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Enhancement of antifouling and antibacterial properties of PVC hollow fiber ultrafiltration membranes using pristine and modified silver nanoparticles
Accepted Manuscript Title: Enhancement of antifouling and antibacterial properties of PVC hollow fiber ultrafiltration membranes using pristine and modified silver nanoparticles Authors: A. Behboudi, Y. Jafarzadeh, R. Yegani PII: DOI: Reference:
S2213-3437(18)30103-9 https://doi.org/10.1016/j.jece.2018.02.031 JECE 2223
To appear in: Received date: Revised date: Accepted date:
17-10-2017 4-12-2017 19-2-2018
Please cite this article as: A.Behboudi, Y.Jafarzadeh, R.Yegani, Enhancement of antifouling and antibacterial properties of PVC hollow fiber ultrafiltration membranes using pristine and modified silver nanoparticles, Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2018.02.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of antifouling and antibacterial properties of PVC hollow fiber ultrafiltration membranes using pristine and modified silver nanoparticles A. Behboudia,b, Y. Jafarzadeha,b,*, R. Yegania,b a
Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran Corresponding author's email: [email protected]
Abstract
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b
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran
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In this study, PVC based hollow fiber ultrafiltration membranes incorporated with pristine and modified silver nanoparticles were prepared by wet spinning method. Silver nanoparticles were modified by silica
based on Stöber method and the results of FTIR and FESEM analyses revealed that silver/silica
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nanoparticles were successfully synthesized. Fabricated membranes were then characterized by
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FESEM, EDX, contact angle, pure water flux, porosity, mechanical strength and antibacterial tests. It
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was found that all hollow fiber membranes had the same asymmetric structure and the presence of
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nanoparticles had no significant effect on the morphological structure of the membranes. The results of EDX analysis showed that the modification of silver nanoparticles improved their dispersion throughout
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the membranes. At the same content of nanoparticles, hydrophilicity, pure water flux and tensile strength of PVC/modified Ag membranes were more than that of PVC/Ag membranes. Moreover, the
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results of antibacterial test revealed that PVC/modified Ag membranes exhibited wider inhibition zones
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compared to PVC/Ag membranes. Finally, neat PVC, 1.5 wt % PVC/Ag and 1.5 wt % PVC/modified Ag membranes were used in a submerged membrane bioreactor (SMBR) system. The results showed that antifouling properties and COD removal of 1.5 wt % PVC/modified Ag membrane were
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considerably higher than that of the other membranes. Keywords: Polyvinyl chloride; Hollow fiber membrane; Silver nanoparticle; Membrane Bioreactor. 1. Introduction During last decades, membrane bioreactor (MBR) technology has been widely used in treatment of municipal and water recycling which involves a biological treatment and solid-liquid separation [1]. MBR systems combine conventional activated sludge (CAS) and membrane filtration processes and 1
offer some advantages like reduced plant volume, less sludge production and higher separation efficiency [2-4]. Nevertheless, membrane fouling is the main drawback of MBRs and restricts widespread application of them [5]. It has been shown that the main cause of membrane fouling in MBRs is the formation of the biofilm on the membrane surface which usually consists of organic matters and microbial cells [6]. In other words, adsorption of natural organic matters (NOMs) on the
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membrane surface and within its pores results in organic fouling whereas bacteria produce a biofilm on the membrane surface which usually composed of polysaccharide [7]. Membrane characteristics, feedbiomass characteristics and operating conditions are three most important parameters that affect
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membrane fouling in MBR systems [4] and many efforts have been made in each category to mitigate fouling. It has been accepted that membrane characteristics such as hydrophilicity, surface roughness
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and pore size influence fouling propensity of membranes [1, 8]. Increasing the hydrophilicity of a
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membrane can improve its antifouling properties [9] even though some contradictory results have been reported [10]. To enhance hydrophilicity and antifouling properties of polymer membranes, different
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approaches such as embedding of inorganic nanoparticles into polymer matrices [11-13], surface
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modification of membranes [14, 15] and blending of polymers [16, 17] have been widely used. For
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membranes used in MBR systems, incorporation of antibacterial and antifouling agents seems to be an effective manner to mitigate biofouling phenomenon [18, 19].
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Due to their antibacterial activity, silver nanoparticles have attracted lots of attention in preparation of antibacterial and antifouling polymeric membranes. Polysulfone/silver composite membranes were
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prepared by Zodrow et al. and the results revealed that antibacterial activity and biofouling resistance of membranes increased [20]. Zhang et al. prepared biogenic silver/PES membranes using phase inversion method and showed that permeate flux, the hydrophilicity and antibacterial properties of
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composite membranes increased due to the presence of biogenic silver [21]. In another work, PES/silver composite UF membranes were fabricated using polyvinylpyrollidone (PVP) of different molecular weights and the results showed that membrane with 2 wt.% Ag and PVP 360,000 Da exhibits 100% inhibition against Escherichia coli which was attributed to the antibacterial properties of silver [22]. Silver nanoparticles can also be embedded into polymer membranes in other ways. Zhang et al. showed that modification of halloysite nanotubes (HNTs) with silver nanoparticles enhanced the hydrophilicity 2
and water permeation of modified HNTs/PES membranes [23, 24]. In another work, silver nanoparticles were attached on silica particles and then embedded into PES membranes. The results showed that pure water flux and antibacterial properties of SiO2- Ag/PES membranes were higher than that of pure PES membrane [25]. The main obstacles of silver embedded mixed matrix membranes are agglomeration/distribution of
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silver nanoparticles within the polymer matrix and the long-term stability of them during membrane filtration process. It has been reported that silver loss during membrane processes is related to the poor adhesion between polymer and silver [26]. To prevent agglomeration and loss of silver nanoparticles in
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mixed matrix membranes, some researchers followed the method of immobilization of silver
nanoparticles on other particles. Huang et al [18] synthesized Ag-SiO2 nanoparticles to immobilize
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silver nanoparticles on the surface of silica and incorporated them into PES membranes to overcome
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dispersion problem. They found that the immobilization process was successful and resulted in membranes with good dispersion of nanoparticles throughout the membranes which, consequently, led
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to membranes with excellent antibacterial and anti-biofouling properties [18]. Li et al. assembled silver
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nanoparticles on graphene oxide (GO) sheets and embedded the synthesized Ag-GO composites into
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PVDF membranes. The results showed that the antibacterial properties of Ag-GO-PVDF membranes were improved and the rate of silver release in Ag-GO-PVDF membranes was slower than that of Ag-
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PVDF one [27].
Recently, polyvinyl chloride (PVC) has attracted lots of attentions as a polymer membrane because of
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its excellent properties such as chemical stabilization, corrosion resistance, mechanical strength and low cost [16, 17, 28]. However, the hydrophobic nature of PVC leads to membranes with high fouling propensity and consequently, the application of PVC membranes in MBR systems have been limited.
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Several methods have been proposed to enhance antifouling properties of PVC membranes which can be classified into three categories including blending with other polymers, surface modification and incorporation of inorganic nanoparticles. As mentioned, incorporation of antifouling agents is an effective manner to mitigate biofouling phenomenon in membranes used in MBR systems. Therefore, the aim of the present work is to study PVC hollow fiber membranes embedded with silver nanoparticles. Both pristine and modified silver nanoparticles were embedded into PVC membranes to 3
find out the effect of modification of silver nanoparticles on the performance of the membranes. For modification of nanoparticles, we used a method proposed by Quinsaat et al. [29] to mitigate the agglomeration of silver nanoparticles within the membranes. In this method, silver nanoparticles were modified by silica based on Stöber method. This method also provides more hydroxyl groups on silver nanoparticles without considerable effect on their antibacterial properties, as shown later. The modified
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nanoparticles were then embedded into PVC hollow fiber membranes. 2. Experimental 2.1. Materials
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PVC (MW=90000) was supplied by Arvand Petrochemical Company, Iran. Silver nanopowder with particle size of 30-50 nm was purchased from US Research Nanomaterials, USA. Polyethylene glycol
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(PEG) with molecular weight of 200 Da as pore former and 1-methyl 2-pirrolidone (NMP) as polymer
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solvent were purchased from Merck. Ammonium hydroxide, ethyl alcohol and tetraethoxysilane (TEOS) were purchased from Sigma-Aldrich. E. coli as standard gram-negative bacteria was provided
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by Persian Type Culture Collection (PTCC), Iran. Luria-Bertani (LB) was purchased form Merck.
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Influent wastewater and adapted activated sludge were supplied by the wastewater treatment plant of
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Dana Pharmaceutical Company, Iran and used as MBR feed. The chemical oxygen demand (COD) of the wastewater was about 1200 mg/L.
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2.2. Synthesis of modified silver nanoparticles Silver/silica nanoparticles were synthesized based on the method described elsewhere [29]. 1 g of silver
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nanoparticles was dispersed in 200 mL ethanol. The suspension was then treated with 7.7 mL ammonium hydroxide whilst stirring gently. Afterward, 10 mL of TEOS was added to the solution at 25º C. The final mixture was stirred rigorously for 24 hours. The mixture was then diluted with acetone,
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centrifuged at 12000 rpm for 15 minutes and decanted. The washing procedure was repeated three times and modified nanoparticles were finally dried and stored. 2.3. Preparation of hollow fiber membranes and membrane module The dope solutions were prepared by dissolving certain amounts of PVC, NMP, nanoparticles and PEG. Nanoparticles were dispersed and sonicated in NMP for at least 1 h. Then, PEG was added to the dispersion and stirred for about 15 min. After that, PVC was gradually added to the mixture and stirred 4
overnight and allowed to degas. Hollow fiber membranes were prepared by wet spinning technique with the conditions summarized in Table 1. Water was used as non-solvent bath and bore fluid. After the formation of hollow fibers, they were stored in water bath for 24 h to complete solvent exchange and finally were dried. In this research, nine hollow fiber membranes were prepared and the composition of dope solutions is listed in Table 2. A membrane module consisting of 50 hollow fibers was fabricated
ca. 16 cm and the surface area of membranes in module was 0.02 m2.
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(Fig. 1) to evaluate the performance of membranes. The effective length of hollow fibers in module was
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Table 1. Spinning conditions for preparation of hollow fiber membranes. Parameter
Value
Vessel Pressure [bar]
2
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Spinneret Temperature [ºC]
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Spinneret Outer Diameter [µm]
Bore Fluid Rate [ml/min]
M
Bore Fluid Temperature [ºC]
A
Spinneret Bore Diameter [µm]
25
800 350 3 25 30
Take-up Speed [m/min]
2.4
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Coagulation Bath Temperature [ºC]
Table 2. Composition of dope solutions for the hollow fiber membranes. 5
PVC, %
PEG/PVC, %
Ag/PVC, %
Modified Ag/PVC, %
M1
17
15
0
0
M2
17
15
0.5
0
M3
17
15
1.0
0
M4
17
15
1.5
0
M5
17
15
2.0
0
M6
17
15
0
0.5
M7
17
15
0
1.0
M8
17
15
0
1.5
M9
17
15
0
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Membrane
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N
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2.0
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Fig. 1. The module of hollow fiber membranes.
