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Omniphobic Polyvinylidene Fluoride (PVDF) Membrane for Desalination of Shale Gas Produced Water by Membrane Distillation Chanhee Boo, Jongho Lee, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03882 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016
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Environmental Science & Technology
Omniphobic Polyvinylidene Fluoride (PVDF) Membrane for Desalination of Shale Gas Produced Water by Membrane Distillation
Chanhee Boo, Jongho Lee, and Menachem Elimelech*
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States
* Corresponding author: email:
[email protected]; Tel. +1 (203) 432-2789
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ABSTRACT
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Microporous membranes fabricated from hydrophobic polymers such as polyvinylidene fluoride
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(PVDF) have been widely used for membrane distillation (MD). However, hydrophobic MD
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membranes are prone to wetting by low surface tension substances, thereby limiting their use in
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treating challenging industrial wastewaters, such as shale gas produced water. In this study, we
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present a facile and scalable approach for the fabrication of omniphobic polyvinylidene fluoride
7
(PVDF) membranes that repel both water and oil. Positive surface charge was imparted to an
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alkaline-treated PVDF membrane by aminosilane functionalization, which enabled irreversible
9
binding of negatively charged silica nanoparticles (SiNPs) to the membrane through electrostatic
10
attraction. The membrane with grafted SiNPs was then coated with fluoroalkylsilane
11
(perfluorodecyltrichlorosilane) to lower the membrane surface energy. Results from contact
12
angle measurements with mineral oil and surfactant solution demonstrated that overlaying SiNPs
13
with ultralow surface energy significantly enhanced the wetting resistance of the membrane
14
against low surface tension liquids. We also evaluated desalination performance of the modified
15
membrane in direct contact membrane distillation with a synthetic wastewater containing
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surfactant (sodium dodecyl sulfate) and mineral oil, as well as with shale gas produced water.
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The omniphobic membrane exhibited a stable MD performance, demonstrating its potential
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application for desalination of challenging industrial wastewaters containing diverse low surface
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tension contaminants.
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TOC Art
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INTRODUCTION
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Natural gas from unconventional shale plays has the potential to secure energy supply and reduce
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carbon dioxide emissions by minimizing the dependence on fossil fuel energy production.1
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Breakthroughs in horizontal drilling and hydraulic fracturing technologies have allowed access
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to vast natural gas resources from shale formation.2 However, environmental impacts of shale
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gas production, including wastewater management and potential groundwater contamination,
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remain major challenges for efficient exploitation of this emerging energy resource.3, 4
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The Environmental Protection Agency (EPA) estimates 25,000−30,000 new shale gas wells
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were developed annually in the United States between 2011 and 2014, with two to four million
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gallons of water used for drilling and hydraulic fracturing of each well.5, 6 Shale gas produced
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water comprises water trapped in underground formation and flowback from hydraulic
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fracturing.7 This waste stream is characterized by high total dissolved solids (TDS)
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concentrations and hydrocarbon content (i.e., oil and grease) due to the nature of formation water
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and high levels of chemical additives used in hydraulic fracturing.8 Shale gas produced water is
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challenging to treat by conventional water treatment practices due to its high salinity and
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complex physicochemical composition.6
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Membrane distillation (MD) is an emerging technology that shows promise for efficient
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desalination of high salinity industrial wastewaters.9, 10 MD desalination is driven by a vapor
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pressure gradient between a hot feed stream and a cold permeate (distillate) stream. Performance
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of this thermal-based separation process is only slightly sensitive to the feed salinity, rendering
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MD suitable for desalination of highly saline shale gas produced water.6, 11 The temperature of
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shale gas produced waters, which originates from geothermal heat sources at well sites, can be as
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high as 100 ºC.6 Hence, the elevated temperature of produced water could be exploited to drive
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MD desalination.
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Despite the advantages of applying MD to produced water desalination, potential wetting of
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the MD membrane remains a practical shortcoming that hinders its implementation. Shale gas
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produced water contains high levels of low surface tension contaminants, such as oil and grease,
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organic solvents, and surfactants.12 These low surface tension substances can potentially wet
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conventional hydrophobic MD membranes, resulting in failure of the MD desalination process.
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Omniphobic membranes resisting wetting to both water and low surface tension liquids hold
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promise for MD application in treatment of shale gas produced water.13, 14 Omniphobicity can be
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realized by constructing surfaces with ultralow interfacial energy and re-entrant structures that
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together allow a metastable Cassie-Baxter state for the liquid−solid−vapor interface.15 Numerous
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approaches have been attempted to create surfaces that feature a re-entrant structure using
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electrochemical16 and plasma etching,17 anodization,18 and photolithography19, and to achieve
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ultralow surface energy by silanization19 and coating20, 21 of fluoroalkyl molecules. However,
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these techniques have mostly focused on fabricating omniphobic surfaces for impermeable
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prototype substrates (e.g., silicon wafer)22 or fabric materials.23
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Microporous polyvinylidene fluoride (PVDF) membranes have been widely used for MD
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applications because of their hydrophobic nature, excellent chemical compatibility, and physical
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robustness.24 The PVDF membrane structure properties that are ideal for MD, such as high
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porosity and sub-micrometer pore size, can be easily controlled by the well-established phase
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separation fabrication technique.25 However, conventional hydrophobic PVDF membranes are
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prone to wetting by low surface tension substances. Furthermore, surface engineering of PVDF
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membranes to achieve desired chemical and physical properties is challenging because PVDF is
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chemically inert and possesses no surface groups.
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This study presents a scalable approach to fabricate omniphobic membranes for desalination
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of shale gas produced water by membrane distillation. Specifically, we demonstrate surface
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modification of a conventional hydrophobic PVDF microporous substrate to produce an
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omniphobic membrane that exhibits surface wetting resistance to low surface tension substances.
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Desalination performance of the omniphobic membrane was evaluated in direct contact
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membrane distillation with synthetic wastewaters containing low surface tension contaminants,
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including surfactants and mineral oil. Produced water from shale gas industry was further
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employed to evaluate MD performance of the omniphobic membrane. Implications of the results
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for the use omniphobic membranes in MD to treat challenging industrial wastewaters are
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evaluated and discussed.
