Study on the fouling behavior of silica nanocomposite modified polypropylene membrane in purification of collagen protein
Graphical abstract
Introduction
The membrane is at the heart of every membrane process and can be considered as a permselective barrier or interface between two phases (Mulder, 1996). Membrane separation processes are currently applied in various fields such as water and wastewater treatment, medicine, pharmacy, food and beverage industries (Pubby et al., 2009, Zhu et al., 2013, Xia et al., 2003, Bottino et al., 2002). Recently, membrane-based processes gained importance in biotechnology due to their ability for size and/or charge based protein separation. Pressure-driven processes of microfiltration, ultrafiltration and nanofiltration are usually applied for protein separation and purification (Saxena et al., 2009, Shen et al., 2009, Jafarzadeh and Yegani, 2015, Jafarzadeh et al., 2015a). Collagen based proteins have recently attracted great attention due to their extensive applications in biotechnology, tissue engineering and biomedical science (Xiong et al., 2009, Duan et al., 2009). However, there are few reports about Collagen proteins purification using membranes (Shen et al., 2009, Jafarzadeh and Yegani, 2015, Jafarzadeh et al., 2015a). Despite the advantages of membrane processes, membrane fouling is one of the major limitations for practical applications of membrane processes. Membrane fouling is the accumulation of substances on the surface and/or within the pores which results in the deterioration of the membrane performance. Fouling is a complicated phenomenon which is efficiently influenced by operating conditions such as trans-membrane pressure, cross-flow velocity, and temperature; feed characteristics such as the type, size and concentration of foulant, pH of feed and also membrane properties including hydrophilicity, roughness and the type as well as the size of pores (She et al., 2009, Wang and Tang, 2011). The interaction between membrane surfaces and solution components plays an important role in the extent of membrane fouling (Nady et al., 2011). It is commonly accepted that a hydrophilic membrane surface provides better performance than a hydrophobic membrane surface against fouling (Hilal et al., 2005, Liang et al., 2012). Unfortunately, most polymeric materials; used for membrane fabrication, are weakly hydrophilic such as Polyethylene (PE), Polyvinylidenefluoride (PVDF), Polysulfone (PSf), Polyethersulfone (PES) and polypropylene (PP).
PP is superior to many other polymers in terms of mechanical strength, chemical stability, thermal and chemical resistance and low cost. Therefore, PP is a very promising material for separating membranes (Xu et al., 2013, Yu et al., 2007, Hu et al., 2006). However, PP is a hydrophobic material since it lacks any polar functional group and consequently PP membranes can be easily fouled and cause a series of problems in practical application. Hence, lots of efforts have been devoted to enhance the hydrophilicity of the polymeric membranes (Zhu et al., 2013, Xu et al., 2013, Yu et al., 2005, Song et al., 2014, Yan et al., 2008).
Membrane modification can be divided into six main groups: coating, blending, grafting, chemical treatment, composite and combination of aforementioned methods (Nady et al., 2011).
Among all modification methods, organic–inorganic composite membranes demonstrate promising performances and are predicted to gather the intrinsic properties of both polymeric and inorganic membranes and give interesting advantages of the hybrid membrane such as great thermal and chemical resistances, good separation performances and excellent adaptation to the severe operating conditions (Xu et al., 2009).
In general, the composite membranes can be fabricated by two methods; the first method is blending the inorganic filler or its precursors with dope solution and the second method is direct incorporation of an inorganic phase into the polymeric membrane substrate (Jafarzadeh and Yegani, 2015, Jafarzadeh et al., 2015a, Liang et al., 2012, Zhang and Wang, 2013, Liang et al., 2014). Regarding the first method, Jafarzadeh et al. fabricated ZnO/PE composite membrane by dispersing hydrophilic ZnO nanoparticles into the polyethylene casting solution and showed that the antifouling property of the composite membrane has considerably improved (Jafarzadeh et al., 2015a). Linag et al. (2012) fabricated anti-irreversible fouling PVDF membrane by adding ZnO nanoparticles with different dosage as an additive into the membrane matrix for the modification of the internal surfaces of membrane pores. The multi-cycle filtration experiments showed that the modified PVDF membranes demonstrated significant anti-irreversible fouling property. All the modified membranes achieved almost 100% water flux recovery after physical cleaning, whereas the raw membrane only reached 78% recovery (Liang et al., 2012). Liang et al. (2014) prepared PVDF/SiO2 hybrid membranes by the thermally induced phase separation followed by an in situ sol–gel process. The surface hydrophilicity, pure water flux and mechanical properties were obviously improved with the formation of SiO2 particles in the hybrid membranes. Moreover, the hybrid membranes possess excellent anti-compression property due to the supporting effect of inorganic SiO2 particles.
