Nanometric Graphene Oxide Framework Membranes with Enhanced

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Environmental Science & Technology

Nanometric Graphene Oxide Framework Membranes with Enhanced Heavy Metal Removal via Nanofiltration

Yu Zhang†, Sui Zhang‡, Tai-Shung Chung†, ‡, *



NUS Graduate School for Integrative Science and Engineering,

National University of Singapore, 28 Medical Drive, Singapore 117456



Department of Chemical and Biomolecular Engineering,

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

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Abstract

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A novel dual-modification strategy, including (1) the crosslinking and construction of a GO

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framework by ethylenediamine (EDA) and (2) the amine-enrichment modification by

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hyperbranched polyethyleneimine (HPEI), has been proposed to design stable and highly

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charged GO framework membranes with the GO selective layer thickness of 70 nm for

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effective heave metal removal via nanofiltration (NF). Results from sonication experiments

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and positron annihilation spectroscopy confirmed that EDA crosslinking not only enhanced

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structural stability, but also enlarged the nanochannels among the laminated GO nanosheets

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for higher water permeability. HPEI 60K was found to be the most effective post-treatment

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agent that resulted in GO framework membranes with a higher surface charge and lower

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transport resistance. The newly developed membrane exhibited a high pure water

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permeability of 5.01 L m-2 h-1 bar-1 and comparably high rejections towards Mg2+, Pb2+, Ni2+,

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Cd2+ and Zn2+. These results have demonstrated the great potential of GO framework

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materials in wastewater treatment and may provide insights for the design and fabrication of

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the next generation 2D-based NF membranes.

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Keywords:

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Graphene oxide framework, hyperbranched polyethyleneimine, nanofiltration, heavy metal

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1. INTRODUCTION

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The scarcity of fresh water is a global challenge due to the rapid growth in population and

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industrialization 1. This leads to a great demand on exploring cost-effective and highly

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efficient water reuse and desalination technologies 2. Compared with traditional wastewater

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treatments such as chemical, precipitation and sorption methods, nanofiltration (NF)

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technology has several advantages, including smaller footprint, lower operation cost and

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energy consumption, and elimination of chemical residuals 3-7. One important application of

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NF is for heavy metal removal, which is achieved by combining the size exclusion and the

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Donnan exclusion separation mechanisms 8-11.

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The ideal NF membrane should have a narrow pore size distribution to achieve a high

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selectivity and a thin and highly porous structure to ensure a good permeability. Generally,

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polymeric NF membranes have acceptable separation performance and are widely used

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owing to easy fabrication with relatively low costs. However, polymeric membranes have

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some drawbacks including poor chemical and thermal resistance, fouling and physical aging

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12, 13

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weaknesses of polymeric membranes and achieve high separation performance

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Nevertheless, inorganic NF membranes are expensive and only used for special applications.

. Inorganic porous membranes, such as Al2O3, can address the aforementioned 14

.

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Recently, carbon-based materials, e.g. graphene and its derivative graphene oxide (GO), have

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shown promising to be membrane materials for their easy accessibility, high chemical and

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mechanical stabilities 15-18. GO is only one atom thick with a lateral length of several hundred 3

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nm (Figure S-1)

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self-assemblable

20-22

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achievements have been made for various applications, including gas separation

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pervaporation dehydration 25-28, and ultra/nanofiltration 29-32.

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. This high aspect ratio makes GO nanosheets highly stackable and . By controlling the d-spacing of GO membranes, remarkable 23, 24

,

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Nair et al. found that a sub-micrometer-thick GO membrane can completely block the

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passage of liquids and gases under a dry state while facilitating the permeation of water vapor

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by the empty inter-space between the nonoxidized regions of GO sheets. However, hydration

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would increase the d-spacing of the GO membrane when it was immersed in water and allow

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a faster permeation of molecules with sizes smaller than 0.45 nm while blocking ions or

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molecules larger than that 34. Inspired by Nair’s findings, Huang et al. reported a modified

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nanostrand-channeled GO membrane using copper hydroxide nanostrands as a sacrificial

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template to enhance the membrane permeability

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membranes of tens-of-nanometer thick for dye retention with similar rejection performance

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but much higher water flux than commercial NF membranes 30. These results suggested that

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laminar GO membranes are a promising candidate to develop NF membranes with an

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ultrahigh permeability.

