The properties and filtration efficiency of activated carbon polymer composite membranes for the removal of humic acid

Desalination 313 (2013) 166–175

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The properties and filtration efficiency of activated carbon polymer composite membranes for the removal of humic acid Li-Luen Hwang a, Jyh-Cherng Chen b, Ming-Yen Wey a,⁎ a b

Department of Environmental Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC Department of Safety, Health and Environmental Engineering, Hungkuang University, Taichung 433, Taiwan, ROC

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

► AC/polymer composite membranes were prepared and characterized. ► AC and PEG contents affected the performances of the membranes. ► Surface hydrophilicity and porosity of the membranes were affected by PEG content. ► Filtration efficiency of the membranes was affected by the AC and PEG contents.

AC particles

PEG directly affects the pore

a r t i c l e

i n f o

Article history: Received 13 August 2012 Received in revised form 16 December 2012 Accepted 21 December 2012 Available online 20 January 2013 Keywords: Activated carbon Filtration Composite membrane Hydrophobic Humic acid

a b s t r a c t To improve the filtration efficiency and permeability of polymer membranes simultaneously during water treatment, different ratios of activated carbon (AC) and polyethylene glycol (PEG) were added into polyphenylsulfone (PPSU)/polyetherimide (PEI) polymers to prepare the novel composite polymer membranes. The results showed that the addition of AC significantly affected the membrane morphology, pore size distribution, porosity, and chemical properties. With this increase in AC concentration, the filtration flux and permeability of the AC/PPSU/PEI/PEG composite membrane improved. The addition of hydrophilic pore-formation agent PEG helped to increase the surface hydrophilicity and porosity of the AC/PPSU/PEI/PEG composite membranes. The intrinsic membrane resistances decreased with the rising concentration of PEG. The optimum component of the AC/PPSU/PEI/PEG composite membrane was 0.25/35/5/6 wt.%, and the corresponding membrane permeability and humic acids (HAs) removal efficiency were 184 L m−2 h−1 and 80%, respectively. © 2012 Elsevier B.V. All rights reserved.

⁎ Corresponding author at: National Chung Hsing University, Department of Environmental Engineering, 250 Kuo Kuang Rd., Taichung 402, Taiwan, ROC. Tel.: + 886 4 22852455; fax: + 886 4 22862587. E-mail address: [email protected] (M.-Y. Wey). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.12.019

1. Introduction As water recycling and water treatment quality requirements have become more numerous and more strict, the development and

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application of membrane separation technology has been attracting increasing attention [1–5]. Nowadays, commonly used advanced membrane separation is particularly attractive because it allows for the removal of various pollutants simultaneously and for the continuous and stable production of high-quality water [5–8]. With the increasing awareness of environmental protection concerns and the need for sustainable development, membrane separation is starting to be considered the water purification method with the most potential because of its low cost, high efficiency, lack of secondary pollution, and water recycling capability. Membranes can remove various pollutants through the separation mechanisms of impaction, diffusion, electrostatic interaction, hydrophobic property, and adsorption among others [9]. Many efforts have been made recently to develop different modification methods for improving the pore size distribution and hydrophilicity/ hydrophobicity of membranes [10–12]. The results indicated that the pore size distribution, porosity, and hydrophilicity of membranes were related to the different additives (opening reagents, poreformation reagents, and dispersants) of different molecular weights and concentrations used for the membrane formation process. Different membrane preparation methods (wet, dry, and dry–wet phase inversion techniques) also affected the hydrophilicity/hydrophobicity, electric properties, mechanical strength, filtration flux, and separation efficiency of these membranes [13–15]. The rapid flux decline in filtration flux caused by the fouling of single or multiple organic polymer membranes is a significant problem in membrane systems [16–19]. To overcome problems like this, as well as the constraints of single organic membranes, various composite membranes can be developed using the polymerization method, and the membrane properties and performances can be modified by changing the component ratios and the preparation conditions. The composite membranes are usually made by blending organic polymers with various organic or inorganic materials. The addition of organic/inorganic materials to organic polymers has been known to improve the permeability, chemical resistance, and removal efficiency of the polymer membranes. The decline of filtration flux during water treatment can therefore be mitigated effectively [11,12,20,21]. The influences of different particles and compositions on the chemical and physical properties of composite membranes have also been investigated [22–24]. Composite membranes have often been prepared and modified by blending different particles with the polyvinylidene fluoride (PVDF) polymer, and the most-used particles were SBA-15, silica (SiO2), titanium (TiO2), alumina (Al2O3), zirconia (ZrO2), lithium perchlorate (LiClO4), and inorganic salts with small molecular weights [11,17,23,25–32]. The particle contents in the prepared composite membranes were 0.02–1.0 wt.%, and the thermal stability, filtration flux, water content, and porosity of the membranes increased with increasing particle content. The properties and morphologies of the hydrophobic PVDF membranes also changed. Other studies on composite membranes that blended different polymers with particles were performed by adding Fe3O4 (1.0, 5.0, and 9.0 wt.%), Al2O3 (0.01, 0.03, 0.05, 0.1, and 0.2 wt.%), ZrO2 (0.01, 0.03, 0.05, 0.07, and 0.1 wt.%), and SiO2 (0, 0.5, 1, 2, and 4 wt.%) to polysulfone (PSf) and poly(ether sulfones) (PES) polymers [25,27,29–33]. The results of these studies indicated that the addition of particles indirectly modified the membrane structures and morphologies. The PSf and PES polymers are similar to polystyrene (PS), polypropylene (PP), polyethylene (PE), and polyvinylidenefluoride (PVDF), in that they are hydrophobic and have good mechanical strength, ductility, and chemical resistance. However, their high densities have a large influence on the filtration flux. The addition of particles to the membranes could increase their hydrophilicity, filtration flux, thickness, thermal stability, porosity, and water content. The relationships between composite membrane filtration and pollutants have not yet been investigated in detail. Therefore, the filtration mechanism, chemical resistance, and aging phenomenon of composite membranes need to be further studied.

