Superhydrophobic modification of TiO2 nanocomposite PVDF membranes for applications in membrane distillation

Journal of Membrane Science 415–416 (2012) 850–863

Contents lists available at SciVerse ScienceDirect

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

Superhydrophobic modification of TiO2 nanocomposite PVDF membranes for applications in membrane distillation Amir Razmjou a, Ellen Arifin a, Guangxi Dong a, Jaleh Mansouri a,b,n, Vicki Chen a a b

UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Cooperative Research Centre for Polymers, Notting Hill, VIC 3168, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 February 2012 Received in revised form 21 May 2012 Accepted 3 June 2012 Available online 15 June 2012

Superhydrophobic membrane for the application in membrane distillation was generated by creating a hierarchical structure with multilevel roughness via depositing TiO2 nanoparticles on microporous PVDF membranes by means of a low temperature hydrothermal (LTH) process. The TiO2 coated membranes were then fluorosilanized using a low surface energy material H, 1H, 2H, 2H-perfluorododecyltrichlorosilane. A variety of techniques such as capillary flow porometry, TEM, SEM, XPS, KI test, liquid entry pressure (LEP) measurement and contact angle goniometry were applied to explore the effects of surface modification on the surface chemistry, structure and performance of the membranes. The anti-fouling performance of virgin and modified membranes were examined in a direct contact membrane distillation (DCMD) process using sodium chloride and humic acid solution as a model feed. Results showed that the modification was mechanically and thermally robust and photoactive. The liquid entry pressure (LEP) and water contact angle were increased from 120 kPa and 1251 to 190 kPa and 1661, respectively. The fluorosilanization of TiO2 nanocomposite PVDF membranes did not compromise the mean pore size. It was also appeared that the TiO2 coating not only contributes in engineering the hierarchical structure but also provides sites (OH functional groups) for the hydrolyzed silane coupling agent to be anchored forming a robust uniform water repellent film. The filtration results also showed that the pure water flux of the modified membrane was lower than that of the virgin membrane particularly at higher temperatures. However, the sodium chloride DCMD test showed that the permeate conductivity of the virgin membrane was increased sharply whereas it was not changed for the modified membrane over the period of the experiment. A 20 h fouling DCMD experiment with humic acid did not show a reduction in flux for virgin and modified membranes. However, a substantial reduction in flux was observed with the addition of 3.775 mM CaCl2 into the solution due to the formation of complexes with humic acid and consequent particles coagulation and precipitation on the membrane surface. Although both virgin and modified membranes showed similar fouling behaviors, a significantly higher flux recovery was found for modified membrane compared to the virgin membrane. & 2012 Elsevier B.V. All rights reserved.

Keywords: Superhydrophobic surface Multi-level hierarchical roughness Titania Surface functionalization Fouling

1. Introduction Recently, a considerable attention has been devoted to the superhydrophobic and superoleophobic surfaces which have a water or oil contact angle greater than 1501 [1]. This remarkable property of extreme liquid wettability has been widely applied in both fundamental research and practical applications [2,3]. Superhydrophobic surface in the nature have prompted many researchers to mimic the micro or nano scale structures of the superhydrophobic biosurfaces [4]. A variety of techniques such as n Corresponding author at: UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia. Tel.: þ 61 2 9385 4328; fax: þ 61 2 9385 5966. E-mail address: [email protected] (J. Mansouri).

0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.06.004

plasma treatment, lithography, sol gel technology, nanoparticle deposition on smooth or roughened substrates, fluoroalkylsilane coatings and phase separation of a multi-component mixture were used to fabricate superhydrophobic surfaces. These routes can generate superhydrophobic surfaces via reducing the surface free energy of a rough surface or roughening a low surface energy material or a combination of both [3,5]. The physicochemical principle of wetting a surface by a liquid has been well established [3,6,7]. When a water droplet rests on a solid surface, three interfacial forces (liquid–vapor surface tension, solid–vapor interfacial tension and solid–liquid interfacial tension) are in a thermodynamic balance [3,8]. These balanced forces determine whether the droplet spreads into a thin film or reaches to an equilibrium shape. The surface wettability of solid is governed by both chemistry and geometrical structure of the

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

surface. Surface chemistry dictates the surface tensions at microscopic level but geometrical structure governs how these forces act upon the liquid [1]. Therefore, surface wettability can be tuned by varying one of these two parameters. Reducing the surface free energy by the functionalization of low surface energy materials particularly fluorosilanes is the most common practice for the generation of superhydrophobic surfaces. Alternatively, attempts have been made to generate a hierarchical nanostructure surface morphology with multilevel surface roughness in order to modulate surface wettability toward extremes [9,10]. Hydrophobic membranes can be used in membrane distillation (MD) for water purification, brine management, heavy metal removal, food industry and purification of pharmaceutical products [11]. The major advantage of these processes is the ability to use low-grade energy (typically at 50–90 1C) to drive the mass transport as the feed does not have to reach boiling point. In membrane distillation (MD), vapor pressure difference, as a result of temperature gradient, transports only volatile molecules from the high temperature feed (typically a saline solution) across the porous hydrophobic membrane to produce purified condensed liquid on the low temperature permeate side. Direct contact MD (DCMD) which condenses the vapor directly into a cold liquid stream is the most common configuration for MD. However, there are other configurations such as air gap membrane distillation (AGMD) where the vapor condenses onto a cold solid surface before being collected, and vacuum enhanced membrane distillation (VMD) where vapor is extracted under vacuum and condensed separately [12]. In practice, sustaining a stable flux and controlling the heat and mass transfer throughout the module is a main obstacle to commercial implementation of MD processes. Sustained operations are compromised by pore wetting, fouling (scaling, biofilm, particulate and colloidal fouling), temperature and concentration polarization at the membrane interface, and heat loss via conduction through the membrane [11]. Pore wetting occurs when the feed liquid partially or entirely penetrates or floods the pores and consequently contaminates the product water and reduces the permeate flux. Increasing the hydrophobicity and decreasing the maximum pore size between 10 nm and 1 mm can reduce the risk of pore wetting. Microporous polymeric membranes such as polyvinylidene fluoride (PVDF), polypropylene (PP), polytetrafluoroethylene (PTFE) and polyethylene (PE) were widely used for MD since they are intrinsically hydrophobic [12]. However, pore wetting may still take place even for these membranes if the membrane surface being in direct contact with a solution containing surface active components or if the transmembrane pressure (TMP) exceeds the liquid entry pressure (LEP) [11]. LEP which is also called pore entry pressure is the point that liquid will penetrate or flood the pores and its magnitude is affected by hydrophobicity, pore size and pore shape [13]. Shifting the hydrophobicity toward superhydrophobicity helps to introduce an air gap between water/oil drop and the surface [14]. This air gap provides an opportunity to increase the allowable pore sizes before pore wetting occurs, consequently allowing higher mass flux [15]. Membrane fouling by organic and inorganic species (scaling) in the feed water reduces the permeate flux and process efficiency by depositing on the membrane surface and plugging the pores. Under extreme cases, foulant build up can reduce feed channels, causing high feed pressure drops. This increases the chance that the LEP is exceeded in parts of the module [11]. To control fouling researchers have tried feed pretreatment, increasing feed flow rate, hydraulic and chemical cleaning, altering the hydrophilicity of the membrane surface, reducing roughness and changing the membrane surface charges [16]. Since these techniques impose additional cost and energy usage, a more efficient way of fouling mitigation is greatly needed. Shifting the wettability of the surface toward superhydrophobicity could reduce the interaction between the feed water

