Incorporation of Cellulose Nanocrystals (CNCs) into the Polyamide

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Environmental Processes

Incorporation of cellulose nanocrystals (CNCs) into the polyamide layer of thin-film composite (TFC) nanofiltration membranes for enhanced separation performance and antifouling properties Langming Bai, Yatao Liu, Nathan Bossa, An Ding, Nanqi Ren, Guibai Li, Heng Liang, and Mark R. Wiesner Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04102 • Publication Date (Web): 03 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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Environmental Science & Technology

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Incorporation of cellulose nanocrystals (CNCs) into the polyamide

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layer of thin-film composite (TFC) nanofiltration membranes for

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enhanced separation performance and antifouling properties

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Langming Bai,† Yatao Liu,† Nathan Bossa,‡,§ An Ding,† Nanqi Ren,† Guibai Li,† Heng Liang,*†

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Mark R. Wiesner*‡,§

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†

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of Technology, 73 Huanghe Road, Nangang District, Harbin, 150090, P.R. China

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‡

State Key Laboratory of Urban Water Resource and Environment (SKLUWRE), Harbin Institute

Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina

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27708, United States

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§

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North Carolina 27708, United States

Center for the Environmental Implications of NanoTechnology (CEINT), Duke University, Durham,

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*Corresponding author: Tel.: +86 451 86282252; fax: +86 451 86282252; e-mail:

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[email protected]

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* Corresponding author: Phone: 919-660-5292; fax: 919-660-5219; e-mail: [email protected].

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ACS Paragon Plus Environment


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ABSTRACT

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To achieve greater separation performance and antifouling properties in a thin-film composite

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(TFC) nanofiltration membrane, cellulose nanocrystals (CNCs) were incorporated into the

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polyamide layer of a TFC membrane for the first time. The results of Fourier transform infrared

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spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the successful

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formation of the CNC-polyamide composite layer. Surface characterization results revealed

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differences in the morphologies of the CNC-TFC membranes compared with a control membrane

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(CNC-TFC-0). Streaming potential measurements and molecular weight cutoff (MWCO)

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characterizations showed that the CNC-TFC membranes exhibited a greater negative surface

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charge and a smaller MWCO as the CNC content increased. The CNC-TFC membranes showed

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enhanced hydrophilicity and increased permeability. With the incorporation of only 0.020 wt%

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CNCs, the permeability of the CNC-TFC membrane increased by 60.0% over that of the

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polyamide TFC without CNC. Rejection of Na2SO4 and MgSO4 by the CNC-TFC membranes was

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similar to that observed for the CNC-TFC-0 membrane, at values of approximately 98.7% and

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98.8%, respectively, indicating that divalent salt rejection was not sacrificed. The monovalent ion

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rejection tended to increase as the CNC content increased. In addition, the CNC-TFC membranes

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exhibited enhanced antifouling properties due to their increased hydrophilicity and more

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negatively charged surfaces.

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INTRODUCTION

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Membrane-based processes currently comprise the state-of-the-art technologies for water

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purification, reclamation and supply.1 Nanofiltration (NF), as a type of pressure-driven membrane,

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has properties in between those of ultrafiltration (UF) and reverse osmosis (RO) membranes. To

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date, most commercially available NF membranes are thin-film composite (TFC) membranes,2

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composed of a thin polyamide selective layer fabricated by interfacial polymerization. NF exhibits

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excellent separation performance, can be highly automated, requires a small plant footprint and is

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readily adaptable to modular configurations at various scales.3 NF has been broadly applied in

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water treatment to achieve drinking water purification, wastewater reuse, seawater desalination,

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ion separation, and solvent purification.4-7 However, membrane fouling and high energy

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requirements are major obstacles for the extensive application of NF.8-10 Fouling-related problems,

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such as reduced water flux, diminished selectivity, diminished lifespan and increased energy

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consumption, limit the application and popularization of NF membranes.11 Therefore, to adopt NF

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in cost-effective and facile desalination processes, the membrane permeability and selectivity,

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along with the antifouling properties of the TFC NF membranes, must be improved.

