Anti-Fouling Ultrafiltration/Microfiltration (UF/MF) Membranes Based on Interfacial Assembly of Functional Materials to Membrane Surface

Abstract

Membrane filtration processes are a relatively simple and cost-effective method to obtain high-quality purified water. Membrane processes are typically classified into four categories i.e., reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) by their pore size and operation pressure. Among them, UF and MF processes have been widely used in various industrial and domestic fields due to purified water productivity and affordable water quality. However, membrane fouling, which is defined as the deposition of contaminants (organic, biological and inorganic materials) on the membrane surface or into membrane pores, decreases the membrane performance and increases operating cost by additional process such as membrane cleaning or replacement. In order to solve membrane fouling phenomena, a variety of reactive functional materials have been introduced to the UF/MF membranes. Nonetheless, these membranes gradually lose anti-fouling properties by leaking the anti-fouling layers due to lack of interaction between the anti-fouling agent and the membrane surface or accumulating irreversible contaminants on the membrane surface. To overcome these limitations, in this study, anti-fouling materials were introduced onto the membrane surface with strong interactions to prepare sustainable anti-fouling active membrane. In addition, reversibly functionalized surface layers were introduced onto the membrane surface by thermo-responsive covalent bonding in order to provide regenerable and convertible functionalities to the membrane. First, an anti-scaling membrane was developed by introducing a high-density positive charge to a poly(vinylidene fluoride) (PVDF) membrane to suppress membrane scaling in Ca2+/silica-rich wastewater. Positively charged modifiers were synthesized by conjugating an amphiphilic polymer (Brij S10) and branched poly(ethylene imine) (b-PEI) at various molar ratios, and these were then implanted to PVDF membranes during the phase-inversion process. ATR FT-IR spectra revealed that the positive modifiers successfully anchored onto the surface of the membrane by hydrophilic-hydrophobic phase recognition. As introducing the positive charge on the membrane surface, the membranes showed positive surface charge and their pure water permeability (PWP) increased due to the protonation of b-PEI. Anti-scaling properties were also confirmed to be improved by filtration tests using a Ca2+/silica-rich feed solution, which results from the repulsion of metal ion by the positively charged branch on the membrane. In addition, the water flux recovery by simple membrane backwashing of the modified membrane was double that of the neat PVDF membrane. Second, a sustainable anti-biofouling membrane was developed by covalently immobilizing silver nanoparticles (Ag NPs) onto PVDF membrane surface mediated by a thiol-end functionalized linker. FE-SEM and EDXS measurements revealed that the Ag NPs were highly bound and dispersed to the PVDF membrane due to the strong affinity of the Ag NPs with the thiol-modified linkers, which had been anchored to the PVDF membrane. The membrane performed well under water permeability and particle rejection measurements, despite the high deposition of AgNPs on the surface of membrane. The Ag-PVDF membrane nanocomposite significantly inhibited the growth of bacteria on the membrane surface, resulting in enhanced anti-biofouling property. Importantly, the Ag NPs were not released from the membrane surface due to the robust covalent bond between the Ag NPs and the thiolated PVDF membrane. Third, a regenerable anti-fouling membrane was developed via the formation of a dynamic peel-and-stick of hydrophilic poly(ethylene glycol) (PEG) layer onto the surface of a poly(tetrafluoroethylene) (PTFE) membrane, using thermo-responsive reversible covalent bonding. In order to attach a peelable-and-stickable hydrophilic layer onto a membrane surface, a maleimide end-modified PEG layer was coupled with a furan-modified PTFE membrane by reversible Diels-Alder (DA) cycloaddition reaction. The combined results of ATR FT-IR, XPS and FE-SEM measurements clearly revealed that the maleimide end-modified PEG was successfully coupled with the furan-modified PTFE membrane surface by DA reaction. In addition, the hydrophilic PEG layer was readily and repeatedly reformed on the membrane surface by a thermally driven dynamic peel-and-stick process. The PEG-coupled PTFE membrane showed effective anti-fouling performance against a highly concentrated silica colloidal aqueous solution. In particular, the anti-fouling property was remarkably recovered after regeneration of the hydrophilic layer through the peel-and-stick process. Finally, a convertible membrane platform was verified by changing membrane surface introduced materials using dynamic peel-and-stick process. SiO2 and Ag NPs were selected as membrane surface modifying materials to prepare inorganic and metallic nanomaterials surface functionalized membrane, respectively. The combined results of FT-IR and 1H NMR analyses showed that the maleimide derivatives were successfully synthesized. They were effectively functionalized at the surface of SiO2 and Ag NPs for introducing maleimide moiety. The various surface analyses results indicated that the SiO2 and Ag NPs were successfully coupled with the furan-modified PTFE membrane by DA reaction. As expected, the inorganic and metallic layers were repeatedly regenerated on the surface of the membrane through the peel-and-stick process, which was verified by ATR FT-IR, XPS, FE-SEM/EDXS results. In particular, surface chemical and morphological analyses were disclosed that the surface introduced functionalities were converted into desired other functionalities by the peel-and-stick process. In this study, sustainable anti-fouling membranes have been developed by introducing various anti-fouling materials i.e, cationic, biocidal, hydrophilic materials, to UF/MF membranes with strong attractive force. In particular, anti-fouling membrane prepared though thermo-reversible bonding can overcome irreversible membrane fouling which is considered as a limitation for conventional anti-fouling membranes and convert surface functionality. Therefore, it is expected that this approach opens up the possibility a novel platformable membrane separating system.