851

solution and membrane surface thereby reducing the fouling propensity. Zhang et al. investigated the anti-biofouling properties of superhydrophobic coating for marine application [17]. They observed no microorganism traces in the first week of immersion whereas for the unmodified substrate, fouling was occurred within a day. However, the superhydrophobic coating showed limited long term antifouling performance in the complicated real sea water environment. Privett et al. synthesized a superhydrophobic xerogel coating with a mixture of nanostructured fluorinated silica colloids, fluoroalkoxysilane, and a backbone silane, which exhibited a significant reduction in the bacterial adhesion of Staphylococcus aureus and Pseudomonas aeruginosa [18]. Ma et al. [15] prepared a narrowed pore size superhydrophobic glass membrane with the integrated arrays of nanospiked microchannels for membrane distillation applications, which reduced fouling during operation and performed significantly better than the polymeric membrane under the same conditions. Similar results were observed by Hendren et al. where alumina AnodiscTM ceramic membranes were made hydrophobic via surface functionalization with different fluorosilane chemicals [13]. The modified ceramic membranes displayed better performance in comparison to its polymeric membrane counterpart tested under similar conditions. Although inorganic membranes can be operated at elevated temperature (500–800 1C for metallic membranes and over 1000 1C for ceramic membranes) and harsh conditions with high resistance to corrosive liquids and gases [19,20], the fabrication of these membranes is expensive and complicated. Thus, they usually are used in those aggressive situations (chemical or thermal) where the polymeric membranes do not perform well. Module design limitations, large footprint due to low surface-to-volume ratio and sealing difficulty at high temperature are the major concerns with the inorganic membranes [20]. As the operating temperature in MD is usually below 90 1C, improving the antifouling performance of the polymeric membranes via superhydrophobic modification at low temperature is attractive for commercial implementation of membrane distillation. Another benefit of the development of a superhydrophobic membrane is the limitation of the heat loss. The heat loss due to conduction tends to decrease the evaporation driving force thus compromising vapor flux. Increasing the membrane thickness reduces the heat conduction. However, a trade-off between heat conduction and vapor diffusion exists, as increasing the thickness may result in a large diffusion resistance of vapor molecules. Since the thermal conductivity of air/gases is an order of magnitude lower than that of polymeric membranes, increasing the porosity and void volume [11] or introducing an air gap by means of superhydrophobic modification could minimize the heat loss by conduction across the membrane. Temperature polarization which causes a difference between the local temperature near membrane surface and bulk temperature of the feed can reduce evaporation driving force for mass transfer up to 80% [11]. If the concentration of the feed increases, concentration polarization along with temperature polarization may further lower the driving force and flux. High shear rates and baffling in the conventional shell and tube configuration could be used to reduce the boundary layer thickness and polarization, but the high feed pressure across the module increases the potential for pore wetting (exceeding the LEP) due to the liquid intrusion into the pores as trans membrane pressure increases [11]. With this regard, superhydrophobic modification may offer another advantage to suppress the polarization since such modification usually raise the LEP [13], thus higher feed pressure and shear rates can be applied to minimize the polarizations. While a number of nanocomposite materials could be used to roughen the surface, TiO2 offers some advantages. Titanium dioxide is one of the most widely used semiconductor with the annual world consumption of above 4.4 million tonnes because it is stable, nontoxic and abundant. Its applications ranged from paints, plastics,

852

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

paper and cosmetic products to photocatalysts, gas sensors and electrical and photochromic devices. These broad applications are based on the nature of TiO2 as a semiconductor material with a very low reflective index. In semiconductors, the energy band gaps are not too large and if the semiconductor adsorbed light or photon with energy equal or greater than the band gap, electrons will jump from valance band (VB) in to the conduction band (CB) and generate holes (electron deficiencies). These pair of electrons and holes (charge carriers) can be recombined by releasing heat. Alternatively, these charge carriers can be presented at the semiconductor surface to carry out photo-oxidation or photo-reduction reactions with chemical species [21] The photocatalytic ability of TiO2 coatings has been applied for the generation of self-cleaning surfaces by turning the TiO2 film into superhydrophilic surface under UV illumination. In our previous work, we have shown that the incorporation of TiO2 nanoparticles without UV illumination into polymeric membranes has a significant contribution in the fouling mitigation in pressure driven membrane processes [21,22]. The surface architecture and chemistry of the TiO2 nanocomposite membranes can be tuned via coating by a low temperature hydrothermal (LTH) process. Hierarchical structure with multilevel roughness generated during TiO2 coating by LTH process changed the wettability of the membrane surface toward extreme (superhydrophilic or superhydrophobic) [21]. Although TiO2 superhydrophobic surface modifications have been recently reported on solid high thermal resistance substrates [10,23], there are a very limited number of reports of superhydrophobic modifications of polymeric membranes using nanoparticles such as TiO2. Sun et al. prepared a superhydrophobic microstructured polyethersulphone (PES) membrane via a surface silica sol treatment followed by fluoroalkyl silanization [24]. The contact angle of the modified membrane was 1541 compared to 751 for the original PES membranes. However, membrane performance and thermo-mechanical modification stabilities were not investigated. The manipulation of wettability of PES and two other different macroporous polymeric membranes were also investigated by Urmenyi et al. [25]. They first coated macroporous membranes by silica via alkaline hydrolysis of tetraethoxysilane. The hydroxyl groups on the deposited silica allowed further functionalization with hydrophilic aminated and hydrophobic (octadecyl or fluorinated) silylating agents. After the fluorination of silicaPES membranes, the contact angle increased from 621 to 1421 with a significantly high LEP of three bars. Although the authors believe that the agents were covalently bonded to the surface, no results regarding the thermo-mechanical stabilities of the silylating agents was reported. In addition, the fouling performance of the modified membranes was not investigated. In this paper, macroporous microfiltration PVDF membranes were modified such that the surface wettability shifts toward superhydrophobicity via increasing the surface roughness by TiO2 coating through the LTH process and subsequently reducing the surface energy by surface fluorosilanization. Surface chemistry, hydrophobicity and surface structure of the modified membranes were examined by XPS, contact angle and SEM techniques, respectively. Stability of the coating was also investigated. The performance of the membranes in direct membrane distillation process before and after modification was investigated using NaCl, humic acid and calcium chloride as model foulants.

1H, 1H, 2H, 2H-perfluorododecyltrichlorosilane (FTCS) were purchased from Sigma–Aldrich. Titanium (IV) iso-propoxide (TTIP) (98%) as TiO2 precursor and 2,4-pentanedione were purchased from Acros Organics. Toluene and absolute ethanol as solvent were supplied by Ajax Finechem. Perchloric acid (70%), sodium chloride (NaCl) and calcium chloride (CaCl2) were purchased from Chem-Supply. 2.2. TiO2 coating on the surface of PVDF membranes 2.2.1. TiO2 coating solution preparation TiO2 precursor sol for coating purpose was prepared by mixing anhydrous ethanol with 2,4-pentanedione, perchloric acid, and titanium (IV) iso-propoxide (TTIP) and ultra-pure Milli-Q water at room temperature to form a stable sol with a pH value of 1.2. A separate solution was prepared by mixing 0.504 g of Pluronic F127 as templating agent in anhydrous ethanol at 50 1C. Both solutions were first stirred for 1 h separately, and then they were mixed and stirred for another hour. The molar ratio of each component in the resulting sol was TTIP:Pluronic F127:2,4pentanedione:HClO4:H2O:ethanol ¼ 1:0.004:0.5:0.5:0.45:4.76. Although the TEM image (Fig. 1) showed that the crystal size of TiO2 is around 10 nm, due to the agglomeration, its average particle size from dynamic light scattering (DLS) in the sol solution was about 52 nm. 2.2.2. Coating of TiO2 nanoparticles onto the PVDF membrane The TiO2 nanoparticles were coated on the surface of PVDF membranes by the low temperature hydrothermal process developed in our previous work [21]. The membranes were dip-coated in the coating solution under the conditions of 0.2 mm/s coating speed and 8 s holding time. The coated membranes were dried in the oven at 120 1C for 1 h. After the heat treatment, the membranes were transferred into a vial filled with Milli-Q water for hot water bath treatment at 90 1C for 2 h. At the end of the 2 h, the membranes were gently rinsed with Milli-Q water and irradiated with UV light for 6 h to decompose any remaining organic residuals. These membranes are labeled as ‘‘TiO2–PVDF’’ membranes.

10 nm 2. Experimental 2.1. Materials Commercial polyvinylidene fluoride (PVDF) flat-sheet membrane HVHP (Millipore, nominal pore size: 0.45 mm, porosity: 75%) was used in this study. Pluronic F127, humic acid (HA), and

Fig. 1. TEM image of TiO2 nanoparticles derived from sol–gel solution.

