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Janus membranes with asymmetric wettability via a layer-by-layer coating strategy for robust membrane distillation
Journal of Membrane Science 603 (2020) 118031
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Janus membranes with asymmetric wettability via a layer-by-layer coating strategy for robust membrane distillation Meng Li a, b, Kang Jia Lu b, Lianjun Wang a, Xuan Zhang a, **, Tai-Shung Chung b, * a
Key Laboratory of New Membrane Materials, Ministry of Industry and Information Technology, School of Environmental and Biological Engineering, Nanjing University of Science & Technology, Nanjing, 210094, China b Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore
A R T I C L E I N F O
A B S T R A C T
Keywords: Asymmetric wettability Layer-by-layer Wetting/fouling resistance Membrane distillation Thermal consumption
Commercial membranes for membrane distillation experience severe wetting and fouling phenomena, which largely hinder their practical applications. To address these issues, a novel strategy to fabricate the Janus membrane is proposed via a facile layer-by-layer coating of Teflon® AF1600 and polydopamine (PDA) on an oxygen plasma treated commercial polytetrafluoroethylene/polypropylene (PTFE/PP) membrane. The resultant membrane has a unique asymmetric surface wettability. In brief, the top layer of the Janus membrane is strongly hydrophobic (145.1 � 1.8� ), while the bottom layer exhibits underwater superoleophobicity (154.2 � 2.2� ) and in-air hydrophilicity (33.5 � 1.2� ). As a result, the Janus membrane possesses a significantly enhanced wetting resistance toward common liquids including pure water, surfactant, ethanol, and crude oil, compared to the unmodified membrane. Due to the excellent anti-fouling property, the Janus membrane also shows a rather robust desalination performance (i.e., a slight decline in water flux and negligible salt passage) when treating a multi-component saline wastewater containing both humic acid and crude oil, ending up with a high water recovery ratio of 50%. Overall, the Janus membrane developed in this work is highly promising for the water reclamation from highly saline wastewater containing complex organic compounds.
1. Introduction Water reclamation, particularly from high-salinity seawater or/and multi-component wastewater, has become an important issue over the past decades [1,2]. To date, membrane-based technologies offer the most promising pathway for water desalination and wastewater treat ment due to their easy operation and environmentally friendly charac teristics [3]. Nevertheless, it should be mentioned that most pressure-driven processes can hardly be used in the treatment of high ly saline water because the energy consumption significantly increases with the concentration increase of the brines [4]. Compared to conventional pressure-driven processes, membrane distillation (MD) has attracted extensive attentions, as its driving force is mainly associated with the gradient of water vapor pressure across the membrane, rather than the osmotic pressure [5–8]. However, mem brane wetting and fouling phenomena are two critical challenges that hinder the wide applications of MD membranes, especially for treating saline wastewaters containing organic compounds (i.e., surfactants,
crude oil, humic acid (HA), etc.) [6,9–11]. For instance, the surfactants will decrease the membrane wetting resistance by reducing the surface tension of the feed solution, resulting in a severe salt passage and worsened MD performance. Besides, these contaminants (i.e., HA and crude oil) can adhere readily to the surfaces of conventional MD mem branes by hydrophobic-hydrophobic or electrostatic interactions [9,10], causing the undesired blocking or wicking of membrane pores as well. To address the above issues, designing an omniphobic MD membrane seems promising, as the omniphobic membranes were proved to show excellent wetting resistance toward both surfactant-containing water and other low surface tension liquids (e.g. organic solvents) [6,12–15]. However, it should be noted that organic pollutants, such as alginate, HA, oil droplet, etc., could still attach to the membrane surface by the hydrophobic-hydrophobic interactions, resulting in a more severe membrane fouling [9,16,17]. In light of this, the concept of Janus membranes, with an asymmet rical surface wettability, has been proposed in recent years [10,19–29]. An incorporated hydrophilic layer could effectively repel the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhang), [email protected] (T.-S. Chung). https://doi.org/10.1016/j.memsci.2020.118031 Received 28 January 2020; Received in revised form 29 February 2020; Accepted 4 March 2020 Available online 8 March 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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hydrophobic foulants, and thus endows the MD membranes with lower fouling propensities. However, regardless of the extensive fabrication strategies of Janus membranes, e.g. electrospinning/spraying [22], surface coating [23,24], grafting [25,26], electrospinning [27], and vaporization-induced phase inversion [28], the poor interfacial compatibility or weak bonding combination between two layers (hydrophobicity—hydrophilicity) still remains the critical challenge for the membrane stability [10,25]. Additionally, the hydrophilic coating is commonly performed on a conventional hydrophobic substrate directly, which is readily wetted by the feed solution containing low surface tension components (e.g., surfactants) [10,25]. Therefore, having a membrane that possessing both excellent anti-fouling and anti-wetting properties would be paramount for a robust MD operation but still rather challenging. The lack of robust MD membrane materials, especially toward treating highly saline wastewater containing surfactants and various organic compounds, motivated us to develop an MD membrane that can well sustain and steadily operate at harsh environments. Considering the nature of mussel-inspired materials (e.g., dopamine [30,31]) and the rather low surface energy of Teflon® series products [6,14], we have designed and fabricated, for the first time, a Janus membrane with asymmetric wettability by a facile layer-by-layer coating of Teflon® AF1600 and polydopamine (PDA) on an oxygen plasma treated com mercial polytetrafluoroethylene/polypropylene (PTFE/PP) membrane. The Teflon® AF1600 coating was employed to improve the membrane wetting resistance by increasing the surface hydrophobicity while the PDA coating was applied to enhance the fouling resistance to organic matters due to its strong hydrophilicity. Physicochemical characteriza tions of the newly developed Janus membrane were thoroughly char acterized, in terms of morphology, surface properties, wetting and
fouling resistance, and so forth. Specifically, its water reclamation from highly saline wastewater containing organic compounds was systemat ically evaluated. Associated economic concerns, including the thermal efficiency (TE) and specific thermal energy consumption (STEC) were also analyzed in detail. 2. Experimental 2.1. Materials and chemicals Commercial flat-sheet PTFE/PP membranes for MD processes were provided from Sterlitech Corporation. The membrane consisted of two layers; namely, a selective PTFE top layer and a highly porous and rough PP substrate. Its detailed specifications, including pore size, water con tact angle, LEP, and porosity are summarized in Table 1. Dopamine hydrochloride (>98%), tris (hydroxymethyl) aminomethane hydro chloride (Tris-HCl, >99%), sodium dodecyl sulfate (SDS, >99%), so dium chloride (NaCl), and humic acid (HA) were procured from Sigma Aldrich. Ethanol and crude oil were purchased from Arkema Inc. Teflon® AF1600 and Galden HT70 heat transfer fluid were acquired from DuPont Co. Ltd. and APP System Services. All other reagents and solvents were used as received. Deionized (DI) water with a minimum resistance of 18.4 MΩ-cm (Millipore) was used throughout the work. 2.2. Design of the Janus membrane and reference membranes Fig. 1 illustrates the fabrication processes for all kinds of membranes. To fabricate the Janus membrane, a layer-by-layer coating procedure was employed in this work. In brief, the pristine PTFE/PP membrane was first cleaned by ethanol and DI water for 10 min, and then dried in a
Table 1 Physical properties of the pristine PTFE/PP MD membrane. Membrane
Pore Size (μm)
Water Contact Angle (� ) (top layer)
LEPa) (bar) (top layer)
Porosity (%)
PTFE/PP membrane
0.20 � 0.02
128.1 � 1.2
2.10 � 0.32
82.2 � 3.7
a
LEP: liquid entry pressure.
Fig. 1. Schematic illustration of the fabrication processes of different MD membranes. 2
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Fig. 2. (A) Digital pictures of both layers of the Janus and the pristine PTFE/PP membranes; (B) Chemical structures of PTFE and Teflon® AF1600; (C) Possible reaction pathways of PDA from a self-polymerization reaction of DA.
vacuum oven before use. In order to obtain a robust and durable coating, the bottom (PP) layer was treated with oxygen plasma at a power of 200 W for 2 min to further enhance the surface roughness for subsequent coating procedures. The resultant membrane was named as the PTFE/PP (P) membrane. Next, the PTFE/PP (P) membrane was completely immersed in a coating solution consisting of 0.05 wt% Teflon® AF1600 in Galden HT70 solvent only for 20 s, and then dried in air. The resultant membrane was named as PTFE/PP (P, AF1600). Owing to the low boiling temperature of the solvent (i.e., 70 � C), Teflon® AF1600 would quickly precipitate and deposit on the membrane surface. Finally, the Janus membrane was fabricated by coating a hydrophilic layer on the bottom layer of PTFE/PP (P, AF1600) via a mussel-inspired dopamine (DA) self-polymerization, as described elsewhere [20,23,24,29]. Spe cifically, the PTFE/PP (P, AF1600) membrane was fixed in a custom-designed mold with an effective area of 20.0 cm2, and a 10 mM Tris-HCl buffer solution containing 2.0 g L 1 of DA was then poured into the mold, allowing for a complete contact with the bottom layer for 16 h, yielding the target Janus membrane. Digital pictures of Janus mem brane, chemical structures of PTFE and Teflon® AF1600, and possible self-polymerization mechanisms of PDA are provided in Fig. 2. Other membranes were also fabricated as references for comparison. As described in Fig. 1, the Janus-D membrane was prepared by directly coating a PDA hydrophilic layer on the bottom layer of the pristine PTFE/PP membrane with the same conditions mentioned above. While the PTFE/PP (AF1600) membrane was fabricated by directly immersing the pristine PTFE/PP membrane into the Teflon® AF1600 coating so lution (0.05 wt%) for 20 s without the initial plasma treatment. After the modifications, all the membranes were rinsed with DI water for several times, and then dried at 80 � C for 24 h under vacuum for subsequent characterizations.
