PMMA-g-PEO polymer blend membranes for membrane bioreactor

Journal of Membrane Science 466 (2014) 211–219

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Quantitative evaluation of fouling resistance of PVDF/PMMA-g-PEO polymer blend membranes for membrane bioreactor Hiroki Minehara n, Kouichi Dan, Yohito Ito, Hiroo Takabatake, Masahiro Henmi Toray Industries, Inc., 2-1 Sonoyama 3-chome, Otsu, Shiga 520-0842, Japan

art ic l e i nf o

a b s t r a c t

Article history: Received 7 July 2013 Received in revised form 5 April 2014 Accepted 21 April 2014 Available online 14 May 2014

Fouling resistance of various kinds of membrane materials including blends of poly(vinylidene difluoride) (PVDF) and copolymer of methyl methacrylate (MMA) and poly(ethylene glycol) methyl ether methacrylate (PVDF/PMMA-g-PEO) was quantitatively evaluated by measuring the amount of model fouling compounds adsorbed on the surface of membrane materials using a surface plasmon resonance (SPR) sensor. The order of the amount of adsorption in the adsorption tests negatively correlated with hydrophilicities of membrane materials. PVDF/PMMA-g-PEO demonstrated superior fouling resistance comparing to pure PVDF and other materials. Pure PVDF and PVDF/PMMA-g-PEO membranes with different length of PEO side chains were fabricated in non-solvent induced phase separation (NIPS) method. Membrane structure and surface composition were analyzed with scanning electron microscopy (SEM) and X-ray photoelectoron spectroscopy (XPS) respectively. Moreover, shortterm and long-term activated sludge filtration experiments were conducted. The rate of increase of differential pressure during filtration process correlated with the amount of adsorbed BSA in the adsorption tests. & 2014 Elsevier B.V. All rights reserved.

Keywords: Membrane fouling Poly(vinylidene difluoride) Poly(ethylene oxide) methacrylate Surface plasmon resonance Membrane bioreactor

1. Introduction Membrane bioreactor (MBR) system, in which membranes are applied to biological wastewater treatment for biomass separation, is an attractive alternative to conventional activated sludge treatment using secondary sedimentation due to their compact design and high quality outputs [1]. MBR also allows high concentration of mixed liquor suspended solids (MLSS) and low production of excess sludge, enabling high removal efficiency of biological oxygen demand (BOD) and chemical oxygen demand (COD), and

Abbreviations: ADBN, 2,20 -azobis (2,4-dimethyl) valeronitrile; BOD, Biological oxidation demand; BSA, Bovine serum albumin; COD, Chemical oxidation demand; cPE, chlorinated polyethylene; DMF, Dimethyl formamide; DMSO-d6, deuterium dimethyl sulfoxide; EPS, Extracellular polymer substances; FT-IR, Fourier transmission – infrared spectroscopy; MBR, Membrane bioreactor; MF, Microfiltration; MLSS, Mixed liquor suspended solid; MMA, Methyl methacrylate; NF, Nanofiltration; NIPS, Non-solvent induced phase separation; NMP, N-methyl pyrrolidone; NMR, nuclear magnetic resonance; PEO, Poly(ethylene oxide); PES, Poly (ether sulfone); PET, Poly(ethylene telephthalate); PMMA, Poly(methyl methacrylate); PP, Poly propylene; PS, Polysulfone; PVDF, Poly(vinylidene difluoride); QCM-D, Quartz crystal microbalance – dissipation; SEM, Scanning electron microscopy; SPR, Surface plasmon resonance; THF, Tetrahydrofurane; UF, Ultrafiltration; wt%, weight per cent; XPS, X-ray photoelectron spectroscopy n Corresponding author. Tel.: þ 81 77 5338446; fax: þ81 77 5338695. E-mail address: [email protected] (H. Minehara). http://dx.doi.org/10.1016/j.memsci.2014.04.039 0376-7388/& 2014 Elsevier B.V. All rights reserved.

water reclamation. However, besides the obvious advantages of MBR, membrane fouling in MBR restricts their widespread application because it reduces productivity and increases maintenance and operating costs [1,2]. On the basis of many studies on membrane fouling, it was determined that extracellular polymer substances (EPS) is the key component affecting fouling in MBR [3–5]. EPS mainly consists of humic acid, polysaccharides and proteins. The major components of foulants were identified as proteins and polysaccharides through FT-IR and 13C NMR analysis [6,7]. To better control membrane fouling, it is necessary to elucidate the relationship between membrane fouling and membrane properties. In the initial stage of fouling, adsorption phenomena play an important role. Kimura et al. examined that membrane fouling of different MF/UF membranes with EPS collected from different origins [7]. In their paper, it was reported that the hydrophilic fraction of EPS caused irreversible membrane fouling and hydrophilic membranes showed high antifouling properties. Hashino et al. examined the effects of different kinds of membrane materials on membrane fouling with bovine serum albumin (BSA) [8]. They evaluated the amount of adsorbed BSA on the surface of the membrane materials, poly(ethylene-co-vinyl alcohol) (EVOH), polyether sulfone (PES) and Poly(vinylidene fluoride) (PVDF) using quartz crystal microbalance with the dissipation monitoring (QCM-D) method. In their study, the degree of BSA adsorption correlated with the hydrophilicity of the polymers. Among the three membranes, the smallest