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MATERIALS AND METHODS
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Materials and Chemicals. ACS grade sodium hydroxide (NaOH, J.T. Baker, Phillipsburg,
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NJ), (3-Aminopropyl)triethoxysilane (99%, APTES) (Sigma-Aldrich, St. Louis, MO), anhydrous
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ethanol (100%, Decon Labs, Inc., PA), silica nanoparticles (SiNPs) with an average diameter of
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0.1 µm (AngstromSphere Monodispersed Silica Powder, Fiber Optic Center Inc., MA), and
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(heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane (FDTS, Gelest Inc., PA) were used for
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the surface modification of the PVDF substrate.
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Surface Modification of PVDF Membrane. A flat sheet polyvinylidene fluoride
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(PVDF) membrane with a nominal pore size of 0.45 µm and an average thickness of 125 µm was
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used as a substrate (HVHP, Millipore, Billerica, MA). The PVDF substrate was immersed in a
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7.5 M NaOH solution at ~70 °C for three hours to generate hydroxyl surface functionality. To
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ensure complete soaking of PVDF substrate in the NaOH solution, the membrane was prewetted
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by ethanol. An alkaline treated PVDF substrate was then soaked in 1% v/v APTES in anhydrous
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ethanol for one hour under gentle stirring to functionalize the surface with amine terminal groups,
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rendering the PVDF substrate positively charged. The APTES-functionalized PVDF substrate
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was immersed in an aqueous SiNP solution for one hour under gentle mixing. APTES
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functionalization and SiNP coating were executed with only the top surface of the membrane in
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contact with the suspension. The aqueous SiNP solution was prepared by dispersing 0.05 wt%
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SiNPs in acetate buffer with an ionic strength of ~1 mM. The pH of the suspension was adjusted
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to 4 to promote electrostatic attraction between the negatively charged SiNPs and the positively
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charged PVDF substrate.
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After attaching SiNPs to the PVDF substrate, the membrane was coated with
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perfluorodecyltrichlorosilane (FDTS) via vapor-phase silanization to lower the membrane
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surface energy. An SiNP-coated PVDF membrane was fixed on the wall of a glass bottle (Septa
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jars, Thermo Fisher Scientific, Inc., MA) using heat resistant tape (Kapton® Polyimide, Uline,
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WI). The bottle was nitrogen purged for 20 minutes through the septa cap. After nitrogen
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purging, 100 µL FDTS was placed in a small petri dish inside the bottle. Then, the SiNP-coated
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PVDF membrane was silanized in a vacuum oven (vacuum pressure at ~80 kPa and temperature
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at ~100 °C) for 17 hours.
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The stability of surface coating with SiNPs and FDTS was assessed by reevaluation of SEM
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images and contact angles after the modified membranes were subjected to harsh chemical and 5 ACS Paragon Plus Environment
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physical stresses. Chemical stress was applied by exposing the modified PVDF membrane for 20
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minutes to a pH 2 solution (HCl), a pH 12 solution (NaOH), or a 1.0 M NaCl solution, followed
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by thorough rinse with DI water. Physical stress was exerted by immersing the membranes in a
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sonicating water bath (Fisher Scientific F60) for 20 minutes.
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Membrane Characterization. Surface morphology of the modified PVDF membranes
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was investigated by scanning electron microscopy (SEM, Hitachi SU-70). Before imaging,
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membrane samples were sputter-coated with a chromium layer (BTT-IV, Denton Vacuum, LLC,
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Moorestown, NJ). To obtain cross-section images, membranes were dried at room temperature
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and freeze-fractured using liquid nitrogen before the chromium coating. Acceleration voltage of
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5.0 kV was applied to image all membrane samples.
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Zeta potential of membrane surface was evaluated by a streaming potential analyzer utilizing
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an asymmetric clamping cell (EKA, Brookhaven Instrument, Holtsville, NY) as described
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elsewhere.26 Measurements were performed with a solution containing 1 mM KCl and 0.1 mM
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KHCO3. Electrophoretic mobility of the silica nanoparticles (SiNPs) was measured in a
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background electrolyte of 1 mM NaCl using a Zeta Potential Analyzer (ZetaPals, Brookhaven
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Instruments, Holtsville, NY) at different pH values. The zeta potentials of SiNPs were calculated
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from electrophoretic mobility measurement by using the tabulated data of Ottewill and Shaw.27
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The elemental composition of the membranes was investigated by Fourier transform infrared
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(FTIR) spectroscopy with attenuated total reflectance (ATR). Absorbance spectra were measured
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with 32 scans of each sample at a spectral resolution of 1 cm-1. Background measurements in air
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were collected before each membrane sample measurement.
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Contact angles of the modified PVDF membranes for liquids with a wide range of surface
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tensions, including water (γ = 72.8 mN/m), 0.1 mM SDS in 1.0 M NaCl (γ ≈ 38 mN/m), and
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mineral oil (γ ≈ 31 mN/m) were measured by a goniometer (OneAttension, Biolin scientific
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instrument) using the sessile drop method. A 5-µL liquid droplet was placed on the membrane
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sample and photographed using a digital camera for three minutes. The left and right contact
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angles were analyzed from the digital images by a post-processing software (OneAttension
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software). The measurements were conducted on a minimum of two random locations with three
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different membrane samples, and the data were averaged.
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Model and Shale Gas Produced Water Feed Solutions. NaCl (1.0 M) solution at
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60 °C and DI water at 20 °C were employed as feed and permeate streams, respectively, for the
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initial two hour DCMD runs. After stabilization, sodium dodecyl sulfate (SDS) or mineral oil
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was added to the feed solution every two hours during the DCMD experiment to evaluate
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wetting resistance of the control and the modified PVDF membranes. The SDS concentrations in
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the feed solution after sequential SDS addition were 0.05, 0.1, and 0.2 mM. Surface tensions
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with varying SDS concentrations in DI water and 1.0 M NaCl solution were determined by a
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tensiometer (Sigma 300, KSV Instrument, Helsinki) using the Wilhelmy Plate method. Mineral
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oil-in-water emulsion at concentration of 1% v/v was prepared by adding 0.1% v/v Tween80®
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(non-ionic surfactant) (Sigma-Aldrich, St. Louis, MO) as an emulsifier. The mineral oil
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concentrations in the feed solution after addition of emulsion every two hour DCMD were 0.001,
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0.005, and 0.01% v/v, which correspond to 8, 40, and 80 mg/L. Size of mineral oil droplets in the
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feed was analyzed by dynamic light scattering (DLS) measurements (ZetaPals, Brookhaven
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Instruments, Holtsville, NY). The feed solution (initial volume of 1 L) was replenished every 50
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mL loss to maintain variation of SDS or mineral oil concentration within 5%.