platinum. For the measurements of cross-section, membrane samples were fractured in liquid nitrogen. Chemical and elemental compositions of membrane surfaces were examined by X-ray photoelectron spectros copy (XPS, PHI Quantera II, Japan). Fourier transform infrared (FTIR) spectra were obtained using an FTIR spectrometer (Bruker Vertex 70, Germany) to qualitatively analyze the variations of functional groups on membrane surfaces after modifications. Additionally, in-air contact an gles (CAs) with different liquids, such as DI water (~72.8 mN m 1), 0.1 mM SDS in 3.5% NaCl solutions (~43 mN m 1), crude oil (~30 mN m 1), and ethanol (~22.1 mN m 1) [6,10,15], and underwater oil contact angles (UOCAs) with crude oil were examined using an optical contact angle measuring instrument (DataPhysics, OCA 25, Germany). At least three measurements were made at different locations on each membrane surface and their averages were recorded with error bars. Other characterizations of MD membranes, including LEP and porosity, were provided in Supporting Information. 2.4. MD performance tests using direct contact membrane distillation (DCMD) All experiments were conducted on a lab-scale DCMD set-up with an effective membrane area of 4.0 cm2 (Fig. S1), and the temperatures for the feed and distillate solutions were fixed at 70 � C and 20 � C, respec tively, indicating a temperature difference of 50 � C. The flow rates of the hot feed and the cold distillate streams were fixed at 0.2 L min 1 and 0.1 L min 1, respectively, to overcome the influence of forward osmosis and ensure the unambiguous detection of wetting [6,10]. Thermometers were inserted at different locations to monitor the temperatures, as shown in Fig. S1. For each test, the entire system would run for at least 1 h for stabilization before collecting the data. Water flux (Jw, L m 2 h 1, measured by the weight change of the distillate solution) and salt rejection (%, measured by the electric conductivity change of the distillate solution) were calculated by the following Eqs. (1) and (2) [5]:
2.3. Membrane characterizations Morphologies of membrane samples were investigated by field emission scanning electron microscopy (FESEM, JEOL JSM-6700LV, Japan). Before imaging, all samples were sputter-coated with
Jw ¼
3
Δm=ρ Am Δt
(1)
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Table 2 Compositions wastewater.
of
the
simulated
the membrane was wetted. In brief, SDS concentrations in feed solutions were fixed at 0.05, 0.1, 0.2, 0.4, and 0.6 mM along with the experiment proceeding. Although the feed concentration exerts minor influences on water flux during the MD process [6], the feed solution was replenished by manually adding DI water periodically, to keep a constant volume and feed concentration. In separate experiments, the feed solutions consisting of two typical contaminants (i.e., HA and crude oil) were selected to evaluate the fouling resistance for all membranes [9,10,18]. Specifically, 500 mL of 3.5% NaCl solutions containing 0.25 g of HA (500 mg L 1) and 0.25 g crude oil (500 mg L 1) were prepared separately and directly used as feed solutions in DCMD tests. To prepare the oil-in-feed solution, a ho mogenizer was employed to disperse the oil in the solution at 10,000 rpm for 5 min. DI water was also manually added to maintain the NaCl and contaminant concentrations, as mentioned above.
multi-component
Constituents
Value
Conductivity (mS cm 1) Ca2þ (mg L 1) Mg2þ (mg L 1) Naþ (mg L 1) 1 NHþ 4 (mg L ) Kþ (mg L 1) Cl (mg L 1) SO24 (mg L 1) SDS (mg L 1) Humic Acid (mg L 1) Crude Oil (mg L 1)
25.7 454 1423 10815.6 74 210 14296 107.8 15 200 50
where Δm represents the weight change of the distillate, ρ refers to the density of the permeate (~1 g cm 3), Am is the effective membrane area, and Δt is the experimental duration. R¼1
ΔðVd Cd Þ=Jw Am Δt Cf
2.4.2. Water reclamation from the simulated multi-component wastewater Table 2 lists the compositions of a model multi-component waste water, including inorganic salts [5,32], organic compounds [33–35], crude oil [36], and surfactants [5,37]. As most the current studies only simulated the feed water containing part of the above components, we therefore reviewed the literatures, estimated the approximate range for each constituent and prepared the simulated wastewater (500 mL) comprising all the representative constituents. In water reclamation tests, the Janus membrane was employed in the DCMD operation with the bottom layer facing the feed solution. Each experiment was stopped at a water recovery ratio of ~50% (i.e., ca. 250 mL of pure water was reclaimed), corresponding to a concentration factor (CF) of 2.0. The pristine PTFE/PP membrane was also tested under the same conditions for comparison.
(2)
where Cf and Cd represent the salt concentrations of the feed and distillate solutions, respectively. Vd refers to the total distillate volume, and ΔðVd Cd Þ indicates the salt mass change in the distillate during the time of Δt. Notably, in MD experiments, membrane orientations were deter mined according to the function of each layer, and clearly stated in Table S1. 2.4.1. Anti-wetting and anti-fouling tests To evaluate the anti-wetting properties of all membranes, an amphiphilic agent, SDS, was gradually added to the feed solution until
Fig. 3. (A) WCAs and (B) corresponding digital pictures on both layers of the fabricated membranes with different DA deposition durations. (C) LEP and porosity of different PTFE/PP membranes. 4
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hydrophobicity of PTFE/PP (P, AF1600) is higher if an O2 plasma treatment is initially conducted to the bottom layer. This surprising result may arise from the increased surface roughness caused by the plasma treatment [38,39]. Fig. S3 shows a comparison of morphology on PP layers before and after the plasma treatment and confirms our hypothesis. The lowered surface energy endows the PTFE/PP (P, AF1600) membrane with a superior anti-wetting ability for retaining liquids with low surface tensions [6,14]. Fig. S4 plots the evolution of contact angles of PTFE/PP and PTFE/PP (P, AF1600) membranes as a function of time with different liquids. Both layers of PTFE/PP (P, AF1600) can resist a model surfactant solution made of 0.1 mM SDS in 3.5% NaCl and pure ethanol, while an instant wicking phenomenon is observed for the pristine PTFE/PP membrane. It’s worth mentioning that the coating layer exhibits an excellent sta bility with the membrane body, as evidenced by the almost unchanged WCAs before and after the ultrasonification treatment (Table S2). Although there is no chemical bond formed during the physical coating process, the strong hydrophobic-hydrophobic interactions between Teflon® AF1600 and the hydrophobic membrane would be highly responsible for the entire membrane integrity [6,14]. Having verified the above membrane properties, DA was eventually coated and formed a hydrophilic PDA layer on the bottom layer of the PTFE/PP (P, AF1600) membrane. Since DA molecules can selfpolymerize in the Tris-HCl buffer solution via Michael addition and Schiff-base reaction [40], the generated PDA polymer is expected to well adhere to the substrate by the strong π-π stacking interactions, covalent bonding between the catechol and amino groups [31,40]. As shown in Fig. 3A and B, the WCAs of the bottom layers for Janus membranes exhibit a sharp decline with the duration of DA deposition, suggesting the formation of a PDA layer. The most optimal duration of DA depo sition for Janus membranes was finally determined to be 16 h, as further extending the duration did not quite affect the surface hydrophilicity of the bottom layer. Therefore, the PTFE/PP (P, AF1600) membrane modified by DA for 16 h is referred to as the Janus membrane thereafter for the following studies. Fig. 3C also summarizes and compares the key parameters, i.e., porosity and LEP, of all membranes. Due to the
Fig. 4. Water fluxes and salt rejections of different membranes under different orientations by using a 3.5% NaCl solution as the feed solution. Here, Top-FS refers to top layer faces the feed solution; Top-FS: top layer faces the feed so lution; Bottom-FS: bottom layer faces the feed solution.
3. Results and discussion 3.1. Fabrication and characterizations of the Janus membrane Designing an optimal substrate is critical for the success of fabri cating a robust Janus membrane. Although the pristine PTFE/PP membrane has a high LEP value of 2.10 � 0.32 bar (Table 1), its top and bottom layers have relatively low water contact angles (WCA) of 128.1 � 1.2� and 115.1 � 1.4� , respectively, indicating poor wetting resistance for both layers. In contrast, both modified membranes, PTFE/PP (AF1600) and PTFE/PP (P, AF1600), have greater surface WCA values than the pristine one after coating Teflon® AF1600 with a per fluorinated structure on the membrane surface, as shown in Fig. S2. Interestingly, among these two modified ones, the surface
Fig. 5. FESEM images of pristine PTFE/PP and Janus membranes. (A) (D) Top layer, (B) (E) bottom layer, and (C) (F) cross-section. (a) (b) are enlarged images of the cross-section for Janus membranes. 5
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Fig. 6. (A) FTIR spectra and (B) XPS spectra of PTFE/PP and Janus membranes.
