High Performance Nanofiltration Membrane for Effective Removal of

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High Performance Nanofiltration Membrane for Effective Removal of Perfluoroalkyl Substances at High Water Recovery Chanhee Boo, Yunkun Wang, Ines Zucker, Youngwoo Choo, Chinedum O. Osuji, and Menachem Elimelech Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01040 • Publication Date (Web): 31 May 2018 Downloaded from http://pubs.acs.org on May 31, 2018

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Environmental Science & Technology

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High Performance Nanofiltration Membrane for Effective Removal of Perfluoroalkyl Substances at High Water Recovery

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Chanhee Boo,† Yunkun Wang,§†* Ines Zucker,† Youngwoo Choo,†

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Chinedum O. Osuji,† and Menachem Elimelech†*

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§

Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China

†

Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286

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Corresponding author: email: [email protected]; Tel: +86 (531) 88365400

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†*

Corresponding author: email: [email protected]; Tel: +1 (203) 432-2789

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ABSTRACT

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We demonstrate the fabrication of a loose, negatively charged nanofiltration (NF) membrane

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with tailored selectivity for the removal of perfluoroalkyl substances with reduced scaling

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potential. A selective polyamide layer was fabricated on top of a polyethersulfone support via

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interfacial polymerization of trimesoyl chloride and a mixture of piperazine and bipiperidine.

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Incorporating high molecular weight bipiperidine during the interfacial polymerization enables

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the formation of a loose, nanoporous selective layer structure. The fabricated NF membrane

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possessed a negative surface charge and had a pore diameter of ~1.2 nm, much larger than a

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widely used commercial NF membrane (i.e., NF270 with pore diameter of ~0.8 nm). We

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evaluated the performance of the fabricated NF membrane for the rejection of different salts (i.e.,

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NaCl, CaCl2, and Na2SO4) and perfluorooctanoic acid (PFOA). The fabricated NF membrane

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exhibited a high retention of PFOA (~90%) while allowing high passage of scale-forming

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cations (i.e., calcium). We further performed gypsum scaling experiments to demonstrate lower

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scaling potential of the fabricated loose porous NF membrane compared to NF membranes

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having a dense selective layer under solution conditions simulating high water recovery. Our

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results demonstrate that properly designed NF membranes are a critical component of a high

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recovery NF system, which provide an efficient and sustainable solution for remediation of

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groundwater contaminated with perfluoroalkyl substances.

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TOC Art

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INTRODUCTION

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Perfluoroalkyl substances (PFASs) are emerging contaminants that are persistent in the

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environment, bio-accumulative, and toxic even at trace concentrations. 1-5 PFASs are used in

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production of firefighting foams and their widespread use, particularly at airports, oil refineries,

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and military bases, has led to severe groundwater contamination. 6-9 Because of their unique

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physicochemical properties, such as strong carbon-fluorine bond and low vapor pressure, PFASs

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are highly resistant to degradation by chemical or biological processes. 10-12

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Nanofiltration (NF) is a low-pressure membrane-based separation process widely used in

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water and wastewater treatment.13, 14 NF has the potential to provide an effective solution for the

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removal of perfluorooctanoic acid (PFOA) — an important class of PFASs with significant

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environmental concerns due to widespread industrial use 15 and frequent detection in drinking

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water resources7 — from groundwater at high water recoveries.16, 17 Separation of PFOA by NF

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is based on size (steric) exclusion and electrostatic interactions. 18 PFOA has relatively high

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molecular weights (414 g/mol) and thus can be retained by NF membranes via size exclusion.