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Shale gas produced water from the Permian Basin in Texas (provided by Gradiant
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Corporation) was pre-filtered through 16-µm filter (Ashless Grade 44, GE Whatman, PA) to
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remove large particulate and suspended matters before they were supplied to DCMD
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experiments. The major composition and key properties of the pre-filtered shale gas produced
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water were analyzed by a third-party analyzing laboratory (Environmental Service Lab, Inc., PA)
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(Table S1). The biological activity of the shale gas wastewater was minimized by storing the
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sample at 4 °C. The DCMD experiments with shale gas wastewater were conducted until the
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feed solution (initial volume of 1 L) was concentrated by 50% (or until the permeate volume
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reached 500 mL).
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Membrane Distillation Performance Tests. The MD performance of the control and
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modified PVDF membranes was evaluated by a laboratory-scale direct contact membrane
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distillation (DCMD) unit. The membrane was inserted into a custom-built transparent acrylic cell
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with channel dimensions of 77-mm in length, 26-mm in width, and 3-mm in depth. The effective
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membrane area exposed to feed and permeate (distillate) streams was 20.0 cm2. Spacers were
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inserted in both feed and permeate channels to support and maintain the membrane geometry in
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the cell. The temperatures of feed and permeate solutions were maintained at 60 and 20 °C,
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respectively. A slightly higher cross-flow rate for the feed stream, 0.4 L/min (cross-flow velocity
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of 8.5 cm/s), than for the permeate stream, 0.35 L/min (cross-flow velocity of 7.5 cm/s), was
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used to facilitate the detection of MD membrane wetting.
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The water vapor flux, Jw, across the membrane was determined by measuring the increase in
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permeate weight. Electric conductivity of the permeate solution was monitored to assess the
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NaCl concentration in the permeate stream, CP. The cumulative salt mass in the permeate stream,
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VPCP , was divided by the volume of water vapor flux to calculate the permeate salt
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concentration, from which the salt (NaCl) rejection, RNaCl, was determined using
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V C /J A t R NaCl = 1 − P P w m 100 CF
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where Vp, is the volume of permeate stream, CF is the initial NaCl concentration in the feed (1.0
183
M), Am is the membrane area, and t is time.
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RESULTS AND DISCUSSION
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Surface Properties of Modified Omniphobic Membrane. Silica nanoparticles (SiNPs)
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were used for the omniphobic surface modification of a PVDF substrate because SiNP is
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abundant in hydroxyl functional groups that allow surface fluorination via well-established silane
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chemistry to lower the membrane surface energy.28 Further, spherical SiNP is a geometry that
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provides a re-entrant structure for substrate surface, which is critical for achieving surface
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omniphobicity.29
(1)
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SiNPs possessing negative surface charge were attached to aminosilane functionalized PVDF
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substrate via electrostatic attraction (Figure 1). First, a PVDF substrate was treated with high
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concentration NaOH solution (7.5 M) under high temperatures (~70 °C) to generate hydroxyl
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functional groups on PVDF polymer chains. Mechanisms for the formation of hydroxyl groups
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on a PVDF chain by alkaline treatment are well described in previous studies.30, 31 After alkaline
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treatment, the PVDF substrate was soaked in an aminosilane solution (APTES 1% v/v in
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anhydrous ethanol) to functionalize the surface with amine terminal groups. The APTES
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covalently binds to the hydroxyl groups on the PVDF substrate via hydrolysis and condensation, 8 ACS Paragon Plus Environment
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rendering the substrate surface positively charged. Positive surface charge of the APTES-
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functionalized PVDF membrane enables binding of the negatively charged SiNPs via
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electrostatic attraction. Grafted SiNPs were then coated with perfluorodecyltrichlorosilane
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(FDTS) to lower the surface energy of the membrane.
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Zeta potentials of the control, alkaline treated, and amine functionalized PVDF membranes
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were evaluated to confirm a binding mechanism between the SiNPs and the PVDF substrate
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(Figure 2A). The control PVDF substrate was found to be negatively charged over the entire pH
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range investigated, consistent with other reported data.32, 33 Because the control PVDF membrane
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does not possess a fixed surface charge, the observed negative zeta potential may be attributed to
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the adsorption of anions, such as OH- and Cl- from the background electrolyte, onto the
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membrane surface, as proposed elsewhere.34, 35 Zeta potential of the PVDF substrate became
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more negative and stable over the whole pH range after treatment with alkaline solution (7.5 M
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NaOH). The PVDF membrane became hydrophilic after alkaline treatment due to the formation
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of hydroxyl groups.30,
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substrate is likely because the hydrophilic surface offers closer proximity for anions (i.e., OH-
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and Cl-) from the background electrolyte. Functionalization of the substrate surface with APTES
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reduced the negative zeta potential (i.e., more positive charge) of the PVDF membrane with an
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extrapolated isoelectric point of ~5.2 (Figure 2A). The SiNP coating was performed under
218
suspension pH at 4, where the zeta potential of the amine functionalized PVDF substrate is
219
positive while that of the SiNPs is negative.
31
Increased negative surface charges of the alkaline treated PVDF
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Surface morphology of the PVDF substrates before and after SiNP coating was investigated
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by scanning electron microscope (SEM). For comparison, top-down and cross-section SEM
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images of the control PVDF and the SiNP-coated PVDF membranes are shown in Figure 2C.
223
The top-down SEM image shows that dense SiNP coating formed on the surface of the PVDF
224
substrate (Figure 2C-b). In addition, the cross-section SEM image of the SiNP-coated membrane
225
(Figure 2C-d) indicates that SiNPs penetrated into the porous substrate and created a 3-5 µm
226
thick SiNP coating layer. A re-entrant structure developed by a spherical or cylindrical surface
227
geometry creates a local kinetic barrier for the transition from the metastable Cassie-Baxter state
228
to the fully wetting Wenzel state for low surface tension liquids. Results from the SEM images
229
clearly show a re-entrant surface structure achieved by coating with spherical SiNPs. In addition,
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SiNP coating inside the pores (Figure 2C-d) is expected to offer multiple kinetic barriers, and
231
thus further increase membrane wetting resistance against low surface tension liquids.
