Sorption isotherm results

Sorption Modeling. Pesticide sorption is characterized by describing sorption isotherms using the Freundhch equation, S = Kj, where S is the pesticide sorbed concentration, C is the pesticide solution concentration after equdibration, and fy and N are constants. Although other equations have been used, the Freundhch has satisfactodly described experimental sorption results for a wide range of pesticides in a variety of sods. The value of N is usually <1 and between 0.75 and 0.95, which indicates that pesticides are proportionally more sorbed at low solution concentration than at high solution concentration. 

Generally the above expressions adequately describe sorption isotherms of the type reported in Fig. 1, giving good agreement with experimental results in non-... 

Typically, sorption isotherms are constructed for a single food ingredient or food system. An alternative approach is to plot the moisture content versus water activity (or relative vapor pressure) values for a variety of as is food ingredients and food systems. The result is a composite food isotherm (Figure 17). The composite isotherm fits the typical shape observed for a sorption isotherm for an individual food system, with a few products falling above or below the isotherm curve (chewing gum, honey, raisins, bread, and colby and cheddar cheeses). Slade and Levine (1991) were the first to construct such a plot using moisture content and aw values from van den... 

A sorption isotherm expresses the quantity of material adsorbed per unit mass of adsorbent as a function of the equilibrium concentration of the adsorbate. The necessary data is derived from experiments where a specified mass of adsorbent is equilibrated with a known volume at a specific concentration of a chemical and the resultant equilibrium concentration is measured in solution [33]. 

It is important to differentiate between the two different types of sorption/ desorption tests (i. e.,batch and column-leaching), and the sorption characteristics determined from one should not be confused with the other. Sorption isotherms obtained with batch equilibrium tests are applied mainly to solid suspensions. The physical model, assumed with this situation, is one of a completely dispersed solid particle system, where all solid particle surfaces are exposed and available for interactions with the contaminants of concern. In contrast, column-leaching tests are performed with intact solid samples, and the sorption characteristics obtained from them are the results of contaminant interactions with a structured system where not all-solid particle surfaces are exposed or available for interactions with the contaminants. 

Popper and Bariska (1972) studied the moisture sorption properties of wood chemically modified with acetic (or phthalic) anhydride and analysed the results using Brunauer-Emmett-Teller (BET) theory and the H-H model. Acetylation was found to reduce the number of sorption sites, whereas little effect was noted with phthaloylation. By dividing the sorption isotherm into a monolayer component and a multilayer component using the H-H model, it was shown that there was a large reduction in the... 

As discussed above, hysteresis loops can appear in sorption isotherms as result of different adsorption and desorption mechanisms arising in single pores. A porous material is usually built up of interconnected pores of irregular size and geometry. Even if the adsorption mechanism is reversible, hysteresis can still occur because of network effects which are now widely accepted as being a percolation problem [21, 81] associated with specific pore connectivities. Percolation theory for the description of connectivity-related phenomena was first introduced by Broad-bent et al. [88]. Following this approach, Seaton [89] has proposed a method for the determination of connectivity parameters from nitrogen sorption measurements. 

The overloading of the stationary phase is related to the maximum solute concentration. Cm, at which the sorption isotherm associated with equilibrium distribution underlying chromatographic retention ceases to be linear. That deviation results in a broadening and deformation of the peak profile. Since this review deals with chromatographic phenomena and optimization we consider thermodynamics as beyond its scope. 

If a species sorbs to a crystalline mineral surface, in an ideal case, no more sorption occurs when all the available surface sites are occupied and concentrations in the solid phase remain constant ( ). In reality surface precipitation may occur, resulting in a sorption isotherm schematically shown in Fig. 4 ( ). 

To illustrate this a model transesterification reaction catalyzed by subtilisin Carls-berg suspended in carbon dioxide, propane, and mixtures of these solvents under pressure has been studied (Decarvalho et al., 1996). To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents. Water sorption isotherms were measured for the enzyme in each solvent. Catalytic activity as a function of enzyme hydration was measured, and bell-shaped curves with maxima at the same enzyme hydration (12%) in all the solvents were obtained. The activity maxima were different in all media, being much higher in propane than in either CO2 or the mixtures with 50 and 10% CO2. Considerations based on the solvation ability of the solvents did not offer an explanation for the differences in catalytic activity observed. The results suggest that CO2 has a direct adverse effect on the catalytic activity of subtilisin. 

Figure 2.1.6.9 and Table 2.1.6.4 present the N2-sorption isotherms and the results of the characterization of the Al-MCM-41 parent material and of the by ion exchange and impregnation onto Al-MCM-41 immobihzed (S,S)-Co( 11)-Jacobsen complexes. The N2-sorption isotherms of the loaded materials clearly illustrate that (S,S)-Co(II)-Jacobsen complex is deposited in the inner surface of the pores... 

The swelling of clays in water results from the extra hydration of the interlamellar cations (Fig. 77). This is the best known example of the important phenomenon of intercalation, which is simply the insertion of guest species into an accommodating host, usually, but not exclusively, a layered solid. The degree of swelling, however, is governed by the nature of the interlamellar cation and the sorption isotherm often exhibits steps, as so often occurs with clathrates. 

Humidity control The question of humidity control was examined in several ways. The moisture sorption isotherm showed moisture levels from 3-6% over a relative humidity range of 20-95%. Also, samples held at 65°C for over 2 weeks remained within this range of water content, which was within pharmacopeial specification. Finally, samples which were heated did not show a change in crystal form by X-ray powder diffraction, such as partial conversion to an anhydrate. Based on these results, humidity control was not deemed necessary for comparative stress studies. Samples to be compared were all stored under identical conditions, i.e., in the same oven. Any local change in humidity around each sample was therefore a function of the sample itself. 

The mean hydraulic conductivities for the aquifer sand determined using the two laboratory methods (constant-head and falling-head permeameters) were 5 10"4 m sec"1 and 2 1 O 4 m sec"1, respectively. The isotherm results showed that the sand had negligible sorption capacity for either chromate 01 PCE. 

Using the above equation we calculated retardation factors of 44 for chromate and 39 for PCE. These pilot-test retardation factors are very close to the estimates of 42 and 29 calculated in section 3.1 for chromate and PCE based upon their laboratory sorption isotherms. The pilot test results thus confirm that the field barrier performance of SMZ may be predicted reasonably well from laboratory characterization of contaminant interactions. 

Figure 4 shows phenanthrene and naphthalene sorption isotherms to kaolinite covered with varying levels of sorbed surfactant these levels of surfactant coverage correspond to the different regions existing in the surfactant sorption isotherms discussed earlier (Fig. 1). The linearity of each isotherm was evaluated using Freundlich and linear sorption models. It is apparent from Fig. 4 and Table 4 that HOC partitioning to kaolinite with and without adsorbed surfactants results in linear or near-linear isotherms. As the amount of surfactant adsorbed on the kaolinite surface increased, the sorption of phenanthrene and naphthalene to the solid phase also increased. However, upon normalizing by the amount of sorbed surfactant present, the sorbed surfactant partition coefficient (Kss) decreased with increasing sorbed surfactant amounts (Table 4).

Table II Some results of the analysis of the NH3 sorption isotherms on beechwood.

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