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PFOA removal can be further enhanced via electrostatic repulsion by tailoring the NF membrane

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surface and pore charge, because PFOA with a low dissociation constant (pKa of –0.1) is

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negatively charged at the pH of natural waters.19 Several studies have investigated the removal of

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PFOA by commercial NF membranes and reported relatively high retention rates ~90%. 20, 21

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Minimizing waste (brine) streams by achieving high water recovery is crucial for successful

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application of NF to treat PFOA contaminated groundwaters. Current NF systems are limited to

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low recoveries (75-80%) mainly due to inorganic scaling because groundwaters contain high

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levels of scale-forming inorganic species, including calcium, magnesium, and silica in the form

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of silicic acid.22,

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environmental impacts associated with brine management and, thus, enhance the feasibility and

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sustainability of NF technology for groundwater remediation. 24, 25

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There is a critical need to increase water recovery to reduce costs and

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Nanofiltration membranes have a thin selective layer with pores at the nanometer scale. 26

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While NF membranes poorly reject monovalent salt, they exhibit high retention rates for scale-

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forming species such as calcium, sulfate, and silicic acid.23, 27 Such high rejection brings about a

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significant concern for membrane scaling, especially when designing high water recovery NF

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systems.28 Therefore, the development of novel NF membranes that provide high PFOA removal 3 ACS Paragon Plus Environment


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and reduced scaling potential is of paramount importance to make NF an efficient and

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sustainable technology for groundwater remediation.

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In this study, we demonstrate the fabrication of a loose, negatively charged NF membrane

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with tailored rejection of perfluoroalkyl substances and scale-forming cations from feed

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solutions simulating groundwater. The performance of the fabricated membrane for salt and

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perfluorooctanoic acid removal was evaluated and compared to commercial NF membranes. The

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selective polyamide layer of the fabricated NF membrane was extensively characterized to relate

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membrane performance to the physicochemical properties of the selective layer. We further

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demonstrated a low gypsum scaling potential of the fabricated NF membranes, highlighting the

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potential application of NF membranes with tailored selectivity for high recovery NF systems to

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treat PFAS-contaminated groundwaters.

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MATERIALS AND METHODS

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Materials and Chemicals. Polyethersulfone (PES) ultrafiltration membranes (LX-300K,

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Synder FiltrationTM, Vacaville, CA) with a molecular weight cut-off of 300 kDa were used as a

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substrate. The PES membranes were prewetted by immersing in deionized (DI) water for at least

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24 h prior to use. Trimesoyl chloride (1,3,5-benzenetricarbonyl trichloride, TMC, 98%),

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piperazine (PIP, 99%), 4,4′-Bipiperidyl dihydrochloride (BP, 97%), triethylamine (TEA, ≥99%),

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and sodium hydroxide (NaOH) from Sigma-Aldrich (Sigma-Aldrich, St. Louis, MO) and hexane

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from J.T. Baker (ACS reagent, Phillipsburg, NJ) were used for interfacial polymerization (IP) of

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the polyamide (PA) selective layer.

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The performance of the fabricated NF membranes was compared to a commercial NF

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membrane, denoted NF270 by the manufacturer (FilmTec Corp., Minneapolis, MN). As

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indicated by the manufacturer, the NF270 membrane comprises a semi-aromatic piperazine-

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based, polyamide layer on top of a microporous polysulfone (PSf) support. The membranes were

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received as flat sheet samples. They were gently washed with deionized (DI) water to remove

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any preservatives and were stored in DI water at 4 °C.

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Thin-Film Composite Nanofiltration Membrane Fabrication. TFC NF membranes

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were fabricated by forming a polyamide (PA) selective layer on top of the PES support

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membrane via interfacial polymerization (IP). For obtaining a dense PA layer structure, an 4 ACS Paragon Plus Environment

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aqueous solution of 1.0 wt % PIP was used while a mixture of 0.5 wt % PIP and 0.5 wt % BP

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was employed to construct a loose, porous selective layer during the interfacial polymerization.

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The NF membranes fabricated using 1.0 wt % PIP and a mixture of 0.5 wt % PIP and 0.5 wt %

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BP are hereafter denoted as PIP and PIP + BP, respectively. To catalyze the reaction, 0.5 wt %

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TEA and 0.15 wt % NaOH were added to both aqueous amine solutions. For IP reaction, the PES

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support membrane (10 cm  15 cm) was tightly taped onto a clean glass plate with a water-proof

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tape (Fisherbrand Colored Labeling Tape, Fisher Scientific, Inc., MA), with the skin layer facing

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upward. Approximately 15 mL of aqueous amine solution was dispensed onto the surface of the

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support membrane and allowed to contact for 90 s. The amine solution was then poured off, and

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residual amine solution was removed from the membrane surface using an air knife. Next, the

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monomer-saturated support membrane was immersed in 0.15 wt % TMC in hexane for 30 s,

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resulting in the formation of a thin-film composite polyamide layer. The composite membranes

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were then air dried for 120 s, rinsed thoroughly, and stored in DI at 4 °C.