Fig. 7. (A) In-air contact angles and (B) corresponding captured images of the PTFE/PP and Janus membranes.
construction of multiple layers that may cause pore blocking, the overall membrane porosity gradually decreased along with the layer-by-layer coating procedures. However, the LEP values of both top and bottom layers for PTFE/PP (P, AF1600) and Janus membranes all increase after the Teflon® AF 1600 coating, which is consistent with the results pre dicted by the Young-Laplace equation (Eq. S(1)). Moreover, the sepa ration performances of these membranes tested under different orientations are displayed in Fig. 4. Despite of the highest water flux for the Janus-D membrane, it suffers a significant rejection decline, indi cating that it can hardly be employed as an MD membrane. By contrast, the Janus membrane exhibits a relatively high flux without compro mising the rejection in comparison with other membranes, suggesting it is the best choice for the subsequent study. Notably, the flux enhance ment for the Janus membrane could be ascribed to the excellent wettability of the hydrophilic coating (PDA) on their feed-facing sur faces, helping drag more water molecules to membrane surfaces for the subsequent vaporization, as compared to the intrinsically hydrophobic
PTFE/PP membrane [24,29]. In addition, dopamine (DA) could pene trate in the substrate pores, and decrease the thickness of the hydro phobic layer, resulting in a decrease in the mass transfer resistance [29, 41–43]. The micro-scale morphologies of all membranes were studied by means of FESEM observations. Compared to the virgin PTFE/PP mem brane, no significant pore blockage is identified from the surface image of the Janus membrane (Fig. 5D), which could be ascribed to the low concentration of the Teflon® AF1600 solution during the coating. However, the original bottom layer of the PTFE/PP membrane, a PP woven fabric (Fig. 5B), is completely covered by a dense polymeric thin layer of ca. 3 μm in thickness (Fig. 5E and F), further demonstrating the existence of PDA. In the FTIR spectra of the Janus membrane (Fig. 6A), new peaks at 1295, 725, and 985 cm 1 are observed on its top layer, verifying the successful coating of Teflon® AF1600 [6]. Meanwhile, broad absorption peaks appear at 3300 and 1620 cm 1 due to the for – O, C– – N, and C– – C [44,45], confirming the mation of –OH, C– 6
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self-polymerization of DA on the bottom layer. Furthermore, due to the shielding effect of the PDA layer, the intensity of –CF3 absorption [6] becomes much weaker in the bottom of the Janus membrane (Fig. 6A), as compared to the spectra of its top layer. Besides, a new O1s signal at 510 eV and N1s signal at 400 eV appear in the XPS spectra, all of which represents a solid conclusion for the successful fabrication of the Janus membrane (Fig. 6B). 3.2. Surface wettability of the Janus membrane The wetting resistance of the pristine PTFE/PP and the Janus membranes was investigated by measuring their in-air contact angles using liquids with different surface tensions and underwater ones using crude oil on the top and bottom layers. Fig. 7 shows the contact angle results and captured images. Due to the hydrophobic nature, the PTFE/ PP membrane fails to retain test liquids with low surface tensions [i.e., ethanol (~22.1 mN m 1) and crude oil (~30 mN m 1)] [6,10], and is prone to be instantly wicked by them (Fig. 7A). The top layer of the Janus membrane, nevertheless, exhibits higher in-air contact angles for all testing liquids. No testing liquid is able to wick the Janus membrane, which can be ascribed to the existence of lower-surface-energy Teflon [6]. The bottom layer of the Janus membrane shows much a low in-air water contact angle of 33.5 � 1.2� but an exceptionally high underwater oil contact angle of 154.2 � 2.2� , which prevents any testing liquids from penetrating through the membrane pores. Such phenomenon re sults from the combination of the hydrophilic PDA layer that allows all liquids to wet it but the Teflon® AF1600 coated hydrophobic layer underneath that ensures no liquid can penetrate through it [10,46].
Fig. 8. (A) Anti-wetting performances of PTFE/PP and Janus membranes as a function of time using different SDS concentrations. (B) A short-term antiwetting experiment of the PTFE/PP and the Janus membrane using an SDS concentration of 0.4 mM. The fluxes were normalized by their own initial fluxes (17.4 and 19.7 L m 2 h 1 for PTFE/PP and Janus membranes, respectively). Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution.
3.3. Anti-wetting and anti-fouling performance of the Janus membrane For comparison, the anti-wetting performance of PTFE/PP mem branes was firstly examined under DCMD using 3.5% NaCl solutions
Fig. 9. Membrane distillation performance of PTFE/PP and Janus membranes as a function of time using (A) HA-contained saline wastewater and (B) crude oilcontained saline wastewater. The water fluxes were normalized (initial fluxes of 17.4 and 19.7 L m 2 h 1 for PTFE/PP and Janus membranes, respectively) and the salt rejections were monitored over the entire experiments. Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution.
7
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Fig. 11. Variations of thermal efficiency (TE) and specific thermal heat con sumption (STEC) of Janus and PTFE/PP membranes with the permeate volume. Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution.
surface, forming a cake layer that blocks the membrane pores. Regard less of the high rejection to the salt, an apparent decline in flux is found from the beginning for the commercial PTFE/PP membrane. It drops to merely 50% of its initial value after 28 h (Fig. 9A) due to the severe fouling and increase in mass transfer resistance [9,48]. By contrast, the Janus membrane possesses a much gentle decline and maintains a water flux of about 90% at the end of the test, suggesting its superior anti-fouling property. The sample color changes shown in Fig. 9A before and after fouling tests are consistent with flux data. The fouling becomes more significant for the pristine membrane when a saline solution containing crude oil is used as the feed. As shown in Fig. 9B, a sudden decline in salt rejection occurs at about 5 h during the test, which is a clear signal of membrane wetting. Since the crude oil can readily wick into the hydrophobic membrane pores owing to its low surface tension, correspondent membrane fouling may not behave like a flux decline but lead to an instant function loss. In contrast, the Janus membrane still exhibits excellent MD performance during the entire test, only shows a slight decrease in water flux and negligible changes in salt rejections. This experimental finding agrees well with the oil contact angle measured underwater previously. As the oil could be effectively repelled due to a hydrophilic-hydrophobic exclusion effect [10], it is thus paramount to have an MD membrane with a hydrophilic layer, especially when treating with oily saline wastewaters. Overall, although both commercial and Janus membranes exhibit comparable anti-wetting performance, the formal membrane fails with organic fouling experi ments. This result clearly indicates the possible adhesion of organic pollutants onto the commercial membrane surface, wicking into the pores via the attractive hydrophobic interaction, and thus results in the severe membrane fouling. On the contrast, the hydrophilic layer (PDA coating) of the Janus membrane can effectively repel them and avoid membrane fouling. Besides, the existence of Teflon® AF1600 also could ensure the Janus membrane not to be wicked by organic pollutants.
Fig. 10. Water reclamation tests from simulated multi-component wastewater using Janus and PTFE/PP membranes. (A) Variations of the normalized fluxes and conductivities in the distillate as a function of permeate water volume. (B) Variation of water fluxes with time for both membranes. Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution. The con ductivity of the multi-component wastewater (feed solution) is 25.7 mS cm 1.