167

In most studies, inorganic materials such as SiO2 and Al2O3 were used as additives when preparing composite membranes [7,34–37]. However, the possibility of adding AC into single or multiple organic polymers for the preparation of composite membranes has rarely been mentioned. Ballinas et al. [38] investigated the characters of hybrid activated carbon/polysulfone (PSf) composite membranes in a continuous operation system. The results showed that the permeability of hybrid activated carbon/polysulfone (PSf) composite membranes was influenced by the loading and particle size of activated carbon. The selectivity was higher for low carbon loadings and small particle sizes. The performance of a hybrid activated carbon/polysulfone (PSf) composite membrane was better than non-hybrid ones. AC particles are hydrophobic in natural water and have uniform electron distributions and low affinity for polar molecules [9,35]. The addition of AC into the composite membrane should change its chemical properties, pore size distribution, porosity, and its filtration flux. We have investigated the characteristics and performance of blended membranes with different polyphenylsulfone (PPSU)/polyetherimide (PEI) ratios [12]. The results indicate that the negatively charged PPSU polymer exhibits good resistance to HAs and that membrane fouling is mitigated. However, the filtration and flux of HAs was only 18.69 L m − 2 h − 1. To increase the HAs removal efficiency and membrane permeability, AC/PPSU/PEI/PEG composite membranes with different component ratios were prepared and tested. The surface morphology, hydrophilicity/hydrophobicity, and surface roughness of each composite membrane was characterized using a field emission scanning electron microscope (FE-SEM), contact angle meter, and atomic force microscope (AFM). Finally, the performance of modified composite membranes was evaluated based on the basis of pure water filtration flux (membrane permeability), hydraulic resistance, HAs separation efficiency, and antifouling properties. 2. Experimental 2.1. Materials The major components of an AC/PPSU/PEI/PEG composite membrane were polyphenylsulfone (PPSU, MW: 53,000–59,000 g/mol) and polyetherimide (PEI, MW: 529 g/mol). Polyethylene glycol (PEG, MW: 200 g/mol) was used as the pore-formation reagent, and coconut shell-based activated carbon (AC) (China Activated Carbon Industries Ltd., Taiwan) was the additive particle. N-Methyl-2-pyrrolidone (NMP) was used as the solvent. HAs are known as a major precursor to causing the formation of various disinfection by-products (DBPs) in water treatment processes. Some researchers used various separation and analysis techniques to determine the molecular weights of HAs in the range of 0.5–500 kDa [7,39,40]. The simulated water for the composite membrane tests was prepared by dissolving 1.4 g of HAs powder (Sigma-Aldrich) into 1 L of deionized water and was pre-filtered using a filter with a mean pore size of 2.5 μm to remove the larger HAs. The pH value of simulated water was controlled at 6.7 ±0.1 by 0.1 M HCl or NaOH [2,41,42]. The HAs concentration in the feed solutions and resultant permeate solutions was determined by a UV–Vis spectrometer (UV–Vis/DRS; Perkin Elmer Lambda 35) at a wavelength of 254 nm [43–45]. The HAs concentration in the pre-filtered solution was 523 mg/L. 2.2. Membrane preparation All the AC/PPSU/PEI/PEG composite membranes with different component ratios were fabricated using the wet phase inversion technique [12,26,32]. The different compositions of AC/PPSU/PEI/PEG composite membranes are shown in Table 1. PPSU and PEI polymers were dissolved in NMP solvent and mixed with different ratios of AC particles (0.25, 0.5, and 1 wt.%) and with pore-formation agent PEG (6 wt.% and 12 wt.%), respectively. The polymer solutions and AC particles were