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Surface coatings of the PVDF substrate with SiNPs and FDTS were confirmed by attenuated
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total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy measurement. Figure 2B
234
shows the 1400−800 wavenumber region of the FTIR spectra, covering the characteristic
235
absorbances of the distinct functional groups in the control PVDF substrate and SiNP-, and
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FDTS-coated PVDF membranes. After surface coating with SiNPs, a new peak at ~ 1110 cm-1
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which corresponds to silanol groups on the SiNPs was detected. An increase in absorbance
238
intensity at ~1140 cm-1, which is ascribed to CF2 symmetric stretching mode, was observed after
239
coating with FDTS, substantiating successful surface fluorination via vapor-phase silanization.
240
Figure 1.
241
Figure 2.
242
Wetting Resistance of Modified Omniphobic Membrane. Contact angles for the
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control and the modified PVDF membranes were measured to evaluate the surface wetting
244
resistance to liquids with a wide range of surface tensions, including water (γ = 72.8 mN/m), 0.1
245
mM sodium dodecyl sulfate (SDS) in 1.0 M NaCl (γ ≈ 38 mN/m), and mineral oil (γ ≈ 31 mN/m).
246
We have selected SDS and mineral oil as representative surface wetting agents found in shale
247
gas wastewater streams. The SDS solution was prepared with high salinity (1.0 M NaCl) to
248
mimic the high TDS shale gas wastewater. Also, we note that the surface tension of ionic
249
surfactant (i.e., SDS) solution is significantly reduced under high salt concentrations because
250
electrolytes screen the charge of the polar head groups, leading to a higher surfactant density at
251
the liquid-air interface as described in previous studies.36
252
Contact angles of liquids with different surface tensions on the control and the modified
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PVDF membranes are shown in Figure 3. Due to the dynamic behavior of droplets of certain
254
testing liquids, the contact angles were monitored for three minutes. The control PVDF substrate
255
maintained a relatively high (~110°) and stable water contact angle during three minutes of
256
observation. However, the control PVDF substrate failed to resist wetting by surfactant (SDS)
257
solution and mineral oil (Figure 3A). The contact angle of 0.1 mM SDS solution reduced from
258
~100° to ~55° during the three minute measurement interval and complete wetting was found
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two minutes after placing mineral oil on the control PVDF substrate. The modified PVDF
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membrane exhibited superhydrophobicity with a water contact angle >150° (Figure 3B). The
261
observed significantly high water contact angle on the modified PVDF membrane is attributed to
262
vapor/air pockets developed between SiNPs attached on the substrate and ultralow surface
263
energy achieved by FDTS coating. Notably, the modified PVDF membrane exhibited high
264
(>130°) and stable (for three minute observation) contact angles with both SDS solution and
265
mineral oil. An omniphobic surface that repels both water and low surface tension liquids (e.g.,
266
oil and alcohol) is available only when the surface has features with a re-entrant structure and
267
ultralow surface energy. The observed wetting resistances of the modified PVDF membrane to
268
SDS solution and mineral oil demonstrate that substrate coatings with SiNP and FDTS
269
successfully imparted surface omniphobicity to the control PVDF membrane.
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The irreversibility of SiNP and FDTS coatings on the substrate surface and subsequent
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membrane wetting resistance were assessed by subjecting the modified PVDF membranes to
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harsh chemical (i.e., pH 2, pH 12, or 1.0 M NaCl) and physical (i.e., bath sonication for 20
273
minutes) stresses. SEM micrographs taken after stress protocols showed no significant difference
274
compared to the results obtained from the SiNP-coated membrane not subjected to stresses (see
275
Figure S1 in the Supporting Information). The results indicated that electrostatic attraction
276
between the negatively charged SiNPs and the positively charged PVDF substrate was strong
277
enough to render the SiNP coating irreversible. Liquid contact angles can be used as a proxy to
278
appraise the stability of SiNP and FDTS coatings because the apparent contact angle is
279
determined by both surface morphology and chemistry. Figure 4 presents the contact angles of
280
water, 0.1 mM SDS in 1.0 M NaCl, and mineral oil on the modified PVDF membranes before
281
and after chemical and physical stresses. Contact angles did not change significantly for all
282
tested liquids, compared to those analyzed immediately after surface modification (indicated as
283
initial in Figure 4). We attribute the observed stable wetting resistance of the modified PVDF
284
membrane to the robust SiNP binding by electrostatic attraction and the strong covalent Si−O−Si
285
linking between FDTS and SiNP achieved via silanization (Figure 1). The confirmed stable
286
wetting resistance of the modified PVDF membrane ensures a long-term omniphobic
287
functionalization under typical MD operational conditions.
288
Figure 3.
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Figure 4.
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Desalination Performance with High Salinity, Low Surface Tension Feed Water.
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To investigate the desalination performance of the control PVDF and modified omniphobic
293
membranes with feed water containing low surface tension contaminants, we performed direct
294
contact membrane distillation (DCMD) experiments using feed solutions with varying
295
concentrations of sodium dodecyl sulfate (SDS, anionic surfactant) and mineral oil. SDS and
296
mineral oil were sequentially introduced to the feed every two hours during the DCMD
297
experiment after an initial two hours stabilization period with a 1.0 M NaCl feed solution. Water
298
(vapor) flux and salt (NaCl) rejection were monitored for eight hours.
299
Surfactants are amphiphilic organic compounds comprising a hydrophilic polar head and a
300
hydrophobic non-polar tail.37 Below the critical micelle concentration (CMC), diffusion of
301
surfactant towards the air-liquid interface is triggered by the hydrophobic tail, which results in a
302
decrease of the solution surface tension. When surfactants are present in a solution of high
303
salinity, they significantly reduce the surface tension of the medium because the charge of the
304
surfactant polar head group is screened by counter ions, leading to a higher surfactant density at
305
the liquid-air interface.38 Surface tensions of the SDS solution measured using a tensiometer
306
were much lower in 1.0 M NaCl than in DI water (Figure S2-A, Supporting Information). We
307
added 0.05, 0.1, and 0.2 mM SDS into the feed solution (1.0 M NaCl at 60 °C) every two hours
308
during the DCMD experiments to progressively lower the feed surface tensions to ~53, ~42, and
309
~33 mN/m, respectively.36
310
A slight increase in water flux and a decrease in salt rejection were observed with the control
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PVDF membrane after the addition of 0.05 mM SDS to the feed solution (Figure 5A). Because
312
we applied a slightly higher cross flow rate for the feed stream than the permeate stream (cross
313
flow velocities of 8.5 cm/s versus 7.5 cm/s for feed and permeate streams, respectively), the feed
314
solution penetrates through the MD membrane to the permeate side when pores are wetted. The
315
increase in water flux and the concomitant decrease in salt rejection observed for the control
316
PVDF membrane are evidence of pore wetting due to the reduced solution surface tension
317
resulting from the added surfactant (SDS) in the feed. Further SDS addition to the feed (i.e., 0.1
318
mM SDS) leads to a more drastic water flux increase and sudden decrease of salt rejection,
319
suggesting more severe or even complete membrane pore wetting. The modified omniphobic 12 ACS Paragon Plus Environment
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membrane, on the other hand, showed stable water flux and complete salt rejection even after the
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addition of 0.2 mM SDS to the feed, which corresponds to a solution surface tension of ~33
322
mN/m, demonstrating wetting resistance of the omniphobic membrane to the low surface tension
323
feed water.