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Membrane Characterization. Zeta potential of membrane surface was evaluated by a

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streaming potential analyzer utilizing an asymmetric clamping cell (EKA, Brookhaven

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Instruments, Holtsville, NY) as described elsewhere.29 Measurements were performed with a

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solution containing 1 mM KCl and 0.1 mM KHCO3. Electrolyte solution flows into the cell were

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generated by pressure ranging from 0 to 300 mbar driven by a mechanical pump. The induced

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streaming potential was measured using Ag/AgCl electrodes mounted at each end of the

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clamping cell.

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The carboxylic group density of the polyamide selective layer was measured using the silver

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elution method.30 First, a 10-mM stock silver nitrate solution (ACS reagent, ≥99% from Sigma)

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was prepared in 1 wt % nitric acid (trace-metal grade from Sigma) and further diluted to 40 M

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and 1 M in DI water to obtain solutions for silver binding and rinsing, respectively. Then,

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solution pH was adjusted to 7 and 10.5 using sodium hydroxide (0.1 M) and nitric acid (1 wt %).

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Membranes were first wetted in DI water for at least 24 h prior to measurement after which

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circular membrane samples (2.0 cm2) were punched and the fabric backings were physically

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removed. The remaining polyamide/support (i.e., either PES or PSf) films were immersed twice,

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each for 10 min, in 10 mL of the 40 M silver nitrate solution at pH 7 or 10.5 to bind silver one-

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to-one with ionized carboxyl groups. After the binding step, isolated polyamide/support films 5 ACS Paragon Plus Environment


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were immersed four times, each time for 7 min, in 10 mL of 1  silver nitrate solution at the

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same pH used during binding to rinse off unbound silver. After each step, film samples were

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gently touched against Kim wipes to minimize solution carryover. Following the wash steps,

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isolated polyamide/support films were immersed in 5 mL of 1 wt % nitric acid for 30 min to

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protonate the carboxyl groups and elute the bound silver ions. After removing films, the silver

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ion concentration in the solution was determined using ICP-MS.

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Elemental mapping of the PA selective layer was performed by energy-dispersive X-ray

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analysis (EDX) in a scanning transmission electron microscope (STEM). The NF membrane

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samples were embedded in epoxy resin (SPI-PON 812, Structure Probe, Inc.), followed by

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curing at 60 °C overnight. The samples were then cross-sectioned by ultramicrotome (Leica EM

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UC7) with a diamond knife (Diatome Ultra 45). A thin specimen of ~100 nm thick cross-

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sectional film was floated on the water trough of the diamond knife and then transferred onto

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lacey carbon grids (Ted Pella). STEM-EDX analysis was conducted on TEM (FEI Tecnai Osiris)

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with an accelerating voltage of 200 kV, equipped with a Super-X EDS detector.

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The thickness of the selective layer was determined by imaging the isolated PA film using

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atomic force microscopy (AFM).31-33 The NF membranes were cut into a small piece (~ 1  1

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cm2) and the fabric backings were physically removed. The isolated polyamide/support films

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were placed on a silicon wafer with two ends of the film being pressed by a glass slide to avoid

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film folding and wrinkling during sample preparation. Then, a few drops of dimethylformamide

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(DMF) were placed on the film to dissolve the PES or PSf support. After a few minutes, the PES

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or PSf dissolved and DMF solution was carefully wiped off using Kim wipes without touching

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the film. This procedure was repeated several times to achieve a complete dissolution of the

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support and to have a transparent free polyamide film tightly fixed on the silicon wafer.

