Preparation and Characterization of Novel Polyethersulfone Hybrid

Article pubs.acs.org/IECR

Preparation and Characterization of Novel Polyethersulfone Hybrid Ultrafiltration Membranes Bending with Modified Halloysite Nanotubes Loaded with Silver Nanoparticles Jingyi Zhang,† Yatao Zhang,*,† Yifeng Chen,† Lei Du,‡ Bing Zhang,† Haoqin Zhang,† Jindun Liu,*,† and Kaijuan Wang§ †

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R. China Department of Bioengineering, Zhengzhou University, Zhengzhou 450001, P. R. China § College of Public Health, Zhengzhou University, Zhengzhou 450001, P. R. China ‡

ABSTRACT: In this study, poly(4-vinylpyridine) (P4VP) was first grafted onto the surface of halloysite nanotubes (HNTs) via in situ polymerization, and then, silver ions were immobilized on P4VP via complex reaction. Finally, silver ions were reduced to silver nanoparticles (Ag NPs). Polyethersulfone (PES) ultrafiltration membranes bending with modified HNTs loaded with Ag NPs were prepared via phase inversion. FT-IR spectra and TGA results showed that HNTs were modified successfully. The contact angle data indicated that the hydrophilicity of the membranes was enhanced by the addition of modified HNTs. The permeation properties of the hybrid membranes were significantly superior to the pure PES membrane, especially when the modified HNTs content was 3%; the pure water flux of the membrane reached the maximum at 396.5 L·m−2·h−1, which was about 251.5% higher than that of the pure PES membrane, and the rejection was slightly affected by the addition of the modified HNTs. The microstructure of the membranes was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that the structure of membrane was not obviously affected by addition of the modified HNTs. Antibacterial activity of the hybrid membrane was evaluated with the viable cell count method using antibacterial rate against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The antibacterial rates of the hybrid membranes against E. coli and S. aureus were about 99.9% and 99.8%, respectively.

1. INTRODUCTION Nowadays, membrane technology has been proven to be an efficient and attractive approach for separation, concentration, and purification in industry because it is faster, energy-saving, and environmentally friendly and does not involve any phase change.1,2 Especially, ultrafiltration (UF) has received an increased attention in the food, in pharmaceutical and biotechnological industries, in pure water production, in wastewater treatments, etc.3−6 Polyethersulfone (PES) has been widely used as a UF membrane material in many industrial fields due to many good performances such as high mechanical property and heat distortion temperature, good heat-aging resistance, and environmental endurance, as well as easy processing.7−10 Membrane fouling is one of the most common and serious problems in the UF processes, which incurs high operating cost and makes membrane technology less favorable in water treatment application. Despite that we have made efforts to develop better antifouling membranes and improve foulingcontrol strategies, membrane fouling still occurs over time, so there are still a lot of things for us to do. Membrane fouling can be divided into two major areas: organic fouling caused by natural organic matter contained in raw water that accumulated on the membrane surface as a cake layer and biofouling caused by microorganisms that adhered to the membrane surface.11 It is known that many factors, especially hydrophilicity, can affect the membrane fouling,12 but PES is hydrophobic in nature which is controlled by its structure and is easily © 2012 American Chemical Society

susceptible for fouling. Thus, it is necessary to modify the PES membrane surface by physical or chemical methods in order to improve its hydrophilicity. Many efforts have been devoted to improving the hydrophilicity of PES; particularly, blending with inorganic materials, especially nanoparticles, has attracted much interest due to their convenient operation and mild conditions.10 A variety of nanoparticles have been introduced to modify PES membranes, such as SiO2,5 Al2O3,13 ZrO2,14 TiO2,15 carbon nanotube,16 and so on. The research of Celik et al.16 showed that the presence of finely dispersed inorganic nanoparticles in the polymer matrix was proven very useful in the improvement of membrane antifouling performance. Membrane biofouling was initially induced by microbes that attached to and grew on the surface of the membranes in use, and for the long-term operation, there would be the undesirable accumulation of microorganisms and the build-up of biofilm on the membrane surface, which resulted in the decline of membrane flux and the increase of the operational pressure and energy cost. Therefore, it is necessary to endow the membrane with a self-antibacterial property for inhibiting the development of biofilm, and it is considerably important to avoid the attachment of microorganisms to the membrane surface. The common strategy in preventing membrane Received: Revised: Accepted: Published: 3081