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amount of BSA adsorbed on the surface of hydrophilic EVOH and the biggest amount on the surface of hydrophobic PVDF. PVDF is a widely used material for microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF) because of its chemical resistance, thermal and mechanical properties. One of the most important requirements for PVDF membranes in waste water treatment application is to reduce the nonspecific adsorption of biomolecules such as EPS with hydrophobic interaction. Various strategies have been applied to modify PVDF membrane surface, such as polymer coating [9–11], surface graft polymerization [12–16] and polymer blends of hydrophilic polymers [17–19], hybridization of inorganic nanoparticles [20,21]. Surface coating is usually unstable and liable to be washed away during the operation process. Surface graft polymerization usually needs a multi-step and expensive process including high energy irradiation, which makes surface grafting not suitable for an industrial scale production. On the other hand, blend polymer membranes can be fabricated with a single-step. Therefore blending method is considered to be suitable for large scale production. In particular, membranes fabricated from blends of PVDF and a freeradically synthesized amphiphilic polymer having a methacrylate backbone and hydrophilic poly(ethylene oxide) side chains (PVDF/ PMMA-g-PEO) exhibited substantially reduced flux decline during filtration of a protein solution relative to pure PVDF membranes having similar pore structure [17,18]. The relatively hydrophobic PMMA provides compatibility with PVDF and water insolubility during filtration process, while hydrophilic side chains deliver fouling resistance. The fouling resistance of PVDF/PMMA-g-PEO membranes which has different length of PEO side chains was qualitatively evaluated by comparing the degree of flux decline in the filtration of aqueous solution of bovine serum albumin (BSA). However, the fouling resistance of PVDF/PMMA-g-PEO membranes needs to be quantitatively analyzed to further understand the fouling behavior of the membranes. Along with QCM method [8], surface plasmon resonance (SPR) is also known to be a strong method to analyze the adsorption phenomena of bio-molecular substances in biomedical research [22]. SPR is capable of detecting the amount of adsorption at the interface of the SPR sensor chip and the aqueous protein solution when the refractive index (n) of an adsorbate differs from that of pure water. Because of the smaller sensitive area of the SPR surface, fewer molecules are necessary for the same surface density. Smaller flow-through cells with smaller sample volumes should be possible compared to the QCM. The SPR-technique exploits the fact that, at certain conditions, surface plasmons on a metallic substrate such as gold can be excited by photons, thereby transforming a photon into a surface plasmon. Gold is a typical example of metal that generates surface plasmons. Fig. 1 shows the most common geometric setup of SPR system. Incoming monochromatic, p-polarized light is reflected from the back of the glass–gold interface. Whenever a plasmon is excited, one photon disappears, producing a dip in the intensity of reflected light at that specific angle. A plot of reflected intensity versus the angle of

Fig. 1. Basic components of an SPR biosensor: Light passes through the prism and a slide with a thin (  48 nm) gold layer that is mounted on the prism. It reflects off the gold and passes back through the prism to a detector. Changes in reflectivity versus angle or wavelength give a signal that is proportional to the mass of protein adsorbed near the surface.

incidence shows a minimum at the resonance angle, Θm, corresponding to the excitation of surface plasmons at the gold solution interface, as shown in Fig. 1. There will be a shift in Θm values depending on the changes in the refractive index of the interfacial region adjacent to the gold surface [23]. In this study, the fouling resistance of PVDF/PMMA-g-PEO membranes was quantitatively evaluated using SPR system to understand the mechanism underlying their high fouling resistance. Adsorption test using model foulants solution was conducted for nine kinds of membrane materials including PVDF/PMMA-g-PEO blend polymers with different length of PEO side chains. The correlation between the amount of adsorbed foulants and hydrophilicity of the materials was discussed based on the obtained data. PVDF/PMMA-g-PEO and PVDF membranes were fabricated in NIPS method for MBR operation. The effect of the length of PEO side chains on the fouling resistance of PVDF/PMMA-g-PEO membranes was also discussed analyzing these data.