. They attributed this fast transport of water vapor to the low-friction nanocapillaries formed

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. Han et al. also prepared ultrathin GO

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However, since the integrity of pure GO films is solely maintained by hydrogen bond

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interactions between the oxygen-containing groups of GO nanosheets, pure GO films suffer 4

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from low structural and physicochemical stability. When the membrane is immersed in an

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aqueous solution, the hydration effect will destroy the hydrogen bond and enlarge the

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d-spacing among GO nanosheets, leading to a significant expansion of the nanochannels in

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the membrane. Therefore, there is an urgent need to improve the GO membrane design and

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self-assembly technique to enhance its stability under various conditions 18, 35. So far studies

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on the separation of small ionic species using GO membranes are limited. It was reported that

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GO based membranes had lower rejections towards ions than common NF membranes 29, 30.

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Several approaches have been proposed to address these issues, including layer-by-layer

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(LbL) dip-coating 36 and construction of GO frameworks 27. However, the LbL membranes

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assembled via electrostatic interaction tend to swell up in ionic solutions, while fully

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crosslinked GO framework membranes suffer from a low liquid water permeability.

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In this study, a novel dual-modification strategy, including (1) the crosslinking and

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construction of a GO framework by ethylenediamine (EDA) and (2) the amine-enrichment

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modification by hyperbranched polyethyleneimine (HPEI), is designed to fabricate stable and

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highly charged NF membranes. The first step includes the mixing of EDA with the GO

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solution and then deposits it on a polycarbonate substrate via a pressure-assisted assembly

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technique, while the second step modifies and stabilizes the as-prepared membrane by HPEI.

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Different chemical, physicochemical and morphological characterizations are conducted to

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investigate the stability and surface properties of the pristine and modified membranes. The

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newly developed GO composite membrane shows good stability, high water permeability and 5

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rejections towards a variety of cationic heavy metal ions, including Pb2+, Ni2+, Cd2+ and Zn2+.

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This study may provide useful insights on the design and fabrication of new generation NF

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membranes.

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2. EXPERIMENTAL

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2.1. GO framework membrane preparation

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Prior to the GO assembly process, the substrates were modified with polydopamine to

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enhance the adhesion between the substrate and the GO layer 37. Polydopamine coating has

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been widely employed in membrane fabrication processes to increase substrate hydrophilicity

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as well as to provide feasible modification sites 38, 39. Firstly, the coating solution (2 mg/mL)

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was prepared by dissolving 0.16 g dopamine hydrochloride into an 80 mL 0.01 M tris buffer

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at pH 8.5. The substrates were then immersed in the solution under ambient temperature for 3

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hours. During this period, dopamine will undergo self-polymerization and form an adhesive

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layer on the substrate to provide GO nanosheets anchor sites (Figure S3). After that, the

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substrates were rinsed in DI water to remove the residues.

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In order to prepare the GO framework, a certain amount of GO solution was fully dissolved in

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a 20 mL 1 wt% EDA water solution. The blend was then filtrated through the modified

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polycarbonate membrane in a dead-end filtration cell under 1 bar. After the filtration, the

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membrane was rinsed in DI water to remove the residual EDA. GO framework membranes

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fabricated under this condition were referred to as GO&EDA. For comparison, a control

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membrane was prepared following the same procedure but without EDA in water. This 6

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membrane was referred to as pristine GO.

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The surface charge properties of the as-fabricated GO&EDA membrane were further

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modified by immersing it into an aqueous solution containing either 1 wt% amine HPEI 60K,

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PEI 2K or dendrimer G(2,0) for a certain period of time and rinsed by DI water. The resultant

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membrane is denoted as GO&EDA_HPEI 60K if it is modified by HPEI 60K.

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2.2. Pure water permeability and salt rejection of the GO membranes

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Pure water permeability (PWP, L m-2 h-1 bar-1, abbreviated as LMH bar-1) and salt rejection

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(R, %) of these composite GO membranes were tested at 1 bar by a dead-end permeation

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cell at room temperature (Figure S4). The effective membrane area was 3.14 cm2. PWP of

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each membrane was measured with DI water and the salt rejection was determined using a

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1000 ppm ion solution (i.e., NaCl, MgCl2). The membrane was firstly conditioned under the

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NF mode for 2 hours before collecting permeate samples. During the test, the feed solution

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was stirred at 500 rpm. PWP and R are calculated as follows: 

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PWP = 

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 = 1 −  × 100%



(1) (2)

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where Q is the water flow rate (L/h) at the permeate side of DI water. A is the effective

