Omniphobic Hollow-Fiber Membranes for Vacuum Membrane Distillation

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Energy and the Environment

Omniphobic hollow fiber membranes for vacuum membrane distillation Kang Jia Lu, Jian Zuo, Jian Chang, Hong Nan Kuan, and Tai-Shung Chung Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00766 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Environmental Science & Technology

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Omniphobic hollow fiber membranes for

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vacuum membrane distillation

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Kang Jia Lua, Jian Zuob, Jian Changa, Hong Nan Kuana, Tai-Shung Chunga,*

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a

Department of Chemical & Biomolecular Engineering,

National University of Singapore, 4 Engineering Drive 4, Singapore 117585 b

Singapore Institute of Technology, 10 Dover Drive, Singapore 138683

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* Corresponding author. Tel.: 65 6516 6645; fax: 65 6779 1936.

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E-mail address: [email protected] (T. S. Chung)

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ACS Paragon Plus Environment


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Abstract

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Management of produced water from shale gas production is a global challenge. Vacuum

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membrane distillation (VMD) is considered as a promising solution because of its various

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advantages. However, low surface tension species in produced water can easily deposit on the

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membrane surface and cause severe fouling or wetting problems. To solve the problems, an

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omniphobic polyvinylidene difluoride (PVDF) hollow fiber membrane has been developed via

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silica nanoparticle deposition followed by a Teflon® AF 2400 coating in this study. The

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resultant membrane shows good repellency towards various liquids with different surface

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tensions and chemistries, including water, ethylene glycol (EG), cooking oil and ethanol. It

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also exhibits stable performance in 7-h VMD tests with a feed solution containing up to 0.6

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mM of SDS. In addition, the effects of surface energy, surface morphology as well as

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nanoparticle size on membrane omniphobicity have been systematically investigated. This

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work may provide valuable guidance to molecularly design omniphobic VMD membranes for

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produced water treatment.

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Key words: omniphobic surface; vacuum membrane distillation (VMD); silica nanoparticle;

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Teflon® AF 2400 coating

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TOC Art 60 nm

250 nm

PVDF

PVDF

hydrophobic

omniphobic

42 43 44 45 46 47 48 49 50 51 52 53 54 55 56


400 nm

Teflon® AF 2400 coating


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1. Introduction

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Because of the explosive increase in global energy demand, shale gas is gaining worldwide

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attention as one of the most promising alternative energy sources.1–3 Nevertheless,

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environmental concerns associated with shale gas production have never ceased.4–6 One of the

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biggest environmental issues is the management of produced water generated during shale gas

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production. It is challenging to treat the produced water owing to its complex physicochemical

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composition and high total dissolved solid (TDS) concentration.7 Membrane distillation (MD)

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is one of the emerging technologies showing promise to treat and recycle the produced water.4

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MD is a thermally driven membrane separation process that involves the transport of water

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vapor across a hydrophobic membrane.8–12 Thanks to the special driven force – vapor pressure

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difference, MD has a theoretical 100% rejection towards non-volatile species.13,14 More

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importantly, MD is not sensitive to feed concentrations.15–19 This unique property distinguishes

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MD from other membrane processes and makes it especially suitable for the treatment of high

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salinity produced water. Moreover, MD process can leverage the elevated temperature of

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produced water to drive the process, which will significantly lower the energy cost of MD 4.

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Among the four common MD configurations – namely, direct contact membrane distillation

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(DCMD), air gas membrane distillation (AGMD), sweeping gas membrane distillation (SGMD)

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and vacuum membrane distillation (VMD), VMD is selected in this current study for a

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potentially higher permeation flux and an improved thermal efficiency.20–22 Despite all the

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above-mentioned advantages, it is still challenging to use VMD to treat produced water. One of

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the major reasons is that the high content of low surface tension species in produced water can

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cause severe fouling and wetting issues.23–25

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A potential solution is to develop an omniphobic membrane, which has strong repellency

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towards various chemicals with a wide range of surface tensions. Successful deisgns of

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omniphobic surfaces have been reported by Tsujii et al., Tuteja et al. and Hensel et al., and these

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valuable studies reveal that a surface should have a re-entrant morphology and low surface

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tension in order to achieve the onmiphobic status.26–29 To apply the concept of omniphobic

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surface on membrane applications, several pioneering works have been conducted. Elimelech

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et al. grafted silica nanoparticles (SiNPs) onto the membrane surface followed by a

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fluoroalkylsilane coating. The resultant membranes exhibit excellent anti-wetting ability in

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treating surfactant added brine solutions via DCMD.30–33 Woo et al. developed an omniphobic

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polyvinylidene difluoride (PVDF) membrane by electospinning and CF4 plasma modification.

