Improvement Of The Antifouling Characteristics Of Polypropylene Microporous Membranes By Surface Modifications

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. In an attempt to improve the antifouling characteristics of these membranes in a submerged membrane bioreactor (SMBR), membrane surface modifications by NH₃ and CO₂ plasma treatment, plasma induced immobilization of α-allyl glucoside (AG) and poly(N-vinyl-2-pyrrolidone) (PVP), graft polymerization of acrylic acid (AA) and acrylamide (AAm) under UV irradiation, and nonionic surfactant Tween adsorption were investigated systematically. To assess the relations between surface modification and antifouling characteristics in the SMBR, membrane filtration for activated sludge was carried out using synthetic wastewater. The relative experiments and results are summarized as below.Surface modifications of polypropylene hollow fiber microporous membranes (PPHFMMs) were performed by NH₃ and CO₂ plasma treatments. Structural and morphological changes of the membranes were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Water contact angle was measured by the sessile drop method. Results indicated that the tensile strength, the rate of elongation, the water contact angle, and pure water flux of the modified membrane decreased with decreasing the initial NH₃ pressure in the plasma reactor. However, the water contact angle increased with the storage time of the membranes, which indicated "hydropobicity recovery". On the other hand, static water contact angle on the modified membrane reduced obviously at first and then keptalmost constant with the increase of CO₂ plasma treatment time. The pore size and porosity, and pure water flux of the membranes after plasma treatment increased at first then decreased with the increase of plasma treatment time. The tensile strength and the rate of elongation after plasma treatment decreased with the increase of plasma treatment time.PPHFMMs were surface-modified by the nitrogen plasma-induced immobilization of AG and PVR The immobilization degree had the optimal plasma treatment time of 4 min (3.5 wt.%). Water contact angle on the AG modified membrane showed a minimum value of 64°, approximately 64° lower than that on the unmodified membrane. Pure water flux increased with the increase of immobilization degree. Results of FT-IR/ATR and XPS clearly indicated that PVP was successfully immobilized on the membrane surface. The immobilization degree increased with the increase of plasma treatment time up to 12 min (6.8 wt.%), then it decreased with further increase of plasma treatment time. Water contact angle on the PVP-modified membrane was approximately 57° lower than that on the unmodified membrane. The total surface free energy and its polar component increased with the increase of the immobilization degree.PPHFMMs were also surface-modified by the sequential photoinduced graft polymerizations of AA and AAm, which were confirmed by FT-IR/ATR analysis. Water contact angle of the AA grafted membrane decreased with the increase of grafting degree (GD). It was 61° lower than that of the nascent one. The relative pure water fluxes of the studied membranes depending on GD and pH value were studied. It was found that the relative pure water flux increased with the increase of GD and the decrease of pH value, which demonstrated these membranes were responsive to pH changes. Water contact angle of the AAm grafted membrane decreased also with the increase of the grafting degree. Nevertheless, it was 94° lower than that of the nascent one. The pure water flux has the optimal grafting degree of 20.28 wt.%.Surface modification by physical adsorption of a series of non-ionic surfactants including Tween 20, Tween 40, Tween 60, Tween 80 and Tween 85, was accomplished on PPHFMMs. The adsorption curve showed a two-platform character similarly. Differences in the degree and curve shape of adsorption resulting from such factors as concentration, temperature, as well as water cleaning time were observed for Tween 85 from other Tweens. FT-IR/ATR analysis and FE-SEM observation showed that the adsorption of Tween on the membrane was effective and occurred mainly in the pores at low adsorption amount, and on the membrane surface also at high adsorption value.After operated in the SMBR, the flux recovery increased by 23.0, 37.0, 29.0, 10.0, 23.0, 55.0 and 115.0%, and relative flux ratio increased by 17.0, 58.0, 90.0, 79.0, 152.0, 32.0 and 52.0% for the NH₃ and CO₂ plasma treated, AG and PVP immobilized, AA and AAm grafted, Tween absorbed PPHFMMs, respectively. It was also found that the Pearson correlation between the total surface free energy and flux recovery was significant, which indicated that the PVP-immobilized membrane with higher total surface free energy would possess excellent antifouling characteristics.