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Table 1 Composition of casting solution for preparing AC/PPSU/PEI/PEG composite membranes. Composite membrane code

SA SA SA SA SA SA SA SA SA SA SA SA

1 2 3 4 5 6 7 8 9 10 11 12

Casting solution compositions (wt.%) PPSU

PEI

PEG

AC

35 35 35 35 35 35 35 35 35 30 30 30

5 5 5 5 5 5 5 5 5 10 10 10

– – – 6 6 6 12 12 12 12 12 12

0.25 0.5 1 0.25 0.5 1 0.25 0.5 1 0.25 0.5 1

mixed thoroughly by being stirred at 500 rpm, and then being heated at 60 °C for 2 h to form a homogeneous casting solution. Subsequently, the solution was uniformly spread on a glass plate using a casting knife. The solvent remaining in the cast film evaporated at room temperature overnight. The casted membrane was gently immersed in a water bath for 6 h, and the water temperature was controlled at 28 ± 2 °C to ensure complete precipitation and membrane formation. After immersion for 6 h in the water bath, the membranes were peeled off the glass plate and subsequently rinsed with deionized water to remove residual solvent and pore-forming agent (PEG). The finished membranes were stored in a mixture solution of deionized water/ methanol (1:1) for the next filtration test. 2.3. Characterization of membranes 2.3.1. Surface morphology The surface and cross-section of the AC/PPSU/PEI/PEG composite membranes were examined using a FE-SEM (Hitachi JSM-5610LVS). Before these observations were performed, the dry membranes were fractured in liquid nitrogen and pre-treated with Au/Pd sputtering to make the membranes have good electrical conductivity to the membranes. 2.3.2. AFM observation The mean surface roughness of a composite membrane (Ra) was examined by using an atomic force microscopy (AFM, Veeco DI-3100) in dynamic force mode with a 10 μm scanner and an SI-DF40 (spring constant= 42 N/m) cantilever.

2.3.3. Contact angle (CA) measurement The hydrophilicity of the membranes was estimated using a drop shape analyzer (KRÜSS, Model: DSA100). The membranes were put on a flat glass and a water droplet was dropped on the membrane surface. The static contact angle between the membrane and the droplet was measured at every 30 s for 3 min. 2.3.4. Particle size and surface characteristics analyses The particle size distributions of HAs and AC particles were measured by a laser analyzer (FRITSCH Analysette 22 COMPACT). The results (Fig. 1 (a)) showed that the AC particles had three peaks of particle sizes at 0.01–0.1 μm, 1–10 μm, and 10–50 μm. As shown in Fig. 1(b), the HAs solution had two peaks of particle sizes at 0.005–0.1 μm and 0.5–50 μm. The specific surface area, pore volume, micro-, meso-, and macro-pore volumes, average pore size, and pore size distribution of an AC particle in Table 2 were measured at 77 K by gravimetric method with a BET-201-AEL vacuum microbalance (BET-201-AEL, Porous Materials Inc., New York) and calculated from the N2 adsorption–desorption isotherms by the BET method. 2.3.5. Equilibrium water content The equilibrium water content (EWC) of the membranes was analyzed in a constant sample area of 5 cm×5 cm. The membrane samples were immersed in water for 24 h and the wet weights of membranes were measured after removing the excess water by absorbent paper. After drying the samples in a vacuum oven at 60 °C for 48 h [46], the dry weights were also obtained. The equilibrium water content was calculated with Eq. (1): ECWð% Þ ¼