October 28, 2011 December 26, 2011 January 9, 2012 January 9, 2012 dx.doi.org/10.1021/ie202473u | Ind. Eng.Chem. Res. 2012, 51, 3081−3090

Industrial & Engineering Chemistry Research

Article

Figure 1. Basic principles of the reactions which modified HNTs.

biofouling is often to add biocides or antibacterial agents.17 Several antibacterial agents have been introduced to prepare antimicrobial membranes, such as TiO2,18−20 copper ions,21,22 silver ions, or silver nanoparticles,23−25 and various organic antibacterial agents.26,17 Especially, silver ions or silver nanoparticles have attracted considerable interest just because silver has been well-known and widely used for a long time as an effective antibacterial metal, and it exhibits powerful antimicrobial activity and has a broad antibacterial spectrum toward many different bacteria.27 However, silver ions will easily be washed away if they are simply dispersed in the membrane without any chemical interaction. As we all know that nitrogen has a lone pair or lone pairs of electrons that can bind a proton or a metal ion through an electron pair sharing to form a complex, some materials that have nitrogen such as PVP24 and chitosan28,29 have been used to bind silver ions. Whereas Qiu et al.21 used poly(4-vinylpyridine) (P4VP) brushes to immobilize copper(II) ions onto the membrane surface through coordination chemistry for P4VP, brushes have the ability of complexing transition metals by the pyridine ring and thus can act as a multidentate ligand. The interaction between pyridine ring and silver ions is more intense than that between nitrogen and silver ions. Thus, we choose P4VP immobilized silver nanoparticles as antibacterial agents to construct a antimicrobial membrane. Halloysite nanotubes (HNTs) are naturally occurring alumino-silicates clays (Al2Si2O5(OH)4·2H2O), which are affluent in China as well as other locations around the world.30 HNTs possess hollow nanotubular structure in the submicrometer range and large specific surface area; furthermore, there are a few hydroxyl groups on the surface, which indicates that HNTs possess much better dispersion property and are easily modified. Because of the physical and chemical properties, HNTs have been applied in many fields used as adsorbents,31 catalyst supports,32 and so on. Compared with other nanosized materials, naturally occurring HNTs are easily obtained and much cheaper than other nanoparticles such as carbon nanotubes (CNTs). More importantly, the

unique tubular structure and hydrophilic group of HNTs are similar to that of CNTs.33 Therefore, HNTs may have the potential to replace the expensive CNTs as hydrophilic additives. In this study, for the purpose of getting a novel PES ultrafiltration membrane which possesses both organic antifouling and antibacterial properties, PES hybrid ultrafiltration membranes bending with modified HNTs loaded with silver nanoparticles (Ag NPs) were prepared. In order to get the modified halloysite nanotubes loaded with silver nanoparticles, a series of reactions including in situ polymerization took place to make modifications of HNTs. Fourier transforminfrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM) were used to characterize the modified HNTs. The separation performance and water contact angle of the membranes were investigated in detail. The surface morphology of the membranes was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). In addition, antibacterial activity of the hybrid membrane was evaluated with the viable cell count method using antibacterial rate against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus).