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2.4. Characterization of membranes
The FT-IR spectra of modified nanoparticles were taken on Tensor 27 FTIR spectrometer (Bruker,
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Germany). A field emission scanning microscope (FESEM; MIRA3 FEG-SEM, Tescan) was used to visualize the morphology of the prepared membranes as well as modified silver/silica nanoparticles.
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Samples of membranes were fractured in liquid nitrogen and coated with gold by sputtering before observation. In case nanoparticles, silver/silica nanoparticles were dispersed in ethanol, dropped onto a microscope slide and prepared for observation. FESEM device was equipped with dispersive X-ray
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analysis (EDX) detector to inspect the element composition as well as dispersion nanoparticles within the membranes. Hydrophilicity of the membranes was evaluated by a contact angle goniometer (PGX, Thwing-Albert Instrument Co., USA) in a sessile drop model at room temperature. It should be noted that flat sheet membranes were used to measure contact angle and these membranes were prepared from the same dope solution but following the method described in our previous work [9]. The average of 5
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measurements was reported to minimize the experimental errors. Porosity of membranes was calculated by measuring the weights of them in dry and wet states. For each membrane, 5 coupons were cut and immersed in water for 48 h. The following equation was used for calculation of porosity: ɛ (%) =
(𝑚𝑤 −𝑚𝑑 )/ 𝜌𝑤 ((𝑚𝑤 −𝑚𝑑 )/ 𝜌𝑤 +𝑚𝑑 / 𝜌𝑝 )
× 100
(1)
where ɛ is the porosity of membrane, 𝑚𝑤 and 𝑚𝑑 are mass of wet and dry membrane, respectively, 𝜌𝑤
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is water density and 𝜌𝑝 is polymer (PVC) density. Mechanical properties (stress and elongation) of
prepared membranes were evaluated using a tensile testing machine (Santam STM-5, Iran) at an
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extension rate of 10 mm/min. Three samples of each membrane with length of 150 mm were tested and the average values were reported. To determine pure water flux (J0) of membranes, each membrane
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pressure of 0.7 bar which was the maximum available TMP.
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module was submerged in water bath and connected to a vacuum pump to provide transmembrane
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2.5. Antibacterial test
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The antibacterial property of the fabricated membranes was evaluated following a method described elsewhere [30]. E. coli NovaBlue as the standard Gram-negative bacteria was incubated at 37º C in
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Luria-Bertani (LB) overnight. Before dispersion on LB plate, the bacterial concentration was diluted to 106 CFU/ml. Samples of membranes were laid on top of the plate. After 24 hours of incubation at 37 ºC
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halo zones were formed around samples which represent inhibited anti-bacterium areas related to the
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presence of silver nanoparticles. The inhibition zones were evaluated through observation in order to compare anti-bacterial property of samples qualitatively.
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2.6. MBR set-up and operational conditions Among fabricated membranes, three of them including neat PVC, 1.5 wt % unmodified silver and 1.5 wt % silver/silica embedded PVC hollow fiber membranes were selected to be used in MBR system. A lab-scale submerged membrane bioreactor apparatus with an effective volume of 25 L was used in this work. Air was fed to the bioreactor underneath the membrane module using a diffuser to provide continuous aeration of sludge and maintain the dissolved oxygen for microorganisms at desired level. 7
Aeration also provided a shear stress to hinder deposition of the microorganisms on the surface of hollow fiber membranes. The module was connected to a vacuum pump to provide transmembrane pressure. The operational conditions of the MBR are shown in Table 3. It should be noted that the operating TMP of all three membranes in MBR was set to 0.1 atm which was below the critical TMP of them as will be shown later. For a specific feed-membrane system, critical flux (Jc) is defined as the
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maximum flux above which fouling is observed, and below which permeate flux dose not decline with time [4, 5, 31]. The critical TMP is the transmembrane pressure at which the critical flux is measured.
The critical flux of three membranes was evaluated following the TMP-step method described
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elsewhere [32]. Permeate flux of membranes during MBR filtration was measured based on the
collected water weight at sub-critical conditions for 6 h. COD removal of membranes was determined using the following equation:
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𝐶𝑂𝐷
𝐶𝑂𝐷 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 (%) = (1 − 𝐶𝑂𝐷𝐸 ) × 100
(2)
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𝐼
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where 𝐶𝑂𝐷𝐸 and 𝐶𝑂𝐷𝐼 are CODs of effluent (permeate) and influent (feed), respectively. COD of
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permeates was evaluated by absorbance method. As mentioned, influent wastewater of Dana
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Pharmaceutical Company with COD of 1200 mg/L was used as the MBR feed.
Table 3. The operation conditions for the MBR system. Value
TMP [bar]
0.1
HRT [h]
24
SRT [day]
25
Aeration [m3.m-2.h-1]
2.5
MLSS [mg/L]
7500-8000
Temperature [ºC]
25
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MBR Parameters
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2.7. Antifouling properties After 360 min MBR operation, each membrane module was submerged in pure water bath to measure
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PWF after fouling (J1). Then the module was brought out, submerged in ethanol and connected to nitrogen balloon. The fouled membranes were back-flushed under the flow of nitrogen at 1 bar. After
back-flushing, pure water flux of membranes was measured again (J2). Fouling properties of the
𝐽0 −𝐽1 𝐽0
𝑅𝐹𝑅 =
𝐽2 −𝐽1 𝐽0
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𝐽2 𝐽0
(3) (4) (5) (6)
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𝐹𝑅 =
𝐽0 −𝐽2 𝐽0
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𝐼𝐹𝑅 =
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𝑇𝐹𝑅 =
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membranes were evaluated using the following equations:
where TFR, RFR, IFR and FR are total fouling ratio, reversible fouling ratio, irreversible fouling ratio
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can be estimated [33].
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and flux recovery, respectively. These are fouling parameters by which fouling properties of membranes
3. Results and discussion
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3.1. Characteristics of silver/silica nanoparticles Fig. 2 shows the FTIR spectra of neat and modified silver nanoparticles. In silver/silica nanoparticles, new adsorption peaks at 1092, 790 and 496 cm-1 are ascribed to the Si-O-Si and Si-O asymmetric
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stretching vibrations [34, 35]. The broad peak at around 3450 cm-1 is assigned to the OH group on silver/silica nanoparticles [36]. Moreover, the FESEM images of neat and modified silver nanoparticles (Fig. 3) shows that silver nanoparticles were modified with silica. Therefore, it can be concluded that silver nanoparticles were successfully modified by silica.
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Fig. 2. FTIR spectra of pristine and modified silver nanoparticles.
Fig. 3. FESEM images of pristine (top) and modified (bottom) silver nanoparticles.
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3.2. Morphology of membranes Fig. 4 shows the cross sectional morphologies of PVC/Ag and PVC/Ag/silica hollow fiber membranes prepared in this work. As shown, all the composite membranes displayed the typical asymmetric structure consisting of a finger-like porous sublayer and a dense skin layer which is characteristic structure of membranes prepared via non-solvent induced phase separation method. As observed, the
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structure of all membranes is almost the same and the presence of neat and modified silver nanoparticles had no significant effect on the structure of membranes. However, the results of porosity test listed in Table 4 revealed that the porosity of PVC/Ag/silica membranes were higher than that of PVC/Ag ones.
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This may imply that the size of macrovoids slightly increased at the presence of silver/silica
nanoparticles in comparison with neat silver nanoparticles. It has been accepted that inorganic
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nanoparticles affect thermodynamics and kinetics of phase separation of dope solution during
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membrane preparation [9]. As shown in Fig. 2, silver/silica nanoparticles contain hydroxyl groups which accelerates the exchange between water (non-solvent) and NMP (solvent) during phase
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separation process and subsequently, results in larger macrovoids.
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Fig. 4. FESEM images and EDX-Mapping of PVC/Ag and PVC/Ag/silica hollow fiber membranes.
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(a1) and (a2): M2, (b1) and (b2): M3, (c1) and (c2): M4, (d1) and (d2): M5, (e1) and (e2): M6, (f1) and (f2): M7, (g1) and (g2): M8, (h1) and (h2): M9.
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Fig. 4 also shows backscattered FESEM images or EDX mapping of membranes which indicates the dispersion of nanoparticles throughout the membranes. The red and green dots in the figure represent Ag and Si elements, respectively. It is clearly seen that both neat and modified silver nanoparticles were dispersed uniformly throughout the membrane. The agglomeration of neat silver nanoparticles is visible in case of 2.00 wt % PVC/Ag membrane, whereas there is no considerable agglomeration in 2.00 wt %
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PVC/Ag/silica membrane. This result indicates that coating of silver nanoparticles with silica prevents the agglomeration of them within the PVC membranes.
Table 4. Characteristics of prepared hollow fiber membranes. Pure water flux, kg/m2hr
Contact Angle, degree
Porosity, %
M1
72 ± 5
86.1 ± 2
71.2 ± 0.2
M2
130 ± 4
80.0 ± 1
73.1 ± 0.1
M3
142 ± 5
78.2 ± 1
M4
142 ± 2
78.1 ± 2
M5
144 ± 7
76.4 ± 3
M6
238 ± 3
M7
384 ± 1
M8
418 ± 0.4
M9
418 ± 7
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Membrane
73.4 ± 0.1 74.6 ± 0.1 74.4 ± 0.1 75.2 ± 0.2
48.7 ± 2
76.4 ± 0.1
44.2 ± 4
78.1 ± 0.2
44.2 ± 6
79.8 ± 0.2
N
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61.5 ± 4
3.3. Hydrophilicity and pure water flux
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The contact angle between water droplet and fabricated (flat sheet) membranes are tabulated in Table
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4. It can be seen that the incorporation of neat and modified silver nanoparticles into PVC membranes
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decreased contact angle which means that the hydrophilicity of composite membranes enhanced at the presence of neat and modified silver nanoparticles. The reduction in contact angle of fabricated
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membranes is due to the hydrophilic properties of Ag and SiO2. However, modified silver nanoparticles resulted in more hydrophilic membranes in comparison with those PVC/Ag membranes. For example,
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the contact angle value of the 2.00 wt % PVC/Ag/silica membrane is 44.2º whereas it is 76.4º for 2.00 wt % PVC/Ag membrane. It means that silver/silica nanoparticles improve the hydrophilicity of PVC membrane more than that of neat silver nanoparticles. The difference between the hydrophilicity of
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PVC/Ag/silica and PVC/Ag membranes can be attributed to the presence of hydroxyl as well as Si-O groups in the former. Hydrophilicity is an important characteristics which affects the antifouling properties and antibacterial activity of polymer membranes and it has been accepted that hydrophilic membranes are more fouling resistant due to repelling of hydrophobic foulants [18, 32, 37]. Table 4 also shows that neat and modified silver nanoparticles have a positive effect on the pure water flux of PVC membranes. It can be seen that PWF increased from 72 kg/m 2h for neat PVC membrane 13
to 144 kg/m2h for 2.00 wt % PVC/Ag membrane. Interestingly, PVC/Ag/silica membranes exhibited higher PWF in comparison with PVC/Ag membranes. For example, the PWF of 2.00 wt % PVC/Ag/silica membrane was 418 kg/m2h, almost three times of 2.00 wt % PVC/Ag membrane. In general, the PWF of a membrane is mainly determined by its hydrophilicity [27, 38] and as mentioned,
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the hydrophilicity of PVC/Ag/silica membranes were higher than that of PVC/Ag membranes.