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Nanomaterials have been incorporated into the polyamide selective layer to produce a thin-film

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nanocomposite (TFN) membrane. The TFN concept was first proposed by Jeong et al. in 2007.12

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They incorporated zeolite NaA nanoparticles into a polyamide layer by interfacial polymerization,

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and the permeability of the TFN membrane was dramatically improved. Since then, various

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nanoparticles, such as silica,13 titanium dioxide (TiO2),14 metal-organic frameworks (MOFs),15

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covalent organic frameworks (COFs),16 silver,17 carbon nanotubes (CNTs)18 and graphene oxide

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(GO)19, have been incorporated to improve the hydrophilicity, permselectivity and fouling 3


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resistance of the TFC membranes.20-22 Increases in the water flux of 100% over that of a pure TFC

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NF membrane have been reported when the addition of arginine (Arg) reached 40% PIP.23 Zhang

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et al. reported that a TFN membrane incorporated with graphene oxide quantum dots (GOQDs)

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exhibited enhanced antifouling properties over a TFC NF membrane.24 However, the possible

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release of nanoparticles has raised perceived health and safety concerns.25 Previous investigations

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showed that single-walled carbon nanotubes (SWCNTs) and graphene-based materials present

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noticeable cytotoxicity to human and animal cells.26,27 To avoid negative health effects, low

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environmental impact nanomaterials may become competitive candidates as additives in water

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treatment membranes.

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As one member of the family of cellulose nanomaterials, cellulose nanocrystals (CNCs) are

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made from renewable and sustainable resources and show low environmental, health, and safety

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footprints.28 CNCs can be facilely produced by the acid hydrolysis of different sources, such as

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wood, cotton, hemp, wheat, and straw.29 A single CNC particle is rod-shaped with high aspect

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ratio and is highly crystalline with few amorphous regions. CNCs exhibit excellent mechanical

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properties and have been shown to improve both the Young’s modulus and elasticity in filled

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nanocomposites.30 In addition, CNCs are highly hydrophilic due to the abundant hydroxyl groups

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(OH) on their surfaces. These attributes make CNCs attractive agents for improving membrane

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performance. In our previous work, polyethersulfone (PES) UF membranes blended with CNCs

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showed increased porosity, larger pore sizes, and greater surface hydrophilicity, all of which lead

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to greater water permeability.31 Metalized CNC nanocomposites (MNC) were used as supports to

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prepare TFC FO membranes. The MNC-TFC membranes exhibited high water flux and enhanced

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selectivity.32 CNCs can also be used as the interlayer of an NF membrane, and a triple-layered 4



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composite exhibited a high permeation of 34 LMH/bar and a high monovalent/divalent ion

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separation ratio.33

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The effects of CNCs on the TFC membrane morphology, physicochemical properties,

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separation performance and fouling resistance should be investigated. For a TFC membrane, the

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polyamide active layer determines the overall permeability, selectivity and antifouling

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performance. However, few studies on TFC NF membranes with CNCs incorporated in their

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polyamide layer have been conducted to date. Taking both separation efficiency and antifouling

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performance into account, we incorporated CNCs as nanofillers in the polyamide layer to prepare

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TFC NF membranes.

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MATERIALS AND METHODS

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CNCs, as slurries of 6.2 wt% solids, were obtained from the Process Development Center of the

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University of Maine. The CNC slurries were freeze dried under vacuum conditions and the dried

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powders were stored for further use. Polyethersulfone (PES) membranes were received from

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Microdyn-Nadir GmbH (UP150) with a nominal molecular weight cutoff (MWCO) of 150 kDa.