2. MATERIALS AND METHODS 2.1. Materials. Polyethersulfone (PES) was supplied by BASF Company. Polyvinylpyrrolidone (PVP), N,N-dimethylacetamide (DMAc), and other common materials were purchased from Kermel. KH-792 silane coupling was purchased from Merck Company. 4-Vinylpyridine (Acros, 95%) was used after vacuum distillation. All reagents were analytical reagent grade without further treatment. The powder of halloysite nanotubes (HNTs) was refined from clay minerals in Henan province, China. The test strains, E. coli (8099) and S. aureus (ATCC6538), used for this study were provided by College of Public Health of Zhengzhou University. 2.2. Modification of HNTs. In order to get the modified halloysite nanotubes loaded with silver nanoparticles, a series of 3082

dx.doi.org/10.1021/ie202473u | Ind. Eng.Chem. Res. 2012, 51, 3081−3090


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cast with a casting knife with the thickness of 0.3 mm onto a glass plate at room temperature. The nascent membrane was evaporated at 25 °C for 30 s and then immersed in a water coagulation bath with the temperature of 40 °C. After complete coagulation, the membrane was kept in deionized water until used, but the deionized water should be replaced every day. 2.4. Characterization of Modified HNTs. 2.4.1. Fourier Transform-Infrared Spectroscopy (FT-IR). To observe the chemical structure changes of HNTs before and after modification, FT-IR spectra of pure HNTs and modified HNTs were performed at 2 cm−1 resolution with Thermo Nicolet IR 200 spectroscope (Thermo Nicolet Corporation, USA). Typically, 64 scans were signal-averaged to reduce spectral noise. 2.4.2. Thermal Analysis. Thermogravimetric analysis (TGA) measurements were carried out using a TG-DTA,DT-40 system (Shimadzu, Japan). The samples were heated from 0 to 800 at 10 °C per min under flowing nitrogen. 2.4.3. Transmission Electron Microscopy (TEM). A FEI Model TECNAI G2 transmission electron microscope (200 KV acceleration voltages) was used to study the nanotubular shapes of the HNTs. The samples for analysis were ground in ethanol and agitated in a glass vial to disperse the particles within the solvent. The suspended particles were transferred to and allowed to dry on a copper grid (400 meshes) coated with a strong carbon film. 2.5. Characterization of the Membranes. 2.5.1. Permeation Properties. A cross-flow filtration system was used to characterize the permeation properties of the PES/ modified HNTs hybrid membranes. All filtration experiments were conducted at a constant trans-membrane pressure of 100 kPa and a system temperature of 25 ± 2 °C. Then, the PEG20000 solution and PVA30000-70000 solution were measured after a total of 50 mL of permeate were collected, respectively, and the concentrations of PEG20000 and PVA30000-70000 were obtained by UV spectrophotometer. The flux was defined as:

