Antifouling enhancement of poly(vinylidene fluoride) microfiltration membrane by adding Mg(OH)2 nanoparticles

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Abstract

Mg(OH)2 nanoparticles were mixed into poly(vinylidene fluoride) (PVDF) with PEG as an additive to prepare hybrid membrane by phase inversion. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) measurements were performed, and the ultrasonic stability, porosity, hydrophilicity, permeation and bovine serum albumin (BSA) and Escherichia coli (E. coli) adsorption of the membrane were investigated. As revealed by FTIR spectra, large amount of –OH groups were exist in the hybrid membrane due to the addition of Mg(OH)2 nanoparticles, and these –OH groups are responsible for the hydrophilicity increase of the modified membrane. BSA and E. coli adsorption measurements confirm that hybrid membrane exhibited higher antifouling property than the original PVDF membrane, as the number of adsorbed proteins and bacterial cells on the membrane surface were reduced dramatically. PEG and Mg(OH)2 had mutual effects on the resulting structures and properties including porosity and permeability of the membrane. The superiority of the PVDF/Mg(OH)2 membrane in filtering bacterial solution was significant, as the resistances with respect to time decreased dramatically with 10 wt.% Mg(OH)2 added, suggesting it would be very effective in preventing flux losses caused by biofilm formation.

Highlights

► Novel PVDF/Mg(OH)2 hybrid membrane was prepared by phase inversion. ► PVDF/Mg(OH)2 hybrid membrane exhibits good antifouling property. ► The added Mg(OH)2 nanoparticles alter the structure and properties of membrane. ► The PEG and Mg(OH)2 have mutual effects on the resulting properties of membrane.

Introduction

Poly(vinylidene fluoride) (PVDF) is a well known polymer for its excellent chemical and oxidation resistance, and good physical and thermal stability. All these properties make PVDF an attractive membrane material for microfiltration [1], ultrafiltration [2], pervaporation [3] and membrane distillation [4]. However, due to its inherent hydrophobic characteristic, PVDF membrane often suffers serious flux losses due to membrane fouling.

Membrane fouling, which is a severe problem for liquid membrane separations driven by pressure, limits the industrialization of the membrane technology. A number of novel strategies for fouling mitigation have emerged, including the use of additives [5], sludge granulation [6], hydrodynamic optimization [7], and membrane surface modification [8]. With the advent of nanotechnology, investigations on the effect of nanomaterials on biofilms and fouling have been reported [9]. Fullerene C60 [10] and silver nanoparticles [11] have been studied for their interaction with the biofilm. Recent works have also focused on blending inorganic nanomaterials to reduce irreversible membrane fouling, such as SiO2 [12], TiO2 [13], Al2O3 [14] and so on, leading to an increase in membrane permeability and a better control of membrane surface properties.

The phase inversion process is one of the most common methods in the preparation of synthetic polymeric membranes. The casting solution is cast as a thin film and immersed into a non-solvent coagulation bath. Due to the exchange of solvent and non-solvent, phase separation of casting solution is induced to form polymer-rich phase and polymer-lean phase, and the membrane is thereby obtained [15]. The additives in the casting solution affect the morphology and performance of the prepared membrane. Among them, polyethylene glycol (PEG) is known to promote pore formation in the polymeric membranes and thus enhance the permeation properties [16], [17], [18].

Mg(OH)2, often regarded as a good example of a material with structurally bound OH group, creates significant interest in the role played by the OH groups [19]. It is an important and widely used inorganic flame retardant due to its environmental favored, readily available and low cost properties [20]. In our previous work, we found that Mg(OH)2 nanoplatelets were effective antibacterial agents against several bacteria and successfully prepared antibacterial paper [21], [22]. In the present study, hydrophilic Mg(OH)2 nanoparticles were used as inorganic fillers to prepare PVDF/Mg(OH)2 hybrid membrane by phase inversion method in order to prevent flux losses caused by membrane bio-fouling. The hybrid membranes were characterized, and the effects of the additive of Mg(OH)2 nanoparticles were also discussed.

Section snippets

Materials

PVDF powder was purchased from General Electric Plastics (USA) and dried in a vacuum oven at 105 °C for 10 h prior to be used. PEG with molecular weight of 600 Da, an additive as pore-former on the structure formation of membranes, was purchased from Damao Chemical Co. (Tianjin, China). Fig. 1 shows the scanning electron microscopy (SEM) image of Mg(OH)2 nanoparticles (also from Damao Chemical Co.), whose average size is about 350 nm × 150 nm × 25 nm. Dimethylacetamide (DMAc, reagent grade) was obtained

Chemical components of the hybrid membrane

The FTIR spectra of Mg(OH)2 nanoparticles, the PVDF/10 wt.% Mg(OH)2 hybrid membrane and the pure PVDF membrane are presented in Fig. 2. It can be seen that the spectrum of the modified membrane (b) contains an absorbance at 567 cm−1 and a strong and sharp absorbance at 3699 cm−1 assigning to the stretching vibration of Mg–O and O–H bands respectively in Mg(OH)2 nanoparticles the same as those shown in (a), which are absent in (c). While the strong and broad absorbance bands at 1617 cm−1 and 3446 cm−1

Conclusion

Organic–inorganic PVDF/Mg(OH)2 hybrid membrane with PEG as an additive was prepared by phase inversion process. As revealed by FTIR spectra, large amount of –OH groups were exist in the hybrid membrane due to the addition of Mg(OH)2 nanoparticles, and these –OH groups are responsible for the hydrophilicity increase of the modified membrane. BSA and E. coli adsorption measurements confirm that hybrid membrane exhibited the antifouling property. SEM results demonstrate a formation of sponge-like

Acknowledgments

We appreciate the financial support of the National Science Fund for Distinguished Young Scholars of China, the Program for New Century Excellent Talents in Chinese Universities supported by the Education Ministry of China (NCET-06-0272), and the National Natural Science Foundation of China (20776026, 21006008).

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