3.4. Mechanical properties
Mechanical strength of a membrane is an important factor that limits its industrial application. The
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mechanical properties of prepared hollow fiber membranes were measured and summarized in Table 5. The results showed that tensile strength of PVC membrane was 12.2 MPa and addition of neat and
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modified silver nanoparticles improved tensile which can be attributed to the reinforcement effect of
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inorganic additive in polymer matrix. At the same content, however, PVC/Ag/silica membranes were stronger than PVC/Ag membranes. This difference may be related to the dispersion of nanoparticles
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within the membranes. As evidenced by EDX analyses, silver/silica nanoparticles were dispersed
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uniformly throughout the membranes whereas agglomeration of neat silver nanoparticles was observed
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specially at higher content of nanoparticles. It has been accepted that the agglomeration of inorganic nanoparticles in polymer matrix reduces mechanical strength of the composite membrane [39]. On the
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other hand, Table 5 shows that the elongation at break of PVC/Ag membranes decreased with increasing silver content whereas addition of silver/silica nanoparticles enhanced the elongation. It means that the
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modification of silver nanoparticles with silica increases the interaction between nanoparticles and polymer matrix and consequently, improves interfacial viscoelastic deformation of PVC/Ag/Silica
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composite membrane.
Table 5. Mechanical properties of prepared hollow fiber membranes. Membrane
Tensile stress, MPa
Elongation at break, %
M1
12.2 ± 0.1
18.0 ± 0.2
14
12.7 ± 0.1
16.0 ± 0.7
M3
13.2 ± 0.2
16.0 ± 2
M4
14.1 ± 0.1
14.0 ± 0.5
M5
14.1 ± 0.2
8.0 ± 0.1
M6
12.9 ± 0.3
18.0 ± 1
M7
13.8 ± 0.4
33.4 ± 2
M8
15.0 ± 0.2
40.7 ± 2
M9
15.2 ±. 04
44.2 ± 1
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M2
3.5. Antibacterial activity
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The antibacterial properties of prepared membranes were determined qualitatively using the zone
inhibition test and the results are shown in Fig. 5. By inhibition zone, we mean clear area (dark zone) around the membranes with no bacterial growth [40]. It can be seen that the neat PVC membrane had
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no antibacterial activity as evidenced by bacteria growth near the PVC membrane surface. On the
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other hand, halo zones were observed around the composite membranes and the width of the zones
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increased with increasing nanoparticles content. This can be attributed to the efficient antibacterial
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property of silver nanoparticles. Interestingly, PVC/Ag/silica membranes exhibited wider inhibition zones compared to PVC/Ag membranes due to the presence of hydroxyl groups in silver/silica
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nanoparticles which can attack to bacteria leading to cell death [41]. It should be noted that the modification of silver nanoparticles by silica does not mean that a silica layer is formed around them.
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In fact, silica forms a porous media around silver nanoparticles [12, 34]. Therefore, electrostatic activity of silver nanoparticles attracts microbial cells toward membrane surface [42]. Consequently,
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the microbial cell would be damaged due to presence of hydroxyl groups and the silver annihilates the
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penetrated cells.
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Fig. 5. Inhibition zone of hollow fiber membranes in antibacterial test.
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3.6. MBR operation
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As mentioned, the critical flux of three membranes including neat PVC, 1.5 wt % unmodified silver
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and 1.5 wt % silver/silica embedded PVC hollow fiber membranes was evaluated following the TMPstep method and the results were depicted in Fig. 6. It can be seen that the permeation flux of membranes
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increased linearly with increasing TMP. However, after a critical TMP the slope of flux variation changed. In other words, flux-TMP results can be divided into two regions namely low-flux region in
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which no fouling occurs and high-flux region in which membrane fouling becomes significant. In both regions, flux-TMP results are linear and the intersection of two straight lines for a specific membrane is determined as the critical flux-TMP point. Fig. 6 shows that the critical TMP of neat PVC, 1.5 wt %
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PVC/Ag and 1.5 wt % PVC/Ag/silica membranes were 0.12, 0.13 and 0.16 atm, respectively. Considering the critical flux as the flux above which fouling occurs by formation of a cake layer, these results indicate that neat PVC membrane started to be fouled by sludge at low transmembrane pressure. In other words, incorporation of 1.5 wt % pristine and modified silver nanoparticles into PVC membrane enhanced its fouling resistance and critical TMP. As shown later, antifouling properties of composite membranes have been improved accordingly. 16
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Fig. 6. Variation of TMP and filtration flux during measurement of critical flux.
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The results of filtration during MBR operation for three membranes are shown in Fig. 7. It can be seen that both PVC/Ag and PVC/Ag/silica membranes exhibited higher flux than the neat PVC membrane. Filtration flux of neat PVC hollow fiber membrane started from 61 kg/m2h and after 6 h, declined to 12
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kg/m2h meaning that neat PVC membrane preserved 20% of its initial flux after 6 h filtration. For 1.5
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wt % PVC/Ag membrane, however, the initial and final flux values were 128 1nd 58 kg/m2h, respectively which shows that the presence of silver nanoparticles in the PVC/Ag membrane increased fouling resistance. This can be attributed to the antibacterial properties of silver nanoparticles which
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prevent attachment of bacteria onto PVC/Ag membrane surface. On the other hand, flux of 1.5 wt % PVC/Ag/silica membrane did not decrease over 6 h and the flux remained nearly at a constant value of 418 kg/m2h indicating that the coating of silver nanoparticles by silica layer not only prevents the agglomeration of them within the PVC/Ag/silica membrane, but also enhanced the antifouling and antibacterial properties of it.
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It might seem that the coating of silver nanoparticles by silica layer prevents antibacterial properties of silver nanoparticles. However, the thickness of silica layer around the silver nanoparticles was about 3 nm as shown in FESEM images of nanoparticles. Even though the exact mechanism of antibacterial property of silver nanoparticles is not clearly known, but the electrical attraction between bacteria and silver nanoparticles seems to be an important factor in the mechanism. On the other hand, Quinsaat
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reported that the coating of silver nanoparticles by silica shell of thickness up to 6.6 nm had no significant effect on the conductivity of silver nanoparticles [29]. Therefore, silver/silica nanoparticles
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A
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in PVC/Ag/silica membrane preserved their antibacterial property.
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Fig. 7. Flux-time behavior of the three hollow fiber membranes.
The difference in flux-time behavior of three membranes can also be attributed to the difference in the hydrophilicity of them because the hydrophilicity of a membrane benefits its antifouling properties [6]. According to the data in Table 4, the contact angle of neat PVC, 1.5 wt % PVC/Ag and 1.5 wt % PVC/Ag/silica membranes were 86.1º, 77.8º and 46.0º, respectively which shows that PVC/Ag/silica membrane was more hydrophilic than two other membranes. 18
In addition to flux-time behavior of membranes, fouling parameters can also be useful in determination of antifouling properties of membranes. These parameters include TFR, RFR, IFR and FR and Table 6 shows the results for three membranes used in MBR operation. It can be seen that TFR of neat PVC membrane was the highest (72.2%) and the presence of pristine silver nanoparticles decreased the TFR value to 45.3% for 1.5 wt % PVC/Ag membrane. Nonetheless, 1.5 wt % PVC/Ag/silica membrane had
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the least TFR value meaning that there was almost no flux decline due to the fouling phenomenon for this membrane. Moreover, comparing the RFR/TFR ratios of the membranes revealed that the main
portion of fouling in 1.5 wt % PVC/Ag/silica membrane was reversible but almost half of the fouling
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in neat PVC membrane was reversible. This ratio was 63% for 1.5 wt % PVC/Ag membrane and indicates that pristine silver nanoparticles also enhanced reversible portion of fouling phenomenon
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which can be easily cleaned by hydraulic cleaning method. The results in Table 6 reveals that even
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though antifouling properties of PVC/Ag membrane are better than that of neat PVC membrane, but incorporation of modified silver nanoparticles resulted in membranes with outstanding fouling
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Table 6. Fouling parameters and COD removal of prepared membranes. TFR, %
RFR, %
IRF, %
FR, %
COD Removal, %
M1
72.2
31.4
30.8
70.2
66.67
M4
45.3
28.7
16.6
83.4
78.64
1.5
0.2
99.8
94.00
1.7
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Membrane
The performance of three membranes used in MBR system was evaluated by measuring the COD of effluent and the results were summarized in Table 6. It can be seen that embedding pristine and modified silver nanoparticles into PVC membrane improved COD removal. This result can be attributed to the surface pore size of membranes as well as the antibacterial properties of silver nanoparticles. The FESEM images of the outer surface of membranes M1, M4 and M8 (Fig.8) showed that there are larger 19
pores on the surface of neat PVC membrane whereas the size of pores decreased considerably for pristine and modified silver embedded PVC membranes. On the other hand, antibacterial properties of silver and silver/silica nanoparticles prevent attachment of sludge on the membrane surface and prohibit the pass of sludge species through the membrane. In addition, COD removal for 1.5 wt % PVC/Ag/silver membrane was 94% which was higher than the removal efficiency of conventional activated sludge
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system. COD removal along with pure water flux are usually considered as the performance criteria of a membrane and therefore, it can be concluded that 1.5 wt % PVC/Ag/silver membrane can be selected
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as a high performance membrane to be used in MBR system.
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Fig. 8. FESEM images of outer surface and cross section of three membranes. (a1) and (a2): M1, (b1) and (b2): M4, (c1) and (c2): M8 To compare the results of the best membrane in the present work with other membranes, a comparison was made and the results were shown in Table 7. It can be seen that PVC membrane embedded with
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1.5 wt. % modified silver nanoparticles maybe have a potential application in MBR systems.
Table 7. Comparison of performance and antibacterial property between 1.5 wt. % modified Ag/PVC membrane in this work with other membranes containing Ag. Performance
Antibacterial Activity
Ag-Am-PES
Over 95% BSA removal
AgNPs-PDA/PSf
84% BSA removal
nAg-polysulfone
100% viral removal
1.5 wt. % modified Ag/ PVC
94% COD removal
A high antibacterial property of the membrane containing Ag nanoparticles was confirmed by halo zone test and antibacterial efficiency was about 99.99% for E.Coli by shake flask method. Membrane displayed excellent antibacterial property with a clear halo zone and more 99.9% sterilization ratio for E.Coli and B.subtilis. No growth of P.mendocina and E.Coli after 72 hours of incubation. Halo zone test revealed larger inhibition zone around Ag/Silica embedded membranes representing excellent antibacterial property for E.Coli
Reference [7]
[30] [20] This work
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Membrane
Conclusion
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PVC hollow fiber ultrafiltration membranes with different contents of pristine and modified silver nanoparticles were prepared by wet spinning method. Silver nanoparticles were modified by a silica shell of 3 nm thickness as confirmed by FTIR and FESEM analyses. Even though all hollow fiber membranes had the same asymmetric structure and the presence of nanoparticles had no significant effect on the morphological structure of the membranes, however, the results showed that the
21
silver/silica nanoparticles were dispersed uniformly throughout the membranes in comparison with pristine nanoparticles. The results of contact angle measurement revealed that the hydrophilicity of PVC/Ag/silica membranes were lower than that of PVC/Ag membranes which was attributed to the hydroxyl group in the silver/silica nanoparticles. Therefore, pure water flux and antibacterial properties of PVC/Ag/silica membranes were more than that of PVC/Ag membranes. To compare the effect of
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pristine and modified silver nanoparticles on the performance of the membranes, neat PVC, 1.5 wt % PVC/Ag and 1.5 wt % PVC/Ag/silica membranes were used in a submerged membrane bioreactor
system. Both nanoparticles had a positive effect on the PVC membrane. However, antifouling properties
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and COD removal of 1.5 wt % PVC/Ag/silica membrane were considerably higher than the other
membranes. These results indicate that PVC membranes embedded with modified silver nanoparticles
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are high performance membranes which possess good antifouling and antibacterial properties and can
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be used in industrial MBR systems.