reactions including in situ polymerization took place. Figure 1 showed the basic principles of the reactions. The modification mainly involved the following steps. 2.2.1. Amino Grafting on HNTs. Before the reaction, HNTs should be dried at 400 °C for 5 h to remove the adsorbed water molecules. First, about 60 mL of KH-792 silane coupling was added into 150 mL of toluene by shaking, and then, 10 g of HNTs was dispersed into the solution. Thereafter, the resulting mixture was stirred under backflow at 125 °C for 24 h. At last, the modified HNTs was obtained by centrifugation and washed with isopropanol for 4−5 times. Then, the product was dried in a vacuum drying chamber at 60 °C. 2.2.2. Double-Bond Grafting on HNTs. Five grams of modified HNTs after amino grafting, 20 mL of acryloyl chloride, and 100 mL of methylene chloride were added into a 250 mL three-necked flask equipped with a reflux condenser. Under the continuous stirring, the reaction was carried out at 40 °C for 4 h. When the reaction came to the end, the final precipitation was washed with methylene chloride for 4−5 times and then was obtained by centrifugation. Finally, the modified HNTs were dried in a vacuum drying chamber at 60 °C. 2.2.3. In Situ Polymerization on HNTs. Four grams of modified HNTs after double-bond grafting was dispersed into 150 mL of 4-vinylpyridine (4VP) buffer solution, and the mixture was placed under N2 conditions for 15 min; thereafter, 1.57 g of azobisisobutyronitrile (AIBN) was added into the mixture, and the in situ polymerization reaction was carried out under N2 conditions at 70 °C for 24 h. Then, the product was washed with methanol aqueous solution for about 5 times and obtained by centrifugation. Finally, the modified HNTs were dried in vacuum drying chamber at 60 °C. 2.2.4. Immobilization of Silver Nanoparticles. Two grams of modified HNTs after in situ polymerization, 1 mL of AgNO3 solution with the concentration range of 0.1−0.4 mol/L, and 100 mL of methanol were added into a 250 mL conical flask, and then, the mixture was stirred continuously on the magnetic stirrer for half an hour. Thereafter, the mixture was oscillated for 24 h in a constant temperature oscillator at 20 °C and 250 r/min rotational speeds. Then, the product was washed with methanol aqueous solution for about 5 times and obtained by centrifugation. Finally, the modified HNTs were dried in a vacuum drying chamber at 60 °C. NaHB4 (1.2 g), all the modified HNTs after loading with silver ions, and 5 mL of 0.01 mol/L NaOH solution were added into a 250 mL conical flask, and some deionized water was added until the pH of the mixture was 8.0. After stirring for half an hour on the magnetic stirrer, the final precipitation was washed with deionized water and then obtained by centrifugation. Finally, the final modified HNTs were dried in vacuum drying chamber at 60 °C. 2.3. Preparation of Membranes. Pure PES membrane and PES/modified HNTs hybrid flat membranes were prepared by phase inversion methods. First, modified HNTs (0, 0.01, 0.02, and 0.03 HNTs/PES ratios, w/w, respectively) were added into DMAc and stirred continuously for half an hour for good dispersion. After dispersing modified HNTs in solvent, PES (18 wt %), PVP (8 wt %), and acetone (0.8 wt.%) were dissolved in the dope solution by continuous stirring at room temperature for 12 h to obtain a uniform and homogeneous casting suspension. The casting solution was ultrasonicated to remove air bubbles; after air bubble removal, the casting solution was

J=

V A×T

(1)

where V is the volume of the permeate pure water (L), A is the effective area of the membrane (m2), and T is the permeation time (h). In our experiments, the effective area of the membranes was 22.2 cm2. Rejection was calculated by the following equation:

⎛ Cp ⎞ %R = ⎜1 − ⎟ × 100 Cf ⎠ ⎝

(2)

where Cp is the concentration of PEG20000 or PVA3000070000 in permeate and Cf is the concentration of PEG20000 or PVA30000-70000 in the feed. 2.5.2. Water Contact Angles. Water contact angles (θ) were measured at 25 °C and 50% RH on a contact angle system (OCA20, Dataphysics Instruments, Germany) for the evaluation of the membrane hydrophilicity. Five μL of water was carefully dropped on the top surface, and the dynamic contact angles were determined using the high speed optimum video analysis system. To minimize the experimental error, the contact angle was measured at five random locations for each sample and then the average was reported. 2.5.3. Scanning Electron Microscopy (SEM). Samples of the membranes were frozen in liquid nitrogen and then fractured. 3083



