) and rds(w), the polar and dispersive component of the total surface energy of membrane top surface; rs(w)w, the interfacial free energy of membrane top surface; ΔFpws, the change in free energy when the protein in water is brought from infinite to equilibrium distance and the equilibrium distance between protein and surface is taken to be approximately 0.16 nm in this study; Apws, Hamaker constant

bound with the membrane top surface and therefore are much easier to be washed off compared to the pristine membrane. The surface energy of membranes and the strength of the interactions between the BSA and the membrane top surface are tested and calculated, as shown in Table 3. It can be seen that with the successful grafting of chitosan, the total surface energy of the top surface of the membrane increases. Moreover, the polar component dominates in the total surface energy due to the grafting of polar groups such as −OH and −NH2 from chitosan onto the relatively nonpolar BPPO top surface. Based on eq 5, a low value for the solid−water interfacial free energy is obtained when the values of the individual surface free energy components (polar and dispersive) of the polymer top surface approach those of their respective counterparts of water (the polar and dispersive surface energy of water is 51 and 21.8 mN· m−1, respectively). It can be seen from Table 3, with chitosangrafting, the polar component of composite membrane surface approach to the value of the polar surface energy of water while the difference of the dispersive component of membranes and the counterpart of water becomes larger. However, the interfacial free energy of top membrane surface decreases with the grafting of chitosan. This means that in chitosan/ BPPO composite membranes, polar component is responsible and the main contributor for the reduction of interfacial free energy. The strength of the interactions between BSA and polymer surface was evaluated via Hamaker constant (Apws). A high value of the Hamaker constant means strong interactions between two bodies. It can be seen from Table 3 that the Hamaker constant of chitosan/BPPO composite membranes is lower than that of the pristine membrane, indicating that BSA was loosely bound with the polymer membrane surface on the composite membrane surface. Specifically, the chitosan/BPPO4h membrane has the lowest Hamaker constant, which means that the BSA adsorbed onto such a top membrane surface is much easier to be washed off during cleaning; thus high flux recovery is obtained. This is consistent with the flux recovery results as shown in Table 2. Besides the antifouling property, the antibacterial property of the chitosan-grafted BPPO membrane is performed. Figure 8 shows the antibacterial results of the pristine BPPO membrane and the optimum chitosan/BPPO-4h membrane. To better evaluate the bacterial-adsorption and bacteria-killing ability of the membranes, the E. coli loosely adsorbed onto surface and strongly attached to membrane surface were both retrieved by washing membranes with PBS buffer (indicated as washing solution) and ultrasonication in PBS buffer afterward (indicated as ultrasonication solution). Compared with the pristine BPPO membrane, the composite membrane shows strong antibacterial ability and the total number of recovered viable E. coli from

Figure 8. Photographs showing the bacterial culture plates of E. coli (a) from the washing off solution; (b) from ultrasonication solution and (c) quantitative characterization of viable E. coli recovered from membranes by washing and sonication. The pristine and chitosan/ BPPO-4h composite membranes were selected for the test.

the chitosan/BPPO-4h sample is about 30% of that of the BPPO membrane. And this is equal to 70% E. coli inactivation. Figure 9 also presents the SEM images of E. coli in contact with pristine and composite membranes. Most of the cells on the pristine BPPO membranes appeared healthy and their cell membranes were intact. In contrast, some of bacterial cells on chitosan/BPPO composite membranes had lost their membrane integrity. Specially, the impacted cells displayed a shrinking shape, as Figure 9c shows, or were totally broken, as Figure 9d shows. The antibacterial property of chitosan has been welldocumented.41,42 The exact mechanism of the antibacterial action of chitosan is still unknown but different mechanisms have been proposed: one proposed mechanism is that positively charged chitosan can interact with negatively charged cell membranes, causing an increase in membrane permeability, and as a result, the leakage of proteinaceous and other intracellular constituents occurs; another possible mechanism is that chitosan can chelate trace metals, resulting in the inhibition of enzyme activities of bacteria.30,31 In this study, the improved 14979