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References
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[1] L. Marbelia, M.R. Bilad, A. Piassecka, P.S. Jishna, P.V. Naik, I.F.J. Vankelecom, Study of PVDF asymmetric membranes in a high-throughput membrane bioreactor (HT-MBR): Influence of phase inversion parameters and filtration performance, Separation and Purification Technology, 162 (2016) 6-13. [2] B. Bienati, A. Bottino, G. Capannelli, A. Comite, Characterization and performance of different types of hollow fibre membranes in a laboratory-scale MBR for the treatment of industrial wastewater, Desalination, 231 (2008) 133-140. [3] H. Hazrati , J. Shayegan, Influence of suspended carrier on membrane fouling and biological removal of styrene and ethylbenzene in MBR, Journal of the Taiwan Institute of Chemical Engineers, 64 (2016) 59-68. [4] P. Le-Clech, V. Chen, T.A.G. Fane, Fouling in membrane bioreactors used in wastewater treatment, Journal of Membrane Science, 284 (2006) 17-53. [5] J.A. Kharraz, M.R. Bilad, H.A. Arafat, Simple and effective corrugation of PVDF membranes for enhanced MBR performance, Journal of Membrane Science, 475 (2015) 91-100. [6] C. Zhao, X. Xu, J. Chen, G. Wang, F. Yang, Highly effective antifouling performance of PVDF/graphene oxide composite membrane in membrane bioreactor (MBR) system, Desalination, 340 (2014) 59-66. [7] I. Sawada, R. Fachrul, T. Ito, Y. Ohmukai, T. Maruyama, H. Matsuyama, Development of a hydrophilic polymer membrane containing silver nanoparticles with both organic antifouling and antibacterial properties, Journal of Membrane Science, 387 (2012) 1-6. [8] A. Drews, Membrane fouling in membrane bioreactors—Characterisation, contradictions, cause and cures, Journal of Membrane Science, 363 (2010) 1-28. [9] A. Behboudi, Y. Jafarzadeh, R. Yegani, Preparation and characterization of TiO2 embedded PVC ultrafiltration membranes, Chemical Engineering Research and Design, 114 (2016) 96-107. [10] H.H.P. Fang, X. Shi, Pore fouling of microfiltration membranes by activated sludge, Journal of Membrane Science, 264 (2005) 161-166.
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[11] J.H. Jhaveri, C.M. Patel, Z.V.P. Murthy, Preparation, characterization and application of GOTiO2/PVC mixed matrix membranes for improvement in performance, Journal of Industrial and Engineering Chemistry, 52 (2017) 138-146. [12] A. Akbari, R. Yegani, B. Pourabbas, A. Behboudi, Fabrication and study of fouling characteristics of HDPE/PEG grafted silica nanoparticles composite membrane for filtration of Humic acid, Chemical Engineering Research and Design, 109 (2016) 282-296. [13] Y. Jafarzadeh, R. Yegani, M. Sedaghat, Preparation, characterization and fouling analysis of ZnO/polyethylene hybrid membranes for collagen separation, Chemical Engineering Research and Design, 94 (2015) 417-427. [14] H. Chen, W. Ma, Y. Xia, Y. Gu, Z. Cao, C. Liu, H. Yang, S. Tao, H. Geng, G. Tao, H. Matsuyama, Improving amphiphilic polypropylenes by grafting poly(vinylpyrrolidone) and poly(ethylene glycol) methacrylate segments on a polypropylene microporous membrane, Applied Surface Science, 419 (2017) 259-268. [15] T. Zhang, K. Zhang, J. Li, X. Yue, Simultaneously enhancing hydrophilicity, chlorine resistance and anti-biofouling of APA-TFC membrane surface by densely grafting quaternary ammonium cations and salicylaldimines, Journal of Membrane Science, 528 (2017) 296-302. [16] A. Behboudi, Y. Jafarzadeh, R. Yegani, Polyvinyl chloride/polycarbonate blend ultrafiltration membranes for water treatment, Journal of Membrane Science, 534 (2017) 18-24. [17] L.-F. Fang, B.-K. Zhu, L.-P. Zhu, H. Matsuyama, S. Zhao, Structures and antifouling properties of polyvinyl chloride/poly(methyl methacrylate)-graft-poly(ethylene glycol) blend membranes formed in different coagulation media, Journal of Membrane Science, 524 (2017) 235-244. [18] J. Huang, H. Wang, K. Zhang, Modification of PES membrane with Ag–SiO2: Reduction of biofouling and improvement of filtration performance, Desalination, 336 (2014) 8-17. [19] X. Zhu, R. Bai, K.-H. Wee, C. Liu, S.-L. Tang, Membrane surfaces immobilized with ionic or reduced silver and their anti-biofouling performances, Journal of Membrane Science, 363 (2010) 278286. [20] K. Zodrow, L. Brunet, S. Mahendra, D. Li, A. Zhang, Q. Li, P.J.J. Alvarez, Polysulfone ultrafiltration membranes impregnated with silver nanoparticles show improved biofouling resistance and virus removal, Water Research, 43 (2009) 715-723. [21] M. Zhang, K. Zhang, B. De Gusseme, W. Verstraete, Biogenic silver nanoparticles (bio-Ag0) decrease biofouling of bio-Ag0/PES nanocomposite membranes, Water Research, 46 (2012) 20772087. [22] H. Basri, A.F. Ismail, M. Aziz, Polyethersulfone (PES)–silver composite UF membrane: Effect of silver loading and PVP molecular weight on membrane morphology and antibacterial activity, Desalination, 273 (2011) 72-80. [23] J. Zhang, Y. Zhang, Y. Chen, L. Du, B. Zhang, H. Zhang, J. Liu, K. Wang, Preparation and Characterization of Novel Polyethersulfone Hybrid Ultrafiltration Membranes Bending with Modified Halloysite Nanotubes Loaded with Silver Nanoparticles, Industrial & Engineering Chemistry Research, 51 (2012) 3081-3090. [24] Y. Chen, Y. Zhang, H. Zhang, J. Liu, C. Song, Biofouling control of halloysite nanotubesdecorated polyethersulfone ultrafiltration membrane modified with chitosan-silver nanoparticles, Chemical Engineering Journal, 228 (2013) 12-20. [25] H. Yu, Y. Zhang, J. Zhang, H. Zhang, J. Liu, Preparation and antibacterial property of SiO2– Ag/PES hybrid ultrafiltration membranes, Desalination and Water Treatment, 51 (2013) 3584-3590. [26] D.G. Yu, M.Y. Teng, W.L. Chou, M.C. Yang, Characterization and inhibitory effect of antibacterial PAN-based hollow fiber loaded with silver nitrate, Journal of Membrane Science, 225 (2003) 115-123. [27] J. Li, X. Liu, J. Lu, Y. Wang, G. Li, F. Zhao, Anti-bacterial properties of ultrafiltration membrane modified by graphene oxide with nano-silver particles, Journal of Colloid and Interface Science, 484 (2016) 107-115. [28] Z. Zhou, S. Rajabzadeh, L. Fang, T. Miyoshi, Y. Kakihana, H. Matsuyama, Preparation of robust braid-reinforced poly(vinyl chloride) ultrafiltration hollow fiber membrane with antifouling surface and application to filtration of activated sludge solution, Materials Science and Engineering: C, 77 (2017) 662-671.
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[29] J.E.Q. Quinsaat, F.A. Nuesch, H. Hofmann, D.M. Opris, Dielectric properties of silver nanoparticles coated with silica shells of different thicknesses, RSC Advances, 3 (2013) 6964-6971. [30] L. Huang, S. Zhao, Z. Wang, J. Wu, J. Wang, S. Wang, In situ immobilization of silver nanoparticles for improving permeability, antifouling and anti-bacterial properties of ultrafiltration membrane, Journal of Membrane Science, 499 (2016) 269-281. [31] R.W. Field, D. Wu, J.A. Howell, B.B. Gupta, Critical flux concept for microfiltration fouling, Journal of Membrane Science, 100 (1995) 259-272. [32] H. Etemadi, R. Yegani, M. Seyfollahi, The effect of amino functionalized and polyethylene glycol grafted nanodiamond on anti-biofouling properties of cellulose acetate membrane in membrane bioreactor systems, Separation and Purification Technology, 177 (2017) 350-362. [33] Y. Jafarzadeh, R. Yegani, Analysis of fouling mechanisms in TiO2 embedded high density polyethylene membranes for collagen separation, Chemical Engineering Research and Design, 93 (2015) 684-695. [34] A. Akbari, R. Yegani, B. Pourabbas, Synthesis of poly(ethylene glycol) (PEG) grafted silica nanoparticles with a minimum adhesion of proteins via one-pot one-step method, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 484 (2015) 206-215. [35] Z. Zhu, J. Jiang, X. Wang, X. Huo, Y. Xu, Q. Li, L. Wang, Improving the hydrophilic and antifouling properties of polyvinylidene fluoride membrane by incorporation of novel nanohybrid GO@SiO2 particles, Chemical Engineering Journal, 314 (2017) 266-276. [36] Q. Zhang, J. Jiang, F. Gao, G. Zhang, X. Zhan, F. Chen, Engineering high-effective antifouling polyether sulfone membrane with P(PEG-PDMS-KH570)@SiO2 nanocomposite via in-situ sol-gel process, Chemical Engineering Journal, 321 (2017) 412-423. [37] M. Pasmore, P. Todd, S. Smith, D. Baker, J. Silverstein, D. Coons, C.N. Bowman, Effects of ultrafiltration membrane surface properties on Pseudomonas aeruginosa biofilm initiation for the purpose of reducing biofouling, Journal of Membrane Science, 194 (2001) 15-32. [38] Y. Zhao, Z. Xu, M. Shan, C. Min, B. Zhou, Y. Li, B. Li, L. Liu, X. Qian, Effect of graphite oxide and multi-walled carbon nanotubes on the microstructure and performance of PVDF membranes, Separation and Purification Technology, 103 (2013) 78-83. [39] Y. Yang, H. Zhang, P. Wang, Q. Zheng, J. Li, The influence of nano-sized TiO2 fillers on the morphologies and properties of PSF UF membrane, Journal of Membrane Science, 288 (2007) 231238. [40] J.A. Prince, S. Bhuvana, K.V.K. Boodhoo, V. Anbharasi, G. Singh, Synthesis and characterization of PEG-Ag immobilized PES hollow fiber ultrafiltration membranes with long lasting antifouling properties, Journal of Membrane Science, 454 (2014) 538-548. [41] E.-R. Kenawy, S.D. Worley, R. Broughton, The Chemistry and Applications of Antimicrobial Polymers: A State-of-the-Art Review, Biomacromolecules, 8 (2007) 1359-1384. [42] G. Franci, A. Falanga, S. Galdiero, L. Palomba, M. Rai, G. Morelli, M. Galdiero, Silver Nanoparticles as Potential Antibacterial Agents, Molecules, 20 (2015).
