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Engineering Graphene Oxide Laminate Membranes for Enhanced Flux and Boron Treatment with Polyethylenimine (PEI) Polymers Changwoo Kim, Siyuan An, Junseok Lee, Qingqing Zeng, and John D. Fortner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18545 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Materials & Interfaces

Engineering Graphene Oxide Laminate Membranes for Enhanced Flux and Boron Treatment with Polyethylenimine (PEI) Polymers

Changwoo Kim, Siyuan An, Junseok Lee, Qingqing Zeng, and John D. Fortner* Department of Energy, Environmental and Chemical Engineering Washington University in St. Louis, St. Louis, MO, 63130 United States

*To whom correspondence should be addressed: John D. Fortner: Tel: +1-314-935-9293; Fax: +1-314-935-5464; Email: [email protected]

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Abstract In this work, we have developed and characterized flux enhanced graphene oxide laminate (GOL) membranes by increasing interlayer (layer-to-layer) spacing using multi-branched polyethylenimine (PEI) polymers with varied molecular weights and by controlling the graphene oxide (GO) oxidation extent.

For these assemblies, water flux was demonstrated

to increase by as much as ca. 30-fold compared to GO only laminate controls.

PEI

embedded GOL membranes also had better methyl orange (MO) rejection performance than GO laminate controls due to the dilution effects (i.e. water is transported through the assembly much faster than MO).

Further, boron removal is demonstrated via functionalized

PEI with D-glucono-1,5-lactone, containing a high density of boron chelating groups, which can also be recycled/recovered with high efficiency.

Keywords Graphene oxide membrane; Boron removal; d-spacing; Methyl orange; Polyethylenimine

Introduction Graphene and graphene oxide (GO) have received considerable attention as promising platform materials for water treatment membranes due to low wall friction1, high chemical stability, and relative high mechanical strength.2

In addition, GO can be easily derivatized

inorganically and/or organically, allowing for a number of oxygen-containing functionalities, including hydroxyl, epoxy, carboxyl, and carbonyl groups.3

Structurally, graphene-based

membranes are typically classified as either nonporous graphene (NPG) or graphene oxide laminates (GOL).4

On the basis of theoretical simulations, NPG has been described as a

potentially ideal membrane for water treatment and desalination.5 2


However, to date,

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fabrication of large scale NPG membranes remains technically limited due to rippling, warping, and general instability of atomically thin, large surface area graphene sheets.6-7

As

an alternative, GOL membranes have been proposed and demonstrated whereby water transport is between GO layers (i.e. d-spacing GO layer-to-layer distance).

For example, Xu

et al. demonstrated that water flux and salt rejection of GOL membrane can be increased via forming interlayer structured water channel through the deposition rate control of GO flakes.8 Chen et al. controlled d-spacing distances of GOL membranes via utilization of strong cationπ interactions (with GO).9

Abraham et al. developed GOL membranes with exceptional

molecular cutoff control by preventing swelling of d-spacing through physical confinement.10 Amadei et al. verified that oxidation extent of GO influences the architectural (including dspacing) of GOL; GO having higher oxygen content forms larger d-spacing, resulting effective water permeability.4

To increase the water flux of GOL membranes even further,

Yi et al. fabricated crumpled GO, which allows high water permeation via vertically tortuous nanochannels.11 Table S1 compares GOL membrane performance regimes for related literature reports, including this work, along with GOL assembly methods and interlayer dspacing values. Despite boron being an essential nutrient for plants and animals, the European Union (EU), South Korea, and Japan have set a limit at the 1.0 mg/L level for drinking water.12 Depending on seasonal variations, seawater contains 5.0 - 6.0 mg/L of boron.13

For

seawater reverse osmosis (SWRO) water treatment processes, boron rejection rates (for practical operation) is only 78 - 80%, which can be insufficient to meet regulations.14

To

address this paradigm, SWRO plants can increase the feedwater pH and/or install a secondary RO system.13

In some cases, SWRO plants have incorporated other treatment processes,

namely ion exchange and/or adsorption membrane filtration (AMF).12 3


To dates, boron


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separation/treatment has not been demonstrated with GOL-type membranes. In this work, flux-enhancement of GOL membranes are achieved by increasing layer d-spacing through the placement of multi-branched PEI polymers, of varied molecular weight, between GO layers and by controlling GO oxidation extent.