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Piperazine (PIP, 99% purity), 1,3,5-benzene-tricarbonyl trichloride (TMC, 99% purity), n-hexane

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(anhydrous 95%), and bovine serum albumin (BSA) were purchased from Sigma-Aldrich (MO,

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USA). Polyethylene glycol (PEG, Mw 200, 300, 400, 600, 1000) was obtained from Tokyo

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Chemical Industry Co., Ltd. (Tokyo, Japan). Inorganic salts, including sodium sulfate (Na2SO4),

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magnesium sulfate (MgSO4) and sodium chloride (NaCl), were of analytical grade and provided

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by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals and solvents were

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used as received without further purification. Deionized water (18.2 MΩ/cm) produced by a

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Milli-Q purification system (Millipore, USA) was used to prepare aqueous solutions and to rinse

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containers.

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Characterization of CNCs

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CNCs were characterized by Fourier transform infrared spectroscopy (FTIR), Particle size

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distribution (PSD) and transmission electron microscopy (TEM). Equipment information and

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measurement procedures were described in the Supporting Information.

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Preparation of CNC-TFC membranes

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The control TFC membrane without CNCs (CNC-TFC-0) and the composite CNC-polyamide

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membranes (CNC-TFC) were prepared as thin films deposited by interfacial polymerization on

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PES UF membranes. The fabrication conditions of the CNC-TFC membranes are summarized in 6



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Table S1. A schematic illustration of the preparation process for the CNC-TFC membranes is

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shown in Figure 1. The aqueous solution was prepared by dissolving PIP (1.0 wt%) and a given

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amount of CNC particles (0.001, 0.005, and 0.020 wt%) in 1 L deionized water at pH 11. The PES

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substrate was first immersed in aqueous solution for 10 min. Then, excess solution was completely

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removed using filter paper and an air gun. Afterwards, a 0.4 wt% TMC/n-hexane solution was

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poured onto the surface of the membrane support. The membrane was dipped in organic solution

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for 2 min to obtain a polyamide layer through interfacial polymerization between PIP and TMC.

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The obtained membrane was heated in an oven at 70 °C for 5 min for further polymerization and

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evaporation of the organic solution. The resultant membranes are referred to as CNC-TFC-1,

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CNC-TFC-5, and CNC-TFC-20 corresponding to the CNC concentration in the polyamide film.

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For example, CNC-TFC-1 was fabricated with 0.001 wt% CNCs. Control TFC membranes

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(termed CNC-TFC-0) with no CNC content were also fabricated via the same procedures.

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Figure1. Schematic illustration of the CNC-TFC membranes preparation by interfacial polymerization.

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Characterization of the CNC-TFC membranes

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Fourier transform infrared spectroscopy (FTIR, Spectrum One B, Perkin Elmer, Inc., USA) was

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used to investigate the chemical structure of the CNC-TFC membranes. The spectra were recorded

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from 4000 to 400 cm-1 at a resolution of 4 cm-1. The surface composition of the CNC-TFC

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membranes was characterized by X-ray photoelectron spectroscopy (XPS, ThermoFisher

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ESCALAB 250Xi, USA) using a monochromatic Al Kα X-ray source (1486.6 eV). The

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penetration depth was less than 10 nm, and the take-off angle of the photoelectrons was set to 90°.

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The morphologies of the membranes were examined by scanning electron microscopy (SEM;

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Hitachi SU8010, Japan) with an accelerating voltage of 15 kV. To obtain top views, membrane

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samples were dried at room temperature. All of the membrane specimens were sputtered with gold

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(Hummer 6.2 Vacuum Sputter) before the tests. The surface topography of the membranes was

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characterized by atomic force microscopy (AFM, Digital Instruments Dimension 3100, Veeco, NY,

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USA). Tapping mode was used to scan the membrane surface using a silicon cantilever (probe

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type TESP-V2). The TESP-V2 cantilever had a spring constant of 42 N/m, resonant frequency of

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320 kHz, and nominal tip radius of 8 nm. The membrane samples were scanned over an area of 5

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µm × 5 µm. The surface roughness parameters, including Rq, Ra and Rmax, were extracted from the

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images using the Nanoscope software (Version 6.14). The surface zeta potential of the investigated

