Stable Superhydrophobic Ceramic-Based Carbon Nanotube

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Stable Superhydrophobic Ceramic-based Carbon Nanotube Composite Desalination Membranes Yingchao Dong, Lining Ma, Chuyang Y. Tang, Fenglin Yang, Xie Quan, David Jassby, Michael J. Zaworotko, and Michael Dominic Guiver Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01907 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Stable Superhydrophobic Ceramic-based Carbon Nanotube Composite Desalination Membranes Yingchao Dong*a, Lining Maa, Chuyang Y. Tangb, Fenglin Yanga, Xie Quana, David Jassbyc, Michael J. Zaworotkod and Michael D. Guiver* *e a

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

b

Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China

c

Department of Civil and Environmental Engineering, University of California Los Angeles, Los Angeles 159310, United States of America State

d

Department of Chemical & Environmental Sciences, Bernal Institute, University of Limerick, Limerick V94 T9PX, Republic of Ireland

e

Key Laboratory of Engines, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China

Corresponding authors: Prof. Yingchao Dong Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China Tel: +86-411-84706328 E-mail: [email protected]

Co-corresponding authors: Prof. Michael D. Guiver State Key Laboratory of Engines, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, China Tel: +86-158-22869113 E-mail: [email protected]; [email protected]

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Abstract: Membrane distillation (MD) is a promising process for the treatment of highly saline wastewaters. The central component of MD is a stable porous hydrophobic membrane with a large liquid-vapor interface for efficient water vapor transport. A key challenge for current polymeric or hydrophobically modified inorganic membranes is insufficient operating stability, resulting in some issues such as wetting, fouling, flux and rejection decline. This study presents an overall conceptual design and application strategy for a superhydrophobic ceramic‒based carbon nanotube (CNT) desalination membrane having specially designed membrane structures with unprecedented operating stability and MD performance. Superporous and superhydrophobic surface structures with CNT networks are created after quantitative regulation of in situ grown CNT. The fully-covered CNT layers (FC-CNT) exhibit significantly improved thermally and superhydrophobically stable properties under an accelerated stability

test. Due to the distinctive structure of the superporous surface network, providing a large liquid-vapor superhydrophobic interface and interior finger-like macrovoids, the FC-CNT membrane exhibits a stable high flux with a 99.9% rejection of Na+, outperforming existing inorganic membranes. Under simple and nondestructive electrochemically assisted direct contact MD (e-DCMD), enhanced anti-fouling performance is observed. The design strategy is broadly applicable to be extended toward fabrication of high performance membranes derived from other ceramic or inorganic substrates, and additional applications in wastewater and gas treatment. Keywords: Membrane distillation; Ceramic membrane; Carbon nanotube; Operating stability; Superhydrophobicity; High performance


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D

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esalination of seawater is becoming a widely used approach to alleviate

fresh water scarcity.1-4 However, large-scale membrane seawater desalination via reverse osmosis (RO) process creates highly saline wastewater as an undesirable by-product, which when discharged back to the sea, causes extensive damage to the marine environment. Moreover, considerable amounts of high salinity industrial wastewaters are frequently discharged untreated on land, increasing surface water and groundwater salinity and thus imposing negative environmental impacts.5-7 Abundant resources such as valuable minerals and green osmotic energy, i.e. salinity gradient power, are available in these highly saline wastewaters and can be therefore recovered by membrane crystallization and energy extraction technologies such as pressure retarded osmosis, reverse electrodialysis, and associated processes as subsequent unit operations after being concentrated to a certain level.4, zero-liquid

discharge

and

solution

concentration,

5

With the goal of

thermal

desalination

technologies are usually used to concentrate these wastewaters, but low energy efficiency, and high investment and operational costs remain significant problems.8,

9

Membrane distillation (MD) is an emerging desalination

technology driven by the vapor pressure difference across a porous hydrophobic membrane.10-12 Using low-grade waste heat,13 and novel Joule14 or photothermal heating,15-17 MD is highly competitive as an economic and green solution for treating high salinity wastewaters that cannot be treated effectively by conventional techniques such as RO and electrodialysis (ED)5,

18, 19

or

emerging methods such as microbial desalination cells (MDC),20 capacitive 4 ACS Paragon Plus Environment

