Highly Permeable Thin-film Composite Forward Osmosis Membrane

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Highly Permeable Thin-film Composite Forward Osmosis Membrane Based on Carbon Nanotube Hollow Fiber Scaffold with Electrically Enhanced Fouling Resistance Xinfei Fan, Yanming Liu, Xie Quan, and Shuo Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05341 • Publication Date (Web): 02 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Environmental Science & Technology

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Highly Permeable Thin-film Composite Forward Osmosis Membrane Based

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on Carbon Nanotube Hollow Fiber Scaffold with Electrically Enhanced

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Fouling Resistance

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Xinfei Fan, Yanming Liu, Xie Quan,* Shuo Chen

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Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education,

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China), School of Environmental Science and Technology, Dalian University of Technology, Dalian

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116024, China

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*Corresponding author e-mail: [email protected]

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1

ACS Paragon Plus Environment


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Abstract：：Forward osmosis (FO) is an emerging approach in water treatment, but its application is

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restricted by severe internal concentration polarization (ICP) and low flux. In this work, a

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self-sustained carbon nanotube hollow fiber scaffold supported polyamide thin film composite (CNT

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TFC-FO) membrane was firstly proposed with high porosity, good hydrophilicity and excellent

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electro-conductivity. It showed a specific structure parameter as low as 126 µm, suggesting its

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weakened ICP. Against a pure water feed using 2.0 M NaCl draw solution, its fluxes were 4.7 and 3.6

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times as high as those of the commercial cellulose triacetate TFC-FO membrane in the FO and

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pressure retarded osmosis (PRO) modes, respectively. Meanwhile, the membrane showed excellent

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electrically assisted resistance to organic and microbial fouling. Its flux was improved by about 50%

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during oil-water simulation separation under 2.0 V voltage. These results indicate that the CNT

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TFC-FO membrane opens up a frontier for stably and effectively recycling potable water from

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electrochemical FO process.

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Keywords: fouling mitigation, water treatment, forward osmosis, carbon nanotube, electrochemistry

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TOC Art:

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Introduction

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Water scarcity is one of the global crises in the 21th century, and membrane separation provides

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promising solutions for addressing this worldwide issue. Among various membrane technologies,

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forward osmosis (FO) has attracted growing attention in recent years to providing clean water.1-3 FO

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is an osmotically driven membrane process with no or low hydraulic pressure requirement.

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Compared to pressure-driven membrane processes, FO delivers the advantages of low energy

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consumption, efficient water recovery, low fouling propensity, and easy fouling removal.4 However,

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despite the significant application advancements, FO has not been widely used in industries primarily

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due to the absence of high-performance membranes.

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The conventional FO membranes are typically asymmetric in a thin-film composite (TFC)

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membrane structure.5-8 These TFC membranes are composed of a densely active layer on the top of a

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porous sublayer. As a result, the design of TFC membranes offers the possibility to optimize their

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active layer and sublayer separately, which favors improving the performance of the final TFC-FO

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membranes.9 Specially, the FO sublayers should possess a low structural parameter (S) to reduce the

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internal concentration polarization (ICP) phenomenon.4, 10 For the FO processes, ICP is recognized

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as the predominantly responsible for the decline in both effective osmotic driving force and water

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production. It can reduce more than 80% flux at higher draw solution concentration. However, the

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conventional sublayers, which are fabricated via phase inversion method, usually present isolated

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channels in a tortuous 1D architecture.11 Such structure is one of the bottlenecks for developing

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high-flux FO membranes because it would form an un-turbulent barrier to the diffusions of draw

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solute and water.

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Recently, constructing 3D architecture with interconnected pores opens a fascinating insight into 3



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developing high-flux TFC-FO membrane. For example, the electrospun scaffold-like nanofibrous

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matrixes usually present high porosity (σ), low tortuosity (τ) and easily controllable thickness (t).12-14

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These unique structure features ensure the electrospun nanofibrous matrixes with a potentially low S

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value (S=σ·τ/t). As the ICP degree is positively relates to S, constructing TFC-FO membranes on

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electrospun scaffold-like sublayers provides an efficient approach for breaking the ICP bottleneck

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and achieving high FO flux. However, despite low fouling tendency during the FO processes, a

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higher flux enables a greater hydrodynamic force. The increased hydrodynamic force would drag the

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foulants toward to the membrane surface and then cause more severe membrane fouling. Finally,

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these fouling, particularly during wastewater treatment, in turn lead to an additional hydraulic

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resistance which then lowers the effective osmotic pressure and water permeability. Developing

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novel strategies to mitigate membrane fouling is a long-active area in the FO processes.4,

10

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According to the electrically enhanced fouling resistance on pressure-driven membranes,15,

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applying a small external potential might be an alternative and scatheless approach to significant

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fouling mitigation on the FO membranes. However, one critical challenge is the electrospun

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polymeric matrixes are intrinsically insulate.