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

2.3. Fluorination of TiO2–PVDF membrane surface 2.3.1. FTCS solution preparation The toluene was initially purged in a 250 mL container using N2 for 1 h, then it was quenched down to a temperature between 0 1C and  5 1C using an ice bath and/or freezer of below 0 1C to avoid reaching the transition temperature (Tc) of FTCS (0 1C). Keeping the temperature of reaction below transition temperature leads to a highly packed, ordered phase with minimal surface energy [26] whereas reaction above Tc causes a disordered phase [27]. While keeping the container in the ice bath, FTCS (0.5 wt%) was mixed with toluene until it is completely dissolved using a magnetic stirrer. NaCl was added to the ice bath to keep the temperature as low as possible during stirring. The container was also sealed and the experiment was performed in the fume cupboard under a continuous air stream to avoid the absorption of moisture into the solution. 2.3.2. Fluorination of TiO2 coated PVDF membranes by a vacuum filtration The pre-dried TiO2 coated membranes (in ambient condition) were fixed to a Buchner funnel that is attached to a conical flaskvacuum pump. The membranes were initially filtered with minimum volume of toluene for wetting purpose. The sufficient amount of prepared FTCS solution was then filtered through the membranes. A minimum vacuum was applied to ensure the uniform coating on the surface. At the end of the filtration, the membranes were removed and rinsed with 10 mL of fresh toluene to remove any residuals on the surface. The fluorinated membranes were then transferred into a covered petri-dish and were placed in the oven at 120 1C for 2 h. Finally, backwashing with ethanol (30 vol%) was performed for 5 min at a pressure of slightly higher than the LEP of ethanol solution to open the pores plugged by residuals from fluorosilanization step. These membranes are labeled FTCS–TiO2–PVDF membranes. 2.4. Membrane characterization 2.4.1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) Field emission scanning electron microscopy (Hitachi S3400, S900 and FEI Nova Nano SEM 230) was used to observe the surface microstructure of membranes. Dry membrane samples were sputtered under vacuum with a thin layer of chromium to render them conductive. The Philips CM200 field emission gun transmission electron microscope (FETEM) was used to measure the particles crystal sizes. 2.4.2. Contact angle measurement Contact angle measurement is a direct method to characterize the hydrophobicity of a solid surface. The contact angle of the membrane samples were measured using the contact angle goniometer (KSV Cam 200 instrument, Finland) by the sessile drop method. A droplet of water was formed on the flat surface of the membrane using a syringe and the contact angle of which was captured by a high speed camera. The average of at least five measurements was reported. The advancing and receding contact angles were calculated in two independent methods of tilting base and add/remove volume method [4,5]. The difference between the advancing and receding contact angles was also reported as contact angle hysteresis. 2.5. Surface free energy of membrane Acid–base (van Oss) approach was applied to calculate the surface free energy of membrane. In this method, contact angles against at

853

least three liquids with known parameters used in our previous work [21] are measured and the surface free energy of the membrane was calculated from a set of three first order linear equations. 2.5.1. X-ray photoelectron spectroscopy (XPS) XPS was performed using a VG Scientific (UK) surface analysis system with a monochromated Al Kalpha, 1486.6 eV 200 W radiation source and ‘‘Fix Analysis Transmittance’’ (FAT) mode for recording the analysis data. Soft X-rays were irradiated onto the sample to eject electrons from the core energy levels of the atoms present. The low characteristic energies electrons within the top few atomic layers of the surface escaped into the high vacuum spectrometer and were analyzed to generate a spectrum. The detailed peak shapes using curve resolving provide precise chemical specification and the peak energy position provides the elemental analysis. XPS typically has a detection depth of about 5–10 nm. 2.5.2. Liquid entry pressure (LEP) measurement LEP points and their corresponded fluxes of membranes were measured using a dead-end filtration set-up with Milli-Q water. Pressure on the feed side was increased step-wise while allowing it to stabilize for a couple of minutes after each increment. The pressure at which the first drop of permeate was obtained was the LEP point and the corresponding permeation rate under this pressure was considered as the flux at LEP point. 2.5.3. Capillary flow porometry A Capillary Flow Porometer (model no: CFP-1200-AEXL) from PMI Porous Materials Inc. was used to conduct the ‘‘bubble-point’’ test in order to measure the mean flow pore size and bubble pressure point. Wet-up/dry-down mode was used for the porometry test on flat sheet membranes with the diameter of 22.25 mm. In order to wet the samples, one side of the membranes was filled with GalWick liquid with surface tension of 15.9 mN/m prior to the measurements. Each experiment was repeated at least three times and the average was reported. 2.5.4. Membrane performance in a direct contact membrane distillation (DCMD) process The MD performance of virgin and FTCS–TiO2–PVDF flat sheet membranes with the approximate contact area of 14.4 cm2 was examined in a direct contact membrane distillation (DCMD) setup (Fig. 2). A horizontal configuration and counter current flow was implemented to achieve the highest temperature driving force through the module. Table 1 shows the operating parameters used during the MD experiments. Two synthetic feed solutions used in this experiment were sodium chloride solution (3.5 wt%) to model seawater and humic acid solution to model natural organic foulant. In the membrane distillation process, the concentration of humic acid for the fouling experiment is varied between 20 mg/L to 100 mg/L. In this study high concentration of humic acid, 150 mg/L with and without 3.775 mM CaCl2, was used to potentially accelerate the fouling process.

3. Results and discussions 3.1. Evidence of TiO2 and FTCS on the membrane surface The presence of TiO2 nanoparticles and FTCS molecule was examined by XPS analysis (Fig. 3). Ti was detected for membrane coated with TiO2 and its amount was decreased after coating with FTCS as was expected. Presence of N in the XPS spectra of FTCS– TiO2–PVDF membrane might be due to the leaching out of PVP and its migration to the surface by chemicals used in the sol [21].

854

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

Fig. 2. Schematic diagram of the labscale direct contact membrane distillation setup.

Table 2 Contact angles for virgin and modified PVDF membranes with TiO2 and/or FTCS deposition.

Table 1 Operating parameters for DCMD experiments. Parameters

Foulant

Feed inlet temperaturea Permeate inlet temperaturea Feed inlet Pressure Permeate inlet pressure Feed flow rateb Permeate flow ratec Feed volume Reynolds number at feed sideb Reynolds number at permeate sidec

Membrane

Sodium chloride (3.5 wt%)

Humic acid (150 mg/L with/out CaCl2 3.775 mM)

70 1C 25 1C 20 kPa 20 kPa 720 mL/min 700 mL/min 1.3 L 2727 1191

70 1C 25 1C 5 kPa 15 kPa 300 mL/min 460 mL/min 2L 1136 783

Contac angle (deg.) Water

Glycerol

Contact angle hysteresis (deg.) 30% (vol) mono-ethanol amine (MEA)

PVDF (virgin) 1251 7 11 991 7 61 1151 7 31 TiO2–PVDF 981 7 131 1011 7 51 821 7 161 FTCS–PVDF 1461 7 51 1401 7 101 1371 7 51 FTCS–TiO2–PVDF 1631 7 31 1661 7 11 1501 7 31

a The difference between feed inlet and outlet temperature was approximately 3 1C while the difference between permeate outlet and inlet was 1.5 1C. b Data were measured at 70 1C. c Data were measured at 25 1C.

291 471 171 21

7 7 7 7

31 91 81 11

The increase in the percentage of O may be an indication of the increase in the OH functional group from TiO2 coating. The significant increase of weight percentage of F and the presence of Si on the membrane coated with TiO2 and FTCS confirmed the presence of FTCS on the membrane. 3.2. Assessment of hydrophobicity

FTCS-Titania-PVDF

Titania-PVDF C O N Si Ti F

Virgin PVDF

0

20

40

60

At.% Fig. 3. Surface composition (wt%) of virgin PVDF, TiO2–PVDF and FTCS–TiO2–PVDF membranes.