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Prior to AFM imaging, multiple parallel dents were developed on the isolated PA films using

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a metal precision glide needle (18 G  1 in, Becton Dickinson & Co., NJ) without damaging the

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silicon wafer. A border region (10  10 m) between the PA film and the silicon wafer was

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scanned by AFM using a Bruker Dimension FastScan AFM with a FastScan-B tip (5 nm tip

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radius, Bruker, Billerica, MA) in tapping mode at a scan rate of 3 Hz. The obtained AFM images

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were analyzed using the Section function of Nanoscope Analysis v1.5 (Bruker) to determine the

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PA layer thickness. 6 ACS Paragon Plus Environment

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Estimation of NF Membrane Pore Size. The average pore size of the NF membrane

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was determined based on a pore transport model that incorporates steric (size) exclusion and

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hindered convection and diffusion from the rejection data of reference inert organic solutes. 34

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Neutrally charged, low molecular weight organic molecules — erythritol, xylose, and dextrose

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(≥99%, Sigma-Aldrich, St. Louis, MO) — were used as the reference organic tracers. 35 Prior to

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NF experiments, the membrane was pre-compacted under 13.8 bar (200 psi) hydraulic pressure

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with DI water as the feed for 4 h. After compaction, each organic solute was injected to the feed

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to obtain a concentration of 40 mg/L (as total organic carbon, TOC) and subsequent experiments

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were conducted at 4.1, 6.2, 8.3, and 10.3 bar with a crossflow velocity of 21.4 cm/s and a feed

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temperature of 25.0  0.5 °C. The permeate flux was recorded after the system was run for 1 h at

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each pressure. Permeate and feed samples were taken for TOC analysis (TOC V-CSH, Shimadzu

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Corp., Japan) to determine the rejection of the reference organic solutes. Details on the pore size

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estimation based on the pore transport model using the measured rejection of the reference

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organic solutes are provided in the Supporting Information.

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Salt and PFOA Rejection Tests. Single salt rejections were evaluated using 2, 10, and

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20 mM of sodium chloride (NaCl), calcium chloride (CaCl 2), and sodium sulfate (Na2SO4)

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solutions. Ion rejections were measured from NF experiments using a salt solution prepared by

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mixing 5 mM NaCl, 5 mM CaCl2, and 5 mM Na2SO4 with a total ionic strength of 35 mM.

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Retention of PFOA (Perfluorooctanoic acid, 96%, Sigma-Aldrich, St. Louis, MO) by NF

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membranes was evaluated using feed solutions containing 1 mg/L of PFOA in DI water and in

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the combined salt solutions. Feed solution pH was adjusted to 7.0  0.1 during NF experiments.

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Prior to each experiment, the membranes were compacted for at least 4 h at 6.9 bar (100 psi)

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using DI water until there was no variation in permeate flux. The feed solution temperature was

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kept constant at 25.0  0.5 °C throughout the experiment. To compare rejection under the same

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permeate flux conditions, the hydraulic pressures were set to approximately 6.9 bar (100 psi) for

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the PIP + BP and PIP membranes and 3.1 bar (45 psi) for the NF270 membrane. Permeate and

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feed samples were collected after the system was equilibrated for 1 h at each condition and

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analyzed as described below. Rejection performance of NF membranes was determined by

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comparing the species concentration in the feed (Cf) and permeate (Cp) samples (i.e., Robs = 1-

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Cp/Cf). 7 ACS Paragon Plus Environment


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Concentrations of NaCl, CaCl2, and Na2SO4 in the feed and permeate were measured using a

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calibrated conductivity meter (Oakton Instruments, Vernon Hills, IL). Ion chromatography (IC,

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Dionex, Sunnyvale, CA) equipped with CS14 and AS14A IonPac separation columns was used

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to quantify cation and anion concentrations, respectively. PFOA concentration was determined

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using a high-performance liquid chromatograph (HPLC, Agilent 1290 Infinity Series) coupled

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with a mass spectrometer (Agilent 6550A iFunnel Q-TOF MS). A sample volume of 1 µL was

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injected into a C18 column (Eclipse Plus, 1.8 m, 4.6  50 mm) using a mobile phase gradient of

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solvent comprising 40% methanol and 60% water with 20 mM ammonium acetate at a flow rate

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of 0.5 mL min-1. Methanol rate was increased from 40% to 98% in 5 min, held for 1 min,

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dropped back to 40%, followed by a 1-min stabilization. The Q-TOF MS was operated using an

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electrospray ionization (ESI) interface in negative mode. Linearity was determined from 0.1 µg

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L-1 to 100 g L-1 using calibration curves (R2 ≥ 0.99), with a limit of quantification (LOQ) of 10

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ng L-1.