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In the past decades, carbon nanotubes (CNTs) with great aspect ratio have been widely employed

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in preparing nanofibrous mats for pressure-driven membrane applications.17-19 In addition to the

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competitive S value comparing to electrospun flat sheets, such membranes with randomly entangled

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CNT meshes possess fascinating electrical/electrochemical properties. Moreover, the non-woven

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networks of entangled CNTs has been recently fabricated into a hollow fiber architecture composed

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of finger-like macrovoids and interconnected pores.20, 21 For FO processes, hollow fiber membranes

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are desirable because of their high packing density for small footprint systems. Therefore, these 4


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material and structure uniqueness might ensure the CNT hollow fiber as an alternative sublayer for

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developing FO membrane with high flux and good fouling resistance under electrochemical

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assistance.

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To verify the feasibility of this hypothesis, a TFC-FO membrane was fabricated by coating a

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polyamide (PA) active layer onto a CNT hollow fiber scaffold (CNT TFC-FO) through wet-spinning

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and interfacial polymerization methods. Its FO performance was investigated in comparison with a

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standard commercial cellulose triacetate (CTA) TFC-FO membrane and a polyether sulfone (PES)

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TFC-FO membranes. The effects of electrochemical assistance on the organic fouling, biofouling and

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gypsum scaling were also studied under a low DC voltage.

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Materials and Methods

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Materials and Chemicals. CNTs (multi-walled carbon nanotubes, diameter: 60~100 nm; length:

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5~15 µm) were supplied by Shenzhen Nanotech Port Co. Ltd. 1,3-phenylenediamine (MPD, >99%),

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triethylamine and 1,3,5-benzenetricarbonyl trichloride (TMC, 98%) were purchased from

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Sigma-Aldrich. N,N-dimethylacetamide (DMAc, anhydrous, 99.8%), sodium chloride (NaCl),

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sulfuric acid (H2SO4, 98%), nitric acid (HNO3, 65%), and polyvinyl butyral (PVB) were obtained

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from Sinopharm Chemical Reagent Co., Ltd. Unless otherwise specified, all chemicals with

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analytical grade were used as received without further purification. Additionally, the commercially

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asymmetric CTA FTC-FO membrane with 50 µm thickness was acquired from Hydration

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Technologies Inc. (Albany, Oregon), as it has received popular recognition as the benchmark

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membrane during the FO studies.

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Preparation of CNT Hollow Fiber Sublayer. The CNT hollow fiber sublayer was prepared by

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the wet-spinning method (Figure S1).21 Briefly, the original CNTs were oxidized by H2SO4/HNO3 5



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solution. Then, 1.0 g oxidized CNTs were dispersed homogeneously in 8.5 g DMAc solution with

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0.5 g PVB to yield dope suspension (Figure S2). After degasification, the dope was dispensed

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through a home-made spinneret by an injection pump, and directly immersed into a water

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coagulation bath. The bore-fluid was DMAc/water solution with volume ratio of 75/25. Finally, the

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obtained hollow fiber sublayer was dried and calcinated at 1000 °C for 2 h in N2 atmosphere.

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Preparation of CNT TFC-FO Hollow Fiber Membranes by Interfacial Polymerization.

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Interfacial polymerization was performed on the outer surface of the CNT hollow fiber sublayer. In

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brief, the CNT sublayer was immersed into isopropyl alcohol for better wetting. After rinsed with DI

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water, the sublayer was immersed in 2.5 w/v% MPD aqueous solution for 120 s. Then, the excess

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MPD solution was removed from the saturated sublayers by an air knife. After sealed both ends by

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resin, a 0.15 w/v% TMC solution with n-hexane as solvent was brought into contact with the outer

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surfaces of the MPD saturated sublayer. After reaction for 60 s to form an ultrathin PA layer, the

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nascent membrane was taken out and cured in DI water at 90 °C for 120 s, then rinsed with a 200

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ppm NaOCl aqueous solutions for 120 s, followed by rinsing with a 1000 ppm NaHSO3 aqueous

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solutions for 30 s. Finally, the prepared CNT TFC-FO hollow fiber membranes were cured at 90 °C

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for 300 s and then stored in DI water at 4 °C.