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deionization (CDI),21 and electrochemically-mediated desalination (EMD).22 The widespread adoption of MD is hindered predominately by the lack of high performance hydrophobic membranes. The central component of MD is an operationally stable porous membrane with a large liquid-vapor hydrophobic interface

for

efficient

water

vapor

transport,

having

thermal

and

superhydrophobic stabilities, anti-wetting and anti-fouling properties. For most hydrophobic polymeric membranes, and more robust inorganic membranes

derived

from

Al2O3,

ZrO2,

and

Si3N4

etc.,

which

are

hydrophobically-modified by fluoroalkylsilane (FAS), insufficient operating stability usually results in structure and performance degradation such as wetting, fouling, scaling, and flux and rejection decline, especially during long-term MD operation.11,

12,23-25

A major limitation for inorganic MD

membranes is the weak bonding between FAS molecules and membrane surfaces, resulting in hydrophobicity degradation, membrane wetting and the introduction of potentially toxic substances.23 Recently, thermally stable imparted hydrophobicity has been achieved for Si3N4 membranes coated with an inorganic nano-SiNCO hydrophobic modifier, but the hydrophobicity and flux remain moderate.26 Carbon nanotubes (CNTs) have outstanding physical and chemical properties, such as high hydrophobicity, large specific surface area, good thermal, mechanical and chemical stabilities, and excellent electro-conductivity.27-30 Thus, CNTs can be used as modifiers to improve performance for separation applications including MD, but mostly for polymeric membranes.31-34 Herein, we demonstrate a new approach for the design, fabrication and application of a robust high performance superhydrophobic ceramic-CNT 5 ACS Paragon Plus Environment

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membrane having specially designed membrane structures with unprecedented thermal and superhydrophobic

operating stability (Fig.

1, see

more

experimental details in the SI), in which an interconnected CNT network is directly constructed in situ on the surface and within the long-channel finger-like macrovoids of ceramic substrates via facile chemical vapor decomposition (CVD). In this novel design, by systematically optimizing CVD parameters, two different membrane surface structures are quantitatively regulated: partially-covered CNT network (PC-CNT) and fully-covered CNT network (FC-CNT), which are studied by MD performances under various conditions. Different from traditional membrane structures, the distinct FC-CNT surface structure provides a superporous and superhydrophobic interlayer, which allows water to exist in a Cassie-Baxter state, thus greatly reducing contact area between feed water and solid membrane surface as well as providing a large water-vapor interface to significantly enhance MD flux without membrane wetting. The excellent thermal and superhydrophobic stability allows it to withstand thermally, mechanically, and chemically harsh conditions often encountered in long-term MD operation. Additionally, we demonstrate how this electrically conducting FC-CNT surface structure can be exploited to markedly mitigate membrane fouling in situ, in a simple and nondestructive electrochemically assisted DCMD (e-DCMD), in place of established chemical or physical backwashing cleaning protocols.


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Figure 1. An illustration of the fabrication and DCMD process of ceramic-based CNT membranes with details such as the key steps, sample images, SEM structure images and simplified structure models.

Materials and Methods. Methods and experimental details are given in the Supporting Information. Results and Discussion. One of the key problems for developing ceramic-CNT membranes is to avoid generating defects and breakage during the fabrication process, by utilizing mechanically stronger thermally stable ceramic substrate with a hierarchical macro-porous structure. Sintering temperature is a dominant factor affecting its structure and mechanical properties.35, 36 The substrate sintered below 1250 °C was more easily fractured during CVD (Fig. S17) due to its insufficient strength (below 51.3±4.3 MPa), 7 ACS Paragon Plus Environment