ð1Þ

where Ww and Wd are the weights (g) of a wet sample and dry sample, respectively. 2.4. Filtration test of membrane A laboratory-scale membrane filtration system was designed and setup to test the AC/PPSU/PEI/PEG composite membranes. As shown in Fig. 2, the system is equipped with automatic filtration and backwash units and the flow and pressure monitoring/controlling devices. The membrane test was conducted in a cross-flow filtration cell with an effective plate area of 105 cm 2. Each test membrane was initially compacted with deionized water at the rising pressures from 98.4 kPa to 490.67 kPa and kept at 490.67 kPa for 30 min. Subsequently, the membrane filtration test was conducted at a water pressure of 294.4 kPa and a feed flow rate of 2400 mL/min for 120 min

30

14

a)

b) 12

Different mass ratio (%)

25

Different mass ratio (%)

Ww−Wd  100 Ww

20 15 10 5 0 0.0001

10 8 6 4 2

0.001

0.01

0.1

1

10

Particle size distribution (µm)

100

1000

0 0.0001

0.001

0.01

0.1

1

10

Particle size distribution (µm)

Fig. 1. Particle size distributions of (a) AC and (b) HAs.

100

1000

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169

Table 2 Physical characteristics of AC particles. Sample

AC

Specific surface area (m2g−1)

Pore volume (cm3g−1)

524

0.431

Micro-, meso-, and macro- pore volume (cm3g−1) Micro b2 nm

%

Meso 2–50 nm

%

Macro >50 nm

%

0.226

52

0.194

45

0.011

3

[11,47,48]. A big feed tank of ca. 20 L was used to avoid temperature variations of feeding water due to the centrifugal pump. The temperature of feeding water was maintained constant at 28±2 °C.

Rð% Þ ¼

Q A  ΔT

ð2Þ −2

ð4Þ

3. Results and discussion 3.1. Structural characteristics of different AC/PPSU/PEI/PEG composite membranes

2.4.2. Membrane resistance (Rm) The membrane resistance is defined as the pressure drop from the permeation of pure water through a membrane. The membrane resistance is equal to the tolerance of a membrane to the hydraulic pressure during the filtration process [11,47,48]. It can be determined using Eq. (3). ΔP μ  Jw1

  Cp 1−  100 Cf

where Cp and Cf (mg mL − 1) are the HAs concentrations in the filtrate and feeding water, respectively (Table 5).

−1

where Jw1 (kg m h ) is the pure water flux (PWF), Q (kg) is the weight of filtrate, A (m2) is the membrane area, and ΔT (h) is the filtration time.

Rm ¼

The surface morphologies of the AC/PPSU/PEI/PEG composite membranes were observed by FE-SEM. The blending of AC particles, PEI polymers, and a pore-formation agent PEG with the PPSU casting solution can improve the porous structure of PPSU membranes. As shown in Figs. 3 and 4, all the membranes exhibited similar asymmetric and sponge-like structures. The pore size distributions in the wall and on the surface of the sponge-like structures were increased along with the AC contents in a casting solution. When 1 wt.% of AC particles was added, the AC incorporation could be observed within the macrovoids and it induced broadening (Fig. 4), thus increasing membrane structure

ð3Þ

where ΔP is the permeability pressure (294.4 kPa) and μ is the viscosity of pure water.

Pressure and flux feedback control

Agitator v

CM

A

F

Concentrated tank

Raw water tank/ Feed tank P

F P

peristaltic pump

Membrane module Permeate tank

Backwash water tank

i Mac

Electronic

peristaltic pump P:Pressure transducer

3.29

2.4.3. HAs removal efficiency (R) The performances of the blend membranes were tested by the filtration of HAs. The HAs concentration in the feed solutions and resultant permeate solutions was determined by a UV–Vis spectrometer (UV–Vis/DRS; Perkin Elmer Lambda 35) at a wavelength of 254 nm [43–45]. The HAs removal efficiency was defined as Eq. (4).

2.4.1. Pure water flux (PWF) The pure water flux (PWF, Jw1) through the membrane was calculated from the data of a 120 min filtration test with Eq. (2). Jw1 ¼

Avg. pore size (nm)

F:Flow meter

balance CM:Conductivitymeter

Fig. 2. Schematic diagram of a membrane filtration system.

