Excellent wetting resistance and anti-fouling performance of PVDF membrane modified with superhydrophobic papillae-like surfaces

Author’s Accepted Manuscript Excellent wetting resistance and anti-fouling performance of PVDF membrane modified with superhydrophobic papillae-like surfaces Ziyi Wang, Yuanyuan Tang, Baoan Li www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(17)30939-0 http://dx.doi.org/10.1016/j.memsci.2017.06.073 MEMSCI15385

To appear in: Journal of Membrane Science Received date: 3 April 2017 Revised date: 26 June 2017 Accepted date: 26 June 2017 Cite this article as: Ziyi Wang, Yuanyuan Tang and Baoan Li, Excellent wetting resistance and anti-fouling performance of PVDF membrane modified with superhydrophobic papillae-like surfaces, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.06.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Excellent wetting resistance and anti-fouling performance of PVDF membrane modified with superhydrophobic papillae-like surfaces

Ziyi Wanga,d, Yuanyuan Tanga*, Baoan Lib,c* a

School of Environmental Science and Engineering, Southern University of Science and Technology, Shenzhen 518055,

China b

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin

300354, China c

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300354, China

d

School of Chemical Engineering and Technology, Nankai University, Tianjin 300072, China

[email protected] [email protected] *

Corresponding authors.

Abstract With hydrophobicity as a basic requirement for membrane used in membrane distillation (MD) process, the membrane wetting and fouling can be effectively alleviated with superhydrophobic surface. Since the applications of existing methods were usually restricted by certain limitations, it is highly essential to develop a facile and effective approach to fabricate a superhydrophobic surface for MD process. In this study, a one-step method was employed for superhydrophobic coating with papillae-like structure on polyvinylidene fluoride (PVDF) membrane surface. The variations in surface morphology and water contact angle (WCA) were explicated as functions of PVDF and polybasic alcohol mixture (PG) contents in coating solution, the immersion temperature and time. The optimal modification was conducted in a 45 °C coating solution for 35 s with 2 wt% of PVDF and 30 wt% of PG, resulting in a superhydrophobic surface with a maximum WCA of 156.8°. Moreover, the performance of the modified membrane was compared with the original membrane in MD process with different feeds. The modified membrane demonstrated the excellent wetting and fouling resistance after superhydrophobic modification. Through the efficient one-step fabrication for superhydrophobic papillae-like surfaces, this study proposed a promising strategy to improve the performance of PVDF membrane in the MD process.

Graphical abstract

Keywords membrane modification; superhydrophobic; micro/nano-papillae structure; wetting resistance; anti-fouling performance

1. Introduction Membrane distillation (MD) is a thermally driven membrane separation process, in which only vapor can pass through the porous hydrophobic membrane while liquid is intercepted [1]. The hydrophobicity is a basic requirement for MD process that can prevent the liquid from passing through the membrane pores in the industrial applications. However, the application of hydrophobic membrane is usually limited by its disadvantages of pore wetting and surface fouling. If the hydrophobic surface of the membrane is modified to be superhydrophobic on the feed side, the pore wetting and fouling of the membrane can be effectively alleviated [2, 3]. With water contact angle (WCA) greater than 150° and a sliding angle (SA) less than 10°, the surface can be identified as superhydrophobic [4, 5]. Therefore, the development of superhydrophobic membrane surface is essential [6] and thus has gathered increased attention in both fundamental research and industrial applications of membrane [7, 8]. Studies on superhydrophobic surface have been emphasized on processes such as oil/water separation [9, 10], photo responsive behavior [11, 12], medical diagnosis and therapy [13], and targeted drug delivery [14].