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Characterization of CNT Sublayer and CNT TFC-FO Membrane. The microstructure and

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surface morphology of both CNT sublayer and CNT TFC-FO membrane were characterized on a

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scanning electron microscopy (SEM, Hitachi S4800). Fourier transform infrared spectrum (FT-IR,

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Bruker Vertex 70) and X-ray photoelectron spectroscopy (XPS, EscaLab 250i) were used to analyze

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the component of the sublayer and TFC-FO membrane. For the CNT sublayer, its measurements of

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pure water flux, pore size distribution, porosity, water contact angle and electric conductivity were 6


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described in Supporting Information.

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According to the reported standard protocols,22-26 water permeability and salt permeability were

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evaluated by testing the membrane under the pressurized cross-flow RO modes. Briefly, the water

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permeability coefficient (A) was determined from pure water flux (25 °C) under the applied

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trans-membrane pressure (∆P) of 0~1.0 bar for hollow fiber membranes and 0~8.0 bar for plate sheet

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membrane, respectively. The salt rejection (Rs) was obtained from conductivity measurements of the

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feed and permeate water by filtering 500 mg/L NaCl solution at 1.0 bar for hollow fiber membranes,

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while 2000 mg/L NaCl solution at 8.0 bar for plate sheet. The salt permeability coefficients (B),

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which is an intrinsic property of FO membranes, were calculated based on the average rejection

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value (3 replicates) at a given pressure using the solution-diffusion theory:6

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1− R   J  B = Jw ⋅   ⋅ exp − w   R   k  where ∆π denotes the osmotic pressure difference across the membrane.

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Lab-Scale Forward Osmosis and Anti-fouling Tests under Electrochemical Assistance. The

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forward osmosis tests were carried out in a lab-scale cross-flow filtration module. The feed and draw

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solutions were kept at 25 °C and fed concurrently into the membrane module at flow rate of 25 cm/s.

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Each membrane was tested under both PRO mode (active layer facing draw solution) and FO mode

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(active layer facing feed solution). The change in feed solution weight was monitored by a balance.

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The water flux (Jw, L/(m2·h), abbreviated LMH) was calculated as follows: Jw =

∆m ρ ⋅ M ⋅ ∆t

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where ∆m (kg) is the weight change of feed solution over a predetermined time ∆t (h) in the FO

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process; M is the effective membrane surface area (m2) and ρ is the water density of 1.0 kg/L.

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Salt concentration in feed was determined by conductivity measurement using a NaCl calibration 7



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curve. Salt reverse-diffusion, Js (g/(m2·h), gMH) was calculated from the increase of feed

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conductivity: Js =

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( C t ⋅ mt − C 0 ⋅ m0 ) ρ ⋅ M ⋅ ∆t

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where Ct and C0 denote the salt concentration (g/L) after and before FO tests, mt and m0 are the

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weights (kg) of the feed after and before FO test, respectively.

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The organic fouling test was performed by treating feed water containing 100 mg/L humic acid

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(HA), 1.0 mM CaCl2 and 10 mM NaCl. The biofouling test was carried out by using LB solution

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containing 3× 107 cfu/mL. Gypsum scaling was tested by feed water containing 35 mM CaCl2, 20

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mM Na2SO4, and 19 mM NaCl (gypsum saturation index (SI) of 1.3). The adsorbed HA, E. coli and

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gypsum on the fouled membrane surface were observed by SEM. The oil/water separation was

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performed by treating feed water containing 50000 mg/L emulsified oil with 2.0 wt% surfactant.

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After operating for 6 h, the oil was desorbed from membrane and re-suspended in water, which was

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then measured by a total organic matter (TOC) analyzer (TOC-VCPH, Shimadzu). The zeta potential

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of emulated oil droplets was measured by Malvem nano-ZS90. During the electrically assisted FO

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processes, the membrane worked as cathode and a Ti mesh as anode (distance of 3 mm). The voltage

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was supplied by an outer DC power.