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a) SA1

b) SA2

PEG directly affects the pore

c) SA3

AC particles

PEG directly affects the pore

Fig. 3. FE-SEM images for the surface view of AC/PPSU/PEI/PEG composite membranes with different AC weight ratios: (a) SA1 (b) SA2 (c) SA3.

porosity. The porosity of 1 wt.% of the AC/PPSU/PEI/PEG composite membrane was larger than that of 0.25 wt.% of the AC/PPSU/PEI/PEG composite membrane. Liao et al. [28] found that increasing the contents of SBA-15 in the PVDF membranes could enhance the membrane porosity, since SBA-15 was dispersed into the finger-like structures and formed smaller pores. Maximous et al. [29–31], Yan et al. [16], and Taurozzi et al. [49] reported similar results for the ZrO2/PES,

a) SA1

b) SA2

Al2O3/PES, Al2O3/PVDF, and Ag/PSf/PVP composite membranes, respectively. Comparing the FE-SEM images in Figs. 3(a–c) and 5(a–i), we can see that the surfaces of the SA1–3 and SA4–12 composite membranes exhibited significantly different morphologies with the addition of AC particles and PEG in the PPSU/PEI membranes, respectively. All the AC/PPSU/PEI/PEG composite membranes were rough and had porous

AC particles

PEG directly affects the pore

c) SA3

AC particles

PEG directly affects the pore

Fig. 4. FE-SEM images for the cross-section view of AC/PPSU/PEI/PEG composite membranes with different AC weight ratios: (a) SA1 (b) SA2 (c) SA3.

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a) SA4

b) SA5

AC particles

171

d) SA7

g) SA10

e) SA8

h) SA11

PEG directly affects the pore

c) SA6

AC particles

PEG directly affects the pore

f) SA9

AC particles

i) SA12

PEG directly affects the pore

Fig. 5. FE-SEM images for the surface view of AC/PPSU/PEI/PEG composite membranes with different weight ratios: (a–c) SA 4–6 (d–f) SA 7–9 (g–i) SA 10–12.

surfaces. This meant that the surface morphology and characteristics of these composite membranes could affect the filtration flux and permeability. The SA1–3 composite membranes were prepared without PEG. Fig. 3(a–c) shows rough surfaces with rising contents of AC. The SA4–6 in Fig. 5(a–c) and SA7–9 in Fig. 5(d–f) show that the composite membranes had an irregular morphology and larger pore size when the PEG content was increased from 6 wt.% to 12 wt.%. These results indicate that the addition of the PEG additive can increase the ratio of large pores on the surface while having little effect on the hydrophilicity of composite membranes [50,51]. Comparing the FE-SEM pictures in Figs. 4(a–c) and 6(a–c), the composite membrane SA1–3 had more porous structures and bigger pores than the one in SA4–6 due to the addition of PEG in the AC/PPSU/PEI composite membranes. The cross-sectional pictures of composite membranes SA4–6 in Fig. 6(a–c) and SA7–9 in Fig. 6(d–f) show that the composite membranes had more porous and spherical sponge-like structures when the PEG content increased from 6 wt.% to 12 wt.%. The porosity of the composite membranes increased with the rising contents of PEG. The surface areas, porous structures, number of bigger pores, and the dispersion of AC particles in the composite membranes were also increased. Comparing the surface and cross-sectional morphologies of AC/PPSU/PEI/PEG composite membrane SA7–9 with SA10–12, the pore size, the surface hydrophilicity and porosity were increased as the PEI concentration increased from 5 to 10 wt.%. On the other hand, the intercalated and exfoliated structures formed on the composite membrane surface during the wet phase inversion process because of the different relative diffusion rates of solvent and non-solvent exchanges, and due to the diffusion of the solvent from the polymer solution into the water bath. The

