Ultrafiltration pore size distribution

Ultrafiltration (UF) is an important component in wastewater treatment and in food industry [109,110]. With increasing concerns and regulations in environment as well as in food safety, the process of ultrafiltration has become more critical, whereby new technology development to provide faster and more efficient water treatment is not only necessary but also urgent. Currently, conventional polymeric UF membranes are prepared mainly by the phase immersion process, typically generating an asymmetric porous structure with two major limitations (1) relatively low porosity and (2) fairly broad pore-size distribution [111,112],... 

With anodic oxidation very controlled and narrow pore size distributions can be obtained. These membranes mounted in a small module may be suitable for ultrafiltration, gas separation with Knudsen diffusion and in biological applications. At present one of the main disadvantages is that the layer has to be supported by a separate layer to produce the complete membrane/support structure. Thus, presently applications are limited to laboratory-scale separations since large surface area modules of such membranes are unavailable. 

Particles smaller than the largest pores, but larger than the smallest pores are partially rejected, according to the pore size distribution of the membrane. Particles much smaller than the smallest pores will pass through the membrane. Thus, separation of solutes by microporous membranes is mainly a function of molecular size and pore size distribution. In general, only molecules that differ considerably in size can be separated effectively by microporous membranes, for example, in ultrafiltration and microfiltration. 

Application Qearly one important application of microporous materials in which the effectiveness is critically dependent on the monodispersity of the pores is the sieving of proteins. In order that an ultrafiltration membrane have high selectivity for proteins on the basis of size, the pore dimensions must first of all be on the order of 25 - ioOA, which is the size range provided by typical cubic phases. In addition to this, one important goal in the field of microporous matmals is the attainment of the narrowest possible pore size distribution, enabling isloation of proteins of a very specific molecular weight, for example. Applications in which separation of proteins by molecular weight are of proven or potential importance are immunoadsorption process, hemodialysis, purification of proteins, and microencapsulation of functionally-specific cells. 

Inorganic membranes have also been studied. Thus, AFM has been used to probe the surface morphology and pore structure of micro- and ultrafiltration membranes, both in contact and noncontact mode, the latter being very suitable for soft and delicate materials. One of the first reports concerned alumina microfiltration membranes (Anapore) [45] and the authors performed statistical analysis to obtain the pore size distribution from the AFM... 

Macromolecular fractionation, which involves high-resolution separation of solutes having comparable molecular weights using ultrafiltration, is challenging primarily due to the broad pore-size distribution of ultrafiltration membranes. This implies that purely size-based fractionation is not feasible using membranes currently available. The development of advanced membranes with narrow pore-size distributions could make fractionation more feasible. With currently available membranes... 

Separation processes as a whole have grown in importance because of increasingly stringent requirements for product purity [1]. Among the different membrane techniques, pressure-driven processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) were the first to undergo rapid commercialization [2-A], These processes basically differ in pore size distribution of membranes used and the types of compounds recovered. A typical schematic of the exclusion of various compounds through different membrane processes is illustrated in Figure 42.1. 

In the presence of solutes with small molecular weights, concentration polarization is likely to occur but with much less effect than in the case of ultrafiltration as explained in Section 12.2.1. A theoretical model concerning separation of sucrose and raffinose by ultrafiltration membranes has been proposed by Baker et al. [53] which assumes transport of solvent and solute exclusively through pores. This model can apply to ceramic nanofilters as they exhibit a porous structure with a pore size distribution. The retention characteristics of a given membrane for a given solute is basically determined by its pore-size distribution. The partial volume flux jy through the pores which show no rejection to the solute can be expressed as a fraction of the total volume flux fy. 

The ability to determine the pore size and pore size distribution for porous membranes has existed for a number of years (11,40-42). Recent advances have permitted the determination of pore size and distribution for finely porous ultrafiltration membranes (, 1 ). Determination of i j, (the average cross-sectional area of the transport corridor) for the skin layer of reverse osmosis and tight ultrafiltration membranes as well as pervaporatlon membranes at present remains a challenge, although advances are being made in this direction (9,10,48-50). 