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Scaling Experiments. The protocol for scaling experiments comprised the following steps.

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First, a new membrane coupon was placed in the NF cross-flow unit and compacted under 13.8

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bar (200 psi) hydraulic pressure with DI water as the feed for 4 h. Crossflow velocity and feed

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solution temperature were maintained at 21.4 cm/s and 25.0  0.5 °C, respectively. After

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compaction, the system was depressurized while allowing the feed flows to the membrane by

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maintaining the crossflow velocity of 21.4 cm/s. Calcium chloride (1 M), calcium sulfate (1 M),

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and sodium chloride (5 M) stock solutions were prepared in advance and filtered with a 0.45 m

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cellulose acetate membrane filter (Corning, Tewksbury, MA). Stock solutions were added to the

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feed reservoir to obtain a scaling solution comprising 15 mM CaCl 2, 10 mM Na2SO4, and 10 mM

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NaCl, with a gypsum (CaSO4·2H2O) saturation index (SI) of 0.54. The feed solution pH was

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adjusted to 7.0  0.1. The scaling solution was fully mixed and equilibrated for 1 h without

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applying pressure (i.e., no permeate flux). After equilibrium, the hydraulic pressures were set to

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about 7.2, 12.4, and 6.6 bar for the PIP + BP, PIP, and NF270 membranes, respectively, to

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achieve an identical initial permeate flux of ~80 L m -2 h-1. The gypsum scaling experiments were

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conducted for 20 h at a feed crossflow velocity of 15.0 cm/s and temperature of 25.0  0.5 °C in

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a recycling mode (i.e., the permeate was recycled back to the feed solution such that the gypsum

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saturation index was kept constant over time). 8 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION

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Polyamide Selective Layer Characteristics. We target the fabrication of NF membranes

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having a selective layer with a loose, nanoporous structure and a negative surface charge to

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facilitate passage of scale-forming cations and to enhance rejection of negatively charged PFOA

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via Donnan (charge) exclusion. Interfacial polymerization is the state-of-the-art technique

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allowing the synthesis of a thin, highly cross-linked selective polyamide layer. 36 Typical NF

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membranes fabricated via interfacial polymerization of piperazine (PIP, MW of 86.1 g/mol) and

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trimesoyl chloride exhibit fairly high salt rejection, especially for salts with divalent ions, such as

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CaCl2, MgCl2, and Na2SO4.37, 38 To achieve our goal of fabricating NF membranes with reduced

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cation rejection, we incorporated bipiperidine (BP) having a higher molecular weight (168.3

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g/mol) than PIP during interfacial polymerization. Bipiperidine comprises two piperidine units

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bound to each other by a carbon-carbon single bond, allowing chain extension and/or

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crosslinking polymerization to form a loose polyamide structure as described in Figure S1. 39, 40

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Specifically, we used a mixture of 0.5 wt % PIP and 0.5 wt % BP to obtain a loose, nanoporous

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selective layer, while a dense polyamide layer was constructed employing 1.0 wt % PIP; these

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membranes are denoted as PIP + BP and PIP, respectively.

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To verify the structural properties of the selective layer, we evaluated the average pore size

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of the PIP + BP membrane and compared it to the PIP and commercial NF270 membranes. The

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real (intrinsic) retention of inert organic tracers (Rr) was obtained from the observed retention

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(Ro) by accounting for the effect of concentration polarization using eq S3. Real retentions of the

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organic tracers by the PIP + BP, PIP, and NF270 membranes at different permeate water fluxes

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(i.e., transmembrane pressures) are presented in Figure S2.

