Nanocomposite Membrane with Different Carbon Nanotubes Location

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Nanocomposite membrane with different carbon nanotubes location for nanofiltration and forward osmosis applications Xiangju Song, Li Wang, Lili Mao, and Zhining Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01575 • Publication Date (Web): 18 Apr 2016 Downloaded from http://pubs.acs.org on April 23, 2016

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ACS Sustainable Chemistry & Engineering

Nanocomposite membrane with different carbon nanotubes location for nanofiltration and forward osmosis applications Xiangju Song,† Li Wang,† Lili Mao,‡ Zhining Wang*† †

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education,

Ocean University of China, 238 Songling Road, Qingdao, 266100, Shandong Province, China ‡

College of Chemistry and Chemical Engineering, Ocean University of China, 238

Songling Road, Qingdao, 266100, Shandong Province, China

*Corresponding author: [email protected]

KEYWORDS Nanocomposite membrane, CNTs, Nanofiltration, Forward osmosis, Antifouling

ABSTRACT

We investigated the effect of CNTs location on the property and performance of the prepared membranes. Four different types of membranes were prepared, including (1) thin film composite (TFC, polyamide active layer on polysulfone substrate), (2) 1

ACS Paragon Plus Environment


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nanocomposite supported thin film composite (nTFC, polyamide active layer on CNTs embedded polysulfone substrate), (3) thin film nanocomposite (TFN, CNTs incorporated polyamide active layer on polysulfone substrate), and (4) nanocomposite supported thin film nanocomposite (nTFN, CNTs incorporated polyamide active layer on CNTs embedded polysulfone substrate). The water permeability followed the sequence of nTFN > TFN > nTFC > TFC. However, the rejection and salt permeability exhibited the opposite trend. The incompatibility between CNTs and polymers provided nanocorridors, where both water and solutes could pass through. nTFN membrane exhibited the highest porosity and lowest structural parameter. Moreover, nTFN membrane possessed the best antifouling capacity by preventing foulants to attach the surface and clog the substrate pores. This work offered some systematic knowledge to design novel membranes with improved performance for desalination and water purification applications.

INTRODUCTION

Water purification utilizing polymeric membranes have emerged as an efficient process, because of its high separation performance and low energy consumption.1-2 However, the common polymeric membranes are limited by the trade-off between permeability and selectivity. Recently, the development of nanomaterials and their use in membranes has paved a new way to make a breakthrough.3-6 The nanocomposite membranes fabricated by using nanomaterials as filler can effectively improve the performance and overcome the trade-off of polymeric membranes. 2


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According to membrane structure and location of nanomaterials, nanocomposite membranes can be classified into two categories: mixed-matrix membranes (MMMs) and thin-film nanocomposite (TFN) membranes.6-7 MMMs are heterogeneous membranes, which typically consist of nanomaterials dispersed in a polymeric matrix. TFN membranes consist of an ultra-thin barrier layer supported on a porous substrate and nanomaterials are embedded in the thin-film layer to improve the membrane physicochemical properties and performance. Both MMMs and TFN membranes are more permeable than nano-fillers absent polymer membrane. The properties of the incorporated nanomaterials, such as size and charge etc., will significantly affect the properties of the nanocomposite membranes, thus alter their filtration performance.8-13 Additionally, the location of the embedded nanomaterial fillers in nanocomposite membranes also plays an important role in maintaining high water permeability.6-7 In current study, we use carbon nanotubes (CNTs) as the nano-filler to systematically investigate the effect of different locations of the embedded nanomaterials on membrane performance. CNTs have been widely applied because of their unique structure.14 In particular, water molecules transport trough CNT tunnels are reported to be dramatically accelerated because of the smooth internal walls, resulting in CNT-modified membranes the potential for superior water permeance without compromise of selectivity.15-16 Moreover, the structure of CNTs allows the specific modification of the pore entrance and the out walls to enhance selective transport and facilitate the incorporation of CNTs in nanocomposite membranes.17 3