For many years, polymeric membranes have been widely utilized in practical appHca-tions without having precise information on their pore size and pore size distribution, despite the fact that most commercial membranes are prepared by the phase inversion technique, and the performance of those membranes is known to be governed by their pore characteristics in a complicated manner [1]. These pore characteristics are influenced both by the molecular characteristics of the polymer and by the preparative method [2]. Crudely, membranes applied for pressure-driven separation processes can be distinguished on the basis of pore diameter as reverse osmosis (RO, < 1 nm), dialysis (2-5 nm), ultrafiltration (UF, 2-100 nm), and microfiltration (MF, 100 nm to 2 J,m). Nanofiltration (NF) membranes are a relatively new class and have applications in a wide range of fields [3]. The pore sizes of NF lie between those of RO and UF membranes. 

FIGURE 5.24 Pore size distribution curves of PL (curves 1-3) and PB (curves 4-6) series of membranes with 5, 10, and 30 kDa. (Adapted from J. Membr. ScL, 309, Jeon, J.-D., Kim, S. J., and Kwak, S.-Y., H nuclear magnetic resonance (NMR) cryoporometry as a tool to determine the pore size distribution of ultrafiltration membranes, 233-238, 2008, Copyright 2008, with permission from Elsevier.)... 

Jeon, J.-D., Kim, S.J., and Kwak, S.-Y. 2008. H nuclear magnetic resonance (NMR) ciyoporometry as a tool to determine the pore size distribution of ultrafiltration membranes. J. Membr. Sci. 309 233-238. 

This implies that microfiltration membranes are porous media containing macropores and ultrafiltration membranes are also porous with mesopores in the top layer. Hence, the definition porous covers both the macropores and mesopores. With membranes of these type it is not the membrane (material) which is characterised but the pores in the membrane. Here the pore size (and pore size distribution) mainly determines which particles or molecules are retained and which will pass through the membrane. Hence, the material is of little importance in determining the separation performance. On the other hand, with dense pervaporation/gas separation membranes, no fixed pores are present and now the material itself mainly determines the performance. 

At the lowest pressures the largest pores will be filled with mercury. On increasing the pressure, progressively smaller pores will be filled according to eq. IV - 3. This will continue until ail the pores have been filled and a maximum intrusion value is reached. It is possible to deduce the pore size distribution from the curve given in figure IV - 10, because every pressure is related to one specific pore size (or entrance to the pore ). The pore sizes covered by this technique range from about 5 nm to 10 pm. This means that all microfiltration membranes can be characterised as well as a substantial proportion of the ultrafiltration membranes. 

Figure I - 20 gives the cumulative pore volume and the pore size distribution for a PPO poly(phenylene oxide) ultrafiltration membrane determined by thermoporometry [12]. Figure I - 21 gives the pore size distribution of a ceramic membrane determined by two methods gas adsorption-desorption and thermoporometry [13]. Both curves (and hence both methods) are in good agreement with each other. Similar results were found by Cuperus for Y-aJumina membranes [14].

Various pressure-driven membrane processes can be used to concentrate or purify a dilute (aqueous or non-aqueous) solution. The characteristic of these processes is that the solvent is the cominueous phase and that the concentration of the solute is relatively low. The particle or molecular size and chemical properties of the solute determine the structure, i.e. pore size and pore size distribution, necessary for the membrane employed. Various processes can be distinguished related to the panicle size of the solute and consequently to membrane structure. These processes are microfiJtration, ultrafiltration, nanofiltration and reverse osmosis. The principle of the four processes is illustrated in figure VI - 2. 

Composite membranes constitute the second type of structure frequently used in reverse osmosis while most of the nanofiltration membranes are in fact composite membranes. In such membranes the toplayer and sublayer are composed of different polymeric materials so that each layer can be optimised separately. The first stage in manufacturing a composite membrane is the preparation of the porous sublayer. Important criteria for this sublayer are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer ... 

Among the different CRP techniques that are available, surface-initiated ATRP and RAFT have been most extensively used to grow polymer brushes from the membrane and pore surfaces [129,139-143]. Both methods can tailor and manipulate the structure and length of the grafted chains on the membrane and pore surfaces, which in return, are able to tune the pore size and pore size distribution to avoid pore blocking. Furthermore, ATRP and RAFT can produce more uniform and smoother surfaces, which have a more prominent effect on protein adsorption and fouling during the microfiltration or ultrafiltration process, than conventional radical polymerization. 

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