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The obtained real retentions were used to estimate the NF membrane average pore size using

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the membrane pore transport model described earlier. The estimated pore radii are consistent for

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the different organic reference solutes as summarized in Table S1 of Supporting Information.

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Based on these results, we conclude that the PIP + BP membrane has an average pore radius of

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0.61 nm, which is much larger than the radii of the PIP (0.47 nm) and NF270 (0.41 nm)

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membranes. The larger pore size of the PIP + BP membrane compared to that of the PIP and

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NF270 membranes demonstrates that incorporating BP during the interfacial polymerization

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forms a polyamide network with a relatively loose, nanoporous structure. 9 ACS Paragon Plus Environment


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Rejection of charged solutes by electrostatic (Donnan) exclusion is directly related to the

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charge characteristics of NF membranes.41 We investigated the charge properties of the PIP + BP,

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PIP, and NF270 membranes by determining the zeta potential and carboxyl group density of the

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polyamide selective layer. As shown in Figure 1a, all NF membranes display a negative surface

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charge at solution pH of 7 and 10; the magnitude of the negative zeta potentials follows the order

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of NF270 > PIP > PIP + BP membranes. The polyamide selective layer of desalination

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membranes fabricated via interfacial polymerization of trimesoyl chloride (TMC) and amine-

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based monomers (i.e., BP or PIP) inherently possesses an outer layer of negative fixed charges

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resulting from hydrolysis of unreacted acyl chlorides of TMC to carboxyl groups. 42 The carboxyl

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groups of the polyamide film have a dissociation constant (pKa value) of ~5.2.43, 44 The observed

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negative zeta potential of the PIP + BP, PIP, and NF270 membranes at the investigated pH range

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is attributed to the ionized surface carboxyl groups.

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FIGURE 1

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Streaming potential analysis allows qualitative evaluation of the membrane surface charge

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characteristics. However, this method does not quantify the fixed charge density but rather

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determines the zeta (electrokinetic) potential, which is determined by the combined effects of

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charged groups on the surface as well as ions near the membrane surface. 45 In addition, zeta

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potential does not account for functional groups buried within the polyamide polymer network or

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inside the pores,30,

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mechanisms of NF membranes.

45

although such information is critical to understand the separation

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To better quantify NF membrane charge characteristics, we measured the carboxylic group

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density of the PA layer using the recently developed silver elution method. 30 This method allows

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for the quantification of areal carboxyl density based on the assumption that small, cationic silver

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ions are bound one-to-one with ionized carboxyl groups that are present on the membrane

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surface as well as those buried within the polyamide network. 44 As presented in Figure 1b, the

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areal carboxyl group densities of the PIP + BP, PIP, and NF270 membranes measured at pH 7

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were in the range of 0.7 – 1.7 sites/nm2, while much higher values were obtained at pH 10.5

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ranging from 3.8 – 13.0 sites/nm2. These values are relatively small compared to those reported

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for commercial reverse osmosis (RO) membranes,30 likely due to the thinner polyamide film of

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NF membranes,46,

47

which has lower degree of cross-linking formed by the semi-aromatic 10 ACS Paragon Plus Environment

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piperazine-based monomer compared to that formed on RO membranes by the fully aromatic m-

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phenylenediamine monomer.

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The carboxylic group density measured at pH 7 follows the order of NF270 > PIP > PIP +

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BP membranes, which correlates well with the results obtained from zeta potential measurements

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(Figures 1a-b). At a solution pH 10.5, the measured carboxylic group densities were much higher

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than those measured at pH 7 for all membranes. Additionally, the PIP membrane was found to

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have much higher carboxylic group density than the PIP + BP and NF270 membranes at a

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solution pH of 10.5. Such a substantial difference in carboxyl group densities at low (pH 7) and

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high (pH 10.5) solution pH is consistent with our previous observation of a series of commercial

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RO membranes.30 We attribute the results to the likely increase in acidity within the polyamide

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film during the silver binding process. Considering the relatively high carboxylic group

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concentration within the polyamide film, which is estimated to be 0.2–0.7 M, 44, 48 a substantial

