Fabrication of Janus Membranes for Desalination of Oil-Contaminated

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Fabrication of Janus Membranes for Desalination of Oil-Contaminated Saline Water Mahdi Mohammadi Ghaleni, Abdullah Al-Balushi, Shayan Kaviani, Elham Tavakoli, Mona Bavarian, and Siamak Nejati ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16621 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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ACS Applied Materials & Interfaces

Fabrication of Janus Membranes for Desalination of Oil-Contaminated Saline Water


Mahdi Mohammadi Ghaleni1, †, Abdullah Al Balushi1, †, Shayan Kaviani1, Elham Tavakoli2, Mona Bavarian1, and Siamak Nejati1*

1

Department of Chemical and Biomolecular Engineering 2

Department of Mechanical and Materials Engineering The University of Nebraska-Lincoln Lincoln, NE 68588-8286, USA

KEYWORDS: Surface functionalization, membrane distillation, water desalination, oil-water separation, underwater oleophobic, chemical modification.

†: These authors contributed to this work equally.

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ABSTRACT Desalination of oil-contaminated saline water using membrane distillation requires hydrophobic membranes with underwater superoleophobic surfaces. For designing such membranes, the chemistry and morphology of the interfacial domains in contact with the contaminated water need to be adjusted such that a stable water layer, adhering to the surface, prevents oil droplets from wetting the membrane. In this article, we present an approach that relies on the controlled functionalization of the surface of Polyvinylidene Fluoride (PVDF) membranes; we adjust the surface topography of the membranes and introduce chemical heterogeneity to them. We show that the morphology of the PVDF surface can be altered by adjusting the composition of the nonsolvent bath, used for the phase inversion process. Also, we render the surface of the membranes hydrophilic by using an alkaline chemical bath solution. The membrane morphology and effectiveness of our chemical treatment were confirmed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Fourier-transformed infrared spectroscopy (FTIR), and zeta potential measurements. A stable underwater contact angle, higher than 150o, was observed for both canola oil (ρ~0.913 gcm-3, γ~31.5 mNm-1) and Hexane (ρ~0.655 gcm-3, γ~18 mNm-1). We evaluated the performance of both pristine and functionalized membranes in a laboratory-scale direct contact membrane distillation (DCMD) setup and desalinated a saline solution contaminated with 500 ppm canola oil. Our results show that oil does not wet the functionalized membrane during the desalination process. The average permeates flux and salt rejection values for the functionalized membranes were 45±5 liter per meter square per hour (Tfeed=70 oC, Tdistillate=20 oC) and 99.99%, respectively.


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1. INTRODUCTION Water scarcity, one of the most severe global challenges, necessitates the development of methods to enable enhanced water recovery and reuse.1–3 To date, among all classes of water desalination and treatment methods, membrane-based processes have received the most attention due to their low energy demand, lower environmental impact, and their applicability.3–5 Nonetheless, the application of membrane-based technologies in water treatment encounters challenges where the feed solution is contaminated with organic compounds such as industrial oil and waste.6–9 Given recent reports on the increase of industrial waste and oil leakage incidents,10 and the large volume of produced water from fracking practices,9 specific considerations are necessary for designing membrane-based technologies suitable for treating these waters. Membrane distillation (MD) is a promising candidate for desalination of seawater and brackish water contaminated with oil.6,9,11,12 In a typical direct contact membrane distillation (DCMD) process, a hydrophobic membrane acts as a physical barrier between the feed (hot saline water) and a cold distillate stream, allowing for water vapor to diffuse through the porous domains but hindering water from pasing through. The driving force in MD is the gradient of water vapor pressure induced by the temperature difference across the membrane.4,13 Compared with other desalination methods, such as reverse osmosis (RO), MD can provide a theoretical, non-selective, 100% rejection value for non-volatile species.14–16 Additionally, the transmembrane mass transfer in MD is not sensitive to the water salinity.17,18 MD can be coupled with affordable energy resources (e.g., solar and waste heat) due to its low temperature (50-80 °C) requirement.1,19–22 Also, compared to the pressure-driven processes such as RO, the low-pressure requirement in MD (atmospheric pressure) reduces the fouling propensity of the membranes.18,23 Membranes used in the MD process are made of hydrophobic materials (typically, polymeric materials) to hinder the passage of liquid water. Nonetheless, hydrophobic materials are prone to wetting by organic compounds and fluids.24,25 This susceptibility results from the longrange molecular interactions between organic contaminants and polymer surface, increasing the organic fouling propensity of hydrophobic membranes.9,26,27 Therefore, to desalinate oilcontaminated water resources through MD, membranes with special wettability and surface chemistry are needed. To create an underwater oleophobic membrane, both morphological and chemical properties of the membrane surface should be properly tailored.5,28–31 3 ACS Paragon Plus Environment