2. Experimental 2.1. Materials Poly(vinylidene difluoride) (PVDF, KF-850 polymer made by Kureha Chemical Industry), methyl methacrylate, 2,20 -azobis (2,4dimethyl) valeronitrile (ADBN), 2,20 -azobis (isobutyronitrile) (AIBN) and poly(ethylene glycol) (PEG) (Mw ¼ 20,000) methyl methacrylate (MMA) were purchased from Sigma Aldrich. Bovine serum albumin (BSA), dextran (Mw ¼2,000,000), ethyl acetate, methanol, petroleum ether, tetrahydrofurane, N-methyl pyrrolidone (NMP), N,N-dimethyl formamide (DMF), hexane, ethanol, tetrahydrofuran (THF), deuterated dimethyl sulfoxide (DMSO-d6), chlorinated polyethylene, poly(propylene) (PP; Mw ¼14,000), poly(ethylene telephthalate) (PET; Mw ¼ 18,000), polysulfone (PS; Mw ¼35,000), and PMMA (Mw ¼10,000) were purchased from Wako Pure Chemical Industries. Poly(ethylene glycol) methyl ether methacrylate, referred to herein as poly (oxyethylene methacrylate) (POEMn, where n denotes the number of repeat units), POEM9, POEM23 and POEM90 were made by Shin Nakamura Chemical. PVDF (Kynars) was provided by Arkema Inc. Polyethersulfone E2010 (PES) and polysulfone S2010 (PS) were provided by BASF. Poly(ethylene-co-vinyl alcohol) F101B (EVAL) was purchased from Kuraray. All chemicals and solvents were reagent grade, and were used as received. 2.2. Synthesis of PMMA-g-PEO PMMA-g-PEO was synthesized following a slightly modified version of the thermally induced radical polymerization approaches previously reported [18]. MMA and POEMn were dissolved in ethyl acetate at room temperature to make a solution containing 30% (wt/ wt) monomer in a 300 mL, three-necked, round-bottomed flask equipped with a mechanical stirrer, reflux condenser and rubber septum inlet. AIBN was added and the reaction solution was placed in a water bath preheated to 60 1C in nitrogen atmosphere. The polymerization was allowed to proceed for 4.5 h. The polymer was precipitated in a 9:1 mixture of petroleum ether and methanol. For purification, the product was washed with water and reprecipitated in similar petroleum/methanol mixture three times then dried in a vacuum oven at room temperature overnight. Gel permeation chromatography was conducted on a Tosoh HLC8220 system equipped with RI-8020 differential refractometer and two TSKgels α-M columns (7.8  300 mm). The measurements were performed in NMP at 40 1C using a flow rate of 0.2 mL/min. The columns were calibrated with several narrow polydispersity polystyrene samples. POEMn content was determined by 1H nuclear

H. Minehara et al. / Journal of Membrane Science 466 (2014) 211–219

213

2.5. Model organic foulants adsorption test

magnetic resonance (NMR) spectroscopy in DMSO-d6 using a JEOL EX-270 (270 MHz) spectrometer.

To evaluate the fouling resistance of PBMs comparing with various polymer materials, PP, PET, PVDF, PS, PMMA, PES, EVAL, PVDF/PMMA-g-PEO9, PVDF/PMMA-g-PEO23, PVDF/PMMA-g-PEO90 were coated on a SPR chip. The blend ratio of PVDF/PMMA-g-PEO was 7/3 wt/wt for each sample. SPR sensor (Biacore 3000, GE Healthcare UK Ltd., UK) was used to evaluate the amount of organic foulants adsorption on the surface of membrane materials. BSA and dextran was chosen as model organic foulants, because previous study showed that foulants on the membrane surface mainly consist of proteins and polysaccharides [24]. The surface of sensor chip of Biacore 3000 is made of gold. Various membrane materials were coated on the surface as the same procedure for the sample preparation of water contact angle measurement except for using gold chips instead of glass plates as a substrate. The test solutions were flowed on the surface of the polymer coated chip and the amount of adsorbed compound was measured by monitoring the change in the SPR signal. Equilibrium adsorbed amount of organic molecules was quantitatively evaluated. A 0.16–2.5 wt% solution of BSA and 0.32– 5.0 wt% solution of dextran were used for the experiments. The model foulant solution was flowed at the rate of 30 μl/min for 8 min, then RO water was flowed at the same rate for 15 min. Mass loading on the gold chip coated with membrane materials was calculated assuming that a signal change of 1 response unit (RU) equals a mass loading of 1 pg/mm2 [25].

2.3. Water contact angle Water contact angle of various membrane materials were measured using a contact angle goniometer (Drop Master 300, Kyowa interface science Co., Japan) at room temperature. PVDF/PMMA-gPEO, PP, PET, PVDF, PS, PMMA, PES, and EVAL samples were prepared from casting solutions of 1% (wt/wt) polymer in DMF. The casting solution was cast by pipette onto a glass plate and the solvent was evaporated in vacuo overnight. The cPE sample was prepared using toluene as a solvent.