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and it could not withstand the tension resulting from CNT growth. Higher temperature sintering increased both strength and spinel phase, but decreased porosity and pore size (Section S1.4), which are disadvantages for gas or vapor transport during both CVD and MD. Through systematic optimization (Figs. S4-S8), the substrate sintered at 1300 °C was selected because of its sufficient strength, suitable macroporous structure composed of long-channel finger-like macrovoids (Fig. S9), thermal and structural stabilities, and a much higher packing density (Table S4). These characteristics are ideal for nano-catalyst loading and reduction, CVD reaction for CNT in situ growth (Fig. S10) and DCMD operation (Fig. S22). CNTs were grown through thermal cracking of methane on Ni nano-catalyst on the surface and within the long-channel finger-like macro-voids of the ceramic substrate (Fig. S12), which was verified by XRD, Raman and TEM (Fig. S13, Section S2.4). The Raman intensity ratio of D to G bands (ID/IG) of ~1.03 (Fig. S13b) indicates the presence of amorphous residual carbon in the ceramic-CNT membranes37 which are required to be pretreated to prevent its release into water stream during MD (Fig. S18). Growth of multi-walled CNTs (MWCNTs) is confirmed with wall thickness of ~10 nm and diameter of ~40-50 nm (Fig. S13c). Composite membrane structures can be quantitatively regulated by Ni nano-catalyst loading, reaction temperature and time (Figs. S14-16, Section S2.5). Considering different reaction times (like nano-catalyst loadings and reaction temperatures), two markedly different membrane structures are clearly distinguishable based on CNT yield (Fig. 2 and Fig. S16). Low yields of CNT resulted in partially-covered CNT network structures on the substrate surface (PC-CNT membrane). Indeed, some isolated micro-regions are not covered by 8 ACS Paragon Plus Environment

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CNTs (Fig. 2b1), indicated by yellow arrows, where some underlying and uncovered ceramic particles and open pores are still clearly visible. In the case of high yield of well-grown CNTs (Figs. 2c1 and 2c2), a randomly interconnected continuous CNT network layer (~4 µm thickness) fully covering the substrate (FC-CNT membrane) is observed as a distinct surface structure with extremely high surface porosity of 80.1±1.0%, which is much higher (~ 9.2 times) than that of traditional membrane surfaces with low surface porosity of 8.7±1.4% (Fig. S24). Besides growth on the ceramic substrate surface, CNTs are also constructed in situ along the channel walls of the finger-like macro-voids (Figs. 2b2, 2c2, Figs. S23) due to low CH4 transport resistance combined with ample space for in situ growth of CNTs in the macro-porous substrate structure. Furthermore, after pretreatment, the FC-CNT membranes show good structural and mechanical stability, even when subjected to rapid water cross-flow (Fig. S18), because of the strong adhesion of in situ grown CNTs onto the substrate. Simplified structural representations of the substrate, PC-CNT and FC-CNT membranes are illustrated in Figs. 2a3, 2b3 and 2c3. Quantified growth of CNTs led to a gradual evolution in membrane surface morphology and properties, as well as membrane structure. Particularly, membrane surfaces are transformed in the order of hydrophilicity ‒ hydrophobicity ‒ superhydrophobicity for the substrate (water contact angle ~17°), PC-CNT (water contact angle ~102°) and FC-CNT (water contact angle ~170°), respectively (Figs. 2a4, 2b4 and 2c4). The FC-CNT membrane has a very high water contact angle of ~170°, 10 times higher than the substrate, indicating superhydrophobicity characteristics (Fig. S25), which significantly inhibits water penetration and membrane wetting by lowering the interfacial 9 ACS Paragon Plus Environment

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energy between feed solution and membrane surface.38

Figure 2. (1) Surface SEM images, (2) cross section SEM images, (3) simplified structural model representations (black color: CNT; light green color: spinel-based substrate) for cross-sectional structures and (4) water contact angles measured at room temperature (~25 °C) on the membrane surfaces of (a) spinel-based ceramic substrate, (b) PC-CNT membrane (fabrication conditions: 30 wt.% Ni(NO3)2 concentration for Ni nano-catalyst loading, CVD reaction time 60 min and CVD reaction temperature 650 °C for in situ growth of CNT) and (c) FC-CNT membrane (fabrication conditions: 30 wt.% Ni(NO3)2 concentration for Ni nano-catalyst loading, CVD reaction time 180 min and CVD reaction temperature 650 °C for in situ growth of CNT). (Notes: The yellow arrows indicate some isolated micro-regions not covered by CNTs (uncovered open pores and ceramic particles) of the PC-CNT membranes. The inset SEM image in Fig. 2c2 clearly shows a superhydrophobic network layer of CNT with a thickness of ~ 4 µm on the surface of the FC-CNT membrane).