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membranes was analyzed with an electrophoretic analyzer (Anton Paar Surpass, Ashland, VA,

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USA) according to the Helmholtz–Smoluchowski equation. Potassium chloride (KCl) was used as

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the electrolyte, whereas hydrochloric acid (HCl) and potassium hydroxide (KOH) were used for

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pH titration. The membrane samples were immersed into 1 mM KCl solution at pH 3.0−10.0 in

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the test. The concentration of polyethylene glycol (PEG) solute was determined using a total 8



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organic carbon analyzer (Liquid TOC, Elementar, Germany). The hydrophilicity of the membranes

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was quantified by contact angle measurements using a contact angle goniometer (Kruss EasyDrop

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Goniometer, Hamburg, Germany). The measurement range of the contact angle goniometer is

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0-180° with the deviation of ± 0.1°. At least 10 contact angle measurements were performed and

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recorded for each type of membrane analyzed.

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Separation performance tests

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The separation performance of the CNC-TFC membranes was evaluated using a cross-flow NF

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cell (CF042D, Sterlitech, USA). The max operating pressure of the device is 69 bar and the active

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membrane area is 42 cm2. Each membrane sample was compacted at 0.7 MPa for 12 h using

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deionized (DI) water to reach steady state. After compacting, the pure water flux and salt rejection

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were determined under 0.6 MPa at 25 °C with a cross-flow velocity of 20 L h−1. The pure water

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flux was calculated as in equation 1.

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J=V/(A×∆t)

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where J is the permeation flux (L·m-2·h-1) (LMH), V is the volume of the permeate (L), A is the

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membrane area (m2) and ∆t is the permeate collection interval (h).

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The rejection properties of the CNC-TFC membranes were investigated using 2000 mg/L Na2SO4,

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MgSO4, and NaCl. The salt concentrations of the solutions were measured by an electrical

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conductivity tester (DDSJ-308A, Shanghai Precision & Scientific Instrument Inc., China). The salt

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rejection ratio was obtained as in equation 2.

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R = ቀ1- P ቁ ×100%

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where Cp (mg/L) and Cf (mg/L) are the concentrations of the permeation and feed solution,

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respectively.

(1)

C

(2)

Cf

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Antifouling performance tests

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To investigate the antifouling properties of the TFC-CNC membranes, 500 mg/L HA was used

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as a representative humic foulant. The membrane was first stabilized with DI water at 0.4 MPa for

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2 h to reach a steady flux, which was recorded as the initial water flux (Jw). Then, the DI water

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was replaced by a predetermined concentration of HA solution. HA filtration was continued for 4

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h under cross-flow mode, and the steady flux of HA (Js) was recorded. After that, the used

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membranes were cleaned with DI water. Finally, the steady pure water flux of the cleaned

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membranes (Jr) was measured again at 0.4 MPa. The total flux decline ratio (FDR) and flux

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recovery ratio (FRR) were used to evaluate the antifouling properties of the CNC-TFC membranes,

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which can be calculated using the following expressions.

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FDR = ቀ1- J s ቁ ×100% J

(3)

w

Jr

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FRR =

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Typically, a lower FDR and higher FRR indicate better antifouling properties of the membranes.

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RESULTS AND DISCUSSIONS

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Characterization of the CNCs

Jw

×100%

(4)

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Characterization of the CNCs was conducted by FTIR, particle size distribution (PSD) analysis

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and TEM, respectively. The results are shown in Figure S1. FTIR analysis was used to

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characterize the functional groups of the CNCs (Figure S1a). The peaks between 3200 and 3600

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cm-1 correspond to O-H stretching vibrations. The peaks at approximately 2902 and 1431 cm−1 are

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assigned to C-H and -CH2 stretching vibrations, respectively.34 The peaks observed at 1643 and

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1060 cm−1 are the O-H stretching vibration of absorbed water and the pyranose ring ether band of

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cellulose, respectively.31 The FTIR results indicated the CNCs were hydrophilic. To investigate the 10



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dispersibility of the CNCs in aqueous solution, the PSD of the CNCs in DI water was measured

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and is shown in Figure S1b. Results showed that no further agglomeration of CNCs occurred as

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CNC concentration increased, indicating their great dispersibility in the aqueous solution

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(Supporting Information). A TEM image of the CNCs is shown in Figure S1c. The CNCs

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exhibited great dispersibility, with single particles separated from each other. A CNC particle

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showed a rod-like structure with dimensions ranging from 200 to 300 nm in length and

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approximately 15 to 30 nm in width.