Currently, most hydrophobic microporous membranes employed in MD process are commercial microfiltration membranes, which are usually prepared from the hydrophobic materials such as polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). Among them, PVDF has received widespread attention due to its excellent chemical stability, mechanical performance, pliability, as well as wide processing temperature. Existing technologies to fabricate superhydrophobic surface include sol-gel method [15-18], plasma treatment [19, 20], chemical etching [21-23], electrospinning [24-26], template [27-29], layer-by-layer self-assembly [30-32], chemical vapour deposition (CVD) [33, 34], etc. Technologies above were adopted by many researchers to fabricate the superhydrophobic modified PVDF membrane for MD process. Razmjou et al. [35] deposited TiO2 nanoparticles on microporous PVDF membranes by CVD to generate superhydrophobic surface with a WCA of 166° for MD process, resulting in a significant improvement of the flux recovery when comparing with the original membrane. Liao et al. [36] successfully fabricated superhydrophobic PVDF nanofiber membranes with high WCA above 150° by electrospinning method, which achieved high and stable MD flux. Another type of superhydrophobic membrane for MD process was generated via spraying a mixture of polydimethylsiloxane and hydrophobic SiO2 nanoparticles on PVDF flat sheet membranes [37]. Yang et al. [38] developed a high performance superhydrophobic PVDF flat sheet membrane with a WCA of 162.4° for direct contact membrane distillation (DCMD) by CF4 plasma surface modification, and the water flux of the superhydrophobic membrane was about 30 % higher than that of the original membrane. Although superhydrophobic membranes with good performance were successfully fabricated by the technologies above, the application of the aforementioned methods was mostly restricted due to the existence of certain limitations. Besides long processing time and high cost of raw materials, large concentration of pores has always been generated due to great shrinkage of membrane associated with the gelation and drying process when using sol-gel method [39]. While in plasma process, the economic efficiency should be taken into consideration due to its high consumption of energy source and automotive control of various operational parameters [40]. The etching technique is potentially harmful for the environment, not even to mention the long etching time and high cost [41]. And the electrospinning process has always been susceptible to the fluctuations in voltage, humidity and the conductivity of the collection target [42]. Besides long duration and cumbersome prefabrication process of the template method, chemical medium are inevitably needed to remove the template and cause further contamination [43]. Many substrates are not thermally stable at high reaction temperature during CVD, and chemical precursors are always required to keep high vapor pressure, making the CVD a costly and environmentally hazardous operation [44].

Therefore, it is highly essential to develop a facile and effective approach for the fabrication of superhydrophobic surface for MD process. In this study, a novel and simple solvent-nonsolvent method was introduced to prepare superhydrophobic surface with micro/nano papillae structure for PVDF hollow fiber membrane. Parameters such as morphology, WCA, porosity, mean pore size and pore size distribution were tested and analyzed to optimize the desirable modification condition. Subsequently, the performances of the modified membrane were extensively investigated and compared with the original membrane in permeability, wetting resistance and fouling resistance. With excellent wetting and fouling resistance, this study effectively presented a one-step method to fabricate superhydrophobic papillae-like surfaces for MD process. 2. Materials and methods 2.1 Superhydrophobic modification of PVDF membranes A desired amount of PVDF powder (FR-904, Shanghai 3F New Material Co. Ltd., China) was dissolved in N, N-dimethylacetamide (DMAC, AR, Tianjin Jiangtian Chemical Technology Co. Ltd., China), and stirred for 30 minutes at room temperature. Then, a certain content of Propylene glycol (PG) was added into the solution and stirred until a homogeneous solution was formed. Two sides of the PVDF hollow fiber membranes (already fabricated in the lab) were sealed by epoxy resin, and then immersed in the coating solution with immersion time ranging from 5 s to 60 s. After that, the wet membranes were moved into a water bath immediately for coating solidification. After being washed by deionized water for several times, the solidified membranes were dried in the air for further testing. Detailed of different modification conditions were summarized in Tab.S1. 2.2 Membrane characterization After being freeze-fractured in liquid nitrogen and sputter coated with gold (Hitachi, E1020), the outer surface of membranes was observed by a field-emitting Scanning Electron Microscope (SEM, Hitachi, S-4800). The WCA and SA were measured at room temperature by the sessile drop method via a contact angle instrument (Dataphysics OCA15EC, Germany) with a tilting table. The adopted water droplet volume is 5 μL. The tilting angle of the table is adjustable within the range of 0° to 60°, and allows for subsequent measurement of the SA. The roughness of the membrane was tested under tapping mode by an Atomic Force Microscopes (MFP-3D-Stand Alone, Asylum Research, USA), and was denoted as Ra (the arithmetical mean deviation of the profile). For the measurement of WCA, SA, and Ra above, three replicates were randomly selected on the membrane surface. The average value of each item was illustrated on the corresponding SEM image of the tested membrane surface and further summarized in Table S1 of the

