CF4 plasma modified highly interconnective porous polysulfone membranes for direct contact membrane distillation (DCMD)

Desalination 369 (2015) 105–114

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CF4 plasma modified highly interconnective porous polysulfone membranes for direct contact membrane distillation (DCMD) Miaomiao Tian a,b, Yong Yin a, Chi Yang a, Baolong Zhao a, Jianfeng Song a, Jindun Liu b,⁎, Xue-Mei Li a,⁎, Tao He a,c,⁎⁎ a b c

Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, China School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, China School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, China

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

• Dual-bath coagulation process yielded membranes with highly open surface. • Surface hydrophobicization was achieved by CF4 plasma modification. • PSf membranes showed as twice as high a MD flux as that of PVDF membranes. • Heat and mass simulation revealed a low tortuosity for PSf membranes.

a r t i c l e

i n f o

Article history: Received 9 December 2014 Received in revised form 17 April 2015 Accepted 1 May 2015 Available online 16 May 2015 Keywords: Dual-bath coagulation Polysulfone Membrane distillation Plasma modification Tortuosity

a b s t r a c t Preparation of MD membranes with a low tortuosity and high DCMD flux is reported. Hydrophilic flat sheet polysulfone (PSf), blended with polyvinylpyrrolidone (PVP) membranes were prepared by dual-bath coagulation phase inversion, where N-methylpyrrolidone (NMP)/water mixtures were used as the first coagulant before immersion into a water bath. CF4 plasma modification was utilized to render the originally hydrophilic membranes hydrophobic. The membranes were characterized by scanning electron microscopy (SEM), water contact angle, X-ray photoelectron spectroscopy (XPS), liquid entry pressure (LEPw), and gas permeability. Results demonstrated that the first coagulation bath acted as a dense skin layer remover in comparison to a PSf membrane prepared in a single water bath. CF4 plasma modified porous PSf membranes using pure NMP as the first coagulant showed a contact angle of 144° and a DCMD flux of 53.33 kg/m2 · h (Tf = 70.3 °C), which was 80% higher than a commercial PVDF membrane with comparable performance stability. Heat and mass simulation modeling results demonstrated that the tortuosity of the PSf membranes is close to 1 in contrast to 2.5 for the commercial PVDF membranes. This result provides some theoretical understanding of the origin of the high performance of PSf MD membranes. © 2015 Elsevier B.V. All rights reserved.

⁎ Corresponding authors. ⁎⁎ Correspondence to: T. He, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai, China. E-mail addresses: [email protected] (X.-M. Li), [email protected] (T. He).

http://dx.doi.org/10.1016/j.desal.2015.05.002 0011-9164/© 2015 Elsevier B.V. All rights reserved.

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1. Introduction

2. Modelling

Membrane distillation (MD) is a separation process driven by vapor pressure gradient across a porous hydrophobic membrane [1]. MD is characterized by the high quality of produced water, and potentially applicable for seawater desalination, wastewater treatment, food industry, removal of volatile organic compounds from water etc. [2]. Among various MD configurations, direct contact membrane distillation (DCMD) is one of the most widely investigated processes for its simplicity and ease of implementation. In DCMD, a cold liquid stream is allowed to circulate at the permeate side of the membrane to condense the vapor that has migrated from the feed side. High performance membranes, modules, and high energy efficiency are the key issues for the development of DCMD [3,4]. As a physical barrier, a high permeate flux, good thermal stability, and suitable hydrophobicity are important characteristics of a MD membrane [5]. For membranes of a certain thickness and pore size, the most relevant membrane properties related to the permeability are the membrane pore size and pore size distribution, porosity and the tortuosity, according to heat and mass transfer models developed by Fane et al. [6]. and Qtaishat et al. [7]. Various methods have been used for preparation of membranes with optimized pore size, porosity and tortuosity: stretching [8], thermally induced phase separation (TIPS) [9], non-solvent induced phase separation (NIPS) [10,11], and plasma assisted polymerization [12,13]. Microporous polymeric membranes such as poly(vinylidenefluoride) (PVDF), polypropylene (PP), and polytetrafluoroethylene (PTFE) have a low surface energy and high hydrophobicity [1]. However, membranes of less open structure with poor interconnectivity resulted in high mass transfer resistance. Hydrophilic membranes, although having higher porosity and intrinsically higher permeability, are not applicable directly for MD. Several research groups have reported the application of hydrophilic materialbased membranes for MD application whereas the membranes were rendered hydrophobic via various approaches including plasma polymerization [14,15], migration of hydrophobic macromolecules to the surface [16,17], and co-extrusion of hydrophilic and hydrophobic materials [18]. However, above approaches often yielded membranes with extra deposited layers on the membrane surfaces, which cause increased mass transfer resistance, and thereby a relatively low flux. Via a CF4 plasma surface treatment, we have reported the conversion of a hydrophilic PES hollow fiber membrane to hydrophobic ones with a water contact angle of about 120°; a MD flux of 66.4 kg/m2 · h has been demonstrated with rejection of 99.97% at a feed temperature of 73.8 °C [19]. Careful examination of the CF4 plasma treated membranes by atomic force microscopy has revealed that there was no apparent extra layer deposited. In addition, it was hypothesized that hydrophilic base membranes may have more interconnected pore structure leading to a lower tortuosity, thereby a high MD flux. However, there has not been a direct proof yet. Herein, the tortuosity of hydrophilic membranes is investigated using a dual-bath coagulation process based on polysulfone (PSf)/ polyvinylpyrrolidone (PVP) blend membranes in order to quantify the effect of the membrane tortuosity on the membrane flux. The membrane surface morphology was controlled using a solvent/ water mixture as the first coagulation bath, followed by a water bath [20–22]. The membranes were converted hydrophobic via a CF4 plasma treatment process [23,24]. The influence of the composition of the first coagulation bath was studied. The morphology, hydrophobicity, gas permeability, pore size, porosity, liquid entry pressure of water (LEPw) and DCMD performance were characterized in order to correlate the surface morphology and wettability with membrane performance. Finally, the tortuosity of the membranes is estimated based on a heat and mass simulation model in order to elucidate the advantages of using hydrophilic substrate membranes for MD processes.