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Cross section and surface of the membranes were sputtered with gold and then transferred to the microscope. The morphology of the cross section and surface of the membranes were inspected by SEM using a JEOL Model JSM-6700F scanning electron microscope (Tokyo, Japan). 2.5.4. Atomic Force Microscopy (AFM). For analyzing the surface morphology and roughness of the membrane surface, atomic force microscopy was employed using the AFM apparatus (DI Nanoscope IIIa, Veeco, USA). Small squares of the prepared membranes (approximately 1 cm2) were cut and glued on glass substrate. The membrane surfaces were imaged in a scan size of 5 nm × 5 nm. 2.5.5. Transmission Electron Microscopy (TEM). In order to observe the distribution of modified HNTs in the membranes, transmission electron microscopy (TEM) measurement was carried out with a FEI model TECNAI G2 transmission electron microscope operated at 200 kV. The membranes were embedded in epoxy resin, and cross sections with a thickness of 50 nm were obtained by sectioning with a Leica Ultracut UCT ultramicrotome. Then, these thin sections were mounted on the carbon-coated TEM copper grids. 2.5.6. X-ray Photoelectron Spectroscopy (XPS). In order to confirm the presence of silver on the membrane surface, the chemical composition of the membrane surface was analyzed by the XPS (AXIS Ultra, Kratos, England). For the XPS analysis, the base pressure of the analyzer chamber was about 5 × 10−7 Pa. The survey spectra (from 0 to 1400 eV) were recorded. During the wide-scan, peak for C 1s was observed at binding energy 284.7 eV. All readings were calibrated with the corresponding C 1s as the standard for the correction of charging effects. 2.5.7. Tests of Antibacterial Activity. Antibacterial activity of the hybrid membrane was evaluated with the viable cell count method using antibacterial rate against E.coli and S. aureus. E. coli and S. aureus were inoculated in 5 mL of LB liquid nutrient medium, respectively, and oscillated for 12 h at 37 °C and 220 r/min rotational speed until the exponential growth phase was reached. The actual number of cells used for a given experiment was determined by the standard serial dilution method. The pure PES membrane and the PES hybrid membrane (0.03 g) were cut and sterilized by autoclaving for 20 min. To test the antibacterial activity, the membranes were added into the 5 mL solution inoculated by about 106 CFU (colony-forming units)/ mL of E. coli and S. aureus, respectively, which was then incubated at room temperature. At the same condition, a suspension culture without any membrane was used as blank sample. After 24 h, membranes were retrieved from cultures and washed by normal saline. The wash solutions were collected and diluted with deionized water until their concentrations becomes 10−3 of the original value. Dilution solution (0.2 mL) was spread onto LB culture medium, and all plates were incubated at 37 °C for 24 h. The numbers of colonies on the plates were determined by the plate count method.

Figure 2. FT-IR spectra of raw HNTs (a), modified HNTs after amino grafting (b), after grafted double-bond (c), and after in situ polymerization (d).

Figure 3. TGA curves of raw HNTs (a), modified HNTs after amino grafting (b), after grafted double-bond (c), and after in situ polymerization (d).

band at 910 cm−1.34,35 Compared with the raw HNTs, the modified HNTs after amino grafting (b) exhibited a new extensive absorption peak at 3451 cm−1 and a moderate peak at 1644 cm−1, which were the NH2 stretching vibration and inplane bending vibration, respectively. Thus, the new peaks could indicate that the silane coupling was successfully grafted onto raw HNTs. From the curve (c), we could find a wide absorption peak at 1654 to 1630 cm−1, and it was due to the stretching vibration of teritary amide, which was the proof that the modification of double-bond grafting onto HNTs made good play. Compared with curve (c), there were two new visible peaks in the curve (d). The first peak was at 2932 cm−1, and it was CH2 stretching absorption. Another peak was at 1610 cm−1, which was the characteristic absorption peak of pyridine ring. We knew that the in situ polymerization had taken place successfully from the frontal result. Figure 3 gave the TGA curves of raw HNTs (a) and modified HNTs after amino grafting (b), after double-bond grafting (c), and after in situ polymerization (d). Raw HNTs

3. RESULTS AND DISCUSSION 3.1. Characterization of HNTs. Figure 2 showed the FTIR spectra of raw HNTs (a) and modified HNTs after amino grafting (b), after double-bond grafting (c), and after in situ polymerization (d). The spectrum of raw HNTs showed two Al2OH stretching absorption bands at 3699 and 3628 cm−1, each OH being linked to two Al atoms, in-plane Si−O−Si stretching (1095 and 1032 cm−1), and a single Al2OH bending 3084



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Figure 4. TEM images of raw HNTs (a) and modified HNTs (b).

