The production of polysulfone (PS) membrane with silver nanoparticles (AgNP): Physical properties, filtration performances, and biofouling resistances of membranes

Journal of Membrane Science 428 (2013) 620–628

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

The production of polysulfone (PS) membrane with silver nanoparticles (AgNP): Physical properties, filtration performances, and biofouling resistances of membranes Derya Y. Koseoglu-Imer

a,c

, Borte Kose b,c, Mahmut Altinbas b,c, Ismail Koyuncu b,c,n

a

Gebze Institute of Technology, Department of Environmental Engineering, Gebze, Kocaeli 41400, Turkey Department of Environmental Engineering, Civil Engineering Faculty, Istanbul Technical University, Maslak, Istanbul 34469, Turkey c National Research Center on Membrane Technologies, Istanbul Technical University, Maslak, Istanbul 34469, Turkey b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 August 2012 Received in revised form 24 October 2012 Accepted 26 October 2012 Available online 5 November 2012

The polysulfone (PS) membranes were prepared by adding different amounts of silver nanoparticleAgNP (0–1 wt%) into the dope solution. The bare PS and AgNP entrapped PS membranes (AgNP-PS composite membrane) were tested for physical properties with water permeability, MWCO (molecular weight cut-off), AFM, contact angle and SEM analyses, filtration performances by using the model protein (BSA) and carbohydrate (dextran) solutions and biofouling resistances by using a real activated sludge. AgNP addition improved the protein and carbohydrate filtration performances of bare PS membrane. The results of biofouling experiments showed the AgNP-PS composite membranes having lower absorptive and pore fouling values than bare PS membrane. The ionic silver loss from membrane during pure water filtration was measured using inductive-coupled plasma spectrometer (ICP) and the results showed the minimum silver loss from the composite membranes. The results of disk diffusion test showed that the composite AgNP-PS membranes decreased the growth of bacterial colonies. However PCR-DGEE technique showed that there were not significant differences in microbial community population density along the bare and composite membranes. & 2012 Elsevier B.V. All rights reserved.

Keywords: Phase inversion Polysulfone membrane Silver nanoparticle (AgNP) Biofouling

1. Introduction The membrane processes have been increasingly used for drinking water and wastewater treatment. In spite of the advantages of membrane systems, the fouling can be accepted as the main problem and the biggest obstacle to their broader application of membrane technologies [1,2]. In particular, for membrane bioreactor (MBR) systems, the bacterial cells and also their metabolic substances (e.g., soluble or bound extracellular polymeric materials) cause to the biofouling problems of membranes. The formation of biofouling is unwanted because it decreases the permeate flux and thus increases the operation and maintenance costs. Fouled membranes are traditionally cleaned with several physical and chemical methods but these methods increase the cost and time of process and also cause the degradation of membrane materials [3]. In recent years the new techniques have been applied for biofouling control such as the production of fouling resistant n Correspending author at: Department of Environmental Engineering, Civil Engineering Faculty, Istanbul Technical University, Maslak, Istanbul 34469, Turkey. Tel.: þ90 212 2856543; fax: þ 90 212 285 3781. E-mail address: [email protected] (I. Koyuncu).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.10.046

membrane materials or the modification of membrane surfaces using nanoparticles. The nanoparticles (NPs) are defined as the particles having the size of 1–100 nm and they have unique magnetic, electrical, optical, mechanical and structural properties [4,5]. Moreover, some nanoparticles such as silver (Ag), copper (Cu), zinc oxide (ZnO), and titanium oxide (TiO2) show high toxicity to a broad spectrum of microorganisms including bacteria, fungi, viruses and yeasts and have been studied as antibacterial agents in different areas [6,7]. TiO2 nanoparticle is mostly used as the photocatalytic disinfectant or photo-killing using for significant number of microorganisms and has unique properties of chemical stability, biocompatibility, and high photoactivity with relatively low cost but its photo activation limits the wide application [8,9]. ZnO nanoparticle is one of the most important multifunctional semiconductor materials and also can be used for the application of photo-catalysis and production of anti-bacterial materials [10]. Ag nanoparticle (AgNP) has large surface-to-volume ratio and thus it serves as a sustained local supply for Ag þ ions and provides a prolonged prevention of bacterial adhesion [11,12]. Compared with other nanoparticles, AgNP is widely used commercially in different fields because AgNP has excellent antibacterial performance and low cytotoxicity to human cells [4].