2.4. Membrane preparation PVDF/PMMA-g-PEO membranes (PBMs) were produced on a polyester nonwoven fabric by immersion precipitation method. PVDF and PMMA-g-PEO were dissolved in DMF at 90 1C to make a solution containing 18% (wt/wt) polymer. PEG (Mw ¼20,000) and pure water were added as a pore-forming agent and a non-solvent respectively. After the solvent solution was cooled to 30 1C, this was applied on a polyester nonwoven fabric having a density of 0.48 g/cm3 and a thickness of 220 μm and immediately immersed in pure water at 20 1C for 5 min. The nonwoven fabric was immersed in hot water at 80 1C three times to remove DMF and PEG. PVDF membrane was also fabricated as control. Membranes were stored in pure water until use. PVDF/PMMA-g-PEO membranes in Table 2 are labeled according to the general format, PBMn-m, where PBM is an abbreviation of polymer blend membrane, the letter n indicates the number of PEO repeating unit of PMMA-g-PEO, the last letter m indicates the blend ratio (wt/wt) of PMMA-g-PEO against PVDF. The obtained membranes were observed in the scope of 9.2 μm by 10.4 μm by scanning electron microscopy (SEM) at a magnification of 10,000 times.

2.6. Filtration studies Water permeation flux of the obtained membranes was measured using water purified with a membrane dialyzer (Filtrizer BGPQ; Toray Medical Co. Ltd., Japan). The membranes were cut into a small circle with a diameter of 4 cm and placed in a cell. The filtration was carried out by the use of the water head difference placing a reservoir tank at a height of 1 m (9.8  103 Pa) at 25 1C. The permeate was weighed continuously with a digital scalar connected to PC. The permeation flux (J0) was calculated from the

Table 1 Synthesis conditions and physical characteristics of PMMA/POEMn copolymers (PMMA-g-PEO). Polymer name

MMA (mol/L)

POEMn (mol/L) n¼ 9

PMMA-g-PEO9 PMMA-g-PEO23 PMMA-g-PEO90

2.4 2.4 2.4

n¼ 23

AIBN (mol/L)

Mw

Mw/Mn

Composition (mol%, POEMn)

0.012 0.012 0.012

245,000 274,000 261,000

1.8 1.9 1.7

64/36 86/14 91/9

n¼90

0.31 0.14 0.04

Table 2 Membrane casting solution composition and preparation condition. Membrane

PVDF BPM9-10 BPM9-30 BPM23-10 BPM23-30 BPM90-10 BPM90-30 a

Casting solution composition (wt%) PVDF

PMMA-g-PEO

PEG

Water

DMF

13.0 11.7 9.1 11.7 9.1 11.7 9.1

– 1.3 (n¼9)a 3.9 (n ¼9)a 1.3 (n¼23)a 3.9 (n ¼23)a 1.3 (n¼90)a 3.9 (n ¼90)a

5.0 5.0 5.0 5.0 5.0 5.0 5.0

3.0 3.0 3.0 3.0 3.0 3.0 3.0

79.0 79.0 79.0 79.0 79.0 79.0 79.0

n indicates the number of PEO repeating unit.

Casting temperature (1C)

Coagulation bath temperature (1C)

30 60 60 48 48 48 48

90 90 90 90 90 90 90

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permeation weight during the first minute after 5 min preliminary permeation using the following equation: J0 ¼

ðQ =106 Þ  μ 60  A  P

ð1Þ

where Q is the weight of permeate (g/min), μ is the viscosity of water, A is the membrane area (m2), and P is the applied pressure. The viscosity can be calculated by the following equation:

μ ¼ 5:75  10  4  t 2  5:19  10  2  t þ 1:94

ð2Þ

where t is the temperature of the water. Rejection of fine polystyrene latex particles with a diameter of 88 nm was measured using a stirred dead-end filtration cell (Amicon 8050) in which an effective membrane area was 13.4 cm2. The particles were dispersed in pure water at a concentration of 20 mg/L. The membrane filtration of the latex particles was conducted at 10 kPa (regulated using compressed nitrogen gas) and a stirrer speed of 600 rpm. Rejection values were obtained by measuring the concentration of the particles by UV spectroscopy using Cary 500i UV–vis dual-beam spectrometer. The concentrations were quantified using UV absorbance at 202 nm. Filtration resistance was also measured to compare the fouling resistance during filtration process with each membrane. Activated sludge filtration was performed for each filtration membrane. The activated sludge used in this study was obtained from rural community sewerage in Shiga, Japan. At first, pure water was passed through a single membrane for 5 min to make the flux stable with an applied pressure of 20 kPa and a stirrer speed of 600 rpm. Then, the reservoir tank was emptied and refilled with an activated sludge dispersion solution with a concentration of 1 g/L. Filtration of the activated sludge was conducted and the flux was measured as the same for pure water filtration. After the filtration, the tank was emptied again and the cell was disconnected from the apparatus and washed with pure water using a shaker (MS1 minishaker, IKA Co., Japan) at a shaking rate of 1000 cycles per minute as a filtration membrane remained in the cell. The Filtration resistance was measured again. Filtration resistance before and during the filtration of activated sludge and after washing with pure water were calculated for each membrane using the following equation: R¼