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membrane area (m2) and ∆P is the trans-membrane pressure (bar). Cp and Cf are the

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concentrations of the ion solution in the permeate and feed, respectively, which are

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determined by a conductivity meter (Metrohm AG). 7

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2.3. Membrane characterizations

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X-ray photoelectron spectroscopy was employed to determine the surface chemistry of the

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membranes using a monochromatized Al Kα X-ray source (1486.6 eV photons) at a

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constant dwell time of 100 ms and a pass energy of 40 eV. The chemical structure of the GO

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framework layer was analyzed by Fourier transform infrared spectroscopy (FTIR) (Bio-Rad

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FTS-3500) over the range of 400-4000 cm-1. The polydopamine coating was not applied on

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the polycarbonate membrane surface for the specific sample where the GO layer was peeled

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off from the substrate, A pellet was prepared by grinding a 0.5 mg GO layer sample with

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99.5 mg KBr powder and compressed under 8 bar, which was then characterized by the

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direct transmittance mode. The total number of scans for each sample was 16.

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The morphologies of the GO framework membranes were observed by field emission

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scanning electron microscopy (FESEM JEOL JSM-6700LV). Prior to FESEM observation,

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the membrane was freeze-dried before fractured in liquid nitrogen. The membrane samples

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were then fixed on stubs and coated with a thin layer of platinum under a vacuum condition

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by a Jeol JFC-1100e Ion Sputtering device. The GO nanosheets and membrane thicknesses

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were examined by an Atomic force microscope (AFM) (Agilent Technologies, USA) under

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the tapping mode. A membrane stability test was carried out via sonicating the membrane

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sample under the degas mode by Elmasonic (S 30 H) for 10 minutes.

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The surface charge properties of the pristine GO, GO&EDA and GO&EDA_HPEI 60K

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framework membranes were analyzed by a SurPASS electrokinetic analyzer (Anton Paar

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GmbH, Austria). A 450 mL 0.01 M NaCl solution was used to measure the ζ-potential of the

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membranes under neutral pH. In order to determine the ζ-potential as a function of pH from

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2.5 to 10, the 0.01 M NaCl solution was firstly auto-titrated with 0.1 M HCl to pH 2.5

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followed by auto-titration with 0.1 M NaOH to pH 10.

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The microstructural evolution of GO framework membranes was investigated by Doppler

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broadening energy spectroscopy (DBES). This measurement was conducted using positron

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annihilation spectroscopy (PAS) in our laboratory. 22Na was used as the source of positrons

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and experiments were performed at a counting rate of 3000-4000 counts s-1. For each

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spectrum, a total of one millions counts was taken. The detailed testing procedures have

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been elaborated elsewhere

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used to determine the microstructural changes along the membrane depth profile. The mean

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depth of the materials associated with the incident positron energy can be calculated by the

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following equation:

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40

. S- parameter derived from the annihilation spectra, can be



Z ( ) =    × .

(3)

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Where Z is the mean depth (nm),  is the material density (g cm-3) and E+ is the incident

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positron energy. The average density employed in the fitting was calculated via the weight

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and volume of the membrane as 0.95 g cm-3.

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3. RESULTS AND DISCUSSION

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3.1. Chemical

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membranes

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Figure 1 shows the FTIR spectra of the pristine GO, GO&EDA and GO&EDA_HPEI 60K

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membranes under the transmission mode. The pristine GO (Figure 1a) has characteristic

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peaks of –OH stretching at 3400 cm-1, -C=O stretching at 1732 cm-1 and -OH stretching at

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1414 cm-1 in -COOH, C-O-C stretching at 1224 cm-1, which well corresponds to its

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structure as illustrated in Figure S-2a in the Supporting Information 17. After crosslinked by

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EDA, new peaks appear at about 1357 cm-1 and 3250 cm-1 (Figure 1b), which are

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representatives of the –CN in secondary amine and the -NH bond of primary amines,

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respectively 41. In addition, compared with the spectrum of pristine GO, the epoxy content

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almost disappears and the intensity of carboxyl group decreases significantly. This indicates

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that crosslinking reactions and electrostatic interaction have taken place between the amine

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group of EDA and the epoxy or carboxylic acid group of GO. For GO&EDA_HPEI 60K, a

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new peak can be observed at 1570 cm-1, which is attributed to the abundant secondary

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amine on the HPEI chains. In addition, part of the free amine groups of HPEI molecules

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may also react with the residual epoxy group in GO to contribute to the peak intensity.

and

morphological

characterizations

of

go

framework

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The reactions are further confirmed by XPS as shown in Table S-1 in the Supporting

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Information. The pristine GO membrane contains an O/C mass ratio of 0.4 and no nitrogen

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content could be detected. An increment in N1s content and a decline in O1s content for 10

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GO&EDA and GO&EDA_HPEI 60K are observed Correspondingly, O-C=O, C=O, C-O

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and C-C signals are detected on the surface of the pristine GO membrane in Figure S-5a.