Article

woven fabric by dip-coating PVA (polyvinyl alcohol). Sep. Purif. Technol. 2008, 61, 276−286. (2) Mockel, D.; Staude, E.; Guiver, M. D. Static protein adsorption, ultrafiltration behavior and cleanability of hydrophilized polysulfone membranes. J. Membr. Sci. 1999, 158, 63−75. (3) Sadana, A. Protein adsorption and inactivation on surfaces influence of heterogeneities. Chem. Rev. 1992, 92, 1799−1818. (4) Ostuni, E.; Chapman, R. G.; Holmlin, R. E.; Takayama, S.; Whitesides, G. M. A survey of structure-property relationships of surfaces that resist the adsorption of protein. Langmuir 2001, 17, 5605−5620. (5) Jeyachandran, Y. L.; Mielezarski, E.; Rai, B.; Mielczarski, J. A. Quantitative and qualitative evaluation of adsorption/desorption of bovine serum albumin on hydrophilic and hydrophobic surfaces. Langmuir 2009, 25, 11614−11620. (6) Fang, F.; Szleifer, I. Effect of molecular structure on the adsorption of protein on surfaces with grafted polymers. Langmuir 2002, 18, 5497−5510. (7) Kang, S.; Asatekin, A.; Mayes, A. M.; Elimelech, M. Protein antifouling mechanisms of PAN UF membranes incorporating PAN-gPEO additive. J. Membr. Sci. 2007, 296, 42−50. (8) Rana, D.; Matsuura, T. Surface modifications for antifouling membranes. Chem. Rev. 2010, 110, 2448−2471. (9) Lensun, L.; Smith, T. A.; Gee, M. L. Partial denaturation of silicaadsorbed bovine serum albumin determined by time-resolved evanescent wave-induced fluorescence spectroscopy. Langmuir 2002, 18, 9924−9931. (10) Kim, J.; Somorjai, G. A. Molecular packing of lysozyme, fibrinogen, and bovine serum albumin on hydrophilic and hydrophobic surfaces studied by infrared-visible sum frequency generation and fluorescence microscopy. J. Am. Chem. Soc. 2003, 125, 3150− 3158. (11) Ying, P. Q.; Jin, G.; Tao, Z. L. Competitive adsorption of collagen and bovine serum albumin - effect of the surface wettability. Colloids Surf., B 2004, 33, 259−263. (12) Tyszler, D.; Zytner, R. G.; Batsch, A.; Brugger, A.; Geissler, S.; Zhou, H. D.; Klee, D.; Melin, T. Reduced fouling tendencies of ultrafiltration membranes in wastewater treatment by plasma modification. Desalination 2006, 189, 119−129. (13) Gancarz, I.; Pozniak, G.; Bryjak, M. Modification of polysulfone membranes 1. CO2 plasma treatment. Eur. Polym. J. 1999, 35, 1419− 1428. (14) Kim, K. S.; Lee, K. H.; Cho, K.; Park, C. E. Surface modification of polysulfone ultrafiltration membrane by oxygen plasma treatment. J. Membr. Sci. 2002, 199, 135−145. (15) Gancarz, I.; Pozniak, G.; Bryjak, M. Modification of polysulfone membranes 3. Effect of nitrogen plasma. Eur. Polym. J. 2000, 36, 1563−1569. (16) Combe, C.; Molis, E.; Lucas, P.; Riley, R.; Clark, M. M. The effect of CA membrane properties on adsorptive fouling by humic acid. J. Membr. Sci. 1999, 154, 73−87. (17) Gonzalez Munoz, M. P.; Navarro, R.; Saucedo, I.; Avila, M.; Pradanos, P.; Palacio, L.; Martinez, F.; Martin, A.; Hernandez, A. Hydrofluoric acid treatment for improved performance of a nanofiltration membrane. Desalination 2006, 191, 273−278. (18) Maartens, A.; Swart, P.; Jacobs, E. P. Membrane pretreatment: A method for reducing fouling by natural organic matter. J. Colloid Interface Sci. 2000, 221, 137−142. (19) Lee, H.; Amy, G.; Cho, J. W.; Yoon, Y. M.; Moon, S. H.; Kim, I. S. Cleaning strategies for flux recovery of an ultrafiltration membrane fouled by natural organic matter. Water Res. 2001, 35, 3301−3308. (20) Reddy, A. V. R.; Mohan, D. J.; Bhattacharya, A.; Shah, V. J.; Ghosh, P. K. Surface modification of ultrafiltration membranes by preadsorption of a negatively charged polymer I. Permeation of water soluble polymers and inorganic salt solutions and fouling resistance properties. J. Membr. Sci. 2003, 214, 211−221. (21) Ma, X.; Su, Y.; Sun, Q.; Wang, Y.; Jiang, Z. Enhancing the antifouling property of polyethersulfone ultrafiltration membranes