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Compared to a superhydrophobic surface in contact with an oil-contaminated water solution, shown in Figure 1, an underwater superoleophobic surface generally possesses one critical property: a stable water layer on the surface prevents oil and organic compounds from adhering to the surface and blocking the membrane pores.28,32

Figure 1. The schematic of (a) the configuration of feed, membrane, and distillate domains in the direct contact membrane distillation process, (b) hydrophobic membrane surface in contact with oil-contaminated feed stream, and (c) an underwater oleophobic surface in contact with oilcontaminated feed stream. Recently, several studies have demonstrated that the structural and chemical surface modification of hydrophobic membranes is an effective way to tailor the properties of MD membranes for desalination of oil-contaminated water.6–9,33–36 To date, the reported surface modification schemes are focused on the addition of either a hydrophilic or an omniphobic layer on the top of a hydrophobic membrane. These schemes are composed of several steps, and in most cases, the addition of the extra layer leads to pore blockage. 7,37–39 Consequently, the permeate flux for the modified membranes is reduced after the surface functionalization process. Here, we present a scalable approach to fabricate symmetrical PVDF membranes with the desired surface topography, and to functionalize the surface of these membranes. We developed a one-step process to render the surface of hydrophobic PVDF membranes hydrophilic while keeping the bulk of materials hydrophobic. Using this approach, we created a stable, oil-wettingresistant membrane (under-water oil repellent) for desalination of oil-contaminated water


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resources. We evaluated the performance and durability of the fabricated membranes in the DCMD process using a synthetic oil-water emulsion as a feed model. 2. Materials and methods 2.1. Materials and Chemicals Polyvinylidene fluoride (PVDF) (MW: 530 kDa), triethyl phosphate (TEP) (97%) were purchased from Sigma Aldrich. Potassium hydroxide (KOH, >85%), sodium chloride (NaCl, >99%), hexane (>98.5%), ethanol (99.5%), and isopropyl alcohol (IPA, >99.5%) were purchased from Fisher Scientific. Deionized (DI) water was obtained from a Simplicity® ultrapure water purification system (Millipore, Billerica, MA). Ultrapure nitrogen was purchased from Matheson Gas Company. Porefil® was purchased from Porometer, Belgium. The canola oil was purchased from Wesson Oil. 2.2. Membrane Fabrication The polymer solution was prepared by dissolving 12 wt.% PVDF (530 kDa) pellets in TEP. The solution was stirred overnight at 125 °C and 400 RPM on a hotplate (Corning Inc.). We left the polymer solution to rest at room temperature for six hours to cool and become free of air bubbles. Then, using a casting knife (Gardco), the solution was cast on a glass plate at a speed of approximately 5 cm/s (room temperature; relative humidity ~52%). The membrane thickness was adjusted by changing the gate height of the casting knife. After casting, the glass plate was submerged in a non-solvent (coagulation) bath within five seconds to induce the non-solvent induced phase separation (NIPS). The coagulation bath was prepared by mixing IPA and DI water at different volume ratios; the volumetric fraction of IPA in the coagulation bath was varied from 30 v.% to 70 v.%. After casting, the membranes were left on the glass plate for five minutes in the coagulation bath. Subsequently, we transferred the membranes to a pure DI water bath to remove the residual solvent. After complete solvent removal, the membranes were rinsed with ethanol and then dried for 12 hours in a temperature-controlled oven (Quincy Lab, 20 GC) set at 75 oC. 2.3. Chemical Modification Approach To functionalize the surface of the cast membranes and render the surface hydrophilic, we assembled the dried PVDF membranes into a custom-made modification cell. The cell consisted 5 ACS Paragon Plus Environment


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of 10 cm x 10 cm high-density polyethylene (HDPE) spacer resting on a rubber gasket (Natural Rubber, McMaster-Carr), that was placed on the membrane surface. The whole assembly was clamped to a glass plate support to create a solution well for surface functionalization. Figure 2 shows the schematic of the cell. 30 mL of concentrated KOH solution (T~25 oC) was transferred to the cell. The cell was placed on a shaker (Fisher Scientific) set at 50 RPM and the modification time varied from 3 to 36 hours. We covered the entire cell with parafilm (VWR International) to prevent water evaporation. After the reaction, the membrane surface was thoroughly rinsed using DI water for one minute. Then, the cell was refilled with DI water and placed on the shaker (50 RPM) for another 30 minutes to remove any trace of KOH. The overhead solution was discarded, and the membrane was stored in DI water until tested for performance in a laboratory-scale DCMD set up.