Supporting Information. Bubble point method was employed to measure porosity, mean pore size and pore size distribution of the membrane. The membrane was first immersed in the buffer tank filled with anhydrous alcohol, and then taken out to be weighted immediately after removing anhydrous alcohol on the outer surface. 2.3 Performance test of the original and the modified membrane The fundamental permeability of the fabricated membrane was evaluated in MD process with the experimental apparatus as illustrated in Fig.S2. The PVDF hollow fiber membranes were first assembled and encapsulated into a module. A hot sodium chloride solution (3.5 wt%, NaCl, AR, Tianjin Jiangtian Chemical Technology Co. Ltd., China) was employed as feed and reserved in a thermostatic tank. The water vapor was evaporated from feed side and transferred across the membrane into the permeation side. The permeated water vapor was condensed by a cooler and then flowed into the distillate reservoir through the lumen of the membranes with a magnetic pump. In order to evaluate the permeability at different temperatures, the hot feed (at temperature ranging from 50 ± 1 °C to 90 ± 1 °C) and cold distillate (at temperature of 25 ± 1 °C) flowed across the outside and inside of the membranes at the same speed in the experimental process. The permeation amount was measured by an electrical balance and recorded in a computer afterwards. The MD flux of the membranes was calculated based on the inner surface area of membranes by the following equation:

F

M d l t

Eq.(1)

where F is the permeate flux (kg·m-2·h-1), ΔM is the quantity of distillate (kg), d is the diameter of the membranes (m), l is the length of the membranes (m) and t is the system running time (h). To further estimate the long-term performance of both original and modified membranes, the MD process was extended to 240 h at the feed inlet temperature of 70 °C. The wetting resistance of both original and modified membranes was evaluated by two approaches. On the one hand, these two membranes were immersed in 70 °C hot water, the WCA of membranes were measured every 2 h to monitor the wetting conditions of the membrane surfaces within 24 h. On the other hand, in a 70 °C MD process taking water as feed, anhydrous alcohol was added 4 h after the beginning of the experiment and then the system was running continuously for 20 h. The MD flux of the original and modified membranes were monitored every hour according to Eq.(1). The organic resistance experiment was carried out with feed of 150 mg·L-1 humic acid (AR, Tianjin Jiangtian Chemical Technology Co. Ltd., China) and 3.775 mmol·L-1

calcium chloride (CaCl2, AR, Tianjin Jiangtian Chemical Technology Co. Ltd., China). The MD flux of the original and modified membranes was detected hourly at a continuous running for 24 h. Furthermore, the inorganic resistance measurement was performed using mixed solutions of 4.3 wt% NaCl, 0.1 wt% CaCl2 and 0.1 wt% magnesium chloride (MgCl2, AR, Tianjin Jiangtian Chemical Technology Co. Ltd., China) as feed. The MD flux of two membranes was also measured every hour in a 24 h continuously running process. After the inorganic resistance experiment, the surface morphologies of two membranes were observed by SEM testing. Then the membranes were cleaned by immersion in the stirring pure water for 30 min at the room temperature to wash off the foulants. Afterwards, the permeability and surface morphologies of two membranes were characterized by MD process and SEM observation, respectively. 3. Results and discussion 3.1 The mechanisms for the modification of membrane surfaces

Fig.1 The formation process of micro/nano-papillae

Fig.2 The surface morphologies of the original membrane (a) and the modified membrane (b). A much rougher surface was observed after the modification, suggesting that the surface morphology of the membrane can be obviously changed by this process. Figure 1 shows a schematic diagram illustrating the preparation of coating solution and the processes for membrane surface modification. A desired amount of PVDF was dissolved in DMAC and the polymer molecular chain was dispersed homogeneously in the solution. Then, a certain concentration of PG was added into the solution to induce the change of PVDF molecular chain to unimers because of the hydrogen bonding among molecules [45]. Under the driving force of free energy difference, the generated unimers were proposed to aggregate into micelles at critical micelle concentration (CMC), which is defined as the point where there is a sharp increase in the number of PVDF molecules associate into micelles [46]. When the two sides sealed hollow fiber membranes were immersed in the coating solution, the micelles-contained solution was found to adhere on the outer surface of the membrane. The obtained wet membrane was then moved into a coagulating bath (water) for further solidification. With mass transfer between the coating solution and water, the micelles adhered on the membrane surface relaxed and rearranged gradually during this solidification process, achieving the formation of papillae in micro-and nano-scale. Figure 2 shows a much rougher surface of the modified membrane (Fig.2b) when compared with the original one (Fig.2a), suggesting that the surface morphology of the membrane can be obviously changed by this modification process. According to the aforementioned procedure, it is proposed that the final morphology of the modified membrane surface can be controlled by PG content, PVDF polymer concentration in the coating solution, immersion temperature and time. Therefore, the effect of modification parameters above should be further discussed in the following sessions. 3.2 The effect of fabrication and modification conditions on the characteristics of the membranes