Figure 9. SEM images of E. coli attached on (a,b) the pristine BPPO membrane and (c,d) chitosan/BPPO-4h composite membrane.

antifouling property of chitosan/BPPO composite membranes can be attributed to the antibacterial nature of chitosan. Further study is required to test the stability of chitosan/BPPO composite membranes in the long-term fouling operation and the introduction of pore-former in chitosan layer to maintain the flux while improving the antifouling and antibacterial property.

4. CONCLUSIONS Chitosan can be easily introduced onto the top of a BPPO membrane and chemically bonded with a BPPO polymer without other cross-linkers and harsh reaction conditions. Due to the cross-linking effect of chitosan, the flux slightly decreased but with obvious improved rejection. The introduction of polar groups of chitosan onto the top surface of BPPO membranes increase the top surface energy, especially the polar component of the total surface energy, and as a result, reduce the strength of interaction between foulants and membrane top surface. Therefore, foulants are much more easily desorbed and consequently higher flux recovery is obtained in chitosangrafted BPPO composite membranes. Moreover, due to the antibacterial property of chitosan itself, the composite membranes also show improved antibacterial property. Therefore, such low-cost chitosan/BPPO composite ultrafiltration membranes are promising for practical industrial filtration applications.

■

AUTHOR INFORMATION

Corresponding Author

*H. Wang. Tel.: +61399053449. E-mail: huanting.wang@ monash.edu. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was in part supported by the Australia-India S & T Fund and the Australian Research Council (ARC). The authors acknowledge the use of the facilities and the assistance of staff at the Monash Centre for Electron Microscopy. H.W. thanks the ARC for a Future Fellowship (FT100100192).

■

REFERENCES

(1) Zhang, C.-H.; Yang, F.-L.; Wang, W.-J.; Chen, B. Preparation and characterization of hydrophilic modification of polypropylene non14980



