Retention by adsorption

Lukaszewicz et al14 has postulated an adsorptive mechanism to explain the excellent retention of bacteria by membranes with some pores larger than the microorganism. He contends that the adsorptive sequestrations of the membrane are more important than the geometric restraints of sieving. 

Davis et alls investigated the retention of 0.05 and 0.005 y Au colloids by capillary-pore and tortuous-pore membranes. Table 2.4 shows clearly that the tortuous-pore membranes retain particles much smaller than the rated pore size. Indeed, a 5 y pore size will retain 60% of the 0.05 y colloidal particles and 18% of the 0.0005 y colloid. On the other hand, capillary-pore membranes retain less than 1% of either. Tortuous-pore membranes have 25 to 50 times more internal surface area for adsorption than capillary-pore membranes, and the tortuous path also results in a greater likelihood of small particles contacting the pore-wall. 

These results suggest that a tortuous-pore configuration is best for cleanup applications where the removal of all particles from the process stream is desired. On the other hand, the capillary-pore configuration is best for fractionation of particles. For example, capillary-pore membranes have been used in fractionating silver colloids to improve resoltuion on photographic films. 

For dilute process streams, product may be lost via adsorption on the membrane. The recovery of this product may be improved by pretreating the membrane such that most of the adsorption sites are occupied. For example, in the data of Hahn et al16 (Table 2.5), polio virus adsorption on cellulosic tortuous-pore membranes was significantly higher than that on polycarbonate capillary-pore membranes, (i.e.. The virus recovery is low due to adsorption.) The recov ery was improved from 5 to 76% by pretreating the membrane with a beef extract solution. 

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A displacement experiment was conducted where a 50-ppm xanthan solution in 15,500 ppm NaCl was injected into a Clashach sandstone core to determine the amount of polymer retained. The Clashach sandstone is more than 99.5% quartzitic and has a low concentration of clays. Consequently, retention by adsorption is expected to be low. Table 5.63 summarizes polymer concentration data from this run when 0.995 PV of polymer solution was injected followed by 0.938 PV of brine. The PV of the core was 303 cm, and the porosity was 0.175. Assuming that polymer retention was irreversible, use material-balance calculations to estimate the amount of polymer retained. The length of the core was 100 cm, and the cross-sectional area was 17.35 cm. Express the polymer retention as mg/g of rock. The density of the solid matrix may be taken as 2.65 g/cm. ... 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

Immobilization by adsorption onto a surface such as activated carbon or to an ion-exchange resin gives a reversible and relatively weak bond, but this can be sufficient to increase the retention time in a flow system to acceptable levels. Recall Section 10.6 where it is shown that the residence time of an adsorbed species can be much larger than that of the mobile phase, in essence giving more time for catalysis. 

Irrespective of the nature of the retention that is due to adsorption, solubility, chemical binding, polarity or molecular filtration, the column does retain some components longer than others. When in the gas phase the components are moved toward the column outlet, they are selectively retarded by the stationary phase. Consequently, all components pass through the column at varying speeds and emerge in the inverse order of their retention by the column materials. The aforesaid process may be outlined schematically as shown in Figure 29.1. 

Abstract The aim of this work was to study the simultaneous effect of amount of clay, activation temperature, contact time, pH, and size of the adsorbent on the retention of oil-grease thermally activated illite by adsorption. The values obtained for the percentage of oil-grease removed ranged from 93.87% for 110°C up to 66.73% for 900°C. The adsorption experiment showed surface that the stronger heat treatment the most effective adsorption of oil-grease. 

Various methods ofachieving preconcentration have been applied, including Hquid -hquid extraction, precipitation, immobihzation and electrodeposition. Most of these have been adapted to a flow-injection format for which retention on an immobihzed reagent appears attractive. Sohd, sihca-based preconcentration media are easily handled [30-37], whereas resin-based materials tend to swell and may break up. Resins can be modified [38] by adsorption of a chelating agent to prevent this. Sohds are easily incorporated into flow-injection manifolds as small columns [33, 34, 36, 39, 40] 8-quinolinol immobilized on porous glass has often been used [33, 34, 36]. The flow-injection technique provides reproducible and easy sample handhng, and the manifolds are easily interfaced with flame atomic absorption spectrometers. 

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