Guanidyl-functionalized graphene/polysulfone mixed matrix ultrafiltration membrane with superior permselective, antifouling and antibacterial properties for water treatment

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Abstract

Mixed matrix membranes blended with graphene-based nanomaterials have great potential in water and wastewater treatment on account of their multiple functionalities. To solve the complicated biofouling problem and diversify the applications of membranes, novel synergistic antibacterial guanidyl-functionalized graphene/polysulfone (GFG/PSF) mixed matrix ultrafiltration membranes were prepared by a non-solvent induced phase separation method. The guanidyl-functionalized graphene nanosheets were achieved by a two-step grafting process consisting of amination and guanidination and exhibited high dispersibility in the casting solution, which showed good compatibility with the polymer matrix. Besides the advantages of partially reduced graphene oxide (GO) nanosheets in creating a stronger interaction with the bacterial cell membrane to destroy the bacteria, the induced bidendate binding between guanidyl groups and phosphate groups on the cell wall can make high sterilization rate even at low concentrations. Different techniques including XRD, FTIR, XPS, SEM, TEM, EDX, contact angle meter, filtration and antibacterial experiments were employed to characterize and investigate the performance of nanosheets and membranes. Compared with pure PSF membrane, the GFG/PSF mixed matrix membranes not only exhibited superior permeability and prominent antifouling property performance toward bovine serum albumin (BSA), but also displayed excellent antimicrobial activity and long-term duration toward Escherichia coli and Staphylococcus aureus.

Introduction

With the development of material science and nanotechnology, low-pressure membrane processes such as microfiltration, ultrafiltration (UF) and nanofiltration (NF) have played an important role in alleviating the extreme shortage of water resources and serious environmental pollution [1], [2], [3]. Ultrafiltration membrane with pore size between microfiltration and nanofiltration can effectively remove smaller colloids, proteins and other macromolecules in water and wastewater treatment. However, the membrane fouling problems severely limit the application of membrane technology [4], [5], [6]. In general, membrane fouling can be divided into the following four types: organic pollution, inorganic pollution, particulate/colloid pollution and biological contamination [7]. Among them, biological pollution is the most complicated pollution phenomenon, in which the bacteria irreversibly adhere to the membrane and rapidly grow up to form a biomembrane [8]. The biomembrane will gradually evolve into a layer of extracellular polymeric substances (EPS) containing polysaccharides, proteins, glycoproteins, lipoproteins and other biomacromolecules which are very difficult to remove. Usually, the formation of biomembranes may sharply decrease the permeability of the membrane [9]. Although several technological ways for controlling membrane biofouling have been explored and applied to some extent, it is still very hard to completely eliminate bacteria simply by feed pretreatment or/and membrane cleaning [10]. Therefore, seeking new materials and effective approaches to solve the biofouling problem of membranes is of paramount importance.

Recently, the preparation of organic-inorganic hybrid membranes by blending has become a hot topic for improving the performance of polymer membranes. Compared with traditional polymer membranes, the polymer membranes blended with nanomaterials such as GO [11], Al2O3 [12], multiwalled carbon nanotubes (MWCNTs) [13], ZnO [14] and TiO2 [15] exhibit better permeation and/or antifouling performance. Among them, GO has received increasing attention due to its outstanding physicochemical properties such as electrical conductivity, thermal conductivity, high surface area and mechanical strength [16], [17]. Meanwhile, graphene and graphene-based nanomaterials can induce bacterial inactivation by wrapping bacteria from the environment, pierce into bacterial cells by extracting phospholipids from the edges and induce oxidative stress, showing the potential to prevent biological pollution [18]. However, like other nanomaterials, GO material has a big tendency to aggregate in organic solvents, leading to deterioration of materials and consequently degrading the permeability and antifouling characteristics of graphene-based composite membranes. Accordingly, uniform dispersion of nanofillers in the mixed matrix membranes and the compatibility between nanoparticles and polymer matrix are the key points to achieve high-quality ultrafiltration membranes.

In this regard, surface functionalization has been attempted to endow graphene-based nanosheets with suitable dispersibility. Lang et al. prepared modified polyvinylidene fluoride (PVDF)/perfluorosulfonic acid (PFSA) hollow fiber ultrafiltration membrane by mixing with sulfonated graphene oxide (SGO) nanosheets. Due to the strong charge of the sulfonic acid group, the miscibility between GO and the polymer matrix was improved. The composite membrane exhibited an increased pure water flux of 174.2 L m−2 h−1 [19]. Zhang et al. synthesized a hyperbranched polyethylenimine (HPEI) functionalized GO and mixed the HPEI-GO into polyethersulfone casting solution to fabricate a hybrid ultrafiltration membrane by phase inversion method. HPEI was used to enhance the compatibility between polymer matrix and GO nanosheets, and the mixed matrix membrane displayed a preferable antifouling and antibacterial properties against E. coli because of the added HPEI-GO nanosheets, but the pure water flux of the mixed matrix membrane was reduced to 77.4% compare to the original membrane [20]. Although some useful progresses were made in the above preliminary researches, there are very limited reports aiming to tackle the uniformity problem in the fabrication of GO-based ultrafiltration membranes. Moreover, how to effectively enhance the permeability together with enough antibacterial function through the surface modification of GO nanosheets still remains a big challenge.