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Characterization of the CNC-TFC membranes

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The surface chemical, morphological and physicochemical properties of the fabricated

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membranes were characterized by FTIR, XPS, SEM, AFM, zeta potential analysis, and pore size

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investigation, as follows.

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Surface Chemistry. FTIR analysis was performed to characterize the surface functional groups

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of the PES support and the fabricated CNC-TFC membranes, and the results are shown in Figure

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2a. For the PES support, the prominent peaks at 1578, 1488, and 1244 cm-1 are attributed to

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stretching vibrations of the benzene ring, C-C bond, and aromatic ether bond, respectively.23 For

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the CNC-TFC membranes, new peaks were observed at approximately 1625, 1440 and 3200–3600

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cm-1. The peak appearing at 1625 cm-1 is assigned to C=O stretching vibrations of the -CO-NH-

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groups.20 The peak at 1440 cm-1 is related to the O-H stretching vibration of the carboxylic groups,

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which were generated by the hydrolysis of unreacted acyl chlorides during interfacial

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polymerization.35 In addition, the broad O-H peak at 3200–3600 cm-1 is ascribed to the presence of

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carboxyl groups.36 The FTIR results suggested the successful interfacial polymerization between

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PIP and TMC. Note that the penetration depth of FTIR ranges from 0.5-1.0 µm, exceeding the 11


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thickness of the thin polyamide layer.13 In addition, due to the low incorporation amount, the

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characteristic peaks of CNCs may be covered by the strong peaks of the PES support and

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polyamide layer. Hence, characteristic peaks of CNCs were not observed by FTIR analysis.

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Figure 2. Surface chemical characterization of the membranes: (a) FTIR, (b) XPS survey spectra, (c) C1s spectrum of CNC-TFC-0, (d) C1s spectrum of CNC-TFC-1, (e) O1s spectrum of CNC-TFC-5 and (f) N1s spectrum of CNC-TFC-20.

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The surface elemental composition of the CNC-TFC membranes was characterized by XPS, and 12



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the results are shown in Figure 2b-f. As seen in Figure 2b, the strong characteristic peaks of the

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investigated membranes at 284.8 eV and 531.2 eV are ascribed to the binding energy of C1s and

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O1s. The occurrence of the N1s peak of the CNC-TFC membranes at 399.8 eV and the

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disappearance of the S2s and S2p peaks of the PES support indicated the formation of a polyamide

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layer on the surface of the PES support.22 Furthermore, the C1s core-level spectra of the PES

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support, CNC-TFC-0 and CNC-TFC-1 are shown in Figure S2, Figure 2c and Figure 2d,

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respectively. The C1s spectrum of the PES membrane could be resolved into three peak

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components. The binding energies of 284.6 and 286.4 eV are attributed to C-C and C-O,

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respectively, while the peak at 285.2 eV is attributed to C-S in the PES structure.37 For the

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CNC-TFC-0 and CNC-TFC-1 membranes (Figure 2c,d), the C1s component peaks at binding

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energies of 285.8 and 287.8 eV are attributed to C-N and N-C=O, respectively, confirming the

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existence of -NHCO- species.38 To further prove the formation of the polyamide layer, XPS O1s

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and N1s core-level spectra were measured. For the CNC-TFC-5 membrane (Figure 2e), the

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oxygen species located at 530.9 and 532.6 eV are assigned to O=C-N and O=C-O, respectively.39