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We use CNTs to elucidate the potential rules of barriers layer and supporting substrate by introduction of nano-fillers in forward osmosis (FO) and nanofiltration (NF) applications. Four different types of nanocomposite membranes were prepared by interfacial polymerization. The resultant membranes are described as (1) thin film composite (TFC, polyamide active layer on polysulfone substrate), (2) nanocomposite supported thin film composite (nTFC, polyamide active layer on CNTs embedded polysulfone substrate), (3) thin film nanocomposite (TFN, CNTs incorporated polyamide active layer on polysulfone substrate), and (4) nanocomposite supported thin film nanocomposite (nTFN, CNTs incorporated polyamide active layer on CNTs embedded polysulfone substrate). Membrane properties, separation performance, and antifouling capability are presented and discussed. Our work not only compared the effect of CNTs locations on the performance of the prepared membranes, but also provided some systematic knowledge to design novel membranes for water purification application. EXPERIMENTAL Materials CNTs (20 nm in diameter, 0.5~2 µm in length, purity > 95%) were obtained from Nanjing XFNANO Materials Tech Co., Ltd (China). Polysulfone (PSf-3500P) was purchased from Solvay Advanced Polymers (China). M-phenylenediamine (MPD, > 99%) was supplied by Sigma-Aldrich Chemical Co., Ltd (China). Dopamine hydrochloride (DA, 98%) and trimesoyl chloride (TMC, 98%) were supplied by Aladdin (China). Other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd. All chemicals were 4


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used as received. Milli-Q water (Barnstead, compact ultrapure water system) with a resistivity of 18.2 MΩ·cm was used. Membrane preparation Support membranes were prepared by dissolving 18 g PSf and 8 g polyethylene glycol 400 (PEG-400) into 74 g dimethylacetamide (DMAc) and stirring by a magnetic stirrer for 24 h at 70˚C. For CNTs incorporated nanocomposite substrate membranes, certain amount of CNTs (0.05 wt%, 0.15 wt%, and 0.30 wt%) were dispersed into DMAc by ultrasound dispersion for 30 min prior to PSf addition. The casting solutions were degassed for more than 24 h at room temperature. Afterwards, the resultant solutions were casted on a glass plate using a homemade casting knife with 80 µm thickness and the casting speed was maintained at approximately 10 cm/s. The glass plate was then immediately immersed into a coagulant bath filled with fresh DI water at room temperature to induce phase separation. The resultant substrate membranes were stored in DI water bath for 24 h before using. TFC and TFN membranes were then formed via interfacial polymerization atop the substrate membranes. Pure PSf substrates were used for TFC and TFN membranes and nanocomposite substrates were used for nTFC and nTFN membranes. To prepare TFC and nTFC membranes, substrate membranes were immersed into a 2 mg/ml dopamine (DA) Tris buffer solution (pH = 8.5) and were shook for 12 h. The modified membranes were rinsed by DI water to remove the remaining PDA. Then the polydopamine (PDA) coated membranes were immersed in 2 w/v% MPD solution for 2 min, the remaining MPD solution was removed by using filter papers. Subsequently, the resultant membranes were 5



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soaked in 0.1 w/v% TMC/n-hexane solution for 0.5 min. Both TFN and nTFN membranes were formed by dispersing CNTs (0.05 wt%, 0.10 wt%, and 0.15 wt%) in DA solution. PDA was used to provide surface with better wetting property and adsorb more CNTs in the active layer.18 After the interfacial polymerization reaction, the resultant membranes were treated at 80 °C in oven for 5 min, and then were washed and stored in fresh DI water for at least 24 h before test. Characterization The dispersed CNTs were deposited on a holey carbon film coated copper grid and dried naturally in air for 2 h prior measurement. The structural morphology was observed by using a JEM-2100 (JEOL, Japan) working at 200 kV. A scanning electron microscope (SEM S-4800 Hitachi, Japan) was used to investigate the surface and cross-sectional morphology of CNTs and membranes. Samples were air dried and sputter-coated with gold for 50 seconds prior to SEM measurement. For cross-sectional analysis, membranes were frozen in liquid N2 and cracked to obtain cross sections. Fourier transform infrared spectroscope (FTIR) and micro-Raman were used to analyze the chemical properties of CNTs and membranes. A Bruker-Tensor 27 FTIR spectrometer (Bruker, Germany) with attenuated total refection (ATR) function was applied to record FTIR spectra. Raman spectrum was recorded by a Raman inVia microscope (Rainshaw, UK) at λ = 532 nm and 10 mW. All the tested samples were dried in a vacuum desiccator for 24 h before measurement.