Article

through surface adsorption-crosslinking of poly(vinyl alcohol). J. Membr. Sci. 2007, 300, 71−78. (22) Hvid, K. B.; Nielsen, P. S.; Stengaard, F. F. Preparation and characterization of a new ultrafiltration membrane. J. Membr. Sci. 1990, 53, 189−202. (23) Nunes, S. P.; Sforca, M. L.; Peinemann, K. V. Dense hydrophilic composite membranes for ultrafiltration. J. Membr. Sci. 1995, 106, 49− 56. (24) Susanto, H.; Ulbricht, M. Photografted thin polymer hydrogel layers on PES ultrafiltration membranes: Characterization, stability, and influence on separation performance. Langmuir 2007, 23, 7818− 7830. (25) Tang, B. B.; Xu, T. W.; Gong, M.; Yang, W. H. A novel positively charged asymmetry membranes from poly(2,6-dimethyl-1,4phenylene oxide) by benzyl bromination and in situ amination: membrane preparation and characterization. J. Membr. Sci. 2005, 248, 119−125. (26) Lin, X.; Liang, X.; Poynton, S. D.; Varcoe, J. R.; Ong, A. L.; Ran, J.; Li, Y.; Li, Q.; Xu, T. Novel alkaline anion exchange membranes containing pendant benzimidazolium groups for alkaline fuel cells. J. Membr. Sci. 2013, 443, 193−200. (27) Lin, X.; Varcoe, J. R.; Poynton, S. D.; Liang, X.; Ong, A. L.; Ran, J.; Li, Y.; Xu, T. Alkaline polymer electrolytes containing pendant dimethylimidazolium groups for alkaline membrane fuel cells. J. Mater. Chem. A 2013, 1, 7262−7269. (28) Lin, X.; Wu, L.; Liu, Y.; Ong, A. L.; Poynton, S. D.; Varcoe, J. R.; Xu, T. Alkali resistant and conductive guanidinium-based anionexchange membranes for alkaline polymer electrolyte fuel cells. J. Power Sources 2012, 217, 373−380. (29) Liu, X. F.; Guan, Y. L.; Yang, D. Z.; Li, Z.; De Yao, K. Antibacterial action of chitosan and carboxymethylated chitosan. J. Appl. Polym. Sci. 2001, 79, 1324−1335. (30) No, H. K.; Park, N. Y.; Lee, S. H.; Meyers, S. P. Antibacterial activity of chitosans and chitosan oligomers with different molecular weights. Int. J. Food Microbiol. 2002, 74, 65−72. (31) Rabea, E. I.; Badawy, M. E. T.; Stevens, C. V.; Smagghe, G.; Steurbaut, W. Chitosan as antimicrobial agent: Applications and mode of action. Biomacromolecules 2003, 4, 1457−1465. (32) Boom, R. M.; Wienk, I. M.; Vandenboomgaard, T.; Smolders, C. A. Microstructures in phase inversion membranes. 2. The role of a polymeric additive. J. Membr. Sci. 1992, 73, 277−292. (33) Smolders, C. A.; Reuvers, A. J.; Boom, R. M.; Wienk, I. M. Microstructures in phase-inversion membranes. 1. Formation of macrovoids. J. Membr. Sci. 1992, 73, 259−275. (34) Rinaudo, M.; Pavlov, G.; Desbrieres, J. Influence of acetic acid concentration on the solubilization of chitosan. Polymer 1999, 40, 7029−7032. (35) Razmjou, A.; Mansouri, J.; Chen, V. The effects of mechanical and chemical modification of TiO2 nanoparticles on the surface chemistry, structure and fouling performance of PES ultrafiltration membranes. J. Membr. Sci. 2011, 378, 73−84. (36) Ruckenstein, E.; Lee, S. H. Estimation of the equilibrium surface free-energy components of restructuring solid-surfaces. J. Colloid Interface Sci. 1987, 120, 153−161. (37) Lee, S. H.; Ruckenstein, E. Adsorption of proteins onto polymeric surfaces of different hydrophilicities - a case-study with bovine serum-albumin. J. Colloid Interface Sci. 1988, 125, 365−379. (38) Norde, W. Adsorption of proteins from solution at the solidliquid interface. Adv. Colloid Interface Sci. 1986, 25, 267−340. (39) Perreault, F.; Tousley, M. E.; Elimelech, M. Thin-film composite polyamide membranes functionalized with biocidal graphene oxide nanosheets. Environ. Sci. Technol. Lett. 2014, 1, 71−76. (40) Peeva, P. D.; Million, N.; Ulbricht, M. Factors affecting the sieving behavior of anti-fouling thin-layer cross-linked hydrogel polyethersulfone composite ultrafiltration membranes. J. Membr. Sci. 2012, 390−391, 99−112. (41) Wang, C.; Yang, F.; Zhang, H. Fabrication of non-woven composite membrane by chitosan coating for resisting the adsorption

of proteins and the adhesion of bacteria. Sep. Purif. Technol. 2010, 75, 358−365. (42) Musale, D. A.; Kumari, A.; Pleizier, G. Formation and characterization of poly(acrylonitrile)/chitosan composite ultrafiltration membranes. J. Membr. Sci. 1999, 154, 163−173.

14981


Synthesis and Characterization of Chitosan-Grafted BPPO

Recommend Documents