D.Y. Koseoglu-Imer et al. / Journal of Membrane Science 428 (2013) 620–628

The mode of action of AgNP include the direct interaction of the AgNPs and the target, the release of silver ions (Ag þ ) and the generation of reactive oxygen species (ROS) [6]. Chamakura, et al. described in detail the steps of AgNP antibacterial mechanism including (a) indirect generation of reactive oxygen species (ROS); (b) direct interaction of silver with proteins and lipids in the cell wall and proteins in the cytoplasmic membrane and (c) potential interaction with DNA [13]. Furthermore several studies reported that the size and shape of the AgNPs influence the antimicrobial behavior, as well as its oxidation number, Ag1 or Ag þ , in the matrix. [8,14]. Some investigators suggested that AgNP might act as a ‘‘Trojan horse’’, bypassing typical cell barriers and then releasing Ag þ ions [15]. Therefore, Ag þ ions interact with the thiol groups of enzymes and proteins of cells thus they damage the bacterial respiration and transport systems across the cell membrane. However AgNPs lead to high antimicrobial activity compared to bulk silver metal because it has a high fraction of surface atoms [16]. The combination of membrane chemistry and antibacterial properties of AgNPs may solve the biofouling problem in membrane systems. At the prevention of biofouling problem, AgNP can be applied by directly coating on the surface of the membranes or by blending in the polymer matrix of the membrane during phase inversion technique [17,18]. From the studies of AgNP-polymeric membrane preparation, it can be said that AgNPs lead the more hydrophilic membrane and improved the selectivity of membrane [19,20]. Moreover, the membranes with AgNP showed slower decrease in permeate flux and also the cells adhering to the membrane surface having AgNP lost their activity quickly [21]. However, the fundamental relationship between the characteristics of membranes with AgNPs and biofouling mechanism have not been well understood. This study has focused on identifying the relationship between physical properties and fouling resistances of bare and composite membranes. The membranes were prepared by adding different amounts of AgNP (0–1.0 wt%) to the PS/PVP (18/2 wt%) dope solution. Scanning electron microscopy (SEM), contact angle, and atomic force microscopy (AFM) techniques were used to characterize the membranes. The performances of membranes were investigated using permeability test and the filtration of model protein (BSA) and carbohydrate (Dextran) solutions and also the real activated sludge. Finally, the anti-biofouling efficiencies of membranes were evaluated by the PCR-DGGE analysis and disk diffusion tests.

2. Material and methods 2.1. Chemicals Polysulfone (PS, Mw 22,000 Da), polyvinylpyrrolidone (PVP10, M w  10,000 Da), N-methyl-2-pyrrolidinone (NMP) and AgNP (particle size o100 nm) were purchased from Sigma-Aldrich. Polyethylene glycol (PEG) having different molecular weight of 4400, 10,000, 20,000, 35,000 Da were purchased from Aldrich. Microbiology agar used in preparing plates for disk diffusion test was obtained from Merck. All chemicals in this study were used without further purification.

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(AgNP-PS composite membrane), different ratios of AgNP (0.25– 0.5–1.0%, w/w) was dispersed in NMP using an ultrasonication probe at 20 min until completely dissolved. PVP (2%) and PS beads (18%) were then added to this solution as previously described. The casting solutions of bare PS and AgNP-PS composite were then spread with a casting knife gap setting of 200 mm at 100 mm/s casting shear on a glass plate. The casting films were left 10 s. for evaporation and then the glass plates were immediately immersed in a deionized water bath (pH¼ 6.8 and T¼ 24 1C) to obtain polymer precipitation. After 10 min, the membranes were removed from the water bath and washed thoroughly with deionized water and stored in deionized water including 100 mg/ L sodium azide at 4 1C to inhibit biogrowth for 2 weeks. 2.3. Membrane characterisation techniques The contact angles of the membranes were measured on KSV Attension Theta contact angle device using scissile drop mode on dried membranes. A small droplet of distilled water was delivered onto membrane surfaces, and waited 5 s and then the image of the droplet onto membrane surface was taken at 10 frame and 1 s interval. For ensuring the accuracy of the measurement, the analyses were performed at 4 different locations on the membranes. The surfaces of membranes were scanned by atomic force microscopy (AFM, Digital Instruments) to determine the roughness of membranes. Before AFM observations, the membranes were washed with ethanol and distilled water, followed by drying at room temperature. The membrane samples were fixed on a slide glass and scanned over 10.0  10.0 mm. AFM was performed under tapping mode with a scanning rate of 6.104 Hz. Obtained data were analyzed with the software of Nanoscope 3.0 and the images were in the height mode. The surface morphologies of the bare and composite membranes were directly observed by SEM (Philips-XL30 SFEG) in high vacuum mode after coating with gold to observe the pore structure. Before the SEM analysis, the membrane samples were immersed ethanol/water solution at room temperature followed by step dehydration with 25, 50, 75 and 100% ethanol for 10 min. The membranes were then dried at room temperature to be ready for SEM scan. 2.4. Permeability and molecular weight cut off (MWCO) experiments Permeability and MWCO experiments were carried out by using a dead-end stirred cell filtration system (Sterlitech, HP4750) pressurized by nitrogen gas. The module had a membrane area of 14.8 cm2. In all further experiments, the membranes were firstly compacted for 10 bar at 1 h. The flux profile over time was monitored online gravimetrically. Hydraulic membrane permeability was measured at different transmembrane pressures within the range 2–4 bar and at least three measurements from different membrane samples were averaged. The molecular weight cut off (MWCO) values of membranes were characterized by the rejection performance of 100 mg L  1 polyethylene glycol (PEG) solute of varying molecular weights (4400, 10,000, 25,000 and 30,000 Da). PEG concentrations in permeate and feed solution of these test solutes were determined by using a combustion-infrared method on Shimadzu V-CPN total organic carbon (TOC) analyzer. 2.5. Silver analysis in the permeate samples