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decrease in local pH within the polyamide film is postulated; that is, the local pH within the

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polyamide film would be much lower compared to the bulk solution pH employed during the

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silver binding procedure. Thus, when a bulk solution pH of 10.5 was employed, the majority of

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carboxylic groups within the polyamide film remain deprotonated/ionized (COO -), while only a

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fraction of carboxyl groups exist in ionized form within the film at bulk solution pH of 7.0. It can

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be concluded that the areal density measured at pH 7.0 corresponds to the carboxyl groups on the

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membrane surface, while that measured at pH 10.5 corresponds to groups buried within the

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polyamide network.30 The remarkably high density of carboxylic groups of the PIP membrane

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observed at pH 10.5 (Figure 1b) based on this proposed mechanism is further discussed in the

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following subsections.

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We conducted STEM-EDX elemental mapping to depict the cross-section images of the PA

317

films. The polyamide selective layer is rich in nitrogen while the underlying PSf or PES support

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layer contains sulfur but no nitrogen, providing elemental contrast in STEM-EDX analysis

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(Figures 2a-1 to a-3). Although the STEM-EDX elemental mapping was limited to characterize

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the distinct shape of the protruding polyamide layer nodules,49 it allowed visualization of the

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relative polyamide layer thickness for the PIP + BP, PIP, and NF270 membranes. The PA layer

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thickness of the NF270 membrane is likely in the range of 20–30 nm (Figure 2a-3), similar to

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values reported from other high-resolution microscopy and surface characterization studies. 31, 50 11 ACS Paragon Plus Environment


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A relatively high and thicker signal of nitrogen was detected for the PIP membrane (Figure 2a-2),

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indicating that the PIP membrane may have a thicker PA selective layer than the other two

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membranes.

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FIGURE 2

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To achieve a more quantitative analysis of the selective layer thickness, the PA film was

329

imaged by atomic force microscopy (AFM). The isolated PA film on the silicon wafer was

330

scratched using a precision glide needle (Figure S4). A border region between the PA film and

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the silicon wafer was scanned using AFM to determine the PA layer thickness as shown in

332

Figures 2b-1 to b-3. The average PA layer thicknesses of the PIP + BP, PIP, and NF270

333

membranes were 43  2, 61  7, and 29  4 nm, respectively. Consistent with the previous

334

STEM-EDX elemental mapping analysis, the PIP membrane had a thicker PA layer than the PIP

335

+ BP and NF270 membranes. In particular, the thickness of the PIP membrane PA layer was

336

more than two times higher than that of the NF270 membrane. The measured polyamide layer

337

thickness explains the previously observed higher carboxylic group density of the PIP membrane

338

at pH 10.5 compared to that of the other two membranes (Figure 1b). The thicker PIP membrane

339

PA layer contains more carboxylic groups buried within the polyamide network or inside the

340

pores.

341

The investigated structural and charge properties of the PIP + BP, PIP, and NF270

342

membranes are schematically visualized in Figures 2c-1 to c-3. As we intended, the PIP + BP

343

membrane had a pore diameter of ~1.2 nm, much larger than the pore diameters of the PIP

344

(~0.94 nm) and NF270 (~0.80 nm) membranes. All membranes possess negative surface and

345

pore charge, which is attributed to the carboxylic groups of the polyamide film. The thickness

346

normalized carboxyl group density, indicated as “COO-” in Figures 2c-1 to c-3, was calculated

347

by dividing the total carboxylic group density at pH 10.5 (shown in Figure 1b) by the average PA

348

layer thickness measured using AFM (shown in Figure 2b). It is assumed that the carboxyl areal

349

density shown in Figure 1b is based on a dimensionless unit square value. The thickness

350

normalized carboxylic group density is the highest for the PIP membrane (0.21 mn -1), followed

351

by the NF270 (0.19 nm-1) and PIP + BP (0.11 nm-1) membranes.


Page 12 of 29

Page 13 of 29


352

In the following subsections, we relate the polyamide selective layer characteristics discussed

353

above to the salt and PFOA rejection behavior of the NF membranes. We further depict the

354

underlying rejection and scaling mechanisms of the fabricated membranes.