Since WCA has been reported as the main index to characterize the hydrophobicity of membrane surface with the aid of Ra and SA [47-49], their values for each membrane have been considered to extensively study the effect of fabrication and modification conditions. Moreover, a positive relationship between WCA and Ra has been proposed by Wenzel [48] and Cassie and Baxter [49], and widely accepted by researchers followed. Besides that, SA has also been stated to have a predictable relationship with WCA [50]. Therefore, although the variations of WCA, SA, and Ra were extensively described, an intensive discussion was specially carried out on the surface hydrophobicity and WCA during a variety of fabrication and modification processes. 3.2.1 The effect of PVDF concentration

Fig.3 Effect of PVDF concentration ranging from 1 wt% to 6 wt% on surface morphologies, WCA, SA and Ra of membranes at the fixed PG content of 20 wt%, immersion temperature of 45 °C and immersion time of 30 s. The WCA and Ra of the membrane increased first and then decreased with the continuous addition of PVDF in the coating solution, while the SA showed a contrary tendency. The WCA reached its maximum of 142.2° at the PVDF concentration of 2 wt%. Figure 3 illustrated the surface morphologies of the modified membranes with a variety of PVDF concentrations. As the important indexes to evaluate the hydrophobicity of a material, the value of WCA, Ra and SA of each membrane was measured and the results were shown in Fig.3. The WCA of the original membrane is measured as 89.7, while a significant increase to 139.8° was found after the surface modification of the membranes with PVDF concentrations of 1 wt%. Furthermore, the WCA first grew up with the increase of PVDF concentration until reaching the maximum value of 142.2°, and then

decreased eventually with further adding PVDF in the coating solution. Similarly, the Ra also increased until reaching the maximum value of 8.2 m with PVDF concentration of 3 wt%, and then decreased afterwards. However, a contrary tendency was observed for the SA value with continuous addition of PVDF in the coating solution. It has been proven that the PVDF molecules dispersed separately as unimers at relatively low concentration because of the addition of PG nonsolvent in coating solution [51]. When the coated membranes were immersed in water, the subsequent diffusion of PG and DMAC into water led to a rougher coating on the membrane surface (Fig.3) compared with the original one (Fig.2a) [52]. Therefore, when water was dropped on the surface of modified membrane, the contact area between the surface and water will be greatly minimized due to the existence of air trapped in the grooves below water and a higher WCA was achieved [53]. The formation of micelle from unimers aggregation only occurs in coating solutions with PVDF molecules reaching CMC, under the driving force of free energy difference between unimers and the as formed micelle [54]. During the immersion of the coated membranes, the polymer chains rearranged and crystallized into the interconnected micro/nano-papillae structure on the membrane surface [55]. In this experiment, the maximum WCA of 142.2° (Fig.3b) was obtained when the PVDF concentration reached CMC. However, any further increase of PVDF concentration after CMC reached made the transformation of PVDF polymer from dispersed phase to continuous phase, resulting in the relatively smoother surfaces and the corresponding decrease of the WCAs value (Fig.3c-Fig.3f) after the solidification. Therefore, the surface structure of the membrane, which is directly related with its further performance during MD process, is significantly influenced by the concentration of PVDF in the coating solution. 3.2.2 The effect of PG content in coating solution