The MD fluxes can be predicted by a mathematical model developed by Fane et al. [7,25]. In the DCMD process, a linear relationship between the mass flux (Jm) and the water vapor pressure difference through the membrane distillation coefficient (Bm) is assumed:
 J m ¼ Bm pm f −pmp

ð1Þ

where pmf and pmp are the partial water vapor pressures (Pa) of water at the feed and permeate sides of the membrane/liquid interface, respectively. The values can be evaluated using Antoine equation based on the temperature at the feed/membrane interface (Tmf) and the permeate/membrane interface (Tmp), as follows:   3841 P v ¼ exp 23:328− T−45

ð2Þ

where Pv is the water vapor pressure (Pa), and T is the corresponding temperature (K) [26]. It should be noted that the Antoine equation is only valid for water in the present form. For other solvents, those data have different values. The membrane distillation coefficient, Bm, is normally determined according to the mass transfer mechanisms, which is mainly affected by the Knudsen number, Kn, defined as the ratio of the mean free path (λ) of the transported molecules to the pore size (diameter, d) of the membrane, i.e. K n ¼ λd. Three regions can be identified: Knudsen region (Kn N 1), continuum region (Kn b 0.01) and transition region (0.01 b Kn b 1). λ can be calculated by the following equation: kB T λ ¼ pffiffiffi 2πPσ 2

ð3Þ

where σ is the collision diameter (2.641 × 10− 10 m for water vapor), kB is the Boltzmann constant (1.38 × 10− 23 J/K), and P is the mean pressure within the membrane pores (≈ 100 KPa). The obtained λ is 1.46 × 10− 7 m, and the pore size of the membranes in this experiment ranged from 0.27 μm to 0.6 μm. Thus, the values of Kn are in the range of 0.24–0.54, and the mass transport falls into the transition region. Accordingly, the membrane distillation coefficient is determined as the following [6,27]: " #−1   3 τδ πRT 1=2 τδ P a RT Bm ¼ þ 2 εr 8M ε PD M

ð4Þ

where ε, τ, δ and r are the porosity, pore tortuosity, thickness (m) and pore radius (m) of the membranes, respectively. M is the molecular weight of water (18 × 10 − 3 kg/mol), R is the gas constant (8.314 J/mol · K), P a is the air pressure (Pa), and D is the water diffusion coefficient (m2 /s). Except tortuosity, all other parameters in Eq. (4) are either constants or measureable. Therefore, the membrane tortuosity can be evaluated by fitting the MD water flux. 3. Experimental 3.1. Materials and chemicals PSf (P-3500NT) was purchased from Solvay. PVP (Mw = 360,000 Da) was obtained from BASF. N-methylpyrrolidone (NMP), sodium hypochlorite and sodium chloride, all AR grade, were supplied by Sinopharm Chemical Reagent Co. Ltd. and used as received. Commercial PVDF flatsheet membranes GVHP (Millipore, nominal pore size: 0.22 μm, porosity: 75%, and thickness: 125 μm) were used in this study. Deionized water was used throughout the experiments.