Growth of CNTs also led to a decrease in water or gas permeability because of increased hydrophobicity and pore blockage (Fig. S20a). The liquid entry 10 ACS Paragon Plus Environment

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pressure (LEP) of FC-CNT membrane increased to ~2.0 bar, which is sufficiently high to hinder water penetration during DCMD. In contrast, the PC-CNT membrane has a low LEP of only ~0.6 bar, allowing membrane wetting to occur. For these two membranes, the nitrogen gas flux varies linearly with pressure (Fig. S20b). When compared to the substrate (175 m3·m-2·h-1 at 0.1 bar), PC-CNT membrane shows a reduction of ~68% (56 m3·m-2·h-1) in nitrogen gas permeability. However, despite a higher density of CNTs in the FC-CNT membrane, there is only a minor further reduction of ~14.8% (30 m3·m-2·h-1) because of minimal pore blockage by CNTs. Smaller pore size and lower porosity generally result in lower permeation flux.39 The substrate exhibits large pore sizes (0.3-0.6 µm) (Fig. S20c), while the PC-CNT and FC-CNT membranes have relatively smaller pore sizes ranging from 0.17 to 0.26 µm and 0.13 to 0.16 µm, respectively. Although the PC-CNT membrane has a high water flux of ~41.4 L·m-2·h-1, salt rejection is very low and gradually decreases from 69.8 to 60.6% with distillate conductivity increasing from 17.2 to 22.8 mS cm-1 after 18 h DCMD operation (Fig. 3a), indicating that membrane wetting progressed with time due to its less hydrophobicity (Fig. 2b4). The feed easily distilled through the open pores not covered by CNTs (Fig. 2b1), which is further verified by the SEM-EDS analysis of finger-like macrovoids where both Na and Cl were detected (Fig. S23a). In contrast, the FC-CNT membrane maintains a slightly decreased, but still high and stable water flux of ~37.1 L·m-2·h-1 and an excellent salt rejection above 99.9% with a distillate conductivity of ~0.09 µS·cm-1 (Fig. 3b). This remarkably high salt rejection is due to the combined superhydrohobicity and thermal stability with strong substrate/CNT adhesion 11 ACS Paragon Plus Environment

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during MD operation, which prevents membrane wetting on the FC-CNT membrane interface with fully covered CNTs layer (Fig. 3c). High and stable flux, outperforming all existing state-of-the-art inorganic membranes (Fig. 3e, Table S5), is attributed to the distinctive membrane structure composed of: (1) a superporous (extremely high 80.1±1.0% surface porosity) and superhydrohobic surface CNT network layer (Figs. 2c1, S16d and S24), which provides a stable large liquid-vapor interface as compared with usual unmodified membrane structures (8.7±1.4% surface porosity), and (2) long-channel finger-like macrovoids (~77%, Fig. S9b) further enhancing water vapor transport. In combination, these two features significantly accelerate the vaporization of hot feed solution and subsequent vapor transportation across the FC-CNT membrane during DCMD operation. Moreover, although CNTs have high thermal conductivity, a high energy efficiency of ~74.7% is obtained for the FC-CNT membrane, which is higher than hollow fiber CNT membranes (71.4%) and much higher than some polymeric membranes such as PVDF (67.7%) and PP (65.4%)40. High energy efficiency indicates low heat loss, mainly due to specially designed membrane materials and structural morphology, and high water flux, greater than existing inorganic MD membranes (Fig. 3e and Table S5). The surface CNT network layer (~4 µm thickness) (Fig. 2c2) serves as the primary barrier for anti-wetting, with thermally and superhydrophobically stable and superporous characteristics, while interior CNT networks situated along the long channels of the finger-like macrovoids serve as the secondary superhydrophobic barrier for further anti-membrane-wetting. Moreover, CNTs within FC-CNT composite membranes have a superhydrophobic outer surface and high specific surface area, thereby further increasing the overall vapor 12 ACS Paragon Plus Environment

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transport.30, 32 Unlike the PC-CNT membrane, indeed, no sodium or chloride ions were detected by SEM-EDS in the FC-CNT membrane (Fig. S23b), providing further supporting evidence for its excellent anti-wetting performance and confirming that the FC-CNT membrane presents superior and more stable MD performance than the PC-CNT membrane.