Not only do PEI

functionalized GOL membranes have enhanced water flux (30-fold compared to the GO membranes), but they also demonstrate better methyl orange (MO) removal performance than GOL controls.

Further, we demonstrate that these assemblies can be specifically modified

for target species.

When these assemblies were functionalized with D-glucono-1,5-lactone

(termed GO@PEI@GDL), enhanced boron removal during filtration is achieved via a wellknown chelation mechanism.

Once saturated, GO@PEI@GDL membranes can be

efficiently regenerated through control of solution pH (i.e. an acidic wash).

Results and Discussion Synthesis and Characterization of GO Materials GO was synthesized by the modified Hummers method, whereby the extent of oxidation was controlled through reaction temperature (detailed information is described in Experimental).15 Slightly oxidized GO (SOGO), moderately oxidized GO (MOGO), and highly oxidized GO (HOGO) were synthesized at 20°C, 30°C, and 50°C, respectively.

The oxidation extent of

GO was determined by XPS via 1S carbon (C1S) peak energies (characterization results are in Fig. S1.).

As-synthesized, GO was then surface functionalized using multi-branched PEI

(GO@PEI).

For this, we prepared HOGO with four different MW of PEI (0.6K, 1.8K, 10K,

and 25K), which are termed [email protected], [email protected], HOGO@PEI10K, and HOGO@PEI25K, respectively.

SOGO, MOGO, and HOGO were also functionalized using

25K MW of PEI (termed here as SOGO@PEI25K, MOGO@PEI25K, and HOGO@PEI25K, 4


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respectively). chelating

In addition, D-glucono-1,5-lactone, containing high density of boron

groups,16

was

HOGO@PEI25K@GDL.

incorporated

into

HOGO@PEI25K,

which

is

termed

Fig. 1(a) shows schematic diagrams of GO, GO@PEI, and

GO@PEI@GDL, along with corresponding TEM images. HOGO@PEI25K, and HOGO@PEI25K@GDL.

We further characterized HOGO,

FTIR transmittance spectra in Fig. 1(b)

identifies key chemical features of GO, GO@PEI, and GO@PEI@GDL.

Specifically, due

to hydroxyl functionalization, GO has a significant OH stretch signal (3400-3200 cm-1).16 After PEI functionalization, GO@PEI demonstrates negligible OH stretch response as amine groups of PEI form covalent amide bonds with the carboxyl or hydroxyl group of GO.17 Upon D-glucono-1,5-lactone functionalization, transmittance of GO@PEI@GDL shows the presence of (new) OH groups, which is due to the fact that D-glucono-1,5-lactone to PEI (on the surface GO) leads to PEI-gluconamide (containing five OH functional groups per gluconamide).18

Carbon 1S XPS data of the GO, GO@PEI, and GO@PEI@GDL matches

FTIR spectra (Fig. 1(c)).

The C1S spectra of GO, GO@PEI, and GO@PEI@GDL, are

deconvoluted into three types of carbon oxidation states, represented as C-C, C-O, and C=O with binding energies centered at 284.5, 286.5, and 287.8 eV, repsectively.

GO contained

45.9%, 47.6%, and 6.6% of C-C, C-O, and C=O, respectively; GO@PEI had 80.1%, 12.6%, and 7.2%, and GO@PEI@GDL possessed 78.5%, 20.0%, and 1.5%.

Flux-enhanced GOL Membranes As shown by others,4,

8, 10

water flux of GOL membrane can be enhanced by control of

interlayer d-spacing (previous studies are summarized in Table S1).

Different from

previous approaches though, we demonstrate multi-branched PEI polymers as simple, flexible, and effective multifunctional spacing agents, as shown in Fig. 2. 5


The thickness of


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GOL increased upon PEI (25K) coating (shown in SEM images in Fig. 2), indicative of layerto-layer distance increasing (d-spacing) as the total mass of GO related materials was held constant.

XRD analysis of GO, GO@PEI, and GO@PEI@GDL are shown in Fig. S2.

As

reported by others, for unmodified GO stacking assemblies, a sharp 2θ response is observed near 11°, whereas for surface functionalized and interlayer modified GO membranes (GO@PEI and GO@PEI@GDL) such a response is not observed, indicating d-spacing(s) are increased and are irregular and/or uneven. We evaluated water flux of GO@PEI membranes using [email protected], [email protected], HOGO@PEI10K, and HOGO@PEI25K along with a control membrane of just HOGO (Fig. 3(a)).