containing different SDS concentrations. As shown in Fig. S5, when the bottom PP surface faces the feed solutions, a quick wetting phenomenon is observed for the pristine PTFE/PP membrane when the SDS content reaches 0.2 mM. Clearly, the bottom layer of the commercially available PTFE/PP membrane has a high wettability. In contrast, the anti-wetting performance can be significantly improved by (1) using the Janus membranes as shown in Fig. 8A or (2) just simply reversing the testing orientation (i.e., the top layer facing to the feed solution). The maximum SDS dosage could be elevated as high as 0.4 mM that is comparable or higher than those reported in the literatures [6,13,25]. Interestingly, although a hydrophilic PDA layer is in contact with the feed solution for the Janus membrane, comparable MD performance is still achieved. One possible explanation is the existence of a superhydrophobic layer un derneath it so that the membrane could effectively impede the low-surface-tension liquid to penetrate it, even if the liquid has already passed through the PDA layer. Therefore, the Janus membrane displays exceptionally high wetting resistance and MD performance in a 60-h stability test (Fig. 8B), as evidenced by its stable flux and salt rejection over the entire testing duration, whereas the wicking phenomenon within 29 h is observed for the pristine membrane. Since a hydrophilic layer generally possesses a better anti-fouling property [10,19,25], the following MD tests are conducted using the Janus membrane with its bottom layer facing the feed solution. Two organic substances (i.e., HA, and crude oil) are used as the model con taminants to evaluate its fouling resistances. Fig. 9A and B shows the results. As a typical contaminant in natural water and wastewater ef fluents [35,47], HA can readily adhere to the hydrophobic membrane
3.4. Treatment of multi-component wastewater The feasibility of the Janus membrane in treating multi-component wastewater is evaluated, together with the pristine PTFE/PP mem brane for comparison. It should be noted that the risk of membrane wetting and fouling would be increased rapidly during the DCMD pro cess, due to the continuously increases in organic component concen trations in the feed solution. As shown in Fig. 10, the PTFE/PP membrane suffers an apparent water flux decline of 45.3% from 17.2 to 9.4 L m 2 h 1 and a dramatic increase in permeate conductivity from 5.7 to 1200 μS cm 1 by the end of experiment. This phenomenon suggests that severe membrane wetting and fouling occur because HA and oil 8
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molecules are prone to adhere to the hydrophobic membrane surface or enter the torturous PP fabric network. Thus, a significant performance decline is inevitable. On the other hand, the Janus membrane only ex periences a mild flux decline of 12.0% from 19.1 to 16.8 L m 2 h 1 and a slight salt passage (i.e., conductivity increases from 6.5 to 86.7 μS cm 1) under the same testing conditions. The performance enhancement arises from the fact that the newly deposited superoleophobic PDA layer not only mitigate the adhesion of hydrophobic oil [10] but also repels HA, while the hydrophobic layer underneath it blocks the SDS penetration [6,9,10]. As a consequence, it only takes ~35 h for the Janus membrane to do the reclamation process up to a 50% water recovery, whereas a one-third longer time is required for the pristine PTFE/PP membrane (Fig. 10B). Fig. 11 shows the essential energy-related parameters, i.e., thermal efficiency (TE) and specific thermal heat consumption (STEC) as a function of permeate volume for DCMD systems consisting of Janus and PTFE/PP membranes. The detailed calculations are explained in Sup plement Information [49,50]. The DCMD system comprising the Janus membrane has milder changes in both TE and STEC than that consisting of the commercial PTFE/PP membrane because the former has a less flux decline than the latter. Specifically, the TE value of the Janus membrane is reduced by 18.9% at a water recovery of 50%, whereas it is a drastic decrease of 49.1% for the unmodified membrane. Accordingly, the former only has a 15.8% increase in STEC, while the latter has an in crease of 78.7%. Clearly, DCMD systems made of the Janus membrane should have a much lower energy consumption than those made of the PTFE/PP membrane. Notably, the calculated values of STEC seem relatively high probably because a small membrane area of only 4.0 cm2 and a low water flux are employed in the calculations. The STEC value could be significantly reduced if a large membrane is employed in practical uses.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Natural Science Foundation of China (21774058, 51778292), the Natural Science Foundation of Jiangsu Province (BK20180072), and the Fundamental Research Funds for the Central Universities (30918012201). We also acknowledge the support from the China Scholarship Council(CSC) for Meng Li (Grant No. 201906840105). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2020.118031. References [1] J.R. Werber, A. Deshmukh, M. Elimelech, The critical need for increased selectivity, not increased water permeability, for desalination membranes, Environ. Sci. Technol. Lett. 3 (2016) 112–120. [2] R.A. Tufa, E. Curcio, E. Brauns, W. Van Baak, E. Fontananova, G.D. Profio, Membrane distillation and reverse electrodialysis for near-zero liquid discharge and low energy seawater desalination, J. Membr. Sci. 496 (2015) 325–333. [3] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [4] P. Wang, T.S. Chung, Recent advances in membrane distillation processes: membrane development, configuration design and application exploring, J. Membr. Sci. 474 (2015) 39–56. [5] X.Y. Du, D. Wang, W. Wang, J.J. Fu, X.M. Chen, L.J. Wang, W. Yang, X. Zhang, Electrospun nanofibrous polyphenylene oxide membranes for high-salinity water desalination by direct contact membrane distillation, ACS Sustain. Chem. Eng. 7 (2019) 20060–20069. [6] Y.M.L. Chen, K.J. Lu, T.S. Chung, An omniphobic slippery membrane with simultaneous anti-wetting and anti-scaling properties for robust membrane distillation, J. Membr. Sci. 591 (2020), 117572. [7] L.H. Chen, A. Huang, Y.R. Chen, C.H. Chen, C.C. Hsu, F.Y. Tsai, K.L. Tung, Omniphobic membranes for direct contact membrane distillation: effective deposition of zinc oxide nanoparticles, Desalination 428 (2018) 255–263. [8] B.J. Deka, E.J. Lee, J.X. Guo, J. Kharraz, A.K. An, Electrospun nanofiber membranes incorporating PDMS-Aerogel superhydrophobic coating with enhanced flux and improved antiwettability in membrane distillation, Environ. Sci. Technol. 53 (2019) 4948–4958. [9] Y.Z. Tan, J.W. Chew, W.B. Krantz, Effect of humic-acid fouling on membrane distillation, J. Membr. Sci. 474 (2015) 39–56. [10] Y.X. Huang, Z.X. Wang, J. Jin, S.H. Lin, Novel Janus membrane for membrane distillation with simultaneous fouling and wetting resistance, Environ. Sci. Technol. 51 (2017) 13304–13310. [11] Z.X. Wang, D.Y. Hou, S.H. Lin, Composite membrane with underwater-oleophobic surface for anti-oil-fouling membrane distillation, Environ. Sci. Technol. 50 (2016) 3866–3874. [12] K.J. Lu, Y.M.L. Chen, T.S. Chung, Design of omniphobic interfaces for membrane distillation – a review, Water Res. 162 (2019) 64–77. [13] W. Wang, X.W. Du, H. Vahabi, S. Zhao, Y.M. Yin, A.K. Kota, T.Z. Tong, Trade-off in membrane distillation with monolithic omniphobic membranes, Nat. Commun. 10 (2019) 1–9. [14] K.J. Lu, J. Zuo, J. Chang, H.N. Kuan, T.S. Chung, Omniphobic hollow-fiber membranes for vacuum membrane distillation, Environ. Sci. Technol. 52 (2018) 4472–4480. [15] C.H. Boo, J.H. Lee, M. Elimelech, Omniphobic polyvinylidene fluoride (PVDF) membrane for desalination of shale gas produced water by membrane distillation, Environ. Sci. Technol. 50 (2016) 12275–12282. [16] W. Qin, J. Zhang, Z. Xie, D. Ng, Y. Ye, S. Gray, M. Xie, Synergistic effect of combined colloidal and organic fouling in membrane distillation: measurements and mechanisms, Environ. Sci. Water Res. Technol. 3 (2017) 119–127. [17] Z. Wang, S. Lin, The impact of low-surface-energy functional groups on oil fouling resistance in membrane distillation, J. Membr. Sci. 527 (2017) 68–77. [18] X.W. Du, Z.Y. Zhang, K.H. Carlson, J. Lee, T.Z. Tong, Membrane fouling and reusability in membrane distillation of shale oil and gas produced water: effects of membrane surface wettability, J. Membr. Sci. 567 (2018) 199–208. [19] M.M. Ghaleni, A. Al Balushi, S. Kavakoli, M. Bavarian, S. Nejati, Fabrication of Janus membranes for desalination of oil-contaminated saline water, ACS Appl. Mater. Interfaces 10 (2018) 44871–44879.
4. Conclusion In this study, a Janus membrane with asymmetric wettability was fabricated via a layer-by-layer coating strategy with the aid of Teflon® AF1600 and PDA using a commercial PTFE/PP membrane as the sub strate. The resultant membrane exhibited strong hydrophobicity on its top layer with a WCA of 145.1 � 1.8� , while its bottom layer displayed hydrophilicity in-air with a WCA of 33.5 � 1.2� and superoleophobicity underwater with an oil contact angle of 154.2 � 2.2� . Due to the multiple-layered architecture, the Janus membrane could readily with stand common foulants existing in saline waters, e.g. surfactant, HA, and crude oil, and thus exhibited excellent MD performance over a 50-h DCMD test without fluctuations in water flux and salt rejection. More over, the Janus membrane also showed a rather stable MD behavior when treating multi-component wastewater, whereas significant wet ting or fouling were observed for the commercial PTFE/PP membrane. As a result, a higher TE and a correspondent lower STEC were obtained for the former membrane than the latter one. Overall, the newly developed Janus membrane has great potential for wastewater recla mation applications consisting of complex compositions. Author Statement Meng Li: Conceptualization, Methodology, Validation, Resources, Investigation, Data curation, Visualization, Writing- Original draft preparation. Kang Jia Lu: Characterization, Validation, Writing- Reviewing. Lianjun Wang: Conceptualization, Funding acquisition, Reviewing. Xuan Zhang: Conceptualization, Methodology, Funding acquisition, Writing- Reviewing and Editing. Supervision Tai-Shung Chung: Writing- Reviewing and Editing, Supervision.
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[20] X.B. Yang, Z.X. Wang, L. Shao, Construction of oil-unidirectional membrane for integrated oil collection with lossless transportation and oil-in-water emulsion purification, J. Membr. Sci. 549 (2018) 67–74. [21] H.C. Yang, Y.S. Xie, J.W. Hou, A.K. Cheetham, V. Chen, S.B. Darling, Janus membranes: creating asymmetry for energy efficiency, Adv. Mater. 30 (2018), 1801495-1801506. [22] L.L. Lan, X.B. Yang, J. Long, X.Q. Cheng, D. Pan, Y.F. Huang, L. Shao, Universal unilateral electro-spinning/spraying strategy to construct water-unidirectional Janus membranes with well-tuned hierarchical micro/nanostructures, Chem. Commun. 56 (2019) 478. [23] X.B. Yang, L.L. Yan, F.T. Ran, A. Ral, J. Long, L. Shao, Interface-confined surface engineering constructing water-unidirectional Janus membrane, J. Membr. Sci. 576 (2019) 9–16. [24] N.G.P. Chew, Y.J. Zhang, K. Goh, J.S. Ho, R. Xu, R. Wang, Hierarchically structured Janus membrane surfaces for enhanced membrane distillation performance, ACS Appl. Mater. Interfaces 11 (2019) 25524–25534. [25] C.X. Li, X.S. Li, X.W. Du, T.Z. Tong, T.Y. Cath, J. Lee, Antiwetting and antifouling Janus membrane for desalination of saline oily wastewater by membrane distillation, ACS Appl. Mater. Interfaces 11 (2019) 18456–18462. [26] Z. Wang, Y. Wang, G. Liu, Rapid and efficient separation of oil from oil-in-water emulsions using a Janus cotton fabric, Angew. Chem. Int. Ed. 55 (2016) 1291–1294. [27] Z.G. Zhu, Z.Q. Liu, L.L. Zhong, C.J. Song, W.X. Shi, F.Y. Cui, W. Wang, Breathable and asymmetrically superwettable Janus membrane with robust oil-fouling resistance for durable membrane distillation, J. Membr. Sci. 563 (2018) 9–16. [28] Y. Zhang, M. Barboiu, Dynameric asymmetric membranes for directional water transport, Chem. Commun. 51 (2015) 15925–15927. [29] H.C. Yang, W.W. Zhong, J.W. Hou, V. Chen, Z.K. Xu, Janus hollow fiber membrane with a mussel-inspired coating on the lumen surface for direct contact membrane distillation, J. Membr. Sci. 523 (2017) 1–7. [30] X.B. Yang, L.L. Lan, F.T. Ran, Y.F. Huang, D. Pan, Y.P. Bai, L. Shao, Mussel-/ diatom-inspired silicified membrane for high-efficiency water remediation, J. Membr. Sci. 597 (2020), 117753. [31] D.X. Guo, Y.R. Xiao, T. Li, Q.F. Zhou, L.G. Shen, R.J. Li, Y.C. Xu, H.J. Lin, Fabrication of high-performance composite nanofiltration membranes for dye wastewater treatment: mussel-inspired layer-by-layer self-assembly, J. Colloid Interface Sci. 560 (2020) 273–283. [32] N.G.P. Chew, S.S. Zhao, C. Malde, R. Wang, Polyvinylidene fluoride membrane modification via oxidant-induced dopamine polymerization for sustainable directcontact membrane distillation, J. Membr. Sci. 563 (2018) 31–42. [33] X.P. Li, H.T. Shan, M. Cao, B.A. Li, Facile fabrication of omniphobic PVDF composite membrane via a waterborne coating for anti-wetting and anti-fouling membrane distillation, J. Membr. Sci. 589 (2019) 117262–117275. [34] Y.C. Woo, Y.J. Kim, M.W. Yao, L.D. Tijing, J.S. Choi, S.H. Lee, S.H. Kim, H.K. Shon, Hierarchical composite membranes with robust omniphobic surface using layer-bylayer assembly technique, Environ. Sci. Technol. 52 (2018) 2186–2196.