Figure 3. DCMD performance (flux, salt rejection and distillate conductivity) of (a) PC-CNT and (b) FC-CNT membranes, (c) water contact angle variations with treatment time of FC-CNT membrane (red solid circle), SiNCO- (blue solid circle) and FAS- (black solid circle) modified ceramic membranes26 under accelerated thermal stability testing (the inset in the upper Fig. 3c: optical photographs and water contact


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angle values of water droplets on the FC-CNT membrane at different treatment times), (d) Hydrophobic loss comparison of FC-CNT membrane, Si3N4-SiNCO ceramic membrane and Si3N4-FAS ceramic membrane after accelerated stability test for 24 h, (e) Comparison of water flux and water contact angle (hydrophobicity) (f) Comparison of salt rejection and water contact angle (hydrophobicity) between existing state-of-the-art hydrophobic inorganic MD membranes (such as ceramic-, metal- and carbon-based membranes) reported in the literature and ceramic-CNT (FC-CNT) composite membrane fabricated in this work (red solid star) under three different MD processes: DCMD (solid circles or star), VMD (vacuum membrane distillation) (hollow circles) and SGMD (sweeping gas membrane distillation) (hollow circles).

One key challenge for hydrophobically-modified ceramic membranes is the limited operating lifetime stabilities of their hydrophobic coatings, including thermal and hydrophobic stability.23, 26 Under accelerated stability tests (Fig. 3c), the

FC-CNT

membrane

consistently

maintained

outstanding

superhydrophobicity, with only very slight variations in water contact angle ranging from 169 to 168° after prolonged exposure times. Unlike the rapid deterioration of hydrophobicity of FAS-modified membranes, even those showing hydrophilicity above 12 h, nano-SiNCO-modified Si3N4 ceramic membrane shows relatively much better thermal and hydrophobic stability, with water contact angle values of 142-136° for different treatment times.26 These water contact angle values are indicative of hydrophobicity, but not superhydrophobicity as reported in the present work. After a 24 h accelerated stability test, the loss percentage of hydrophobicity of FC-CNT membrane is as low as 0.59%, which is much lower than FAS-modified ceramic membrane (43.26%) and nano-SiNCO-modified ceramic membrane (4.23%) (Fig. 3d). We ascribe this outstanding operating stability to the high thermal stability of the superhydrophobic and superporous CNT layers as well as robust adhesion between the CNTs and ceramic substrate, which combine to achieve substantial long-term superhydrophobic and thermal stability. In summary, besides 14 ACS Paragon Plus Environment

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excellent superhydrophobic and thermal operating stability, the FC-CNT membrane outperforms all existing inorganic membranes (such as ceramic, CNTs and metal membranes) in terms of hydrophobicity, water flux and salt rejection, especially for DCMD (Figs. 3e and 3f, Table S5), which are key performance indicators for stability and efficiency of MD membranes. The thermal stability in air (Fig. S26) shows the FC-CNT membrane is stable up to ~500 °C. Considering the high gas permeability (Fig. S20) and excellent thermal stability in air, therefore, it is likely that the application of ceramic-CNT membranes can be extended to hot gas purification. An increase in salt concentration from 35 to 105 g·L-1 resulted in a decrease in water flux of FC-CNT membrane from 37.5 to 11.2 L·m-2·h-1 due to both decreased driving force and increased membrane fouling (Fig. 4a).25,

26

An

increase in feed temperature from 60 to 80 °C resulted in an increase in water flux from 10.3 to 37.5 L·m-2·h-1 (Fig. S27). In two cases, the salt rejection (>99.9%) was unaffected by both salt concentration and temperature. For high salinity water (70 g·L-1 NaCl), the FC-CNT membrane still exhibits quite stable DCMD performance with high water flux (23.70-25.9 L·m-2·h-1) and high rejection (above ~99.6%) over the whole operation duration (Fig. 4b). During MD, membrane organic fouling is a major problem, which usually results in membrane wetting and consequently performance degradation.41, 42 Established cleaning protocols using acidic, alkaline or oxidative chemicals invariably result in some problems such as membrane structure and performance degradation, as well as the need for energy consumption, cleaning agents and their subsequent disposal.43 Notably, by additionally exploiting the excellent electro-conductivity of the FC-CNT surface structure (Fig. S21), humic acid 15 ACS Paragon Plus Environment