Fig.4 Effect of PG content ranging from 10 wt% to 60 wt% on the surface morphologies, WCA, Ra and SA of membranes at a fixed PVDF concentration of 2 wt% and immersion temperature of 45 °C for 30 s. The WCA and Ra declined after reaching their maximum value at the PG content of 30 wt%, while the SA showed an opposite tendency. PG is a kind of low molecular weight nonsolvent, and has been reported to have high swelling capacity for PVDF [56]. The addition of PG nonsolvent in coating solution resulted in the generation of unimers by the intertwining of PVDF molecular chains and their consecutive aggregation into micelles. Figure 4 demonstrates the surface morphologies, WCA, SA, and Ra of the modified membranes with different contents of PG added in the coating solution. Both WCA and Ra increased with the addition of PG content in the coating solution and reached their maximum values of 146.9° and 7.4 µm, respectively, when the coating solution contains 30 wt% of PG. However, an opposite tendency was found for the value of SA which decreased firstly and then increased with the continuous addition of PG. As mentioned in Section 3.1, the driving force for micellization is caused by the free energy difference between micelles and the dissociated unimers in the coating solution. Therefore, the thermodynamic force for micelles formation varied a lot with different PG contents in the coating solution. With an initial PG content of 10 wt%, the micelles existed as semi-gelified state in the coating solution, resulting in gelatinous and swollen structure of the solidified surface (Fig.4a). An increase in PG content to 30wt% enhanced the intertwining of PVDF molecular chains in coating solution, and the micelles were solidified into the regular interconnected micro/nano-papillae coating on the membrane surface with WCA of 146.9° (Fig.4c). With further addition of PG content, the

gelation of the coating solution generated a much smoother gelatinous structure as shown in Figs.4d-4f, resulting in decreasing roughness and WCA of membrane surface. Therefore, a moderate PG content (30 wt%) improved the aggregation of unimers to micelles and achieved the successful formation of micro/nano-papillae surface. 3.2.3 The effect of immersion temperature

Fig.5 Effect of immersion temperature ranging from 30 °C to 55 °C on surface morphologies, WCA, SA, and Ra of membranes at a fixed PG content of 30 wt%, PVDF concentration of 2 wt% and immersion time of 30 s. The WCA and Ra first increased and then decreased with the elevated immersion temperature, while the SA showed a contrary tendency. The interconnected micro/nano-papillae surface and the maximum WCA of 150.3° were obtained when the immersion temperature is 45 °C. Figure 5 displays the surface morphologies, WCA, SA, and Ra of the modified membranes at different immersion temperatures. From Fig. 5, a similar tendency was found for the WCA and Ra, which increased firstly with elevated immersion temperature and then decreased after reaching the maximum value of 150.3° and 11.3 µm, respectively. But the SA exhibited the contrary tendency when compared with the variations of WCA and Ra with elevated immersion temperature. The interconnected micro/nano-papillae surface and the maximum WCA of 150.3° were obtained at the immersion temperature of 45 °C (Fig.5d). Critical micellization temperature (CMT) is identified as the point polymers aggregation occurs and leads to the formation of intermolecular micelles, which is considered as a valuable factor that determines the micellization process. When the temperature is below

CMT, the PVDF molecules chains dispersed in solution as unimers [57-59]. It is well understood that there exists hydrogen bonding between PVDF and PG molecules. The increment of temperature enhanced molecular thermodynamic movement and induced the cleavage of hydrogen bonds between PVDF and PG. Therefore, the polymer molecules intertwined with each other and the micelles are formed [60]. However, the continuous increase in solution temperature caused the dissolution of membrane surface during the immersion process. Therefore, the PVDF concentration in the coating solution increased consequently, and a membrane with smoother surface and lower WCA was formed as discussed in Section 3.2.1 (Fig.5e-5f). 3.2.4 The effect of immersion time

Fig.6 Effect of immersion time ranging from 5 s to 60 s on the surface morphologies, WCA, SA, and Ra of membranes at the fixed PG content of 30 wt%, PVDF concentration of 2 wt%, and immersion temperature of 45 °C. The WCA and Ra showed a decline trend after the first increasing, while the SA exhibited an opposite tendency. The interconnected structure is formed and the maximum WCA is 156.8° at the immersion time of 35 s. Figure 6 summarized the surface morphologies, WCA, SA, and Ra of membranes modified within the immersion time ranging from 5 s to 60 s at 45 °C with PG content of 30 wt% and PVDF concentration of 2 wt% in coating solutions. The WCA and Ra of the membrane increased to 156.8° and 14.0 µm respectively at 35 s, and decreased afterwards with the extended immersion time. However, the value of SA decreased firstly to 3.1° at 35 s, and then increased when the immersion time was prolonged. The interconnected