6


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The hydrophilicity of composite membrane was characterized through the static contact angle method by using a DSA100 contact angle meter (Kruss, Germany). The membranes were dried for 24 h before measurement, and then the contact angles were measured with a 5 µL DI water drop using the sessile drop method. The membrane porosity (ε) was calculated by gravimetric method, as defined in the following equation:18 =

( )

[

( )

+

]

(1)

where m1 and m2 (g) are the wet and dry weights of membrane, ρw (g/ml) is the density of water, and ρp (g/ml) is the density of PSf. To minimize the experimental error, all the reported values were the average values of at least five replicates. Membrane performance Nanofiltration (NF) performance of the prepared membranes was measured by using 2000 ppm MgCl2 as the feed solution under 4 bar pressure in a cross-flow permeation cells. The effective membrane area for NF test is 19.625 cm2. Before NF test, the membrane was compacted using DI water for 1 h under 8 bar. The NF water flux (J) and salt rejection (R) were calculated by the following equations:19-20 = ∆ ⁄( ∆) = ( − )⁄

(2) (3)

where ∆Wfeed is the weight change of feed solution, Am is the membrane effective area, and ∆t is the permeation time. Cf and Cp are the salt concentration in the feed and permeate. Forward osmosis (FO) performance was measured by a lab-scaled cross-flow FO unit with an effective area of 36 cm2. DI water and 2 mol/L MgCl2 were respectively used as 7



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feed and draw solutions. In the FO test, the active layer of membranes was faced to the feed solution (i.e., FO mode). The average crossflow velocity was 7.8 cm/s. Forward water flux (Jv) and reversed solute flux (Js) were calculated according to the following equations: 21-22 = ∆ ⁄( ∆)

(4)

= ∆ ⁄( ∆)

(5)

where ∆Cf is the concentration change of feed solution and Vf is the volume of feed solution. Water permeability (A) and solute permeability (B) was calculated as follows:23 = ⁄∆

(6)

! = (1 − )(∆ − ∆#)⁄

(7)

where ∆P is transmembrane pressure difference, ∆# is osmotic pressure difference across the membrane. In addition, the structural parameter (S) of the supporting membranes can be determined in accordance to the classical internal concentration polarization (ICP) model as expressed in equation (8):23-24 %

+,-./0 12

$ = & ()* +, '

344- 1&' 12

5

(8)

where D is the solute diffusion coefficient in water, πdraw and πfeed are the osmotic pressures of the draw solution and feed solution. The antifouling capacities of prepared membranes for NF process were evaluated by using humic acid (HA). HA is a principal component of natural organic matter (NOM) resulting from biodegradation of animal and plant residue. It was widely used as a natural 8


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organic matter in fouling test.25 Before fouling studies, the membranes were compacted under 8 bar pressure, and the initial flux was got using 2000 ppm MgCl2 solution. Then the feed solution was changed to the mixed solution of 5 mg/l HA and 2000 ppm MgCl2 and the fouling test was carried out for 1 h. The process was repeated three times to evaluate the antifouling capacity of the membrane with different CNTs loading locations. In FO process, the active layer was faced to the feed solution (i.e., FO mode). After 1 h stabilization, the initial flux was measured by using 2 mol/L MgCl2 as DS and DI water as FS. Then 5 mg/l HA was added to the FS. To clean the membrane, DI water was used as FS and DS for 0.5 h. The recovered water flux was measured by using 2 mol/L MgCl2 and DI water as DS and FS respectively for 1 h. RESULTS AND DISCUSSION CNTs characterization The SEM and TEM images of CNTs are presented in Figure 1A and B. The CNTs exhibited uniform diameter around 20~30 nm, while the length varied from 0.5 to 2 µm. FTIR and Raman are powerful techniques to provide the chemical information of CNTs. The FTIR spectrum of CNTs (Figure 1C) was consistant with the previously reported data of pristine CNTs.26-27 The peak at 2917 cm−1 was assigned to streching vibration of the C-H groups. The presence of CH2/CH3 groups in CNTs indicated the defects in the graphic structure, which were generated during the production process. The band at around 1450 cm−1 was induced by the streching of aromatic rings. The streching mode of C−C−O appeared at 1034 cm−1, which could be explained by the introdution of carboxyl groups 9



during the purification stage of the commercial CNTs production.28 The hydroxyl streching at 3300 cm−1 could be attributed to the absorbed water molecules. Figure 1D shows the Raman spectrum of the D-band (1338 cm−1) and the G-band (1574 cm−1) for CNTs. The D-band, disorder band, was the dispersion of the disordered graphite structure or sp3-hybridized carbons of the nanotubes. G-band corresponded to a splitting of the E2g stretching mode of graphite, which was assigned to the movement of two neighboring carbon atoms in opposite directions, characteristic of highly oriented pyrolitic graphite.29-30 The ratio of intensities of these two bands (ID/IG) suggested an indication of the relative degree of defects or functionalization of the nanotubes. ID/IG of our CNTs sample was about 1.05, which was consistant with the value of unmodified CNTs reported in literature.31 A

B

C

D D

4000

G

Intensity (a.u.)