coagulated the composite membrane surface also hampered the diffusion, and the surface layers filled with polymer chains, further contributing to the formation of these structures [28,46–48]. 3.2. Hydrophilicity and hydrophobicity of different AC/PPSU/PEI/PEG composite membranes The hydrophilicity and hydrophobicity of the AC/PPSU/PEI/PEG composite membranes with different AC and PEG contents using a water contact angle meter. Table 3 and Fig. 7 show the contact angles on the top layers (the surface between the membrane and air) and the bottom layers (the surface between the membrane and the glass plate) of the composite membranes. The results indicate that the AC/PPSU/PEI composite membranes SA1–3 were more hydrophobic than the AC/PPSU/PEI/PEG composite membranes SA4–6 and SA7–9. The addition of PEG can effectively improve the hydrophilicity and the density of PPSU polymers as well as increase the porosity and the PWF of membranes [21–23]. PEG is hydrophilic, neutral, non-toxic, biodegradable, and non-repulsive [52–54]. The addition of PEG enhances pore formation and the hydrophilicity of AC/PPSU/ PEI composite membranes. When the PEG contents increased from 6 wt.% to 12 wt.%, the contact angles of composite membranes decreased from 116.6±0.5° (SA4–6) to 81.8±0.4° (SA7–9), and the hydrophilicity of the membrane increased. 3.3. PWF, Rm, and Ra of different AC/PPSU/PEI/PEG composite membranes The PWF and Rm of the AC/PPSU/PEI/PEG composite membranes with various compositions were tested and measured using a membrane

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a)SA 4

d)SA7

g)SA10

b)SA5

e)SA8

h)SA11

AC particles

PEG directly affects the pore

c)SA6

f)SA9

i)SA12

AC particles

PEG directly affects the pore

Fig. 6. FE-SEM images for the cross-section view of AC/PPSU/PEI/PEG composite membranes with different weight ratios: (a–c) SA 4–6 (d–f) SA 7–9 (g–i) SA10–12.

filtration system. Table 3 shows the PWFs, the intrinsic membrane resistances, the contact angles, and the HAs removal efficiencies of different AC/PPSU/PEI/PEG composite membranes. The PWF of AC/PPSU/PEI/PEG composite membrane SA1–3 increased from 271.14 L m − 2 h − 1 to 535.86 L m − 2 h − 1 as the AC particle content increased. The addition of AC particles directly affects the pore size and pore distribution as well as the filtration flux and the HAs Table 3 EWC, contact angle, PWF, Rm, and HAs removal efficiency of various AC/PPSU/PEI/PEG composite membranes.

SA SA SA SA SA SA SA SA SA SA SA SA a b c

1 2 3 4 5 6 7 8 9 10 11 12

15 21 22 25 37 51 43 56 59 54 53 68

b

Contact angle (θ) (°) Top

Contact angle (θ) (°) Bottom

PWF (L m−2 h−1)

130.8±0.1 129.3±0.1 128.5±0.0 116.6±0.5 91.5±0.4 85.7±0.4 81.8±0.4 81.6±0.4 80.7±0.1 79.6±0.2 78.5±0.3 77.7±0.3

86.9±0.7 97.0±0.3 106.7±0.3 80.4±0.2 73.5±0.7 82.3±0.1 77.8±0.6 74.7±1.3 78.1±0.2 81.0±0.4 80.5±0.4 77.2±0.4

271.14 428.29 535.86 183.64 284.31 385.79 252.93 452.57 479.14 257.43 260.00 294.71

EWC: equilibrium water content. PWF: pure water flux. Rm: membrane resistance.

150

Rmc

HAs (*108, m−1) removal (%) 14.0 7.1 8.9 20.7 13.4 9.9 15.1 8.4 8.0 10.7 14.6 12.9

74 59 51 80 75 70 76 69 61 69 63 59

SA1-3 SA4-6 SA7-9

120

Contact angle (O)

Composite EWC membrane (%) code

a

removal efficiency. On the other hand, the PWFs of the AC/PPSU/PEI composite membranes SA4–6 and SA7–9 also increased along with the rising contents of PEG. The PWFs of composite membranes SA4–6 and SA7–9 were not higher than those of SA1–3 except SA8. This may be due to the aggregations of AC particles in the structure of composite membranes SA4–6 and SA7–9, and the consequent shrinking of membrane pores. During the membrane filtration process, a fouling of the membrane often occurs and this causes different levels of membrane

90

60

30

0 0.00

0.25

0.50

0.75

1.00

1.25

Activated carbon composition (wt.%) Fig. 7. Water contact angle of different AC/PPSU/PEI/PEG composite membranes.