2.2. Preparation of bare PS and AgNP-PS composite membranes For bare PS membrane preparation, first PVP (2%, w/w) was added to NMP solvent and was stirred until it was completely dissolved. PS beads (18%, w/w) were added slowly into this solution and stirred at room temperature for 24 h in order to form a homogeneous solution. For PS membrane with AgNP

300 mL of deionized (DI) water was filtered at dead-end stirred filtration cell, and the total silver concentrations in the permeate were analyzed using ICP device. Total silver concentrations were quantified using a Perkin-Elmer (Norwalk, CT) Optima 3000 DV Inductively Coupled Plasma-Optical Emission Spectrometer (ICPOES). All measurements were carried out in the axial mode at

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328.068 nm. A 999 70,2 mg mL  1 standard solution from Inorganic Ventures including silver, water and %5 HNO3 (v/v) was used as an internal standard for calibration as recommended by the ICP manufacturer and correlation coefficient was 0.9998. The detection limit of silver for the ICP was 0.01 mg L  1. 2.6. Protein and carbohydrate filtration tests The separation experiments were carried out using the protein and carbohydrate solutions. The bovine serum albumin (BSA) (100 mg L  1, pH 6 in phosphate buffer) and the dextran (Mw  70,000 Da, 100 mg L  1) solutions were used for determining of protein and carbohydrate filtration performances, respectively. Filtration experiments were carried out at 3 bar for 1 h using the dead-end stirred filtration cell. 2.7. Activated sludge experiments A real activated sludge was used for the biofouling experiments and taken from the aerated tank in a post biological municipal treatment plant located Istanbul-Turkey and cultivated in a laboratory scale aeration tank treating synthetic wastewater for one week. Glucose, (NH4)2SO4 and peptone, K2HPO4 and KH2PO4 were used as carbon, nitrogen and phosphorus sources, respectively. Additional trace metals and alkalinity (NaHCO3) were also supplied to the tank. The sludge characteristics are summarized in Table 1. For the activated sludge filtrations were conducted at 3 bar and 1 h using the dead-end stirred filtration cell. 2.8. Membrane fouling analysis The resistance values of bare and composite membranes in activated sludge filtration were calculated according to equations given below; Rt ¼

DP

ð1Þ

m:J

where, J is the permeate flux (m3 m  2 s  1), DP is the transmembrane pressure (Pa), m is the viscosity of permeate (Pa.s), and Rt is the total filtration resistance (m  1) and can be described as the sum of various resistances as follow: Rt ¼ Rm þRp þ Rc

ð2Þ 1

where Rm is the membrane resistance (m ), Rp is the pore blocking resistance (m  1) and Rc is the cake resistance (m  1). Each resistance can be calculated as follow: Rt ¼

Rm ¼

Rp ¼

DP

ð3Þ

m:Jt DP

ð4Þ

m:J0 DP

m:J1

Rm

ð5Þ

Rc ¼ Rt Rp Rm

ð6Þ

Table 1 Activated sludge characteristics. Parameters

Values (mg L  1)