Transmittance (a.u.)

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3500

3000

2500

2000

1500

1000

600

-1

Wave number (cm

800

1000

1200

1400

1600

-1

Raman shift (cm

)

10


)

1800

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Figure 1. Characterizations of CNTs. (A) SEM image, (B) TEM image, (C) FTIR spectrum, and (D) Raman spectrum of CNTs. Membrane characterization To confirm the formation of CNTs incorporated nanocomposite membranes, the micro-Raman spectra are presented in Figure 2. As shown in Figure 2A, the PSf substrate exhibited two characteristic peaks of the aromatic ring at 1460cm−1 and 1524 cm−1. The peaks at 1021 cm−1, 1110 cm−1, and 1149 cm−1 were ascribed to the stretching vibration of sulfone group and stretching vibration of ether group of PSf.18 The PDA coated PSf membrane showed two new peaks at 1295 cm−1 and 1370 cm−1, which were assigned to the stretching vibration of hydroxyl group and catechol group of PDA.32 For the CNTs incorporated PSf membrane (nPSf) and PDA coated nPSf membrane, two wide peaks at 1338 cm−1 (D-band) and 1574 cm−1 (G-band) appeared. According to Figure 2B, the TFC membrane exhibited a characteristic peak of carbonyl group at 1635 cm−1, which was induced by formation of polyamide. For nTFC, TFN, and nTFN membranes, the broadened G and D bands revealed the successful incorporation of CNTs in the prepared membranes. A

B nPSf+PDA

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nPSf

PSf+PDA

TFN0.05

nTFC0.15 TFC

PSf 600

nTFN0.15/0.05

800

1000

1200

1400

1600

600

800

1000

1200

1400 -1

Raman shift (cm-1)

Raman shift (cm )

11


1600


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Figure 2. Raman spectra of (A) PSf, PDA coated PSf, nPSF, and PDA coated nPSf, (B) TFC, nTFC0.15, TFN0.05, and nTFN0.15/0.05 membranes. Figure 3 illustrates the top view and cross sectional SEM images of the TFC, nTFC0.15, TFN0.05, and nTFN0.15/0.05 membranes. Typical asymmetric structure can be observed with a dense active layer on top of a highly porous sublayer for all the prepared membranes. The ridge and valley morphology of the active layer indicated the successful polymerization of MPD with TMC.33 In comparison, the TFN and nTFN membranes exhibited ascendant ridge-valley structure, which varied the surface roughness. The shape and size of finger like pores demonstrated no critical change by incorporation of CNTs in different positions of the nanocomposite membranes. (A) TFC

(B) TFC

(C) nTFC0.15

(D) nTFC0.15

(E) TFN0.05

(F) TFN0.05

(G) nTFN0.15/0.05

(H) nTFN0.15/0.05

Figure 3. SEM top view and cross sectional images of the (A, B) TFC, (C, D) nTFC0.15, (E, F) TFN0.05, and (G, H) nTFN0.15/0.05 membranes. Figure 4 shows the three-dimensional AFM images of the prepared membranes. Referring to Figure 4A and 4B, the RMS slightly increased from 18.25 nm of TFC to 23.36 nm of nTFC by addition of 0.15wt% CNTs in the substrate. The increased RMS could be attributed to the agglomeration in PSf matrix and/or exposure of CNTs in the substrate. The 12


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unevenness of CNTs incorporated PSf substrate may adversely influence the integrity of the polyamide layer subsequently formed on it. The presence of CNTs in active layer induced higher nodular points on the surface, and then resulted in more obvious enhancement of RMS (Figure 4C). As expected, the embedment of CNTs in both the substrate and active layer led to the highest RMS value of 48.17 nm (Figure 4D). It is important to note that the higher roughness the more area available for membrane transport, which favored the improvement of membrane flux.34 (A) TFC