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fouling is significantly mitigated in situ using simple and nondestructive e-DCMD by manipulating the electrostatic interactions between humic acid and FC-CNT membrane. At open circuit (i.e. without electrochemical assistance, Fig. 4c), the water flux decreased by ~22% (19.6 L·m-2·h-1) after 6 h (compare with ~ 6.4% reduction to 23.9 L·m-2·h-1 for 70 g·L-1 NaCl without humic acid, Fig. 4b), while salt rejection was still maintained at a high level (~99.4%). This was due to membrane fouling generated by the accumulation of hydrophilic humic acid on CNTs surface, resulting in a decrease in hydrophobicity with water contact angle decreasing from 170 to 122° after 12 h DCMD operation (Fig. 4e). After back-washing cleaning with hot deionized water for 10 min, the water flux was recovered completely, indicating a weak physical adsorption of humic acid molecules on CNTs. In contrast, when the FC-CNT membrane was applied as cathode at -2.0 V under negative polarization, even for as long as 12 h DCMD operation (Fig. 4c), a more stable and higher flux of 24.3 L·m-2·h-1 was achieved with a stable and very low distillate conductivity (~0.2 µS·cm-1), indicating that humic acid membrane fouling was significantly mitigated (Fig. 4f) during e-DCMD due to stronger electrostatic repulsion. This is further verified by the enduring hydrophobic membrane surface with a water contact angle of ~134° (the inset in Fig. 4f). However, when applied as anode at +2.0 V under positive polarization, both flux and salt rejection decreased steeply with operation time, which, unlike the other two cases, could only be partially recovered by hot water back-washing cleaning (only ~53.3% recovery for flux), due to more severe and stronger humic acid fouling (Fig. 4g) with a hydrophilic membrane surface (water contact angle = 43°). Humic acid is negatively charged and hydrophilic in water44 and CNTs are also negatively charged.45, 46 16 ACS Paragon Plus Environment

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Therefore, electrostatic interactions between humic acid and CNTs might be a crucial factor influencing membrane organic fouling (Figs. 4h, 4i and 4j). Inherently, FC-CNT membrane shows anti-fouling performance for humic acid due to electrostatic repulsion (Fig. 4h). More specifically, when working as cathode, the electronegativity of the FC-CNT membrane was enhanced, resulting in enhanced electrostatic repulsion between humic acid and CNTs, thus significantly mitigating in situ membrane fouling (Fig. 4i). When working as anode, surface CNTs were positively charged, and the electrostatic attraction between humic acid and CNTs dominates, resulting in more severe membrane fouling. Thus, by employing e-DCMD, the FC-CNT membrane is capable of maintaining a stable and high MD performance for high salinity water containing humic acid. Inspired by this phenomenon, by strategically regulating electronegative characteristics via employing ceramic-CNT membranes as different electrodes under different voltages during e-DCMD, membrane anti-fouling is expected to be extended to other organic or inorganic foulants with different electric charges in wastewaters.


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Figure 4. (a) Initial DCMD performance (water flux, salt rejection and distillate conductivity) of FC-CNT membrane as a function of salt concentration, (b) variations of flux, salt rejection and distillate conductivity with MD process operating time for the treatment of high salinity water (70 g·L-1 NaCl) with hot deionized water backwash cleaning after MD operation for 6 h, (c) flux and (d) salt rejection variations with MD


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operation time for high salinity water with humic acid organic foulant (70 g·L-1 NaCl and 30 mg·L-1 humic acid) with hot deionized water backwash cleaning after MD operation for 6 h during e-DCMD operation, membrane surface SEM images and water contact angle values (insets) for probing and characterizing humic acid fouling on CNTs surface layer when FC-CNT membranes worked as different electrodes (e) open circuit, (f) cathode and (g) anode after 12 h e-DCMD operation. Representations and electrostatic interaction simplified models on the FC-CNT membrane surface during e-DCMD operation under different electrochemical assistance protocols: (h) no polarization (open circuit), (i) negative polarization (FC-CNT membrane as cathode) and (j) positive polarization (FC-CNT membrane as anode).