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For the GO&EDA framework membrane, the peak intensities of O-C=O, C=O and C-O

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decline while a new peak of C-N appears due to the introduction of EDA into the GO layers

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(Figure S-5b). Figure S-6a shows the peaks of primary, secondary and quaternary amine

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groups on the surface of GO&EDA, indicating the presence of free amine, formation of

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covalent bonding and electrostatic interaction between GO and EDA. The peak intensity of

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C-N in GO&EDA_HPEI 60K increases substantially owing to the C-N bond of HPEI 60K

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on the membrane surface (Figure S-5c). In the meantime, the peak intensities of C-O and

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C=O further decrease while the O-C=O peak disappears, which may be due to the formation

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of the covalent bond and ionic interaction between oxygen-containing groups of GO with

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amine groups.

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Figure 2b and 2c elucidates the chemical structures on the surfaces of GO&EDA and

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GO&EDA_HPEI 60K membranes. It is worth noting that EDA may either perpendicularly

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crosslink two parallel GO nanosheets or two adjacent nanosheets. Moreover, there might be

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only one amine group of EDA reacting with GO nanosheets, which explains the presence of

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amine group in the FTIR spectrum of the GO&EDA composite membrane. Therefore, the

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dual modification strategy; namely, the first step by EDA to crosslink the GO sheets and the

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second step by HPEI to impart the surface with abundant positive charge, works well on the

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GO membranes. 11

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The structural integrity of the GO layer over the polycarbonate support is greatly enhanced

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by the dual modifications as evidenced in Figure S-7 in the Supporting Information, which

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shows the digital images of pristine GO and GO&EDA_HPEI 60K after a vigorous

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sonication process. Originally, both samples have the same brown appearance indicative of a

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thin GO layer. After the sonication process. ¾ of the pristine GO membrane becomes

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translucent, implying most GO nanosheets have been removed by the sonication, whilst

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GO&EDA_HPEI 60K maintains its appearance. This phenomenon suggests the structural

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stability of GO&EDA_HPEI 60K has been improved as compared with pristine GO.

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In order to optimize the preparation conditions, Figure S-8 shows the PWP and MgCl2

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rejection of GO framework membranes as functions of GO loading and immersion duration

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in the HPEI 60K solution. The GO framework membrane comprising 0.09375 mg GO

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(specific deposition: 0.083 g/m2) and being modified by 1 wt% HPEI 60K aqueous solution

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for 20 min has the best balanced separation performance; namely, a MgCl2 rejection of 96.3%

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± 1.3% and a PWP of 5.01 ± 0.24 LMH bar -1. It is therefore chosen as a representative of

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the GO&EDA_HPEI 60K for the subsequent studies.

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Figure 3a-d displays the surface and cross section morphologies of the substrate and the

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composite GO&EDA_HPEI 60K membrane. The polycarbonate substrate has a smooth

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surface with uniform pores of 0.2 µm in diameter. After the deposition of GO nanosheets, a 12

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well packed laminated film is formed on the substrate surface (Figure 3b). The wrinkles on

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the top surface of the GO membrane are formed from the edges and folding of GO

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nanosheets. The thickness of the GO layer was measured by FESEM and AFM. Figure 3d

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shows its FESEM cross section image on the substrate with a thickness of less than 100 nm,

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while Figure 3e displays its height profile along the line across the membrane surface drawn

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by AFM with an average thickness of 69 nm. Since the specific GO amount for this NF

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membrane is only 0.083 g/m2, the newly developed NF membrane could be cost effective

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and prospective for practical wastewater treatment.