RMS = 18.25 nm

(C) TFN0.05

RMS = 35.85 nm

(B) nTFC0.15

RMS = 23.36 nm

(D) nTFN0.15/0.05

RMS = 48.17 nm

Figure 4. AFM images (scan size 2×2 µm2, z-scale 500 nm) of (A) TFC, (B) nTFC0.15, (C) TFN0.05, and (D) nTFN0.15/0.05 membranes. The hydrophilicity of membrane was evaluated by measuring the water contact angles. The contact angle data are listed in Table 1. The PSf and nPSf substrate membranes were more hydrophobic than other membranes. Contact angle decreased from 83.1° of PSf to 63.2° of TFC, indicating the formation of a relative hydrophilic surface due to the 13



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polyamide layer on PSf substrate. The contact angle value correlated well with the previous reported data of polyamide TFC membrane.33 The nTFC membranes exhibited similar contact angles with TFC membrane, which suggested that the embedded CNTs in the nPSf substrate did not have significant influence on the surface hydrophilicity. In addition, CNTs in the active layer showed negligible effect on the wetting performance of the TFN and nTFN membranes. According to our early study, it could be attributed to the lack of hydrophilic groups at the unmodified CNTs surface and the cover of polyamide layer.18 Table 1. Contact angle and porosity of different membranes Membranes

Contact angle (º)

Porosity (%)

PSf nPSf TFC nTFC0.05 nTFC0.15 nTFC0.30 TFN0.01 TFN0.05 TFN0.10 TFN0.15 nTFN0.15/0.05

83.1 ± 3.1 79.3 ± 2.2 63. 0 ± 2.6 63. 2 ± 2.4 63.5 ± 2.6 63.3 ± 2.4 64.1 ± 1.8 63.2 ± 2.1 63.5 ± 1.9 64.2 ± 2.0 63.6 ± 1.7

63.3 ± 2.7 65.8 ± 1.9 64.0 ± 2.5 67.6 ± 1.7 69.5 ± 2.1 68.2 ± 1.8 67.1 ± 1.9 67.4 ± 2.5 67.3 ± 1.9 66.9 ± 1.7 70.6 ± 1.9

Table 1 also summarizes the porosity of different membranes. PSf showed a high porosity of 63.3% because of the porous finger like structure. When CNTs were added to the PSf and formed a mixed matrix membrane, the porosity of nPSf increased to 65.8%. Incorporation of CNTs induced some nanocorridors between CNTs and PSf polymers, where water molecules could adsorb. During the filtration process, these nanocorridors 14


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provided more water channels and facilitated the water transfer trough membrane. The TFC membrane possessed similar porosity with PSf, which demonstrated that the dense and thin active layer had only marginal effect on the porosity. While nPSf was used as substrate, the nTFC membrane exhibited higher porosity than TFC membrane. The porosity increased from 67.6% to 69.5% with the increase of CNTs component in the substrate from 0.05 wt% to 0.15 wt%. This was ascribed to the nanocorridors in the nPSf induced by CNTs. It was noteworthy that further addition of CNTs (0.30 wt%) resulted in a slight decline in porosity, causing by the agglomeration of CNTs and the increased casting solution viscosity.35 The increased viscosity delayed the diffusion of solvent from solution and hindered the inside diffusion of water into the membrane. Slower demixing led to lower porosity and more dense membrane.11 When CNTs was loaded in the active layer, some voids existed between CNTs and polymer due to their limited compatibility. The voids enhanced the porosity and favored the water permeation.22 nTFN exhibited the highest porosity, resulting from the dual contribution of CNTs in both PSf substrate and polyamide active layer. Membrane performance The NF and FO performance of the four kinds of membranes are presented in Figure 5. According to Figure5A, the pure water flux of TFC membrane was 20.5 L·m-2·h-1 (LMH). All three nanocomposite membranes exhibited higher water flux relative to TFC. For nTFC and TFN membranes, the pure water reached the highest value at CNTs loading of 0.15 wt% (nTFC0.15) and 0.05 wt% (TFN0.05) respectively. It was reported that 15



incorporation of CNTs, either in the substrate or in the active layer, likely provided hollow inside nanochannels and nanocorridor network, and these new water channels favored water permeation.31, 36 However, further loading of CNTs led to a decline of pure water flux. The change of pure water flux as increasing CNTs component was consistent with the variation of porosity (Table 1), implying the voids induced by CNTs played an important role in improving the water permeability. When CNTs were embedded in both substrate and active layer, the nTFN membrane possessed the best water flux among all the membranes. The reason might be that CNTs not only enhanced the porosity but also increased the roughness of the nanocomposite substrate supported nanocomposite coating film. A

