Anti-bacterial properties of ultrafiltration membrane modified by graphene oxide with nano-silver particles

Journal of Colloid and Interface Science 484 (2016) 107–115

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Regular Article

Anti-bacterial properties of ultrafiltration membrane modified by graphene oxide with nano-silver particles Jingchun Li, Xuyang Liu, Jiaqi Lu, Yudan Wang, Guanglu Li, Fangbo Zhao ⇑ College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China

g r a p h i c a l a b s t r a c t

AgNO3

GO

Ag/GO

membrane casng soluon phase inversion

a r t i c l e

i n f o

Article history: Received 12 July 2016 Revised 23 August 2016 Accepted 24 August 2016 Available online 30 August 2016 Keywords: GO-Ag composites Polyvinylidene fluoride (PVDF) Antibacterial Anti-biofouling

a b s t r a c t To improve the anti-biofouling properties of PVDF membranes, GO-Ag composites were synthesized and used as membrane antibacterial agent by a simple and environmentally friendly method. As identified by XRD, TEM and FTIR analysis, AgNPs were uniformly assembled on the synthesized GO-Ag sheets. The membranes were prepared by phase inversion method with different additional amounts (0.00– 0.15 wt%) of GO-Ag composites. The GO-Ag composites modified membranes show improved hydrophilicity, mechanical property and permeability than unmodified PVDF membrane. Specially, the antibacterial properties and inhibition of biofilm formation were greatly enhanced based on conventional inhibition zone test and anti-adhesion of bacterial experiment. The modified membranes also reveal a remarkable long-term continuous antimicrobial activity with slower release rate of Ag+ compared to AgNPs/PVDF membrane. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction Ultrafiltration (UF) has a wide application in drinking water production and wastewater treatment due to its low cost and high liquid separation efficiency [1,2]. However, membrane fouling

⇑ Corresponding author. E-mail address: [email protected] (F. Zhao). http://dx.doi.org/10.1016/j.jcis.2016.08.063 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.

problems always exist in an emerging separation/desalination membrane process, which limits the practical applications. For all types of fouling, biofouling is the most complicated and inevitable in membrane separation process, since the biofilm is not easily removed and results in a decrease in water flux and selectivity [3,4]. In addition, higher operating and maintenance cost is required for cleaning of biofouling on membrane by mechanical or chemical approaches [5]. Hence, to overcome such disadvan-

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tages, various strategies have been proposed to suppress the biofouling of membrane process in recent years [6,7]. A common strategy employed to prevent biofilm growth and membrane fouling is the addition of antibacterial nanoparticles, such as titanium dioxide (TiO2), silicon dioxide, and zinc oxide (ZnO), to the membrane backbone materials during membrane preparation [8–10]. Another effective way to mitigate the membrane fouling was to increase the membrane layer hydrophilicity [11]. A more hydrophilic membrane surface was usually less susceptible to be fouled under the same separation conditions [12]. Polyvinylidene fluoride (PVDF) is a common material for ultrafiltration membrane preparation for its excellent chemical stability and thermal performance [13]. However, the inherent hydrophobicity of the PVDF membrane is easy to be contaminated by bacteria and causes biofouling on the surface or in the pores of membranes during wastewater treatment process [14]. Based on this mechanism, a number of works have been reported to increase the hydrophilic properties of PVDF membranes by blending metallic and non-metallic nanoparticles into PVDF matrix [1,15,16]. In recent years, graphene oxide (GO) has been used as modifier in polymer membranes to enhance the mechanical strength and

hydrophilic properties of membranes due to its large specific surface area and abundant oxygen-containing surface groups [17– 19]. Due to its strong inhibitory and biocidal effects [20,21] silver nanoparticles (AgNPs) and silver composite have been used to decrease membrane biofouling [22–24]. In our previous study, the hydrophilicity, permeability and in situ antibacterial properties were improved for the Ag-embedded nano-sized titanium dioxide (Ag-n-TiO2) modified polyvinyl chloride (PVC) membranes [25]. However, the nanoscale AgNPs are hard to be fixed in the membrane firmly by physical blending and thus Ag+ is easily isolated from chemical groups in the complex and actual water condition [26]. Silver nanoparticles assembled on graphene oxide sheets (GO– Ag) have been exploited as a novel antibacterial agent [27]. GO’s functional groups provide ideal nucleation sites for AgNPs. Therefore, AgNPs can strongly attach to the GO surface [27]. In addition, GO can be easily dispersed into polymer matrix, and increases the compatibility of membrane casting solution. Therefore, in this paper we report the fabrication of a novel type of PVDF mixed membrane by embedding GO-Ag composites. Since few of literatures have reported the application of GO-Ag composites as

AgNO3

Fig. 1. Principles of the reactions of modified GO.