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The membranes were tested for AGMD with surfactant added real RO brine as the feed solution

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and showed stable performance.34

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The current study aims to simplify the fabrication of omniphobic membranes and to

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systematically investigate the effects of surface energy and surface morphology on membrane

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performance. Lu et al. have reported a facile method to lower the membrane surface energy by

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Teflon® AF 2400 coating.35 Compared with the conventional silanization or plasma treating

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method, it is much easier to conduct the Teflon® AF 2400 coating. In addition, the surface

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energy could be lowered with minimal changes of the membrane morphology by carefully

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tuning the coating conditions. As a result, one can study the effects of membrane surface energy

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and surface morphology separately. Moreover, the effects of nanoparticle size on membrane

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omniphobicity will also be studied, which is rarely reported before. The omniphobic treatment

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will be conducted on PVDF hollow fibers and the resultant membranes will be tested in VMD

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with sodium dodecyl sulfate (SDS) added brine solutions as feeds.

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2. Materials and methods

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2.1 Dope preparation and membrane fabrication

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Hollow fiber membranes were fabricated via a non-solvent induced phase separation (NIPS)

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method. In order to achieve superior mechanical properties, dual-layer membranes consisting

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of a hydrophobic PVDF outer layer and a hydrophilic polyetherimide (PEI) inner substrate

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were designed and prepared. Spinning conditions and dope formulations were summarized in

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Table S1. The information of the materials used can be found in the supporting information.

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Detailed information about the dope preparation and the hollow fiber spinning procedure can

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be found elsewhere.36

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2.2 Syntheses of silica nanoparticles

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Silica nanoparticles with different particle sizes were prepared by the Stöber method according

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to the formula shown in Table S2. Pre-determined amounts of ammonia hydroxide and water

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were well mixed in 100 mL of anhydrous ethanol, followed by the addition of tetraethyl

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orthosilicate (TEOS). The reactions then occurred at room temperature. Subsequently, the

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mixtures were centrifuged at 10,000 rpm for 30 min. The obtained silica nanoparticles were

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washed with isopropanol (IPA) for three times to remove the unreacted reagents and finally

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dried in an oven at 80 °C for 24 h.

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2.3 Omniphobic modification of membranes

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The procedure of the omniphobic modification is illustrated in Figure 1. Before any treatment,

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one end of the fiber was sealed with epoxy to minimize the diffusion of treating solutions into

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the lumen side of the hollow fiber. The fiber was firstly reacted with a 7.5 M sodium hydroxide

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(NaOH) solution at 50 °C for 1 h to generate hydroxyl groups on the membrane surface.

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Afterwards, the treated fiber was thoroughly washed by de-ionized (DI) water and ethanol, and


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then completely dried in open air. In the next step, the fiber was immersed in an ethanol solution

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comprising 1.5 wt% 3-Triethoxysilylpropylamine (APTES) for 1 h in order to render positive

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charge onto the membrane surface. The optimization of the alkaline treatment condition and

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APTES treatment duration can be found in the supporting information. The membrane was

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then transferred to a DI water solution containing 0.5 wt% silica nanoparticles for 1 h followed

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by complete drying in open air. To lower the surface energy of the membrane, it was dip-coated

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in a HT-70 Galden heat transfer fluid consisting of 0.025 wt% Teflon® AF 2400 for 30 s. The

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special solvent has a low boiling temperature of 70°C. It evaporated away quickly within a few

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seconds, leaving Teflon® AF 2400 on the membrane surface.

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Membranes are denoted as D-PVDF-x-T where x (= 60, 250 or 400) represents the diameters

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(nm) of nanoparticles and T means the Teflon® coating. D-PVDF-60 refers to the membrane

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coated with 60 nm nanoparticles only while D-PVDF-T represents the membrane coated with

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Teflon® AF 2400 only.