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Results and Discussion

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Properties of CNT Sublayer. According to the SEM images in Figure 1, a hollow fiber structure

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with macro-voids sandwiching from inner to outer layers can be observed. This structure arises from

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the instantaneous polymer precipitation of PVB contacting with coagulation bath. It keeps consistent

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with those of conventional polymeric membranes from wetting spinning.25, 26 After PVB pyrolysis at

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high temperature, structure collapses did not occur. Moreover, the entangled CNTs constructs the 8


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hollow fiber architecture with a feature of interconnected pores like electrospun fibrious matrixes

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(Figure 1c~e). Attributed to the co-presence of macro-voids and interconnected scaffold, the CNT

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sublayer presents a high porosity of 93%. This value is much higher than 60~80% of the polymeric

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hollow fibers. In addition, the pore size of 194 nm suggests that the CNT sublayer is a microfiltration

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membrane. A water contact angle (WCA) of 37° indicates that it possesses a good hydrophilic

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property. A high pure water flux of about 5000 LMH at 0.6 bar implies its low membrane resistance

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for water transport. The result of mechanical strength reveals the CNT sublayer has a measured

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tensile strength of 4.4 MPa, higher than that of the electrospun polymeric sublayer.14 Therefore, the

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prepared CNT sublayer possesses desirable properties (e.g. high porosity, good hydrophilicity and

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low water transport resistance) as potential sublayer for developing high-flux TFC-FO membrane.9, 27

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Figure 1. SEM images of CNT hollow fiber: cross-section (a~c), inner surface (d) and outer surface (e).

Properties of TFC-FO Hollow Fiber Membrane. The complete TFC-FO hollow fiber membrane 9



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is comprised of a salt-rejecting PA active layer interfacially polymerized on the outer surface of the

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CNT hollow fiber. As presented in Figure 2a, the outer-view SEM image of the TFC-FO membrane

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shows a typical ridge-and-valley morphology like the conventional PA layer.5, 8 Moreover, the PA

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active layer (thickness: about 210 nm) is integrally bonded with CNTs (Figure 2b), indicating strong

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bonding between PA active layer and its sublayer. In addition, the cross-section and inner surface of

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the final TFC-FO membrane display a similar interconnected porous structure to the original CNT

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sublayer (Figure S6). These results indicate that the sublayer was not affected during interfacial

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polymerization except for a thin PA layer deposited on its outer surface.

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Figure 2. SEM images of outer-view (a) and cross-section (b) of TFC-FO membrane based on CNT hollow fiber

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sublayer, FT-IR (c) and XPS (d) spectrum of CNT sublayer and final CNT TFC-FO membrane.

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To further verify PA active layer formation, both the final TFC-FO membrane and original CNT

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sublayer were characterized by FT-IR and XPS. Compared with the original CNT sublayer, the

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formation of PA active layer is evidenced by the appearance of some additional absorption peaks in

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the FT-IR spectrum of the TFC-FO membrane (Figure 2c). The new peaks at 1541 and 1672 cm-1

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correspond to the bending vibration of –N-H (amide II peak) and stretching vibration of –C=O

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(amide I peak), respectively. The absorption peak at 1613 cm-1 is ascribed to the aromatic ring 10


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breathing vibration in the PA molecule. All these results confirm the success of PA layer formation in

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the TFC-FO membrane, which was further evidenced by XPS spectrum. As presented in Figure 2d,

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an additional N1s peak, arising from the nitrogen element of the amide group in the PA layer, appears

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at the binding energy of 400 eV after the interfacial polymerization.

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FO Water Flux and Salt Reverse Transport. To evaluate the performance of CNT TFC-FO

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membrane, both osmotic water fluxes (Jw) and reverse salt fluxes (Js) were investigated under FO

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and PRO modes by using 2.0 M NaCl as draw solution against a DI water feed. Meanwhile, a

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commercial CTA TFC-FO planar membrane (without macrovoids and interconnected pore structure)

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and PES TFC-FO hollow fiber membrane (with macrovoids but no scaffold structure) were taken as

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the control membranes to reflect the sublayer structure effect on the FO performance (Figure 3 and

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S7). According to the experimental results (Figure 3a and 3b), the CNT TFC-FO membrane displays

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high Jw of 61.0 and 81.9 LMH with relatively low Js of 8.8 and 11.2 gMH for respective FO and

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PRO mode. In the case of PES TFC-FO membrane, although it exhibits lower Js of 5.9 and 9.5 gMH,

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its Jw is only 30.7 and 59.0 LMH for FO and PRO modes, respectively. Specific salt flux (Js/Jw) is

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usually used to evaluate the FO performance by determining the amount of draw solute loss per unit

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of water production.14, 28 It is noteworthy that the Js/Jw of CNT TFC-FO membrane (FO: 0.144 g/L,

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PRO: 0.137 g/L) is lower than those of the PES TFC-FO membrane (FO: 0.192 g/L, PRO: 0.161