Fig. 2. Schematic diagram of the fabricated processes of GO-Ag/PVDF membrane.

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J. Li et al. / Journal of Colloid and Interface Science 484 (2016) 107–115 Table 1 Compositions of membrane casting solutions with different GO-Ag composites loading. Membrane

GO-Ag (wt%)

PVDF (wt%)

PVP (wt%)

DMAC (wt%)

M-0 M-0.05 M-0.10 M-0.15

0 0.05 0.10 0.15

17% 17% 17% 17%

3% 3% 3% 3%

80% 79.95% 79.90% 79.85%

antibacterial agent to modified PVDF-based membranes, this study focuses on the novel modifier to enhance the antibacterial properties and mechanical properties of the modified PVDF membranes. 2. Experimental section 2.1. Materials PVDF (Shenyang Chemical Industry Co., Ltd., China) was employed as the base polymer. N,N-dimethyl acetamide (DMAC, >99.5%, AR, Bodi Co., Ltd., China), polyvinyl pyrrolidone (PVP, AR, Lanji Co., Ltd, China), and flake natural graphite powder were obtained from Qingdao Tianhe Co., Ltd. (China). H2SO4 (purity, 98%), sodium nitrate (NaNO3), hydrogen peroxide solution (5%), KMnO4 and ethanol applied in GO preparation were purchased from Fuyu Co., Ltd. (China). Silver nitrate (AgNO3, >99.8%, AR, Tianjin Kemiou Chemical Agent Co., Ltd., China) and isonicotinic acid (>99.5%, AR, Tianjin Guangfu Fine Chemical Research Institute) were used to synthesize GO-Ag composites. The distilled water was used as the non-solvent for the polymer precipitation. 2.2. Synthesis and characterization of GO-Ag composites The precursor of GO was produced by the Hummers method [28]. GO-Ag composites were prepared via a two-step solution intercalation method. Firstly, 0.1 g isonicotinic acid was added into 150 mL toluene under room temperature and vibration (125 r/ min). Subsequently, 0.05 g GO nanosheets were added into the mixture. The resulting mixture was stirring during reflux for two hours at 60 °C to synthesize the modified GO. Next, the obtained were vacuum drying at 60 °C. Then, 0.04 g GO, 1 mL of 0.1 mol/L AgNO3 solution, and 100 mL methanol were mixed in 250 mL erlenmeyer flask under the condition of 30 min magnetic stirring. After that, the mixture was shaken at the rate of 125 r/min (25 °C), subsequently, the mixture was centrifuged with washing by methanol for several times, and finally, the sample was obtained by drying at 60 °C. Fig. 1 shows the basic principles of the reactions. The presence of Ag nanoparticles was characterized by Transmission electron microscopy transmission electron microscopy (TEM, JEM-2000EX, Japan). Before the measurement, GO-Ag composites were suspended in 200 mL ethanol followed by ultrasonic treatment for 5 min and then the 0.2 mL of dispersion solutions were transferred to a copper grid (400 meshes) coated with a carbon film. The chemical structures of the GO-Ag composites were characterized using a NEXUS 670 Fourier transform infrared spectrometer (FT-IR, PE-100, USA). Prior to testing, the particles were dried at 60 °C in a vacuum oven for 24 h to remove the absorbed water. The structure of GO was analyzed by powder XRD (Rigaku, Japan) using a TTR-III diffract meter under Cu Ka radiation (k = 0.15406 nm). 2.3. Membrane preparation The schematic diagram of the fabricated processes of membrane is presented in Fig. 2. Both pure PVDF membranes and GOAg/PVDF flat sheet membranes were prepared via the immersion