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The peak at 531.8 eV is ascribed to the -OH group of the CNCs, which indicates the existence of

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CNCs in the polyamide layer. Figure 4f presents the deconvoluted N1s core-level spectrum of the

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CNC-TFC-20 membrane. Two peaks can be observed at 399.3 eV and 399.9 eV, which correspond

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to the O=C-N bond and N-C bond of the amide group, respectively.24

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The quantitative analyses of the elemental content of the PES support and the CNC-TFC

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membranes are tabulated in Table S2. The N/O atomic ratio of the CNC-TFC membranes

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increased with increased CNC content, indicating an increase in crosslinking in the CNC-TFC

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membranes.40 This result suggested that the polyamide structure of the CNC-TFC membranes was 13


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changed upon incorporation of CNCs. The XPS results were well consistent with the FTIR results,

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confirming the successful formation of the polyamide layer atop the PES support.

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Morphological characterizations. The surface morphologies of the investigated membranes

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were characterized by SEM and AFM. As shown in Figure 3a, the PES support exhibited a smooth

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surface with pores uniformly distributed on the membrane surface. The surface of the CNC-TFC

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membranes was made up of rough nodules and spherical globules, which were attributed to the

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crosslinking between PIP and TMC. The observed nodular structure was a typical structure of the

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polyamide membrane surface fabricated through interfacial polymerization.41 Visually, no obvious

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membrane pores could be observed on the surface of the CNC-TFC membranes, implying that the

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PES substrates were completely covered by the polyamide layers. This result was consistent with

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the XPS analysis (Figure 2b), presenting the disappearance of the characteristic peak of sulfur in

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the CNC-TFC membranes. As seen in Figure 3b-d, the CNC-TFC membranes exhibited a more

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nodular-like structure as the CNC content increased. Interestingly, the surface of CNC-TFC-20

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(Figure 3e) exhibited a ridge-and-valley structure, which may have been caused by the continuous

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distribution of spherical globules. The different surface morphologies of the CNC-TFC

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membranes were attributed to the presence of CNCs in the polyamide layer. The -OH groups of

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the CNCs may react with excess TMC, affecting the self-inhibiting reaction of interfacial

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polymerization and leading to a different surface structure.42 In addition, the length of a single

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CNC particle and its agglomeration diameter may also affect the interfacial polymerization

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reaction. Hence, the presence of CNCs changed the surface morphology of the polyamide layer.

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The membrane surface roughness of the CNC-TFC membranes was characterized by

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three-dimensional (3D) AFM images (Figure 3f-j). The bright and dark areas represent the peaks 14



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and valleys of the membrane surface. Mathematical analysis of the AFM images was performed,

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and the parameters are tabulated in Table S3. The parameters Rq, Ra, and Rmax represent the

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root-mean-square of the height deviations, average plane roughness, and maximum roughness,

285

respectively. As seen from Figure 3f, the gaps between peaks and valleys on the PES support were

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uniform and smooth, indicating the low surface roughness of the investigated membranes. The

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surface roughness of the CNC-TFC membranes increased gradually with increased CNC

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concentration in the aqueous solution, indicating an increase in the water permeability of the

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CNC-TFC membranes.43 The Ra values agreed with the 3D images that the CNC-TFC membranes

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contained more variable and broader ridges. The ridges seen in the AFM images also corresponded

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with the obvious nodular-like structure seen in the SEM images. The incorporation of hydrophilic

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CNCs into the polyamide layer may accelerate the diffusion of PIP from the aqueous phase to the

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organic phase during the interfacial polymerization reaction, leading to a different surface

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morphology.44 In addition, the size of the CNC particles may exceed the thickness of the

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polyamide layer, which gives the CNC-TFC membranes a rougher surface. Overall, the surface

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roughness of the CNC-TFC membranes increased only slightly over that of the control TFC

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membrane.