M. Tian et al. / Desalination 369 (2015) 105–114

3.2. Membrane preparation The membranes were prepared via a dual-bath coagulation phase inversion approach as shown in Fig. 1. The polymers, PSf (16.5 g) and PVP (16.5 g), were dissolved in NMP (67 g) under mechanical agitation at 65 °C to form a homogeneous casting solution. The solution was filtrated using a 40 μm stainless steel metal filter, and degassed in an oven at 65 °C overnight. The solution was cast onto a glass plate, using a casting knife with a gap of 300 μm, at ambient temperature (23 °C) and ~45% relative humidity. The initial film was immediately immersed into water or a NMP/H2O mixture for 5 s followed by immersion in water. After complete coagulation, the membranes were soaked in sodium hypochlorite solution (38.5 g/l) for 24 h to degrade the most of the PVP residue within the membrane. Afterwards, the membranes were rinsed and soaked in deionized water for at least 12 h to remove residual chemicals before further experiments. For a given sample, the variation in the membrane thickness was typically b10%. A label of PSf-X, where X represents the volume percentage of NMP in the first bath, was given to each sample. For example, PSf-80 means a membrane prepared in a first bath with 80 vol.% NMP. The PSf membranes were then dried in a vacuum oven at 30 °C overnight. Then an IoN 40 plasma system (PVA TePla Co. Ltd) equipped with parallel plate electrodes coupled with RF plasma reaction system was utilized for CF4 plasma treatment of the PSf membranes. The CF4 plasma modification was carried out at a glow discharge power of 200 W for 30 min as published elsewhere.

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3.3.2. Contact angle (CA) Water contact angle measurement is a direct indication of the surface hydrophobicity. The water contact angle was measured by a contact angle goniometer (Maist Drop Meter A-100P) equipped with a high speed charge-coupled device (CCD) camera via the sessile drop method. The samples were taped on a glass plate. Water contact angles were determined by placing a water droplet of 5 μL on the membrane surface followed by image analysis At least five points were examined for each sample with a variation in the CA typically b2%. 3.3.3. Liquid entry pressure of water (LEPw) LEPw is a quantitative measure of the resistance of the hydrophobic membrane towards wetting. LEPw is inverse to the maximum pore size and proportional to the cosine of the CA (Eq. (5)).

LEP w ¼

−2Bγ L cosθ r max

ð5Þ

where B is a geometric factor determined by pore structure, which is taken as a constant for the PSf membranes; γL is the liquid surface tension (72.8 mN/m). The pressure introduced from a compressed nitrogen gas was adjusted at a stepwise manner and maintained for about 3 min. At the point when water was observed visually at the backside of the membrane facing air, the set pressure was recorded as the LEPw. The measurement was repeated at least three times and a typical variation in the pressure was less than 10%.

3.3. Membrane characterization 3.3.1. Scanning electron microscopy (SEM) The morphology of the membrane top and bottom surfaces and the cross-section were characterized by scanning electron microscopy (HITACHI TM-1000). The cross-section sample was prepared by cryogenic breaking of a wet sample in liquid nitrogen. All the samples were coated with a thin layer of gold before imaging.

Step 1: Casting PSf solution

3.3.4. Pore size and gas permeability Pore size and gas permeability was determined with Capillary Flow Porometry (Porolux 1000). The nitrogen permeation through each dry membrane was measured at 0.1 bar. To measure the pore size and pore size distribution, the membranes were wetted with commercial low surface tension liquid Porefil (surface tension of 16 mN/m). The test was managed with a program consisting of wet-run and dry-run.

PSf-0

Step 3: Immersing in water bath

PSf/PVP/NMP Glass plate

Step 2: Immersing in solvent/water bath

PSf-80 PSf-90 PSf-100

Membranes in different mixture bath Fig. 1. Flow chart of the membrane preparation process with a dual coagulation bath process. Step 1, PSf solution was cast on a dry and clean glass plate at ambient temperature and relative humidity; Step 2, the nascent film cast was immersed in a NMP/water mixture for 5 s; and Step 3, the film was immersed in water bath to solidify PSf.

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The detailed process was reported in our previous work [23]. Each membrane was measured at least three times with a typical variation b10%.

permeate flux (J, kg/m2 · h) was calculated based on the mass of water transported from feed to permeate (Δm, kg) divided by time interval (Δt, h) and membrane area (A, 0.03 m2 in this experiment) as

3.3.5. Membrane porosity The membrane porosity was assessed gravimetrically [19,28,29]. The mass of the hydrophilic membrane samples, mwet, was measured after carefully removing the water from the membrane surface. The membrane was then dried at 30 °C under vacuum. Once the membrane's mass stabilized, the sample dry mass, mdry, was measured. The porosity of the membranes was calculated as

J¼

ε¼

mwet −mdry  100% ρH 2 O  A  δ

ð6Þ

where ρH2 O is the density of water (1.0 × 103 kg/m3), A is the area of the tested membrane (m2), and δ is the membrane thickness (m). Each membrane was repeated at least three times and the average was reported.