324
The effect of oil in the feed solution on MD desalination performance of the control and
325
modified PVDF membranes is presented in Figure 5B. A homogeneous oil stock solution was
326
prepared by mixing mineral oil (1% v/v) and non-ionic surfactant (0.1% v/v) in water. The
327
prepared oil emulsion was sequentially added to the feed to achieve mineral oil concentrations of
328
8, 40, and 80 mg/L. The average diameter of the mineral oil droplets in the feed solution
329
measured by dynamic light scattering (DLS) was ~0.7 µm (Figure S2-B, Supporting
330
Information). The prepared feed solution simulates the wastewaters contaminated by emulsified
331
oils with droplet sizes below several micrometers, which are challenging to treat by conventional
332
pre-treatment practices, such as oil skimmers, coalescers, settling tanks, and depth filters.39
333
At a relatively low mineral oil concentration (0.001% v/v or 8 mg/L) in the feed, the control
334
PVDF membrane exhibited water flux decline without a significant change in salt rejection
335
(Figure 5B). Nonpolar oils easily adhere to the PVDF membrane by hydrophobic interactions,40
336
thereby causing membrane fouling.41 The observed water flux decline without significant change
337
in salt rejection at 0.001% v/v mineral oil concentration in the feed is likely because the water
338
and salt passage was blocked by the accumulated oils within the porous structure. At a higher
339
mineral oil concentration (0.005% v/v or 40 mg/L), however, the control PVDF membrane was
340
totally wetted by the feed solution as evidenced by drastically increased water flux and reduced
341
salt rejection, similar to the results obtained with the SDS feed solutions.
342
In contrast, the omniphobic membrane showed stable water flux and complete salt rejection
343
with feed solution mineral oil concentrations up to 0.01% v/v or 80 mg/L (Figure 5B). Mineral
344
oil droplets are expected to adhere to the omniphobic membrane via nonpolar-nonpolar
345
interaction as they adsorb on hydrophobic surfaces. However, contrary to the hydrophobic
346
control PVDF membrane, the modified omniphobic membrane resists wetting by mineral oils,
347
thereby preventing penetration of the oils into the porous substrate. Also, mineral oil droplets
348
adhered to the membrane can be readily removed by hydrodynamic shear force in cross-flow
349
DCMD operation due to the ultralow interfacial energy of the omniphobic surface.42, 43 13 ACS Paragon Plus Environment
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350
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Figure 5.
351
Desalination Performance with Shale Gas Produced Water. We further tested the
352
desalination performance of the modified omniphobic membrane by employing a brine from the
353
shale gas industry as a feed stream in DCMD. The shale gas brine has very high total dissolved
354
solids (TDS) concentration (101,000 mg/L) and contains substances which can potentially foul
355
and wet the MD membranes (Table S1, Supporting Information). In particular, oil and grease,
356
surfactants, and organic loading in the shale gas wastewater feed threaten the anti-wetting
357
properties of conventional hydrophobic membranes.
358
Figure 6 presents flux decline curves, expressed as normalized water flux, and permeate
359
conductivity obtained from DCMD experiments for the control PVDF and the omniphobic
360
membranes with the shale gas wastewater as feed solution (60 °C) and DI water as permeate
361
solution (20 °C). To allow meaningful comparison of membrane performance at different initial
362
water fluxes (23.5 and 13.6 L m-2 h-1 for the control PVDF and the omniphobic membranes,
363
respectively), data are presented as a function of cumulative permeate volume. Experiments were
364
continued until the feed recovery reached 50% (or permeate volume of 500 mL was attained),
365
which took 13.3 hours for experiments with the control PVDF membrane and 16.2 hours for the
366
omniphobic membrane. Other conditions were identical to those employed in the DCMD
367
experiments described in the previous subsection.
368
As presented in Figure 6, the omniphobic membrane experienced a much lower flux decline
369
and reduced salt passage compared to the control PVDF membrane. The observed significant
370
performance deterioration of the control PVDF membrane can be attributed to various factors,
371
including wetting, fouling, and scaling of the membrane, given the complex physicochemical
372
composition of the shale gas produced water used. Of these, membrane wetting is expected to be
373
the main factor that caused performance degradation of the control PVDF membrane in the early
374
DCMD stage because wetting is a phenomenon that affects MD performance much faster than
375
fouling and scaling.44, 45 Results obtained from a DCMD experiment using a clean 1.7 M NaCl
376
feed solution (salinity identical to the shale gas produced water) without surface wetting
377
substances further demonstrated a stable desalination performance of the control PVDF
378
membrane in the early MD stage (Figure S3-A of Supporting Information).
14 ACS Paragon Plus Environment
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379
The omniphobic membrane exhibited stable MD performance with shale gas wastewater feed
380
(Figure 6B), which was comparable to the results obtained using a clean NaCl feed solution with
381
a salinity (~1.7 M NaCl) identical to the shale gas produced water (Figure S3-B of Supporting
382
Information). Although in both cases (i.e., shale gas produced water and 1.7 M NaCl solution
383
feed) a slight flux decline and a small conductivity increase in the permeate were observed in a
384
long-term DCMD run, likely caused by intrinsic membrane defects, the omniphobic membrane
385
exhibited much more stable desalination performance than the control PVDF membrane. The
386
observed stable desalination performance in MD with a shale gas produced water sample
387
containing a variety of low surface tension contaminants is attributable to the enhanced wetting
388
resistance of the omniphobic membrane achieved by surface modification with silica
389
nanoparticles and low surface energy materials (i.e., fluoroalkylsilane). Figure 6.