P  S  tp L  ηp

ð3Þ

where R is the membrane resistance, S is the membrane area, tp is the temperature of the permeate, L is the volume of the permeate and ηp is the viscosity of the permeate. 2.7. Activated sludge filtration testing The filtration of activated sludge was conducted with an averaged mixed liquor suspended solids (MLSS) concentration of 20.6 g/L and a fluid volume of 8.6 L. The reactor had a volume of 10 L. The module is composed of three elements made of PVDF membrane and PBMs with effective membrane size of 144 cm2 in each side. Three elements were immersed in the reactor and the permeate was suctioned at the rate of 42 L/m2/h with a constant flow pump (SMP-23, Tokyo Kasei). Suctioning was performed for 10 min with 1 min intervals using a digital 24 h time switch (KS1500A, Koizumi Computer). The differential pressure was measured with a digital manometer (DM-2, Shibata Technology Ltd.). A long-term filtration test using the reactor which had a volume of 30 L was also conducted with MLSS concentration of 7.7 g/L, a fluid volume of 24.1 L. Module composed of six elements made of PVDF membranes or PBMs with the effective membrane size of 144 cm2 in each side. The procedure was almost the same as described above except that filtration and washing procedure was repeated and the differential pressure was logged using a data logger (Fig. 2).

Fig. 2. Activated sludge filtration system.

3. Results and discussion 3.1. Membrane characterization Membrane surface properties were characterized by SEM observation and filtration performance measurements. SEM micrographs of PVDF and PBMs with the content of PMMA-g-PEO is 10 or 30 wt% are shown in Figs. 3 and 4. As seen in Fig. 3a, pure PVDF membrane exhibits surface morphology of sparse,  50 nm circular pores which is characteristic of PVDF membrane fabricated by immersion precipitation. PBMs also have circular pores approximately the same diameter of the PVDF membrane. Mayes et al. previously reported that PBMs fabricated on a flat optical mirror by immersion precipitation method with a polymer solution with higher concentration of 18–20 wt% had the porosity of 0.004–0.016 and the average pore diameter of  50 nm. On the other hand, membranes in this study had higher porosity of 0.030–0.100 with almost the same size of the average pore size ( 50 nm). This is ascribed to the lower polymer concentration of the solution and the usage of bigger pore forming agent, PEG, with molecular weight of 20,000 instead of glycerol. Tables 3 and 4 summarize the results from XPS analysis. The surface mole fractions of additive were obtained from the ratio of the COO and C-F peak areas. These mole fractions were converted to weight fractions of PVDF and PMMA-g-PEO unit and PMMA and PEO unit were also calculated using the copolymerization ratio of PMMAg-PEO and the repeat unit molecular weights. 1H NMR spectroscopy of the dissolved membranes indicated that PMMA-g-PEO composition in bulk membrane were only 15–18 wt%. The surface composition of PMMA-g-PEO9 in PBM9-30 was 38 mol% and higher than that of bulk composition of 28 mol% indicating that PMMA-g-PEO9 locally segregated on the surface. In this study, surface segregation was not observed for PBM23-30 and PBM90-30. The results of water permeability test and filtration test of polystyrene latex particles with a diameter of 88 nm are shown in Table 5. The water permeability of the blend membranes decreased according to the increase of the content of PMMA-g-PEO. This is probably because the compatibility of hydrophilic PMMA-g-PEO and water decelerates the speed of phase separation to form a lower asymmetric structure. All membranes exhibited high particle rejection more than 96%. These results indicated that PVDF and PBMs were sufficiently prepared without a significant amount of defects and similar structure enough to discuss the effect of membrane material itself. 3.2. Adsorption test using model foulants The adsorption isotherms using model foulants solution of BSA and dextran were shown in Figs. 5 and 6 respectively. The obtained data are plotted against the water contact angle of each membrane material. The fouling resistance was different among these materials.

H. Minehara et al. / Journal of Membrane Science 466 (2014) 211–219

215

Fig. 3. Surface and cross-section SEM images of PVDF and PVDF/PMMA-g-PEO9 membranes: (a) for PVDF membrane; (b) for PBM9-10; (c) for PBM9-30 (scale bar is 6 μm).

Fig. 4. Surface SEM images of PVDF/PMMA-g-PEO23 and PVDF/PMMA-g-PEO90 membranes: (a) PBM23-10; (b) PBM23-30; (c) PBM90-10; (d) PBM90-30 (scale bar is 6 μm).