L.-L. Hwang et al. / Desalination 313 (2013) 166–175

100

640

25

90

560

80 480

PWF (L m-2h-1)

70

15

10

400

60

320

50 40

240

SA1-3 SA4-6 SA7-9

160 5 80 0 0.00

0.25

0.50

0.75

1.00

1.25

0 0.00

Activated carbon composition (wt.%)

0.25

0.50

0.75

1.00

30 20

HAs removal efficiency (%)

SA1-3 SA4-6 SA7-9

20

Rm (*108 , m-1)

173

10 0 1.25

Activated carbon composition (wt.%)

Fig. 8. Membrane hydraulic resistance (Rm) of different AC/PPSU/PEI/PEG composite membranes.

Fig. 10. HAs removal efficiency and pure water flux of different AC/PPSU/PEI/PEG composite membranes.

resistances. The mechanism and types of membrane resistance can be assessed using the series resistance model. Fig. 8 shows that the hydrophobic AC/PPSU/PEI/PEG composite membrane had better chemical stability for the filtration of hydrophobic contaminants. The results indicate that the composite membranes with higher hydrophobic PPSU content would increase the Rm and decrease the PWF. Table 3 shows the Rm of different AC/PPSU/PEI/PEG composite membranes. The Rm of AC/polymer composite membranes SA4 and SA7 were decreased from 20.7×108 m−1 to 15.1×108 m−1 as the content of hydrophilic PEG increased from 6 wt.% to 12 wt.%. Similarly, the Rm of AC/PPSU/ PEI/PEG composite membrane SA6 and SA9 were decreased from 9.9 × 108 m −1 to 8.0 × 108 m−1 as the content of hydrophilic PEG

increased from 6 wt.% to 12 wt.%. The addition of AC particles also directly affects the pore size, increases the filtration flux, and decreases the Rm. Therefore, the modification of hydrophobic membranes by the hydrophilic polymer and AC particle contents are effective in increasing the permeability of the hydrophobic membrane during filtration. In addition to the pore size and pore distribution, the hydrophilicity and mechanical strength of the composite membranes can also affect the filtration flux significantly. The roughness of the membrane is related to the porosity of the membrane surface, and affects the filtration efficiency of composite membranes [55–57]. The surface roughness of AC/PPSU/PEI/PEG composite membranes SA1, SA4 and SA7 was measured using AFM.

a) SA 1

b) SA 4

Ra=360.0

Ra=72.1

c) SA 7

Fig. 9. AFM images of the top surfaces view of AC/PPSU/PEI/PEG composite membranes.

Ra=119.0

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Table 4 The comparisons of different modification methods on the performances of different composite membranes. Membrane material and content (wt.%)

Addition agent and content (wt.%)

Preparation method

Membrane contact angle (θ) (°)

PWF

Modified membrane

Ref. no

PSf (15)a

MWNTs (2)b

Phase inversion method

67

PVP rejection = 49%

[20]

PSF (15)c

Fe3O4 (0.27)d

Phase inversion method

58, 73

lysozyme rejection = 70%

[25]

Phase inversion method

70.6

6 m3/m2 day 12.5 L/m2 h 596 L/m2 h 60 L/m2 h 93 L/m2 h 183.64 L/m2 h

BSA rejection = 48%

[26]

lysozyme rejection = 65%

[42]

NOM rejection = 22%

[58]

HAs rejection = 80%

This work

PES (15)

e

TiO2 (0.5)

f

PPESK (15)g

TiO2 (3.0)

Phase inversion method

20.0

PES (20)

MWNTs (0.5)

Phase inversion method

54

PPSU/PEI (35/5)

AC (0.25)

Phase inversion method

116.6

a b c d e f g

PSf: polysulfone. MWNTs: multi-walled carbon nanotubes. PSF: polysulfone. Fe3O4: magnetite. PES: polyethersulfone. TiO2: titanium. PPESK: poly(phthalazinone ether sulfone ketone).