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Figure 3 SEM images of (a) the PES support, (b) CNC-TFC-0, (c) CNC-TFC-1, (d) CNC-TFC-5, and (e) CNC-TFC-20 and AFM images of (f) the PES support, (g) CNC-TFC-0, (h) CNC-TFC-1, (i) CNC-TFC-5 and (j) CNC-TFC-20.

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Surface charge and MWCO characterizations. The measurements of streaming potential and

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associated calculations of zeta potential near the surface of the CNC-TFC membranes were

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determined over a pH range of 3.0–10.0. As seen in Figure 4a, the calculated zeta potential of all

302

investigated membranes changed from positive to negative with an increase in pH. The CNC-TFC

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membranes presented a more negatively charged surface as the CNC content increased. The

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isoelectric points (IEPs) of CNC-TFC-0, CNC-TFC-1, CNC-TFC-5, and CNC-TFC-20 were

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observed at pH values of 4.99, 4.67, 4.26 and 3.99, respectively. For the TFC membrane fabricated

306

by interfacial polymerization, the unreacted amine of PIP and the carboxylic acid of TMC

307

hydrolyzed by acid chloride dominated the charge behavior of the membrane surface. The

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incorporation of CNCs in the polyamide layer may disturb the formation of a continuous network

309

between PIP and TMC, leading to more polyamide network terminals and imparting a higher

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negative charge.23 In addition, CNC particles were reported to have a negative surface charge with

311

an IEP of 2.4, which also increased the negative charge at the surface of the CNC-TFC membranes.

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The surface charge plays a role in the salt rejection due in part to the Donnan effect. Generally, the

313

pH used for both nanofiltration tests and fouling tests ranged from 6.0 to 9.0, at which all of the

314

CNC-TFC membranes investigated in this study were negatively charged.

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The MWCOs of the CNC-TFC membranes were evaluated by the rejection of a neutral

316

polyethylene glycol (PEG) solute due to its low interaction with the membranes. PEG 200, PEG

317

400, PEG 600 and PEG 1000 were used as feed solutions. The MWCOs of the investigated

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membranes were taken as the molecular weight of the PEG molecule that was 90% rejected by the

319

membrane.39 The PEG rejections of the CNC-TFC membranes are presented in Figure 4b. As seen,

320

the MWCOs are 771, 573, 552, and 504 Da for the CNC-TFC-0, CNC-TFC-1, CNC-TFC-5, and 17


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321

CNC-TFC-20 membranes, respectively. The decrease in MWCO of the CNC-TFC membranes as

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the CNC content increased was possibly due to the hydroxyl groups on CNC surfaces which

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provides more reaction site resulted in the polyamide layer much denser.45 In addition, the

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well-dispersed CNC particle constructed tortuous and elongated diffusion pathway as molecular

325

barriers, suppressing the transport of PEG molecules.46 The possible pores blockage with addition

326

of CNCs may also benefit for PEG rejection. Collectively, the CNC-TFC membranes had a

327

smaller MWCO as the CNC content increased.

328 329 330 331 332

Figure 4. (a) Zeta potential of the CNC-TFC membrane surfaces as a function of solution pH and (b) PEG rejections by the CNC-TFC membranes as a function of PEG molecular weight. The PEG rejection experiments were conducted using Amicon® Stirred Cells (UFSC40001) at 0.4 MPa and 25.0 °C.

333

Permeability and separation performance of the CNC-TFC membranes

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The permeability and contact angle of the CNC-TFC membranes were investigated, and the

335

results are presented in Figure 5a. The water permeabilities of CNC-TFC-0, CNC-TFC-1,

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CNC-TFC-5 and CNC-TFC-20 were 10.30, 12.86, 14.10 and 16.45 LMH/bar, respectively. With

337

the incorporation of only 0.020 wt% CNCs in the polyamide layer, the permeability of

338

CNC-TFC-20 increased by 60.0% over that of the CNC-TFC-0 membrane, indicating the great

339

effect of CNCs in enhancing the water flux through the TFC membranes. As shown in Figure 5a,