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g/L). The lower Js/Jw ratio reflects a better FO efficiency on the CNT TFC-FO membrane. As their

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active layers were prepared in the same method, the difference in FO performance between CNT and

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PES TFC-FO membranes might arise from their sublayers. It can be found in Table S2 that the

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porosity of CNT sublayer (93%) is much higher than that of the PES sublayer (79%). Such high

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porosity is attributed to that the CNT sublayer possesses both macro-voids and interconnected mesh 11



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structures, while PES sublayer only with macro-voids. On the other hand, the CNT sublayer presents

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a WCA (37°) lower than the PES sublayer (62°), suggesting the CNT sublayer is more hydrophilic.

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The high hydrophilicity is because the CNTs were oxidized by acid treatment before sublayer

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preparation. Such treatment can introduce hydrophilic oxygen-containing groups (e.g. ‒COOH, ‒OH,

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etc.) onto the CNT surface. According to the reported works,9, 13, 14, 29, 30 both high porosity and good

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hydrophilicity are favorable to a lower water diffusion resistance and a higher concentration gradient

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between the two sides of the active layer on the FO membranes. As a result, such fascinating

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properties of CNT sublayer might be the primary reason that endows the CNT TFC-FO membrane

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with enhanced mass transfer and improved water permeability (Figure 3c and 3d). To confirm this,

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the structural parameter (S) is calculated to express the contribution of the sublayers on the ICP

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based on the intrinsic properties (including A, Rs and B) of the two TFC-FO membranes (Table 1). It

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is noteworthy that the CNT TFC-FO membrane possesses a significantly low S of 126 µm, which is

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nearly 1/3 of that of the PES TFC-FO membrane. As S is an intrinsic membrane property positively

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indicating the ICP degree, the result implies that a weaker ICP effect formed in the CNT hollow fiber

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sublayer. Combined with the flux comparison, the weakening ICP is substantiated to contribute to

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higher flux on the CNT TFC-FO membrane. Thus, the higher flux on the CNT TFC-FO membrane is

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evident from its lower S.

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Figure 3. Water flux (a) and reverse salt flux (b) of the TFC-FO membranes (Conditions: 2.0 M NaCl as the draw

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solution against a DI water feed at the same crossflow velocity of 25 cm/s and temperature of 25 °C), schematic

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images of ICP in the CNT TFC-FO (c) and PES TFC-FO (d) membranes.

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Table 1. Transport properties and structural parameters of TFC-FO membranes Water permeabilitya

Salt rejectionb

Salt permeability

Structural parameterc

(A, L/(m2·h·bar))

(Rs, %)

(B, L/(m2·h))

(S, µm)

CNT

2.45 ± 0.10

92.6 ± 1.4

0.119 ± 0.041

125.57 ± 7.51

PES

1.93 ± 0.09

90.7 ± 0.9

0.573 ± 0.106

324.18 ± 28.87

CA

0.68 ± 0.01

88.4 ± 1.2

0.120 ± 0.026

634.26 ± 41.51

Sample

a

A was obtained at 0~1.5 bar for CNT (PES) samples, and 0~8.0 bar for CA sample (Figure S8). b Rs was measured

at a fixed crossflow velocity of 25 cm/s using pressure of 1.0 bar for CNT (PES) samples (500 mg/L NaCl feed), and 8.0 bar for CA sample (2000 mg/L NaCl feed). c S was calculated based on experiments in FO mode using 1.0 M NaCl as draw solution and deionized water as feed. All experiments were performed at 25 °C. 231

On the other hand, both TFC-FO hollow fiber membranes showed much better performance than

232

the commercial CTA TFC-FO planar membrane under both FO and PRO modes. Particularly, the 13



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CNT TFC-FO membrane presents S of 6 times lower than the standard CTA TFC-FO membrane. Its

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Jw values of 4.7 times higher than the commercial CTA TFC-FO membrane in the FO mode, and 3.6

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times higher in the PRO mode. Meanwhile, the Js/Jw of CNT TFC-FO membrane is only about 1/4

236

and 1/5 of those of standard CTA TFC-FO membrane under FO and PRO modes, respectively. A

237

quick comparison to the reported TFC-FO membranes is also taken and presented in Table S3. It can

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be found that the CNT TFC-FO hollow fiber membrane displays the FO performance better than or

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comparable to those reported TFC-FO membranes. Therefore, this work establishes the potential

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prospect of using CNT hollow fiber scaffold as a new sublayer for interficially polymerized TFC-FO

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membranes with low ICP and high flux.