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Figure 1. The diagram of the coating procedure

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2.4 Characterizations

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Surface scans of the pristine, silica nanoparticle and Teflon® coated hollow fiber membranes

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were performed by an X-ray photoelectron spectroscopy (XPS). Membrane morphology and

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topology were observed by a field-emission scanning electron microscope (FESEM) and an

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atomic force microscopy (AFM), respectively. Membranes’ surface porosities, mean and

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maximum pore sizes were analyzed with the help of ImageJ software (NIH) using FESEM

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images as discussed elsewhere.37 Membranes’ overall pore size distributions were measured

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by a CFP-1500 AE capillary flow porometer (PMI, Vista, CA). Overall porosities were

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obtained by calculating the dry/wet mass differences by immersing the membranes in IPA.

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Dynamic contact angles of hollow fibers in four liquids (DI water, ethylene glycol (EG),

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cooking oil and ethanol) and surface tension values of the liquids were measured by a

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tensiometer. To verify the stability of the silica nanoparticle coating, the FESEM pictures of

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the outer membrane surfaces after VMD tests were taken. The VMD permeate solutions were

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collected and the concentrations of silicon have been tested using an iCAP 6200 ICP

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spectrometer. Details of the characterization methods can be found in the support information.

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2.5 VMD tests

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VMD tests were conducted using a laboratory setup depicted in Figure 2. Before testing,

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hollow fiber membranes were firstly assembled into a module with an average length of 10 cm

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and mounted onto the setup. The initial feed solution was model seawater of 3.5 wt.% NaCl.

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After the first 60 min operation, SDS was added into the feed solution at an incremental rate

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of 0.2 mM for every 2 hours. The final SDS concentration was 0.6 mM. The surface tension

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values of the 3.5 wt.% NaCl solutions containing 0.2, 0.4 and 0.6 mM SDS were measured to

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be 34.7, 30.2, 25.8 mM/m, respectively. The feed solution was controlled at 70 ⁰C using a

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heater (F12, Julabo) and circulated to the shell side of membranes at a flow rate of 0.2 L/min,


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while the lumen side was maintained at around 20 mBar with the aid of a vacuum pump. Vapor

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permeate on the lumen side was condensed by liquid nitrogen. The mass and conductivity of

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the permeate were measured by an accurate balance (A & D, GR-200) and a conductivity meter

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(Lab 960, Schott), respectively. VMD tests for each sample were repeated for three times and

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the average data were recorded.

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Figure 2. Schematic diagram of the VMD set-up

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The flux of the membrane was calculated by the following equation: 𝐽𝐽 =

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∆𝑤𝑤

𝐴𝐴×𝑡𝑡

(1)

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where ∆𝑤𝑤 (kg) is the permeate mass collected during time t (h); A (m2) is the total area of the

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the following equation:

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outer membrane surface; t (h) is the test duration. The membrane rejection was obtained from

𝛽𝛽 = (1 −

𝐶𝐶𝑝𝑝

𝐶𝐶𝑓𝑓

) × 100

(2)

where 𝐶𝐶𝑝𝑝 and 𝐶𝐶𝑓𝑓 are the salt concentrations of the permeate and feed solutions, respectively. ACS Paragon Plus Environment


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3. Results and discussions

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3.1 Engineering re-entrant structures with a low surface energy

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To examine the nanoparticle attachment and Teflon® AF 2400 coating, XPS survey spectra

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were taken for D-PVDF, D-PVDF-60 and D-PVDF-60-T and Figure 3 shows the results. The

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pristine PVDF surface exhibits two major emissions at 680 eV and 280 eV which are assigned

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to binding energies of F 1s and C 1s respectively.38 From the spectrum of the silica nanoparticle

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coated membrane, these two peaks diminish, while peaks for O 1s (525 eV), Si 2s (145 eV), Si

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2p (96 eV) emerge.39 When Teflon® AF 2400 is applied, the F 1s emission reappears, indicating

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the successful coating. It is interesting to notice that weak peaks for Si 2s and Si 2p could still

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be observed after the Teflon® AF 2400 coating. This is ascribed to the fact that the coating

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layer thickness is thinner than the XPS depth limit of 10 nm with the designated coating

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condition.35 This is critical in order to ensure the surface morphology is not affected so that the

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effects of surface morphology and surface energy can be studied separately.