MLSS SMPp SMPc EPSp EPSc

40107 45 627 5 57 0.8 2387 17 337 4

where Jt is the steady state flux at the activated sludge (L m  2 h  1) filtration, J0 is the initial steady state flux (L m  2 h  1) of distilled water and J1 is the steady state flux of distilled water (L m  2 h  1) after the removing of cake layer. In order to measure the adsorptive fouling between the biological materials in activated sludge and membrane surface, the following steps were orderly carried out: (a) The distilled water flux of a fresh membrane (J0) was determined using the stirred filtration cell for three replicates (b) 100 mL of activated sludge was poured into the stirred filtration cell without transmembrane pressure thus the biological materials could contact with the top surface of the membrane (c) After one hour, the membrane was taken out and the surface of its briefly rinsed with water and the distilled water flux of the membrane was measured again (J2). The relative flux reduction of distilled water (FRdw) was calculated as follows:   J FRdw ¼ 1 2 x100 ð7Þ J0 2.9. Bacterial analyses 2.9.1. Disk diffusion method The activated sludge was firstly centrifuged at 6000 rpm for 20 min. After the supernatant was decanted, the phosphate buffer saline (PBS) was added into the tubes and the sludge was centrifuged again at same values. This procedure was repeated three times to ensure that all nutrients in the activated sludge were removed for creating the environment without any nutrients. The activated sludge was diluted up to 100 times with PBS. 1 mL of this suspension was pipetted into an agar plate and then spread throughout the surface under the sterile conditions. The membranes were rinsed gently with ethanol and PBS and then placed on the agar surface to cover an area in the middle of the plate. This was done at three sets for all membrane types. These agar plates were then placed in an incubator at 37 1C for 48 h. After that, the agar plates were visually observed for the growth of bacteria colonies. 2.9.2. Molecular methods and DGGE analysis Activated sludge and bare and composite membranes were subjected to DNA extraction according to manufacturer’s instruction (Zymo Research, Irvine, CA, U.S.A.) after mechanical disruption by bead beating. DGGE-PCR of extracted DNAs was performed with U968-GC (50 -AAC GCG AAG AAC CTT AC-3) forward and L1401 (50 -GCG TGT GTA CAA GAC CC-3) reverse primers [22] specific for variable region between V6 and V8 of the 16S rRNA gene. U968-GC had a 40-base GC-clamp (CGC CGG GGG CGC GCC CCG GGC GGG GCG GGG GCA CGG GGG G) linked to the 50 end of the primer to improve the separation of the DNA fragments. The cycling conditions for DGGE-PCR was as follow; initial denaturation at 95 1C for 5 min then, 35 cycles of denaturation at 95 1C for 30 s, primer annealing at 56 for 40 s and extension at 72 1C for 1 min. After the last cycle, a final elongation of 72 1C for 5 min and immediate cooling to 4 1C were applied. The GC clamp PCR-products were separated in 8% (w/v) polyacrylamide gels (37.5:1 acrylamide:bisacrylamide) containing a linear 35–58% denaturant gradient (100% denaturant corresponds to 7 M urea and 40% deionized formamide) in 1 X (TAE) buffer at a constant voltage of 100 V and a temperature of 60 1C for 16 h [23]. Gels were stained using silver nitrate and scanned at GS-800TM Calibrated Densitometer (Bio-Rad, Hercules, CA, USA). The processing of the DGGE gels was performed using BioNumerics software package Gel Compar II Software version 6.5 (Applied Maths, Kortrijk, Belgium). The DGGE banding patterns reflecting microbial consortia was examined by clustering

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analysis. The gel were normalized and the dendrogram of DGGE patterns were constructed using an Unweighted Pair Group Method with an Arithmetic Mean method. The Dice correlation (binary) coefficient matrice was applied. Since the clustering analysis of the PCR-DGGE patterns could be affected by various factors, such as position bias in gels, band assignment, and different settings in the BioNumerics software, the optimal position tolerance and optimization setting were calculated using the tolerance and optimization analysis program supplied with the BioNumerics software package to ensure that there were better matches for the band patterns.

3. Results and discussion 3.1. Characteristics of bare PS and composite AgNP-PS membranes The bare and composite membranes were characterized by SEM, AFM, contact angle, pure water flux (permeability) and MWCO analysis. Fig. 1(a)–(d) shows the SEM images of the outer surfaces of bare and composite PS membranes. Pores can be observed as only cracks onto bare PS membrane surfaces but after the addition of AgNP, most of the cracks disappeared on the composite AgNP-PS membranes. As can be seen from Fig. 1b, 0.25AgNP-PS composite had very homogeneous surface. However, it can be observed from Fig. 1c and d that the some of AgNPs were located along the 0.5AgNP-PS and 1.0AgNP-PS composite membrane surfaces and the numbers of AgNP spheres and aggregates increased with the increasing of AgNP ratios. Increasing AgNP

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amount in dope solution may cause that some AgNPs without homogeneous mixing with polymer or PVP moved to the polymer-rich phase (upper part of glass film) during phase inversion process and then these AgNPs may accumulate onto membrane surface. SEM images of membranes were taken after physically washing with ethanol–water solution. For this reason, it could be thought that these AgNP aggregates onto surface are strongly-bound with polymer and are not easily dragged with physical forces. AFM analysis was done for determining the roughness of bare and composite membrane surfaces. AFM images for membranes are presented in Fig. 2(a)–(d) and a summary of the surface roughness values of membranes are presented in Table 2. The roughness value of bare PS membrane decreased from 15.7 to 9.7, 10.1 and 14.2 nm according to AgNP ratios (0.25–0.5–1.0 wt%, respectively) after the addition of AgNP. 0.25AgNP-PS composite membrane had the lowest roughness value. Therefore it was seen that the roughness values of the AgNP-PS composite membranes increased with increasing the ratio of AgNP. AgNP aggregates onto membrane surface could cause the increase in surface roughness. The contact angle values of bare and composite PS membranes are given at Table 2. It was clearly seen that the contact angle of bare PS membrane decreased with the addition of AgNP. The bare PS membrane had highest contact angle value (7071.21). However, the contact angle values of AgNP-PS composite membranes decreased from 6771.51 for 0.25AgNP-PS membrane to 56 72.41 for 1.0AgNP-PS membrane. These values were similar to the literature. Zodrow et al. (2009) found that bare PS and 0.9AgNP-PS