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3.2. Effects of EDA crosslinking and HPEI modification on rejection

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performance in NF processes

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Figure 4a compares the PWP values of the pristine GO, GO&EDA and GO&EDA_HPEI

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60K membranes. GO&EDA has the highest PWP of 9.79 ± 2.20 LMH bar-1, which is

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significantly higher than that of the pristine GO membrane. One possible explanation is that

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the orientation of GO nanosheets in the pristine GO membrane is highly ordered and

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densely packed

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For perpendicularly crosslinked GO nanosheets, the expansion of d-spacing under the wet

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condition is retarded, which would reduce water permeability. In contrast, if EDA

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crosslinks two adjacent nanosheets, it would enlarge water transport channels and increase

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PWP. Similarly, the incorporation of free amine and un-ordered EDA molecules among GO

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nanosheets may introduce defects and lower their packing density, thus increasing PWP.

42

. For GO&EDA, EDA may crosslink GO nanosheets in different ways.

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These competing mechanisms result in an enhanced pure water permeability for the

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GO&EDA membrane. The S- parameter values measured by DBES spectroscopy confirm

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our hypotheses. Figure 5 shows the S- parameter as a function of the incident positron

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energy for these three membranes. A smaller S- parameter indicates either a smaller free

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volume size or a lower free volume content of the membrane

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(i.e., corresponding to the thin GO layer), the S- parameter of the pristine GO membrane is

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much lower than the other two. It is hence reasonable to conclude that the pristine GO has

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the densest and most ordered packing, while GO&EDA and GO&EDA_HPEI 60K have a

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comparable packing density. Although the packing densities of GO nanosheets in

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GO&EDA_HPEI 60K and GO&EDA are similar, HPEI 60K crosslinks the top layer of

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GO&EDA_HPEI 60K and thus its pure water permeability is reduced to 5.01 ± 0.24 LMH

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bar -1.

25

. At low incident energy

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The ζ-potential of the pristine GO, GO&EDA and GO&EDA_HPEI 60K as a function of

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pH is shown in Figure 6. In a wide pH range from 3 to 11, the pristine GO membrane is

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negatively charged, which is mainly due to the deprotonation of the carboxyl group at the

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edges of GO nanosheets. The GO&EDA is slightly positive charge below pH 4.5 but

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becomes neutral at pH 4.5 and then negative charge at higher pH values owing to the

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complicated functional groups on the GO&EDA membrane surface such as hydroxyl,

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unreacted carboxyl acid, unreacted amine and amide groups. For GO&EDA_HPEI 60K,

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since additional amine groups are introduced on the GO framework surface, the isoelectric 14

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point shifts to pH 10.5 due to the protonation of the free amine groups. Moreover, its

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ζ-potential is around 100 mV at neutral pH, which is higher than most reported data for

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positively charged membranes 43, 44.

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Figure 4 also displays the rejections of these membranes against MgCl2, MgSO4, NaCl and

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Na2SO4 solutions. The pristine GO has the highest rejection towards Na2SO4, which is

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consistent with the literature data 30, then the rejection follows an order of R (Na2SO4) > R

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(MgSO4) > R (NaCl) > R (MgCl2). However, the rejection order is reversed for

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GO&EDA_HPEI 60K. This phenomenon can be explained by the Donnan exclusion effect.

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GO is negatively charged in a wide pH range. Therefore, it tends to extrude co-ions, such as

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SO42- and Cl-1. In order to maintain the electroneutrality of the solutions at each side of the

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GO composite membrane, the counter ions Na+ and Mg2+ have to be rejected as well.

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According to Donnan exclusion theory, the rejection rate is related to the valences of the ion

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species, following the order of Zco-ions/Zcount-ions (Z refers to the valence). On the contrary, the

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GO&EDA_HPEI 60K membrane is positively charged after being crosslinked by

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amine-rich HPEI molecules. In this case, the co-ions are Mg2+ and Na+. The membrane

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thereby shows a higher rejection against divalent cations (Mg2+). Since the GO&EDA has a

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larger free volume and close-to-neutral charge at neutral pH, the rejections towards different

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salts are lower compared with the other two membranes.

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3.3. Effects of different amine-enrichment modifications

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Besides HPEI 60K, PEI 2K and dendrimer G(2,0) were employed to investigate the effects

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of various amine modifications. PEI 2K has a smaller molecular weight and dendrimer G(2,

295

0) is a micelle shaped molecule with abundant amine groups. The post-treatment conditions

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for PEI 2K and dendrimer G(2,0) have been optimized and Table 1 tabulates the best NF

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performance for each membrane.