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Figure 3. XPS spectra of D-PVDF, D-PVDF-60 and D-PVDF-60-T membrane surfaces


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Figure 4 displays the FESEM pictures of the outer membrane surfaces. A comparison between

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Figure 4 (A) and 4 (B) indicates that the membrane morphology does not change much after

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the Teflon® AF 2400 coating. The porosity, average pore size and maximum pore size of D-

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PVDF and D-PVDF-T in Table 1 also show great similarity. The same conclusion can be drawn

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by comparing Figure 4C and 4D. Interestingly, the texture of silica nanoparticles is not affected

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by the Teflon® coating because of the use of a very diluted coating solution and a short coating

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duration. Figure 4 (D) to 4 (F) exhibit the surfaces coated by SiNPs with sizes of 60, 250 and

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400 nm respectively. The synthesized SiNPs have narrow size distributions and cover the entire

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membrane surfaces uniformly. The successful attachment of SiNPs can also be supported by

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AFM images of the outer membrane surfaces as shown in Figure S3.

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Figure 4. FESEM pictures of the outer membrane surfaces (A) D-PVDF, (B) D-PVDF-T, (C)

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D-PVDF-60, (D) D-PVDF-60-T, (E) D-PVDF-250-T, (F) D-PVDF-400-T

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Table 1. Surface and overall porosities and pore sizes of membranes

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3.2 Effects of surface energy and morphology on membrane omniphobicity

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Figure 5 depicts the contact angles of various solutions on the pristine and modified membranes.

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Comparing the pristine membrane with the Teflon® AF 2400 coated membrane, the latter

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exhibits higher contact angles for all the liquids. As mentioned in the previous section, the

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morphology and pore size distribution for both membranes are of great similarity. Therefore,

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the major cause for the difference in contact angle between D-PVDF and D-PVDF-T is the

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surface chemistry. Teflon® AF 2400 has a perfluorinated structure, which brings extra fluorine

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to the membrane substrate. As shown in Table 2, the fluorine to carbon ratio on the membrane

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surface increases from 0.8 to 1.37 after the Teflon® coating that would successfully lower the

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surface energy. The motivation of lowering surface energy can be partially explained by the

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Young’s equation: cosθ = (𝛾𝛾𝑆𝑆 − 𝛾𝛾𝑆𝑆𝑆𝑆 )/𝛾𝛾𝐿𝐿 where θ is the equilibrium contact angle; 𝛾𝛾𝑆𝑆 , 𝛾𝛾𝐿𝐿

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and 𝛾𝛾𝑆𝑆𝑆𝑆 are the surface tensions of solid, liquid and solid/liquid interface, respectively.40 The

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interaction forces between the two contacting materials are of the same kind.26 Under this

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solid/liquid interfacial tension can be approximated by 𝛾𝛾𝑆𝑆𝑆𝑆 = 𝛾𝛾𝑆𝑆 + 𝛾𝛾𝐿𝐿 − 2�𝛾𝛾𝑆𝑆 𝛾𝛾𝐿𝐿 when the 𝛾𝛾 condition, the Young’s equation could be reduced to the following form cosθ = 2� 𝑆𝑆�𝛾𝛾𝐿𝐿 − 1. ACS Paragon Plus Environment

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Therefore, coating membranes with a low surface energy material will be beneficial for

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obtaining large contact angles towards low surface tension liquids.

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Other than the effects of low membrane surface tension, Figure 5 also proves that the

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nanoparticle coating has significant influences on membrane omniphobicity. The nanoparticle

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coated membrane exhibits much larger contact angles for all types of liquids than the

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membrane coated with Teflon® AF 2400 only. Since both D-PVDF-T and D-PVDF-60-T have

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Teflon® AF 2400 on the outmost layer, they should exhibit similar outer surface chemistry.

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Therefore, the imparities in their contact angles are mainly resulted from their different surface

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morphologies. Thus, the effects of surface roughness and structure on omniphobicity have been

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investigated.