Fig. 1. SEM pictures for bare PS and composite AgNP-PS membranes (a) bare PS (b) 0.25AgNP-PS (c) 0.5AgNP-PS (d) 1.0AgNP-PS.

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Fig. 2. AFM pictures for bare PS and composite AgNP-PS membranes (a) bare PS (b) 0.25AgNP-PS (c) 0.5AgNP-PS (d) 1.0AgNP-PS.

Table 2 The roughness and contact angle values of bare and composite PS membrane surfaces. Membrane sample

Roughness (nm)

Contact angle (1)

Bare PS 0.25AgNP-PS 0.5AgNP-PS 1.0AgNP-PS

15.7 9.7 10.1 14.2

70 71.2 67 71.5 60 71.7 56 72.4

composite membrane (0.9 wt% AgNP) had 76.8 74.831 and 68.676.11 [20]. Basri et al. (2011) studied with AgNP-PES composite membrane and found that the addition of AgNP had improved the membrane hydrophilicity [18]. Moreover, the authors described this phenomenon like that Ag particles had low surface tension of pristine PES and so the water can easily spread on membrane surfaces.

Table 3 The permeability and MWCO values of bare and composite PS membranes and the concentration of silver lost from membranes. Membrane sample

Permeability (L m  2 h  1 bar  1)

MWCO (Da)

Silver concentration in filtrate (mg L  1)

Bare PS 0.25AgNP-PS 0.5AgNP-PS 1.0AgNP-PS

31.4 7 6.5 33.5 7 1.8 14.4 7 2.2 12.8 7 1.0

125,813 85,884 76,589 73,728

0 0.14 0.19 0.84

The silver loss values of membranes are also given in Table 3. The values showed that the loss of Ag þ from composite membranes increased with increasing of AgNP loading to the membrane. However, the concentrations of Ag þ in permeate samples was very low compared with AgNP amounts in dope solutions. 3.3. Protein and carbohydrate filtration

3.2. Membrane permeability, MWCO and silver loss Table 3 shows the permeability and MWCO values of membranes and also the silver concentrations in permeate. As apparent from the table, the permeability and MWCO values of membranes marginally decreased as AgNP ratio in the dope solution increased. Sawada et al. explained this phenomena like that an increase in the number and size of the silver nanoparticles on membrane surface caused in the reduction of water permeability because the nanoparticles create a barrier to water transport [24]. From SEM analysis explained at Section 3.1, it can be clearly seen that 1.0AgNP-PS composite membrane had higher AgNP aggregates on the membrane surface and so this membrane had lowest permeability and MWCO values (12.8 71.0 L m  2 h  1 bar  1 and 73,728 Da, respectively).

In order to explore the protein and carbohydrate fouling resistances of bare and composite membranes, the filtration experiments were performed using the model organic foulant (BSA and dextran) in aqueous solution. The permeate volumes of protein and carbohydrate filtrations of membranes are graphically shown in Fig. 3a and b, respectively. The protein permeate volumes of membranes were changed following the order: 1.0AgNP-PS40.5AgNP-PS40.25AgNP-PS 4Bare PS. On the contrary, the carbohydrate permeate volumes of membranes were changed thus: 0.25AgNP-PS40.5AgNP-PS 41.0AgNP-PS4Bare PS. For both filtration experiments, AgNP-PS composite membranes showed a better performance compared to bare PS membrane. Besides the filtration performances of AgNP-PS composite membranes changed with the ratio of AgNP in membranes.

D.Y. Koseoglu-Imer et al. / Journal of Membrane Science 428 (2013) 620–628

30

Bare PS 0.25AgNP-PS 0.5AgNP-PS 1.0AgNP-PS

Permeate volume (ml)

25 20 15 10 5 0 0

70

20

30 Time (min)

40

50

60

20

30 Time (min)

40

50

60

Bare PS 0.25AgNP-PS 0.5AgNP-PS 1.0AgNP-PS

60 Permeate volume (ml)

10

50 40 30 20 10 0 0

10

Fig. 3. Permeate volumes of protein and carbohydrate passed through bare and composite PS membranes (a) Protein (BSA) (b) Carbohydrate (Dextran).