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The PEI 2K post-treatment is unfavorable for both permeability and rejections because the

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resultant membrane has a lower rejection of 89% towards MgCl2 and a significantly

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reduced PWP of 0.39 LMH bar -1. This is due to the fact that the molecular size of PEI 2K is

302

smaller. It can easily block surface pores of the GO layer and react with GO nanosheets 45,

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resulting in a denser surface with a reduced water permeability. For the membrane

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post-treated by dendrimer G(2,0), the rejection remains high while the PWP drops 4-fold.

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This arises from the fact that dendrimer G (2.0) may form a denser crosslinking layer than

306

that from HPEI 60K because HPEI 60K has a longer molecular chain and higher chain

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flexibility. As a consequence, the membrane post-treated by the former has a comparable

308

rejection but a lower PWP. Clearly, HPEI 60K is the most efficient agent to modify the GO

309

framework membrane with higher surface charge but lower transport resistance.

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3.4. Heavy metal rejection of the HPEI 60K modified GO framework membrane

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The newly developed GO&EDA_HPEI 60K membrane was further tested by 1000 ppm 16

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Pb(NO3)2, NiCl2, CdCl2 and ZnCl2 solutions, respectively. High rejections to these divalent

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cationic heavy metal ions have been achieved as summarized in Table 2, especially for

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Pb(NO3)2 and NiCl2. Since the newly developed GO framework membrane is highly

316

positively charged at neutral pH, it can effectively repel the cationic heavy metal ions. Table

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2 shows a benchmarking of the current GO framework membrane with some other NF

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membranes

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this work.

8-10, 46, 47

. A much higher PWP with comparable rejections has been achieved in

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ASSOCIATED CONTENT

322

Supporting Information

323

Materials, AFM images of GO nanosheets, chemical structures of the different compounds

324

used in this work, FESEM images of substrate membrane before and after polydopamine

325

modification, experimental apparatus, XPS C 1s and N 1s narrow scan spectra of different

326

membranes, digital photo images of GO framework membranes, effect of GO loading and

327

post-treatment duration on NF performance and XPS characterizations results. This material

328

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

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AUTHOR INFORMATION

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Corresponding author

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* Tel: +65-65166645. Fax: +65-67791936. Email: [email protected].

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Notes 17

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The authors declare no completing financial interest.

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ACKNOWLEDGEMENT

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The authors would like to acknowledge the financial support from the Singapore National

338

Research Foundation (NRF) Competitive Research Program for the project entitled

339

“Advanced FO membranes and Membrane systems for Wastewater Treatment, Water Reuse

340

and Seawater Desalination” (Grants R-279-000-336-281 and R-278-000-339-281). Special

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thanks are due to Dr. Y. Tang, Dr. K.S. Liao, Ms. J. Gao and Mr. S. Japip for their suggestions

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on the experimental work and paper writing.

343 344

ABBREVIATIONS

345

AFM, atomic force microscopy; DBES , doppler broadening energy spectroscopy; EDA,

346

ethylenediamine; FESEM, field emission scanning electron microscopy; FTIR, Fourier

347

transform

348

polyethyleneimine; LbL, layer-by-layer, NF, nanofiltration; PAS, positron annihilation

349

spectroscopy; PWP, pure water permeability; XPS, X-ray photoelectron spectroscopy.

infrared

spectroscopy;

GO,

graphene

oxide;

HPEI,

hyperbranched

350 351

REFERENCES

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1.

353

P.; Glidden, S.; Bunn, S. E.; Sullivan, C. A.; Liermann, C. R.; Davies, P. M., Global threats

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to human water security and river biodiversity. Nature 2010, 467, 555-61.

Vorosmarty, C. J.; McIntyre, P. B.; Gessner, M. O.; Dudgeon, D.; Prusevich, A.; Green,

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2.

Escobar, I. C.; Van der Bruggen, B., Modern applications in membrane science and

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Dual-layer

475 476 477

24

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478

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TOC Art

479 480

481

25

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C-O-C

-OH hydroxyl

(a)

C=O C=C

(b)

C-OH -CN- secondary amine

-NH secondary amine

(c)

pristine GO GO&EDA GO&EDA_HPEI 60K

-NH Primary amine 4500

Page 26 of 33

4000

3500

3000

2500

Wavenumber

2000

1500

1000

500

0

(cm-1)

Figure 1. FTIR spectra of the GO layers: (a) pristine GO, (b) GO&EDA and (c) GO&EDA_HPEI 60K framework membranes, measured in the transmission mode.

