Ultrafiltration macromolecular

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

Nakao, S-I Nomura, T Kumura, S, Characteristics of Macromolecular Gel Layer Formed on Ultrafiltration Tubular Membrane, AIChE Journal 25, 615, 1979. 

As discussed previously, the technique of microfiltration is effectively utilized to remove whole cells or cell debris from solution. Membrane filters employed in the microfiltration process generally have pore diameters ranging from 0.1 to 10 pm. Such pores, while retaining whole cells and large particulate matter, fail to retain most macromolecular components, such as proteins. In the case of ultrafiltration membranes, pore diameters normally range from 1 to 20 nm. These pores are sufficiently small to retain proteins of low molecular mass. Ultrafiltration membranes with molecular mass cut-off points ranging from 1 to 300 kDa are commercially available. Membranes with molecular mass cut-off points of 3,10, 30, 50, and 100 kDa are most commonly used. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Fig. 4.4 Scheme of pulsed ultrafiltration-mass spectrometry (PUF-MS) to screen chemical mixtures for compounds that bind to a macromolecular receptor. The ultrafiltration membrane traps a receptor in solution, but allows low molecular weight... 

For ultrafiltration, the macromolecular solutes and colloidal species usually have insignificant osmotic pressures. In this case, the concentration at the membrane surface (C ) can rise to the point of incipient gel precipitation, forming a dynamic secondary membrane on top of the primary structure (Figure 7). This secondary membrane can offer the major resistance to flow. 

After the mild-hydrolysis step at 70°, the sialic acids liberated are removed from the sample by dialysis or ultrafiltration at 2°, and the macromolecular material is rehydrolyzed, using, however, the stronger acidic conditions of 0.1 M acid. The dialysis time ranges between 6 and 24 h, depending on the volume and viscosity of the hydrolysis mixture. Therefore, the optimum dialysis time should be evaluated by determinations of sialic acid in the eluate, or by addition of a trace of radioactive Nen5Ac. The dialyzates, or filtrates, are combined, and processed as will be described. By using this procedure, the overall yield of purified sialic acids is 70-80%, and the loss of O-acetyl groups107 is 40%. 

R.W. Baker and H. Strathmann, Ultrafiltration of Macromolecular Solutions with High-flux Membranes, J. Appl. Polym. Sci. 14, 1197 (1970). 

Because of the effect of the secondary layer on selectivity, ultrafiltration membranes are not commonly used to fractionate macromolecular mixtures. Most commercial ultrafiltration applications involve processes in which the membrane completely rejects all the dissolved macromolecular and colloidal material in the feed solution while completely passing water and dissolved microsolutes. Efficient fractionation by ultrafiltration is only possible if the species differ in molecular weight by a factor of 10 or more. 

Guo, L. (2000). Re-examination of cross-flow ultrafiltration for isolating coUoidal macromolecular organic matter in seawater. Mar. Chem. 69, 75-90. 

The C-NMR spectmm of HMW DOM also includes contributions from carboxyl (CO-(OH or NH), 5% of total carbon), and alkyl (CH 14% total carbon) functional groups, which may derive from proteins, lipids, or carbohydrates (deoxy-and methyl sugars). Hydrolysis of HMW DOM followed by extraction with organic solvent yields 4-8% of the total carbon in HMW DOM as acetic acid. Acetyl is easily recognized in the H-NMR spectra of HMW DOM, where it appears as a broad singlet centered at 2 ppm (Figure 6(b)). Free acetic acid and its derivatives are not retained by ultrafiltration, and the acetyl in HMW DOM must be covalently bound to macromolecular material, most likely as an A-acetyl amino sugar. Acetyl contributes up to half the carboxyl carbon in the C-NMR spectrum. 

Ultrafiltration relies on the ability of a membrane to act as a selective barrier. One of the major properties of an ultrafiltration membrane is its ability to act as a sieve for macromolecular substances. The transmission of a particular solute through an ultrafiltration membrane can be expressed in several ways, a commonly used way being in terms of sieving coefficients. The intrinsic or real sieving coefficient (5j) is defined as... 

A concentration process involves removal of a solvent, typically water from a macromolecular solution. Ultrafiltration is the method of choice for large-scale concentration. The selectivity issue involving removal of water from a macromolecular solution using ultrafiltration is trivial. The main challenge in a concentration process is maintaining a high productivity on account of the increased macromolecular concentration in the feed solution. Some of the main applications of macromolecular concentration using ultrafiltration are listed below [2] ... 

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... 

The use of ultrafiltration for macromolecular fractionation is not an established operation in the industry. This field is stiU a domain for fundamental and applied research [15-20]. Some of the projected apphcations are listed as follows ... 

A solution containing the metal ion to be extracted and a water-soluble polymer is delivered into an ultrafiltration unit (Figure 29.5). The feed stream, upstream of the UF system, is adequately stirred to enhance recovery of the radioactive ions. The metallic macromolecular complex is retained while low-molecular-weight solutes pass through the membrane. The efficiency of the process is mainly characterized by the passage of each species through the membrane. The transfer coefficient of a given solute, i, is defined by... 

Reverse osmosis preceded by microfiltration or ultrafiltration is considered as an option for the treatment of radioactive wastes from Romanian nuclear centers. Effective studies are carried on at Research Center for Macromolecular Materials and Membranes, Bucharest and at Institute of Nuclear Research, Pitesti aiming in employing these pressure-driven techniques for cleaning the wastes from decontamination of nuclear installations and reactor primary circuit [34,35]. 

In nuclear industry ultrafiltration was applied in the pretreatment stage before reverse osmosis that needs removal of potential foulants from feed streams. Very often ultrafiltration is combined with precipitation or complexation. Small ions bound by macromolecular chelating agent form complexes, which are retained by UF membrane. Such an enhanced ultrafiltration becomes an efficient separation process with high decontamination factors, sometimes compared with those... 

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