The ultimate reason might be the change of membrane surface characteristics with different AgNP ratio. It should be noted that one of the main factors enhancing the protein fouling onto the membrane surface is hydrophobic interaction between the membrane surface and protein molecules. Therefore, the protein molecules adsorbed on the surface of membrane can be reduced with modifying hydrophobic membrane surface to hydrophilic membrane surface [25]. Wang and Tang [26] described in detail that the effect of membrane properties on BSA fouling was evaluated using four commercial membranes as a reverse osmosis, two nanofiltration an a ultrafiltration membranes. Despite the large differences in their membrane properties, they observed that the long term flux was independent of membrane types and properties, the short term flux during BSA fouling was clearly membrane-dependent. Moreover, it was found that the membranes having smooth, highly hydrophilic and highly negatively charged surface experienced much slower flux decline at initial fouling period. In our study, the protein filtration performance was investigated at the short-term filtration experiment (for 1 h) and so the membrane characteristics especially hydrophobic/ hydrophilic properties could play important role for the protein fouling mechanism. The contact angle value of bare PS membrane decreased due to increase in hydrophilicity after AgNP addition. Among AgNP-PS composite membranes, the 1.0AgNP-PS composite

625

membrane had lowest contact angle value and so this membrane showed the best protein filtration performance whereas bare PS membrane had highest contact angle value and so this membrane showed the worst protein filtration performance. It is seen from Fig. 3b that the carbohydrate permeate volumes of all composite PS membranes are higher than the bare PS membrane. Especially, the permeate volume at 0.25AgNP-PS composite membrane was about three times greater than bare PS membrane. However the performances of composite membranes decreased with increasing AgNP concentration in membranes. The relationship between the molecular weight of dextran and MWCO values of composite membranes could be caused to this phenomena. The molecular weight of dextran used in the experiments was 70,000 Da and the MWCO values of composite membranes were determined as 85,884, 76,589 and 73,728 Da for 0.25AgNP, 0.5AgNP and 1.0AgNP, respectively. The carbohydrate permate volume trend of membranes were similar to the MWCO values change of composite membranes. Although the bare PS membrane had the highest MWCO value (125,813 Da), it had the worst dextran filtration performance. The MWCO values of membranes has linear relationship with pore size of the membranes [27]. For this reason, bare PS membrane had greater membrane pore size compared with composite membranes (as described in Section 3.1). Thus, bare PS membrane had a severe dextran fouling while AgNP addition would protect the membrane surface against dextran molecules. In literature, it was indicated that for average molar masses up to 70,000 Da, bound dextran molecules could either narrow or (completely) block the membrane pores and also the dextran is a flexible coil polymer and is able to deform within the membrane pores [28]. For AgNPPS composite membranes, the deposition of AgNPs both on surface and in the pores of composite membranes could prevent the pore fouling caused by dextran molecules. In addition, the reason of different carbohydrate filtration performances of composite membranes could be that the composite membranes had different surface roughness values. 1.0AgNP-PS composite membrane had the highest roughness value and so this membrane showed the worst carbohydrate filtration performance. On the other hand, 0.25AgNP composite membrane had the lowest roughness value and the highest carbohydrate filtration performance. It could be expected that the pore fouling was more severe at the composite membranes having greater roughness values because as described above the dextran molecules having different polymeric characteristics tend to accumulate onto roughness surfaces. From all experimental results, it could be concluded that 1.0AgNP-PS composite membrane showed the best protein filtration performance while 0.25AgNP-PS composite membrane had the best carbohydrate filtration performance. 3.4. Biofouling study of composite and bare PS membranes with activated sludge The biofouling studies of composite and bare PS membranes were conducted using the filtration of a real activated sludge obtained from post biological municipal treatment plant. The sludge characteristics are given at Table 1. As apparent from table, the protein contents of biological substances (SMP and EPS) were significantly higher than the carbohydrate contents of its. The activated sludge filtration performances of composite and bare membranes were monitored as total permeate volumes and resistance analysis in series model. The trend of permeate volumes of membranes during one hour activated sludge filtration is given at Fig. 4. 0.25AgNP-PS composite membrane had the highest permeate volume and 1.0AgNP-PS composite membrane had the lowest permeate volume after 1 h filtration. The permeate

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70

Bare PS 0.25AgNP-PS 0.5AgNP-PS 1.0AgNP-PS

Permeate volume (ml)

60 50 40 30 20 10 0 0

10

20

30 Time (min)

40

50

60

Fig. 4. The cumulative permeate volume with time of bare and composite membranes during activated sludge filtration.