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Janus membranes with asymmetric wettability via a layer-by-layer coating strategy for robust membrane distillation Meng Li a, b, Kang Jia Lu b, Lianjun Wang a, Xuan Zhang a, **, Tai-Shung Chung b, * a
Key Laboratory of New Membrane Materials, Ministry of Industry and Information Technology, School of Environmental and Biological Engineering, Nanjing University of Science & Technology, Nanjing, 210094, China b Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore
A R T I C L E I N F O
A B S T R A C T
Keywords: Asymmetric wettability Layer-by-layer Wetting/fouling resistance Membrane distillation Thermal consumption
Commercial membranes for membrane distillation experience severe wetting and fouling phenomena, which largely hinder their practical applications. To address these issues, a novel strategy to fabricate the Janus membrane is proposed via a facile layer-by-layer coating of Teflon® AF1600 and polydopamine (PDA) on an oxygen plasma treated commercial polytetrafluoroethylene/polypropylene (PTFE/PP) membrane. The resultant membrane has a unique asymmetric surface wettability. In brief, the top layer of the Janus membrane is strongly hydrophobic (145.1 � 1.8� ), while the bottom layer exhibits underwater superoleophobicity (154.2 � 2.2� ) and in-air hydrophilicity (33.5 � 1.2� ). As a result, the Janus membrane possesses a significantly enhanced wetting resistance toward common liquids including pure water, surfactant, ethanol, and crude oil, compared to the unmodified membrane. Due to the excellent anti-fouling property, the Janus membrane also shows a rather robust desalination performance (i.e., a slight decline in water flux and negligible salt passage) when treating a multi-component saline wastewater containing both humic acid and crude oil, ending up with a high water recovery ratio of 50%. Overall, the Janus membrane developed in this work is highly promising for the water reclamation from highly saline wastewater containing complex organic compounds.
1. Introduction Water reclamation, particularly from high-salinity seawater or/and multi-component wastewater, has become an important issue over the past decades [1,2]. To date, membrane-based technologies offer the most promising pathway for water desalination and wastewater treat ment due to their easy operation and environmentally friendly charac teristics [3]. Nevertheless, it should be mentioned that most pressure-driven processes can hardly be used in the treatment of high ly saline water because the energy consumption significantly increases with the concentration increase of the brines [4]. Compared to conventional pressure-driven processes, membrane distillation (MD) has attracted extensive attentions, as its driving force is mainly associated with the gradient of water vapor pressure across the membrane, rather than the osmotic pressure [5–8]. However, mem brane wetting and fouling phenomena are two critical challenges that hinder the wide applications of MD membranes, especially for treating saline wastewaters containing organic compounds (i.e., surfactants,
crude oil, humic acid (HA), etc.) [6,9–11]. For instance, the surfactants will decrease the membrane wetting resistance by reducing the surface tension of the feed solution, resulting in a severe salt passage and worsened MD performance. Besides, these contaminants (i.e., HA and crude oil) can adhere readily to the surfaces of conventional MD mem branes by hydrophobic-hydrophobic or electrostatic interactions [9,10], causing the undesired blocking or wicking of membrane pores as well. To address the above issues, designing an omniphobic MD membrane seems promising, as the omniphobic membranes were proved to show excellent wetting resistance toward both surfactant-containing water and other low surface tension liquids (e.g. organic solvents) [6,12–15]. However, it should be noted that organic pollutants, such as alginate, HA, oil droplet, etc., could still attach to the membrane surface by the hydrophobic-hydrophobic interactions, resulting in a more severe membrane fouling [9,16,17]. In light of this, the concept of Janus membranes, with an asymmet rical surface wettability, has been proposed in recent years [10,19–29]. An incorporated hydrophilic layer could effectively repel the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhang), [email protected] (T.-S. Chung). https://doi.org/10.1016/j.memsci.2020.118031 Received 28 January 2020; Received in revised form 29 February 2020; Accepted 4 March 2020 Available online 8 March 2020 0376-7388/© 2020 Elsevier B.V. All rights reserved.
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hydrophobic foulants, and thus endows the MD membranes with lower fouling propensities. However, regardless of the extensive fabrication strategies of Janus membranes, e.g. electrospinning/spraying [22], surface coating [23,24], grafting [25,26], electrospinning [27], and vaporization-induced phase inversion [28], the poor interfacial compatibility or weak bonding combination between two layers (hydrophobicity—hydrophilicity) still remains the critical challenge for the membrane stability [10,25]. Additionally, the hydrophilic coating is commonly performed on a conventional hydrophobic substrate directly, which is readily wetted by the feed solution containing low surface tension components (e.g., surfactants) [10,25]. Therefore, having a membrane that possessing both excellent anti-fouling and anti-wetting properties would be paramount for a robust MD operation but still rather challenging. The lack of robust MD membrane materials, especially toward treating highly saline wastewater containing surfactants and various organic compounds, motivated us to develop an MD membrane that can well sustain and steadily operate at harsh environments. Considering the nature of mussel-inspired materials (e.g., dopamine [30,31]) and the rather low surface energy of Teflon® series products [6,14], we have designed and fabricated, for the first time, a Janus membrane with asymmetric wettability by a facile layer-by-layer coating of Teflon® AF1600 and polydopamine (PDA) on an oxygen plasma treated com mercial polytetrafluoroethylene/polypropylene (PTFE/PP) membrane. The Teflon® AF1600 coating was employed to improve the membrane wetting resistance by increasing the surface hydrophobicity while the PDA coating was applied to enhance the fouling resistance to organic matters due to its strong hydrophilicity. Physicochemical characteriza tions of the newly developed Janus membrane were thoroughly char acterized, in terms of morphology, surface properties, wetting and
fouling resistance, and so forth. Specifically, its water reclamation from highly saline wastewater containing organic compounds was systemat ically evaluated. Associated economic concerns, including the thermal efficiency (TE) and specific thermal energy consumption (STEC) were also analyzed in detail. 2. Experimental 2.1. Materials and chemicals Commercial flat-sheet PTFE/PP membranes for MD processes were provided from Sterlitech Corporation. The membrane consisted of two layers; namely, a selective PTFE top layer and a highly porous and rough PP substrate. Its detailed specifications, including pore size, water con tact angle, LEP, and porosity are summarized in Table 1. Dopamine hydrochloride (>98%), tris (hydroxymethyl) aminomethane hydro chloride (Tris-HCl, >99%), sodium dodecyl sulfate (SDS, >99%), so dium chloride (NaCl), and humic acid (HA) were procured from Sigma Aldrich. Ethanol and crude oil were purchased from Arkema Inc. Teflon® AF1600 and Galden HT70 heat transfer fluid were acquired from DuPont Co. Ltd. and APP System Services. All other reagents and solvents were used as received. Deionized (DI) water with a minimum resistance of 18.4 MΩ-cm (Millipore) was used throughout the work. 2.2. Design of the Janus membrane and reference membranes Fig. 1 illustrates the fabrication processes for all kinds of membranes. To fabricate the Janus membrane, a layer-by-layer coating procedure was employed in this work. In brief, the pristine PTFE/PP membrane was first cleaned by ethanol and DI water for 10 min, and then dried in a
Table 1 Physical properties of the pristine PTFE/PP MD membrane. Membrane
Pore Size (μm)
Water Contact Angle (� ) (top layer)
LEPa) (bar) (top layer)
Porosity (%)
PTFE/PP membrane
0.20 � 0.02
128.1 � 1.2
2.10 � 0.32
82.2 � 3.7
a
LEP: liquid entry pressure.