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Figure Caption
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Figr-16Figure Captions Fig. 1. The module of hollow fiber membranes. Fig. 2. FTIR spectra of pristine and modified silver nanoparticles. Fig. 3. FESEM images of pristine (top) and modified (bottom) silver nanoparticles. Fig. 4. FESEM images and EDX-Mapping of PVC/Ag and PVC/Ag/silica hollow fiber membranes.
and (f2): M7, (g1) and (g2): M8, (h1) and (h2): M9.
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Fig. 5. Inhibition zone of hollow fiber membranes in antibacterial test.
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(a1) and (a2): M2, (b1) and (b2): M3, (c1) and (c2): M4, (d1) and (d2): M5, (e1) and (e2): M6, (f1)
Fig. 6. Variation of TMP and filtration flux during measurement of critical flux. Fig. 7. Flux-time behavior of the three hollow fiber membranes.
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Fig. 8. FESEM images of outer surface and cross section of three membranes. (a1) and (a2): M1, (b1)
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and (b2): M4, (c1) and (c2): M8.
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Table 1. Spinning conditions for preparation of hollow fiber membranes. Value
Vessel Pressure [bar]
2
Spinneret Temperature [ºC]
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Spinneret Outer Diameter [µm]
800
Spinneret Bore Diameter [µm]
350
Bore Fluid Rate [ml/min]
3
Bore Fluid Temperature [ºC]
25
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Parameter
Coagulation Bath Temperature [ºC]
30
2.4
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Take-up Speed [m/min]
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PVC, %
PEG/PVC, %
Ag/PVC, %
Modified Ag/PVC, %
M1
17
15
0
0
M2
17
15
0.5
0
M3
17
15
1.0
0
M4
17
15
1.5
0
M5
17
15
2.0
0
M6
17
15
0
0.5
M7
17
15
0
M8
17
15
0
M9
17
15
0
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Table 2. Composition of dope solutions for the hollow fiber membranes.
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1.0 1.5 2.0
Table 3. The operation conditions for the MBR system. Value
TMP [bar]
0.1
HRT [h]
24
SRT [day]
25
Aeration [m3.m-2.h-1]
2.5
MLSS [mg/L]
7500-8000
Temperature [ºC]
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MBR Parameters
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Pure water flux, kg/m2hr
Contact Angle, degree
Porosity, %
M1
72 ± 5
86.1 ± 2
71.2 ± 0.2
M2
130 ± 4
80.0 ± 1
73.1 ± 0.1
M3
142 ± 5
78.2 ± 1
73.4 ± 0.1
M4
142 ± 2
78.1 ± 2
74.6 ± 0.1
M5
144 ± 7
76.4 ± 3
74.4 ± 0.1
M6
238 ± 3
61.5 ± 4
75.2 ± 0.2
M7
384 ± 1
48.7 ± 2
M8
418 ± 0.4
44.2 ± 4
M9
418 ± 7
44.2 ± 6
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Table 4. Characteristics of prepared hollow fiber membranes.
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76.4 ± 0.1 78.1 ± 0.2 79.8 ± 0.2
Table 5. Mechanical properties of prepared hollow fiber membranes. Tensile stress, MPa
Elongation at break, %
M1
12.2 ± 0.1
18.0 ± 0.2
M2
12.7 ± 0.1
16.0 ± 0.7
M3
13.2 ± 0.2
16.0 ± 2
M4
14.1 ± 0.1
14.0 ± 0.5
M5
14.1 ± 0.2
8.0 ± 0.1
M6
12.9 ± 0.3
18.0 ± 1
M7
13.8 ± 0.4
33.4 ± 2
M8
15.0 ± 0.2
40.7 ± 2
M9
15.2 ±. 04
44.2 ± 1
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Table 6. Fouling parameters and COD removal of prepared membranes. TFR, %
RFR, %
IRF, %
FR, %
COD Removal, %
M1
72.2
31.4
30.8
70.2
66.67
M4
45.3
28.7
16.6
83.4
78.64
M8
1.7
1.5
0.2
99.8
94.00
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Membrane
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Table 7. Comparison of performance and antibacterial property between 1.5 wt. % modified Ag/PVC membrane in this work with other membranes containing Ag. Membrane
Performance
Antibacterial Activity
Reference
Ag-Am-PES
Over 95% BSA removal
[7]
AgNPs-PDA/PSf
84% BSA removal
nAg-polysulfone
100% viral removal
1.5 wt. % modified Ag/ PVC
94% COD removal
A high antibacterial property of the membrane containing Ag nanoparticles was confirmed by halo zone test and antibacterial efficiency was about 99.99% for E.Coli by shake flask method. Membrane displayed excellent antibacterial property with a clear halo zone and more 99.9% sterilization ratio for E.Coli and B.subtilis. No growth of P.mendocina and E.Coli after 72 hours of incubation. Halo zone test revealed larger inhibition zone around Ag/Silica embedded membranes representing excellent antibacterial property for E.Coli
[30]
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This work
S2213-3437(18)30103-9 https://doi.org/10.1016/j.jece.2018.02.031 JECE 2223
To appear in: Received date: Revised date: Accepted date:
17-10-2017 4-12-2017 19-2-2018
Please cite this article as: A.Behboudi, Y.Jafarzadeh, R.Yegani, Enhancement of antifouling and antibacterial properties of PVC hollow fiber ultrafiltration membranes using pristine and modified silver nanoparticles, Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2018.02.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement of antifouling and antibacterial properties of PVC hollow fiber ultrafiltration membranes using pristine and modified silver nanoparticles A. Behboudia,b, Y. Jafarzadeha,b,*, R. Yegania,b a
Membrane Technology Research Center, Sahand University of Technology, Tabriz, Iran Corresponding author's email: [email protected]
Abstract
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b
Faculty of Chemical Engineering, Sahand University of Technology, Tabriz, Iran
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In this study, PVC based hollow fiber ultrafiltration membranes incorporated with pristine and modified silver nanoparticles were prepared by wet spinning method. Silver nanoparticles were modified by silica
based on Stöber method and the results of FTIR and FESEM analyses revealed that silver/silica
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nanoparticles were successfully synthesized. Fabricated membranes were then characterized by
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FESEM, EDX, contact angle, pure water flux, porosity, mechanical strength and antibacterial tests. It
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was found that all hollow fiber membranes had the same asymmetric structure and the presence of
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nanoparticles had no significant effect on the morphological structure of the membranes. The results of EDX analysis showed that the modification of silver nanoparticles improved their dispersion throughout
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the membranes. At the same content of nanoparticles, hydrophilicity, pure water flux and tensile strength of PVC/modified Ag membranes were more than that of PVC/Ag membranes. Moreover, the
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results of antibacterial test revealed that PVC/modified Ag membranes exhibited wider inhibition zones
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compared to PVC/Ag membranes. Finally, neat PVC, 1.5 wt % PVC/Ag and 1.5 wt % PVC/modified Ag membranes were used in a submerged membrane bioreactor (SMBR) system. The results showed that antifouling properties and COD removal of 1.5 wt % PVC/modified Ag membrane were
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considerably higher than that of the other membranes. Keywords: Polyvinyl chloride; Hollow fiber membrane; Silver nanoparticle; Membrane Bioreactor. 1. Introduction During last decades, membrane bioreactor (MBR) technology has been widely used in treatment of municipal and water recycling which involves a biological treatment and solid-liquid separation [1]. MBR systems combine conventional activated sludge (CAS) and membrane filtration processes and 1
offer some advantages like reduced plant volume, less sludge production and higher separation efficiency [2-4]. Nevertheless, membrane fouling is the main drawback of MBRs and restricts widespread application of them [5]. It has been shown that the main cause of membrane fouling in MBRs is the formation of the biofilm on the membrane surface which usually consists of organic matters and microbial cells [6]. In other words, adsorption of natural organic matters (NOMs) on the
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membrane surface and within its pores results in organic fouling whereas bacteria produce a biofilm on the membrane surface which usually composed of polysaccharide [7]. Membrane characteristics, feedbiomass characteristics and operating conditions are three most important parameters that affect
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membrane fouling in MBR systems [4] and many efforts have been made in each category to mitigate fouling. It has been accepted that membrane characteristics such as hydrophilicity, surface roughness
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and pore size influence fouling propensity of membranes [1, 8]. Increasing the hydrophilicity of a
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membrane can improve its antifouling properties [9] even though some contradictory results have been reported [10]. To enhance hydrophilicity and antifouling properties of polymer membranes, different
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approaches such as embedding of inorganic nanoparticles into polymer matrices [11-13], surface
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modification of membranes [14, 15] and blending of polymers [16, 17] have been widely used. For
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membranes used in MBR systems, incorporation of antibacterial and antifouling agents seems to be an effective manner to mitigate biofouling phenomenon [18, 19].
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Due to their antibacterial activity, silver nanoparticles have attracted lots of attention in preparation of antibacterial and antifouling polymeric membranes. Polysulfone/silver composite membranes were
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prepared by Zodrow et al. and the results revealed that antibacterial activity and biofouling resistance of membranes increased [20]. Zhang et al. prepared biogenic silver/PES membranes using phase inversion method and showed that permeate flux, the hydrophilicity and antibacterial properties of
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composite membranes increased due to the presence of biogenic silver [21]. In another work, PES/silver composite UF membranes were fabricated using polyvinylpyrollidone (PVP) of different molecular weights and the results showed that membrane with 2 wt.% Ag and PVP 360,000 Da exhibits 100% inhibition against Escherichia coli which was attributed to the antibacterial properties of silver [22]. Silver nanoparticles can also be embedded into polymer membranes in other ways. Zhang et al. showed that modification of halloysite nanotubes (HNTs) with silver nanoparticles enhanced the hydrophilicity 2
and water permeation of modified HNTs/PES membranes [23, 24]. In another work, silver nanoparticles were attached on silica particles and then embedded into PES membranes. The results showed that pure water flux and antibacterial properties of SiO2- Ag/PES membranes were higher than that of pure PES membrane [25]. The main obstacles of silver embedded mixed matrix membranes are agglomeration/distribution of
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silver nanoparticles within the polymer matrix and the long-term stability of them during membrane filtration process. It has been reported that silver loss during membrane processes is related to the poor adhesion between polymer and silver [26]. To prevent agglomeration and loss of silver nanoparticles in
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mixed matrix membranes, some researchers followed the method of immobilization of silver
nanoparticles on other particles. Huang et al [18] synthesized Ag-SiO2 nanoparticles to immobilize
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silver nanoparticles on the surface of silica and incorporated them into PES membranes to overcome
N
dispersion problem. They found that the immobilization process was successful and resulted in membranes with good dispersion of nanoparticles throughout the membranes which, consequently, led
A
to membranes with excellent antibacterial and anti-biofouling properties [18]. Li et al. assembled silver
M
nanoparticles on graphene oxide (GO) sheets and embedded the synthesized Ag-GO composites into
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PVDF membranes. The results showed that the antibacterial properties of Ag-GO-PVDF membranes were improved and the rate of silver release in Ag-GO-PVDF membranes was slower than that of Ag-
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PVDF one [27].
