Unmodified membranes

The conductivity of membranes that do not contain dissolved ionophores or lipophilic ions is often affected by cracking and impurities. The value for a completely compact membrane under reproducible conditions excluding these effects varies from 10-8 to 10 10 Q 1 cm-2. The conductivity of these simple unmodified membranes is probably statistical in nature (as a result of thermal motion), due to stochastically formed pores filled with water for an instant and thus accessible for the electrolytes in the solution with which the membrane is in contact. Various active (natural or synthetic) substances... 

The performance of modified and unmodified membranes evaluated by analyzing the expression of liver-specific functions in terms of albumin production is shown in Figure 19.3. Hepatocytes cultured on unmodified PES membranes produced... 

Fig. 26. Photo-effects on water permeation through a porous poly(vinj4 alcohol) gel with tri dmiyl-methane leucocyanide groups (content, 1.9 mol %). ( ) an unmodified membrane ( ) a coated membrane in the dark (O) a coated membraw under UV irradiation...

Also, the final CdS nanoparticle size is influenced by the pH of the outer solution (the increase in pH decreases the size of the CdS particles formed) as well as by modification of the membrane with ionogenic surfactants such as SDS and CTAB (see Fig. 7). The size of the particles formed is influenced as well by the concentration of the CdS precursor in the inner cavities of the vesicles the increase in concentration enlarges the final nanoparticle size (in the case of the surfactant-unmodified membrane). At the same time, for the CTAB-modified membrane, the relationship is reversed. 

Membranes modified with the weak acid AA monomer are able to reduce irreversible fouling to zero, in contrast to other strongly hydrophilic monomers, such as HEMA and AAG. These compounds increase irreversible fouling relative to the unmodified membrane. ... 

Hilal et al. [17] studied the surface structure of a poly(ether sulfone) membrane, the surface of which was modified by depositing a molecularly imprinted polymer layer. They measured the pore size and surface roughness by AFM. It was reported that an increase in the degree of modification, given by the weight of the imprinted polymer layer, led to a systematic decrease in pore size and an increase in surface roughness. As the pore size decreased, the relative water flux (the ratio of the water flux of the surface-modified membrane to that of the unmodified membrane) also decreased. In other words, the flux decreased as the surface roughness increased. 

A composite ceramic membrane was formed [77] by the graft polymerisation of a hydrophilic polymer, PVP, onto the surface of silica membranes (pore size = 3.0 pm). The flux of an unmodified UF membrane of an oil/water emulsion (4.7%) decreased with time as compared to the flux of the composite membrane. The dechne in flux was caused by fouling and/or the immediate formation of an oil gel layer on the surface of the unmodified membrane. The modified membrane, in contrast, was not only more resistant to adsorption of oil, but also had a higher oil rejection. The performance of the modified ceramic membrane depends upon the configuration of the grafted chains in response to solvent—polymer interactions. Thus, the hydrophific PVP polymer chains tend to expand or extend away from the surface in aqueous solutions, preventing oil adsorption on the membrane surface. Simultaneously, the hydrophific polymer allows the passage of water molecules preferentially over oil. 

Figure 12. (a) Results ofpeel off tests for two-layer assembly made of unmodified membranes and membranes modified with PGMA layer, (b) AFM image (5x5 pm ) of surface of PET membrane covered with PGMA after the peel off test. Vertical scale 30 nm. 

To assess the permeability of each membrane, the permeate flux of the distilled water (/ ) was measured for the SMM-modifled membranes, the unmodified PES membrane, and the commercial PES membrane. It was observed that the SMM-modifled membranes exhibited lower permeation rates than the commercial PES and the unmodified membrane. The flux of the distilled water was highest for the PES commercial membrane (168.6 kg/m h) and for the prepared unmodified PES membrane (127.6 kg/m h). Eor the modified membranes, SMM3/PES and SMM41/PES, the permeate fluxes were lower at 41.3 and 39.5 kg/m h, respectively (Table 1.1). 

A radioactive solution containing Co of the total radioactivity of ca. 4000 counts/100 sec was anployed as a radioactive feed. The respective radioactivity counting rate (pulse/sec), treated as the intensity of the y-radiation of a 5 cm liquid sample containing Co, was measured by a y-analyzer, LG-IB (INCT). The obtained values were compared with the radiation intensity of the Co standard sample. Therefore, the specific activity of the feed (Ap), the retentate (Ar), and the permeate (Ap) were calculated. The parameters (/ ), (DF), and if) were determined when the radioactive solutions were treated. In all the experiments, the transman-brane hydrostatic pressure was maintained at 0.22 MPa and the feed flow rate at 401/h. Substantially high DFs for radioactive Co were obtained for the SMM-modified membranes used in the UF/complexation process 223 for SMM3/PES and 163 for SMM41/PES (Table 1.2). For the unmodified membranes, the DFs were lower 44 for the commercial PES membrane and 75 for the prepared unmodified PES membrane. Additionally, the SMM-modified membranes showed smaller adsorption of the radioactive cobalt than the modified membranes, which was beneficial, taking into account the considered applications. After a 2 h operation, the adsorption of Co by the SMM-modified membranes was four to five times smaller than that of the unmodified PES membrane. 