phase inversion method [29]. PVDF 17 wt% and polyvinyl pyrrolidone (PVP) 3 wt% were applied as the polymer matrix. Four different concentrations of GO-Ag composites (i.e., 0.0, 0.05, 0.10, 0.15 wt%) were added and dispersed in DMAC in a 25 °C ultrasonic bath at 40 kHz for 60 min. Subsequently, the PVDF resin powder and PVP were added to the solution at 60 °C and stirred for 24 h to obtain homogenous solutions for casting. The solution was kept in a drying oven overnight to remove air bubble. The solutions were casted on a glass plate using a membrane casting machine with the thickness of 400 lm and immediately dipped into a distilled water coagulating bath (25 °C). After complete coagulation, the wet membranes were stored in distilled water at least 48 h before the following investigation. Table 1 shows the compositions of the casting solutions. 2.4. Membrane characterization 2.4.1. Viscosities of the casting solutions The viscosity of the casting solutions was measured by a rotational viscometer (NDJ-5S, Shanghai Changji instrument Co., Ltd., China) at 25 °C. 2.4.2. Scanning electron microscopy (SEM) and Energy dispersive spectrometer (EDS) The surface and cross-section of the prepared membranes were characterized by a scanning electron microscope (JSM-6480A, JEOL, Japan) at 10 kV in a high vacuum mode after being coated with approximately 10 nm of gold. Samples of the membranes were frozen in liquid nitrogen and then fractured. At the same time, Energy Dispersive Spectrometer (EDS) was used to examine the GO-Ag composites loaded on the surface of membranes. 2.4.3. Contact angle Contact angle measurements of the membranes were performed using a JYSP-360 contact angle goniometer (Jin Shengxin Co., Ltd., China). The membranes were dried in the air for about 48 h before the measurements. The dynamic water contact angle was measured by placing 2 lL of deionized water on the membrane surface for multiple (P10) measurements to obtain an average value. 2.4.4. Mechanical characterization The mechanical properties of the membranes were measured using an electric elastic yarn strength analyzer (YG020B, Nantong Sansi Co., Ltd., China) at a pull rate of 2 mm/min. Every sample was tested for 5 times to obtain an average value. 2.5. Study of antibacterial properties and anti-adhesion effect of bacteria Antibacterial performance was investigated by inhibition method using E. coli bacteria [30]. One mL of E. coli bacteria suspension was incubated at 37 °C and shaken at 130 rpm for 12 h. Luria-Bertani (LB) agar solid medium was seeded with the bacteria suspension. In order to evaluate antibacterial performance, all the membrane samples (about 10 mm diameter) and worktable were disinfected by autoclaving for 1 h. Membranes were then placed

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1200

14000

(a)

10000 8000 6000 4000

800 600 400 200

2000 0

(b)

1000

Intensity (a.u)

Intensity (a.u)

12000

0

10

20

30

40

50

60

70

80

0

0

10

20

2θ (°)

30

40

2θ (°)

50

60

70

80

Fig. 3. XRD images of (a) GO and (b) GO-Ag composites.

Fig. 4. TEM images of (a) GO and (b) GO-Ag composites.

on Luria–Bertani (LB) agar plates of E. coli incubated at 310 K for 24 h, which ensure the concentration of E. coli within 107– 109 CFU/mL. Anti-bacteria adhesion effect was evaluated by the colonycounting method [31]. Micro-polluted water was used as feed solution through the dead-end filtration cell (Millipore) at a pressure of 0.1 MPa. All the filtration fluid was collected and then 1 mL of solution for further expanded culture was used for solid plate culture medium. 2.6. Membrane performance testing The filtration experiment was conducted using an Amicon 8200 stirred dead-end filtration cell (Millipore) at a pressure of 0.1 MPa. And the membranes had an effective area of 32.15 cm2. Prior to the permeation testing, each membrane was first compacted at 0.20 MPa with distilled water for 20 min to obtain a steady flux. The pure water flux was obtained by Eq. (1) [32]:

J¼

m

q:s:t

ð1Þ

where m, t and S are mass of the permeated water (kg), the permeation time (h), and membrane area (m2), respectively. 2.7. Study of silver ions release To study the kinetics of silver ions released from the prepared GO-Ag/PVDF membranes, 5% nitric acid was used as a simulated fluid. The dried membranes were cut into disks in diameter of 10 mm with the same mass. The pieces of membranes were immersed in 50 mL of the simulated fluid under ultrasonic condition for 10 h. Finally, ICP-MS was used to determine the silver concentration of the suspending fluid.