Recently, polyvinyl chloride (PVC) has attracted lots of attentions as a polymer membrane because of
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its excellent properties such as chemical stabilization, corrosion resistance, mechanical strength and low cost [16, 17, 28]. However, the hydrophobic nature of PVC leads to membranes with high fouling propensity and consequently, the application of PVC membranes in MBR systems have been limited.
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Several methods have been proposed to enhance antifouling properties of PVC membranes which can be classified into three categories including blending with other polymers, surface modification and incorporation of inorganic nanoparticles. As mentioned, incorporation of antifouling agents is an effective manner to mitigate biofouling phenomenon in membranes used in MBR systems. Therefore, the aim of the present work is to study PVC hollow fiber membranes embedded with silver nanoparticles. Both pristine and modified silver nanoparticles were embedded into PVC membranes to 3
find out the effect of modification of silver nanoparticles on the performance of the membranes. For modification of nanoparticles, we used a method proposed by Quinsaat et al. [29] to mitigate the agglomeration of silver nanoparticles within the membranes. In this method, silver nanoparticles were modified by silica based on Stöber method. This method also provides more hydroxyl groups on silver nanoparticles without considerable effect on their antibacterial properties, as shown later. The modified
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nanoparticles were then embedded into PVC hollow fiber membranes. 2. Experimental 2.1. Materials
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PVC (MW=90000) was supplied by Arvand Petrochemical Company, Iran. Silver nanopowder with particle size of 30-50 nm was purchased from US Research Nanomaterials, USA. Polyethylene glycol
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(PEG) with molecular weight of 200 Da as pore former and 1-methyl 2-pirrolidone (NMP) as polymer
N
solvent were purchased from Merck. Ammonium hydroxide, ethyl alcohol and tetraethoxysilane (TEOS) were purchased from Sigma-Aldrich. E. coli as standard gram-negative bacteria was provided
A
by Persian Type Culture Collection (PTCC), Iran. Luria-Bertani (LB) was purchased form Merck.
M
Influent wastewater and adapted activated sludge were supplied by the wastewater treatment plant of
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Dana Pharmaceutical Company, Iran and used as MBR feed. The chemical oxygen demand (COD) of the wastewater was about 1200 mg/L.
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2.2. Synthesis of modified silver nanoparticles Silver/silica nanoparticles were synthesized based on the method described elsewhere [29]. 1 g of silver
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nanoparticles was dispersed in 200 mL ethanol. The suspension was then treated with 7.7 mL ammonium hydroxide whilst stirring gently. Afterward, 10 mL of TEOS was added to the solution at 25º C. The final mixture was stirred rigorously for 24 hours. The mixture was then diluted with acetone,
A
centrifuged at 12000 rpm for 15 minutes and decanted. The washing procedure was repeated three times and modified nanoparticles were finally dried and stored. 2.3. Preparation of hollow fiber membranes and membrane module The dope solutions were prepared by dissolving certain amounts of PVC, NMP, nanoparticles and PEG. Nanoparticles were dispersed and sonicated in NMP for at least 1 h. Then, PEG was added to the dispersion and stirred for about 15 min. After that, PVC was gradually added to the mixture and stirred 4
overnight and allowed to degas. Hollow fiber membranes were prepared by wet spinning technique with the conditions summarized in Table 1. Water was used as non-solvent bath and bore fluid. After the formation of hollow fibers, they were stored in water bath for 24 h to complete solvent exchange and finally were dried. In this research, nine hollow fiber membranes were prepared and the composition of dope solutions is listed in Table 2. A membrane module consisting of 50 hollow fibers was fabricated
ca. 16 cm and the surface area of membranes in module was 0.02 m2.
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(Fig. 1) to evaluate the performance of membranes. The effective length of hollow fibers in module was
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Table 1. Spinning conditions for preparation of hollow fiber membranes. Parameter
Value
Vessel Pressure [bar]
2
U
Spinneret Temperature [ºC]
N
Spinneret Outer Diameter [µm]
Bore Fluid Rate [ml/min]
M
Bore Fluid Temperature [ºC]
A
Spinneret Bore Diameter [µm]
25
800 350 3 25 30
Take-up Speed [m/min]
2.4
A
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Coagulation Bath Temperature [ºC]
Table 2. Composition of dope solutions for the hollow fiber membranes. 5
PVC, %
PEG/PVC, %
Ag/PVC, %
Modified Ag/PVC, %
M1
17
15
0
0
M2
17
15
0.5
0
M3
17
15
1.0
0
M4
17
15
1.5
0
M5
17
15
2.0
0
M6
17
15
0
0.5
M7
17
15
0
1.0
M8
17
15
0
1.5
M9
17
15
0
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Membrane
A
N
U
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2.0
M
Fig. 1. The module of hollow fiber membranes.
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2.4. Characterization of membranes
The FT-IR spectra of modified nanoparticles were taken on Tensor 27 FTIR spectrometer (Bruker,
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Germany). A field emission scanning microscope (FESEM; MIRA3 FEG-SEM, Tescan) was used to visualize the morphology of the prepared membranes as well as modified silver/silica nanoparticles.
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Samples of membranes were fractured in liquid nitrogen and coated with gold by sputtering before observation. In case nanoparticles, silver/silica nanoparticles were dispersed in ethanol, dropped onto a microscope slide and prepared for observation. FESEM device was equipped with dispersive X-ray
A
analysis (EDX) detector to inspect the element composition as well as dispersion nanoparticles within the membranes. Hydrophilicity of the membranes was evaluated by a contact angle goniometer (PGX, Thwing-Albert Instrument Co., USA) in a sessile drop model at room temperature. It should be noted that flat sheet membranes were used to measure contact angle and these membranes were prepared from the same dope solution but following the method described in our previous work [9]. The average of 5
6
measurements was reported to minimize the experimental errors. Porosity of membranes was calculated by measuring the weights of them in dry and wet states. For each membrane, 5 coupons were cut and immersed in water for 48 h. The following equation was used for calculation of porosity: ɛ (%) =
(𝑚𝑤 −𝑚𝑑 )/ 𝜌𝑤 ((𝑚𝑤 −𝑚𝑑 )/ 𝜌𝑤 +𝑚𝑑 / 𝜌𝑝 )
× 100
(1)
where ɛ is the porosity of membrane, 𝑚𝑤 and 𝑚𝑑 are mass of wet and dry membrane, respectively, 𝜌𝑤
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is water density and 𝜌𝑝 is polymer (PVC) density. Mechanical properties (stress and elongation) of
prepared membranes were evaluated using a tensile testing machine (Santam STM-5, Iran) at an
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extension rate of 10 mm/min. Three samples of each membrane with length of 150 mm were tested and the average values were reported. To determine pure water flux (J0) of membranes, each membrane
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pressure of 0.7 bar which was the maximum available TMP.
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module was submerged in water bath and connected to a vacuum pump to provide transmembrane
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2.5. Antibacterial test
M
The antibacterial property of the fabricated membranes was evaluated following a method described elsewhere [30]. E. coli NovaBlue as the standard Gram-negative bacteria was incubated at 37º C in
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Luria-Bertani (LB) overnight. Before dispersion on LB plate, the bacterial concentration was diluted to 106 CFU/ml. Samples of membranes were laid on top of the plate. After 24 hours of incubation at 37 ºC
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halo zones were formed around samples which represent inhibited anti-bacterium areas related to the
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presence of silver nanoparticles. The inhibition zones were evaluated through observation in order to compare anti-bacterial property of samples qualitatively.
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2.6. MBR set-up and operational conditions Among fabricated membranes, three of them including neat PVC, 1.5 wt % unmodified silver and 1.5 wt % silver/silica embedded PVC hollow fiber membranes were selected to be used in MBR system. A lab-scale submerged membrane bioreactor apparatus with an effective volume of 25 L was used in this work. Air was fed to the bioreactor underneath the membrane module using a diffuser to provide continuous aeration of sludge and maintain the dissolved oxygen for microorganisms at desired level. 7
Aeration also provided a shear stress to hinder deposition of the microorganisms on the surface of hollow fiber membranes. The module was connected to a vacuum pump to provide transmembrane pressure. The operational conditions of the MBR are shown in Table 3. It should be noted that the operating TMP of all three membranes in MBR was set to 0.1 atm which was below the critical TMP of them as will be shown later. For a specific feed-membrane system, critical flux (Jc) is defined as the
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maximum flux above which fouling is observed, and below which permeate flux dose not decline with time [4, 5, 31]. The critical TMP is the transmembrane pressure at which the critical flux is measured.
The critical flux of three membranes was evaluated following the TMP-step method described
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elsewhere [32]. Permeate flux of membranes during MBR filtration was measured based on the
collected water weight at sub-critical conditions for 6 h. COD removal of membranes was determined using the following equation:
U
𝐶𝑂𝐷
𝐶𝑂𝐷 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 (%) = (1 − 𝐶𝑂𝐷𝐸 ) × 100
(2)
N
𝐼
A
where 𝐶𝑂𝐷𝐸 and 𝐶𝑂𝐷𝐼 are CODs of effluent (permeate) and influent (feed), respectively. COD of
M
permeates was evaluated by absorbance method. As mentioned, influent wastewater of Dana
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Pharmaceutical Company with COD of 1200 mg/L was used as the MBR feed.
Table 3. The operation conditions for the MBR system. Value
TMP [bar]
0.1
HRT [h]
24
SRT [day]
25
Aeration [m3.m-2.h-1]
2.5
MLSS [mg/L]
7500-8000
Temperature [ºC]
25
A
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MBR Parameters
8
2.7. Antifouling properties After 360 min MBR operation, each membrane module was submerged in pure water bath to measure
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PWF after fouling (J1). Then the module was brought out, submerged in ethanol and connected to nitrogen balloon. The fouled membranes were back-flushed under the flow of nitrogen at 1 bar. After
back-flushing, pure water flux of membranes was measured again (J2). Fouling properties of the
𝐽0 −𝐽1 𝐽0
𝑅𝐹𝑅 =
𝐽2 −𝐽1 𝐽0
A
𝐽2 𝐽0
(3) (4) (5) (6)
M
𝐹𝑅 =
𝐽0 −𝐽2 𝐽0
N
𝐼𝐹𝑅 =
U
𝑇𝐹𝑅 =
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membranes were evaluated using the following equations:
where TFR, RFR, IFR and FR are total fouling ratio, reversible fouling ratio, irreversible fouling ratio
PT
can be estimated [33].
ED
and flux recovery, respectively. These are fouling parameters by which fouling properties of membranes
3. Results and discussion
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3.1. Characteristics of silver/silica nanoparticles Fig. 2 shows the FTIR spectra of neat and modified silver nanoparticles. In silver/silica nanoparticles, new adsorption peaks at 1092, 790 and 496 cm-1 are ascribed to the Si-O-Si and Si-O asymmetric
A
stretching vibrations [34, 35]. The broad peak at around 3450 cm-1 is assigned to the OH group on silver/silica nanoparticles [36]. Moreover, the FESEM images of neat and modified silver nanoparticles (Fig. 3) shows that silver nanoparticles were modified with silica. Therefore, it can be concluded that silver nanoparticles were successfully modified by silica.