Δm A  Δt

ð7Þ

The salt rejection (R) was defined as follows: R¼

  Cp 1−  100% Cf

ð8Þ

where Cp and Cf represent the concentration of NaCl in the permeate and feed, respectively. An online conductivity meter (EC-4300RS, supplied by SUNTEX Instrument Ltd.) was used to monitor the conductivity of permeate. The salt concentration in the permeate was determined based on a calibration curve of the conductivity and salt concentration. 4. Results and discussion

3.3.6. X-ray photoelectron spectroscopy (XPS) The chemical nature of the prepared membrane surface before and after CF4 plasma modification was examined using X-ray photoelectron spectroscopy (K-alpha, Thermo Fisher). Surface survey data was collected followed by high resolution scans over C1s (278–298 eV), N1s (392–402 eV), O1s (525–545 eV), and F1s (675–695 eV). 3.4. Direct contact membrane distillation In the DCMD process, DI water was used at both the feed and permeate side to measure the water fluxes of each membrane at different feed temperatures. After that, 4 wt.% NaCl solution was used as the feed in the MD process for a long time operation to elucidate the stability of the modified membrane PSf-100. A schematic of the DCMD process is shown in Fig. 2. The membranes were placed in a quadrate test cell as a barrier separating the feed and permeate. To maintain a constant feed volume, water was siphoned from a large water reservoir. Feed temperature was varied, but permeate temperature was constant (20 ± 0.2 °C). Both feed and permeate were circulated at 600 ml/min by two gear pumps. The conductivity, permeate weight, temperatures were monitored by transmitters and logged by a computer. The

4.1. Membrane morphology and hydrophobicity The membrane surface morphology was observed by scanning electron microscopy (SEM). There was no observable alteration in the membrane surface morphology after CF4 plasma modification, therefore, only images after surface modification are shown. PSf-0 was prepared using water as the only coagulant for comparison, and it showed a very smooth and dense top surface (Fig. 3). By gradual increase of NMP content in the first bath, it can be seen that membranes with highly open top surface structure are obtained only at a solvent content higher than 50%. This may be explained by the thermodynamic behavior of the liquid–liquid phase separation of the polymer solution in immersion precipitation [30,31]. For PSf-80, small pores can be identified at a magnification of 5000 on the top surface; the pores enlarged significantly for PSf-90. In contrast, completely open top surface was obtained for PSf-100. The bottom surfaces of the PSf hybrid membranes showed homogeneously open pores of similar size, except that PSf-100 has a smaller pore size. All PSf membranes were macrovoid-free in the cross-section with varied structure: PSf-0, PSf-30, and PSf-50 had rather compact cross-section pore structure in comparison to the other three;

Fig. 2. Schematic illustration of the direct contact membrane distillation test setup and process (1) Membrane; (2) Module; (3) The temperature transmitter; (4) Flow meter; (5) Gear Pump; (6) Feed tank; (7) Permeate tank; (8) Thermostatic bath; (9) Cooling bath; (10) Conductivity transmitter; (11) Digital balance; (12) Level controller; and (13) Computer.

M. Tian et al. / Desalination 369 (2015) 105–114

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Fig. 3. Top, cross-section, and bottom surface SEM images of CF4 plasma modified PSf membranes and commercial PVDF membrane. The inset images are photographs of a water droplet (5 μL) on the corresponding membrane surface.

membrane PSf-80 appeared to have a very thin top skin and rather symmetric sub-porous structure and membrane PSf-90 and PSf-100 showed more open top skin layer than the sub-layer. The wettability of the membranes was monitored by water contact angle measurements. Photo images of a water droplet on the surface modified membranes are shown as inset images in Fig. 3. For PSf-0, PSf-30 and PSf-50, the top surface water contact angle values are almost same within the experimental error. Therefore, for above 3 membranes, the concentration of NMP in the second bath had shown negligible effects, because of the very smooth skin surface. For PSf-80, PSf-90 and

PSf-100, the contact angle data are obviously higher than the other three membranes. Careful examination of the surface morphology of the membrane showed that the water contact angle of the membranes is closely related to the membrane surface morphology, where a more open surface corresponds to a higher water contact angle, which is expectable according to Cassie–Baxter theory [32,33]. It should be noted that the membranes were hydrophilic before CF4 plasma modification, and the contact angle measurements showed that water was instantaneously absorbed upon contacting the membrane. The water absorption ability the PSf membranes is mainly ascribed to the presence of PVP,

