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Novel Remediation and Control Technologies
Fabrication of Desalination Membrane with Enhanced Microbial Resistance through Vertical Alignment of Graphene Oxide Xinglin Lu, Xunda Feng, Xuan Zhang, Mary Chukwu, Chinedum O. Osuji, and Menachem Elimelech Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.8b00364 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018
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Environmental Science & Technology Letters
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Fabrication of Desalination Membrane with
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Enhanced Microbial Resistance through Vertical
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Alignment of Graphene Oxide
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Xinglin Lu*, 1, Xunda Feng1, Xuan Zhang2, Mary Chukwu1,
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Chinedum O. Osuji1, and Menachem Elimelech*, 1
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Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, USA 2
Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science & Technology, Nanjing 210094, China
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* Corresponding Authors
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E-mail:
[email protected] (X.L.);
[email protected] (M.E.)
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Abstract
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Biofouling is a major obstacle for efficient and reliable operation of membrane-based
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desalination processes. Innovations in membrane materials and fabrication processes are
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therefore needed to develop antibiofouling strategies. In this study, we utilize the alignability of
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an emerging 2D nanomaterial, graphene oxide (GO), to fabricate a desalination membrane with
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enhanced bacterial resistance. GO nanosheets are dispersed in a polymer solution to form a
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homogenous mixture, which undergoes slow solvent evaporation in a magnetic field to form a
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thin nanocomposite membrane with vertically aligned GO nanosheets. Structural characteristics
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of the fabricated membranes confirm the enhanced exposure of nanosheet edges on the surface
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through the vertical alignment of GO. Notably, the addition and alignment of GO do not
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compromise membrane water permeability and water-salt selectivity. When contacted with
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bacterial cells, membranes with vertically aligned GO nanosheets exhibit enhanced antimicrobial
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activity compared with the non-aligned GO membrane. We attribute the enhanced antimicrobial
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activity to the maximization of edge-mediated interactions through vertical alignment of GO
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nanosheets. Our results suggest the potential of this membrane to delay the onset of biofouling in
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desalination, as well as the promise of the developed platform for the design of antimicrobial
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surfaces for a variety of environmental applications.
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Introduction
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Membrane desalination holds significant promise in addressing the global challenge of water
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scarcity.1 Reverse osmosis (RO), the most-energy efficient desalination process, has already
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become the leading technology in the desalination market.2 The wide application of RO
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desalination technology has also led to the development of other supplemental membrane-based
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desalination technologies, like forward osmosis (FO), which could further expand the realm of
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desalination to the treatment of challenging feed waters (e.g., high salinity or high fouling
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potential waters), thereby achieving greater sustainability (e.g., reduced brine discharge and
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environmental impact).3 Currently, thin-film composite (TFC) membranes, which comprise a
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polyamide selective layer on top of a porous support,4 are the gold standard for RO and FO
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applications,5, 6 owing to their excellent desalination performance (i.e., high water permeability
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and water-salt selectivity).
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Despite the merits of TFC membranes in desalination, biofouling remains the Achilles heel
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in real-world applications of such membranes.7 Biofouling is detrimental to the membrane
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process as it severely decreases water flux and selectivity as well as shortening the lifetime of
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membranes. The intrinsic surface properties of TFC membranes,8, 9 including hydrophobicity,
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roughness, and native carboxyl groups, make this type of membrane intrinsically prone to
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biofouling. Additionally, the highly crosslinked polyamide selective layer is susceptible to attack
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by oxidants, thereby excluding the use of disinfectants, such as chlorine or ozone, as membrane-
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cleaning agents to prevent the growth of biofilm.10 Therefore, substantial efforts have been
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devoted to developing antibiofouling strategies for desalination membranes, including the use of
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biocide-releasing nanomaterials (e.g., silver11 or polymer particles12) or surface chemical
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functionalization (e.g., quaternary ammonium13 or zwitterion polymer brushes14).
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Graphene is an emerging 2-D material with extraordinary physiochemical properties,
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enabling a range of innovations in material science.15 Recent studies have also revealed the
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contact-mediated cytotoxicity of graphene-based nanomaterials (GBNs) toward a range of
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microorganisms,16-18 which allows for a non-depleting and chemical-free antimicrobial strategy.