The contact angle (pure water) of the membrane was increased from 1251 7 11 for virgin PVDF membranes to 1631 7 31 for FTCS–TiO2–PVDF membranes, which indicates that the FTCS– TiO2–PVDF membrane is superhydrophobic (Table 2 and Fig. 4a). An increase in water contact angle was observed for FTCS–PVDF membranes due to the significant reduction in surface free energy (Table 3) and increase in roughness (Fig. 7). From Table 2, a sharp decrease in contact angle hysteresis was observed from 291 for virgin membrane to 21 for FTCS–PVDF membranes. However, the hydrophilicity and contact angle hysteresis increased significantly after coating with TiO2 nanoparticles compared with the virgin membrane due to the increase in roughness as it was shown in our previous work [21]. Water droplets on superhydrophobic surface with low contact angle hysteresis are almost spherical and can be easily rolled off and clean the surface (self-cleaning property) at low tilt angle. As can be seen in Fig. 4b, unlike FTCS–TiO2–PVDF membrane, the virgin and TiO2 coated PVDF membranes did not show self-cleaning property even at high tilt angle. For FTCS–PVDF

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

Before

After

TiO2 – PVDF

PVDF (virgin) t3

t2

t1

855

t4

FTCS – PVDF t3

t2

t1

t4

TiO2 – FTCS – PVDF Fig. 4. Effect of super hydrophobic modifications on the PVDF membranes (a) before and after modification (white: MQ water, brown: humic acid solution, pink, and blue and green: buffer solutions with pH of 4, 7 and 14), and (b) imparting self-cleaning property during modification steps. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

Table 3 Surface free energy, bubble point pressure, liquid entry pressure values and fluxes for virgin and modified PVDF membranes with TiO2 and/or FTCS deposition. Membrane

Surface free energy (mN/m)

Bubble point pressure (kPa)

Liquid (water) entry pressure (LEP) (kPa)

Flux (L/m2 h) at LEPnPure water

PVDF(virgin) TiO2–PVDF FTCS–PVDF FTCS–TiO2–PVDF

14 7 0.4 13 7 1.6 0.57 0.14 0.17 0.01

55 54 55 55

120 90 130 190

450 7 10 210 7 15 40 7 8 30 7 3

membranes, as soon as the droplet released from the syringe (time ¼ t2), it formed spherical shape but the droplet did not roll off at low tilt angle to show self-cleaning property. It should point out here that the droplet rolled off at high tilt angle for this membrane. The FTCS–TiO2–PVDF membrane showed superhydrophobicity behavior not only for pure water but also for other solutions with different pH values and for humic acid solution (Fig. 4a). It is reported that the addition of capillary-inactive substances such as

salts into pure water increases its surface tension and causes a negative adsorption in the surface layer [28]. On the other hand, the addition of surfactant such as humic acid or Bovine Serum Albumin (BSA) could reduce the surface tension of the solution up to a certain level. Further increase in the concentration of humic acid will not change the surface tension of the solution due to the formation of micelles [29]. Contact angle measurements were carried out using different concentrations of modeled fouling solutions of sodium chloride, humic acid and BSA. Contact angle measurements in Fig. 5 show that the hydrophobicity of both virgin and FTCS–TiO2–PVDF membrane did not reduce regardless of the type of solutions. Similar results were observed for BSA (150 7 4.91 and 154 7 71 for 1 and 5 g/L BSA, respectively). As mentioned earlier, liquid surface tension is one of the three interfacial forces that may influence the wettability of the surface [3,8]. In addition to that, morphology plays an important role in the wetting action of a solid [1]. Therefore, the negative effect of humic acid or BSA on the hydrophobicity of the membrane surface could be insignificant in spite of reducing the surface tension of the feed solution. Increasing the concentration of sodium chloride increases the surface tension of water, while increasing the concentration of humic acid decreases the surface tension of water. Thus, similar superhydrophobicity towards liquids of different surface tensions had proven that the high contact angle was mainly due to the

856

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

180

Modified

Control

160

Contact Angle (°°)

140 120 100 80 60 40 20 0 Ultra-pure water

10 g/L NaCl 35 g/L NaCl 70 g/L NaCl

25 mg/L Humic Acid

50 mg/L Humic Acid

100 mg/L Humic Acid

Fig. 5. Contact angles of virgin and modified (FTCS–TiO2–PVDF) membrane using different concentrations of NaCl and humic acid.

change in the surface chemistry of the membrane and not due to the surface tension of liquid. The potential application of using superhydrophobic modified membrane (FTCS–TiO2–PVDF) for CO2 and H2S removal from flue gas was examined by measuring the 30% (vol) mono-ethanol amine (MEA) contact angles on membrane surfaces. MEA is used as an absorption liquid in membrane contactors where the membrane acts as an interface between the feed gas and MEA [30]. As can be seen in Table 2, the MEA contact angle was increased from 1151 for virgin membrane to 1501 for FTCS–TiO2– PVDF membranes. From the table, the FTCS–TiO2–PVDF membranes also showed superoleophobic behavior to relatively high surface tension oils such as glycerol; the glycerol contact angle was increased from 991 to 1661. This demonstrates another importance of this modification route as the superoleophobic coatings can be used in a wide range of applications such as biofouling and stain resistant materials, fluid power systems and also can be used for elastomeric seals and O-rings to prevent swelling [9,31] in organic environments. In addition, this modification technique is easy to implement and requires moderate temperate whereas the current techniques to create superoleophobic surfaces are much more tedious and costly [31].

3.2.1. Thermal and mechanical stability The stability of the coating layers plays an important role in maintaining the performance of membrane. In direct contact membrane distillation (DCMD) applications, FTCS–TiO2–PVDF membranes are exposed to different temperatures and cross-flow velocities which could detach the loosely attached particles. To evaluate the mechanical stability of the coatings, membranes were immersed in Milli-Q water and were sonicated at room temperature for 15 min and the contact angle was measured. The contact angle of the FTCS–TiO2–PVDF membrane was 1601 7 11. Comparison with the initial contact angle (1631 7 31), indicated that the coating was mechanically stable. Thermal stability of the coated membranes was analyzed by immersing the membrane in Milli-Q water at 90 1C for 15 min to assess whether the membrane can still maintain its superhydrophobicity. The contact angle of membrane after the exposure was 1561 7 21 which shows a marginal decrease but still remains in the superhydrophobic range. An increase in temperature reduces the surface tension of water as a result of reduction in the cohesive forces and enhancement in the molecular thermal activity and consequently surface hydrophobicity [32].

3.2.2. Surface morphology The effect of different modification steps on the structure of the membranes in macro scale are presented in Fig. 6. As can be seen in Fig. 6b, the deposition of TiO2 on the PVDF membrane did not significantly change the surface structure, porosity and pore size of the membrane in macro scale as there is a very thin layer of TiO2 on the surface [21]. However, the FTCS modification (Fig. 6c) without TiO2 coating resulted in a loose cake layer of fluorosilane which can be detached from the surface by backwashing or applying high cross flow velocity. Fig. 6d shows the surface structure of FTCS–TiO2–PVDF membranes after back washing. From these images, the overall structure and average pore size of the FTCS–TiO2–PVDF membrane is similar to the virgin membrane (Fig. 6a) although there are some loosely attached large residues on the surface. Despite the little detectable change in surface microstructure of the membrane at macro scale after superhydrophobic modification, its surface structure and pore walls were altered dramatically at nano scale as presented in Fig. 7. The images showed that after modification there is a hierarchical structure with multilevel roughness which plays an important role in shifting the wettability toward superhydrophobicity [9,10]. Fig. 7b shows that the FTCS– TiO2–PVDF membrane surface composed of clusters which demonstrates hierarchical submicron with dual level roughnesses. The size of clusters is approximately around 85 nm in length and 57 nm in width and formed from smaller nano-particles or clusters of diameter around 20 nm. 3.2.2.1. Mechanism of superhydrophobic modification. When a water droplet is placed on the surface, it can follow the contours and roughness of the surface as explained by Wenzel model [33]. According to this model, the degree of roughness should be to amplify the wettability of the surface toward its intrinsic tendency of either roll-up or film formation of the liquid. In other words, if the surface is intrinsically hydrophilic with water contact angle of less than 901, roughening the surface makes it superhydrophilic but if it is hydrophobic with water contact angle greater than 901 roughening turns it into superhydrophobic surface. The Wenzel equation describes the effect of surface chemistry and morphology on the water droplet contact angle cosy ¼ r cosy

e

ð1Þ e

where y is apparent contact angle and y is the contact angle on the flat surface, and the roughness factor ‘‘r’’ is the ratio of the actual