Figure 2. Schematic of the modification cell used to functionalize the surface of PVDF membranes. An HDPE cell is clamped on the top of the PVDF membrane, placed on a glass plate, and the KOH solution was poured into the well. 2.4. Membrane Characterization Scanning electron microscopy (SEM) images were taken using an FEI Helios NanoLab 660 microscope. To obtain the cross-section images, we wet the membranes and freeze-fractured them using liquid nitrogen. To remove the residual moisture, we dried the samples in a vacuum oven set at 60 oC for three hours. After drying, the samples were cut to size, mounted on the SEM stub, and coated with 60 nm of gold, using a Ted Pella sputtering machine (108-Auto) before imaging. Atomic force microscopy (AFM) measurements were carried out under ambient conditions using a Bruker Icon AFM instrument. The measurements were performed in peak force tapping mode (Scansyst-Air) using a silicon tip with a nominal resonant frequency (f0) of 70 kHz and nominal tip radius of 2 nm. Height and phase contrast data were recorded. The measurement was performed over 25 µm2, and the average roughness was determined from the entire sample area. 6 ACS Paragon Plus Environment

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X-ray photoelectron spectroscopy (XPS) was performed using a K-Alpha XPS system (ThermoFisher Scientific). Survey XPS spectra were acquired at 100 W with a pass energy of 200 eV over the range of 0−1350 eV with 1 eV resolution, 100 ms dwell time, and averaged over three scans. High-resolution XPS spectra of C1s, F1s, and O1s core electrons were acquired over a spot of approximately 400 μm with 50 W beam power and averaged five times with 0.1 eV resolutions at 50 eV pass energy with 100 ms dwell time. Fourier-transform infrared spectroscopy (FTIR) measurements were performed using the attenuated total reflection (ATR) module of Bruker Alpha-p. The ATR crystal was cleaned using pure IPA before every measurement. A small (1 cm x 1 cm) sample was cut to size and placed on the diamond crystal of the ATR module. The FTIR measurements were performed using 24 highresolution scans on each sample with a resolution of 4 cm-1. The zeta potential of the pristine and functionalized PVDF membrane surfaces was measured using a streaming-potential analyzer (SurPASS, Anton Paar). The membranes were attached to the parallel planar surfaces of an adjustable gap cell (Anton Paar). The membrane surfaces were separated by ca. 100 µm using the adjusting knobs. The electrolyte solution (1 mM KCl) flowed at different pressures along the gap between two parallel surfaces. The electrolyte flow generated a streaming potential from which the zeta potential of the membrane surface was calculated using the Helmholtz–Smoluchowski equation.40 The pH of the electrolyte was adjusted by automatic addition of KOH or HCl when needed. The water contact angle on the membrane surfaces was measured using an optical tensiometer (Rame-hart, Model 590) and the sessile drop method. A 5 μL DI water droplet was placed on the dried membrane sample. We performed the measurements on three random points on each sample and presented the data with one standard deviation. The membrane porosity was measured using the gravimetric method, as explained elsewhere.41 We performed five independent measurements and presented the data with one standard deviation. The liquid entry pressure (LEP) and gas permeation measurements were performed using the porometer setup described in the Supporting Information, Section 6. For the LEP measurements, the overhead space of the membrane in the filter holder was filled with DI water. Subsequently, the pressure behind DI water was increased gradually. The LEP is reported as the pressure at which a gas flow is detected by the flowmeter, placed inline at the outlet of the 7 ACS Paragon Plus Environment


