Isotherms sorption and desorption

Fig. 3 Water vapor sorption and desorption isotherms for ipratropium bromide at 20°C ( ), 2 month sorption (A), 5 month sorption ( ), 2 and 5 month desorption [Note All 2 month sorption results, except at 53% and 63% relative humidity, were verified at 5 months.]...

Figure 4 also shows the excellent fit to the GAB equation (Eq. 10) of the sorption and desorption isotherms for microcrystalline cellulose. In this regard, this equation offers considerable practical utility in fitting isotherms for these types of materials over the entire relative humidity range, especially in contrast to the BET equation, which usually only fits uptake data up to about 40% relative humidity. As we have mentioned, however, this does not in itself confirm the validity of the GAB model for describing moisture sorption data on these materials. Rather, independent confirmation of the physical meaning is necessary. 

Figure 2.6 A sorption and desorption isotherm, showing hysteresis.

Assumptions inherent in the use of a Koc (or Kom) are that sorption is exclusively to the organic component of the soil all soil organic carbon has same sorption capacity per unit mass equilibrium is observed in the sorption-desorption process and the sorption and desorption isotherms are identical (Green and Karickhoff, 1990). Both Koc and Kd have units of L/kg or cm3/g. 

Sorption and desorption isotherms were obtained for sorption of radionuclides under oxidizing and reducing conditions. The Freundlich equation accurately describes most of these isotherms. Most radionuclides are apparently irreversibly sorbed on each of the geologic solids since the slopes of sorption and desorption isotherms for a given radionuclide are different. This hysteresis effect is very large and will cause a significant delay in radionuclide transport. It, therefore, should be included in modeling radionuclide transport to accurately assess the isolation capabilities of a repository in basalt. 

This equation has been successfully applied to many sorption and desorption reactions of dissolved metals and organic compounds. In the case of irreversible sorption (hysteresis), sorption and desorption isotherms are not identical. However, both sorption and desorption Freundlich isotherm equations can be substituted into the transport equation(2) ... 

This equation can be used to describe one-dimensional transport of radionuclides through porous media (e.g. radionuclide elution curves from laboratory columns packed with interbed solids) assuming instantaneous sorption and desorption. Van Genuchten and coworkers have demonstrated the importance of using both sorption and desorption isotherms in this equation when hysteresis is significant. Isotherm data for sorption and desorption reactions of radionuclides with interbed materials are presented in this paper which can be used to predict radionuclide transport. 

Sorption and Desorption Isotherms. To model radionuclide transport in groundwater through geologic media, it is necessary to mathematically describe sorption and desorption in terms of isotherms. The Freundlich isotherm was found to accurately describe sorption and desorption of all radionuclides studied in the interbed-groundwater systems, except when precipitation of the radionuclide occurred. 

In addition to describing sorption and desorption, isotherms can be used to estimate the solubilities of radionuclides in the groundwater-radionuclide-geologic solid system. For radionuclides that form slightly soluble compounds (e.g., SrC03, Pu02 H20) in these systems, isotherms can define the approximate concentrations above which precipitation, rather than sorption, dominates removal from solution. 

Figure 2. Sorption and Desorption Isotherms for Neptunium Sorption on Mabton Interbed Solids, (a) Oxidizing Conditions, (b) Reducing Conditions.

The experimental approach for obtaining sorption and desorption isotherms as well as hysteresis eflFects is well known and will not be discussed further. However, diflFusion measurements will be given a somewhat more detailed treatment. 

The amount of water sorbed in a solid also decreases as the solid is heated. Complete removal of this type of moisture at 100°C is by no means a certainty, however, as indicated by the drying curves for an organic compound shown in Figure 35-4. After this material was dried for about 70 min at 105°C, its mass apparently became constant. Note, however, that additional moisture was removed by further increasing the temperature. Even at 230°C, dehydration was probably not complete. Commercial vapor sorption analyzers can automate the acquisition of moisture sorption and desorption isotherms. 

Equilibrium Moisture Content-Equilibrium Relative Humidity Relationships Used to Analyze Sorption and Desorption Isotherms of Grains and Oilseeds... 

A number of equilibrium moisture data have been found in the literatnre for peat also. Larsson and Wimmerstedt [21] presented a data for peats of different origin and humification. Results from the investigation are presented in Figure 36.11. The sorption and desorption isotherms at 30°C and for different peats are shown in Figure 36.11a. 

Figure 14. Sorption and desorption isotherms for Se(VI) and Se(IV) reacted vsdth am-Fe(OH)3 at pH 5. Adsorption and desorption points were determined after reaction for 24 h. From Su and Suarez (23).

A recent study reported by LeBoeuf and Weber 49) illustrates the impact of rubbery and glassy matrices on long term sorption/desorption behavior of phenanthrene. In that study, both sorption and desorption isotherm data for these systems were fit to the Freundlich equation, as a simple means for characterizing relative linearity... 

Figures 9 and 10 show phenanthrene sorption and desorption isotherms measured for the peat and the shale. The values of organic carbon-normalized are 21.7 and...

Figure 7. Aqueous-phase sorption and desorption isotherms for rubbery cellulose (T = -45°C) (Adapted from ref 49).

A typical pattern of change in the time-dependent SSA-normalized distribution coefficient, K (t), as a function of t is shown in Figure 11 for a-Al203, silica gel-40 and -100. It is clear from this Figure that sorption is completed within a few minutes, regardless of that facts that i) the two gel samples have enormous internal surface areas and ii) the particles of these solids have diameters of 63-150 pm. Complete reversible sorption is shown in Figure 12 for silica gel-40, in which both sorption and desorption isotherms coincide, indicating no hysteresis. 

Parameters of T equation applied to sorption and desorption isotherms shown in Figures 5, 6 and 7... 

Fig. 39.4 (a) Gas sorption isotherm of 2b at 296 K for N2, Air, O2 and CO2 (b) SectiMial views of dimaeric capsule formed from 2, two calixarenes are colored blue and red while sectiOTied surface kept yellow with green methane molecules, (c) Sorption of acetylene and CO2 at room temperature (d) Hysteresis in sorption and desorption isotherm of acetylene (Reproduced with permission from publisher Refs. [16,26b, 23b]. Copyright 2004,2005, and 2006, Wiley-VCH and Royal Society of Chemistry)... 

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