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which was not completely rinsed away as confirmed by the presence of nitrogen in the XPS spectra (Fig. 4a). Furthermore, according to the XPS survey scan the major elements, as anticipated, are carbon and oxygen, nitrogen, and sulfur for the PSf membranes before surface modification. Fluoride was almost non-detectable, which yields an F/C ratio of 0. Deconvolution of core level C1s peaks indicate that aromatic C_C located around 284.2 eV as indicated in Fig. 4(b), is the main component. The C1s signals at 284.7 eV (FWHM = 1.7 eV), 285.09 eV (FWHM = 1.7 eV), 287.05 eV (FWHM = 1.7 eV), and 290.09 eV (FWHM = 1.7 eV) are assigned to sp2 and sp3 carbons, C–S and C–N bonds, aromatic shake up and satellite shake-up, respectively. After surface modification, F element becomes dominant in the XPS spectra. Although still present, the amounts of N and S become negligible. The F/C ratio increased to 1.61, indicating that CF4 plasma treatment has brought a fluorinated layer to the surface, leading to surface wettability alteration. Deconvolution of the core level C1s peak has revealed 9 signals where CF2–CF2 type of carbon (appearing at 290.19, 291.19, 291.5 eV) is dominant. In addition, −CF3 carbon signal was also present (292.73, 294.25 eV). The C1s signal at 284.7 eV (FWHM = 1.7 eV) was assigned to adventitious carbon, which cannot be affected by fluorination. C1s signals at 286.63 and 289.09 eV (FWHM = 1.7 eV) are assigned to sp2 carbon with one or two F atoms. This core level C1s signal deconvolution confirms the change of the bonding state of carbon elements from primarily aromatic sp2 C to CF2–CF2, indicating that surface modification takes place by deposition of CF2–CF2, and replacement of C–H bonds with C–F and C–CF3. This result is in agreement with our previous finding. In addition, the plasma is in the gaseous form, and thereby it can modify not only the surface of the membranes, but also the interior of the membrane pores, as confirmed by the SEM–EDX data of the cross-section of the membranes in the Supplementary material. The use of dual coagulation for tuning the membrane surface porosity has been reported before. Our finding is in agreement with published

results in that the use of solvent mixture as the first coagulation bath has contributed to a highly open surface morphology [34,35]. Similar morphology was also reported using a dual-layer co-casting technique to produce open porous membranes with high porosity [36–39]. In other words, the first coagulation bath has functioned as the dense skin remover, and thus effectively improved the membrane surface porosity. It should be mentioned that contact angles of the membrane bottom surfaces are approximately 125° with variations of b1° for all the PSf membranes. 4.2. Membrane pore size, gas permeability and LEPw The gas permeability of PSf-0, PSf-30 and PSf-50 was beyond detection limit of the Porolux setup, thus no pore size was reported. The same phenomenon was reported recently that addition of PVP to the polymer solution tends to form dense skin layers [40]. For PSf-80, the mean pore size was 0.27 μm, and it was 0.57 μm for PSf-90 and 0.60 μm for PSf-100. Increase in the gas permeability agrees well with the increase in the mean pore size (Table 1), since a greater mean pore size renders membranes a lower mass transfer resistance. Based on SEM images, pore size analysis and gas permeability results, it can be concluded that the use of dual-bath coagulation can effectively enlarge the membrane surface pore size and reduce the membrane mass transfer resistance, demonstrating a top dense skin layer removal effect. As shown in Table 1, the maximum pore size of PSf membrane increases with the increase of the NMP concentration in the first bath, as a result of the dilution effect of the solvent in the first bath and the two stage demixing of PSf and PVP [41,42]. The open and interconnective porous structure is mainly due to the polymer–polymer (PSf–PVP) phase separation at the second stage. At a higher solvent concentration, the liquid–liquid demixing starts at a lower polymer content (PSf–PVP as one phase); the resulted membrane after the polymer was solidified 1.0x10

5

5

8.0x10

4

1.50×105

6.0x10

4

4.0x10

4

2.0x10

4

2.50×105

(a)

(b)

C1s

Intensity

Intensity

2.00×10

O1s

1.00×105

N1s 5.00×105

C1s

C=C

C-S C-N

aromatic

S1s

C-C

0.0

0.00 1200

1000

800

600

400

200

295

0

290

285

280

Binding Energy (eV)

Binding Energy (eV) 3000 6.00x10

4

4.50x10

4

3.00x10

4

F1s

(c)

(d) 2500

1.50x10

Intensity

Intensity

CF2

2000 1500

CF2-CF2

=CF2

CF3

1000

C=C contamination

4

O1s

500

N1s C1s

0.00 1200

1000

800

600

400

Binding Energy (eV)

200

0

CF3 CF2-CF2

=CF

0 295

290

285

280

Binding Energy (eV)

Fig. 4. XPS survey scan of PSf-100 membrane before (a) and after (c) surface modification, deconvolution of the core level C1s spectra before (b) and after (d) CF4 plasma treatment.

