Adsorption processes isotherms

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es) monolayer-multilayer adsorption, capillary condensation or micropore filling. If the isotherm exhibits low-pressure hysteresis (i.e. at p/p° < 0 4, with nitrogen at 77 K) the technique should be checked to establish the degree of accuracy and reproducibility of the measurements. In certain cases it is possible to relate the hysteresis loop to the morphology of the adsorbent (e.g. a Type B loop can be associated with slit-shaped pores or platey particles). 

The distance requited to approach the constant pattern limit decreases as the mass transfer resistance decreases and the nonlinearity of the equihbrium isotherm increases. However, when the isotherm is highly favorable, as in many adsorption processes, this distance may be very small, a few centimeters to perhaps a meter. 

Favorable and unfavorable equihbrium isotherms are normally defined, as in Figure 11, with respect to an increase in sorbate concentration. This is, of course, appropriate for an adsorption process, but if one is considering regeneration of a saturated column (desorption), the situation is reversed. An isotherm which is favorable for adsorption is unfavorable for desorption and vice versa. In most adsorption processes the adsorbent is selected to provide a favorable adsorption isotherm, so the adsorption step shows constant pattern behavior and proportionate pattern behavior is encountered in the desorption step. 

The adsorption process generally is of an exothermal nature. With increasing temperature and decreasing adsorbate concentration the adsorption capacity decreases. For the design of adsorption processes it is important to know the adsorption capacity at constant temperature in relation to the adsorbate concentration. Figure 11 shows the adsorption isotherms for several common solvents. 

The exact effect of a variation of the temperature on the coverage of the electrode surface by an adsorbed species is dependent on the isotherm obeyed by the system. In general, however, a rise in the temperature of the system will be accompanied by a decrease in this coverage, and this is as would be predicted since an adsorption process would be expected to have a negative entropy. 

N2 adsorption-desorption isotherms revealed that MCs had hi surface area (>1200 m /g) and large pore volume (>1.0 cm /g). From SAXS patterns of the prepared materials, it was confirmed that pores of SBA-15 and CMK-3 retained highly ordered 2-dimensional hexagonal type arrangement [5], while MCM-48 had 3-dimensional cubic type pore structure. It should be noted that a new scattering peak of (110) appeared in the CMK-1 after the removal of MCM-48 template. Furthermore, the pore size of CMK-1 and the wall thickness of MCM-48 were found to be 2.4 nm and 1.3 nm, respectively. This result demonstrates that a systematic transformation of pore structure occurred during the replication process from MCM-48 to CMK-1 [6]. 

Before an antibacterial agent can exert its effect on a cell it must eombine with that eell. This process often follows the pattern of an adsorption isotherm. Clearly, faetors whieh affect the state of the cell surface, as the pH of the eelTs environment must do, must affect, to some extent, the adsorption process. An inerease in the external pH renders the cell surface more negatively charged. Bioeidal agents that are eationie in nature thus bind more strongly to the cell surface with a eonsequent inerease in aetivity. 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

A typical adsorption process in electrocatalysis is chemisorption, characteristic primarily for solid metal electrodes. The chemisorbed substance is often chemically modified during the adsorption process. Then either the substance itself or some fragment of it is bonded chemically to the electrode. As electrodes mostly have physically heterogeneous surfaces (see Sections 4.3.3 and 5.5.5), the Temkin adsorption isotherm (Eq. 4.3.46) is suitable for characterizing the adsorption. 

Spirodela intermedia, L. minor, and P. stratiotes were able to remove Pb(II), Cd(II), Ni(II), Cu(II), and Zn(II), although the two former ions were removed more efficiently. Data fitted the Langmuir model only for Ni and Cd, but the Freundlich isotherm for all metals tested. The adsorption capacity values (K ) showed that Pb was the metal more efficiently removed from water solution (166.49 and 447.95 mg/g for S. intermedia and L. minor, respectively). The adsorption process for the three species studied followed first-order kinetics. The mechanism involved in biosorption resulted in an ion-exchange process between monovalent metals as counterions present in the macrophytes biomass and heavy metal ions and protons taken up from water.112... 