242

Fouling Resistance of FO membranes. Membrane fouling is an inevitable problem in all

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membrane processes, including FO. Thus, the fouling resistance to organic fouling, biofouling and

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gypsum scaling has been investigated on the CNT TFC-FO membrane, and then taken in comparison

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with the two control membranes. Figure 4 displays that the fluxes on the three membranes can reach

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the steady state in 24 h operation for all fouling tests. After the fluxes reaching the steady state, the

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flux loss on the three FO membranes are less than 20%, 31% and 18% for organic fouling,

248

biofouling and gypsum scaling, respectively. These results keep in consistent with the fact of FO

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with low fouling tendency under FO mode.4,

250

membrane can reach steady state after 12, 20 and 16 h during the organic fouling, microbial fouling

251

and gypsum scaling tests, respectively. These values are lower than those on the two control

252

membranes. Moreover, the CNT TFC-FO membrane not only presents less flux loss, but also shows

253

better flux recovery than the PES TFC-FO membrane in all fouling tests. According to the reported

254

works,31-34 CNTs can be used to change the membrane wettability and surface charges for improving

10

Meanwhile, the fluxes on the CNT TFC-FO

14


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fouling resistance. This might be the reason that results in the high antifouling performance on the

256

CNT TFC-FO membrane over PES TFC-FO membrane. In the case of CTA TFC-FO planar

257

membrane with the lowest flux loss and best flux recovery, the primary reason is that its initial water

258

flux is much less than those of the other two TFC-FO hollow fiber membranes. Its lowest initial flux

259

results in minimal fouling. In addition, compared to CTA, the PA active layer on the CNT TFC-FO

260

and PES TFC-FO membranes is more susceptible to foulant adsorption due to its higher surface

261

heterogeneity. However, the CNT TFC-FO membrane presents steady fluxes of 4.6, 4.3 and 4.6 times

262

higher than the CTA TFC-FO membrane in the organic fouling, biofouling, and gypsum scaling tests,

263

respectively (Figure S9). Therefore, the CNT TFC-FO membrane not only possesses high flux, but

264

also has relatively good antifouling ability. (c)100

90 80 70 CTA CNT PES

60

(e)100 Normalized Flux (%)

Normalized Flux (%)

90 80 70 CTA CNT PES

60

50

50 0

4

8

12

16

20

24

4

8

Time (h) After cleaning After fouling

100

90

80

265

12

(d)

16

20

CTA CNT PES

60

0

4

90

80

12

16

20

24

(f)

After cleaning After fouling

100

90

80

70 CTA

CNT

8

Time (h)

70 PES

70

24

After cleaning After fouling

100

70 CTA

80

Time (h) Normalized Flux (%)

Normalized Flux (%)

(b)

90

50 0

Normalized Flux (%)

Normalized Flux (%)

(a)100

PES

Membrane

CNT

Membrane

CTA

PES

CNT

Membrane

266

Figure 4. Comparison of organic fouling (a) and cleaning (b), biofouling (c) and cleaning (d), gypsum scaling (e)

267

and cleaning (f) in FO mode. (Organic foulant: 100 mg/L HA in 10 mM NaCl solution. Bio-foulant: 3×107 cfu/mL

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E. coli LB suspension. Gypsum scaling solution: 35 mM CaCl2, 20 mM Na2SO4, and 19 mM NaCl, gypsum

269

saturation index (SI) of 1.3. FO conditions: crossflow velocity of 15 cm/s and the temperature of 25 °C for both

270

feed and 2 M NaCl drawn solution. Cleaning: DI water (25 °C) as feed to rinse membrane after 12 h fouling tests

271

for 30 min at a crossflow velocity of 25 cm/s.) 15



272

Antifouling Performance of CNT TFC-FO Membrane under Electrochemical Assistance.