To step forward, the searching and introduction of nanomaterials modified with specific functional groups into the hybrid membranes tends to be urgent need. According to our previous study [21], we have demonstrated that the pure water flux of hybrid ultrafiltration membrane with HEMA (2-hydroxyethyl methacrylate) grafted TiO2 nanoparticles increased to 3 times that of the pure polysulfone membrane, which also showed enhanced antifouling properties. It seems that the modification of nanomaterials through grafting organic functional groups should be an effective way to enhance the compatibility between the organic and inorganic materials and thereby effectively improve the permeation and antifouling performance of mixed matrix membranes. As guanidyl polymers have attracted numerous attentions as an effective cationic antimicrobial material with low toxicity in recent years [22], we go to think if guanidyl-functionalized graphene nanosheets can be put into the polymer matrix to achieve high-quality ultrafiltration membranes. Because of the strong non-covalent interaction between the guanidyl group and the phosphate group, the guanidyl group was used as an excellent modifying group for phosphopeptide enrichment. The surface of the bacterial cell wall contains a large number of phosphate groups, and thus the bidendate binding between guanidyl groups and phosphate groups on the cell wall can make high sterilization rate even at low concentrations [23], [24], [25]. Furthermore, guanidyl-functionalized graphene nanosheet can be synthesized to selectively adsorb monophosphate peptides and polyphosphate peptides as a novel bifunctional adsorbent. There are abundant oxygen-containing functional groups on the surface of GO nanosheets, which can be employed as active sites for grafting guanidyl groups [26]. Based on the affinity between the guanidyl group and the phosphopeptide, the guanidyl group may serve as aspecific functional group to increase the compatibility of GO and polymer matrix. Meanwhile, the grafting of guanidyl group can greatly reduce the van der Waals force between the GO sheets, making the single-layer graphene oxide sheet more easily disperse in the polymer casting solution. To date, guanidyl-functionalized graphene as nanofiller has not been applied in the fabrication of mixed matrix membrane.

In this study, we prepared a synergistic antibacterial GFG/PSF mixed matrix membrane by blending guanidyl-functionalized graphene nanosheets into PSF matrix via a phase inversion method. The guanidyl-functionalized graphene nanosheets were obtained by a two-step grafting process consisting of amination and guanidination. After modification, the alkyl chain on the graphene sheets can be used as a soft segment to increase the flexibility and usability of the guanidyl group, resulting in better mixing of GFG and PSF matrix and improved porosity. The effects of the introduced GFG nanosheets on morphology and structure of prepared membranes were evaluated in detail. The separation and antifouling performance of the GFG/PSF mixed matrix membranes were measured by the filtration test of pure water and BSA. The antibacterial properties of the membrane were determined by the inhibition rate against Escherichia coli (E. coli, Gram-negative) and Staphylococcus aureus (S. aureus, Gram-positive).

Section snippets

Materials

All chemicals used were at least of analytical grade. Polyvinylpyrrolidone (PVP, K-30), Thionyl chloride (SOCl2, 99%), 1,6-hexanediamine, Trifluoroacetic acid (TFA, 99%), N,N-Dimethylformamide (DMF, 99.5%) and N,N-dimethylacetamide (DMAc, 99%) was purchased from Aladdin Chemical Reagent Co. O-methylisourea hemisulfate (OMIU, 95%) were purchased from Beijing J&K Scientific Co. Deionized water was obtained by a self-made RO/EDI system with ion contents analyzed and controlled by IRIS Intrepid ICP

Sedimentation test

To observe the dispersibility of GO and GFG nanosheets in organic solvent DMAc, solutions containing GO and GFG nanosheets were first sonicated for 0.5 h and then stood for 12 h. As shown in Fig. 3, the GO nanosheets were easy to agglomerate and formed a deposited layer at the bottom. However, GFG nanosheets were uniformly dispersed in DMAc solution. This observation clearly indicates that grafting guanidyl group on the surface of the GO can effectively prevent the stacking of the GO nanosheets.

Conclusions

To solve the complicated biofouling problem and diversify the applications of membranes, guanidyl-functionalized graphene nanosheets were successfully introduced into polysulfone mixed matrix ultrafiltration membrane via non-solvent induced phase inversion process. The guanidyl-modification of graphene material affected the distribution of nanocomposites in organic casting solution and the formation of ultrafiltration membrane which impressed the characteristics of hybrid membranes. The

Acknowledgements

We thank for financial support the National Natural Science Foundation of China (Grant Nos. 21476206 and 21736009), the Zhejiang Provincial Natural Science Foundation of China (Grant No. LY18B060010) and the Minjiang Scholarship from Fujian Provincial Government. A patent application related to this work has been filed.

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    Co-corresponding author at: Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang 310027, PR China.

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