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

857

Fig. 6. SEM images of (a) virgin, (b) TiO2dip coated, (c) FTCSvacuum filtered and (d) TiO2dip coated and FTCSvacuum filtered PVDF membrane after backwashing at 5 K magnification.

solid/liquid contact area to its vertical projection. According to Wenzel, morphology lies within r whilst the surface chemistry represents by ye. Since the PVDF membranes were coated by FTCS, it can be considered as a new surface which is saturated by fluorinated methyl groups. The maximum contact angle that can be reached on a flat solid surface saturated by CF3 groups is 1201 which corresponds to a surface free energy of 6.7 mJ/m2 [34]. Taking into account the apparent contact angle of fluorinated TiO2–PVDF membrane as 1631 and 1201 for flat surface of FTCS saturated by CF3 groups, the r-value will be 1.91. Using the methodology developed by Xiu et al. [35] and by assuming the clusters as a series of cylinders (85 nm in length and 57 nm in diameter) aligned in a horizontal orientation (top left corner of inset image in Fig. 7b), the calculated roughness factor ‘‘r’’ would be 1.57 (p/2) if water contacts entire top half of each cylinder. Therefore, the apparent contact angle would be around 1401 which cannot be considered as superhydrophobic surface. These deviations from experimental values may come from the fact that the clusters are aligned randomly and the modified surface is more complex than the models with simple flat-topped surface protrusions. It was reported that the complex topography is often more effective at generating superhydrophobic surfaces [3]. In addition to surface complexity, the Wenzel assumption can be breached if the liquid finds it difficult to penetrate into the roughness or texture of the surface due to capillary forces as it is explained by Cassie–Baxter model [36]. As a result, liquid energetically favors to bridge across the top of surface protrusions such that the water droplets sit

upon the patchwork of solids and air gaps and does not follow the surface contours. The Cassie–Baxter equation for solid–liquid–air composite interface (porous) considers the effect of air gaps under the droplets e

cosy ¼ f s ðcosy þ1Þ1

ð2Þ

In Eq. 2 if the solid surface fraction (fs) decreases, the apparent contact angle y increases and in limit approaches 1801 when fs approaches zero. That means increase in roughness reduces the area of contact between the liquid and solid and results in a reduction in the adhesion of a droplet to the solid surface. 3.2.2.2. The effect of multilevel roughness on hydrophobicity. A multilevel roughness (micro and nano) is required to achieve superhydrophobic surfaces. As can be seen in Fig. 6a, the unmodified PVDF membranes consists of pores in microscale with water contact angle of 1251. Therefore, microscale roughness is not sufficient enough to gain superhydrophobicity. However, after surface modifications shown in Fig. 7b, the hierarchical structure with micro and nano roughness leads to a superhydrophobic surface with water contact angle of 1631. Such high water contact angle and wetting resistance can be ascribed to a dramatic increase in the number of narrow and sharp nano-size protrusion in contact with liquid droplet. According to the Cassie–Baxter model, the sharp reduction in solid surface fraction ðlimf s-1 cos1 ðyÞ ¼ 1801Þ resulted in a substantial increase in air gaps which shifts the surface wettability toward superhydrophobicity [10,37].

858

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

on the surface, the fluorosilanization did not significantly changed the mean pore size of the membranes which were firstly coated by TiO2 (Fig. 8). It should be noted that a similar bubble point pressure was obtained for all of the membranes (Table 3). On the other hand, superhydrophobic modifications resulted in a drastic reduction in pure water flux of PVDF membrane at LEP point from 450 L/m2 h for virgin membrane to 30 L/m2 h for FTCS–TiO2–PVDF membranes (Table 3). 3.2.4. Effect of modifications on the LEP As aforementioned, one of the critical membrane characteristics in MD is liquid entry pressure (LEP) above which the pore wetting occurs and results in the contamination of permeate and a significant reduction in the flux [11]. According to Laplace (Cantor) equation, the LEP is directly proportional to the liquid surface tension (g), cosines of liquid–solid contact angle (y) and geometric factor (B) and is reversely proportional to the largest pore radius (rmax) as below [12]: Pliquid Pvapor ¼

2gB cosðyÞ

gmax

o LEP

ð3Þ

Increasing the Reynolds number can reduce the temperature and concentration polarization but increases the chance of exceeding the LEP. Liquid surface tension (g) increases by temperature and reduces at a higher salt concentration. The geometric factor (B) (one for cylindrical pores and less than one for non-cylindrical pores [38]) and rmax are membrane characteristics which can be altered to improve MD performance [13]. As can be seen in Table 3, the superhydrophobic modifications raised the LEP values from 12 kPa for virgin membrane to 19 kPa for FTCS– TiO2–PVDF membranes. This is most likely due to the change in surface chemistry (g cos y) rather than geometry. From the bubble point test results, the maximum pore diameter was around 0.8 mm for virgin and FTCS–TiO2–PVDF membranes. The geometric factor (B ¼ rmax  bubble point pressure/4g) [38] also remained approximately unchanged (0.35). It should be pointed out here that Eq. 3 is based on a series of assumptions that may result in substantial deviation from the observation. Franken et al. suggested that the LEP must be calculated experimentally [39] rather than using Laplace equation, and according to Lawson and Lloyd the equation should only be used as an idealized model to interpret the experimental results [12]. From Table 3, the LEP value for TiO2–PVDF membranes was reduced significantly to 90 kPa as TiO2 deposition reduces the hydrophobicity of the membranes [21,22,40]. An increase of 101 in water contact angle was observed for FTCS–PVDF membranes due to the reduction in surface free energy after fluorosilanization. Fig. 7. Effect of superhydrophobic modification on the PVDF membranes (a) virgin PVDF, (b) FTCS–TiO2–PVDF membrane (insets at top right corner of (a) and (b) are the SEM images at high resolution and at top left corner of (b) is the schematic side view of clusters assumed in a series of cylinders aligned horizontally.

3.2.3. Effect of modifications on the pore size and permeability One of the concerns of any membrane modification is the possibility of pore plugging or constriction [13]. To verify that the modifications have not substantially altered the membrane porosity and pore size, a bubble point test was carried out and the results (shown in Fig. 8) revealed that the mean pore sizes were not changed, the original porosity remained intact after dipcoating of the TiO2 nanoparticles as the TiO2 layer is very thin [21]. This can also be seen by comparing the SEM images presented in Fig. 6a and b. However, a 15% reduction in mean pore size was observed after fluorosilanization of the surface in the absence of TiO2 coated layer (FTCS–PVDF membrane). Although the SEM image in Fig. 6d showed some large residues

3.2.5. Role of TiO2 in the surface modification As noted before, a hierarchical structure with multilevel roughness is necessary to approach extremes of either superhydrophobicity or superhydrophilicity. In our previous paper, we have shown that a hierarchical morphology can be generated by the deposition of TiO2 nanoparticles using templating agents and a low temperature hydrothermal coating process [21]. In addition to engineering the surface architecture, the deposition of TiO2 nanostructure might also provide sites for fluorosilanization via covalent bonding between hydrolyzed silane coupling agents and hydroxyl groups (OH) available on the surface of TiO2 coated layer. The increase of OH functional group was confirmed by XPS results (Fig. 3) which showed the increased level of O1s on the TiO2 coated membrane. The coating mechanism of selfassembled monolayer (SAMs) of organosilane coupling agents is based on a series of hydrolysis and condensation reactions and affinity with the wet surface [41]. The proposed mechanism of the

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

859

Mean Flow Pore DIiameter (Microns)

0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 PVDF

TiO2–PVDF

FTCS–PVDF

FTCS–TiO2–PVDF

Fig. 8. Mean pore diameter (mm) for virgin and modified membranes.