3. Results and discussion 3.1. Characterization of GO-Ag composites For the characterizations of GO and GO-Ag composites, XRD, TEM and FTIR experiments were conducted. XRD spectrum (Fig. 3 (a)) showed the typical diffraction peak of GO at 2h = 10.03° within the structure of GO-Ag composites. The XRD test result confirmed that GO was successfully produced by oxidation of graphite, and some function groups were introduced to the surface and the edges of GO sheets. As shown in Fig. 3(b), the diffraction peaks at 2h = 33°, 44° and 77° were attributed to the diffraction plane of silver crystal on GO surface. TEM image showed the typical lamellar morphology and layer structure of GO with wrinkles and curves (Fig. 4(a)). Fig. 4(b) showed that silver nanoparticles with a spherical shape were uniformly deposited on the GO surface and no particles were observed outside the GO sheets. It was also observed that there were no changes on the lamellar of GO with hydrophilic functional groups when silver decorated on the GO. Fig. 5 shows the FTIR of GO and GO-Ag composites. GO had a strong band at 3412 cm1 majorly associated with OH stretching vibration, the band at 1735 cm1 indicated the C@O stretching of carboxyl group (COOH), the band at 1225 cm1 associated with CAOAC stretching and the band at 1055 cm1 associated with CAO stretching. Compared with the FTIR of GO, after the deposition of the AgNPs on the GO sheets, some peaks in the absorption features shifted largely. A prominent peak of 1390 cm1 was appeared. The reason was ascribed to strong interactions between positive Ag+ and function groups of GO sheets. Hence, the FTIR spectrum indicated the excellent hydrophilic property of GO and GO-Ag composites [33]. XRD, TEM and FTIR results showed the evenly and uniformly deposited silver nanoparticles on the GO sheets. These results were also similar to other reports [34].

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700

15000

650

14000

600 550

GO 1225

1055

GO-Ag

500 12000

450 400

11000

350 300

10000

1389

250

9000

1725

200 150

8000

-0.05

3412

Thickness/mm

Viscosity/ (Pa·s)

Transmitance/%

13000

0.00

0.05

0.10

0.15

100 0.20

GO-Ag content/% Fig. 6. Viscosity of casting solutions and thickness of memranes.

4500

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber/cm-1 Fig. 5. FTIR images of (a) GO and (b) GO-Ag composites.

3.2. Characterization of modified PVDF membranes 3.2.1. Viscosity of casting solutions and thickness of membranes Fig. 6 reveals the viscosities of PVDF casting solution and thickness of membranes with different GO-Ag composites loading. An increase in casting solution viscosity and thickness was observed with the addition of GO-Ag composites. With the increase of viscosity, the motion of polymer solution molecules was inhibited by the flow resistance, resulting in decreased fluidity and worse film-forming. In addition, the high viscosity also decelerates the diffusions of nonsolvent into the polymer during phase separation process. Hence, a thicker and dense membrane was produced [35].

3.2.2. Membrane microstructure and EDS analysis Fig. 7 shows images of surface and cross-section of membranes with different GO-Ag composites contents. All the modified PVDF membranes have typical asymmetric structures. From these SEM images, finger-like voids could be observed as GO-Ag composites concentration increased from 0.05 to 0.15 wt%. GO-Ag composites loading at 0.1 and 0.15 wt% resulted in membranes with larger finger-like pore structure. Haider et al. also reported similar phenomena in the literature [36]. The changes in substrate morphology could be attributed to the higher exchange rate between solvent and non-solvent due to the addition of small amount of hydrophilic GO-Ag composites. Energy dispersive spectrometer (EDS) was used to examine whether the GO-Ag composites have been assembled on the modified membranes. Fig. 8 depicts the EDS elemental analysis of

Fig. 7. SEM images of (a) 0.05 wt% GO-Ag modified membrane, (b) 0.10 wt% GO-Ag modified membrane and (c) 0.15 wt% GO-Ag modified membrane.

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pure PVDF membrane and modified membranes. Compared with the unmodified membrane, it was found that the EDS analyses of the modified membranes contained significant characteristic peaks of silver. These characteristic peaks verified the successful incorporation of GO-Ag structure. In addition, the intensity of silver peaks in EDS spectrum increased with the GO-Ag composite concentrations.