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filter holder. We performed three measurements and presented the data with one standard deviation. For the gas permeation test, we flowed ultrapure nitrogen into the membrane at different pressures ranging from 2 to 40 kPa. The nitrogen permeation through the membrane was recorded using a digital flowmeter, connected to a computer. We performed three measurements and presented the data with one standard deviation. The performances for the control and modified PVDF membranes were evaluated using a laboratory-scale DCMD unit described elsewhere.41 We placed the membrane into a custom-built cell with channel dimensions of 77 mm in length, 26 mm in width, and 3 mm in depth. The effective membrane area exposed to feed and distillate streams was about 19 cm2. Two plastic spacers (Nylon, McMaster-Carr) were used in the feed and permeate channels to support the membrane in the cell. The temperatures of feed and permeate streams were maintained at 70 °C and 20 °C, respectively. The control feed solution was composed of 0.6 M sodium chloride (NaCl) in DI water. To make the oil-water emulsion, we added 0.05 v.% canola oil (ρ=0.913 gcm-3, γ~31.5 mN/m) to the solution and rigorously mixed it for at least one hour at 70 oC to obtain welldistributed oil droplets. A fine plastic mesh (pore size: 100 m) was placed at the entrance of the DCMD module to redistribute the oil droplets before they re-entered the module. The water vapor flux across the membrane (J) was determined by the gravimetric method.41 The distillate conductivity was continuously measured to calculate the NaCl concentration in the distillate stream (Cd). The cumulative salt mass in the distillate stream, VdCd, was divided by the volume of water vapor flux to find the permeate salt concentration and salt (NaCl) rejection:  VC  R NaCl %= 1- d d  ×100  CI JA m t 

(1)

where, Vd is the volume of permeate stream, CI is the initial NaCl concentration in the feed (0.6 M), Am is the tested membrane area, and t is the time. 9 3. RESULTS AND DISCUSSION 3.1. Membrane Morphology The membrane morphologies were adjusted by varying the composition of the nonsolvent (coagulation) bath. All the fabricated membranes, regardless of processing conditions, were self8 ACS Paragon Plus Environment

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supporting and flexible, allowing them to be used in MD operation without additional mechanical support. Figure 3 (a) represents the cross-section image of the membranes made in the coagulation bath that includes 70 v.% IPA. The membranes featured an interconnected symmetrical structure. The cross-section of membranes made using coagulation baths with different compositions, including 30 v.% and 50 v.% for IPA in water, are shown in Supporting Information, Figure S1. We observed that the depth of the skin layer on the top surface, liquid interface, can be adjusted by variying the IPA concentration in the coagulation bath. As shown in Figure S1, a dense skin layer (~1-2 µm) formed on the surface of the membrane fabricated using 30 v.% IPA in the coagulation bath. In contrast, the membrane made using 70 v.% IPA in the coagulation bath has a more open structure and no discernible skin layer at the interface. This trend also can be observed in Figures 3 (b), 3 (d), and 3 (f), which show the top-down structure of the membranes. At low IPA concentrations, the top surface of the membrane quickly solidifies, creating a dense skin layer at the nonsolvent-membrane interface. As a result, the rate of solvent-nonsolvent exchange through the remainder of the membrane slows, leading to an asymmetric structure. On the other hand, by increasing the IPA concentration in the coagulation bath, a delayed phase separation occurs at the top surface of the membrane, resulting in a more open surface and symmetric membrane structure. The effect of IPA concentration in the coagulation bath on the rate of phase separation can be explained by calculating the difference in the Hansen solubility parameters of the nonsolvent– polymer system.42 By adding IPA to the coagulation bath, the difference in the solubility parameter will move from 33.4 MPa0.5 (for PVDF-water) towards 9.7 MPa0.5 (for PVDF–IPA), causing a uniform and slow phase separation through the polymer film, which in turn results in a symmetrical membrane structure.41