fluorosilanization on the surface of PVDF membranes with and without TiO2 was summarized in Fig. 9. FTCS molecule consists of hydrophilic trichlorosilane anchor and fluorinated hydrophobic carbon chain. As illustrated in Fig. 9a, in the presence of water, the hydrophilic trichlorosilane head can hydrolyze to form hydrosilanes or trisilanols. Brownian motion helps the long-chain silanes to have an in-plane lateral mobility which leads to an in-plane reorganization and the formation of densely packed monolayer of vertical chains [26]. In the next stage, the hydroxyl groups in the trisilanol heads can form hydrogen bond to the surface hydroxyl groups of TiO2 and eventually the covalent bonds of Si–O–Ti can take place (Fig. 9b) [41,42]. Under the proper conditions such as favorable distance and orientation, the intermolecular cross-linking between the tri-silanol anchors can also occur which leads to a 2D network of polysiloxane (Fig. 9c) [26]. These lateral interactions of hydroxysilanes and interactions to the surface of TiO2–PVDF membranes may lead to a dense, robust and water repellent layer. Post heat treatment at 1201 could lead to a significant reduction in the density of surface silanol groups (Si–OH) in the favor of Si–O–Ti bonds [43]. Due to the silica sol–gel chemistry [44] and in the absence of Ti–OH groups, the hydrolyzed FTCS molecules can condense and form a network and agglomerations of Si–O–Si in the solution, which limits the formation of covalent bond to the substrate (Fig. 9d). The cloudiness of the coupling agent solution with time could be due to the formation of these agglomerations which can be formed from micelles or other extended structures such as lamellar, hexagonal, and cubic phases [45]. As can be seen in Fig. 9e, the fluorosilanization of PVDF membranes in the absence of TiO2 nanoparticles coating resulted in an extended toothed walls structure which are physically absorbed and can be easily detached from the PVDF substrate.

3.2.6. Durability, stability and photo-catalytic activity of TiO2 PVDF membrane Iodide oxidation test [21] was used in order to investigate the photo-catalytic activity of TiO2–PVDF membrane as its activity has some specialized practical applications in membrane technology. The TiO2 coated PVDF membrane was immersed in 30 mL vial filled with 0.1 M potassium iodide (KI) solution which then irradiated by UV lamp (T10 backlight blue lamp, 20 W) for 6 h. The KI test was also used to examine the durability of the TiO2

OH

CL (n)

CL+3H2O

Si

(n)

OH + 3HcL

Si

OH

CL

H2O

OH O

(n)

(n)

(n)

OH Si

H 2O H 2O

OH Si

H 2O (n)

Si

(n)

Si OH

OH

Ti O

O

OH OH OH O O

O PVDF

H 2O

(n)

Si

Ti

Ti

Ti

O

OH

O

O

Ti O

O

O Ti

(n)

(n)

Si

Si

Si

H 2O

H2O (n)

Si O

O O

O

OH

Ti Ti O O O PVDF

H2O

(n)

Si

(n)

Si

OH OH O OH O OH OH PVDF

Fig. 9. Proposed scheme for the silanizaton of the PVDF membranes (a) hydrolyzation of FTCS, (b) interaction with the surface of TiO2 (c) inplane reticulation, (d) condensation of trisilanols in the soultion in the absence of TiO2 coating, and (e) surface SEM image of FTCS–PVDF membranes.

nanoparticles coated on the PVDF membranes. If the coated membrane is photo-catalytically active, it can oxidize iodide (I  ) to iodine (I2) and changes the KI solution from colorless to yellow. The detailed mechanism of the oxidation process and the effect of UV light on PVDF substrate were discussed in our earlier papers [21,46]. Although the observed color change is

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

qualitatively an indication of photo activity, the peaks of these species were detected by UV–vis absorption spectroscopy at 288 nm and 351 nm for I2 and I3 , respectively, to quantitatively investigate the photoactivity of the surface. The KI test can be repeated during different cycles of UV irradiations to investigate the durability of the photocatalytic activity of TiO2 coated membranes. The virgin membrane did not show the color change and peaks were not observed after 6 h of UV irradiation whereas the yellow color and peaks were observed for TiO2 coated membranes. As presented in Fig. 10, the absorbance at the end of each cycle remained relatively unchanged for both I2 and I3 , which shows good stability and durability of TiO2 coating as well as its photo-catalytic activity. Photo-activity of the TiO2–PVDF membranes also facilitates the re-application of FTCS coating due to the possibility of generation of OH groups at the membrane surface by UV radiation.

100

Flux (L.m-2.hr-1)

860

PVDF Membrane

90

FTCS+TiO2+PVDF membrane

80

Expon. (PVDF Membrane ) Expon. (FTCS+TiO2+PVDF membrane)

70 60 50 40 30 20 10 0 50

60

70

80

100

90

Temperature (°°C) Fig. 11. Effect of feed inlet temperature on permeate flux of both virgin (PVDF) and modified (FTCS–TiO2–PVDF) membrane.

3.3. Membrane performance

1.8 351 nm 288 nm

1.6 Absorbance (nm)

1.4 1.2 1 0.8 0.6 0.4 0.2 0 1

2

3

4

Cycle Fig. 10. Absorbance of the KI solution at 351 nm and 288 nm after exposure of the TiO2–PVDF membrane to four cycles of UV irradiation for 6 h.

25 20 Flux (L.m-2.hr-1)

3.3.1. Effect of feed temperature and pressure on the flux Prior to running the membrane performance test using DCMD set-up, a preliminary analysis was carried out to determine the effect of feed inlet temperatures and pressures on flux. There is a general consensus that permeate flux increases with temperature as the vapor pressure generated from temperature difference between the feed and permeate rapidly increases with temperature [11]. As can be seen in Fig. 11, this behavior was observed for both virgin and FTCS–TiO2–PVDF membranes, which is in a good agreement with the theoretical and experimental results published by Schofield et al. [47] and Banat and Simandl [48] who used similar PVDF (0.4 mm) flat sheet membrane. Results furthermore demonstrated that the FTCS–TiO2–PVDF membrane has a lower average permeate flux. The increase in feed pressure resulted in an increase in feed flow which effect on flux was investigated in Fig. 12. The figure shows that the permeate flux increases to asymptotic level with feed pressure for the DCMD configuration in agreement with other studies [16,48]. The increase in the flow rate may switch the flow into the turbulent flow regime which reduces the thickness of the thermal boundary layer on the membrane surface. This causes the temperature at the membrane surface to be closer to the bulk feed temperature, resulting in the increase of vapor pressure difference across the membrane, thus increasing the permeate flux [11]. However, the increase in feed inlet pressure may also result in exceeding the liquid entry pressure. Therefore, there is a trade-off between

15 10 5 0 45

55

65 Pressure (kPa)

75

Fig. 12. Effect of feed inlet pressure on the permeate flux.

the high flow rate and permeate quality. In these experiments a low feed inlet pressure was applied during the membrane fouling experiment. To determine the flow regime, Reynolds number (Re) was calculated using the following formula: Re ¼

rvD 4rQ ¼ m pmD

ð4Þ

where r is the density of feed solution (i.e. water), Q is the flow rate (mL/s), m is the dynamic water viscosity which reduces by temperature, and D is the hydraulic diameter of the cross flow channel. The Reynolds number values for feed and permeate sides are presented in Table 1. 3.3.2. Effect of superhydrophobic modification on wetting resistance The wetting resistance of membranes were investigated by the injecting of ethanol (15 wt%) to the feed during the pure water direct contact membrane distillation process. In order to see the effect of wetting, the permeate pressure was set at a higher pressure (15 kPa) than the feed (5 kPa). As can be seen in Fig. 13, after the injection of ethanol, the flux of virgin membrane decreased sharply and became negative. This means water flows from permeate to feed side and hence the predominant driving force was pressure difference rather than temperature gradient. The sharp reduction and the negative flux were not observed for FTCS–TiO2–PVDF membranes, which is an indication of a higher wetting resistance for this membrane. However, a gradual reduction in flux was still observed, which is due to a continuous increase of ethanol content on the permeate side where did not have modification. In addition, the water vapor molecules and

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

Ethanol injected (15 wt.%)

30

Flux (L.m-2.hr-1)

20 10 0 0

2

4

-10

6

8

Virgin PVDF membrane FTCS-TiO2-PVDF membrane

-20 -30 -40

Time (hr)

Fig. 13. Pore wetting resistance behavior of virgin (PVDF) and modified (FTCS– TiO2–PVDF) membranes.