and 425 °C compared with raw HNTs which was due to decomposition of the grafted silane coupling. In the TGA curve of modified HNTs after double-bond grafting (c), the weight loss in the temperature range of 250−350 °C corresponded to decomposition of the grafted double-bond, while the weight loss between 350 and 425 °C was due to decomposition of the grafted silane coupling. For the modified HNTs after in situ polymerization (d), the weight loss between 200 and 325 °C, 325 and 375 °C, and 375 and 425 °C were due to decomposition of the grafted P4VP, double-bond, and silane coupling, respectively. We can get the grafted amount of each reaction, according to the flowing computing formula:

grafted amount = Δw /(1 − Δw)

(3)

where Δw is the weight loss. Thus, the total grafted amount of the modification was 0.110 g (P4VP)/g (HNTs). The results of TGA can also indicate that the modification had been successfully carried out from another point of view. Microstructure of raw HNTs and modified HNTs were observed by TEM, and the respective images were shown in Figure 4. From chart (a), which was raw HNTs, we could find that HNTs have a cylindrical shape and contain a transparent central area that runs longitudinally along the cylinder, indicating that the nanotubular particles are hollow and openended, with a length of 0.5−2 mm and an inner diameter of 20−30 nm. The shell thickness is 15−20 nm.33 The large and smooth unhindered pores can provide sufficient space for flowing of the water, which can remarkably improve the water flux of the membrane if HNTs were added into the membrane. In Figure 4(b), which was the modified HNTs, the HNTs had gathered into a mass just because P4VP had entwined around HNTs. Furthermore, a lot of black spots, which were silver nanoparticles of about 10 nm in diameter, were clearly observed on the surface of the modified HNTs, which indicated that silver nanoparticles had been successfully loaded onto HNTs. 3.2. Characterization of the Membranes. 3.2.1. Permeation Properties and Water Contact Angle. The effect of modified HNTs contents on the pure water flux and the rejection of PEG20000 and PVA30000-70000 were shown in Figure 5. As is seen, the permeation properties of the hybrid membranes were significantly superior to the pure PES membrane, and with the increase of the modified HNTs content, the pure water flux of the membranes raised rapidly, especially when the modified HNTs content went to 3%; the pure water flux of the membrane reached the maximum at

Figure 5. Effect of modified HNTs contents on the pure water flux and the rejection of PEG20000 and PVA30000-70000.

Figure 6. Schematic diagram of increase of pure water flux of the membranes.

(a) have a weight loss between 50 and 150 °C, which corresponds to loss of adsorbed water; second weight losses are due to structural dehydroxylation in the temperature range of 450−550 °C.32 For the modified HNTs after amino grafting (b), after double-bond grafting (c), and after in situ polymerization (d), the first weight loss between 50 and 150 °C was also due to loss of adsorbed water, and the weight loss in the temperature range of 450−550 °C was because of structural dehydroxylation, too. For the modified HNTs after amino grafting (b), there was a new weight loss between 250 3085



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Figure 7. SEM images of the pure PES membrane (a) and the hybrid membrane (b): 1, top surface; 2, bottom surface; 3, cross section.

396.5 L·m−2·h−1, which was about 251.5% higher than that of the pure PES membrane, and the rejection of the membranes against PVA30000-70000 was slightly affected by the addition of the modified HNTs. Only the rejection of the membranes against PEG20000 decreased a little, so the hybrid membranes had good permeation properties. Measurement of the water contact angle is commonly used to estimate the hydrophilicity and wetting characteristics of polymer surfaces. A large water contact angle represents a hydrophobic surface, whereas a small water contact angle

implies a hydrophilic surface.36 The pure PES membrane had a contact angle of about 85°, while the contact angle of the hybrid membrane with the modified HNTs content of 3% was 66°, which indicated that the hydrophilicity of the hybrid membrane had been improved due to the addition of the modified HNTs. This result could be explained due to the fact that, during the phase inversion process, hydrophilic HNTs migrated spontaneously to the membrane/water interface to reduce the interface energy.2 3086



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Figure 10. XPS spectra of the surface of PES/HNTs-Ag NPs hybrid membrane.

