Photoinduced graft polymerization of acrylamide on polypropylene microporous membranes for the improvement of antifouling characteristics in a submerged membrane-bioreactor

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Abstract

Polypropylene hollow fiber microporous membranes (PPHFMMs) were surface-modified by the photoinduced graft polymerization of acrylamide (AAm). Structural and morphological changes on the membrane surface were characterized by attenuated total reflection-Fourier transform infrared spectroscopy (FT-IR/ATR) and field emission scanning electron microscopy (FE-SEM). Water contact angles of the membrane surfaces were also measured by the sessile drop method. Results of FT-IR/ATR indicated the successful graft polymerization of AAm onto the membrane surface. Water contact angle of the modified membrane decreased with the increase of the grafting degree. It showed 34.9° for the modified membrane with a grafting degree of 278.4 wt.%, nearly 94° lower than that of the nascent one. The antifouling characteristics of the PPHFMMs before and after surface modification were assessed by the filtration of activated sludge in a submerged membrane bioreactor for synthetic wastewater treatment. It demonstrated that after continuous operation for about 42 h, the flux recovery were 36.9, 63.7% and the relative flux ratio were 1.00, 1.32 for the nascent and modified PPHFMMs, respectively.

Introduction

Polymeric membranes, such as polypropylene microporous membrane, exhibit high potentials for comprehensive applications due to their high void volume, well-controlled porosity, high thermal and chemical stability, and low cost. However, the low energy surface and relatively high hydrophobicity probably lead to membrane fouling. To solve this problem, surface modifications, both chemical and physical, have been widely used to enhance the membrane performance such as flux and selectivity without changing the bulk properties. Surface modification can reduce or endow the membrane with various chemical functionalities [1], [2], [3], [4], [5], [6], [7]. General goals are a decrease in biomacromolecule adsorption [8], an increase in the hydrophilic [9] or hydrophobic [10] character of the surface, the attachment of a biologically active molecule, or an alteration in the lubricity of the surface. Different methods such as UV irradiation [11], [12], [13], [14], plasma treatment [15], [16], [17], gamma irradiation [18], [19], [20], and chemical reaction [21] have been employed to modify the membrane surface. Among these methods UV-assisted graft polymerization is a powerful and highly attractive technology, with low cost of operation and potentially reducing or even avoiding negative effects onto the bulk polymer [22], [23], [24].

On the other hand, there is a growing impetus for wastewater recycle and reuse due to the shortage of water all over the world. Interest in the membrane-bioreactor (MBR) technology for wastewater treatment has increased because of the advantages offered by MBR over the conventional treatment technologies [25]. Negative aspects, however, include membrane fouling and concentration polarization [26], [27], [28], which cause the permeate flux to decline quickly, significantly reduces membrane performance, increases operating costs and shortens membrane life. In general, membrane fouling occurs more seriously on hydrophobic membranes than hydrophilic ones due to the hydrophobic interactions between solutes, microbial cells, and membrane materials [29], [30], [31]. There have been many investigations about the mechanisms of membrane fouling [32], [33], processes to restrict fouling [34], and methods to enhance the flux [35]. As a result, much attention has been made to reduce membrane fouling by modifying hydrophobic materials to relative hydrophilic.

In our previous work, polypropylene microporous membrane were hydrophilized by ammonia and carbon dioxide plasma treatment [36], [37], or by the immobilization of sugar moieties induced by nitrogen plasma treatment [38], and the antifouling characteristics for these membranes in an MBR were improved to some extent. Although the use of plasma treatment processes has many advantages, such as a very shallow modification depth compared to other surface modification techniques, it still has drawbacks. For examples, the chemical reactions in the discharge itself are rather complex, the mechanical strength of the membrane decreases dramatically after plasma treatment, the control and ultimate analysis of the surface chemistry is limited, and it is not convenient to extend its applications on a large scale.

In this work, the surface modification of polypropylene microporous membranes was accomplished by UV irradiation in aqueous acrylamide solutions. The impacts of polymerization conditions on the grafting degree were studied; the modified membranes were characterized with FT-IR/ATR, FE-SEM, and water contact angle measurement as well. The antifouling characteristics in a submerged membrane bioreactor for synthetic wastewater treatment were assessed.

Section snippets

Materials

PPHFMM and polypropylene flat microporous membrane with a porosity of 45.9 ± 3.1% and an average pore diameter of 0.11 ± 0.04 μm were prepared with a melt-extruded/cold-stretched method in our laboratory. The inner and outer diameters of PPHFMM are 240 and 290 μm, respectively. In this study, U-shape PPHFMM modules were carefully fabricated manually. There were 100 bundles of hollow fibers within each module with an area of about 90 cm2. Acrylamide (AAm, 99.9%) was used as received. Benzophenone (BP)

Photoinduced graft polymerization and characterization of PPHFMMs

Fig. 2 shows the effect of UV irradiation time on the grafting degree of AAm on the membrane surface. It was found that the grafting degree increased with the increase of UV irradiation time systematically. This was understandable, because much more surface radicals generated and much more monomers got accessible to the surface radicals with the prolongation of reaction time. The variation of grafting degree with the monomer concentration is depicted in Fig. 3. It can be clearly seen that the

Conclusion

To improve the antifouling characteristics in the submerged membrane-bioreactor for synthetic wastewater treatment, polypropylene hollow fiber microporous membranes were surface-modified by the photoinduced graft polymerization of acrylamide. FT-IR/ATR analysis confirmed the successful graft polymerization of acrylamide on the membrane surfaces by UV irradiation. Contact angle data showed that the hydrophilicity of the surface modified membranes increased systematically with the increase of the

Acknowledgement

Financial support from the National Basic Research Program of China (Grant no. 2003CB15705) is gratefully acknowledged.

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