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Figure 3. Scanning electron microscopy (SEM) images of pristine membranes fabricated using different compositions for the nonsolvent. Here, (a), (b), and (c), respectively, show the cross-section, top, and bottom (glass side) surfaces of the membrane fabricated in a non-solvent bath composed of 70 v.% IPA in DI water; the inset shows that the membrane is sturdy and rollable. Here, (d) and (e) are SEM images of the top and bottom surfaces of a membrane made in 50 v.% IPA in DI water; (f) and (g) are SEM images of the top and bottom surfaces of a membrane made in 70 v.%, IPA in DI water. The composition of the polymer solution was 12 wt.%, PVDF (530 kDa) in TEP. The gate height of the casting knife was adjusted to be approximately 230 µm. The scale bars for (b-g) are 2 µm. To understand the effect of the phase inversion condition on surface morphology, we studied the surface morphology of membranes, using AFM analysis,. Figure 4 shows AFM height images of the membranes fabricated using different IPA concentrations in the coagulation bath. In Figure 4 (a), (c), and (e), the roughness of the membrane top surface significantly changes with increased IPA concentration in the coagulation bath. We calculated the average surface roughness (Ra) values—quantified over an area of 5 µm×5 µm —for 30 v.% IPA, 50 v.% IPA, and 70 v.% IPA samples to be ~86, ~117, and ~243 nm, respectively. Also, the line profiles indicate that the height fluctuation ranged from -200 nm to 200 nm, -300 nm to 300 nm, and -1200 nm to 750 nm for the for the 30 v.% IPA, 50 v.% IPA, and 70 v.% IPA samples, respectively.


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Figure 4. Atomic force microscopy (AFM) images of membrane surfaces fabricated using different nonsolvent bath compositions. Images (a), (c), and (e) are the AFM height images featuring the topography of membrane top surfaces; (b), (d), and (f) are height profiles through the lines indicated in images (a), (c), and (e), respectively. Membranes were fabricated using 12 wt.% PVDF (530 kDa) in TEP and different IPA content in the nonsolvent/coagulation bath: (a) and (b) 30/70 v.%, IPA/water; (c) and (d) 50/50 v.%, IPA/water; (e) and (f) 70/30 v.%, IPA/water. The average roughness and its mean square were estimated over a 5 µm×5 µm sample area. 3.2. Membrane Characteristics Figure 5 (a) displays the liquid entry pressure (LEP) and the porosity of the membranes fabricated using different IPA concentrations in the coagulation bath. The data indicated that the LEP of the membrane varies significantly as a function of IPA concentration in the coagulation bath. Changing the IPA concentration in the coagulation bath, we noticed that the LEP values for the membranes varied by ca. 36%. Similarly, we observed that the membranes’ porosities varied when we fabricated membranes using different compositions for the coagulation baths. As shown in Figure 5(a), by changing the concentration of IPA from 30 v.% to 70 v.% in the coagulation medium, the average porosity was increased, while the LEP was reduced. We attributed this 11 ACS Paragon Plus Environment


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phenomenon to the different kinetics and dynamic trajectory of the phase separation process. At higher IPA concentrations, the phase separation goes through a prolonged metastable (spinodal decomposition) region and crystallization phenomenon, resulting in the growth of larger polymer crystals with the interconnected structures.43 It was previously reported that the formation of larger polymer crystals results in higher void-fraction (porosity) in the membrane structure.44 The surface contact angle of membranes fabricated using different coagulation media was measured with DI water using the sessile drop method. Figure 5 (b) shows that the contact angle on the top and bottom surface of membranes, cast using different volume fractions of IPA in the coagulation media. As the IPA concentration in the coagulation bath was increased from 30 v.% to 70 v.%, the contact angles on the top surface were increased from 102±1.72° to 121±3.2, respectively. However, minimal changes in the contact angles for the bottom surfaces of the membranes were observed. We attributed this phenomenon to the topographical variations of the membrane surfaces, shown earlier in Figure 4. It was previously reported that a higher concentration of IPA in the coagulation bath results in the formation of a more open pore structure with an increased average surface roughness.41 The induced roughness accommodates air pockets at the interface of air-water-membrane, creating a stable Cassie-Baxter wetting state.45 Consequently, the apparent contact angles on the top surface of the membranes increase noticeably. We did not observe any significant change in the morphology of the bottom surface of the membrane; this observation corroborates the similar contact angle values measured for the bottom surface of different membranes. The effect of IPA concentration in the coagulation bath on the surface porosity of the membrane was measured by gas permeation, using an upstream pressure of 40 kPa for nitrogen. Figure 5 (c) shows that the nitrogen flux through the membrane increased by ~150% when the volume fraction of IPA in the coagulation bath increased from 30 v.% to 70 v.%. The observed increase in the nitrogen flow through the membranes is attributed to the increased surface porosity of membranes. The averaged water vapor flux measured under the DCMD operation, shown in Figure 5 (c), displays a similar trend. We confirmed this by measuring the average pore size distribution of the membranes. As shown in the Supporting Information, Table S1, the mean pore size of the membranes increased from 204 nm to 395 nm and 589 nm when the IPA volume fraction in the coagulation bath was increased from 30% to 50% and 70%. 12 ACS Paragon Plus Environment