It seems that the second method would be more effective in modifying the membrane surface property since the inorganic phase is in exposure without the cover and interference from the polymeric matrix (Zhang and Wang, 2013). Regarding this method, there are successful reports (Xu et al., 2013, Zhang and Wang, 2013, Zhang and Wang, 2014). Zhang and Wang (2014) fabricated hydrophobic organic–inorganic composite hollow fiber membrane by incorporating the fluorinated silica (fSiO2) inorganic layer on the polyetherimide (PEI) organic substrate using sol-gel method. The incorporation of the fSiO2 inorganic layer not only offered high hydrophobicity that is beneficial in membrane contactors, but also protected the polymeric substrate from the attacks of chemical absorbents, giving the longer lifespan for membrane. To improve the separation performances of polypropylene membranes, Xu et al. (2013) deposited TiO2 on the membrane pore surface using the atomic layer deposition strategy with and without a plasma pretreatment step on the membrane. The TiO2-deposited membranes showed improved anti-fouling property compared to the unmodified membranes due to enhanced hydrophilicity.
In this study, we report a novel method to fabricate a hydrophilic organic–inorganic nanocomposite membrane using direct incorporation of the silica (SiO2) nanoparticles on the PP membrane substrate via sol–gel process for Collagen protein purification. PP membrane was pre-treated by Piranha solution (mixture of H2O2 and H2SO4) to create hydroxyl functional group (OH) on the membrane surface (Raveshian et al., 2014). The existence of hydroxyl group ensures the chemical grafting of silane coupling agents on to the membrane surface.
A set of experiments including Scanning Electron Microscopy (SEM), Field Emission Scanning Electron Microscopy (FE-SEM), Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy, Energy dispersive X-ray (EDX) spectroscopy, pure water flux, static contact angle, porosity and mechanical strength measurements were carried out to characterize the structural and operational specifications of unmodified and modified PP membranes. Moreover, in order to investigate the impact of membrane modification on the fouling behavior of modified PP membranes, dead-end filtrations of Collagen protein using unmodified and modified PP membranes were carried out and fouling mechanisms during protein filtration were investigated using combined models (Bolton et al., 2006).
Section snippets
Theory of fouling and combined models
Four mechanisms are introduced for membrane fouling during membrane filtration, consisting of complete blocking, intermediate blocking, standard blocking and cake filtration (Hermans and Bredée, 1936). In the case of cake filtration model, foulants form a layer on the membrane surface. This layer grows with time and causes further flux decline (Bottino et al., 2002). Complete blocking assumes that particles seal off pore entrances and prevent flow. Intermediate blocking is similar to complete
Materials
Commercial grade of isotactic polypropylene (iPP, EPD60R, MFI = 0.35 g/10 min) was purchased from Arak Petrochemical Company and used as polymer. Mineral oil (MO) as diluent, acetone as extracting agent and Irganox 1010 as heat stabilizer were purchased from Acros Organics, Merck and Ciba Co., respectively. Reagent grades of sulfuric acid and hydrogen peroxide solutions were purchased from Merck and Sigma Aldrich companies, respectively and used for chemical modification of fabricated membrane.
Membrane morphology observation and EDX
SEM and FE-SEM images of PP-SiO2 composite membranes prepared by approaches 1 and 2 are shown in Fig. 4, Fig. 5, respectively. As shown in Fig. 4, approach 1 results in the formation of SiO2 particles on the membrane surface with an average particle size of 250–300 nm. Moreover, cross-section image confirms that the silica particles were also synthesized on the membrane pore walls, which can improve the hydrophilicity of the bulk of the membrane; however, some pores became smaller due to the
Conclusion
Polypropylene microfiltration membrane was fabricated via thermally induced phase separation method. The fabricated membrane was modified by Stober method. In order to use the Stober method, generation of hydroxyl (OH) functional group was necessary. In this regard, chemical pretreatment with Piranha solution was performed and ATR-FTIR spectroscopy was used to confirm the generation of OH group on the membrane surface. PP-SiO2 composite membranes with high hydrophilicity were successfully
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