Fig. 9 shows that the surface roughness of AC/PPSU/PEI composite membranes was higher than that of AC/PPSU/PEI/PEG composite membranes. When the concentrations of PEG in AC/PPSU/PEI/PEG composite membranes increased from 6 wt.% to 12 wt.%, the surface roughness (Ra) values of SA1, SA4 and SA7 decreased from 360 to 72.1 and 119.0. The addition of AC particles can affect the pore size and the gaps between upper and lower pores in the composite membranes, and such influence can be mitigated by the addition of PEG. In summary, blending AC particles and PEG with polymer membranes can improve the hydrophilicity and surface roughness of composite membranes, and can mitigate membrane fouling and membrane resistance during the filtration process. 3.4. HAs removal efficiencies of different AC/PPSU/PEI/PEG composite membranes Fig. 10 and Table 3 show the relationship between the AC particle contents and the PWF, equilibrium water content (EWC), and HAs removal efficiencies of composite membranes. When the AC particle content increased, the PWF of AC/PPSU/PEI/PEG composite membranes also increased but the HAs removal efficiency decreased. On the other hand, the addition of PEG can increase the number of pores in AC/PPSU/PEI/PEG composite membranes. When 12 wt.% PEG was added into the composite membrane, the PWF and HAs removal efficiencies of composite membrane SA7 were 253 L m−2 h−1 and 76%, respectively. When the PEG concentration was reduced to 6 wt.%, the

Table 5 List of symbols used in this study. Symbols

Unit

Description

Ww Wd Jw1 Q A ΔT Rm ΔP R Cp Cf Td Tg μ θ

[g] [g] [kg m−2 h−1] [kg] [m2] [h] [m−1] [kPa] [%] [mg mL−1] [mg mL−1] [°C] [°C] Pa s [degree]

Wet membrane weight Dry membrane weight Pure water flux The weight of filtrate Membrane area Filtration time Membrane hydraulic resistance Trans-membrane pressure Humic acid removal efficiency Concentration in the filtrate Concentration in feed water Final decomposition temperature Glass transition temperature Viscosity of pure water Contact angle

PWF and HAs removal efficiencies of the composite membrane SA4 were 184 L m −2 h−1 and 80%, respectively. Comparing the FE-SEM images of the composite membranes SA4 and SA7 (Section 3.1), the addition of 6 wt.% PEG into the composite membrane could reduce the porosity and increase the HAs removal efficiency. As the porosity of the composite membrane decreased, the filtration flux decreased and the membrane resistance increased. Moreover, the modification of the hydrophobic polymer composite membrane through the addition of a hydrophilic polymer is effective for increasing the permeability of the hydrophobic membrane during filtration. Comparing the results of this study with the related literatures, our AC/PPSU/PEI/PEG composite membranes were more hydrophobic. The performance of AC/PPSU/PEI/PEG composite membrane for pollutant removal in this study was better than those in the other references (Table 4). 4. Conclusions The AC/PPSU/PEI/PEG composite membranes were prepared using different AC and PEG contents. The morphology, porosity and properties of polymer membranes were improved by the addition of different concentrations of AC and PEG. The PWFs, permeability, intrinsic resistances, hydrophilicity, roughness, HAs removal efficiency and fouling resistance of AC/PPSU/PEI/PEG composite membranes were better than the polymer membranes. The optimum component ratio of the AC/PPSU/PEI/PEG composite membrane was found to be 0.25/35/5/6 wt.%, and the corresponding membrane permeability and HAs removal efficiency were 184 L m−2 h−1 and 80%, respectively. References [1] K. Ghosh, M. Schnitzer, Macromolecular structures of humic substances, Soil Sci. 129 (1980) 266–276. [2] W. Yuan, A.L. Zydney, Humic acid fouling during microfiltration, J. Membr. Sci. 157 (1999) 1–12. [3] I. Ali, V.K. Gupta, Advances in water treatment by adsorption technology, Nat. Protoc. 1 (2007) 2661–2667. [4] N. Ma, Y. Zhang, X. Quan, X. Fan, H. Zhao, Performing a microfiltration integrated with photocatalysis using an Ag–TiO2/HAP/Al2O3 composite membrane for water treatment: evaluating effectiveness for humic acid removal and anti-fouling properties, Water Res. 44 (2010) 6104–6114. [5] S. Shirazi, C.J. Lin, D. Chen, Inorganic fouling of pressure-driven membrane processes: a critical review, Desalination 250 (2010) 236–248. [6] J.H. Kim, K.H. Lee, Effect of PEG additive on membrane formation by phase inversion, J. Membr. Sci. 138 (1998) 153–163. [7] D. Ayhan, Agricultural based activated carbons for the removal of dyes from aqueous solutions: a review, J. Hazard. Mater. 167 (2009) 1–9.

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