355

Single Salt Rejection Performance. Salt rejection behavior of the PIP + BP, PIP, and

356

NF270 membranes was evaluated with sodium chloride, calcium chloride, and sodium sulfate at

357

concentrations of 2, 10, and 20 mM (Figure 3). We employed hydraulic pressure of 6.9 bar (100

358

psi) for the PIP + BP and PIP membranes and 3.1 bar (45 psi) for the NF270 membrane during

359

the NF experiments to compare the rejection performance of all membranes that had different

360

permeabilities (Figure S3), at the same permeate water flux. As shown in Figure 3a, the rejection

361

of NaCl decreased with increasing salt concentration for all three membranes, which is typically

362

observed for negatively charged NF membranes.51, 52 For symmetric salts like NaCl, electrostatic

363

repulsion between co-ions (ions with the same charge as the membrane) and the membrane

364

governs the NF rejection mechanism.18 For this reason, NF membranes exhibit higher NaCl

365

rejection with increasing solution pH because more carboxylic groups on the PA selective layer

366

are deprotonated (COO-) at higher pH, thereby enhancing Cl- rejection by charge repulsion.53

367

Similarly, at higher salt concentration, screening of the charge on the PA selective layer reduces

368

the effectiveness of electrostatic repulsion of Cl - ions, resulting in a decrease of NaCl rejection.

369

We also observe that the PIP + BP membrane showed much lower NaCl rejections of ~50% and

370

~25% at low (2 mM) and high (20 mM) concentrations, respectively, than the PIP and NF270

371

membranes.

372

FIGURE 3

373

It is interesting to note that NaCl rejection of the PIP membrane was higher than that of the

374

NF270 membrane at all salt concentrations tested, despite the PIP membrane having a larger

375

pore size (Table S1) and less negative zeta potential than the NF270 membrane (Figure 1a). The

376

observed higher NaCl rejection of the PIP membrane can be explained by the thickness of the

377

selective polyamide layer. As described in Figures 2c-2 and c-3, the PIP membrane has a

378

polyamide layer much thicker (61 nm) than the NF270 membrane (29 nm), which induces higher

379

resistance to ion transport through the selective layer. This result highlights the fact that in

380

addition to membrane pore size and charge properties, the PA selective layer thickness plays a



381

critical role in governing ion rejection/selectivity as well as water permeability (Figure S3) of NF

382

membranes.

383

Observed trends of CaCl2 rejection as a function of salt concentration were opposite to those

384

observed with NaCl, where the rejection of CaCl 2 increased at higher concentrations for all NF

385

membranes (Figure 3b). Charge-based separation of asymmetric salts, such as CaCl 2 and Na2SO4,

386

is dominated by the ions of higher valency (Ca2+ for CaCl2 and SO42- for Na2SO4) because the

387

electrostatic repulsive or attractive interactions between such ions and the membrane control the

388

rate of salt transport.18, 53 Hence, CaCl2 rejection by the negatively charged PIP + BP, PIP, and

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NF270 membranes is mainly controlled by the rate of Ca 2+ permeation through the PA selective

390

layer. At a relatively low CaCl2 concentration (2 mM), electrostatic attraction between Ca2+ and

391

the negative membrane charge facilitates the transport of Ca 2+, resulting in a low CaCl2 rejection.

392

However, cation transport through the negatively charged membrane is reduced at higher salt

393

concentrations (20 mM in Figure 3b) due to effective charge neutralization and screening of the

394

membrane carboxyl groups, resulting in reduced electrostatic attraction and thus higher rejection

395

of CaCl2. When the effect of charge interaction diminishes at high ionic strengths, size exclusion

396

has a stronger effect on Ca2+ retention. Considering the size of the hydrated Ca2+ ion (radius of

397

~0.41 nm)54 and the estimated pore radii of the PIP (~0.47 nm) and NF270 (~0.41 nm)

398

membranes (Table S1), size exclusion of Ca2+ can be effective at high salt concentrations. In

399

contrast, the PIP + BP membrane showed a much lower CaCl 2 rejection (

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