Modification of microfiltration membranes by alkoxysilane polycondensation induced quaternary ammonium compounds grafting for biofouling mitigation
Graphical abstract
Introduction
In recent years, membrane-based technologies are widely adopted for water and wastewater treatment due to their modularity, reliability, and high-efficiency [1]. However, membrane fouling, particularly biofouling, calls for extensive efforts to develop antibiofouling protocols to mitigate bacterial adhesion, colonization and subsequently biofilm formation on membrane surface [2], [3], [4]. Attempts have been dedicated to biofouling mitigation using biological-based control strategies (e.g., quorum quenching [5], enzymatic disruption [6] or energy uncoupling [7]) and membrane modification [8], [9], [10]. The latter has attracted growing interest due to high-efficiency for biofouling mitigation through constructing antibiofouling membrane surfaces.
Recently, a variety of membrane surface modification methods have emerged for the fabrication of antiadhesive [11], [12], [13] and antibacterial surfaces [14], [15], [16]. It is generally accepted that the antiadhesive protocol involves membrane surfaces modified by hydrophilic materials, such as poly(ethylene glycol) (PEG)-based [17], [18], zwitterionic [19], [20] and glycomimetic [21] compounds. However, this approach is ineffective in preventing bacterial colonization. An attractive alternative is to construct antibacterial membrane surfaces that inactivate microbes and inhibit colonization. This strategy typically involves membrane functionalization with antibacterial agent based on releasable bacteria-killing (such as silver [22], [23], crystalline titanium dioxide [24], [25] and copper nanoparticles [26]) and membrane decoration with bactericidal functionalities for contact killing (e.g., quaternary ammonium compounds (QAC) [27], [28], [29], graphene oxide [30], [31], [32] and carbon nanotubes [33], [34]).
QAC is known to disrupt cell membranes of microorganisms upon contact and has thus been considered as an effective antibiofouling agent [2], [35]. Nevertheless, the traditional approach of grafting/introducing long alkyl chains of QAC often leads to impaired membrane hydrophilicity and often reduced membrane permeability [15], [36]. This inherent disadvantage prompts us to explore alternative ways for incorporating QAC into polymeric membranes.
In this study, we report a new protocol to graft QAC onto polymeric membrane surface. A controlled architecture was created by initially coating a robust polydopamine (PDA)/polyethylenimine (PEI) layer which is beneficial to polycondensation of silicic acid for its abundant positively-charged functional groups, followed by in situ synthesizing a hydrophilic silica nanoparticle layer through silification reaction and then immobilizing QAC on the silica-decorated membrane to form an antibacterial layer via alkoxysilane polycondensation. The presence of silica nanoparticle (NP) layer not only counteracts the negative impact by QAC (e.g., impaired membrane hydrophilicity) but also enables the covalent-bonded QAC on the silica layer to have strong stability, thus providing a highly antibiofouling efficiency. This new QAC-grafting method offers a new dimension for membrane surface modification to possess antibiofouling capability.
Section snippets
Materials and chemicals
A commercial polyvinylidene fluoride (PVDF) microfiltration membrane, received from Millipore (GVWP09050, pore size = 0.22 µm, USA), was used as the base membrane for modification. Prior to use, the membranes were washed by acetone overnight to remove impurities adsorbed on the membrane surfaces, rinsed with deionized (DI) water for about 4 h, and then dried in a vacuum oven at 40 °C to a constant weight.
All chemicals used in this work were of analytical grade unless specified otherwise.
Membrane characterization
Surface chemistries of the pristine and modified membranes were analyzed by ATR-FTIR spectroscopy and XPS. The MD membrane (Fig. 2A) exhibits absorption peaks at 1666 cm−1 and 1539 cm−1, corresponding to N-C-O and C-N-H vibrations of the amide groups in PDA/PEI layer [44], respectively. The peaks located at 1150 cm−1 are associated with Si-O-C groups in MD-Si membranes [15], whereas a new intense band at 1550 cm−1 in the spectra of MD-Si-Q membranes is attributed to the presence of quaternary
Conclusions
The novel approach developed in this study mitigates membrane biofouling via immobilizing QAC onto a silica-decorated membrane surface. This grafting protocol provides a benign and robust method for fabricating antibiofouling membranes with controlled architecture and excellent antibacterial ability without compromising intrinsic membrane separation properties. The grafted QAC also exhibited high stability under chemical cleaning conditions, presenting a long-lasting antibiofouling efficiency.
Acknowledgements
We thank the National Natural Science Foundation of China (Grants 51378371 and 51678423) and the Fundamental Research Funds for the Central Universities of China (Grant 0400219373) for the financial support of the work.
Associated contents
Supporting information
The Supporting Information is available free of charge on the website.
Membrane rejection behaviors (Section S1); Membrane flux recovery tests (Section S2); Picture and schematic of a custom-made cell for membrane preparation (Fig. S1); Images of the pristine membrane M0 and the PDA/PEI-modified membrane MD (Fig. S2); XPS spectra of modified membranes (Fig. S3); Pore size, SA rejection and flux recovery of the control and modified membranes (Fig. S4); SEM images of MD-Si-Q membrane before
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