340

the contact angles of CNC-TFC-0, CNC-TFC-1, CNC-TFC-5 and CNC-TFC-20 were 52.7, 48.6, 18



341

45.1 and 40.9°, respectively, which suggested that the hydrophilicity of the CNC-TFC membranes

342

increased as the CNC content increased. Due to the strong hydrogen bonds between the CNCs and

343

water, the transport rate of water molecules through the polyamide layer was accelerated, leading

344

to an enhancement in the water flux. In addition, the increased surface roughness, as shown in

345

Figure 3, also contributed to the permeability increase. It should be noted that the MWCO of the

346

CNC-TFC membranes showed a tendency to decrease from 771 Da for CNC-TFC-0 to 504 Da for

347

CNC-TFC-20 (Figure 4b). Following methods reported previously47-49, the pore size of the

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CNC-TMC membranes in this study ranged from 0.53–0.67 nm (Supporting Information), which

349

is much greater than the hydration radius of a water molecule (0.27 nm).23 Hence, the decrease in

350

the MWCO may not be the key factor for the CNC-TFC membrane permeability.

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352 353 354

Figure 5. Hydrophilicity and salt rejection of the CNC-TFC membranes: (a) permeability and contact angle, (b) Na2SO4 permeability and rejection, (c) MgSO4 permeability and rejection and (d) 19


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355 356 357

NaCl permeability and rejection. The permeability and salt rejection were determined under 0.6 MPa at 25 °C.

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To evaluate the separation performance of the CNC-TFC membranes, three types of salt solutions,

359

Na2SO4, MgSO4 and NaCl, were used as representative divalent and monovalent salts. The

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permeability and salt rejection of Na2SO4 and MgSO4 are shown in Figure 5b and Figure 5c,

361

respectively. As seen, all of the investigated CNC-TFC membranes exhibited great SO42- rejection

362

performance, with rejection rates of approximately 98.7 and 98.8% for Na2SO4 and MgSO4,

363

respectively. The divalent salts rejection of the CNC-TFC membranes was not sacrificed. In

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addition, these changes of salt rejection were not statistically significant (based on SPSS statistical

365

analysis) compared to the control TFC membrane (CNC-TFC-0). Relationship between the salt

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rejection ratios and the CNC contents in the polyamide layer were shown in Figure S3. Results

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showed that the relationship between divalent salt rejections and CNC incorporation contents are

368

probably non-linear, indicating the rejection of divalent salts was nearly independent of CNC

369

content. The Donnan exclusion effect was mostly responsible for the repulsion of divalent anions.

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The strong electrostatic repulsion between SO42− and the negatively charged surface of the

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CNC-TFC membranes increased as the CNC content increased (Figure 4a), leading to a high

372

repulsion of SO42−.22 In addition, size exclusion also contributed to the great divalent salt rejection

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performance.50 As indicated by the MWCO characterization (Figure 4b), the pore size of the

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CNC-TFC membranes decreased as the CNC content incorporated in the polyamide layer

375

increased. Thus, the size exclusion mechanism played an important role in salt rejection. Moreover,

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the degree of crosslinking in the CNC-TFC membranes increased as the CNC content increased,

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which was also beneficial to salt rejection.42,51-53 The permeability of the divalent salts of the

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CNC-TFC membranes were greater than that of the control TFC membrane, indicating increased 20



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salt filtration efficiency of the CNC-TFC membranes. Different from pure water permeability, the

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divalent salts permeability showed a slight decrease trend as CNC content increased. This was due

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to the increased electrostatic repulsion between the negatively charged membrane surfaces and

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SO42− in the salt solutions and the decreased membrane MWCO of the CNC-TFC membranes,

383

which may have increased the water molecule transport resistance.54,55

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Figure 5d presents the permeability and salt rejection of the CNC-TFC membranes towards

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NaCl. The NaCl rejection of the CNC-TFC membranes tended to increase as the CNC content

386

increased and significant difference was observed (p

Incorporation of Cellulose Nanocrystals (CNCs) into the Polyamide

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