390 391
In summary, we demonstrated a facile and scalable surface engineering technique to fabricate
392
an omniphobic membrane that exhibits wetting resistance to low surface tension substances. Our
393
omniphobic membrane showed a stable performance in MD for desalination of shale gas
394
produced water, demonstrating the potential benefits of omniphobic membranes in treating high
395
salinity wastewaters that contain diverse low surface tension contaminants. Application of MD
396
with omniphobic membranes can be beneficial for a wide range of industries, considering the
397
increasing demand for efficient treatment of challenging industrial wastewaters and stricter
398
regulations for disposal of such wastewaters.
399 400
ASSOCIATED CONTENT
401
Major composition and key properties of pre-filtered shale gas produced water (Table S1); SEM
402
images of modified PVDF membranes taken after chemical and physical stresses (Figure S1);
403
surface tension of SDS solution in DI water and in 1 M NaCl (Figure S2-A) and size distribution
404
of mineral oil droplets (Figure S2-B); water (vapor) flux and permeate conductivity obtained
405
from DCMD experiments for unmodified PVDF and modified PVDF membranes with 1.7 M
406
NaCl feed solution and DI water permeate solution (Figure S3). This material is available free of
407
charge via the Internet at http://pubs.acs.org. 15 ACS Paragon Plus Environment
Environmental Science & Technology
408
ACKNOWLEGMENT
409
We acknowledge the support received from the National Science Foundation through the
410
Engineering Research Center for Nanotechnology-Enabled Water Treatment (ERC-1449500).
411
Facilities used were supported by the Yale Institute of Nanoscale and Quantum Engineering
412
(YINQE) and the Chemical and Biophysical Instrument Center (CBIC) at Yale.
413
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414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459
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REFERENCES 1. Inman, M., The Fracking Fallacy. Nature 2014, 516, (7529), 28-30. 2. Veatch, R. W., Jr.; Moschovidis, Z. A., An Overview of Recent Advances in Hydraulic Fracturing Technology. In Society of Petroleum Engineers. 3. Vengosh, A.; Jackson, R. B.; Warner, N.; Darrah, T. H.; Kondash, A., A Critical Review of the Risks to Water Resources from Unconventional Shale Gas Development and Hydraulic Fracturing in the United States. Environ Sci Technol 2014, 48, (15), 8334-8348. 4. Getzinger, G. J.; O'Connor, M. P.; Hoelzer, K.; Drollette, B. D.; Karatum, O.; Deshusses, M. A.; Ferguson, P. L.; Elsner, M.; Plata, D. L., Natural Gas Residual Fluids: Sources, Endpoints, and Organic Chemical Composition after Centralized Waste Treatment in Pennsylvania. Environ Sci Technol 2015, 49, (14), 8347-8355. 5. https://fracfocus.org/. 6. Shaffer, D. L.; Chavez, L. H. A.; Ben-Sasson, M.; Castrillón, S. R.-V.; Yip, N. Y.; Elimelech, M., Desalination and Reuse of High-Salinity Shale Gas Produced Water: Drivers, Technologies, and Future Directions. Environ Sci Technol 2013, 47, 9569-9538. 7. Gregory, K. B.; Vidic, R. D.; Dzombak, D. A., Water Management Challenges Associated with the Production of Shale Gas by Hydraulic Fracturing. Elements 2011, 7, (3), 181-186. 8. Ahmadun, F. R.; Pendashteh, A.; Abdullah, L. C.; Biak, D. R. A.; Madaeni, S. S.; Abidin, Z. Z., Review of technologies for oil and gas produced water treatment. J Hazard Mater 2009, 170, (2-3), 530-551. 9. Tong, T.; Elimelech, M., The Global Rise of Zero Liquid Discharge for Wastewater Management: Drivers, Technologies, and Future Directions. Environ Sci Technol 2016, 50, (13), 6846-6855. 10. Lin, S. H.; Yip, N. Y.; Elimelech, M., Direct contact membrane distillation with heat recovery: Thermodynamic insights from module scale modeling. J Membrane Sci 2014, 453, 498-515. 11. Xu, P.; Cath, T. Y.; Robertson, A. P.; Reinhard, M.; Leckie, J. O.; Drewes, J. E., Critical Review of Desalination Concentrate Management, Treatment and Beneficial Use. Environ Eng Sci 2013, 30, (8), 502-514. 12. Shih, J. S.; Saiers, J. E.; Anisfeld, S. C.; Chu, Z. Y.; Muehlenbachs, L. A.; Olmstead, S. M. M., Characterization and Analysis of Liquid Waste from Marcellus Shale Gas Development. Environ Sci Technol 2015, 49, (16), 9557-9565. 13. Lee, J.; Boo, C.; Ryu, W.-H.; Taylor, A. D.; Elimelech, M., Development of Omniphobic Desalination Membranes Using a Charged Electrospun Nanofiber Scaffold. ACS Applied Materials & Interfaces 2016, 8, (17), 11154-11161. 14. Lin, S.; Nejati, S.; Boo, C.; Hu, Y.; Osuji, C. O.; Elimelech, M., Omniphobic membrane for robust membrane distillation. Environmental Science & Technology Letters 2014, 1, (11), 443-447. 15. Wang, Z. X.; Elimelech, M.; Lin, S. H., Environmental Applications of Interfacial Materials with Special Wettability. Environ Sci Technol 2016, 50, (5), 2132-2150. 16. Kumar, R. T. R.; Mogensen, K. B.; Boggild, P., Simple Approach to Superamphiphobic Overhanging Silicon Nanostructures. J Phys Chem C 2010, 114, (7), 2936-2940. 17. Aulin, C.; Yun, S. H.; Wagberg, L.; Lindstrom, T., Design of Highly Oleophobic Cellulose Surfaces from Structured Silicon Templates. Acs Applied Materials & Interfaces 2009, 1, (11), 2443-2452. 17 ACS Paragon Plus Environment