Fig. 1. Schematic illustration of the fabrication processes of different MD membranes. 2
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Fig. 2. (A) Digital pictures of both layers of the Janus and the pristine PTFE/PP membranes; (B) Chemical structures of PTFE and Teflon® AF1600; (C) Possible reaction pathways of PDA from a self-polymerization reaction of DA.
vacuum oven before use. In order to obtain a robust and durable coating, the bottom (PP) layer was treated with oxygen plasma at a power of 200 W for 2 min to further enhance the surface roughness for subsequent coating procedures. The resultant membrane was named as the PTFE/PP (P) membrane. Next, the PTFE/PP (P) membrane was completely immersed in a coating solution consisting of 0.05 wt% Teflon® AF1600 in Galden HT70 solvent only for 20 s, and then dried in air. The resultant membrane was named as PTFE/PP (P, AF1600). Owing to the low boiling temperature of the solvent (i.e., 70 � C), Teflon® AF1600 would quickly precipitate and deposit on the membrane surface. Finally, the Janus membrane was fabricated by coating a hydrophilic layer on the bottom layer of PTFE/PP (P, AF1600) via a mussel-inspired dopamine (DA) self-polymerization, as described elsewhere [20,23,24,29]. Spe cifically, the PTFE/PP (P, AF1600) membrane was fixed in a custom-designed mold with an effective area of 20.0 cm2, and a 10 mM Tris-HCl buffer solution containing 2.0 g L 1 of DA was then poured into the mold, allowing for a complete contact with the bottom layer for 16 h, yielding the target Janus membrane. Digital pictures of Janus mem brane, chemical structures of PTFE and Teflon® AF1600, and possible self-polymerization mechanisms of PDA are provided in Fig. 2. Other membranes were also fabricated as references for comparison. As described in Fig. 1, the Janus-D membrane was prepared by directly coating a PDA hydrophilic layer on the bottom layer of the pristine PTFE/PP membrane with the same conditions mentioned above. While the PTFE/PP (AF1600) membrane was fabricated by directly immersing the pristine PTFE/PP membrane into the Teflon® AF1600 coating so lution (0.05 wt%) for 20 s without the initial plasma treatment. After the modifications, all the membranes were rinsed with DI water for several times, and then dried at 80 � C for 24 h under vacuum for subsequent characterizations.
platinum. For the measurements of cross-section, membrane samples were fractured in liquid nitrogen. Chemical and elemental compositions of membrane surfaces were examined by X-ray photoelectron spectros copy (XPS, PHI Quantera II, Japan). Fourier transform infrared (FTIR) spectra were obtained using an FTIR spectrometer (Bruker Vertex 70, Germany) to qualitatively analyze the variations of functional groups on membrane surfaces after modifications. Additionally, in-air contact an gles (CAs) with different liquids, such as DI water (~72.8 mN m 1), 0.1 mM SDS in 3.5% NaCl solutions (~43 mN m 1), crude oil (~30 mN m 1), and ethanol (~22.1 mN m 1) [6,10,15], and underwater oil contact angles (UOCAs) with crude oil were examined using an optical contact angle measuring instrument (DataPhysics, OCA 25, Germany). At least three measurements were made at different locations on each membrane surface and their averages were recorded with error bars. Other characterizations of MD membranes, including LEP and porosity, were provided in Supporting Information. 2.4. MD performance tests using direct contact membrane distillation (DCMD) All experiments were conducted on a lab-scale DCMD set-up with an effective membrane area of 4.0 cm2 (Fig. S1), and the temperatures for the feed and distillate solutions were fixed at 70 � C and 20 � C, respec tively, indicating a temperature difference of 50 � C. The flow rates of the hot feed and the cold distillate streams were fixed at 0.2 L min 1 and 0.1 L min 1, respectively, to overcome the influence of forward osmosis and ensure the unambiguous detection of wetting [6,10]. Thermometers were inserted at different locations to monitor the temperatures, as shown in Fig. S1. For each test, the entire system would run for at least 1 h for stabilization before collecting the data. Water flux (Jw, L m 2 h 1, measured by the weight change of the distillate solution) and salt rejection (%, measured by the electric conductivity change of the distillate solution) were calculated by the following Eqs. (1) and (2) [5]:
2.3. Membrane characterizations Morphologies of membrane samples were investigated by field emission scanning electron microscopy (FESEM, JEOL JSM-6700LV, Japan). Before imaging, all samples were sputter-coated with
Jw ¼
3
Δm=ρ Am Δt
(1)
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Table 2 Compositions wastewater.
of
the
simulated
the membrane was wetted. In brief, SDS concentrations in feed solutions were fixed at 0.05, 0.1, 0.2, 0.4, and 0.6 mM along with the experiment proceeding. Although the feed concentration exerts minor influences on water flux during the MD process [6], the feed solution was replenished by manually adding DI water periodically, to keep a constant volume and feed concentration. In separate experiments, the feed solutions consisting of two typical contaminants (i.e., HA and crude oil) were selected to evaluate the fouling resistance for all membranes [9,10,18]. Specifically, 500 mL of 3.5% NaCl solutions containing 0.25 g of HA (500 mg L 1) and 0.25 g crude oil (500 mg L 1) were prepared separately and directly used as feed solutions in DCMD tests. To prepare the oil-in-feed solution, a ho mogenizer was employed to disperse the oil in the solution at 10,000 rpm for 5 min. DI water was also manually added to maintain the NaCl and contaminant concentrations, as mentioned above.
multi-component
Constituents
Value
Conductivity (mS cm 1) Ca2þ (mg L 1) Mg2þ (mg L 1) Naþ (mg L 1) 1 NHþ 4 (mg L ) Kþ (mg L 1) Cl (mg L 1) SO24 (mg L 1) SDS (mg L 1) Humic Acid (mg L 1) Crude Oil (mg L 1)
25.7 454 1423 10815.6 74 210 14296 107.8 15 200 50
where Δm represents the weight change of the distillate, ρ refers to the density of the permeate (~1 g cm 3), Am is the effective membrane area, and Δt is the experimental duration. R¼1
ΔðVd Cd Þ=Jw Am Δt Cf
2.4.2. Water reclamation from the simulated multi-component wastewater Table 2 lists the compositions of a model multi-component waste water, including inorganic salts [5,32], organic compounds [33–35], crude oil [36], and surfactants [5,37]. As most the current studies only simulated the feed water containing part of the above components, we therefore reviewed the literatures, estimated the approximate range for each constituent and prepared the simulated wastewater (500 mL) comprising all the representative constituents. In water reclamation tests, the Janus membrane was employed in the DCMD operation with the bottom layer facing the feed solution. Each experiment was stopped at a water recovery ratio of ~50% (i.e., ca. 250 mL of pure water was reclaimed), corresponding to a concentration factor (CF) of 2.0. The pristine PTFE/PP membrane was also tested under the same conditions for comparison.
(2)
where Cf and Cd represent the salt concentrations of the feed and distillate solutions, respectively. Vd refers to the total distillate volume, and ΔðVd Cd Þ indicates the salt mass change in the distillate during the time of Δt. Notably, in MD experiments, membrane orientations were deter mined according to the function of each layer, and clearly stated in Table S1. 2.4.1. Anti-wetting and anti-fouling tests To evaluate the anti-wetting properties of all membranes, an amphiphilic agent, SDS, was gradually added to the feed solution until
Fig. 3. (A) WCAs and (B) corresponding digital pictures on both layers of the fabricated membranes with different DA deposition durations. (C) LEP and porosity of different PTFE/PP membranes. 4
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hydrophobicity of PTFE/PP (P, AF1600) is higher if an O2 plasma treatment is initially conducted to the bottom layer. This surprising result may arise from the increased surface roughness caused by the plasma treatment [38,39]. Fig. S3 shows a comparison of morphology on PP layers before and after the plasma treatment and confirms our hypothesis. The lowered surface energy endows the PTFE/PP (P, AF1600) membrane with a superior anti-wetting ability for retaining liquids with low surface tensions [6,14]. Fig. S4 plots the evolution of contact angles of PTFE/PP and PTFE/PP (P, AF1600) membranes as a function of time with different liquids. Both layers of PTFE/PP (P, AF1600) can resist a model surfactant solution made of 0.1 mM SDS in 3.5% NaCl and pure ethanol, while an instant wicking phenomenon is observed for the pristine PTFE/PP membrane. It’s worth mentioning that the coating layer exhibits an excellent sta bility with the membrane body, as evidenced by the almost unchanged WCAs before and after the ultrasonification treatment (Table S2). Although there is no chemical bond formed during the physical coating process, the strong hydrophobic-hydrophobic interactions between Teflon® AF1600 and the hydrophobic membrane would be highly responsible for the entire membrane integrity [6,14]. Having verified the above membrane properties, DA was eventually coated and formed a hydrophilic PDA layer on the bottom layer of the PTFE/PP (P, AF1600) membrane. Since DA molecules can selfpolymerize in the Tris-HCl buffer solution via Michael addition and Schiff-base reaction [40], the generated PDA polymer is expected to well adhere to the substrate by the strong π-π stacking interactions, covalent bonding between the catechol and amino groups [31,40]. As shown in Fig. 3A and B, the WCAs of the bottom layers for Janus membranes exhibit a sharp decline with the duration of DA deposition, suggesting the formation of a PDA layer. The most optimal duration of DA depo sition for Janus membranes was finally determined to be 16 h, as further extending the duration did not quite affect the surface hydrophilicity of the bottom layer. Therefore, the PTFE/PP (P, AF1600) membrane modified by DA for 16 h is referred to as the Janus membrane thereafter for the following studies. Fig. 3C also summarizes and compares the key parameters, i.e., porosity and LEP, of all membranes. Due to the
Fig. 4. Water fluxes and salt rejections of different membranes under different orientations by using a 3.5% NaCl solution as the feed solution. Here, Top-FS refers to top layer faces the feed solution; Top-FS: top layer faces the feed so lution; Bottom-FS: bottom layer faces the feed solution.