273

Enhancing fouling resistance can decrease the frequency of membrane cleaning and replacement,

274

then lowering the economic consumption. As the newly developed CNT TFC-FO membrane

275

possesses a good electric conductivity of 1500 S/m, an integration of FO with electrochemistry was

276

performed to evaluate the electrochemical effects on its antifouling performance. Prior to the

277

antifouling tests, water splitting and flux change under electrochemical assistance were investigated

278

by treating DI water feed. The results suggested no gas evolution and flux changes occurring on the

279

membrane even under 2.0 V voltage (Figure S10 and S11). As expected, the flux loss declines with

280

increasing the voltages on the membrane for organic fouling under the FO mode (Figure 5a). When

281

the applied voltage is 2.0 V, the membrane presents an inapparent flux loss even after 12 h. By

282

contrast, the flux loss was 14.6% under the open circuit condition. These results suggest that organic

283

fouling can be significantly suppressed under the electrochemical assistance. To understand this

284

phenomenon, the fouled membranes were observed by SEM. It can be found that an obvious organic

285

fouling layer has formed on the membrane surface under the open circuit condition. In contrast, the

286

electrically assisted membrane presents similar architecture as the fresh membrane. These results

287

imply the organic foulant accumulation on the membrane surface is significantly mitigated by the

288

electrochemical assistance. It is well known that the HA molecule is negatively charged with

289

electrophoretic mobility of -3.3 × 10-8 m2/(V·s) (10 mM NaCl, pH 7).35 Thus, the electrostatic

290

repulsive force between the HA and cathodic membrane would be enhanced by increasing the

291

voltages. The enhanced repulsive force then prevents the HA attaching to the membrane surface.

292

Furthermore, the effect of electrical field force on the foulant was also considered in the experiment.

293

By calculation in SI, the critical field strength (Ecritical, V/cm), that can counterbalance the convective 16


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Page 17 of 23


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migration of HA toward membrane, is about 5.1 V/cm. As the electric field strength is about 6.7

295

V/cm at 2.0 V voltage, electrical field force is evidenced to drive HA away from membrane surface.

296

To further evaluate the electrically assisted fouling resistance on CNT TFC-FO membrane, both

297

biofouling and gypsum scaling tests were performed. Interestingly, the flux loss also declines to 3.0 %

298

at 2.0 V from 12.9 % under open circuit conditions (Figure 5d) during the biofouling tests (Figure

299

5d). According to the SEM images of the fouled membrane surface, bacterial adhesion occurred on

300

the membrane under the open circuit conditions (Figure 5e), but did not happen on the electrically

301

assisted membrane (Figure 5f). Such results suggest that the electrochemical assistance can also

302

inhibit biofouling formation. As E. coli presents an electrophoretic mobility of -4.7 × 10-8

303

m2/(V·s)),36 both electric field force and electric repelling can prevent microbial movement toward to

304

the membrane surface, mitigating biofouling formation. Unfortunately, the gypsum scaling cannot be

305

inhibited by the electrochemical assistance. More flux loss and gypsum formation occur at higher

306

voltages (Figure 5g, 5h and 5i). According to Mi’s work,37 the PA active layer can form complexes

307

with Ca2+ ions, initiating the formation of gypsum prenucleation clusters and subsequently

308

amorphous gypsum particles on the PA membrane surface. When the membrane worked as the

309

cathode, more Ca2+ ions might be dragged toward to the negatively charged membrane surface,

310

resulting in more severe gypsum scaling (Figure 5h and 5f). Thus, the electrically assisted fouling

311

mitigation in FO was mainly dominated by electrostatic interaction and electric field force (Figure 6),

312

which was further confirmed by the flux changes on anodic membrane in the fouling tests (Figure

313

S12).

17



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314 315

Figure 5. Normalized flux and SEM images of electrically assisted fouling resistance to organical (a~c), microbial

316

(d~f) and scaling (g~i) fouling under FO mode. (Organic foulant: 100 mg/L HA in 10 mM NaCl solution.

317

Bio-foulant: 3×107 cfu/mL E. coli LB suspension. Gypsum scaling solution: 35 mM CaCl2, 20 mM Na2SO4, and 19

318

mM NaCl, gypsum saturation index (SI) of 1.3. FO conditions: crossflow velocity of 15 cm/s and the temperature

319

of 25 °C for both feed and 2 M NaCl drawn solution.) (+)

Feed

Draw

(–)

Feed

Feed (+)

(–)

(a) FO E. coli

320 321

Ca2+

Shearing force

(–)

Draw

(+)

Feed (–)

(+)

(b) Electrically assisted FO HA

Gypsum

Ti mesh

PA layer

Hydrodynamic force

CNT sublayer Electric field force

Figure 6. Mechanism of FO (a) and electrically assisted FO (b) processes.