The total masses of GOL were fixed to 5 mg.

As a function of

PEI loading (and PEI MW), the GO@PEI water flux increased dramatically: 8.7, 220.5, 190.0, 239.3, and 274.1 L/m2·hr·bar for HOGO, [email protected], [email protected], HOGO@PEI10K, and HOGO@PEI25K, respectively.

Further, we investigated the

dependency of water flux on the oxidation extent of GO@PEI, using SOGO@PEI25K, MOGO@PEI25K, and HOGO@PEI25K.

As shown in Fig. 3(b), water flux is also

enhanced with increased oxidation extent of GO@PEI.

Water flux was observed to be

127.6, 168.2, and 274.1 L/m2·hr·bar for SOGO@PEI25K, MOGO@PEI25K, and HOGO@PEI25K, respectively.

Increased oxidation of GO also leads to higher PEI

loadings as carboxyl and hydroxyl groups provide additional sites for PEI binding on the surface of GO.

The mass percentage of PEI for GO@PEI was calculated by comparing the

dry masses of GO@PEI vs. GO.

Mass percentage of PEI increased with increasing

oxidation of GO: 0.5%, 4.6%, and 10.9% for SOGO@PEI25K, MOGO@PEI25K, and HOGO@PEI25K, respectively. Separation performance of flux enhanced GOL membranes was also explored as the 6


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increase in d-spacing likely leads to changes in rejection mechanism(s).

Here we evaluated

and compared methyl orange (MO, 327.33 MW) filtration performance for HOGO@PEI25K membranes and HOGO only laminate controls.

The total mass of GOL was fixed to 5 mg

and all other experimental conditions were identical for systematic comparison.

As shown

in Fig. 4, MO rejection rate of HOGO@PEI25K membranes is higher than that of HOGO membranes.

For both HOGO@PEI25K and HOGO membranes, MO rejection rates

decrease with increasing the filtrate volume ratios, as MO becomes concentrated (at near the membrane) for a dead-end system.

While HOGO laminate controls eventually reached ca.

100% MO breakthrough (after 0.5 V/V0, Fig. 4b), HOGO@PEI25K membranes removed MO at a ca. 15 – 30% rate, depending on pressure, once equilibrium was reached (Fig. 4a). Further, water flux and MO solute flux of HOGO@PEI25K were observed to increase (Fig. S3) in a differential manner, with relatively higher MO rejection compared to water (termed dilution effect(s))13, 19

Further, as a function of pressure, here we compare 0.5 and 1.5 bar,

the mass flux ratios of water to MO significantly increases from 8.3 to 68.0 (dimensionless ratio) for HOGO and HOGO@PEI25K, respectively.

GOL Membranes for Boron Treatment We evaluated boron removal performance of the GO@PEI membranes, using HOGO@PEI25K and controls (the GO membrane and support membrane) at pH 7.0 and 9.5. Support membranes (polyethersulfone) alone do not remove any boron, as shown in Fig. S4. Both the GO membrane and GO@PEI membrane removed little (above 1.0 mg/L of boron passed through the membrane) due to the high permeability of boron due to its uncharged (pH 7) or slightly charged (pH 9.5) speciation.20

To address this, we synthesized and

evaluated GO@PEI@GDL membranes. 7



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Based on the Kozeny-Carman equation, water flux is expected to decrease linearly as the number of deposited layers of GO increases (i.e. total flow channel length).21

Here we

quantify boron removal and water flux as a function of the deposited mass of GO@PEI@GDL.

As shown in Fig. 5(a), water flux of 274.1, 206.0, 121.8, and 92.8

L/m2·hr·bar for 5, 10, 20, and 40 mg deposition were observed, respectively.

Though water

flux did decrease with increasing deposition mass, flux decrease did not correspond with the Kozeny-Carman predictions.

We hypothesize that low friction surfaces of reduced GO,

which enhances water flux, is likely the reason disagreement, as others have suggested.22 Boron removal performance of GO@PEI@GDL membrane increases with corresponding mass of GDL is increased, as shown in Fig. 5(b).