40

JW ( LMH)

30

20

10

T nT FC FC 0 nT .05 FC 0. nT 15 FC 0. T F 30 N 0. 01 TF N 0. 0 TF 5 N 0. 10 TF N 0. 15 0. nTF 15 N /0 .0 5

0

40

80

J

R

20

40

10

20

0

0 15

05

nT

nT

FC

0.

0.

TF

FC

FC 0. 3 TF 0 N 0. 01 TF N0 .0 TF 5 N 0. 10 TF N0 .1 5 0. n T F 15 N /0 .0 5

60

C

30

nT

J (LMH )

B

R (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

16


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20

20

JV

JS

10

5

5

0

0 0. N TF

N0

.0 N0

TF

TF

5

30

.0 N0

TF

0. FC

FC nT

nT

0.

15

05 0.

TF

FC nT

.1 5 0. n T F 15 N /0 .0 5

10

10

15

1

15

C

J V (LMH )

C

J S (gMH)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Performance of the prepared membranes. (A) Pure water flux (JW), (B) NF flux (J) and rejection (R), (C) FO water flux (JV) and reverse salt flux of different membranes. According to Figure 5B, the NF rejection showed an opposite trend to the water flux. The MgCl2 rejections of the three nanocomposite membranes were slightly lower than that of TFC membrane. The low compatibility between CNTs and polymer materials produced abundant voids in the membranes, where not only water but also salt solute (MgCl2 in this study) pass through the membrane. Therefore, the addition of CNTs reduced the membrane efficiency in salt rejection and increased its reverse salt flux (Figure 5C). Table 4 compares the water permeability (A) and salt permeability (B). All the CNTs incorporated nanocomposite membranes exhibited higher water permeability and salt permeability than TFC. Moreover, the nTFN membrane gave the highest A and B. It implied that although water transport was promoted by adding CNTs in the membrane, the salt permeation was increased simultaneously.22 It was also worthy to note that the voids resulted in low structural parameter (S) for the membranes.22,

37

The nTFN membrane possessed the

lowest S of 1429 nm. The S value is a good indicator of the support resistance to diffusion.38 It should be as low as possible to minimize the internal concentration 17



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polarization and improve the water permeability. Similar phenomena were also reported in the previous work by suing zeolite as the nanofiller.38 Incorporation of nanomaterials in the polymer matrix could be an effective way to reduce S value by improving porosity and providing extra water pathways. Table 2. Water permeability (A), salt permeability (B), and structural parameter (S) of the prepared membranes. Membranes

TFC

nTFC

TFN

nTFN

CNTs concentration

A (LMH·bar-1)

B (LMH)

S (µm)

0%

5.2

5.1

3145

0.05%

5.7

5.8

2660

0.15%

6.5

7.0

1669

0.30%

6.1

8.5

1820

0.01%

5.8

7.9

2733

0.05%

6.7

8.3

1637

0.10%

6.0

8.4

1992

0.15%

5.6

8.2

2375

0.15%/0.05%

7.3

8.7

1429

Antifouling capacity of the membranes Fouling is an important and inevitable phenomenon in membrane filtration process. Incorporation of CNTs could improve the antifouling capacity of the membranes. To investigate the antifouling effect of CNTs on the nanocomposite membranes, four kinds of membranes (e.g. TFC, nTFC, TFN, and nTFN) were measured by three cycles of NF and FO fouling and cleaning process employing HA as model foulant. The data are presented in Figure 6 and Figure 7. It was known from Figure 6A that the NF initial flux followed the 18