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Figure S3 and Table 2 show that the surface roughness of D-PVDF-T is close to that of D-

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PVDF-60-T. Even though the nanoparticles introduce some extra roughness, they also fill and

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cover some of surface pores because the maximum surface pore size of D-PVDF-T is 551 nm

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as shown Table 1 and Figure 4. As a result, the overall roughness of the membrane remains

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similar after being coated with 60 nm nanoparticles and the higher contact angles of D-PVDF-

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60-T are mainly due to the re-entrant surface structure created by the nanoparticles. A

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qualitative analysis by Tuteja et al. reveals that the net force on the liquid-vapor interface is

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directed away from the solid surface for a re-entrant textured surface.27,41 Therefore, the liquid

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will be prevented from penetrating into the solid texture and forming a composite solid-liquid-

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vapor interface. Such composite interface, equivalent to a Cassie state,42,43 exhibits apparently

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high contact angles even if low surface tension liquids are applied.

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Figure 5. Contact angles of various solutions on D-PVDF, D-PVDF-T and D-PVDF-60-T

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Table 2. Surface roughnesses and F/C atomic ratios of membrane surfaces

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3.3 Effects of surface energy and morphology on VMD performance

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Figure 6 displays the change of normalized fluxes during 7-h VMD tests. In the first hour of

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tests when the feed solution contains only 3.5 wt% sodium chloride, all the membranes exhibit

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stable fluxes. At the beginning of the second hour, 0.2 mM SDS is added to the feed solution.


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The flux of the pristine membrane increases dramatically upon the SDS addition. This can be

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attributed to the adsorption of hydrophobic tails of SDS onto the membrane surface, leaving

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the hydrophilic heads pointing outwards. The hydrophilic heads will assist water absorption

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and result in a significant increase in flux.24 From the fourth hour, the SDS concentration is

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increased to 0.4 mM. At the same time, the flux of the pristine membrane experiences an

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obvious decline. The flux of the pristine membrane continues decreasing when the SDS

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concentration increases to 0.6 mM. At the end of the 7-h test, the flux drops to only one-half

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of the initial value. The flux decline is possibly due to membrane partial wetting caused by

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SDS adsorption and scaling. In the early stage of the partial wetting, which is also referred as

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surface wetting. Liquid penetration takes place but the water bridge is not formed. The

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membrane contains stagnant liquid water that has poor mass and convective heat transfer. The

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severe mass and temperature polarization leads to a low driving force and flux. On the other

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hand, since no pore is completely wet, there is no pathway for the salt to the permeate solution.

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Thus, the conductivity of the distillate remains unchanged. In fact, the rejections of all the three

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membranes are higher than 99.5% over the tests; therefore, they are not displayed in Figure 6.

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The flux of the Teflon® AF 2400 coated membrane is stable for the first two hours and starts

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to decline from the third hour onwards at a slower rate compared to that of the pristine

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membrane. The sudden increase of the flux was not observed for the Teflon® AF 2400 coated

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membrane, possibly due to the weaker attraction with SDS compared to the PVDF membrane.

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The attractive London dispersion force between the membrane surface and the hydrocarbon

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tail of the SDS is positively related to the geometric mean of the dispersion force components

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of the surface energy values of SDS and the membrane material.44 For PVDF and Teflon® the

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dispersion components of the surface energies are 23.3 and 18.4 mN/m,45 respectively.

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Therefore, the attractive force between SDS and PVDF surface is stronger than that between

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SDS and Teflon® AF 2400 coating. The flux of the nanoparticle and Teflon® AF 2400 coated



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membrane D-PVDF-60-T remains stable over the entire test. The superb performance is due to

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its excellent repellency towards the low surface tension feed.

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Figure 6. Performances of D-PVDF, D-PVDF-T, D-PVDF-60-T with the SDS addition in

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feed solutions. Average fluxes of D-PVDF, D-PVDF-T and D-PVDF-60-T are 24.9, 18.3 and

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14.6 kg m-2 h-1, respectively

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3.4 Effects of silica nanoparticle size on membrane omniphobicity

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The positive effects of nanoparticle coating on membrane omniphobicity has been discussed

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in previous sections. In this section, the effects of different nanoparticle sizes on membrane

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omniphobicity are explored. As shown in Figure 4, the synthesized nanoparticles are of uniform

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sizes and completely cover the membrane outer surfaces. However, the number density of

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nanoparticles per unit area and the particle-particle gap among them vary with their sizes. The

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number density of 60 nm nanoparticles is obviously higher than those of 250 nm and 400 nm

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ones, while the gap between particles is wider for larger nanoparticles. The effects of

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nanoparticle size on membrane omniphobicity are reflected in Figure 7. For liquids with high

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surface tensions such as water and EG, D-PVDF-250-T and D-PVDF-400-T exhibit similar


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repellency as D-PVDF-60-T, if not higher. However, for liquids with surface tensions, the

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membrane coated with 60 nm nanoparticles shows stronger repellency.