PEI-gluconamide is an

excellent boron chelating agent as it forms tetradentate complexes (with boric acid or borate) via four (sterically) optimal OH groups.16

Boron is a pH dependent ion with regard to

sorption and neutrally charged boric acid B(OH)3 dominates at pH below the pKa (9.24), while the borate anion B(OH)4- dominates at pH above the pKa.13

To explore this dynamic,

boron removal was evaluated at pH 5, 7, 9, 9.5, and 11 and removal performance slightly increased with increasing solution pH (Fig. 5(c)).

Coordination number may explain this

behavior; for boron chelation with gluconamide, borate (SP3) has a higher coordination number than boric acid (SP2). 600 mM NaCl (Fig. 5(d)).

Ionic strength effects were also investigated for 100 mM and The effect of salt is negligible, which is in line with other

observations of ionic strength having little effect on boron rejection (by GDL functionalized resins).16

Lastly, GO@PEI@GDL can also be regenerated by simply adjusting flush/wash

solution pH (via 1 N HCl solution (50 mL)).

As presented in Fig. 5(e), boron was

effectively recovered (99%) from the GO@PEI@GDL membrane with only 10 mL of washing solution (1 N HCl).

After that, we evaluated the performance of the regenerated 8


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GO@PEI@GDL membrane.

As shown in Fig. 5(f), after regeneration, the GO@PEI@GDL

membrane performed as well as before regeneration.

Conclusions Increasing the d-spacing of GOL membranes via multi-branched PEI, which is also relatively low cost, is an efficient, flexible, and effective approach for enhanced water flux and solutes (MO) removal.

With regard to boron treatment, D-glucono-1,5-lactone (with PEI)

functionalization is an excellent option, with a high density of boron chelating groups. Further, as demonstrated, membrane regeneration with regard to boron separation, is highly efficient.

Experimental Materials. Ethanol (99.9%), sulfuric acid (H2SO4, 95-98%), graphite (powder, 18.2 MΩ-cm resistivity, 9



Milli-Q, Millipore Corp).

Page 10 of 20

Next, we reheated the mixture at 60°C for 30 min, then added

H2O2 (30%) until gas evolution ceased, in order to ensure the reduction of residual permanganate to soluble manganese ions.

The resulting suspension was filtered, washed

with DI water for 6 times, and dried at 60°C for 24 h to obtain graphite oxide powder. sonicated the graphite oxide power (150 mg) in water (DI, 200 mL) for 2 h.

We After

sonication, the suspension was collected and centrifuged at 10,000 rpm for 2 h.

The

supernatant (GO) was kept in glass bottle. PEI Functionalization. A mixture of 8 mL of GO solution (500 mg/L) and 1 mL of PEI (20 g/L) in the presence of NaOH (2 g/L) was heated at 80°C for 6 h.

After reaction, the

mixture was washed using ethanol (20 ml) and DI water (20 ml), and centrifuged at 8000 rpm for 15 min.

This entire procedure was repeated six times.

The purified PEI-functionalized

GO (GO@PEI) was redispersed in water by probe sonication for 1 h. D-glucono-1,5-lactone Functionalization. A mixture of GO@PEI (5 g), D-glucono-1,5lactone (50 g), and 4-dimethylaminopyridine (4 g) in ethanol (150 mL) was heated at 72°C with vigorous stirring over 24 h.16

As-synthesized samples were washed using ethanol (20

ml) and DI water (20 ml), and centrifuged at 8000 rpm centrifuging for 1 h. processes were repeated until the pH of the solution became neutral.

Cleaning

D-glucono-1,5-lactone

functionalized GO@PEI (GO@PEI@GDL) was stored in DI water and sonicated for 1 h for dispersion. Membrane System. The water flux and boron removal performance of the GOL membrane were explored under a dead end system.

Bulk solution stored in a reservoir (Millipore

Amicon 8050) was directly fed to the membrane, which was mounted in a holder (47 mm, Pall Life Science).

The membrane driving force (pressure) was controlled by nitrogen gas.

Water permeation was measured with an electronic balance (Mettler Toledo ML1502E), with 10


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data automatically logged at 30 s intervals. Methyl orange (MO) Removal Test. MO rejection tests were conducted using HOGO and HOGO@PEI25K membranes.

Initial concentration of MO was 10 ppm at pH 6.0.