difference between the pure PES membrane (a1) and the hybrid membrane (b1) on the top surface, but more large pores emerged on the bottom surface of the hybrid membrane (b2) compared with pure PES membrane (a2), which maybe have a relation to increase of membrane flux. Compared with pure PES membrane (a3), there were more fingerlike pores and thinner skin layer in the hybrid membrane (b3), and the water molecule could pass through the membrane easier, which maybe was another reason of increase of membrane flux. According to the frontal analysis, we know that the basic structure of the membrane was not affected greatly and only a few changes appeared, which may contribute to the increase of membrane flux. Figure 8 gives the 3D AFM images of the pure PES membrane and the hybrid membrane. According to the research of Chung et al.,37 the darkest regions are depressions or pores and the light regions are the highest points. It can be seen that the nodular structure was formed on the surface of pure PES membrane in Figure 8(a), whereas the surface of the hybrid membrane in Figure 8(b) was smooth, which indicated that the roughness of the membrane was reduced due to the addition of modified HNTs. Furthermore, the membrane with high surface roughness indicated high flux and the membrane with smooth surface exhibited low flux,1 but the result of our

Figure 8. TEM images of the dispersion of modified HNTs in the PES hybrid membrane.

There were three possible factors that helped improve the pure water flux of the hybrid membranes: one was the improved hydrophilicity of the hybrid membrane due to the addition of the hydrophilic HNTs; the second one was the large and smooth unhindered pores in the HNTs, which could provide sufficient space for flowing of the water; the last one was increase in pore size of the hybrid membranes, which could be seen through a little decrease of the rejection of the membranes. The role of pore size seems more dominant than the hydrophilicity according to the research of Razmjou et al.8 Therefore, the main factors were the hollow structure of HNTs and increase in pore size, which could be expressed in Figure 6. 3.2.2. Microstructure of Membranes. Figure 7 showed the SEM images of the pure PES membrane and the hybrid membrane. The two membranes had similar asymmetric structure, which was the typical structure of ultrafiltration membranes, with a top dense layer, a porous sublayer, and fully developed macropores at the bottom.36 There was no distinct

Figure 9. AFM images of pure PES (a) and PES/HNTs-Ag NPs hybrid membrane (b). 3087



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Figure 11. Photographs showing the bacterial culture plates of (a) E. coli and (b) S. aureus to PES and PES/HNTs-Ag NPs.

Table 1. Comparison of Antibacterial Properties between Our Work and Other Antibacterial Agents membranes with different antibacterial agents PDAMEMA-grafted MPPM TiO2 entrapped PVDF membranes copper-immobilized membrane membrane loaded with silver nanoparticles

antibacterial efficiency (%) The reductions of E. coli and S. aureus concentration are 100% after a contact time of up to 30 min. Almost complete and faster removal of E. coli occurred (within 1 min of UV light exposure) on the membrane. Antibacterial rate against E. coli is 100%. The antibacterial rates against E. coli and S. aureus are about 99.9% and 99.8%, respectively.

references 17 18 21 this work

seen from the image, for its particle size was too small compared with that of HNTs. 3.2.3. X-ray Photoelectron Spectroscopy (XPS). In order to confirm the presence of silver nanoparticles on the surface of the hybrid membrane, XPS analysis was carried out. As can be seen in Figure 10, the hybrid membrane showed intense peaks at around 373.6 and 367.6 eV, which were known as Ag 3d 3/2 and 3d 5/2 peaks, respectively.28,11 This result indicated that silver nanoparticles existed on the surface of the hybrid membrane and the modification of HNTs was effective. 3.2.4. Antibacterial Effect of the Membranes. Antibacterial activity of the hybrid membrane was evaluated with the viable cell count method using antibacterial rate against E. coli and S. aureus. Compared with pure PES membrane, the antibacterial rates of the hybrid membranes against E. coli and S. aureus were about 99.9% and 99.8%, respectively. The antibacterial effect was shown in Figure 11. As is seen, the hybrid membrane had a good antibacterial property. According to Zhu et al.’s23 research, the antibacterial mechanism of silver ions has been related to their interaction with the thiol (−SH) group of cysteine that normally exists in the cell membrane of a bacterium. Silver ions can react with