M. Tian et al. / Desalination 369 (2015) 105–114 Table 1 Values of gas permeability, mean pore size, maximum pore size, and LEPw for PSf-80, PSf-90, and PSf-100 membranes. Samples

Gas permeability (l/min/cm2/bar)

Mean pore size (μm)

Maximum pore size (μm)

LEPw (bar)

PSf-80 PSf-90 PSf-100

4.86 5.36 5.46

0.27 0.57 0.60

0.39 0.71 0.7

2.64 1.68 2.6

shows larger pores [36,43]. However, LEPw of the membranes did not show a direct relationship to the solvent concentration. LEPw is an indication of the ability of a hydrophobic membrane against wetting in the MD process. According to Laplace equation, LEPw is inversely proportional to the maximum pore size (rmax) and directly proportional to the cosine of the contact angle (cosθ) as Eq. (5). In order to obtain a high LEPw, a high water contact angle and a small maximum pore size are desirable. If the contact angle and the pore geometry of PSf membranes are the same, the LEPw will be inversely related to the maximum pore size. Taking the pore size and the contact angle into consideration, the ratios of LEPw between the membranes are reasonably justified. PSf-100 has a higher contact angle (144.6°) than PSf-90, but with a similar maximum pore size; thus, it is understandable that PSf-100 has a larger LEPw. Similarly, PSf-80 has a higher contact angle (137°) than PSf-90 but with a much smaller maximal pore size; as a result, PSf-80 has a greater LEPw. The LEPw variations in Table 1 agree to the above prediction.

111

Highly interconnective porous membrane matrix and similar bottom skin indicate a similar mass transfer resistance in the matrix section and bottom surface. The highest flux obtained for the membrane PSf100 then resulted from the very open top surface. Since the membrane top surfaces are already quite open, only slight variation was found in the three PSf membranes (Fig. 5). Nevertheless, due to the different morphologies between PVDF and PSf, the difference in the mass transfer resistances between the two types of membranes is significant (over 80% against PVDF). Fig. 6 shows the performance stability of surface modified PSf-100 in a DCMD process as compared to the commercial PVDF membrane. The initial flux was 32.58 kg/m2 · h and 18.56 kg/m2 · h, respectively for PSf-100 membrane and commercial PVDF membrane using 4 wt.% NaCl solution as the feed, and the rejection for two membranes are all above 99.99%. However, over 27 h, the conductivity of the permeate for PSf-100 decreased sharply along the process, while a much milder slope was detected for the commercial PVDF membrane, which was ascribed to the 50% lower flux of the PVDF membrane, resulting in a milder dilution effect. These results have illustrated that the performance stability of the plasma modified prepared PSf membranes is comparable to that of the commercial PVDF membranes but having a higher permeability, and thus may hold potential for large scale applications. 4.4. Permeation activation energy of membranes The transport activation energy of the PSf and PVDF membranes can be calculated using the Arrhenius equation (Eq. (9)).

4.3. Direct contact membrane distillation (DCMD) There was no detectable water flux across the membrane of PSf-0 and PSf-30 for a test period of 2 h, probably due to the very high mass transfer resistance. These results are in line with gas permeability, which was beyond the detection limit of the setup. Fig. 5 shows the effect of the feed temperature on the water fluxes for the PSf-50, PSf-80, PSf-90, PSf-100 and PVDF membranes. The flux increases with the feed temperature due to the anticipated higher vapor pressure (Eq. (2)) [17, 44–46]. PSf-80, PSf-90, and PSf-100 membranes showed similar MD fluxes, nearly twice as high as commercial PVDF membranes. In contrast, PSf 50 showed a 30% lower flux than the commercial PVDF membranes, which was ascribed to the very dense skin surface. The similar MD performance of the three PSf membranes is a reflection of the similar mass transfer resistance across the membrane. According to the mass transfer resistance model, the top skin (surface), the matrix and the bottom skin (surface) are the three main sections determining the membrane MD flux.

  EJ J ¼ J 0 exp − RT

ð9Þ

where, J is the permeation flux (mol/m2 · s), EJ is the activation energy (kJ/mol), R is the gas constant (8.314 × 10−3 kJ/mol · K), T is the absolute temperature (K), and J0 is the pre-exponential factor (mol/m2 · s). EJ can be obtained from the relation of lnJ versus 1/T as Eq. (10). ln J ¼ −

EJ þ ln J 0 RT

ð10Þ

The Arrhenius plots of permeation flux with temperature are shown in Fig. 7. From the slope of the straight lines, the permeation activation energy for the membranes was calculated. As shown in Fig. 7, the slopes of three PSf membranes are nearly overlapping each other, indicative of the same activation energy, whereas PVDF membrane showed a different slope. By fitting the lines, the EJ was about 40.4 kJ/mol for the PSf

60 1.0 0.8

J0=18.56 kg/m2·h

40 30

J0=32.54 kg/m2·h

12

0.6

10

0.4

8

20 PVDF (J/J0)

10 52

6

PSf (J/J0)

0.2

PSf (Conductivity) PVDF (Conductivity)

54

56

58

60

62

64

66

68

70

72

Tf-in (oC) Fig. 5. Effects of the feed temperature on the water flux of the PSf membranes and commercial PVDF membrane using DI water as feed solution.