9
IP T SC R U
A
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ED
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A
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Fig. 2. FTIR spectra of pristine and modified silver nanoparticles.
Fig. 3. FESEM images of pristine (top) and modified (bottom) silver nanoparticles.
10
3.2. Morphology of membranes Fig. 4 shows the cross sectional morphologies of PVC/Ag and PVC/Ag/silica hollow fiber membranes prepared in this work. As shown, all the composite membranes displayed the typical asymmetric structure consisting of a finger-like porous sublayer and a dense skin layer which is characteristic structure of membranes prepared via non-solvent induced phase separation method. As observed, the
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structure of all membranes is almost the same and the presence of neat and modified silver nanoparticles had no significant effect on the structure of membranes. However, the results of porosity test listed in Table 4 revealed that the porosity of PVC/Ag/silica membranes were higher than that of PVC/Ag ones.
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This may imply that the size of macrovoids slightly increased at the presence of silver/silica
nanoparticles in comparison with neat silver nanoparticles. It has been accepted that inorganic
U
nanoparticles affect thermodynamics and kinetics of phase separation of dope solution during
N
membrane preparation [9]. As shown in Fig. 2, silver/silica nanoparticles contain hydroxyl groups which accelerates the exchange between water (non-solvent) and NMP (solvent) during phase
A
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PT
ED
M
A
separation process and subsequently, results in larger macrovoids.
11
IP T SC R U N A M ED PT
Fig. 4. FESEM images and EDX-Mapping of PVC/Ag and PVC/Ag/silica hollow fiber membranes.
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(a1) and (a2): M2, (b1) and (b2): M3, (c1) and (c2): M4, (d1) and (d2): M5, (e1) and (e2): M6, (f1) and (f2): M7, (g1) and (g2): M8, (h1) and (h2): M9.
A
Fig. 4 also shows backscattered FESEM images or EDX mapping of membranes which indicates the dispersion of nanoparticles throughout the membranes. The red and green dots in the figure represent Ag and Si elements, respectively. It is clearly seen that both neat and modified silver nanoparticles were dispersed uniformly throughout the membrane. The agglomeration of neat silver nanoparticles is visible in case of 2.00 wt % PVC/Ag membrane, whereas there is no considerable agglomeration in 2.00 wt %
12
PVC/Ag/silica membrane. This result indicates that coating of silver nanoparticles with silica prevents the agglomeration of them within the PVC membranes.
Table 4. Characteristics of prepared hollow fiber membranes. Pure water flux, kg/m2hr
Contact Angle, degree
Porosity, %
M1
72 ± 5
86.1 ± 2
71.2 ± 0.2
M2
130 ± 4
80.0 ± 1
73.1 ± 0.1
M3
142 ± 5
78.2 ± 1
M4
142 ± 2
78.1 ± 2
M5
144 ± 7
76.4 ± 3
M6
238 ± 3
M7
384 ± 1
M8
418 ± 0.4
M9
418 ± 7
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IP T
Membrane
73.4 ± 0.1 74.6 ± 0.1 74.4 ± 0.1 75.2 ± 0.2
48.7 ± 2
76.4 ± 0.1
44.2 ± 4
78.1 ± 0.2
44.2 ± 6
79.8 ± 0.2
N
U
61.5 ± 4
3.3. Hydrophilicity and pure water flux
A
The contact angle between water droplet and fabricated (flat sheet) membranes are tabulated in Table
M
4. It can be seen that the incorporation of neat and modified silver nanoparticles into PVC membranes
ED
decreased contact angle which means that the hydrophilicity of composite membranes enhanced at the presence of neat and modified silver nanoparticles. The reduction in contact angle of fabricated
PT
membranes is due to the hydrophilic properties of Ag and SiO2. However, modified silver nanoparticles resulted in more hydrophilic membranes in comparison with those PVC/Ag membranes. For example,
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the contact angle value of the 2.00 wt % PVC/Ag/silica membrane is 44.2º whereas it is 76.4º for 2.00 wt % PVC/Ag membrane. It means that silver/silica nanoparticles improve the hydrophilicity of PVC membrane more than that of neat silver nanoparticles. The difference between the hydrophilicity of
A
PVC/Ag/silica and PVC/Ag membranes can be attributed to the presence of hydroxyl as well as Si-O groups in the former. Hydrophilicity is an important characteristics which affects the antifouling properties and antibacterial activity of polymer membranes and it has been accepted that hydrophilic membranes are more fouling resistant due to repelling of hydrophobic foulants [18, 32, 37]. Table 4 also shows that neat and modified silver nanoparticles have a positive effect on the pure water flux of PVC membranes. It can be seen that PWF increased from 72 kg/m 2h for neat PVC membrane 13
to 144 kg/m2h for 2.00 wt % PVC/Ag membrane. Interestingly, PVC/Ag/silica membranes exhibited higher PWF in comparison with PVC/Ag membranes. For example, the PWF of 2.00 wt % PVC/Ag/silica membrane was 418 kg/m2h, almost three times of 2.00 wt % PVC/Ag membrane. In general, the PWF of a membrane is mainly determined by its hydrophilicity [27, 38] and as mentioned,
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the hydrophilicity of PVC/Ag/silica membranes were higher than that of PVC/Ag membranes.
3.4. Mechanical properties
Mechanical strength of a membrane is an important factor that limits its industrial application. The
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mechanical properties of prepared hollow fiber membranes were measured and summarized in Table 5. The results showed that tensile strength of PVC membrane was 12.2 MPa and addition of neat and
U
modified silver nanoparticles improved tensile which can be attributed to the reinforcement effect of
N
inorganic additive in polymer matrix. At the same content, however, PVC/Ag/silica membranes were stronger than PVC/Ag membranes. This difference may be related to the dispersion of nanoparticles
A
within the membranes. As evidenced by EDX analyses, silver/silica nanoparticles were dispersed
M
uniformly throughout the membranes whereas agglomeration of neat silver nanoparticles was observed
ED
specially at higher content of nanoparticles. It has been accepted that the agglomeration of inorganic nanoparticles in polymer matrix reduces mechanical strength of the composite membrane [39]. On the
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other hand, Table 5 shows that the elongation at break of PVC/Ag membranes decreased with increasing silver content whereas addition of silver/silica nanoparticles enhanced the elongation. It means that the
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modification of silver nanoparticles with silica increases the interaction between nanoparticles and polymer matrix and consequently, improves interfacial viscoelastic deformation of PVC/Ag/Silica
A
composite membrane.
Table 5. Mechanical properties of prepared hollow fiber membranes. Membrane
Tensile stress, MPa
Elongation at break, %
M1
12.2 ± 0.1
18.0 ± 0.2
14
12.7 ± 0.1
16.0 ± 0.7
M3
13.2 ± 0.2
16.0 ± 2
M4
14.1 ± 0.1
14.0 ± 0.5
M5
14.1 ± 0.2
8.0 ± 0.1
M6
12.9 ± 0.3
18.0 ± 1
M7
13.8 ± 0.4
33.4 ± 2
M8
15.0 ± 0.2
40.7 ± 2
M9
15.2 ±. 04
44.2 ± 1
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M2
3.5. Antibacterial activity
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The antibacterial properties of prepared membranes were determined qualitatively using the zone
inhibition test and the results are shown in Fig. 5. By inhibition zone, we mean clear area (dark zone) around the membranes with no bacterial growth [40]. It can be seen that the neat PVC membrane had
U
no antibacterial activity as evidenced by bacteria growth near the PVC membrane surface. On the
N
other hand, halo zones were observed around the composite membranes and the width of the zones
A
increased with increasing nanoparticles content. This can be attributed to the efficient antibacterial
M
property of silver nanoparticles. Interestingly, PVC/Ag/silica membranes exhibited wider inhibition zones compared to PVC/Ag membranes due to the presence of hydroxyl groups in silver/silica
ED
nanoparticles which can attack to bacteria leading to cell death [41]. It should be noted that the modification of silver nanoparticles by silica does not mean that a silica layer is formed around them.
PT
In fact, silica forms a porous media around silver nanoparticles [12, 34]. Therefore, electrostatic activity of silver nanoparticles attracts microbial cells toward membrane surface [42]. Consequently,
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the microbial cell would be damaged due to presence of hydroxyl groups and the silver annihilates the
A
penetrated cells.
15
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N
Fig. 5. Inhibition zone of hollow fiber membranes in antibacterial test.
A
3.6. MBR operation
M
As mentioned, the critical flux of three membranes including neat PVC, 1.5 wt % unmodified silver
ED
and 1.5 wt % silver/silica embedded PVC hollow fiber membranes was evaluated following the TMPstep method and the results were depicted in Fig. 6. It can be seen that the permeation flux of membranes
PT
increased linearly with increasing TMP. However, after a critical TMP the slope of flux variation changed. In other words, flux-TMP results can be divided into two regions namely low-flux region in
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which no fouling occurs and high-flux region in which membrane fouling becomes significant. In both regions, flux-TMP results are linear and the intersection of two straight lines for a specific membrane is determined as the critical flux-TMP point. Fig. 6 shows that the critical TMP of neat PVC, 1.5 wt %
A
PVC/Ag and 1.5 wt % PVC/Ag/silica membranes were 0.12, 0.13 and 0.16 atm, respectively. Considering the critical flux as the flux above which fouling occurs by formation of a cake layer, these results indicate that neat PVC membrane started to be fouled by sludge at low transmembrane pressure. In other words, incorporation of 1.5 wt % pristine and modified silver nanoparticles into PVC membrane enhanced its fouling resistance and critical TMP. As shown later, antifouling properties of composite membranes have been improved accordingly. 16
IP T SC R U N
M
A
Fig. 6. Variation of TMP and filtration flux during measurement of critical flux.
ED
The results of filtration during MBR operation for three membranes are shown in Fig. 7. It can be seen that both PVC/Ag and PVC/Ag/silica membranes exhibited higher flux than the neat PVC membrane. Filtration flux of neat PVC hollow fiber membrane started from 61 kg/m2h and after 6 h, declined to 12
PT
kg/m2h meaning that neat PVC membrane preserved 20% of its initial flux after 6 h filtration. For 1.5
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wt % PVC/Ag membrane, however, the initial and final flux values were 128 1nd 58 kg/m2h, respectively which shows that the presence of silver nanoparticles in the PVC/Ag membrane increased fouling resistance. This can be attributed to the antibacterial properties of silver nanoparticles which
A
prevent attachment of bacteria onto PVC/Ag membrane surface. On the other hand, flux of 1.5 wt % PVC/Ag/silica membrane did not decrease over 6 h and the flux remained nearly at a constant value of 418 kg/m2h indicating that the coating of silver nanoparticles by silica layer not only prevents the agglomeration of them within the PVC/Ag/silica membrane, but also enhanced the antifouling and antibacterial properties of it.