micro/nano-papillae structure was formed and the maximum WCA reached 156.8° when the membrane has been immersed in the coating solution for 35 s (Figs.6g). As shown in Fig.6, the number of micro/nano-papillae was not enough to cover the membrane surface within a short immersion time (Figs.6a-6f). A further increase in papillae number with the extended immersion time improved the roughness and the WCA of membrane surface. However, a further extension of the immersion time caused membrane corrosion, resulting in PVDF dilution from the membrane surface. Therefore, as discussed in Section 3.2, if the PVDF concentration is higher than the CMC, the coated membrane surface will be much smoother with lower WCA (Figs.6h-6l). The optimal immersion time was found to be 35 s as illustrated in Fig. 6g, showing the formation of a regular interconnected structure from the interconnection of micro/nano-papillae and the prevention of further PVDF simultaneous dissolution. To achieve a better visualization of the relationship among WCA, SA and Ra, the data in Table S1 was further plotted in Fig.S3 of the Supporting Information. From the results illustrated in Fig.S3, it can be concluded that the Ra is positively correlated to the WCA, but negatively correlated to the SA. Moreover, Figs.S4-S7 demonstrate that there are no obvious changes in the porosity, mean pore diameter and pore size distribution of membranes along with varied PVDF concentrations and PG contents, except for a slight increase in the porosity and mean pore diameter in Figs.S6f-6h and Figs.7h-7l caused by the surface corrosion at higher temperatures (> 45 °C) or within extended immersion time longer than 35 s. On the basis of these findings, it can be concluded that the modification processes only construct a hydrophobic coating on the surface of the membranes without changing the pore structure. In general, the optimal superhydrophobic coating can be achieved by immersing the membrane in 45 °C coating solutions with PVDF concentration of 2 wt%, PG content of 30 wt% for 35 s. 3.3 Characterization of the original and modified membranes 3.3.1 Comparison in permeability between two membranes

Fig.7 MD flux variation of two membranes with a variety of inlet feed temperatures from 50 to 90 C when using 3.5 wt% NaCl solution as feed in a 24 h operation. Figure7 presents the variation in MD flux of both original and modified membranes with inlet feed temperatures ranging from 50 to 90°C, which shows the increase in MD flux of both membranes with the increase of NaCl solution temperatures. The driving force of MD process is caused by the vapor pressure difference between two sides of the membrane, and the pressure is an exponential function of temperature [61]. Therefore, the permeate flux is exponentially increased with feed temperature. As discussed in Section 3.1, the micro/nano-papillae coating on the membrane surface can be successfully formed as a result of the superhydrophobic modification. With an intensified mass transfer resistance, the MD flux of the modified membrane is slightly lower than that of the original membrane at any feed temperature.

Fig.8 MD flux variation of the two membranes with prolonged time to 240 h at operation temperatures of 50 °C (a), 60 °C (b) and 70 °C (c) when using 3.5 wt% NaCl solution as feed.

The MD process has been broadly utilized for wastewater treatment, resource recovery and other valuable applications at operation temperatures no more than 70 °C [62-64]. Therefore, the long term MD operation was conducted at 50-70 °C within 240 h for both modified and original membranes, and the variations in MD flux were illustrated in Fig. 8. Although a higher MD flux was initially observed for the original membrane at the beginning of MD processes, a much slower decrease rate of the MD flux was observed for the modified membrane with the prolonged time at any operation temperature (50, 60, or 70 °C). As the system proceeded, NaCl in the feed would be saturated or oversaturated, and thus began to crystallize on the outer surface of original membrane due to the effect of concentration polarization. The MD flux of the original membrane then declined gradually with the exacerbated membrane fouling. Since the superhydrophobic coating on the outer surface of modified membrane improved the resistance to membrane fouling, the MD flux decreased slowly in comparison with that of the original membrane, showing more excellent long term running performance of the modified membrane. 3.3.2 Comparison in wetting resistance between two membranes