0.0

0

5

Conductivity (µs/cm)

14

J/J0

J (kg/m2·h)

50

16

PSf-50 PSf-80 PSf-90 PSf-100 PVDF

4 10

15

20

25

Time (h) Fig. 6. Normalized flux and conductivity profiles for PSf-100 and commercial PVDF membranes using 4 wt.% NaCl feed solution at 60.2 ± 0.3 °C.

M. Tian et al. / Desalination 369 (2015) 105–114

4.0 3.8

ln J (mol/m2·s)

3.6 3.4 3.2 3.0 2.8

PSf-80 PSf-90 PSf-100 PVDF

2.6 2.4 0.00290

0.00295

0.00300

0.00305

1/T (1/K) Fig. 7. Arrhenius plots of the PSf and PVDF membranes.

membranes and 50.7 kJ/mol for the PVDF membrane. This lower activation energy of the PSf membranes is in line with the higher observed flux of the PSf membranes. The underlying mechanism will be illustrated in further paragraphs. 4.5. Membrane tortuosity One of the main purposes of present study was to improve the membrane surface porosity and pore interconnectivity of the resulting PSf membranes. It is envisioned that the originally hydrophilic membranes possess higher pore interconnectivity than hydrophobic ones. The

60

(a)

Jm1

60

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membrane tortuosity was used as a quantitative parameter to quantify the pore interconnectivity based on the assumption that a highly interconnective porous structure corresponds to a low tortuosity. There is a solid physical background for such argument: statistically, water molecules will diffuse through a shorter distance across the membrane if the pores are more interconnected. In this section, the tortuosity of the membranes prepared in this study is compared with commercial hydrophobic PVDF membranes. Tortuosity cannot be determined experimentally and is generally considered to be related to the porosity of the materials. Various models have been established in relating the porosity with tortuosity, but all these models are empirical and the theoretical origins are not clear. In this study, the tortuosity was calculated based on Mackie's equation [47]; the membrane distillation coefficient and the theoretical flux Jm were estimated by Eqs. (1) and (4), respectively. The tortuosity was then adjusted to minimize the difference between the calculated Jm and experimental flux Jw to less than 5%. Also, τ ≅ 1 − ln ε2 was included for prediction of the maximum flux with the least mass transfer resistance as shown in Fig. 8. For three PSf membranes, the best fitted tortuosity values are in the range between 1.12 and 1.14, very close to 1. However, for the PVDF membrane, the best fitting shows that the PVDF membrane has a tortuosity of 2.5, which is significantly greater than those PSf membranes. For the three PSf membranes tested in this work, one tortuosity value fits well with the experimental data. This indicates that the polymer dope composition (PSf/PVP/NMP) is most probably the dominating parameter in determining the tortuosity, not the NMP concentration in the dual bath. The surface porosity (as shown in Fig. 3) is strongly related to the NMP concentration in the dual bath due to the dilution effect of the solvent in the first bath. But the impact of solvent concentration in the first bath is less important to the membrane matrix. Thus, it is

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Fig. 8. Comparison of experimentally observed flux Jw to theoretical predicted flux Jm at different tortuosity for the different membranes. (a) PSf-80, (b) PSf-90, (c) PSf-100, and (d) PVDF.

