Adsorbents isotherm classifications

Based on their molecular properties as well as the properties of the solvent, each inorganic or organic contaminant exhibits an adsorption isotherm that corresponds to one of the isotherm classifications just described. Figure 5.1 illustrates these isotherms for different organic contaminants, adsorbed either from water or hexane solution on kaolinite, attapulgite, montmorillonite, and a red Mediterranean soil (Yaron et al. 1996). These isotherms may be used to deduce the adsorption mechanism. 

Figure 3.1 The five isotherm classifications according to BDDT. W, weight adsorbed F, adsorbate equilibrium pressure Pq, adsorbate saturated equilibrium vapor pressure P/Pq, relative pressure. Condensation occurs at P/Pq >1.

Of the five isotherm classifications depicted in Fig. 3.1 the types I, IV and V isotherms are associated with porosity. The type I isotherm usually corresponds to microporosity, that is, pores of diameters only slightly larger than adsorbate molecules. The types IV and V isotherms are associated with pores ranging in radius from about fifteen to several hundred angstroms. The type IV isotherm is more frequently encountered with porous adsorbents. 

The nitrogen isotherms for the two carbons are shown in Figure 1(a) in the conventional form of amount adsorbed, n, vs the relative pressure, p/p, and in Figure 1(b) in the semi-log form of vs log p/p. In a new extended isotherm classification [1], the isotherm given by sample A is of Type Ib and that by sample B is a composite isotherm of Types Ib and IVb. From the isotherm shapes in Figure 1(a), we can tentatively conclude that sample A has a wide range of micropores and that sample B is both microporous and mesoporous. 

Type IV isotherms are obviously similar to type II and usually correspond to systems involving capillary condensation in porous solids. In this case, however, once the pores have become filled, further adsorption to form multilayers does not occur and the terminating plateau region results. This would indicate a relatively weak interaction between the adsorbate molecules. While more complex isotherm classifications are available, they generally represent combinations and extensions of the five basic types described above. 

Figure 10.5. Adsorption isotherm classifications covering greater concentrations of adsorbate.

The basis of the classification is that each of the size ranges corresponds to characteristic adsorption effects as manifested in the isotherm. In micropores, the interaction potential is significantly higher than in wider pores owing to the proximity of the walls, and the amount adsorbed (at a given relative pressure) is correspondingly enhanced. In mesopores, capillary condensation, with its characteristic hysteresis loop, takes place. In the macropore range the pores are so wide that it is virtually impossible to map out the isotherm in detail because the relative pressures are so close to unity. 

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Flat Surface Isotherm Equations The classification of isotherm equations into two broad categories for flat surfaces and pore filling reflec ts their origin. It does not restrict equations developed for flat surfaces from being apphed successfully to describe data for porous adsorbents. 

The adsorption capacities of the adsorbents are usually determined from modeling of the adsorption isotherms according to the Giles s classification [36] (Figure 15.1). 

The advantage of equation 17.14 is that it may be fitted to all known shapes of adsorption isotherm. In 1938, a classification of isotherms was proposed which consisted of the five shapes shown in Figure 17.5 which is taken from the work of Brunauer et alSu Only gas-solid systems provide examples of all the shapes, and not all occur frequently. It is not possible to predict the shape of an isotherm for a given system, although it has been observed that some shapes are often associated with a particular adsorbent or adsorbate properties. Charcoal, with pores just a few molecules in diameter, almost always gives a Type I isotherm. A non-porous solid is likely to give a Type II isotherm. If the cohesive forces between adsorbate molecules are greater than the adhesive forces between adsorbate and adsorbent, a Type V isotherm is likely to be obtained for a porous adsorbent and a Type III isotherm for a non-porous one. 

A typical N2 adsorption measurement versus relative pressure over a solid that has both micropores and mesopores first involves essentially a mono-layer coverage of the surface up to point B shown in isotherm IV (lUPAC classification) in Figure 13.1. Up to and near point B the isotherm is similar to a Langmuir isotherm for which equilibrium is established between molecules adsorbing from the gas phase onto the bare surface and molecules desorbing from the adsorbed layer. The volume of adsorbed N2 that covers a monolayer volume, hence the surface area of N2 can then be determined from the slope of the linearized Langmuir plot when P/V is plotted against P ... 