Rc Rp Rm Rt FRdw

4

50

40

3

30

2

20

1

10

0 Bare PS

0.25AgNP-PS 0.5AgNP-PS Membrane type

FRdw (%)

Resistance (x1013) m-1

5

0 1.0AgNP-PS

Fig. 5. The fouling resistance results of bare and composite membranes.

volumes of composite membranes decreased with increasing AgNP ratios. This result was similar to the decreasing of membrane permeability with increasing AgNP ratio in membrane as discussed at Section 3.2. In order to explain in detail the fouling phenomena of membranes, the various filtration resistances of bare and composite membranes was calculated and the results are graphically shown in Fig. 5. Rm values of composite membranes were slightly higher than the bare membrane and increased with increasing AgNP ratio. The membrane resistance is strongly dependent on the membrane characteristics (hydrophilicity, pore size distribution, thickness, etc.) and so in this study it is deemed that the Rm was changed with membrane hydrophilicity and roughness (in Section 3.1). The other resistance results clearly showed that Rp values decreased substantially with the addition of AgNP. This may be considered as lower pore fouling tendency for AgNP-PS composite membranes because AgNP may have served as a protective barrier for membrane pores. Therefore the change of Rp of composite and bare membranes was correlated with the roughness values of membranes taken from AFM analysis because the soluble foulants (such as SMP) become easier into the pores of the membranes with increasing the roughness of membrane surface. Interestingly, the addition of AgNP increased Rc values of membranes. The

structure of cake layer onto membrane surface regulates the development of membrane biofouling. As mentioned above, the protein content in biological substances (EPS and SMP) of activated sludge (from Table 1) was very high comparing with its carbohydrate content. In literature, it was generally considered that the sludge with a higher ratio of proteins to carbohydrate in biological substances had high stickiness and thus favour the development of cake formation [29]. In our study, high protein content in activated sludge caused to high sticky gel-layer formation onto membrane surfaces (visually observed). It was expected that the accumulation of biological substances in cake layer of composite membranes was higher than the bare membrane because AgNPs on the composite membrane surfaces prevented clogging the pores of membranes with these substances and these substances were accumulated onto cake layer. Because of these mechanisms, the structure of cake layers onto AgNP-PS composite membranes was different from the bare membrane. As numerical, Rc values of composite membranes were higher than bare membrane but the cake layer structures of composite membranes were more gel-like and easily removed with physical forces (it was observed during physical cleaning experiments). Moreover, the membrane characteristics may affect the cake resistance onto membrane surface. It is mentioned above that the contact angle of bare PS membrane decreased and so the hydrophilicity of membranes increased with the addition of AgNP (Bare PS 40.25AgNP-PS40.5AgNP-PS4 1.0AgNP-PS) and Rc values of membranes changed as 1.0AgNP-PS 40.25AgNPPS40.5AgNP-PS4Bare PS. For this reason, it may be thought that a relationship exists between the sludge cake resistances and hydrophilicity of membranes and so the increasing hydrophilicity may accelerate the sludge accumulation onto membrane surface. Furthermore, the adsorptive fouling tendencies of bare and composite membranes with biological materials were investigated and FRdw values (the relative flux reduction of distilled water) of membranes were calculated using Eq. (7) as described in Section 2.8. The FRdw values of membranes are graphically given at Fig. 5. FRdw values of membranes changed as 45, 10, 15 and 40% for bare PS, 0.25AgNP-PS, 0.5AgNP-PSand 1.0AgNP-PS membranes, respectively. These results showed that the adsorptive fouling of bare PS membrane was greater than the composite membranes and among composite membranes, the adsorptive fouling increased with increasing the ratio of AgNP. As apparent from the results, the change of roughness and FRdw values of membranes was directly proportional as Bare PS41.0AgNP-PS40.5AgNP-PS40.25AgNP-PS. The increase of membrane surface roughness stimulated the adsorptive capacity of bacterial cells itself and also substances. From all activated sludge filtration results, it could be concluded that the using of AgNP in membrane preparation changed the membrane filtration characteristics (permeate volume, fouling resistances etc.). Therefore, AgNP addition improved the activated sludge filtration performance of bare membrane. Especially, it significantly decreased the pore fouling of membranes. The pore fouling in MBR systems is very serious problem because it causes to the irreversible fouling at membranes and so the cleaning cost of membranes increase dramatically. For this reason, the protection of membrane pores with AgNPs could provide many advantages as well as AgNPs onto membrane surface could restrict the bacterial development with producing ionic silver (Ag þ ). 3.5. Bacterial analyses Fig. 6 shows the results of the disk diffusion test. In this test, first 1 mL of diluted activated sludge was spread on the agar and then the bare and composite membranes having same sizes were put on the agar thus the growth of bacteria colonies was monitored. After 24 h incubation, it was observed that the bacterial growth in plates of AgNP-PS composite membranes

D.Y. Koseoglu-Imer et al. / Journal of Membrane Science 428 (2013) 620–628

Sludge

Bare PS

0.25 AgNP-PS

0.5 AgNP-PS

627

1.0 AgNP-PS

Fig. 6. The results of disk diffusion test.