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18. Fujii, T.; Aoki, Y.; Habazaki, H., Fabrication of Super-Oil-Repellent Dual Pillar Surfaces with Optimized Pillar Intervals. Langmuir 2011, 27, (19), 11752-11756. 19. Zhao, H.; Law, K. Y.; Sambhy, V., Fabrication, Surface Properties, and Origin of Superoleophobicity for a Model Textured Surface. Langmuir 2011, 27, (10), 5927-5935. 20. Ozbay, S.; Erbil, H. Y., Superhydrophobic and oleophobic surfaces obtained by graft copolymerization of perfluoroalkyl ethyl acrylate onto SBR rubber. Colloid Surface A 2015, 481, 537-546. 21. Zhang, G. W.; Lin, S. D.; Wyman, I.; Zou, H. L.; Hu, J. W.; Liu, G. J.; Wang, J. D.; Li, F.; Liu, F.; Hu, M. L., Robust Superamphiphobic Coatings Based on Silica Particles Bearing Bifunctional Random Copolymers. Acs Applied Materials & Interfaces 2013, 5, (24), 1346613477. 22. Zhao, H.; Park, K. C.; Law, K. Y., Effect of Surface Texturing on Superoleophobicity, Contact Angle Hysteresis, and "Robustness". Langmuir 2012, 28, (42), 14925-14934. 23. Leng, B. X.; Shao, Z. Z.; de With, G.; Ming, W. H., Superoleophobic Cotton Textiles. Langmuir 2009, 25, (4), 2456-2460. 24. Liu, F.; Hashim, N. A.; Liu, Y. T.; Abed, M. R. M.; Li, K., Progress in the production and modification of PVDF membranes. J Membrane Sci 2011, 375, (1-2), 1-27. 25. Nejati, S.; Boo, C.; Osuji, C. O.; Elimelech, M., Engineering flat sheet microporous PVDF films for membrane distillation. J Membrane Sci 2015, 492, 355-363. 26. Elimelech, M.; Chen, W. H.; Waypa, J. J., Measuring the Zeta (Electrokinetic) Potential of Reverse-Osmosis Membranes by a Streaming Potential Analyzer. Desalination 1994, 95, (3), 269-286. 27. Ottewill, R. H.; Shaw, J. N., Electrophoretic Studies on Polystyrene Latices. J Electroanal Chem 1972, 37, (Jun), 133-&. 28. Matsumoto, A.; Tsutsumi, K.; Schumacher, K.; Unger, K. K., Surface functionalization and stabilization of mesoporous silica spheres by silanization and their adsorption characteristics. Langmuir 2002, 18, (10), 4014-4019. 29. Boo, C.; Lee, J.; Elimelech, M., Engineering Surface Energy and Nanostructure of Microporous Films for Expanded Membrane Distillation Applications. Environ Sci Technol 2016, 50, (15), 8112-8119. 30. Ross, G. J.; Watts, J. F.; Hill, M. P.; Morrissey, P., Surface modification of poly(vinylidene fluoride) by alkaline treatment 1. The degradation mechanism. Polymer 2000, 41, (5), 1685-1696. 31. Zheng, Z. R.; Gu, Z. Y.; Huo, R. T.; Luo, Z. S., Superhydrophobic poly(vinylidene fluoride) film fabricated by alkali treatment enhancing chemical bath deposition. Applied Surface Science 2010, 256, (7), 2061-2065. 32. Liang, S.; Kang, Y.; Tiraferri, A.; Giannelis, E. P.; Huang, X.; Elimelech, M., Highly Hydrophilic Polyvinylidene Fluoride (PVDF) Ultrafiltration Membranes via Postfabrication Grafting of Surface-Tailored Silica Nanoparticles. Acs Applied Materials & Interfaces 2013, 5, (14), 6694-6703. 33. Han, M. J.; Barona, G. N. B.; Jung, B., Effect of surface charge on hydrophilically modified poly(vinylidene fluoride) membrane for microfiltration. Desalination 2011, 270, (1-3), 76-83. 34. Childress, A. E.; Elimelech, M., Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes. J Membrane Sci 1996, 119, (2), 253268. 18 ACS Paragon Plus Environment
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35. Elimelech, M.; Omelia, C. R., Effect of Electrolyte Type on the Electrophoretic Mobility of Polystyrene Latex Colloids. Colloid Surface 1990, 44, 165-178. 36. Matijevic, E.; Pethica, B. A., The Properties of Ionized Monolayers .1. Sodium Dodecyl Sulphate at the Air-Water Interface. Transactions of the Faraday Society 1958, 54, (9), 13821389. 37. Dominguez, A.; Fernandez, A.; Gonzalez, N.; Iglesias, E.; Montenegro, L., Determination of critical micelle concentration of some surfactants by three techniques. J Chem Educ 1997, 74, (10), 1227-1231. 38. Lu, J. R.; Marrocco, A.; Su, T. J.; Thomas, R. K.; Penfold, J., Adsorption of DodecylSulfate Surfactants with Monovalent Metal Counterions at the Air-Water-Interface Studied by Neutron Reflection and Surface-Tension. J Colloid Interf Sci 1993, 158, (2), 303-316. 39. Zhang, W. B.; Shi, Z.; Zhang, F.; Liu, X.; Jin, J.; Jiang, L., Superhydrophobic and Superoleophilic PVDF Membranes for Effective Separation of Water-in-Oil Emulsions with High Flux. Adv Mater 2013, 25, (14), 2071-2076. 40. Tanford, C., Interfacial Free-Energy and the Hydrophobic Effect. P Natl Acad Sci USA 1979, 76, (9), 4175-4176. 41. Lu, X. M.; Peng, Y. L.; Ge, L.; Lin, R. J.; Zhu, Z. H.; Liu, S. M., Amphiphobic PVDF composite membranes for anti-fouling direct contact membrane distillation. J Membrane Sci 2016, 505, 61-69. 42. Werber, J. R.; Osuji, C. O.; Elimelech, M., Materials for next-generation desalination and water purification membranes. Nature Reviews Materials 2016, 1, 16018. 43. Chen, W. J.; Su, Y. L.; Peng, J. M.; Zhao, X. T.; Jiang, Z. Y.; Dong, Y. A.; Zhang, Y.; Liang, Y. G.; Liu, J. Z., Efficient Wastewater Treatment by Membranes through Constructing Tunable Antifouling Membrane Surfaces. Environ Sci Technol 2011, 45, (15), 6545-6552. 44. Srisurichan, S.; Jiraratananon, R.; Fane, A. G., Humic acid fouling in the membrane distillation process. Desalination 2005, 174, (1), 63-72. 45. Razmjou, A.; Arifin, E.; Dong, G. X.; Mansouri, J.; Chen, V., Superhydrophobic modification of TiO2 nanocomposite PVDF membranes for applications in membrane distillation. J Membrane Sci 2012, 415, 850-863.