17
It might seem that the coating of silver nanoparticles by silica layer prevents antibacterial properties of silver nanoparticles. However, the thickness of silica layer around the silver nanoparticles was about 3 nm as shown in FESEM images of nanoparticles. Even though the exact mechanism of antibacterial property of silver nanoparticles is not clearly known, but the electrical attraction between bacteria and silver nanoparticles seems to be an important factor in the mechanism. On the other hand, Quinsaat
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reported that the coating of silver nanoparticles by silica shell of thickness up to 6.6 nm had no significant effect on the conductivity of silver nanoparticles [29]. Therefore, silver/silica nanoparticles
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PT
ED
M
A
N
U
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in PVC/Ag/silica membrane preserved their antibacterial property.
A
Fig. 7. Flux-time behavior of the three hollow fiber membranes.
The difference in flux-time behavior of three membranes can also be attributed to the difference in the hydrophilicity of them because the hydrophilicity of a membrane benefits its antifouling properties [6]. According to the data in Table 4, the contact angle of neat PVC, 1.5 wt % PVC/Ag and 1.5 wt % PVC/Ag/silica membranes were 86.1º, 77.8º and 46.0º, respectively which shows that PVC/Ag/silica membrane was more hydrophilic than two other membranes. 18
In addition to flux-time behavior of membranes, fouling parameters can also be useful in determination of antifouling properties of membranes. These parameters include TFR, RFR, IFR and FR and Table 6 shows the results for three membranes used in MBR operation. It can be seen that TFR of neat PVC membrane was the highest (72.2%) and the presence of pristine silver nanoparticles decreased the TFR value to 45.3% for 1.5 wt % PVC/Ag membrane. Nonetheless, 1.5 wt % PVC/Ag/silica membrane had
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the least TFR value meaning that there was almost no flux decline due to the fouling phenomenon for this membrane. Moreover, comparing the RFR/TFR ratios of the membranes revealed that the main
portion of fouling in 1.5 wt % PVC/Ag/silica membrane was reversible but almost half of the fouling
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in neat PVC membrane was reversible. This ratio was 63% for 1.5 wt % PVC/Ag membrane and indicates that pristine silver nanoparticles also enhanced reversible portion of fouling phenomenon
U
which can be easily cleaned by hydraulic cleaning method. The results in Table 6 reveals that even
N
though antifouling properties of PVC/Ag membrane are better than that of neat PVC membrane, but incorporation of modified silver nanoparticles resulted in membranes with outstanding fouling
M
A
resistance.
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Table 6. Fouling parameters and COD removal of prepared membranes. TFR, %
RFR, %
IRF, %
FR, %
COD Removal, %
M1
72.2
31.4
30.8
70.2
66.67
M4
45.3
28.7
16.6
83.4
78.64
1.5
0.2
99.8
94.00
1.7
A
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M8
PT
Membrane
The performance of three membranes used in MBR system was evaluated by measuring the COD of effluent and the results were summarized in Table 6. It can be seen that embedding pristine and modified silver nanoparticles into PVC membrane improved COD removal. This result can be attributed to the surface pore size of membranes as well as the antibacterial properties of silver nanoparticles. The FESEM images of the outer surface of membranes M1, M4 and M8 (Fig.8) showed that there are larger 19
pores on the surface of neat PVC membrane whereas the size of pores decreased considerably for pristine and modified silver embedded PVC membranes. On the other hand, antibacterial properties of silver and silver/silica nanoparticles prevent attachment of sludge on the membrane surface and prohibit the pass of sludge species through the membrane. In addition, COD removal for 1.5 wt % PVC/Ag/silver membrane was 94% which was higher than the removal efficiency of conventional activated sludge
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system. COD removal along with pure water flux are usually considered as the performance criteria of a membrane and therefore, it can be concluded that 1.5 wt % PVC/Ag/silver membrane can be selected
A
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PT
ED
M
A
N
U
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as a high performance membrane to be used in MBR system.
20
Fig. 8. FESEM images of outer surface and cross section of three membranes. (a1) and (a2): M1, (b1) and (b2): M4, (c1) and (c2): M8 To compare the results of the best membrane in the present work with other membranes, a comparison was made and the results were shown in Table 7. It can be seen that PVC membrane embedded with
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1.5 wt. % modified silver nanoparticles maybe have a potential application in MBR systems.
Table 7. Comparison of performance and antibacterial property between 1.5 wt. % modified Ag/PVC membrane in this work with other membranes containing Ag. Performance
Antibacterial Activity
Ag-Am-PES
Over 95% BSA removal
AgNPs-PDA/PSf
84% BSA removal
nAg-polysulfone
100% viral removal
1.5 wt. % modified Ag/ PVC
94% COD removal
A high antibacterial property of the membrane containing Ag nanoparticles was confirmed by halo zone test and antibacterial efficiency was about 99.99% for E.Coli by shake flask method. Membrane displayed excellent antibacterial property with a clear halo zone and more 99.9% sterilization ratio for E.Coli and B.subtilis. No growth of P.mendocina and E.Coli after 72 hours of incubation. Halo zone test revealed larger inhibition zone around Ag/Silica embedded membranes representing excellent antibacterial property for E.Coli
Reference [7]
[30] [20] This work
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PT
ED
M
A
N
U
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Membrane
Conclusion
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PVC hollow fiber ultrafiltration membranes with different contents of pristine and modified silver nanoparticles were prepared by wet spinning method. Silver nanoparticles were modified by a silica shell of 3 nm thickness as confirmed by FTIR and FESEM analyses. Even though all hollow fiber membranes had the same asymmetric structure and the presence of nanoparticles had no significant effect on the morphological structure of the membranes, however, the results showed that the
21
silver/silica nanoparticles were dispersed uniformly throughout the membranes in comparison with pristine nanoparticles. The results of contact angle measurement revealed that the hydrophilicity of PVC/Ag/silica membranes were lower than that of PVC/Ag membranes which was attributed to the hydroxyl group in the silver/silica nanoparticles. Therefore, pure water flux and antibacterial properties of PVC/Ag/silica membranes were more than that of PVC/Ag membranes. To compare the effect of
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pristine and modified silver nanoparticles on the performance of the membranes, neat PVC, 1.5 wt % PVC/Ag and 1.5 wt % PVC/Ag/silica membranes were used in a submerged membrane bioreactor
system. Both nanoparticles had a positive effect on the PVC membrane. However, antifouling properties
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and COD removal of 1.5 wt % PVC/Ag/silica membrane were considerably higher than the other
membranes. These results indicate that PVC membranes embedded with modified silver nanoparticles
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are high performance membranes which possess good antifouling and antibacterial properties and can
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be used in industrial MBR systems.
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References
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Figr-16Figure Captions Fig. 1. The module of hollow fiber membranes. Fig. 2. FTIR spectra of pristine and modified silver nanoparticles. Fig. 3. FESEM images of pristine (top) and modified (bottom) silver nanoparticles. Fig. 4. FESEM images and EDX-Mapping of PVC/Ag and PVC/Ag/silica hollow fiber membranes.
and (f2): M7, (g1) and (g2): M8, (h1) and (h2): M9.
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Fig. 5. Inhibition zone of hollow fiber membranes in antibacterial test.
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(a1) and (a2): M2, (b1) and (b2): M3, (c1) and (c2): M4, (d1) and (d2): M5, (e1) and (e2): M6, (f1)
Fig. 6. Variation of TMP and filtration flux during measurement of critical flux. Fig. 7. Flux-time behavior of the three hollow fiber membranes.
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Fig. 8. FESEM images of outer surface and cross section of three membranes. (a1) and (a2): M1, (b1)
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Table 1. Spinning conditions for preparation of hollow fiber membranes. Value
Vessel Pressure [bar]
2
Spinneret Temperature [ºC]
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Spinneret Outer Diameter [µm]
800
Spinneret Bore Diameter [µm]
350
Bore Fluid Rate [ml/min]
3
Bore Fluid Temperature [ºC]
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Parameter
Coagulation Bath Temperature [ºC]
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2.4
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Take-up Speed [m/min]
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PVC, %
PEG/PVC, %
Ag/PVC, %
Modified Ag/PVC, %
M1
17
15
0
0
M2
17
15
0.5
0
M3
17
15
1.0
0
M4
17
15
1.5
0
M5
17
15
2.0
0
M6
17
15
0
0.5
M7
17
15
0
M8
17
15
0
M9
17
15
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Membrane
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Table 2. Composition of dope solutions for the hollow fiber membranes.
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1.0 1.5 2.0
Table 3. The operation conditions for the MBR system. Value
TMP [bar]
0.1
HRT [h]
24
SRT [day]
25
Aeration [m3.m-2.h-1]
2.5
MLSS [mg/L]
7500-8000
Temperature [ºC]
25
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MBR Parameters
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Pure water flux, kg/m2hr
Contact Angle, degree
Porosity, %
M1
72 ± 5
86.1 ± 2
71.2 ± 0.2
M2
130 ± 4
80.0 ± 1
73.1 ± 0.1
M3
142 ± 5
78.2 ± 1
73.4 ± 0.1
M4
142 ± 2
78.1 ± 2
74.6 ± 0.1
M5
144 ± 7
76.4 ± 3
74.4 ± 0.1
M6
238 ± 3
61.5 ± 4
75.2 ± 0.2
M7
384 ± 1
48.7 ± 2
M8
418 ± 0.4
44.2 ± 4
M9
418 ± 7
44.2 ± 6
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Table 4. Characteristics of prepared hollow fiber membranes.
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76.4 ± 0.1 78.1 ± 0.2 79.8 ± 0.2
Table 5. Mechanical properties of prepared hollow fiber membranes. Tensile stress, MPa
Elongation at break, %
M1
12.2 ± 0.1
18.0 ± 0.2
M2
12.7 ± 0.1
16.0 ± 0.7
M3
13.2 ± 0.2
16.0 ± 2
M4
14.1 ± 0.1
14.0 ± 0.5
M5
14.1 ± 0.2
8.0 ± 0.1
M6
12.9 ± 0.3
18.0 ± 1
M7
13.8 ± 0.4
33.4 ± 2
M8
15.0 ± 0.2
40.7 ± 2
M9
15.2 ±. 04
44.2 ± 1
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Membrane
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Table 6. Fouling parameters and COD removal of prepared membranes. TFR, %
RFR, %
IRF, %
FR, %
COD Removal, %
M1
72.2
31.4
30.8
70.2
66.67
M4
45.3
28.7
16.6
83.4
78.64
M8
1.7
1.5
0.2
99.8
94.00
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Membrane
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Table 7. Comparison of performance and antibacterial property between 1.5 wt. % modified Ag/PVC membrane in this work with other membranes containing Ag. Membrane
Performance
Antibacterial Activity
Reference
Ag-Am-PES
Over 95% BSA removal
[7]
AgNPs-PDA/PSf
84% BSA removal
nAg-polysulfone
100% viral removal
1.5 wt. % modified Ag/ PVC
94% COD removal
A high antibacterial property of the membrane containing Ag nanoparticles was confirmed by halo zone test and antibacterial efficiency was about 99.99% for E.Coli by shake flask method. Membrane displayed excellent antibacterial property with a clear halo zone and more 99.9% sterilization ratio for E.Coli and B.subtilis. No growth of P.mendocina and E.Coli after 72 hours of incubation. Halo zone test revealed larger inhibition zone around Ag/Silica embedded membranes representing excellent antibacterial property for E.Coli
[30]
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[20]
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This work