Fig. 7. UPGMA dendrogram analysis for the assessment of similarity between DGGE profiles. The indices for the clustering analysis are optimization 0%; position tolerance 0.6%, minimum height and minimum surface 0% used for the comparison. 0.0–100%, indicating the entire length of each lane.

was much lower than the plates of Bare-PS membrane. Moreover, the number of bacterial colonies decreased with increasing AgNP ratio and so 1.0AgNP-PS composite membrane had the lowest bacterial colonies. This could be due to the high-loading of AgNP in the composite membrane matrix limited the bacterial growth rate. During the disk diffusion test, the Ag þ ions released from the composite membranes and thus the agar medium become enrichment in terms of Ag þ ions. As a result, Ag þ ions attacked bacteria and disturbed its function and so deteriorate the bacterial growth. It is in good agreement with other studies that free silver ions played a considerable role in the toxicity of AgNP suspensions [4,30,31]. The further biological technique was used for finding the differences onto membrane surfaces. The bacterial communities in the activated sludge and the bare and composite membranes were compared by PCR-DGGE analysis. PCR-DGGE profiles were obtained for samples which are depicted as starting inoculum (sludge), no Ag (Bare). 0.25 Ag (0.25 AgNP), 0.5 Ag (0.5 AgNP), 1 Ag (1 AgNP) in Fig. 7. The Dice band-based coincidence index in combination with 0% optimization and 0.6% position tolerance was found to be consistent with the visual inspection and resulted in highest similarity recognition (90%) among samples. From the analysis, it could be said that only the activated sludge was clustered in different from the membrane samples and the addition of AgNP did not significantly affect the bacterial consortium.

addition of AgNP (d) The permeability and MWCO values of membranes decreased with increasing of AgNP ratio. 2. At the protein and carbohydrate filtration, 1.0AgNP-PS composite membrane showed the best protein filtration performance while 0.25AgNP-PS composite membrane had the best carbohydrate filtration performance. Therefore bare PS had the lowest filtration performance for both model foulant. The specific relationships were found as the between protein fouling and membrane hydrophilicity and carbohydrate fouling and membrane surface roughness. 3. At the activated sludge filtration, 0.25AgNP-PS composite membrane had the highest permeate volume and bare PS membrane had the lowest permeate volume. AgNP addition improved the activated sludge filtration performance of bare membrane. Especially, it significantly decreased the pore fouling of membranes. The adsorptive fouling of bare PS membrane was greater than the composite membranes and among composite membranes, the adsorptive fouling increased with increasing the ratio of AgNP. The change of roughness and adsorptive fouling of membranes was directly proportional. 4. At the bacterial analysis, the growth of bacterial colonies decreased with increasing AgNP ratio and the addition of AgNP did not significantly affect the bacterial consortium onto membrane surfaces.

4. Conclusions

Acknowledgement

In this study, the bare PS and AgNP-PS composite membranes were prepared and tested at simultaneous experiments. Major findings are listed as follows:

The first author (Derya Y. Koseoglu-Imer) financially supported by the TUBITAK-B_IDEB (The Scientific and Technological Research Council of Turkey) International Postdoctoral Research Scholarship Programme.

1. The membrane characterization analysis showed that (a) the surface structure of bare PS membrane changed after the addition of AgNP and also AgNP spheres and aggreagates were observed with the increasing of AgNP ratios, (b) the roughness value of bare PS membrane decreased with the addition of AgNP but the roughness values of the AgNP-PS composite membranes increased with increasing the ratio of AgNP (c) the contact angle of bare PS membrane decreased with the

Nomenclature R Cp

the carbohydrate and protein rejections (%) the protein or carbohydrate concentrations of permeate (mg L  1)

628

Cf Rt DP

m J Rm Rp Rc Jt J0 J1

J2 FRdw

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the protein or carbohydrate concentrations of feed (mg L  1) the total filtration resistance (m  1) the transmembrane pressure (Pa) the viscosity of permeate (Pa.s), the permeate flux (m3 m  2 s  1) the membrane resistance (m  1), the pore blocking resistance (m  1) the cake resistance (m  1) the steady state flux at the activated sludge (L m  2 h  1) the initial steady state flux (L m  2 h  1) of distilled water the steady state distilled water flux (L m  2 h  1) after removing the cake layer by flushing with distilled water and sponge the steady state distilled water flux (L m  2 h  1) after removing the adsorptive fouling The relative flux reduction of distilled water (%)

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