The Langmuir adsorption isotherm provides a simple mechanistic picture of the adsorption process and gives rise to a relatively simple mathematical expression. It can also be used to obtain a crude estimate of specific surface areas. More important, from the viewpoint of the chemical engineer, it serves as a point of departure for formulating rate expressions for heterogeneous catalytic reactions. 

Many individuals have developed more elegant theoretical treatments of adsorption processes since Brunauer, Emmett, and Teller published their classic paper. Nonetheless, the BET and Langmuir isotherms are the most significant ones for chemical engineering applications. 

Rhaman and coworkers [112,113] studied the adsorption of lipase on [MgAl] LDH and its biocatalytic activity for butyl oleate synthesis. They demonstrated that up to 277 and 531 mgg-1 of lipase were adsorbed on [MgAl-N03] and [MgAl-Dodecylsulfate] LDH, respectively, showing the highest adsorption capacity of the anionic clays compared to smectite or inorganic phosphate. Recently, we reported the adsorption isotherms of urease on [ZnRAl] LDH under various experimental conditions (pH, buffer) [117]. The kinetic study showed the fast adsorption process (less than 60 min) (Figure 15.3). 

Adsorption isotherms obtained from the model have been shown to agree very closely with the predictions of recently published statistical theories (9,13). While there can be no doubt that the more sophisticated, statistical models provide more information on the nature of the adsorption process and the structure of the adsorbed film, because of its simple form, the macroscopic model can offer a powerful tool for the analysis, interpretation and utilization of adsorption data. 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

Consider the adsorption of a species A with concentration ca in the bulk of the solution. The variation of the coverage 9 with ca, keeping all other variables fixed, is known as the adsorption isotherm. We regard the adsorption process as a reaction between the free sites on the electrode, whose number is proportional to (1 — 6), and the species A in the solution. Using absolute rate theory, we can write the rate of adsorption as ... 

Data on the adsorption of caprylic acid on a hydrophobic (mercury) surface in terms of a double logarithmic plot of Eq. (4.13) Panel a) compares the experimental values with a theoretical Langmuir isotherm, using the same values for the adsorption constant B for both curves. Panel b) shows that the adsorption process can be described by introducing the parameter a, which accounts for lateral interaction in the adsorption layer. Eq. (4.13) postulates a linear relation between the ordinate [= log [0/ 1 - 0)] - 2a 0 / (In 10)] and the abscissa (log c). If the correct value for a is inserted, a straight line results. For caprylic acid at pH 4, a value of a = 1.5 gives the best fit. 

In dilute solutions of surfactants adsorption processes are controlled by transport of the surfactant from the bulk solution towards the surface as a result of the concentration gradient formed in the diffusion layer the inherent rate of adsorption usually is rapid. For non-equilibrium adsorption the apparent (non-equilibrium) isotherm can be constructed for different time periods that are shifted with respect to the true adsorption isotherm in the direction of higher concentration (Cosovic, 1990) (see Fig. 4.10). 

In many works in which the adsorption process is described using the Frumkin isotherm, the solvent activity is considered as constant for every value of ag in view of the fact that the ratio of the molar fractions Xg/Xg is small. Hence,... 

Abstract Removal of catechol and resorcinol from aqueous solutions by adsorption onto high area activated carbon cloth (ACC) was investigated. Kinetics of adsorption was followed by in-situ uv-spectroscopy and the data were treated according to pseudo-first-order, pseudo-second-order and intraparticle drfiusion models. It was fotmd that the adsorption process of these compotmds onto ACC follows pseudo-second-order model. Furthermore, intraparticle drfiusion is efiective in rate of adsorption processes of these compoimds. Adsorption isotherms were derived at 25°C on the basis of batch analysis. Isotherm data were treated according to Langmuir and Freundhch models. The fits of experimental data to these equations were examined. 

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