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Table 3 C1s component peak areas as percentages of the total area of PBMs. Membrane

C–H

CH2–CF2

C–O, C–N

CQO, O–C–O

COO

CF2–CH2

PBM9-30 PBM23-30 PBM90-30

31 22 27

19 28 27

23 16 14

4 4 4

4 2 1

19 28 27

Table 4 Surface composition by XPS analysis of PBMs. Bulk compositiona, mol%

Surface composition, mol% Membrane

PVDF unit

MMA unit

PEO unit

PVDF unit

MMA unit

PEO unit

PBM9-30 PBM23-30 PBM90-30

62 81 85

21 9 5

17 10 10

72 82 85

8 6 3

21 13 13

a

Determined by 1H NMR spectroscopy.

Table 5 Filtration performance of PVDF and PVDF/PMMA-g-PEO membranes.

The number of PEO units of PMMA-g-PEO PMMA-g-PEO blend ratio (wt%) Water permeability (10  9 m3/m2/Pa/s) Particle rejectiona (%) a

PVDF

PBM9-10

PBM9-30

PBM23-10

PBM23-30

PBM90-10

PBM90-30

– – 37 98.0

9 10 48 97.2

9 30 32 96.0

23 10 43 97.8

23 30 29 96.2

90 10 53 98.9

90 30 46 98.8

Polystyrene latex particle dispersions (average particle diameter of 88 nm) were used.

140

3000

120

2500

100

2000

80

1500

60

1000

40 20

500 0

0

0

15

30

45

60

75

90

Water contact angle (o) Fig. 5. The correlation between water contact angle and the amount of adsorption of the BSA on the surface of PP (◊), PET ( ), PVDF (Δ), PS (  ), PMMA ( þ), PES (□), EVAL (  ), cPE (○), PVDF/PMMA-g-PEO9 (■), PVDF/PMMA-g-PEO23 (▲), PVDF/ PMMA-g-PEO90 (●) with BSA concentration of 2.5 g/L.

Hydrophobic polymers such as PP, PVDF, PS, PMMA and PES adsorbed more BSA and dextran than hydrophilic polymers such as EVAL and PVDF/PMMA-g-PEO. The maximum amount of adsorption and equilibrium constant (Ka) was also determined by fitting the following equation on the adsorption isotherms Req ¼

Rmax K a c 1 þ Kac

where Req and Rmax are equilibrium and maximum adsorption amount of model foulant, respectively, Ka is an equilibrium constant of adsorption, c is the concentration of model foulant. Obtained Rmax is shown Table 1. Rmax of all materials was less than 3 mg/m2 (Table 6). The theoretical adsorption amount of monolayer of BSA is

0

15

30

45

60

75

90

Water contact angle (o) Fig. 6. The correlation between water contact angle and equilibrium amount of adsorbed dextran on the surface of PP (◊), PET ( ), PVDF (Δ), PS (  ), PMMA ( þ ), PES (□), EVAL (  ), cPE (○), PVDF/PMMA-g-PEO9 (■), PVDF/PMMA-g-PEO23 (▲), PVDF/PMMA-g-PEO90 (●) with dextran concentration of 5 g/L.

known to be 2.5 mg/m2 for side-on type and 9.0 mg/m2 for end-on type [26]. Therefore the adsorption of BSA on these materials in this condition was considered to be a monolayer adsorption. PVDF/ PMMA-g-PEO blend polymers showed higher fouling resistance to BSA than other materials resulting in more than 10 times lesser amount of adsorption compared to pure PVDF. When the length of PEO side chain increased, the maximum amount of adsorbed BSA decreased from 0.22 to 0.13 mg/m2. It seems that BSA is prevented from reaching the hydrophobic PVDF moiety due to high mobility of long PEO side chains on the surface of the membranes. The results of dextran adsorption tests are shown in Fig. 6. Hydrophilic membranes adsorbed a lesser amount of dextran. However, the equilibrium amount of adsorbed dextran was about 10 times less than that of BSA for all samples at the same concentration of

H. Minehara et al. / Journal of Membrane Science 466 (2014) 211–219

Table 6 Calculated equilibrium constants and maximum amount of adsorbed BSA and dextran.

2500

Dextran

Rmax (mg/m2)

Ka (  104 M  1)

Rmax (mg/m2)

Ka (  104 M  1)