45

700

40

600 500

30

400

25 20 15

PVDF (Flux) FTCS-TiO2-PVDF (Flux)

300

PVDF (Conductivity)

200

Conductivity (µ µS)

Flux (L.m-2.h-1)

35

FTCS-TiO2-PVDF (Conductivity)

10

100

5 0

0 0

2

4

6

8

Time (h) Fig. 14. Flux and permeate conductivity vs. time for virgin (control PVDF) and modified (FTCS–TiO2–PVDF) membrane with 3.5 wt% NaCl; bulk feed temperature 70 1C; bulk permeate temperature 25 1C; feed inlet pressure 20 kPa; Re ¼ 2727.

ethanol as a volatile organic compound were competing with each other to pass through the membrane pores, which could reduce the flux [11]. 3.3.3. Direct contact membrane distillation The first membrane distillation experiment was performed using 1.3 L of 3.5 wt% sodium chloride solution (L: 58.6 mS). At the end of 7 h, 367 g of permeate was collected using virgin membrane, while 323 g was collected using FTCS–TiO2–PVDF membrane. The difference in the quantity of permeate obtained indicates that the FTCS–TiO2–PVDF membrane is more hydrophobic. Although the FTCS–TiO2–PVDF membrane did not show enhancement in permeability (Fig. 14), unlike FTCS–TiO2–PVDF membranes, the conductivity of permeate for virgin membrane significantly increased after 4 h through the experiment (Fig. 14). This could be due to the partial pore wetting which facilitates salt transportation; such behavior was also observed elsewhere [11,48,49]. Another contributory factor is the relatively higher LEP for FTCS–TiO2–PVDF membrane compared to the virgin membrane. The increase in conductivity due to local transmembrane pressure exceeding of the LEP point was not observed throughout the duration of experiment. The second membrane distillation experiment was performed using 2 L of humic acid solution with and without CaCl2. Fig. 15 shows the flux reduction (J/J0) for virgin and FTCS–TiO2–PVDF membranes as a function of

time for 150 mg/L humic acid solution with and without 3.775 mM CaCl2 at PH of 7. From the figure, no flux reduction was observed in the absence of salt throughout 20 h of operation. The experiment was continued for 97 h and the flux reduction was not detected. The addition of divalent cation, Ca2 þ to the HA solution resulted in a significant reduction in normalized flux. For the virgin membrane the reduction reached 0.52, and for FTCS– TiO2–PVDF membranes the flux reduction reached to 0.48. The negatively charged carboxyl functional groups of humic acids bind together by Ca2 þ cations to form bigger molecules and causes coagulation and particle precipitation [50]. Charge screening could also enhance the fouling [51]. The flux reduction in membrane distillation could be due to both effects of first blocking the pore entrances and reducing the available surface area for vaporization and second reducing the heat transfer driving force [11,51]. The fouled membranes were cleaned by 15 min of recirculation of NaOH solution (0.2 wt%, pH 12) at 25 1C on feed side and then the pure water flux was measured to calculate the flux recovery. The flux recovery of FTCS–TiO2–PVDF membranes was 94% whereas it was 59% for virgin membranes. As can be seen in Fig. 16, there are less brownish precipitate of HA on the FTCS–TiO2–PVDF membrane after chemical cleaning compared to the virgin membrane, indicating the modification increases the antifouling performance of the membrane. It should point out here that a more efficient cleaning method such as longer recirculation time or backwashing may increase the flux recovery of the virgin membrane and remove the precipitates as shown by Srisurichan et al. [51]. From the inset images in Fig. 16, the superhydrophobicity of the FTCS–TiO2–PVDF membrane has been preserved over the course of 20 h operation, indicating that modification was robust. However, from industrial point of view, further long-term experiments are essential to assess the distillation performance of the FTCS–TiO2–PVDF superhydrophobic membranes. Since the superhydrophobic modification introduced in this study was achieved at low temperature, it is potentially feasible to recover the superhydrophobicity of the surface by filtering the FTCS solution and heating the membrane module at moderate temperature without the need of dismantling the module.

4. Conclusion A superhydrophobic PVDF membrane with water contact angle of 1631 7 31 was successfully prepared by both generating 1.2

Flux Recovery after chemical cleaning: PVDF membrane = 59% FTCS–TiO2–PVDF Membrane = 94%

1 0.8 J/J0

40

861

0.6 0.4 PVDF membrane (150 mg/L HA and 3.77 mM CaCl2) FTCS-TiO2-PVDF membrane (150 mg/L HA and 3.77 mM CaCl2) PVDF membrane (150 mg/L HA)

0.2 0 0

5

10

15

20

Time (hr) Fig. 15. Flux reduction (instantaneous flux J over the average purewater flux J0) vs. time for virgin PVDF and FTCS–TiO2–PVDF membrane with 150 mg/L humic acid with/out 3.775 CaCl2 at pH of 7; bulk feed temperature 70 1C; bulk permeate temperature 25 1C; feed flow.

862

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

FTCS–TiO2–PVDF one. In the experiment of DCMD using 150 mg/L humic acid solution, the virgin membrane showed insignificant flux reduction toward the end of experiment. However, the addition of Ca2 þ to the HA solution, resulted in a significant flux decline for both virgin and FTCS–TiO2–PVDF membranes due to the coagulation of HA molecules. A significant increase in flux recovery after chemical cleaning for FTCS–TiO2–PVDF membrane was observed, which is an indication of enhancement in antifouling property of membranes.

Acknowledgments The authors acknowledge funding from the Australian Research Council. The authors also thank Dr. Shane Cox for help with the Labview program and Siriluk Matuam for assistance with the fluorosilanization optimization. References

Fig. 16. Virgin PVDF (a) and FTCS–TiO2–PVDF (b) membranes fouled by 150 mg/L HA and 3.775 mM CaCl2 solution for 20 h and cleaned by 0.2 wt% NaOH solution for 15 min recirculation (insets are water droplets after fouling and cleaning).

multilevel roughness and reducing the surface free energy of the membranes via TiO2 coating by a low temperature hydrothermal (LTH) process followed by fluorosilanization of the surface with 1H, 1H, 2H, 2H-perfluorododecyltrichlorosilane (FTCS). The modifications led to the resultant membranes with not only superhydrophobic behavior but also superoleophobicity to glycerol. The FTCS–TiO2–PVDF membranes have also shown good thermal and mechanical resistance which is essential for the membrane application in membrane distillation. The liquid entry pressure (LEP) of water increased significantly from 120 kPa to 190 kPa without compromising the mean pore size due to the change in the surface chemistry of the membrane. The TiO2 coating provided hierarchical structures and also OH functional groups to ensure the uniform functionalization of FTCS. The coating of FTCS on the rough membrane allows its hydrophilic end to be hydrolyzed onto the TiO2 coated membrane and exposing its hydrophobic fluorinated carbon chain. The contact angle measurements showed that membrane still maintained its superhydrophobicity towards high concentrations of model fouling solutions. The FTCS–TiO2–PVDF membrane was also relatively superhydrophobic towards different concentrations of both sodium chloride and humic acid solutions. Membrane performance in direct contact membrane distillation (DCMD) using 3.5 wt% sodium chloride solution did not show a significant improvement in flux and anti-fouling property. However, unlike FTCS–TiO2–PVDF membranes, the permeate conductivity of the virgin membrane increased significantly due to the partial pores wetting, which provided pathways for sodium chloride to permeate through the membrane. Pores wetting was expected to occur with the virgin membrane since it has lower LEP point than the