MO concentration was measured using a UV−vis spectrophotometer (Varian Bio 50).11 used 463 nm wavelength to measure the MO concentration.

The We

As shown in Fig. S5, the 463

nm peak had a strong linear relationship with MO concentration. Boron Removal Test. Boron removal tests were conducted under constant flux (150 L/m2·hr) except for HOGO membrane, which was tested at 65 L/m2·hr due to its low water flux performance.

Every initial boron concentration is 1.5 mg/L.

Boron concentration was

measured using inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer Optima 7300 DV). X-ray Photoelectron Spectroscope (XPS). An XPS spectrometer PHI 5000 VersaProbe II Scanning ESCA Microprobe (Physical Electronics) was used with a monochromatic aluminum 38.6 W X-ray source and a 200.0 μm X-ray spot size with an energy of 26.00 eV at 45.0°. X-Ray Diffraction (XRD). XRD patterns were measured using a powder diffractometer (Bruker d8 Advance X-ray Diffractometer) with Cu-K radiation (1.54 Å). Fourier Transform Infrared Spectrometer (FTIR). Interaction of IR radiation with the samples was measured by FTIR (Thermo Nicolet Nexus 470, Thermo Scientific) Transmission Electron Microscope (TEM). The TEM images were taken by a Tecnai G2 Spirit Twin microscope (FEI, Hillsboro, OR) operated at 120 kV. Scanning Electron Microscopy (SEM). The SEM images were obtained by a field emission scanning electron microscopy (FESEM, NOVA NanoSEM 230)

11



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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX.

Comparison of water flux of GOL membrane reported in published studies,

Oxidation degrees of graphene oxide, XRD of graphene oxide laminate membranes, flux of methyl orange, UV-vis absorbance for methyl orange, and other data of boron removal

Acknowledgements This work is supported by the U.S. Army Corps of Engineers (W912HZ-13-2-0009-P00001) and the US National Science Foundation (CBET 1437820).

XPS measurements were made

possible by a grant from the U.S. National Science Foundation (EAR-1161543).

TEM,

SEM, and FTIR were provided by the Nano Research Facility (NRF) at Washington University in St. Louis. The authors acknowledge Professor James Ballard for reviewing the manuscript.

12


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Figures

Fig 1. (a) Schematics, TEM images, (b) FTIR, and (c) XPS measurements of GO (HOGO), GO@PEI (HOGO@PEI25K), and GO@PEI@GDL (HOGO@PEI25K@GDL).

13



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Fig 2. Schematics and SEM images of GO membrane, GO@PEI membrane, and GO@PEI@GDL

membrane.

Deposited

mass

of

GO

(HOGO),

GO@PEI

(HOGO@PEI25K), and GO@PEI@GDL (HOGO@PEI25K@GDL) on PES membrane was 5 mg.

14


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Fig 3. Water flux was measured as a function of pressure.

(a) HOGO, [email protected],

[email protected], HOGO@PEI10K, and HOGO@PEI25K. MOGO@PEI25K, and HOGO@PEI25K.

(b) SOGO@PEI25K,

GO deposition mass was 5 mg.

15



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Fig 4. Rejection rate and percentage of methyl orange (MO) as a function of filtrated water volume ratio (volume/volume0) and pressure. 6.0.

Initial MO concentration was 10 ppm at pH

(a) HOGO@PEI25K membranes (b) HOGO membranes.

mL)

16


(Initial volume was 100

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Fig 5. (a) Water flux is shown as a function of pressure for different deposition masses of GO@PEI@GDL (5, 10, 20, and 40 mg).

(b-d and f) Effluent boron concentrations,

expressed volume/volume0, measured for GO@PEI@GDL membrane. concentration and volume were 1.5 mg/L and 50 mL, respectively.

Initial boron

(b) Deposition mass-

dependent tests at pH 7, (c) pH-dependent tests with 10 mg of deposition mass, (d) NaCl concentration-dependent test at pH 7, with 10 mg of deposition mass, (f) comparative boron removal performance of GO@PEI@GDL membrane (40 mg deposition) at pH 7, before and after regeneration.

(e) Effluent boron concentrations as volume/volume0, measured during

regeneration of GO@PEI@GDL membrane (40 mg deposition). for regeneration studies.

17


1 N HCl solution was used


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