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Figure 5. The changes in the physical properties of PVDF membranes with a variation of isopropyl alcohol (IPA) in the nonsolvent bath. (a) shows the trend of liquid entry pressure and porosity of different membranes; (b) displays the average values of the water contact angle on the top and bottom surface of the membranes; (c) presents the area nitrogen (N2) and water vapor fluxes for each membrane (the N2 fluxes were acquired at comparable pressure difference across the 13 ACS Paragon Plus Environment


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membrane as the MD process, ~40 kPa). The thickness, averaged from three measurements, of membranes tested in MD was 79.7±4.78 µm, 64.3±1.25µm, and 72.33±4.9 µm for the 70% IPA, 50% IPA, and 30% IPA membranes, respectively. The porosity values are the average from five measurements; the LEP, contact angles, water flux, and nitrogen flux were all averaged from three measurements. The error bars represent one standard deviation. 3.3. Surface modification and chemistry of the reaction 3.3.1

The modification reaction pathways: the effect of membrane dryness Surface treatment of PVDF membranes was performed using different concentrations (1M,

2M, 3M, and 8M) of KOH solution. Depending on our sample preparation, we realized two different pathways. This observation is in agreement with the previous report on the PVDF treatment in strongly basic media.46 The first pathway, shown in Figure 6 (a), displays the HF elimination pathway and formation of conjugated double carbon-carbon bonds through the PVDF chains. Surface treatment with a strong base (OH-) initiates a one-step elimination (E2) reaction with a single transition state. Thus, the carbon-hydrogen and carbon-fluorine bonds break simultaneously, 47,48 where the two leaving groups (hydrogen and fluorine) are antiperiplanar with more favored staggered conformation. The second pathway, displayed in Figure 6 (b), is the nucleophilic substitution (SN) reaction, where the hydroxyl base acts as the nucleophile. SN reaction results in the substitution of fluorine groups by hydroxyl groups. We realized that the primary parameter influence of the reaction pathway in the PVDF surface functionalization is the dryness of the samples. For the membrane dried in the oven after casting, we noticed that, regardless of the KOH concentration and the functionalization time, the nucleophilic substitution is the dominant mechanism. This was confirmed by measuring the water contact angle before and after the functionalization step. As shown in Figure 7 (b), the modified PVDF membrane shows a contact angle of ~62o,, which is very different from that of the pristine membranes (Figure 7 (d), CA ~105o). The reduction of water contact angle values is attributed to the enrichment of the membrane surface with the -OH groups. On the other hand, for the sample that was only air-dried, the modification resulted in a higher elimination reaction. Consequently, discoloration, which is an indication of double bond formation along the PVDF chain, was observed.46,49,50 The contact angle measurements on the wet-


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modified membranes showed a relatively higher value (~97o) than that of dry-modified membranes, ~62o (see Supporting Information, Figure S2). It is expected that for the oven-dried samples, the diffusion of the basic solution into the membrane will be limited; this phenomenon can lead to a predominant substitution reaction (SN2). Therefore, higher hydroxyl groups can be found on the surface, and as a result, the hydrophilicity of the membrane surface increases. In contrast, for the samples that are air-dried, the KOH solution diffuses into the depth of the sample, favoring the E2 reaction, and the dehydrofluorination takes place in the bulk of the samples. This is in agreement with the increased intensity of the brownishred color of the membranes as a function of treatment time.

Figure 6. Schematic of the modification reaction mechanism. (a) shows the E2 elimination reaction mechanism; the inset shows that the color of the membrane treated with an 8M KOH turned brown after three hours of treatment, (b) shows the SN2 reaction mechanism; the inset shows that the color of a dry membrane treated with an 8M KOH did not change during the reaction time of 25 hours. 3.3.2

Surface properties of the functionalized membranes To analyze the surface properties of the functionalized membranes, we performed

Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and 15 ACS Paragon Plus Environment