Conclusion. The conceptual design for a robust inorganic ceramic-CNT structured membrane having thermal and superhydrophobic operating stability is presented for DCMD treatment

of high salinity wastewaters. A

superhydrophobic and superporous CNT network structure is constructed by in

situ CVD growth on the surface of, and within the long-channel finger-like macrovoids of hollow fiber specially-structured ceramic substrates having high thermal and mechanical stabilities, and high permeability. After optimization, two distinct kinds of CNT structure state were observed by SEM, denoted as PC-CNT (partially-covered) and FC-CNT (fully-covered) membranes. The FC-CNT membrane shows superior superhydrophobic and thermally stable properties, when compared with existing state-of-the-art inorganic hydrophobic membranes. Due to its outstanding anti-wetting characteristics and membrane structure consisting of a superporous and superhydrophobic CNT surface layer (large liquid-vapor interface between feed and membrane) and CNT-decorated interior long-channel finger-like macrovoids, the FC-CNT membrane has high and stable water flux and high salt rejection. Moreover, under a simple and nondestructive e-DCMD protocol, the FC-CNT membrane exhibits stable high flux, high salt rejection and more especially excellent fouling resistance against humic acid. Results from the current study will provide useful guidelines for 19 ACS Paragon Plus Environment

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the design and fabrication of other advanced inorganic membranes. These are not expected to be limited to low-cost spinel-based substrates, but should extend to other more common ceramic substrates such as alumina, titania, mullite and zirconia. By full utilization of superhydrophobicity, thermal and electro-conductive properties of CNTs to achieve stable high performance, it is believed that ceramic-CNT membranes fabricated using this design strategy will have a great potential in the DCMD or e-DCMD treatment of not only seawater, high salinity brine and industrial wastewaters, but other wastewaters containing nonvolatile solutes such as heavy metals, macromolecules and colloids and gas purification, which will be a focus in our future work. Future work will also focus on extracting salts or green energy from highly saline wastewaters, by combining membrane crystallization and green energy extraction

technologies

such

as

pressure

retarded

osmosis,

reverse

electrodialysis, and their associated processes, using the membranes fabricated in the current work. ASSOCIATED CONTENT

Supporting Information Supporting

Information

Available:

Details

and

descriptions

of

experimental methods (Fig. S1, Table S2, Fig. S10, Fig. S19 and Fig. S22). Results and discussions on the characterization of raw materials (Figs. S2, Table S1), fabrication, optimization and characterization of ceramic substrates (Figs. S3-S9, Tables S3-S4) and ceramic-CNT composite membranes (Figs. S11-S18, Fig. S20-21), as well as performances for typical ceramic-CNT composite membranes (Figs. S23-S27, Table S5) and preliminary extension to 20 ACS Paragon Plus Environment

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other membrane systems (Figs. S28) (PDF). AUTHOR INFORMATION Corresponding authors: Yingchao Dong ([email protected]) Co-corresponding authors: Michael D. Guiver ([email protected], [email protected])

ORCID: 1, Yingchao Dong: 0000-0003-1409-0994 2, Michael D. Guiver: 0000-0003-2619-6809 Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGEMENTS This work was financially supported by the Fundamental Research Funds for the Central Universities (No. DUT16RC(3)050 and DUT18LAB02), the 111 Program of Introducing Talents of Discipline to Universities (No. B13012), the Haitian Scholar Program from Dalian University of Technology, Nature Science Foundation for Distinguished Young Scholars of Fujian Province (No. 2015J06013). We also thank Miss. Yiran Si, Miss. Peicong Han, Miss. Xueling Wang and Mr. Mao Fu for improving some figure artwork and doing additional experiments, and Dr. Xinfei Fan at DUT and Prof. Yang Zhang (The Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences) for helpful discussions. We would also like to acknowledge the reviewers and editor for helpful feedback and ideas for improving the quality of our manuscript. 21 ACS Paragon Plus Environment

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Stable Superhydrophobic Ceramic-Based Carbon Nanotube

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