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Therefore, the cytotoxicity of GBNs has been harnessed to mitigate biofouling on a variety of
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engineered surfaces,19-21 including desalination membranes22-27
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Further investigations into the physical and chemical mechanisms of graphene-induced -3-
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cytotoxicity highlight the significant contribution of nanosheet edges,28, 29 which facilitate both
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physical piercing and chemical oxidation effects to inactivate bacteria through intimate contact. 28,
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to fully realize the antimicrobial activity of GBNs. Current approaches to introducing GBNs on
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membrane surfaces were either through surface functionalization24,
These insights imply that enhancement of edge exposure on the membrane surface is beneficial
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or incorporation in the
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membrane fabrication process
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resistance, the majority of nanosheets were distributed on the surface with uncontrolled
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orientations or were embedded in the bulk of membranes, thereby obscuring the contribution of
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edge-mediated interactions.
. While these approaches have yielded enhanced biofouling
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Here, we exploit the alignability of graphene oxide (GO) in a magnetic field31, 32 to fabricate
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a desalination membrane with maximized edge exposure and enhanced antimicrobial activity. A
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mixture comprising GO, polymer, and solvent undergoes a slow solvent evaporation process in a
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magnetic field to form a thin composite membrane with vertically aligned GO nanosheets.
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Material properties of the fabricated membranes are extensively characterized to confirm the
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enhanced exposure of nanosheet edges on the surface through the vertical alignment of GO.
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Additionally, performance evaluation using an FO setup demonstrates that the addition and
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alignment of GO do not compromise membrane water flux and water-salt selectivity. When
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contacted with a model bacterium, E. coli, the composite membrane with vertically-aligned GO
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exhibits enhanced antimicrobial activity compared with the non-aligned GO membrane. Our
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results demonstrate a new strategy in fabricating GO-enabled membrane material for biofouling
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mitigation in desalination, which also holds potential for the design of other engineered surfaces.
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Materials and Methods
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Synthesis of PODH Polymer. Polyoxadiazole-co-hydrazide (PODH) was synthesized
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through solution polycondensation, adapting methods from previous studies.33, 34 In brief, 4,4′-
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oxybis(benzoic acid) (OBBA, 1.65 g, 6.4 mmol, Alfa Aesar), hydrazine sulfate (1.0 g, 7.7 mmol,
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Alfa Aesar), and polyphosphoric acid (26.0 g, Sigma-Aldrich) were added to a three-neck flask.
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The reactant mixture was heated to 160 °C under vigorous stirring for 3.5 h. Then, the viscous
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solution was poured into deionized (DI) water (18.2 MΩ·cm, Millipore-Q) to isolate the fiber-
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like PODH polymer, which was subsequently rinsed with 0.5 M NaOH and DI water to -4-
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neutralize the residual acid and remove unreacted monomers. The rinsed PODH polymer was
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vacuum dried at 80 °C for 24 h and stored in a desiccator. The synthesized polymer would
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display characteristic peaks in Fourier transform infrared spectroscopy (FTIR) and 1H nuclear
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magnetic resonance (NMR) spectra in following positions. FTIR (υ, cm−1): 1670 (C=O), 1605,
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1468 (C–C linkage of aromatic rings), 1237 (C−O−C from OBBA), 1085, 1030 (C−O−C from
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oxadiazole ring); 1H NMR (δ, DMSO-d6, ppm): 8.5 (hydrogen of −NH−NH−), 8.3−8.0 (hydrogen
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of an aromatic ring in polyoxadiazole, Ar−H), 7.4−7.0 (hydrogen of an aromatic ring in
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hydrazide, Ar−H).
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Fabrication of GO/PODH Composite Membrane with Vertically Aligned
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Nanosheets. N,N-dimethylformamide (DMF, 99.5%, Sigma-Aldrich) was added to an aqueous
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GO suspension (6.2 g L-1, Graphene Supermarket, material properties provided in the Supporting
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Information, Table S1 and Figure S1) and was subsequently distilled in a rotary vacuum
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evaporator (Heating Batch B-100, Buchi) under controlled conditions (55 °C,