(a)

(b)

3.2.3. Contact angle and water flux of membrane The contact angle was almost in accord with membrane permeability. For instance, the pure PVDF membrane showed the largest water contact angle of 82.06° due to the intrinsic hydrophobic characteristic of PVDF. Addition of 0.05, 0.1 and 0.15 wt% GO-Ag composites decreased the water contact angle to 70.6°, 64.6° and 63.4°, which could be attributed to the hydrophilic functional

Element

Wt%

At%

CK

50.62

61.85

FK

49.38

38.15

AgL

00.00

00.00

Matrix

Correction

ZAF

Element

Wt%

At%

CK

55.62

69.40

FK

33.14

26.15

ClK

10.21

04.32

AgL

01.03

00.14

Correction

ZAF

Matrix

(c)

Fig. 8. EDS images of (a) PVDF membrane, (b) 0.10 wt% GO-Ag modified membrane and (c) 0.15 wt% GO-Ag modified membrane.

J. Li et al. / Journal of Colloid and Interface Science 484 (2016) 107–115

90 Water flux

Contant angle

85 80

150 75 70

100

65

Contant angle (°)

Water flux / (L/m 2 .h)

200

60

50

55 0

0.00

0.05

0.10

50

0.15

GO-Ag content (wt %) Fig. 9. Contact angle and pure water permeation flux of membranes.

600 14

550

12

450

10

400 350

8

300 250

6

200 4

150 100

Elogation at break /%

Tensile strength /cN

500

2

50 0 -0.05

0.00

0.05

0.10

0.15

0 0.20

GO-Ag contnet/wt% Fig. 10. Mechanical property of GO-Ag/PVDF membranes.

groups of silver decorated GO on the surface of the membranes (Fig. 9). These functional groups would make the membrane more hydrophilic. In generally, the water flux through membranes is

113

mainly determined by hydrophilicity of membrane [37]. Loading GO-Ag composites into the membrane casting solution resulted in an increased pure water flux from 110.5 L/m2 h to 177.5 L/ m2 h. GO-Ag composites with hydrophilic functional groups (Fig. 5) could attract water molecules inside the membrane and promoted them to pass through membranes [38]. 3.2.4. Mechanical characterizations The mechanical strength of ultrafiltration membranes is a key factor to evaluate their practical applicability [39]. Fig. 10 shows the mechanical property of pure and modified membranes in terms of tensile strength and elongation at break. As shown in Fig. 10, the modified membranes have a pronounced improvement in elongation at break, compared with the pure PVDF membrane. Furthermore, the tensile strength of the modified membranes increased with the additional concentration of GO-Ag composites of P0.10 wt%. This test result implies that GO-Ag composites have an excellent interface compatibility with PVDF polymer matrix, and can enhance the mechanical stability of modified hybrid membranes. However, when the modified hybrid membranes were blended with a higher concentration of GO-Ag composites (i.e., 0.15 wt%), tensile strength decreased probably due to the aggregation of GO-Ag composites. Hence, the optimal concentration is determined to be 0.10 wt% of GO-Ag composites in PVDF polymer matrix. 3.3. Antimicrobial properties of membranes 3.3.1. Inhibition ring test The inhibition ring test was used to evaluate the antibiotic effect against Escherichia Coli (E. coli). The GO/PVDF membranes (P0.1, P0.15), GO-Ag/PVDF membranes (M0.1, M0.1) and pure PVDF (M0) membrane samples were tested. GO/PVDF membranes were employed to serve as a control test, which helps to clarify the antibacterial property caused by GO itself or GO-Ag composites. As shown in Fig. 11, no antibacterial effect was detected in the pure PVDF membrane. Only a small and faint inhibition ring was observed around the outer edge of the GO/PVDF membrane. Therefore, GO/PVDF membrane has very limited bacterial toxicity against E. coli. In contrast, the GO-Ag/PVDF membrane presents a clear inhibition ring, which indicated the inhibition of the E. coli growth of the GO-Ag composites. In addition, the test result highlighted the significant effect of the incorporated amount of Ag

Fig. 11. Inhibition ring of membranes (M0 pure PVDF membrane. P0.1, P0.15 GO/PVDF membrane. M0.1, M0.15 GO-Ag/PVDF membrane).