Fig.9 WCA variation in 70 °C hot water (a) and MD flux variation when using water as feed within the first 4 h and adding anhydrous alcohol into the feed afterwards (b). In MD process, the outer surface of membrane contacted with hot feed directly, so that the superhydrophobicity should be reasonably stable and durable when exposed to harsh environment. Therefore, the wetting resistance of the membrane surface was evaluated by measuring the WCAs of the surfaces every 2 h and the values were revealed in Fig.9a. Because of weak wetting resistance, the WCA of the original membrane declined significantly due to the partial wetting during the operation. In contrast, owing to the presence of air pockets trapped between the droplet and the membrane surface, the liquid tends to be suspended on the tip of micro/nano-papillae and flow off quickly from the surface. As shown in Fig.9a, the WCA of the modified membrane and original membrane

was measured initially as 158° and 89.7°, respectively. The WCA of the modified membrane remained continuously higher than 150° within 24 h while that of the original membrane declined eventually due to the pore wetting by the water feed, which confirmed the steady superhydrophobicity of the modified membrane in case of harsh environment. Fig.9b exhibits the MD flux of the original and modified membranes when using water as feed within the first 4 h and adding anhydrous alcohol into the feed afterwards. A slight decrease in MD flux was observed from 31.7 to 29.2 kg·m-2·h-1 and from 25.1 to 23.6 kg·m-2·h-1 for the original and modified membranes, respectively, within the first 4 h. After adding the anhydrous alcohol to the feed, the original membrane was wetted consequently. Therefore, the liquid in permeate side passed through the membrane pores and went into the feed side, causing a dramatic decline of MD flux to negative value. However, the modified membrane with unique micro/nano-papillae morphologies can significantly increase the presence of air pockets trapped between the droplet and the membrane surface, which may reduce the contact areas between the liquid and membrane surface but increase the contact areas between the liquid and vapor. Therefore, the water droplet tends to be suspended on the membrane surface and can be removed easily, preventing the liquid from penetrating into the surface cavities. Therefore, even after the anhydrous alcohol added into the feed, only a slight decrease of the MD flux was observed for the modified membrane. After the experiment, it was found that the original membrane was partly wetted while the modified one maintained dry as before. In conclusion, the wetting resistance of the modified membrane was improved significantly compared to the original one. 3.4 Comparison in fouling resistance between two membranes

Fig.10 MD flux variation when using organic solution of 150 mg·L-1 humic acid and 3.775 mmol·L-1 CaCl2 as feed (a) and using inorganic solution of 4.3 wt% NaCl, 0.1 wt% CaCl2 and 0.1 wt% MgCl2 as feed (b). Fig.10a shows the MD flux attenuation of the two membranes in the process of treating organic solution of 150 mg·L-1 humic acid and 3.775 mmol·L-1 CaCl2. Almost the same MD flux of 20 kg·m-2·h-1 was obtained after running 24 h even with an initial difference in MD flux, which shows a slower decrease in the MD flux for the modified membrane in comparison with the original one. The negatively charged carboxyl groups of humic acid have been reported to be easily interacted with the positively charged calcium ions in the solution, which leads to the deposition of humic acid on the membrane surface [65]. Therefore, the mass transfer resistance of the membrane was enhanced and thus the MD flux decreased. However, the accumulation and deposition rate of humic acid will become much slower on the superhydrophobic surface of the modified membrane, resulting in a much slower decrease in the MD flux. Fig.10b illustrates the variation in MD flux of the original and modified membranes when using the inorganic solution of 4.3 wt% NaCl, 0.1 wt% CaCl2 and 0.1 wt% MgCl2 as feed. In the process of experiment, the inorganic salts will deposit on the outer surface of the original membrane, leading to a continuous decrease in the MD flux from 29.7 to 20.8 kg·m-2·h-1 after 24 h. However, with a slightly lower initial MD flux comparing to the original membrane, a much slower decrease (25.1 to 24.2 kg·m-2·h-1) was observed for the modified membrane due to less inorganic salts adhered on its superhydrophbic surface. From the above results, both organic and inorganic contaminants were prone to deposit on the surface of the original membrane. However, the micro/nano-papillae on the surface of the modified membrane kept the liquid suspending on the top of the papillae so as to be easily removed, which is supposed to be beneficial for the fouling resistance.