2.81 1.18 2.31 2.26 0.85 1.99 2.25 1.66 0.22 0.17 0.13

5.64 20.73 48.53 6.26 68.45 6.86 17.98 3.93 3.92 27.09 39.89

1.18 0.34 0.15 0.50 0.45 0.51 0.62 0.73 0.10 o0.01 0.43

2.66 2.03 10.90 4.33 2.32 0.52 5.65 0.92 3.17 9.95 1.43

model compound solution. As for PVDF/PMMA-g-PEO, the maximum amount of adsorbed dextran fluctuated against the length of PEO unlike the case of BSA. Among three PVDF/PMMA-g-PEO, PVDF/ PMMA-g-PEO90 adsorbed the most amount of dextran (0.43 mg/m2) and PVDF/PMMA-g-PEO23 the least (o0.01 mg/m2). This might be because dextran adsorption is mainly caused by physical interaction rather than chemical interaction. Therefore it can be easily affected by the surface morphology and makes the correlation ambiguous [27]. The difference in the order of the amount of adsorption between BSA and dextran was also observed in the other materials. PET and EVAL adsorbed a lesser amount of BSA than expected by their hydrophilicities of the surface. PET and EVAL have hydrophilic functional groups and form partially hydrophilic area where adsorbed BSA easily desorbs (Fig. 7). In contrast to the alternative quantitative method for evaluating fouling resistance, such as radiocounting of adsorbed radiolabeled protein [28], spectroscopy of adsorbed and eluted chromophorelabeled protein [29], XPS analysis of adsorbed colloidal gold [17], the method using a SPR sensor can measure the amount of adsorption quantitatively without any modification of model foulant, which may affect the interactions between polymer and foulant.

Req (ng/m2)

PP PET PVDF PS EVAL cPE PMMA CA PBM9-30 PBM23-30 PBM90-30

BSA

3000

2000 1500 1000 500 0

0

500

1000 1500 2000 2500 BSA concentration (mg/L)

0

1000 2000 3000 4000 5000 Dextran concentration (mg/L)

3000

500 400 Req (ng/m2)

Polymer

217

300 200 100 0

6000

Fig. 7. Adsorption isotherms for BSA (a) and dextran (b) on PP (◊), PET ( ), PVDF (Δ), PS (  ), PMMA ( þ ), PES (□), EVAL (  ), cPE (○), PVDF/PMMA-g-PEO9 (■), PVDF/ PMMA-g-PEO23 (▲), PVDF/PMMA-g-PEO90 (●).

after water flushing. Recovery rate (Rr) is defined as follows: 3.3. Fouling resistance to activated sludge Rr ¼ The results obtained above suggest that PBMs should exhibit lower fouling in activated sludge filtration compared with the pure PVDF membrane. Sludge filtration was conducted using PVDF and PBMs in a protocol where activated sludge concentration of 20.6 g/L is followed by water flushing. Membrane resistance was continuously measured during the process. The results are shown in Table 7. R0 indicates the membrane resistance of virgin membrane which was measured by passing RO water through the membrane under a pressure of 10 kPa. Filtration of activated sludge solution increased the membrane resistance due to membrane fouling. Membrane fouling can be divided into two groups, reversible and irreversible fouling based on the attaching strength. Foulants which are removable from the membrane by washing can be defined as the reversible fouling components and the remained as the irreversible fouling components. Thus, the degree of reversible and irreversible fouling can be evaluated with two values, Rf and Rw calculated as follows: Rf ¼ Rs  R0 Rir ¼ Rw  R0 where R0 is membrane resistance of virgin membrane, Rs is membrane resistance after sludge filtration, Rw is membrane resistance

Rs Rw Rs  R0

The recovery rates of PBMs were higher than that of the pure PVDF membrane as expected. However, when it comes to the effect of the length of PEO side chain of PBMs, fouling resistance enhancing effect was not observed. This may indicate that the filtration and washing process needs to be repeated to clarify the difference between PBMs having different length of PEO side chains. Long term filtration study was also conducted because it has much relevance to application to MBR process, as more fouling mechanisms can be taken into account [30–32]. The filtration process was conducted with PVDF membrane and PBMs using a reactor with a volume of 30 L. The differential pressure was logged during repeated filtration and washing process and plotted against the operation time in Fig. 8. The rate of increase of differential pressure was defined as the slope of the line which connects the origin and the point which the differential pressure reached 20 kPa. This is because in general MBR operation membrane flushing is conducted when the differential pressure reached to 20 kPa. The differential pressure of the pure PVDF membrane rapidly increased soon after the filtration started. On the other hand, PBMs showed higher fouling resistance. The rate of increase of differential pressure was smaller than that of pure PVDF membrane and decreased as the length of PEO side chains of PMMA-g-PEO increased. As shown in Table 8, the

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Table 7 Summary of activated sludge filtration data. Membrane

R0 (  1010 m  1)

Rs (  1010 m  1)

Rw (  1010 m  1)

Rf (  1010 m  1)

Rir (  1010 m  1)

Recovery rate (%)

PVDF PBM9-10 PBM9-30 PBM23-10 PBM23-30 PBM90-10 PBM90-30

4.8 6.6 4.7 6.1 5.8 7.4 4.9

69.7 71.3 78.7 80.7 73.1 74.8 62.9

14.6 14.9 10.1 13.6 12.6 16.1 12.4

64.9 64.7 74.0 74.6 67.4 67.4 58.0

9.8 8.3 5.3 7.5 6.9 8.6 7.5

84.9 87.1 92.8 90.0 89.8 87.2 87.0

R0: Water filtration resistance, Rs: sludge filtration resistance, Rw: membrane resistance after filtration of sludge solution and washing with RO water.