322

Compared to FO mode, PRO mode usually presents much higher water flux at the same osmotic

323

pressure because of its low ICP.4, 10 However, the application of PRO mode is limited due to its

324

higher fouling tendency than FO mode. Therefore, it would be significant if the PRO mode presented 18


Page 19 of 23


325

good antifouling ability for more water production. Here, the antifouling performance in both

326

organic fouling and biofouling tests were investigated in the PRO mode under electrochemical

327

assistance. As expected, the flux loss in both fouling tests were significantly mitigated. These results

328

suggest that the electrically assisted fouling resistance can also achieve in the PRO mode (Figure 7a

329

and 7b). It is because the electric repelling and electric field force dominate the foulant movement

330

away from sublayer surface, like that occurred in FO mode. This would prevent the foulant

331

accumulation and pore blocking. Consequently, the electrochemical assistance also endows the PRO

332

mode with good antifouling ability that provides higher water production over FO mode in water

333

treatment processes. To extend this application, the electrically assisted FO process was applied to

334

separate oil-water mixture in the PRO mode. Figure 7c depicts that the water flux of the FO alone

335

quickly declines to only 36% of its initial value after 6 h. It is consistent with the fast fouling

336

propensity on FO membrane under the PRO mode during oily wastewater.38, 39 This phenomenon

337

might arise from that the oil droplets can easily deposit on the surface and/or entrance into the porous

338

substrate, resulting in pore blocking and water flux loss. Interestingly, the water flux is improved on

339

the cathodic CNT TFC-FO hollow fiber membrane at 2.0 V voltage. Its water flux is still 88% of its

340

initial value after 6 h, suggesting the oil fouling is significantly mitigated by the electrochemical

341

assistance. These results are confirmed by analyzing the amount of TOC on the fouled membrane

342

surfaces. As presented in Figure 7d, the TOC values of desorb and re-suspend oil from

343

electrochemically assisted FO membranes are much lower than that from FO membrane alone. These

344

results can be also attributed to the enhanced electric field force and repulsion interaction between

345

the negatively charged oil droplets (-27 mV) and membrane cathode. Moreover, the electrically

346

assisted FO process showed an acceptably energy consumption of 0.6 kWh/ton water under 19



Page 20 of 23

347

electrochemical assistance (Supporting Information). Therefore, this work provides an alternative

348

sublayer for interfacially polymerized TFC-FO membrane with low ICP and high flux. Meanwhile, it

349

gives a useful insight in applying such membrane with electrically assisted fouling resistance to

350

organic and microbial fouling. 100

(b) Normalized flux (%)

Normalized flux (%)

(a) 100 90

80

70

90 80 70 60 50

Cathode: 2.0 V Open circuit 60

Cathode: 2.0 V Open circuit

40

0

2

4

6

8

10

12

0

2

4

6

Time (h)

(d)

90 80 70 60 Cathode: 2.0 V Open circuit

40 30 0

351

2

4

6

8

10

12

Normalized TOC of fouled oil (%)

Normalized flux (%)

(c) 100

50

8

10

12

Time (h) 100 80 60 40 20 0 Open circuit

Time (h)

Cathode: 2.0 V

Condition

352

Figure 7. Normalized flux of electrically assisted antifouling tests under PRO mode: organic fouling (a), biofouling

353

(b), oil/water emulsion separation (c) and normalized TOC of fouled oil on membrane surface (d). (Organic foulant:

354

100 mg/L HA in 10 mM NaCl solution. Bio-foulant: 3×107 cfu/mL E. coli in LB suspension. Oil/water emulsion

355

solution: 50000 mg/L oil/water emulsion with 2.0 wt% surfactant and 10 mM NaCl. FO conditions: crossflow

356

velocity of 15 cm/s and the temperature of 25 °C for both feed and 1 M NaCl drawn solution.)

357

Associated Content

358

Supporting Information. Preparation of PES sublayer; schematic diagram of wetting spinning;

359

characterization and optimization of CNT sublayer; outer surface and cross-section SEM images of

360

CNT TFC-FO membrane; property comparisons for the three TFC-FO membranes; electrochemical

361

properties of CNT TFC-FO membrane; flux changes on anodic membrane; comparison between the

362

prepared and reported FO membranes; energy consumption calculation. This material is available

363

free of charge via the Internet at http://pubs.acs.org.

364

Author Information 20


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365

Corresponding Author E-mail: [email protected]

366

Notes

367

The authors declare no competing financial interests.

368

Acknowledgment

369

This work was supported by the National Natural Science Foundation of China (No. 21437001 and

370

51708085) and the China Postdoctoral Science Foundation (No. 2016M601314).

371

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Highly Permeable Thin-film Composite Forward Osmosis Membrane

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