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contact angle measurements on the membrane coupons. Figure 7 (a) shows the attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectra of the control and functionalized PVDF membranes. Comparing the spectra of pristine and functionalized PVDF membranes, we can see that the shape of the spectra between 1400 cm-1 and 800 cm-1 –assigned to the characteristic vibration modes of the distinct functional groups of the control PVDF 51–remained fairly unchanged. The only noticeable change was the reduction in the intensity of the peak located at ~1200 cm-1 (related to C-F groups). We attributed this to the removal of fluorine groups from the membrane surface. The appearance of broad OH stretching vibration modes around 3300 cm−1 is an indication of increased OH functional groups on the membrane surface. The FTIR spectra for the membranes treated for different duration times, shown in Supporting Information, Figure S3, suggest that the surface functionalization was successful. We also investigated the effect of KOH concentration (1M, 2M, and 3M) on the surface functionalization process of PVDF membranes. We observed that the required time for successful functionalization of the membrane surfaces increased for a solution with a lower KOH concentration. For instance, when PVDF samples were treated using a 3M KOH, we did not observe the resonance of OH functional groups in the FTIR spectrum of the samples within the first 24 hours (see Supporting Information, Figure S4). This trend is expected as the rate of reaction is directly correlated with the concentration of KOH in the solution. Membrane surface charge also plays a significant role in the wetting properties and fouling of the membrane.52 For this reason, we performed zeta potential measurements over a pH range of 2.5 to 6 (see Supporting Information, Figure S5). The surface of the pristine membrane was found to be negatively charged at pH values higher than 2.7, consistent with other reported data.9,53 However, the membrane surface became more negatively charged after the surface functionalization process. This occurs because a hydrophilic surface offers closer proximity for anions (i.e., OH− and Cl−) from the background electrolyte, resulting in a greater negative surface charge. The contact angle for two different oils (canola oil and Hexane) was also measured to show the oleophobic properties of the membranes in water. As shown in Figure 7 (c), both canola oil (γ~31.5 mNm-1) and Hexane droplets (γ~18 mNm-1) exhibited underwater contact angles higher than 150o. The image analysis showed that the averaged underwater contact angles for the Hexane 16 ACS Paragon Plus Environment

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and canola droplets on the surface are 158o±6.33 and 151o±2.33, respectively. However, both oil droplets wetted the pristine membrane surface in less than five seconds, shown in Figure 7 (e).

Figure 7. (a) FTIR spectra of pristine and functionalized (8M-15h) PVDF membrane surfaces. The O-H stretching vibration modes are visible at the region between 3200 cm-1 to 3750 cm-1 , (b) shows the water contact angle on the modified membrane surface, whereas (c) displays underwater oil (Hexane and canola oil) on the modified membrane surface, (d) represents the water contact angle on the pristine membrane surface; (e) also shows that Hexane immediately wets the surface of the pristine membrane under water. We performed XPS measurements to obtain information about the effect of KOH treatment on the chemistry of membrane surfaces. Figures 8 (a) and 8 (d) compare the XPS survey spectrum of pristine and treated, for 15 hours, using 8M KOH at 25 oC, PVDF. Compared to the pristine spectrum, a reduction in fluorine intensity (F1s) occurs together with an increase in the intensity of the oxygen peak(O1s). As shown in Figure 8 (b), five Gaussian peaks centered at 291.7eV, 17 ACS Paragon Plus Environment


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288.9 eV, 287.7 eV, 287.2 eV, and 284.7 eV were identified in C1s core electron spectrum of pristine PVDF membrane. These peaks are assigned to CF2, C=O (adventitious carbon), C-O (adventitious carbon), CH2, and C-C/C-H, respectively.46,50,54–56 However, as shown in Figure 8 (b), for the functionalized surface, two new peaks were identified at 287.8 eV and 290 eV, attributed to the C-OH and FC-OH groups.46 The presence of FC-OH species strengthens the idea that the SN2 substitution is the dominant pathway, where the defluorination was limited to the surface of membranes. Figures 8 (c) and 8 (f) show the O1s core electron spectra for pristine and modified membranes, respectively. Three peaks centered at ~534.1 eV, ~533 eV and ~531.2 eV were identified in the O1s signal of the pristine membrane — these peaks are assigned to the oxygen species associated with the adventitious carbon contamination, C-O, C-OH, and C=O, respectively. For the modified membrane, however, the relative intensity of C-OH components increased compared to that of C=O species due to added OH groups as a result of the surface modification reaction. Moreover, the new peak that appeared at ~535 eV is attributed to the atmospheric water molecules adsorbed on the surface of functionalized membranes after surface functionalization.