research was not in line with this. The possible reason was that the addition of modified HNTs greatly improved the flux due to its hydrophilicity and hollow structure, and this factor had more important influence compared with that of the roughness of the membrane. More importantly, Razmjou et al.’s8 work showed that a membrane with lower roughness and surface energy has stronger antifouling abilities. Furthermore, foulants are likely to be absorbed in the valleys of a membrane with coarser surfaces, resulting in clogging of the valleys. Therefore, it is important to fabricate a membrane with less surface energy and roughness to improve antifouling ability and performance of the membrane, so we could know that the addition of modified HNTs did not have a negative effect on membrane performance; on the contrary, it had effectively improved the flux and maybe improved antifouling ability of the membrane. The dispersion of modified HNTs in the PES membrane was observed by TEM in Figure 9. The tubular structure of HNTs could be seen clearly, which suggested that the modified HNTs were dispersed uniformly and individually in the membrane and there were few large clusters that might have resulted from particles overlapping or from particles coalescing in the membrane. However, the silver nanoparticles could not be 3088



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cysteine by replacing the hydrogen atom of the thiol group to form S−Ag complex, thus hindering the normal enzymatic function of the affected protease. This kind of denaturing of the enzyme is lethal for living bacteria. Furthermore, when silver nanoparticles are exposed to aqueous environments, some ionic silver species would be produced and released. Therefore, the antibacterial mechanism of silver nanoparticles has been considered to be the same as that of silver ions. We also made a comparison of antibacterial properties between our work and the membranes with other antibacterial agents, and the results were shown in Table 1. As can be seen, the membrane loaded with silver nanoparticles also had high antibacterial activity; additionally, it has better permanence, but other antibacterial agents more or less have some small shortcomings. For instance, TiO2 shows antibacterial activity only when it is in UV light exposure, which is not so convenient in application. Organic antibacterial agents do not possess good stability, and their decomposed products are noxious. Therefore, the membranes loaded with silver nanoparticles maybe have a potential application prospect for water treatment in the future.

Novel polyethersulfone hybrid ultrafiltration membranes bending with modified halloysite nanotubes loaded with silver nanoparticles were prepared via phase inversion, and the permeation, microstructure, and antibacterial effect of the hybrid membranes were investigated. The conclusions were as follows: (1) The results of FT-IR and TGA showed that the modification of HNTs was carried out successfully, and the total grafted amount of the modification was 0.110 g (P4VP)/g (HNTs). From the TEM images, silver nanoparticles of about 10 nm in diameter were clearly observed on the surface of the modified HNTs. (2) After adding the modified HNTs, the pure water flux of the hybrid membrane increased greatly, and the maximum was 396.5 L·m−2·h−1, which was about 251.5% higher than that of the pure PES membrane. The rejection decreased a little, and the hydrophilicity was improved; meanwhile, the surface of the hybrid membrane became smooth. The microstructure of the membrane was not changed greatly from the SEM images. In the TEM images, we could find that the modified HNTs were dispersed uniformly and individually in the membrane. The XPS result showed that silver nanoparticles existed on the surface of the hybrid membrane. (3) The results of antibacterial activity tests showed that the hybrid membrane had a good antibacterial property, and the antibacterial rates against E.coli and S. aureus were about 99.9% and 99.8%, respectively.

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-371-67781734. E-mail: [email protected] (Y.T. Zhang); [email protected] (J.D. Liu).

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REFERENCES

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ACKNOWLEDGMENTS

We gratefully acknowledge the support from National Natural Science Foundation of China (No. 21106137) and Innovation Scientists and Technicians Troop Construction Projects of Zhengzhou City. 3089



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Preparation and Characterization of Novel Polyethersulfone Hybrid

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