Functionalization of ultrafiltration membrane with polyampholyte hydrogel and graphene oxide to achieve dual antifouling and antibacterial properties
Graphical abstract
Introduction
Wastewater purification and reuse are becoming an attractive alternative water resource, mainly for agricultural production, and its use is expected to grow in the near future owing to water scarcity worldwide [1], [2], [3], [4]. Membrane filtration, particularly low-pressure ultrafiltration (UF) and microfiltration (MF), is one of the most advanced, cost-effective, and sustainable wastewater technologies for direct treatment of wastewater [5], tertiary treatment of secondary effluents [6], use in membrane bioreactors (MBRs) [7], and use as a pretreatment step before wastewater desalination [8]. However, one of the main limitations of the most commonly used polymeric UF membranes, such as polyethersulfone (PES) and polyvinyl difluoride (PVDF), is the high fouling propensity of the membrane resulting from its intrinsic hydrophobicity [9], [10], [11]. Membrane fouling is caused by the adsorption and deposition of colloidal particles, suspended solids, macromolecules, and microorganisms on the membrane surface [12], [13]. Fouling results in a severe flux decline, increase in the energy consumption, and shorter membrane lifetime because of frequent chemical cleaning. These negative effects enhance the operational costs and hinder the wide-scale application of membrane-based technologies for wastewater treatment.
Increasing the surface hydrophilicity is an efficient approach to reducing membrane organic fouling and mitigating bacteria adhesion. These beneficial properties are typically attributed to the formation of a hydration layer, which imposes a thermodynamic and steric barrier against the adsorption of foulants [14], [15], [16]. Numerous studies have reported the antifouling modification of hydrophobic UF membranes to enhance hydrophilicity through direct surface coating or chemical grafting of hydrophilic polymers, such as polydopamine (PDA) [17], poly(ethylene glycol) (PEG) [18], zwitterionic polymers (e.g., sulfobetaine) [19], or hydrophilic nanomaterials such as TiO2 [20], ZnO [21], and silica nanoparticles [22]. Although surface antifouling modification using hydrophilic materials successfully delay or even prevent organic fouling and limit initial bacterial deposition, these modifications still cannot completely inactivate bacteria that inevitably attach to the membrane surface, which eventually leads to formation of a biofilm [23]. Thus, developing novel membranes by introducing bactericidal capabilities to the antifouling coating to achieve so-called dual functionality is a promising strategy in reducing both organic fouling and biofouling [24], [25].
Xu et al. entrapped polysulfone membrane with Ag/Cu2O hybrid nanowires to obtain fouling resistance and antimicrobial properties [26]. In another study, Zou et al. synthesized a bifunctional membrane by integrating a zwitterion with antimicrobial peptides [27]. More recently, Wu et al. incorporated a silver-polydopamine nanohybrid in the polysulfone membrane matrix, which resulted in excellent antibacterial and antibiofouling properties [28]. These results demonstrate the great advantage of dual functionality. However, the antibacterial strategies in these studies relied on either dissolution-dependent (i.e., depleting) nanoparticles or environmentally sensitive peptides, which easily lose their antimicrobial activity over time in a complex solution such as wastewater [29]. In addition, the release of biocides is also associated with potential impact to the environment [30].
Graphene oxide (GO), a two-dimensional carbon material, consists of a high density of covalently attached oxygen-containing functional groups on its edges and basal planes, which make it highly hydrophilic [31], [32], [33]. Various studies have reported that introducing GO to membranes, either by blending in the bulk or grafting GO to the surface, improves the membrane antifouling property by reducing the surface roughness and hydrophobicity [34], [35]. GO has also exhibited toxicity toward bacterial cells, thereby effectively inhibiting bacterial growth. Unlike many antibacterial nanoparticles, the antibacterial activity of GO nanosheets is not driven by dissolution but instead by contact between the GO and bacterial cells [36], [37]. The cytotoxicity of GO against a wide variety of microorganisms has been attributed to the disrupted bacteria cell integrity resulting from the atomically sharp edges of GO, which penetrate the bacterial cell [38], [39], [40], and to oxidative reaction with the cell membrane mediated by GO nanosheets [41], [42], [43]. Also, GO is a low-cost, easy to scale, and dispersible and stable in aqueous solution, making it an attractive antibacterial material [44], as demonstrated in many applications, including membranes [14], [30].
Herein, a two-step surface modification strategy was developed for the preparation of a dual antifouling and antibacterial coating for a UF PES membrane. In this approach, unlike the common blending method, only the membrane surface is modified, thereby allowing independent optimization of the highly antifouling zwitterion hydrogel and the antibacterial GO. First, a zwitterionic polyampholyte hydrogel was grafted on the surface of a PES membrane by UV-photoinitiation. Then, GO nanosheets were loaded into the swollen polyampholyte hydrogel via vacuum filtration. The successful incorporation of the GO into the grafted hydrogel was shown by a series of characterization methods (i.e., Raman, SEM, ATR-FTIR, and contact angle measurements) as well as visually by incorporating GO nanosheets into a bulk hydrogel. The antifouling of the dual coating was demonstrated by static adsorption and filtration experiments using protein solution as a model organic foulant, and soluble microbial products extracted from an MBR and a solution containing bacteria intracellular products as real foulants. The antibacterial property of the modified membrane was demonstrated using the contact killing method and filtration experiments. The results clearly confirmed that the dual membrane functionalization with GO and zwitterion hydrogel is a promising strategy to prevent both organic fouling and biofouling in UF applications.
Section snippets
Chemicals and materials
PES polymer was obtained from BASF, Germany, and N-methyl-2-pyrrolidone (NMP) was purchased from J.T. Baker, USA. Vinyl sulfonic acid (VSA) was purchased from Tokyo Chemical Industry, Japan, and [2-(methacryloyloxy) ethyl] trimethylammonium chloride (METMAC), N, N′-Methylenebis (acrylamide) (Bis), sulfuric acid (H2SO4), graphite powder (200 mesh), and bovine serum albumin (BSA) were purchased from Sigma. Na2HPO4 and KCl were purchased from Merck, USA, and HCl (37%), CaCl2, and NaCl were
Membrane surface characteristics
Fig. 1a presents the surface Raman spectra of the three membranes. A new peak at 1360 cm−1, attributed to the specific D band of the GO, appeared on the GO-p-PES membrane surface, confirming the successful loading of GO into the zwitterionic polyampholyte hydrogel matrix [38], [41]. The changes in the membrane surface morphology before and after immobilization of the GO nanosheets (~ 30 nm, as measured by DLS, see Fig. S2) into the hydrogel were further investigated using SEM. From Figs. 1b and
Conclusion
This study demonstrated that grafting of a zwitterionic polyampholyte hydrogel PES membrane followed by loading of the hydrogel with GO nanosheets can be employed as an effective method to produce a GO-functionalized zwitterionic polyampholyte UF membrane. The successful preparation of the GO-p-PES membrane was confirmed by Raman spectroscopy, SEM, and ATR-FTIR analyses. Contact angle measurements demonstrated that the hydrophilicity of the GO-p-PES membrane increased due to the loaded GO into
Acknowledgements
We acknowledge financial support from the United States-Israel Binational Agricultural Research and Development Fund (BARD), Grant IS-4977-16. AB is thankful for partial support from The Israel Science Foundation's Grant No. 373/16. W.Z. is supported by the Kreitman Negev Scholarship for Distinguished Ph.D. Students from BGU. W.C. is supported by the China Scholarship Council for providing a graduate fellowship.
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