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sequence of nTFN0.15/0.05 > TFN0.05 > nTFC0.15 > TFC. Comparing with the CNTs incorporated membranes, TFC exhibited low initial flux and poor antifouling capacity. The ratio of total flux decline (RTD) was ~58% and the ratio of flux recovered (RFR) was 51% for the first cycle. Generally, higher initial flux would lead to more severe fouling and flux decline for a given membrane. However, the nanocomposite membranes showed improved antifouling capacity in spite of their higher initial fluxes. The RTD (first cycle) for nTFC, TFN, and nTFN was 48%, 42%, and 34% respectively. The RFR (first cycle) for nTFC, TFN, and nTFN was 62%, 72%, and 79% respectively. In special, the nTFN membrane showed a RTD of 42% and a RFR of 65% after three cycles of fouling and cleaning (Table 3). CNTs could expose from the active layer and the inert and hydrophobic nature of CNT surface would prevent HA to attach at the surface or clog the substrate pores. Moreover, CNTs incorporated membranes were able to more effectively remove fouling materials, due to the atomically smooth graphitic structure of CNTs.39 A

30

20

2

J (L/m h)

25

15 10 TFC nTFC0.15 TFN0.05 nTFN0.15/0.05

5 0 0

1

2

3

4

5

6

7

5

6

7

Time (h)

B Normalized flux

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8 0.6 0.4 TFC nTFC0.15 TFN0.05 nTFN0.15/0.05

0.2 0.0 0

1

2

19 3

4

Time (h)



Figure 6. Time-dependent (A) flux and (B) normalized flux of TFC, nTFC, TFN, and nTFN membranes in the three cycles of NF operation. Figure 7 presents the antifouling properties of the four kinds of membranes in FO process. The initial forward fluxes showed similar trend with NF test. However, the FO fouling was significantly mitigated compared with NF. The RFR (first cycle) for TFC, nTFC, TFN, and nTFN was approximately 81%, 83%, 88%, and 93% respectively. In contrast to pressure-driven membrane processes, FO is a low fouling process. In FO, organic foulants loosely accumulated and formed a less compact cake layer on the membrane surface due to the low hydraulic pressure employed.40-41 As shown Figure 7B, CNTs component in active layer played a more important role in alleviating the fouling, owing to the low adhension between CNTs and the foulant. Therefore, the TFN and nTFN membranes possessed higher antifouling capacity than the nTFC membrane. Despite the performance, especially the rejection, of our CNTs membranes might not be as high as the previously reported nano-fillers incorporated membranes,42 the emphasis of this work was to provide some systematic knowledge for design and fabrication of novel membranes. A

18 15 12

2

JV (L/m h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

9 6 TFC nTFC0.15 TFN0.05 nTFN0.15/0.05

3 0 0

1

2

3

4

5

6

7

Time (h)

20


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B Normalized flux

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 0.8 0.6 0.4 TFC nTFC0.15 TFN0.05 nTFN0.15/0.05

0.2 0.0 0

1

2

3

4

5

6

7

Time (h)

Figure 7. Time-dependent (A) forward flux and (B) normalized forward flux of TFC, nTFC, TFN, and nTFN membranes in the three cycles of FO operation. Table 3. The ratio of flux recovered (RFR) after the successive three cycles of fouling and cleaning of the prepared membranes. Membranes

RFR (1st)

RFR (2nd)

RFR (3rd)

NF

FO

NF

FO

NF

FO

TFC

50.7

80.6

40.1

74.2

37.5

70.7

nTFC0.15

62.5

82.7

57.5

75.4

48.8

71.2

TFN0.05

72.1

88.1

65.1

86.2

58.6

80.4

nTFN0.15/0.05

78.8

93.2

70.7

92.1

64.8

87.8

CONCLUSIONS We compared the structure, property, and performance of the membranes with different CNTs locations. The water permeability of the prepared membranes followed the sequence of nTFN0.15/0.05 > TFN0.05 > nTFC0.15 > TFC. However, salt permeability showed the opposite trend. The incompatibility between CNTs and polymers offered more channels

21



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for water and salt transport. Despite the high water flux, nTFN exhibited excellent antifouling capacity, resulting from the foulant resistance of the incorporated CNTs. AUTHOR INFORMATION

Corresponding Author * Tel.: +86 53266782017. E-mail address: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21476219), the Science and Technology Development Plan of Shandong Province (2014GSF116006), and the Science and Technology Program for Fundamental Research of Qingdao (13-1-4-251-jch). This is MCTL contribution No.101. REFERENCES 1.

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TOC 27



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Nanocomposite membrane with different carbon nanotubes location for nanofiltration and forward osmosis applications Xiangju Song,† Li Wang,† Lili Mao,‡ Zhining Wang*†

We offered some systematic knowledge to design high performance TFN membranes for desalination and water purification in a sustainable way.

28


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Nanocomposite Membrane with Different Carbon Nanotubes Location

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