Fig. 8. Differential pressure of (a) pure PVDF membrane, (b) PBM9-30, (c) PBM2330 and (d) PBM90-30 in the long-term sludge filtration experiments.

Table 8 Pore size distribution, porosity and the rate of increase of differential pressure of PVDF membrane and PBMs. Membrane

D (nm)

s

єa

ΔP (kPa/h)

PVDF PBM9-30 PBM23-30 PBM90-30

47 89 64 53

0.025 0.069 0.053 0.039

0.074 0.030 0.080 0.076

0.305 0.232 0.079 0.070

D: average diameter, s: standard deviation, є: porosity, ΔP: the rate of increase of differential pressure. a The porosity is defined as the total area enclosed by pore inlets per unit area of separation surface.

Fig. 9. The rate of increase of differential pressure vs. maximum amount of adsorbed BSA for PBM9-30 (■), PMB23-30 (▲), PBM90-30 (●).

brane filtration, the fouling layers were fractionated into upper layer, intermediate layer and lower layer using some cleaning methods. The results showed that the lower layer had a relatively high concentration of bound proteins compared to other layers. This means proteins are the component which deposits on the membrane surface at the early stage of MBR. Based on that, the reason why the fouling resistance of PBMs in MBR correlated with a passive adsorption study using BSA solution can be understood. Hence the measurement of the amount of adsorbed BSA using SPR sensor is a highly effective way to predict fouling resistance of membrane materials for MBR operation.

4. Conclusion additives affected the pore structure such as increase in average pore size and/or porosity and broadening of the pore size distribution. Such changes of average pore size and porosity agrees with the study reported by Hester et al. while the broadening of the distribution did not occur in their study. However, an apparent correlation between fouling resistance and the average pore size or porosity was not observed. On the other hand, the rate of increase of differential pressure was found to positively correlate with the maximum amount of BSA in the adsorption test measured by SPR sensor (Fig. 9). However, the maximum amount of adsorbed dextran did not correlate with it. Dextran adsorption is strongly affected by surface morphology because its adsorption on the membrane materials is mainly caused by physical interaction rather than chemical interaction [27]. Therefore the correlation between the amount of adsorbed dextran and fouling resistance of membrane materials was ambiguous comparing to BSA in this study. This observation can be interpreted in the point of view of the dynamic deposition mechanism during filtration. Bio fouling is the deposition of biopolymers which gradually thickened by the time of operation. Metzger et al. [33] performed a study to reveal a composition of deposited biopolymers in MBR. After mem-

The fouling resistance of PVDF/PMMA-g-PEO was quantitatively evaluated by comparing the amount of adsorption of model fouling compound with various membrane materials using SPR sensor. PVDF/ PMMA-g-PEO9 adsorbed only one-half of BSA and one-eleventh of dextran compared to pure PVDF. The fouling resistance correlated with hydrophilicities of the membrane materials. Hydrophilic PVDF/ PMMA-g-PEO blend polymers exhibited high hydrophilicity and fouling resistance to both of BSA and dextran. On the other hand, hydrophobic PVDF, PP, PET, PS and cPE adsorbed 2 times more amount of BSA and dextran than the blend polymer. Pure PVDF membrane and PVDF/PMMA-g-PEO membranes were fabricated by NIPS method to evaluate fouling resistance to activated sludge. The obtained membranes had pores with diameter of approximately 50 nm and showed pure water permeability of 29–50 L/(m2 bar h) and more than 96% rejection of polystyrene latex with a diameter of 88 nm. The fouling resistance of PVDF/PMMA-g-PEO blend polymer membrane to activated sludge was evaluated using a filtration system with a volume of 10 or 30 L. The fouling resistance was enhanced as the repeating unit of PEO increased from 9 to 90. The rate of increase of differential

H. Minehara et al. / Journal of Membrane Science 466 (2014) 211–219

pressure in the activated sludge filtration was found to show good correlation with the amount of adsorbed BSA in the adsorption test for membrane materials. However, in the plot against the amount of adsorbed dextran, such a correlation was not seen. This could be explained by the fact that the degree of contribution of hydrogen bonding and hydrophobic interaction in the attractive interaction with membrane materials are different between polysaccharides and proteins. This study confirmed that measuring the amount of model foulant adsorption using SPR sensor is a highly effective way to evaluate the fouling resistance of membrane materials for MBR. Quantitative evaluation of fouling resistance can be used not only to elucidate fouling mechanism but also to give a useful guide for exploration of new membrane materials.

Acknowledgments Financial support for this work was provided by the New Energy and Industrial Development Organization (NEDO) of Japan.

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