3. Results and discussion 3.1. Fabrication and characterizations of the Janus membrane Designing an optimal substrate is critical for the success of fabri cating a robust Janus membrane. Although the pristine PTFE/PP membrane has a high LEP value of 2.10 � 0.32 bar (Table 1), its top and bottom layers have relatively low water contact angles (WCA) of 128.1 � 1.2� and 115.1 � 1.4� , respectively, indicating poor wetting resistance for both layers. In contrast, both modified membranes, PTFE/PP (AF1600) and PTFE/PP (P, AF1600), have greater surface WCA values than the pristine one after coating Teflon® AF1600 with a per fluorinated structure on the membrane surface, as shown in Fig. S2. Interestingly, among these two modified ones, the surface
Fig. 5. FESEM images of pristine PTFE/PP and Janus membranes. (A) (D) Top layer, (B) (E) bottom layer, and (C) (F) cross-section. (a) (b) are enlarged images of the cross-section for Janus membranes. 5
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Fig. 6. (A) FTIR spectra and (B) XPS spectra of PTFE/PP and Janus membranes.
Fig. 7. (A) In-air contact angles and (B) corresponding captured images of the PTFE/PP and Janus membranes.
construction of multiple layers that may cause pore blocking, the overall membrane porosity gradually decreased along with the layer-by-layer coating procedures. However, the LEP values of both top and bottom layers for PTFE/PP (P, AF1600) and Janus membranes all increase after the Teflon® AF 1600 coating, which is consistent with the results pre dicted by the Young-Laplace equation (Eq. S(1)). Moreover, the sepa ration performances of these membranes tested under different orientations are displayed in Fig. 4. Despite of the highest water flux for the Janus-D membrane, it suffers a significant rejection decline, indi cating that it can hardly be employed as an MD membrane. By contrast, the Janus membrane exhibits a relatively high flux without compro mising the rejection in comparison with other membranes, suggesting it is the best choice for the subsequent study. Notably, the flux enhance ment for the Janus membrane could be ascribed to the excellent wettability of the hydrophilic coating (PDA) on their feed-facing sur faces, helping drag more water molecules to membrane surfaces for the subsequent vaporization, as compared to the intrinsically hydrophobic
PTFE/PP membrane [24,29]. In addition, dopamine (DA) could pene trate in the substrate pores, and decrease the thickness of the hydro phobic layer, resulting in a decrease in the mass transfer resistance [29, 41–43]. The micro-scale morphologies of all membranes were studied by means of FESEM observations. Compared to the virgin PTFE/PP mem brane, no significant pore blockage is identified from the surface image of the Janus membrane (Fig. 5D), which could be ascribed to the low concentration of the Teflon® AF1600 solution during the coating. However, the original bottom layer of the PTFE/PP membrane, a PP woven fabric (Fig. 5B), is completely covered by a dense polymeric thin layer of ca. 3 μm in thickness (Fig. 5E and F), further demonstrating the existence of PDA. In the FTIR spectra of the Janus membrane (Fig. 6A), new peaks at 1295, 725, and 985 cm 1 are observed on its top layer, verifying the successful coating of Teflon® AF1600 [6]. Meanwhile, broad absorption peaks appear at 3300 and 1620 cm 1 due to the for – O, C– – N, and C– – C [44,45], confirming the mation of –OH, C– 6
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self-polymerization of DA on the bottom layer. Furthermore, due to the shielding effect of the PDA layer, the intensity of –CF3 absorption [6] becomes much weaker in the bottom of the Janus membrane (Fig. 6A), as compared to the spectra of its top layer. Besides, a new O1s signal at 510 eV and N1s signal at 400 eV appear in the XPS spectra, all of which represents a solid conclusion for the successful fabrication of the Janus membrane (Fig. 6B). 3.2. Surface wettability of the Janus membrane The wetting resistance of the pristine PTFE/PP and the Janus membranes was investigated by measuring their in-air contact angles using liquids with different surface tensions and underwater ones using crude oil on the top and bottom layers. Fig. 7 shows the contact angle results and captured images. Due to the hydrophobic nature, the PTFE/ PP membrane fails to retain test liquids with low surface tensions [i.e., ethanol (~22.1 mN m 1) and crude oil (~30 mN m 1)] [6,10], and is prone to be instantly wicked by them (Fig. 7A). The top layer of the Janus membrane, nevertheless, exhibits higher in-air contact angles for all testing liquids. No testing liquid is able to wick the Janus membrane, which can be ascribed to the existence of lower-surface-energy Teflon [6]. The bottom layer of the Janus membrane shows much a low in-air water contact angle of 33.5 � 1.2� but an exceptionally high underwater oil contact angle of 154.2 � 2.2� , which prevents any testing liquids from penetrating through the membrane pores. Such phenomenon re sults from the combination of the hydrophilic PDA layer that allows all liquids to wet it but the Teflon® AF1600 coated hydrophobic layer underneath that ensures no liquid can penetrate through it [10,46].
Fig. 8. (A) Anti-wetting performances of PTFE/PP and Janus membranes as a function of time using different SDS concentrations. (B) A short-term antiwetting experiment of the PTFE/PP and the Janus membrane using an SDS concentration of 0.4 mM. The fluxes were normalized by their own initial fluxes (17.4 and 19.7 L m 2 h 1 for PTFE/PP and Janus membranes, respectively). Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution.
3.3. Anti-wetting and anti-fouling performance of the Janus membrane For comparison, the anti-wetting performance of PTFE/PP mem branes was firstly examined under DCMD using 3.5% NaCl solutions
Fig. 9. Membrane distillation performance of PTFE/PP and Janus membranes as a function of time using (A) HA-contained saline wastewater and (B) crude oilcontained saline wastewater. The water fluxes were normalized (initial fluxes of 17.4 and 19.7 L m 2 h 1 for PTFE/PP and Janus membranes, respectively) and the salt rejections were monitored over the entire experiments. Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution.
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Fig. 11. Variations of thermal efficiency (TE) and specific thermal heat con sumption (STEC) of Janus and PTFE/PP membranes with the permeate volume. Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution.
surface, forming a cake layer that blocks the membrane pores. Regard less of the high rejection to the salt, an apparent decline in flux is found from the beginning for the commercial PTFE/PP membrane. It drops to merely 50% of its initial value after 28 h (Fig. 9A) due to the severe fouling and increase in mass transfer resistance [9,48]. By contrast, the Janus membrane possesses a much gentle decline and maintains a water flux of about 90% at the end of the test, suggesting its superior anti-fouling property. The sample color changes shown in Fig. 9A before and after fouling tests are consistent with flux data. The fouling becomes more significant for the pristine membrane when a saline solution containing crude oil is used as the feed. As shown in Fig. 9B, a sudden decline in salt rejection occurs at about 5 h during the test, which is a clear signal of membrane wetting. Since the crude oil can readily wick into the hydrophobic membrane pores owing to its low surface tension, correspondent membrane fouling may not behave like a flux decline but lead to an instant function loss. In contrast, the Janus membrane still exhibits excellent MD performance during the entire test, only shows a slight decrease in water flux and negligible changes in salt rejections. This experimental finding agrees well with the oil contact angle measured underwater previously. As the oil could be effectively repelled due to a hydrophilic-hydrophobic exclusion effect [10], it is thus paramount to have an MD membrane with a hydrophilic layer, especially when treating with oily saline wastewaters. Overall, although both commercial and Janus membranes exhibit comparable anti-wetting performance, the formal membrane fails with organic fouling experi ments. This result clearly indicates the possible adhesion of organic pollutants onto the commercial membrane surface, wicking into the pores via the attractive hydrophobic interaction, and thus results in the severe membrane fouling. On the contrast, the hydrophilic layer (PDA coating) of the Janus membrane can effectively repel them and avoid membrane fouling. Besides, the existence of Teflon® AF1600 also could ensure the Janus membrane not to be wicked by organic pollutants.