536
19 ACS Paragon Plus Environment
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537 538 539 540
Figure 1. Schematic illustrating the procedure for omniphobic surface modification of PVDF
541
membrane. (A) Hydroxyl groups are generated on the surface of the PVDF substrate by alkaline
542
treatment with a 7.5 M NaOH solution at 70 °C for three hours. (B) Hydroxyl groups are used
543
for grafting of (3-Aminopropyl)triethoxysilane (APTES). (C) Silica nanoparticles (SiNPs) bind
544
to APTES functionalized PVDF substrate via electrostatic attraction. (D) SiNPs are coated with
545
perfluorodecyltrichlorosilane (FDTS) via vapor-phase silanization.
546 547 548 549 550 551 552 553 554 555 556 557 558 559
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Environmental Science & Technology
Zeta Potential (mV)
(A) 60 40 20 0 -20 -40 -60 -80
Amine Functionalized PVDF Control PVDF Alkaline Treated PVDF SiNPs (100 nm)
3
4
5
6
7
8
9
pH
Absorbance (a.u.)
(B)
1400
After FDTS Coating After SiNP Coating Control PVDF 1140 cm-1 1110 cm-1
1200
1000
Wavenumber (cm-1)
800
560 561 562
Figure 2. (A) Zeta potential as a function of pH of silica nanoparticles (SiNPs) and PVDF
563
membranes at various stages of surface modification. (B) ATR-FTIR spectra of control, SiNP-
564
coated, and FDTS-coated PVDF membranes. (C) SEM images depicting top-down and cross-
565
section of control and SiNP-coated PVDF membranes.
566
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Page 22 of 25
Water (72.8 mN m-1) 0.1 mM SDS in 1 M NaCl (~38 mN m-1) Mineral Oil (~30 mN m-1)
Contact Angle (°)
200
(A) Control PVDF
150 100
Complete Wicking
50 200
(B) Omniphobic PVDF
150 100 50 0
567 568
Complete Wicking
60
120
180
Time (sec)
569 570
Figure 3. Variation in static contact angle of (A) control PVDF and (B) modified omniphobic
571
PVDF membranes measured with DI water, 0.1 mM SDS in 1 M NaCl, and mineral oil. A 5-µL
572
liquid droplet was placed on the membrane surface and the contact angle was monitored for three
573
minutes (180 seconds). Error bars represent standard deviations of two contact angles from three
574
different membrane samples. Surface tensions of the liquids are indicated in legends.
575
22 ACS Paragon Plus Environment
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Contact Angle (°)
Page 23 of 25
Initial pH 2 (HCl) pH 12 (NaOH) 1.0 M NaCl Bath Sonicated
210 180 150 120 90
Water
0.1 mM SDS (in 1 M NaCl)
Mineral Oil
576 577
Figure 4. Contact angles of DI water, 0.1 mM SDS in 1 M NaCl, and mineral oil on the
578
membrane surface measured immediately after surface modification (indicated as initial) and
579
after the modified membranes were subjected to the indicated physical and chemical stresses.
580
Contact angles three minutes after placing 5 µL of the testing liquid were determined to be a
581
steady-state value. The variation in liquid volume during the three minute measurement was
582
within 0.5 µL. Error bars represent standard deviations of two contact angles from three different
583
membrane samples.
584 585 586 587 588 589 590 591 592 23 ACS Paragon Plus Environment
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Page 24 of 25
0.05
50
0.1
0.2
(A)
Control PVDF Omniphobic PVDF
100
40 30
90
20 10 0 0
Salt (NaCl) Rejection
Water Flux (L m-2 h-1)
Surfactant Concentration in the Feed (mM)
120
240
80 480
360
Time (min)
593
594
0.001
50
0.005
0.01
(B)
Control PVDF Omniphobic PVDF
100
40 30
90
20 10 0 0
Salt (NaCl) Rejection
Water Flux (L m-2 h-1)
Oil Concentration in the Feed (% v/v )
120
240
360
80 480
Time (min)
595 596
Figure 5. Water flux and salt rejection of control and modified PVDF membranes measured in
597
DCMD using 1 M NaCl at 60 °C with varying (A) SDS and (B) mineral oil concentrations as
598
feed solution and DI water at 20 °C as permeate solution. The SDS concentrations in the feed
599
after sequential additions every two hours were 0.05, 0.1, and 0.2 mM, and the corresponding
600
estimated surface tensions of the feed solutions were ~53, ~42, and ~33 mN/m, respectively. The
601
mineral oil concentrations in the feed after sequential additions every two hours were 0.001,
602
0.005, and 0.01% v/v, which correspond to 8, 40, and 80 mg/L. The feed solution (initial volume
603
of 1 L) was replenished after every 50 mL volume loss to maintain the variation of contaminant
604
(SDS and mineral oil) concentrations within less than 5%.
605 24 ACS Paragon Plus Environment
1.0
150
0.8
100 50
0.6 0
0 100 200 300 400 500
Permeate Volume (mL)
1.2 (B) Omniphobic PVDF
200
1.0
150
0.8
100 50
0.6 0
0 100 200 300 400 500
Permeate Volume (mL)
Permeate Conductivity (µS cm-1)
200
Permeate Conductivity (µS cm-1)
606 607
1.2 (A) Control PVDF
Nomalized Water Flux (J / J0)
Environmental Science & Technology
Nomalized Water Flux (J / J0)
Page 25 of 25
608
Figure 6. Desalination performances of (A) control PVDF and (B) modified omniphobic PVDF
609
membranes with brines from shale produced water. Water vapor flux and permeate conductivity
610
were measured in DCMD until the cumulative permeate volume reached 500 mL, using shale
611
gas wastewater at 60 °C as a feed stream and DI water at 20 °C as a permeate stream. Properties
612
of the pre-filtered (14 µm filter) shale gas wastewater are described in Table S1 (Supporting
613
Information). Initial water flux, J0, was determined by averaging the permeate flux from the
614
onset of the experiment until the permeate volume reached 25 mL. The obtained initial water
615
fluxes were 23.5 and 13.6 L m-2 h-1 for the control and omniphobic PVDF membranes,
616
respectively. Water flux performance is expressed as a normalized water flux, J/J0, based on the
617
initial water flux.
25 ACS Paragon Plus Environment