Figure 8. X-ray photoelectron spectroscopy (XPS) spectra of functionalized PVDF membrane surface compared to that of pristine PVDF surface. Here, (a), (b) and (c) show the 18 ACS Paragon Plus Environment

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elemental survey, C1s core electron, and O 1s core electron spectra of the pristine membrane surface, respectively; however, (d), (e), and (f) present the elemental survey, C1s core electron, and O1s core electron after surface functionalization, respectively. 3.4. Membrane distillation performance To investigate the desalination performance of the control PVDF and modified hydrophilic membranes, we characterized our membranes in a laboratory-scale DCMD setup, using both control and synthetic feed solutions. Figures 9 (a) and 9 (b) present the normalized water flux and salt rejection profiles obtained from DCMD experiments for the pristine and modified membranes. We observed an average permeate flux of ~45±5 LMH for the pristine membrane during the 12hour testing period; both permeate flux and salt rejection remained constant. Additionally, the pristine and functionalized PVDF membranes were tested with a feed solution containing canola oil. The size distribution for the oil droplets was evaluated using direct imaging and image analysis as shown in Supporting Information, Figure S9. Figures 9 (a) and 9 (b) show the performance of the membranes challenged with the oil-contaminated feed solution. As can be seen, the permeate flux of the pristine membrane significantly drops compared to that of the functionalized membrane. This drop in the flux is concomitant with the reduction of salt rejection values. This phenomenon is attributed to the wetting of the membrane by oil and the consequent pore blockage (Supporting Information, Figure S6). As shown in Figure 9 (c), the salt rejection of the pristine membrane decreased significantly during desalination of the oilcontaminated water solution, suggesting the crossover of salt from the feed to the permeate channel. This observation can be explained by the fact that, once oil wets the surface of the pristine membrane, it gradually creates a channel between feed and distillate streams, through which ions can be transferred from the feed to the distillate stream. In contrast, the salt rejection and the permeate flux of the functionalized membrane remained stable during the entire experiment.



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Figure 9. MD performance of the pristine and functionalized membranes. (a) and (b) show membrane distillation performance of pristine and functionalized membranes (50% IPA) during 720 minutes of operation with zero oil in the feed solution, respectively, (c) and (d) show the performance of a pristine and functionalized membrane, respectively, used for desalination of a water sample including 500 ppm (0.05 v.%) canola oil in saline water. 4. CONCLUSION We presented a scalable approach to selectively perform chemical modification on the surface of PVDF membranes for desalination of oil-contaminated water. We fabricated symmetrical membranes with unique surface morphologies and introduced a chemical heterogeneity to these substrates. We identified two different chemical pathways for the alkaline treatment of PVDF membranes which were sensitive to the sample preparation and reaction condition. By controlling 20 ACS Paragon Plus Environment

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the drying process of membranes, we showed that it is possible to favor the desired functionalization scheme, rendering the outermost surface of the PVDF membranes hydrophilic. The DCMD results for desalination of the oil-contaminated salines water showed promising results, suggesting that this approach can be used to separate water from oil emulsion. ASSOCIATED CONTENT − Supporting Information SEM cross-section images of pristine membranes (Figure S1). Images of wet-treated and drytreated membranes after the reaction (Figure S2). FTIR spectra of pristine and dry-treated membranes after different reaction times (Figure S3). FTIR spectra of the PVDF membrane functionalized using low molarity of KOH (Figure S4). Zeta potential of pristine and functionalized membrane surfaces (Figure S5). Images of pristine and functionalized PVDF membranes used in desalination of oil-contaminated saline for five hours (Figure S6). Flow vs. pressure curves obtained by performing the dry/wet flow method on different membranes (Figure S7). A sample of surface pore size analysis before and after chemical modification (Figure S8). Table S1 includes the pore size information of the membranes. The size distribution of oil droplets in saline water acquired using image analysis (Figure S9). − Video file The video shows the interactions of Hexane droplets with the functionalized surface in water (Movie S1). ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from the Bureau of Reclamation, U.S. Department of Interior, through DWPR Agreement R17AC00139. The research was performed in part in the Nebraska Nanoscale Facility, National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience, which are supported by the National Science Foundation under Award ECCS: 1542182, and the Nebraska Research Initiative. REFERENCES (1)

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TOC: abstract graphics


Fabrication of Janus Membranes for Desalination of Oil-Contaminated

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