M. Tian et al. / Desalination 369 (2015) 105–114

reasonable that the tortuosity of the three PSf membranes is very close as fitted by the model. The above results are considered rational in reflecting the tortuosity difference between PSf and PVDF membranes. In addition, based on porosity measurements, we found that all the membranes have shown porosity in the range of 75–78%, among which, PVDF had the highest. The fitted τ values based on the heat and mass simulation (Eq. (4)) of the PSf membranes are close to the predicted ones according to Mackie's equation. The fitted τ value (2.5) for the PVDF membrane deviated significantly from theoretically predicted one (1.12), which we believe was caused by low pore interconnectivity. When the membranes have a similar pore interconnectivity, the tortuosity is mainly determined by porosity. But when the membrane has low pore interconnectivity, the influence of pore interconnectivity on membrane tortuosity is more significant. For the PVDF membrane, the low pore interconnectivity has caused a much higher observed tortuosity than the theoretically predicted one. But for PSf membranes, the theoretical prediction and fitted values are very close, indicating very high pore interconnectivity. More specifically, the high pore interconnectivity of the PSf membranes corresponds to a low tortuosity, low mass transfer resistance, and consequently a high MD performance. In brief, membrane pore interconnectivity is a key factor in determining the mass transfer resistance, and hydrophilic materials plus hydrophobic treatment may be an alternative approach for the preparation of high performance MD membranes. 5. Conclusions This study has reported the preparation of PSf membranes with a highly interconnective pore structure and low tortuosity. The hydrophilic PSf/PVP membranes were made hydrophobic by CF4 plasma modification. The surface modified PSf membranes have shown nearly twice as high a MD flux as that of a commercial PVDF membrane. The underlying mechanism of the high DCMD flux of the PSf membranes was illustrated by a combined Knudsen/diffusion mass transfer model. It was shown that the PSf membranes have a low tortuosity close to 1, which was ascribed to the high pore interconnectivity of the hydrophilic material-based MD membranes. These results have provided both experimental proof and theoretical basis in understanding the high performance of PSf membranes, pointing out new directions for the development of high performance MD membranes. By adjusting the tortuosity, fitting experimental MD flux could be achieved for all membranes. In addition, it was found that membranes with highly open structure tend to have a higher MD flux, which may indicate that MD flux may be positively related to membrane surface porosity. Further research should focus on quantitative relationship between the chemistry, morphology and the surface wettability of the MD membranes with the MD performance. Abbreviations MD membrane distillation DCMD direct contact membrane distillation PSf polysulfone PVP polyvinylpyrrolidone NMP N-methylpyrrolidone AR analytical reagent SEM scanning electron microscopy XPS X-ray photoelectron spectroscopy LEPw liquid entry pressure of water TIPS thermally induced phase separation NIPS non-solvent induced phase separation PVDF Poly(vinylidenefluoride) PP Polypropylene PTFE Polytetrafluoroethylene

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CCD camera charge-coupled device camera FWHM full width at high maximum Nomenclature Tf Temperature of the feed inlet (°C) Jm Mass flux (kg/m2 · h) Bm Membrane distillation coefficient Pmf Partial water vapor pressure of water at the feed side of the membrane/liquid interface Pmp Partial water vapor pressures of water at the permeate side of the membrane/liquid interface Tmf Temperature at the feed/membrane interface (K) Tmp Temperature at the permeate/membrane interface (K) Pv Water vapor pressure (Pa) Kn Knudsen number d Diameter (m) M Molecular weight of water (18 × 10−3 kg/mol) R Gas constant (8.314 J/mol · K) D Water diffusion coefficient mwet Mass of the wet membrane (kg) mdry Mass of the dry membrane (kg) ρH2 O Density of water (1.0 × 103 kg/m3) A Area of the tested membrane (m2) B Geometric factor of the pore structure γL Liquid surface tension (N/m) EJ Permeation activation energy (J/mol) Greek letters λ Mean free path (m) σ Collision diameter (m) kB Boltzmann constant (1.38 × 10−23 J/K) ε Porosity τ Pore tortuosity δ Thickness of membrane (m) r Pore radius (m) Acknowledgments The authors would like to thank the partial financial support from the National Natural Science Fund China (Project nos. 21176119, 51206114) and the National Key Basic Research Program of China (973 Program with project nos. 2012CB932800). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.desal.2015.05.002. References [1] A. Alkhudhiri, N. Darwish, N. Hilal, Membrane distillation: a comprehensive review, Desalination 287 (2012) 2–18. [2] Kevin W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [3] M. Gryta, Fouling in direct contact membrane distillation process, J. Membr. Sci. 325 (2008) 383–394. [4] M. Khayet, T. Matsuura, J.I. Mengual, M. Qtaishat, Design of novel direct contact membrane distillation membranes, Desalination 192 (2006) 105–111. [5] J. Zhang, Z.Y. Song, B.A. Li, Q. Wang, S.C. Wang, Fabrication and characterization of superhydrophobic PVDF for DCMD, Desalination 324 (2013) 1–9. [6] J. Phattaranawika, R. Jiraratananon, A.G. Fane, Effect of pore size distribution and air flux on mass transport in direct contact membrane distillation, J. Membr. Sci. 215 (2003) 75–85. [7] M. Qtaishat, T. Matsuura, B. Kruczek, M. Khayet, Heat and mass transfer analysis in direct contact membrane distillation, Desalination 219 (2008) 272–292. [8] K.I. Kurumada, T. Kitamura, N. Fukumoto, M. Oshima, M. Tanigaki, S.I. Kanazawa, Structure generation in PTFE porous membranes induced by the uniaxial and biaxial stretching operations, J. Membr. Sci. 149 (1998) 51–57. [9] H. Matsuyama, T. Maki, M. Teramoto, K. Asano, Effect of polypropylene molecular weight on porous membrane formation by thermally induced phase separation, J. Membr. Sci. 204 (2002) 323–328.

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