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O

OH HO2C

OH

O

OH

O

O

O

O

CO2H

OH

OH

OH

O

O

OH

HO2C

CO2H

CO2H

OH

OH

OH

CO

OH

O

(a)

OH

HO2C

OH

OH

OH

CO

OH OH

O

OH CO H 2 OH

HO2C

OH HO2C

OH

O

(b)

HO2C

OH

OH

O

(c)

Figure 2. Schematic diagrams of the purposed structures: (a) pristine GO, (b) GO&EDA and (c) GO&EDA_HPEI 60K frameworks. ACS Paragon Plus Environment

Environmental Science & Technology

1 μm

(b)

1 μm

(c)

100 nm

(d)

100nm nm 100

(e)

20 nm

(a)

Page 28 of 33

0

-20 -40

Membrane average thickness: 69.41 ± 3.85 nm

-60 -80 -100 0

2

4

6

8

10

12

Figure 3. The top surface morphology of (a) the Whatman® Cyclopore® polycarbonate membrane and (b) the GO&EDA_HPEI 60K composite membrane. The cross section morphology of (c) the substrate layer and (d) the GO layer. (e) The AFM image of the GO layers on a mica film. ACS Paragon Plus Environment

14 μm

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PWP (LMH bar -1)

(a) 15

10

5

0 pristine GO

Salt Rejection (%)

(b)100

MgCl2

GO&EDA

NaCl

MgSO4

GO&EDA_HPEI 60K Na2SO4

80 60 40 20

0 pristine GO

GO&EDA

GO&EDA_HPEI 60K

Figure 4. (a) The PWP of the three GO framework membranes at a transmembrane pressure of 1 bar, and (b) the rejection performance ofACS theParagon membranes for four different salt solutions (each 1000 Plus Environment ppm).

Environmental Science & Technology

0

0.55

Mean depth (µm) 1.68 3.21 5.08

Page 30 of 33

7.26

9.72

S- parameter

0.52 0.5 0.48 pristine GO

0.46

GO&EDA

0.44

GO&EDA_HPEI 60K

0.42 0

0

5

4.6

10 15 20 25 Incident positron energy (keV)

30

Mean depth (nm) 13.9 26.6 40.0 60.2 80.6 103.1 127.6

S- parameter

0.47 0.46 GO framework layer

0.45 0.44 0.43 0.42 0

0.25

0.5 0.75 1 1.25 1.5 1.75 Incident positron energy (keV)

2

Figure 5 . S- parameters of the three GO framework membranes against the incident positron energy. ACS Paragon Plus Environment

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150

ζ-potential (mV)

100 50 0 0

2

4

6

8

12

pH

-50

-100

10

pristine GO GO&EDA

-150

GO&EDA_HPEI 60K

Figure 6. ζ-potential as a function of pH of the three GO framework membranes. ACS Paragon Plus Environment

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Page 32 of 33

Table 1. Effects of post-treatment agents on NF performance of 1000 ppm MgCl 2 .

Reaction duration*

PWP (LMH bar -1)

Rejection (%)

ζ-potential (mV)

30 s

0.39

89.0

72.6

HPEI 60K

20 min

5.01

96.3

92.6

Dendrimer G(2,0)

20 min

1.20

97.2

101.0

PEI 2K

*

The reaction duration was optimized to the highest rejection performance

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Table 2. A benchmarking of NF membranes for heavy metal removal.

Membrane Chitosan PES composite membrane

PWP (LMH bar -1)

Ion

Testing condition

Rejection (%)

Refs.

3.45

NiCl2

1000 ppm, 10 bar

96.3

6

Pb(NO3)2

93.0

Chelating polymer modified P84

˃1

Pb(NO3)2

1000 ppm, 10 bar

˃ 99

7

Kraton matrimid composite membrane

2.4

Pb(NO3)2

1000 ppm, 10 bar

99.8

8

˃ 98

CdCl2 PBI/PES dual-layer hollow fiber

0.826

Pb(NO3)2

200 ppm, 1 bar

93

42

Dow membrane NF270

13.2

Pb(NO3)2

1000 ppm, 4 bar

≈ 60

43

≈ 68

CdCl2 HPEI modified GO&EDA framework membrane

5.01

Pb(NO3)2

1000 ppm, 1 bar

95.7 ± 0.7

NiCl2

96.0 ± 3.8

ZnCl2

97.4 ± 2.0

CdCl2

90.5 ± 0.1

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This work