Fig. 10. Water reclamation tests from simulated multi-component wastewater using Janus and PTFE/PP membranes. (A) Variations of the normalized fluxes and conductivities in the distillate as a function of permeate water volume. (B) Variation of water fluxes with time for both membranes. Top-FS: top layer faces the feed solution; Bottom-FS: bottom layer faces the feed solution. The con ductivity of the multi-component wastewater (feed solution) is 25.7 mS cm 1.
containing different SDS concentrations. As shown in Fig. S5, when the bottom PP surface faces the feed solutions, a quick wetting phenomenon is observed for the pristine PTFE/PP membrane when the SDS content reaches 0.2 mM. Clearly, the bottom layer of the commercially available PTFE/PP membrane has a high wettability. In contrast, the anti-wetting performance can be significantly improved by (1) using the Janus membranes as shown in Fig. 8A or (2) just simply reversing the testing orientation (i.e., the top layer facing to the feed solution). The maximum SDS dosage could be elevated as high as 0.4 mM that is comparable or higher than those reported in the literatures [6,13,25]. Interestingly, although a hydrophilic PDA layer is in contact with the feed solution for the Janus membrane, comparable MD performance is still achieved. One possible explanation is the existence of a superhydrophobic layer un derneath it so that the membrane could effectively impede the low-surface-tension liquid to penetrate it, even if the liquid has already passed through the PDA layer. Therefore, the Janus membrane displays exceptionally high wetting resistance and MD performance in a 60-h stability test (Fig. 8B), as evidenced by its stable flux and salt rejection over the entire testing duration, whereas the wicking phenomenon within 29 h is observed for the pristine membrane. Since a hydrophilic layer generally possesses a better anti-fouling property [10,19,25], the following MD tests are conducted using the Janus membrane with its bottom layer facing the feed solution. Two organic substances (i.e., HA, and crude oil) are used as the model con taminants to evaluate its fouling resistances. Fig. 9A and B shows the results. As a typical contaminant in natural water and wastewater ef fluents [35,47], HA can readily adhere to the hydrophobic membrane
3.4. Treatment of multi-component wastewater The feasibility of the Janus membrane in treating multi-component wastewater is evaluated, together with the pristine PTFE/PP mem brane for comparison. It should be noted that the risk of membrane wetting and fouling would be increased rapidly during the DCMD pro cess, due to the continuously increases in organic component concen trations in the feed solution. As shown in Fig. 10, the PTFE/PP membrane suffers an apparent water flux decline of 45.3% from 17.2 to 9.4 L m 2 h 1 and a dramatic increase in permeate conductivity from 5.7 to 1200 μS cm 1 by the end of experiment. This phenomenon suggests that severe membrane wetting and fouling occur because HA and oil 8
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molecules are prone to adhere to the hydrophobic membrane surface or enter the torturous PP fabric network. Thus, a significant performance decline is inevitable. On the other hand, the Janus membrane only ex periences a mild flux decline of 12.0% from 19.1 to 16.8 L m 2 h 1 and a slight salt passage (i.e., conductivity increases from 6.5 to 86.7 μS cm 1) under the same testing conditions. The performance enhancement arises from the fact that the newly deposited superoleophobic PDA layer not only mitigate the adhesion of hydrophobic oil [10] but also repels HA, while the hydrophobic layer underneath it blocks the SDS penetration [6,9,10]. As a consequence, it only takes ~35 h for the Janus membrane to do the reclamation process up to a 50% water recovery, whereas a one-third longer time is required for the pristine PTFE/PP membrane (Fig. 10B). Fig. 11 shows the essential energy-related parameters, i.e., thermal efficiency (TE) and specific thermal heat consumption (STEC) as a function of permeate volume for DCMD systems consisting of Janus and PTFE/PP membranes. The detailed calculations are explained in Sup plement Information [49,50]. The DCMD system comprising the Janus membrane has milder changes in both TE and STEC than that consisting of the commercial PTFE/PP membrane because the former has a less flux decline than the latter. Specifically, the TE value of the Janus membrane is reduced by 18.9% at a water recovery of 50%, whereas it is a drastic decrease of 49.1% for the unmodified membrane. Accordingly, the former only has a 15.8% increase in STEC, while the latter has an in crease of 78.7%. Clearly, DCMD systems made of the Janus membrane should have a much lower energy consumption than those made of the PTFE/PP membrane. Notably, the calculated values of STEC seem relatively high probably because a small membrane area of only 4.0 cm2 and a low water flux are employed in the calculations. The STEC value could be significantly reduced if a large membrane is employed in practical uses.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the National Natural Science Foundation of China (21774058, 51778292), the Natural Science Foundation of Jiangsu Province (BK20180072), and the Fundamental Research Funds for the Central Universities (30918012201). We also acknowledge the support from the China Scholarship Council(CSC) for Meng Li (Grant No. 201906840105). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2020.118031. References [1] J.R. Werber, A. Deshmukh, M. Elimelech, The critical need for increased selectivity, not increased water permeability, for desalination membranes, Environ. Sci. Technol. Lett. 3 (2016) 112–120. [2] R.A. Tufa, E. Curcio, E. Brauns, W. Van Baak, E. Fontananova, G.D. Profio, Membrane distillation and reverse electrodialysis for near-zero liquid discharge and low energy seawater desalination, J. Membr. Sci. 496 (2015) 325–333. [3] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [4] P. Wang, T.S. Chung, Recent advances in membrane distillation processes: membrane development, configuration design and application exploring, J. Membr. Sci. 474 (2015) 39–56. [5] X.Y. Du, D. Wang, W. Wang, J.J. Fu, X.M. Chen, L.J. Wang, W. Yang, X. Zhang, Electrospun nanofibrous polyphenylene oxide membranes for high-salinity water desalination by direct contact membrane distillation, ACS Sustain. Chem. Eng. 7 (2019) 20060–20069. [6] Y.M.L. Chen, K.J. Lu, T.S. Chung, An omniphobic slippery membrane with simultaneous anti-wetting and anti-scaling properties for robust membrane distillation, J. Membr. Sci. 591 (2020), 117572. [7] L.H. Chen, A. Huang, Y.R. Chen, C.H. Chen, C.C. Hsu, F.Y. Tsai, K.L. Tung, Omniphobic membranes for direct contact membrane distillation: effective deposition of zinc oxide nanoparticles, Desalination 428 (2018) 255–263. [8] B.J. Deka, E.J. Lee, J.X. Guo, J. Kharraz, A.K. An, Electrospun nanofiber membranes incorporating PDMS-Aerogel superhydrophobic coating with enhanced flux and improved antiwettability in membrane distillation, Environ. Sci. Technol. 53 (2019) 4948–4958. [9] Y.Z. Tan, J.W. Chew, W.B. Krantz, Effect of humic-acid fouling on membrane distillation, J. Membr. Sci. 474 (2015) 39–56. [10] Y.X. Huang, Z.X. Wang, J. Jin, S.H. Lin, Novel Janus membrane for membrane distillation with simultaneous fouling and wetting resistance, Environ. Sci. Technol. 51 (2017) 13304–13310. [11] Z.X. Wang, D.Y. Hou, S.H. Lin, Composite membrane with underwater-oleophobic surface for anti-oil-fouling membrane distillation, Environ. Sci. Technol. 50 (2016) 3866–3874. [12] K.J. Lu, Y.M.L. Chen, T.S. Chung, Design of omniphobic interfaces for membrane distillation – a review, Water Res. 162 (2019) 64–77. [13] W. Wang, X.W. Du, H. Vahabi, S. Zhao, Y.M. Yin, A.K. Kota, T.Z. Tong, Trade-off in membrane distillation with monolithic omniphobic membranes, Nat. Commun. 10 (2019) 1–9. [14] K.J. Lu, J. Zuo, J. Chang, H.N. Kuan, T.S. Chung, Omniphobic hollow-fiber membranes for vacuum membrane distillation, Environ. Sci. Technol. 52 (2018) 4472–4480. [15] C.H. Boo, J.H. Lee, M. Elimelech, Omniphobic polyvinylidene fluoride (PVDF) membrane for desalination of shale gas produced water by membrane distillation, Environ. Sci. Technol. 50 (2016) 12275–12282. [16] W. Qin, J. Zhang, Z. Xie, D. Ng, Y. Ye, S. Gray, M. Xie, Synergistic effect of combined colloidal and organic fouling in membrane distillation: measurements and mechanisms, Environ. Sci. Water Res. Technol. 3 (2017) 119–127. [17] Z. Wang, S. Lin, The impact of low-surface-energy functional groups on oil fouling resistance in membrane distillation, J. Membr. Sci. 527 (2017) 68–77. [18] X.W. Du, Z.Y. Zhang, K.H. Carlson, J. Lee, T.Z. Tong, Membrane fouling and reusability in membrane distillation of shale oil and gas produced water: effects of membrane surface wettability, J. Membr. Sci. 567 (2018) 199–208. [19] M.M. Ghaleni, A. Al Balushi, S. Kavakoli, M. Bavarian, S. Nejati, Fabrication of Janus membranes for desalination of oil-contaminated saline water, ACS Appl. Mater. Interfaces 10 (2018) 44871–44879.
4. Conclusion In this study, a Janus membrane with asymmetric wettability was fabricated via a layer-by-layer coating strategy with the aid of Teflon® AF1600 and PDA using a commercial PTFE/PP membrane as the sub strate. The resultant membrane exhibited strong hydrophobicity on its top layer with a WCA of 145.1 � 1.8� , while its bottom layer displayed hydrophilicity in-air with a WCA of 33.5 � 1.2� and superoleophobicity underwater with an oil contact angle of 154.2 � 2.2� . Due to the multiple-layered architecture, the Janus membrane could readily with stand common foulants existing in saline waters, e.g. surfactant, HA, and crude oil, and thus exhibited excellent MD performance over a 50-h DCMD test without fluctuations in water flux and salt rejection. More over, the Janus membrane also showed a rather stable MD behavior when treating multi-component wastewater, whereas significant wet ting or fouling were observed for the commercial PTFE/PP membrane. As a result, a higher TE and a correspondent lower STEC were obtained for the former membrane than the latter one. Overall, the newly developed Janus membrane has great potential for wastewater recla mation applications consisting of complex compositions. Author Statement Meng Li: Conceptualization, Methodology, Validation, Resources, Investigation, Data curation, Visualization, Writing- Original draft preparation. Kang Jia Lu: Characterization, Validation, Writing- Reviewing. Lianjun Wang: Conceptualization, Funding acquisition, Reviewing. Xuan Zhang: Conceptualization, Methodology, Funding acquisition, Writing- Reviewing and Editing. Supervision Tai-Shung Chung: Writing- Reviewing and Editing, Supervision.
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