Fig.11 The surface morphologies of the two membranes before treating the inorganic solution of 4.3 wt% NaCl, 0.1 wt% CaCl2 and 0.1 wt% MgCl2 (a), after treating the inorganic solution of 4.3 wt% NaCl, 0.1 wt% CaCl2 and 0.1 wt% MgCl2 (b) and after cleaning by pure water for 30 min (c). Fig.11 displays surface morphologies of the original and modified membranes before and after 24 h MD experiment when taking inorganic solution of 4.3 wt% NaCl, 0.1 wt% CaCl2 and 0.1 wt% MgCl2 as feed, and surface morphologies of both membranes after cleaning. A fouling layer was clearly observed on the surface of the original membrane after 24 h MD process (Fig.11a and Fig.11b) due to the gradual accumulation of inorganic salts on the membrane surface. By contrast, the surface morphology of the modified membrane almost unchanged as shown in Fig.11d and Fig.11e, illustrating the excellent inorganic resistance of the superhydrophobic coating. After 24 h proceeding of the system, the MD flux of the original and modified membrane reduced to 70.1% and 93.9% of their initial flux, respectively. Through cleaning by pure water, the fouling layer can still be observed on the original membrane surface (Fig.11c) but the micro/nano-papillae morphology was maintained on the modified membrane surface (Fig.11f), followed by a restored MD flux to 79.2% and 97.4% of their individual initial flux, respectively. When both original and modified membrane were compared, the relatively flat surface of the original membrane indicates a greater barrier for further cleaning. However, with a much weaker adhesion between the foulants and the micro/nano-papillae, the superhydrophobic coating makes the modified membrane cleaned easily and further benefits the surface restoration [66-68]. Conclusion

In this study, a facile and effective solvent-nonsolvent approach was introduced for PVDF hollow fiber membrane modification, and the superhydrophobic coating was achieved with a micro/nano-papillae structure. The roughness and WCA value of the modified membrane surface increased firstly with the increase of PVDF and PG content in the coating solution, but decreased after reaching the maximum value when 3 wt% of PVDF concentration or 30 wt% of PG content was added in the solution. Moreover, with increasing temperature of the coating solution or prolonged immersion time, there was also a first increase and then decrease trend in the roughness and WCA value of the modified membrane surface. The maximum WCA reached 156.8° when the membrane was modified in a 45 C coating solution with 2 wt% PVDF concentration and 30 wt% PG content for 35 s. Both original and modified membrane were characterized and further compared in terms of MD flux, wetting resistance, organic resistance, inorganic resistance and the stability of hydrophobicity. Results illustrated the outstanding stability of the superhydrophobic coating, and excellent wetting resistance and anti-fouling performance to organic and inorganic feed. Acknowledgments The authors gratefully acknowledge Shenzhen Science and Technology Innovation Committee (Grant No. JCYJ20160429191618506), the Science and Technology Project of Tianjin China (Grant No. 12ZCZDSF02200) and the National Key Technology R&D Program of China (Grant No. 2006BAB03A06). References [1] M. Khayet, Membranes and theoretical modeling of membrane distillation: a review, Adv. Colloid. Interfac. 164 (2011) 56-88. [2] R. F ü rstner, W. Barthlott, Wetting and self-cleaning properties of artificial superhydrophobic surfaces, Langmuir 21 (2005) 956-961. [3] J. Zhang, G. Pu, S.J. Severtson, Fabrication of zinc oxide/polydimethylsiloxane composite surfaces demonstrating oil-fouling-resistant superhydrophobicity, ACS Appl. Mater. Inter. 2 (2010) 2880-2883. [4] A.K. An, J. Guo, E.J. Lee, S. Jeong, Y. Zhao, Z. Wang, T. Leiknes, PDMS/PVDF hybrid electrospun membrane with superhydrophobic property and drop impact dynamics for dyeing wastewater treatment using membrane distillation, J. Membrane Sci. 525 (2017) 57-67. [5] Y. Liao, R. Wang, A.G. Fane, Fabrication of bioinspired composite nanofiber membranes with robust superhydrophobicity for direct contact membrane distillation, Environ. Sci. Technol. 48 (2014) 6335-6341. [6] P. Wang, D. Zhang, Z. Lu, Advantage of super-hydrophobic surface as a barrier against atmospheric corrosion induced by salt deliquescence, Corros. Sci. 90 (2015) 23-32.

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Highlights 

A facile and effective one-step solvent-nonsolvent method was developed.



A superhydrophobic coating was achieved with papillae-like surface on PVDF membrane.



The maximum WCA reached 156.8° at the optimal modification condition.



The modified membrane showed excellent performance on wetting & fouling resistance.