314 315

As discussed in section 3.3, a Cassie state with a composite solid-liquid-vapour interface is the

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key to achieve omniphobicity. In the Cassie model, the apparent contact angle θ* is closely

317 318

related to the fraction of solid in contact with the liquid 𝜙𝜙𝑠𝑠 (0 D-PVDF-400-T. Membranes with


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larger silica nanoparticles have smaller mass transfer resistance. The lowest flux of D-PVDF-

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400-T is possibly due to the partial wetting resulted from the unstable solid-liquid-vapor

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interface. All three membranes have rejections higher than 99.5% over the entire tests;

356

therefore, they are not displayed in Figure 8.

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Figure 8. Performances of D-PVDF-60-T, D-PVDF-250-T and D-PVDF-400-T with the SDS

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addition in feed solutions. Average fluxes of D-PVDF-60-T, D-PVDF-250-T and D-PVDF-

359

400-T are 14.6, 16.2 and 11.9 kg m-2 h-1, respectively

360 361

3.6 Stability of the nanoparticle coating

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In order to investigate the stability of the nanoparticle layers as well as the possible

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contamination of the permeate, we measured the silica concentrations in permeate solutions

364

and took pictures of membranes after VMD tests. Figure 9 and Figure S6 show no obvious

365

damage to the nanoparticle layers after VMD tests. Silica content in all the permeate solutions

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are lower than 0.3 ppm as shown in Table 3, which are much lower than the Singapore drinking

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water quality of 3.62 ppm.48



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Figure 9. FESEM images of outer surfaces of (A) D-PVDF-T (B) D-PVDF-60-T (C) D-

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PVDF-250-T (D) D-PVDF-400-T after VMD tests

371 372

Table 3. Silicon concentration in the distillates

373 374 375

Supporting Information

376

Materials; optimization of the alkaline treatment condition; optimization of APTES treatment

377

duration; characterizations; mass transfer resistance

378


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Acknowledgements

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The authors would like to thank Singapore National Research Foundation under its Energy

381

Innovation Research Programme for supporting the project entitled, “Using Cold Energy from

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Re-gasification of Liquefied Natural Gas (LNG) for Novel Hybrid Seawater Desalination

383

Technologies” (Grant number: R-279-000-456-279). Miss Kangjia Lu would also like to thank

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Dr. Dieling Zhao, Dr. Lin Luo and Dr. Gang Han for their valuable advices and kind support.

385 386

387

List of abbreviations and symbols 𝐽𝐽

Permeation flux

∆𝑊𝑊

Change in the permeate mass

Α

Effective membrane area

𝑡𝑡

Test duration

𝐶𝐶𝑝𝑝

Permeate concentration

𝐽𝐽0

Permeation flux for pristine membrane

𝛽𝛽

Separation factor

𝐶𝐶𝑓𝑓

Feed concentration

388

References

389

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Dong, D.; Wang, Y.; Li, X.; Zou, C.; Guan, Q.; Zhang, C.; Huang, J.;Wang, S.; Liu, H.;

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Bai, W.;Liang, F.; Lin, W.; Zhao, Q.; Liu, D.; Zhen Q. Breakthrough and prospect of

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shale gas exploration and development in China. Natl. Gas Ind. B 2016, 3, 12–26.

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(2)

U.S. Energy Information Administration. Technically recoverable shale oil and shale gas resources China. 2015.



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(3)

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U.S. Energy Information Administration. Annual energy outlook 2017 with projections to 2050. 2017.

(4)

Shaffer, D. L.; Arias Chavez, L. H.; Ben-Sasson, M.; Romero-Vargas Castrillón, S.;

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Yip, N. Y.; Elimelech, M. Desalination and reuse of high-salinity shale gas produced

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water: Drivers, technologies, and future directions. Environ. Sci. Technol. 2013, 47

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Omniphobic Hollow-Fiber Membranes for Vacuum Membrane Distillation

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