Ultrafiltration component transport

A different approach is the use of an ultrafiltration membrane with an immobilized chiral component [31]. The transport mechanism for the separation of d,l-phenylalanine by an enantioselective ultrafiltration membrane is shown schematically in Fig. 5-4a. Depending on the trans-membrane pressure, selectivities were found to be between 1.25 and 4.1, at permeabilities between 10 and 10 m s respectively (Fig. 5-4b). 

Transport of contaminants by surface runoff is illustrated in the experimental results of Turner et al. (2004), which deal with the colloid-mediated transfer of phosphorus (P) from a calcareous agricultural land to watercourses. Colloidal molybdate-reactive phosphorus (MRP) was identified by ultrafiltration associated with particles between l am and Inm in diameter. Colloidal P compounds can constitute a substantial component of the filterable MRP in soil solution and include primary and secondary P minerals, P occluded or adsorbed on or within mineral or organic particles, and biocolloids (Kretzschmar et al. 1999). 

Electrolytes and other plasma components with low molecular weights enter the primary urine by ultrafiltration (right). Most of these substances are recovered by energy-depen-dent resorption (see p. 322). The extent of the resorption determines the amount that ultimately reaches the final urine and is excreted. The illustration does not take into account the zoning of transport processes in the kidney (physiology textbooks may be referred to for further details). 

While both of these devices use hollow fiber membranes similar to the primary components of kidney dialyzer units, the difference between the two techniques lies in how the analyte undergoes mass transport into the device. Microdialysis sampling is a diffusion-based separation process that requires the analyte to freely diffuse from the tissue space into the membrane inner lumen in order to be collected by the perfusion fluid that passes through the inner lumen of the fiber. Ultrafiltration pulls sample fluid into the fiber lumen by applying a vacuum to the membrane (Figure 6.1). 

To understand the mechanism of urine concentration, one must retrace the fate of the fluid in the various segments of the kidney. In the glomerulus, the membranes of the Bowman s capsule cells allow passage of all plasma components except protein. The ultrafiltrate is markedly reduced in volume as it passes through the proximal tubule. In fact, only 20% of the original volume reaches the distal portion of the proximal convoluted tube. The volume of the ultrafiltrate is reduced due to passive water reabsorption. Passive means that no known molecular mechanism exists for the transport of water from the lumen of the proximal tubule to the interstitial tissue. However, the movement of water follows that of sodium. In the proximal tubule, sodium is excreted actively into the interstitial tissue, and as a result, the osmotic pressure of the interstitial tissues increases. This draws water from the lumen of the tubule into the interstitial environment of the medulla because the tubule is highly permeable to water. 

Mass transport through porous membranes can be described with the pore model. In accordance with particle filtration, selectivity is determined solely by the pore size of the membrane and the particle or the molecular size of the mixture to be separated. This process is driven by the pressure difference between the feed and permeate sides [83]. The processes described by the pore model include microfiltration and ultrafiltration. Whereas membranes for microfiltration are characterized by their real pore size, membranes for ultrafiltration are defined according to the molar mass of the smallest components retained. 

Ultra-pure water is quite corrosive and it attacks the transport lines, valves and other system components, producing particulates downstream of the polishing filter. A point-of-use filter rated at 0.2 pm or a small ultrafiltration system should therefore be used. The complexity thus implicit in an ultra-pure water system is illustrated in Figure 4.21, which also shows a recycle stream of used wash water. 

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