[1] G. McHale, N.J. Shirtcliffe, M.I. Newton, Super-hydrophobic and super-wetting surfaces: analytical potential? Analyst. 129 (2004) 284–287. [2] P. Roach, N.J. Shirtcliffe, M.I. Newton, Progess in superhydrophobic surface development, Soft Matter 4 (2008) 224–240. [3] N.J. Shirtcliffe, G. McHale, S. Atherton, M.I. Newton, An introduction to superhydrophobicity, Adv. Colloid Interface Sci. 161 (2010) 124–138. [4] C. Dorrer, J. Ruhe, Some thoughts on superhydrophobic wetting, Soft Matter 5 (2009) 51–61. [5] X. Zhang, F. Shi, J. Niu, Y. Jiang, Z. Wang, Superhydrophobic surfaces: from structural control to functional application, J. Mater. Chem. 18 (2008) 621–633. [6] D. Bonn, D. Ross, Wetting transitions, Rep. Prog. Phys. 64 (2001) 1085. [7] B. Drops, W. Pearls, P.-G.d. Gennes, F. Brochard-Wyart, D. Quere, Capillarity and Wetting Phenomena, Springer, Berlin, 2004. [8] G. McHale, N.J. Shirtcliffe, M.I. Newton, Super-hydrophobic and super-wetting surfaces: analytical potential? Anal. 129 (2004) 284–287. [9] A. Steele, I. Bayer, E. Loth, Inherently superoleophobic nanocomposite coatings by spray atomization, Nano Lett. 9 (2008) 501–505. [10] H. Kim, K. Noh, C. Choi, J. Khamwannah, D. Villwock, S. Jin, Extreme superomniphobicity of multiwalled 8 nm TiO2 nanotubes, Langmuir 27 (2011) 10191–10196. [11] M.S. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29. [12] K.W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [13] Z.D. Hendren, J. Brant, M.R. Wiesner, Surface modification of nanostructured ceramic membranes for direct contact membrane distillation, J. Membr. Sci. 331 (2009) 1–10. [14] A. Lafuma, D. Quere, Superhydrophobic states, Nat. Mater. 2 (2003) 457–460. [15] Z. Ma, Y. Hong, L. Ma, M. Su, Superhydrophobic membranes with ordered arrays of nanospiked microchannels for water desalination, Langmuir 25 (2009) 5446–5450. [16] A.M. Alklaibi, N. Lior, Membrane-distillation desalination: status and potential, Desalination 171 (2005) 111–131. [17] H. Zhang, R. Lamb, J. Lewis, Engineering nanoscale roughness on hydrophobic surface—preliminary assessment of fouling behaviour, Sci. Technol. Adv. Mater. 6 (2005) 236. [18] B.J. Privett, J. Youn, S.A. Hong, J. Lee, J. Han, J.H. Shin, M.H. Schoenfisch, Antibacterial fluorinated silica colloid superhydrophobic surfaces, Langmuir 27 (2011) 9597–9601. [19] K. Nath, Membrane Separation Processes, Prentice Hall of India Private Limited, New Delhi, 2008. [20] A. Buekenhoudt, A. Kovalevsky, J. Luyten, F. Snijkers, Basic aspects in inorganic membrane preparation, in: G.L. Enrico (Ed.), Comprehensive Membrane Science and Engineering, Elsevier, Oxford, 2010, pp. 217–252. [21] A. Razmjou, J. Mansouri, V. Chen, M. Lim, R. Amal, Titania nanocomposite polyethersulfone ultrafiltration membranes fabricated using a low temperature hydrothermal coating process, J. Membr. Sci. 380 (2011) 98–113. [22] A. Razmjou, J. Mansouri, V. Chen, The effects of mechanical and chemical modification of TiO2 nanoparticles on the surface chemistry, structure and fouling performance of PES ultrafiltration membranes, J. Membr. Sci. 378 (2011) 73–84. [23] A. Borras, A. Barranco, A.n.R. Gonza!lez-Elipe, Reversible superhydrophobic to superhydrophilic conversion of Ag TiO2 composite nanofiber surfaces, Langmuir 24 (2008) 8021–8026. [24] G.F.L.Xu Dong Sun, Yu Zhong Zhang, Yu Xiang Li, Hong Li, Preparation of super-hydrophobic polyethersulphone membrane by sol–gel method, Adv. Mater. Res. 79-82 (2009) 839.

A. Razmjou et al. / Journal of Membrane Science 415–416 (2012) 850–863

[25] A.M. Urmenyi, A.P. Philipse, R.G.H. Lammertink, M. Wessling, Polymer-in-asilica-crust membranes: macroporous materials with tunable surface functionality, Langmuir 22 (2006) 5459–5468. [26] J.B. Brzoska, I.B. Azouz, F. Rondelez, Silanization of solid substrates: a step toward reproducibility, Langmuir 10 (1994) 4367–4373. [27] R.R. Rye, Transition temperatures for n-alkyltrichlorosilane monolayers, Langmuir 13 (1997) 2588–2590. [28] G. Jones, W.A. Ray, The surface tension of solutions of electrolytes as a function of the concentration. I. A differential method for measuring relative surface tension, J. Am. Chem. Soc. 59 (1937) 187–198. [29] T.F. Guetzloff, J.A. Rice, Does humic acid form a micelle? Sci. Total Environ. 152 (1994) 31–35. [30] K. Simons, K. Nijmeijer, M. Wessling, Gas–liquid membrane contactors for CO2 removal, J. Membr. Sci. 340 (2009) 214–220. [31] A. Tuteja, W. Choi, M. Ma, J.M. Mabry, S.A. Mazzella, G.C. Rutledge, G.H. McKinley, R.E. Cohen, Designing superoleophobic surfaces, Science 318 (2007) 1618–1622. [32] G. G.J, Variation of surface tension of water with temperature, J. Colloid Interface Sci. 30 (1969) 406–412. [33] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988–994. [34] T. Nishino, M. Meguro, K. Nakamae, M. Matsushita, Y. Ueda, The lowest surface free energy based on  CF3 alignment, Langmuir 15 (1999) 4321–4323. [35] Y. Xiu, L. Zhu, D.W. Hess, C.P. Wong, Biomimetic creation of hierarchical surface structures by combining colloidal self-assembly and Au sputter deposition, Langmuir 22 (2006) 9676–9681. [36] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551. [37] N.J. Shirtcliffe, G. McHale, M.I. Newton, G. Chabrol, C.C. Perry, Dual-scale roughness produces unusually water-repellent surfaces, Adv. Mater. 16 (2004) 1929–1932. [38] K. Smolders, A.C.M. Franken, Terminology for membrane distillation, Desalination 72 (1989) 249–262.

863

[39] A.C.M. Franken, J.A.M. Nolten, M.H.V. Mulder, D. Bargeman, C.A. Smolders, Wetting criteria for the applicability of membrane distillation, J. Membr. Sci. 33 (1987) 315–328. [40] A. Razmjou, A. Resosudarmo, R.L. Holmes, H. Li, J. Mansouri, V. Chen, The effect of modified TiO2 nanoparticles on the polyethersulfone ultrafiltration hollow fiber membranes, Desalination 287 (2012) 271–280. [41] A.Y. Fadeev, T.J. McCarthy, A New, Route to covalently attached monolayers: reaction of hydridosilanes with titanium and other metal surfaces, J. Am. Chem. Soc. 121 (1999) 12184–12185. [42] F. Milanesi, G. Cappelletti, R. Annunziata, C.L. Bianchi, D. Meroni, S. Ardizzone, Siloxane–TiO2 hybrid nanocomposites. the structure of the hydrophobic layer, J. Phys. Chem. C 114 (2010) 8287–8293. [43] D.L. Angst, G.W. Simmons, Moisture absorption characteristics of organosiloxane self-assembled monolayers, Langmuir 7 (1991) 2236–2242. [44] J. Livage, M. Henry, C. Sanchez, Sol–gel chemistry of transition metal oxides, Prog. Solid State Chem. 18 (1988) 259–341. [45] B.C. Bunker, R.W. Carpick, R.A. Assink, M.L. Thomas, M.G. Hankins, J.A. Voigt, D. Sipola, M.P. de Boer, G.L. Gulley, The impact of solution agglomeration on the deposition of self-assembled monolayers, Langmuir 16 (2000) 7742–7751. [46] M. Lim, M. Ng, A. Razmjou, J. Mansouri, V. Chen, R. Amal, Low Temperature Synthesis of Titanium Dioxide Coatings on PVDF Membranes, Chemeca, Adelaide, Australia, 2010. [47] R.W. Schofield, A.G. Fane, C.J.D. Fell, R. Macoun, Factors affecting flux in membrane distillation, Desalination 77 (1990) 279–294. [48] F.A. Banat, J. Simandl, Theoretical and experimental study in membrane distillation, Desalination 95 (1994) 39–52. [49] G. Marek, Long-term performance of membrane distillation process, J. Membr. Sci. 265 (2005) 153–159. [50] W. Yuan, A.L. Zydney, Humic acid fouling during microfiltration, J. Membr. Sci. 157 (1999) 1–12. [51] S. Srisurichan, R. Jiraratananon, A.G. Fane, Humic acid fouling in the membrane distillation process, Desalination 174 (2005) 63–72.