UF of BSA solutions modified membranes, especially with NVP, showed higher flux and lower fouling compared with the unmodified membrane (Pieracci et al. 1999)... 

UF of sugarcane juice and BSA solutions modified membranes showed more resistance to fouling and a higher rejection than unmodified membranes relatively high monomer concentration (40 g/1) at medium irradiation times (1.5-3 min) were among the optimal modification conditions to reduce membrane fouling (Susanto et al. 2007)... 

MF of E. coli suspensions a neutral hydrophilic membrane surface was less susceptible to fouling than charged (positively or negatively) membranes (Kochkodan et al. 2006) MF of E. coli suspensions the modified membranes were more resistant to biofouling the number of bacterial cells able to proliferate from countable colonies was reduced for qDMAEM-grafted samples compared with unmodified membranes (Hilal et al. 2003, 2004)... 

MF of activated sludge modified membranes showed better filtration performances in a submerged membrane biweactor than unmodified membranes. AA-grafted membrane had the best antifouUng characteristics (Yu et al. 2008a)... 

MF of activated sludge after continuous operation in a membrane bioreactor for about 70 h, the reduction of water flux was 90.7% for the unmodified membrane and ranged from 80.8% to 87.2% for the modified membranes, increasing with an increase in the length of the grafted chains (Gu et al. 2009)... 

The hydrophilicity of the NH3 plasma-treated PP membranes increased with the increase in plasma treatment time and decreased with the increase in storage time (Yan et al. 2008). The adsorption of BSA on the modified membranes was lower than that on the unmodified-membrane surface and the flux recoveries after water and caustic cleaning for the NH3 plasma-treated membranes for 1 min were 51.1% and 60.7% higher than those for the unmodified membrane. However, the mechanical properties of the membranes decreased after prolonged plasma treatment, thus the optimal plasma treatment time for membrane modification was taken as 1 min. 

Such layers are usually made of highly cross-linked material and show good adhesion to the substrate. Examples of such membrane surface treatment are plasma polymerization of allyl alcohol and allyl amine (Gancarz et al. 2002, 2003). It was shown that the membranes modified with allyl amine do not foul as intensively during UF of the BSA solutions compared with the unmodified membranes. Similar behavior was also shown for membranes modified by the deposition of plasma-polymerized n-butylamine however, in this case, the modified layer deposited on the manbrane surface was not as enriched in amines as the polymer formed from aUyl amine (Gancarz et al. 2002). 

Yu et al. (2006) used air plasma-initiated grafting of polyvinylpyrrolidone (PVP) in attempts to reduce the PP membrane fouling in an SMBR. It was shown that after continuous operation for about 50 h, the flux recovery, reduction of flux, and relative flux ratio for the modified membranes were 53% higher, 17.9% lower, and 79% higher, respectively, than those for the unmodified membranes. The water contact angle on the PVP-immobilized membrane showed a minimum value of 72.3°, approximately 57° lower than that on the unmodified one. 

The hydrophilicity of the MF PP membranes may be increased by increasing the amount of surfactant Tween-20 adsorbed onto the surface or in the pores of the membrane (Xie et al. 2007). The PP membrane modified with a monolayer of the adsorbed surfactant showed higher flux and stronger antifouling ability than the unmodified membrane after operating in an MBR for about 12 days. 

The blend UF PES/PAN membranes treated with aqueous NaOH solutions at room temperature for 24 h showed higher flux recovery ratios compared with the unmodified membranes after the UF of the PEG, dextran, and PPS solutions. The increase in the fouling resistance is believed to be due to the higher hydrophilicity of the modified-manbrane surface (Reddy and Patel 2008). 

Based on the membrane surface properties and the HA properties, various researchers have attempted to change the membrane surface characteristics by surface modification. Different techniques have been performed, such as ion beam irradiation, plasma treatment, redox-initiated graft polymerization, photochemical grafting, and interfacial polymerization (IP). In this chapter, two surface modification techniques, IP and photochemical grafting, are discussed by means of experimental examples. The surface characteristics of the unmodified membrane and the modified membranes are studied and their relationships with irreversible fouling and NF performance are reported. 

The measured average pure water permeabilities (P ) for the unmodified and the modified membranes are summarized in Table 5.1. It can be observed that all the modified membranes showed a lower water permeability than the unmodified membrane (NFPESIO) with a water permeability as low as 0.58 l/m h bar. 

FIGURE 5.3 An ATR-FTIR analysis of the unmodified membrane (NFPESIO) and a modified membrane by IP using 2% w/v BPA and 60 sec reaction time. (From Journal of Membrane Science, 348, Abu Seman, M.N., Khayet, M., and Hilal, N., Nanofiltration thin-film composite polyester polyethersulfone-based membranes prepared by interfacial polymerization, 109-116, Copyright (2010), with permission from Elsevier.)... 

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