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Fig. 12. Colonies formed on the unmodified (a) and modified membrane surface with 0.10 wt% (b), and 0.15 wt% (c) of GO-Ag composites.

nanoparticle on the antibacterial property of the membranes. The antibacterial effect can be explained by the release of Ag+ from the GO–Ag composites to the surface of agar media, which prevented the bacteria growth in the area close to the nanocomposite modified membrane.

3.3.2. Anti-adhesion effect of bacteria Fig. 12 shows much lower concentrations of bacteria attached to modified membranes of 0.10 wt% and 0.15 wt% of GO-Ag composites (Fig. 12(b) and (c)) in contrast with the pure sample (Fig. 12(a)). Only 13 CFU/mL of bacterial communities and 3 CFU/ mL were found in the filtration fluid collected through the 0.1 and 0.15 wt% GO-Ag modified membranes. In contrast, 6.7  103 CFU/mL of bacterial communities appeared in the filtration fluid of the unmodified membrane (See Table 2). Hence, the GO-Ag/PVDF membrane has exhibited significant improvement in the property of anti-bacteria adhesion and prohibited biofilm formation in the short term. These two experiments inferred that GO-Ag/PVDF membranes exhibited novel antibacterial properties and resistance to biofouling. It also demonstrated that GO–Ag nanosystems can be used

in a variety of industrial applications that require materials with antibacterial and anti-adhesion properties. 3.3.3. Release of silver ions from membranes The antimicrobial activity of silver nanoparticles largely depends on the release of silver ions [36,40]. The silver released from the silver modified membrane is beneficial for the antimicrobial performance. However, the rapid depletion of silver has limited its application on long-term continuous membrane operation. In this work, we investigated the release of silver ions from GO-Ag/PVDF (0.1 and 0.15 wt%) blending membranes and silver nanoparticle/PVDF (0.1 and 0.15 wt%) membranes. Fig. 13 shows that AgNPs/PVDF membranes have exhibited a much higher concentration of Ag+ than that of GO-Ag/PVDF membrane. The reason is that physically blended AgNPs could be easily separated out from the membrane. Such fast release of silver ions prevents Ag/ PVDF membrane from long-term continuous application and may also cause secondary pollution of water, or even serious safety problems due to possible high concentrations of silver ions. In contrast, the continuous release of Ag+ concentration of GO-Ag/PVDF membranes has been within the scope of safety [41]. The results found that GO could efficiently control the releasing of Ag+, probably due to the anchoring sites provided by the function groups of GO for the growth of the AgNPs.

Table 2 The bacterial communities of outflow. Membrane

Bacteria on membrane (CFU/L) 6.7  10 1.3  10 3

Concentration of silver/ppm

Pure membrane 0.10% GO-Ag/PVDF membrane 0.15% GO-Ag/PVDF membrane

9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.00

4. Conclusions

3

Herein, we report the successful synthesis of GO-Ag nanocomposites and the application as a novel antibacterial agent to modify PVDF membrane. The GO-Ag composites modified PVDF membranes efficiently inhibited the bacterial growth, exhibited improved anti-bacteria adhesion properties, and prevented the formation of biofilms on the membrane surface. In addition, the GO-Ag/PVDF membranes presented enhanced hydrophilic properties with improved permeability and strengthened mechanical properties. The investigation on silver release kinetics showed much slower rate of Ag+ from the GO-Ag/PVDF membrane than AgNPs/PVDF membrane, which indicated its longer period of continuous operation in the wastewater treatment. The novel properties of GO-Ag PVDF membrane can be attributed to GO’s function groups that served as the chemi-sorption sites for Ag+ and AgNPs.

GO-Ag/PVDFmembrane Silver/PVDFmembrane

Acknowledgement

0.05

0.10

GO-Ag content/wt%

0.15

Fig. 13. Concentration of Ag+ released from the tested samples.

0.20

This work was supported by the National Natural Science Foundation of China (21574033), the Natural Science Foundation of Heilongjiang Province (E201252, LC201404), Project of Applicable Technology Research and Development of Harbin City (2013DB4BG010), and the Fundamental Research Funds for the Central Universities (HEUCF20161014).

J. Li et al. / Journal of Colloid and Interface Science 484 (2016) 107–115

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