Many adsorption isotherms are borderline cases between two or more of the above types. In addition, there are some isotherms which do not fit into Brunauer s classification at all, the most notable being the stepwise isotherms, an example of which is given in Figure 5.6. Stepwise isotherms are usually associated with adsorption on to uniform solid surfaces, each step corresponding to the formation of a complete monomolecular adsorbed layer (see page 133). 

The sorption isotherms can be grouped into five types, according to the classification of Brunauer, Emmet and Teller. l,2 3A However, we prefer a classification, based on the pore size of the adsorbent.5 The IUPAC classification6 of pores is given in table 2.1. 

The equilibrium isotherms for microporous adsorbents are generally of type I form in Brunauer s classification (Fig. 1). Such isotherms are commonly represented by the Langmuir model,... 

Adsorption isotherms are plots of the amount of gas adsorbed at equilibrium as a function of the partial pressure p/p°, at constant temperature. The quantity of gas adsorbed is mainly expressed as the mass of gas (usually g) or the volume of gas reduced to STP (standard temperature and pressure). The majority of isotherms which result from physical adsorption may conveniently be grouped into five classes — the five types I to V included in the classification originally proposed by Brunauer, Deming, Deming and Teller — sometimes referred to simply as the Brunauer classification [2]. The essential features of these types are indicated in Fig. 12.1. 

Bilayer architectures formed in M2(2)3(N03)4 n (where M = Co, Ni and Zn) were one of the first systems of coordination polymers to be shown as porous materials [43]. The bilayer architectures interdigitate with each other leaving small channels in the crystal lattice which were occupied by solvated water molecules. Powder X-ray studies indicate that the water molecules can be removed from the network without causing any distortion or decomposition of the network. The adsorption studies of water removed and dried sample indicated that the material is capable of adsorbing CH4, N2 and 02. About 2.3 mmol of CH4 and 0.80 mmol of N2 or 02 are adsorbed per gram of anhydrous material. The adsorption-readsorption followed the same isotherm, indicating the stability of the network throughout the process. Further, the isotherms for the adsorption-readsorption can be classified as type I in the IUPAC classification [48]. 

Adsorption/separation processes are based on adsorption isotherms (thermodynamics) and intracrystalline diffusivity (kinetics). Figure 16.1 illustrates various shapes of adsorption isotherms depending on the VOC nature, trichloroethylene (TCE) and tetrachloroethylene (PCE), and of the zeolite, MFI with Si/Al > 500 and FAU (Si/Al > 100) (14). The isotherms of VOCs adsorbed on FAU present a more or less S-shape which corresponds to type V of the IUPAC classification. In contrast, the isotherms of VOCs on MFI are more of type I, with the additional particularity of a step at 4 molecules per u.c. for PCE adsorption. The... 

According to the IUPAC classification (Everett, 1972 Sing et al., 1985), the upper limit of the internal micropore width is about 2 nm. A characteristic property of microporous adsorbents is that they give Type I physisorption isotherms (see Figure 1.7). As noted previously, the most distinctive feature of a Type I isotherm is the long, almost horizontal plateau, which extends across most, if not all, of the... 

Adsorbed amounts are commonly presented as an adsorption Isotherm, which is a plot of either or r as a function of the polymer solution concentration at a given temperature. Polymer adsorption typically leads to hlgh-afflnlty (H) Isotherms (see the classification in fig. 2.24). A typical example for a homodlsperse polymer is shown in fig. 5.7. As a rule considerable adsorption occurs even at extremely low concentrations, typically well below 1 g m 3. For somewhat higher concentrations, the Isotherm shows a nearly horizontal part, the pseudo-plateau (often simply referred to as "plateau ). Indicating saturation of the surface. For polydisperse polymers there is usually not such a well-defined pseudo-plateau, and a more rounded isotherm Is found (see sec. 5.3d below). 

The adsorption isotherms of nitrogen at -196 °C for the samples studied are shown in Fig. 4. All the isotherms are type II in the BDDT classification [10]. The BET apparent surface areas values are in Fig. 4. The increase in the temperature of treatment produces a decrease in the amount of nitrogen adsorbed, with an important decrease in the BET areas due to an ordering in the sample structure. The great differences in the BET surface areas of the samples make it possible to measure a wide range of aetive surface areas. 

The amount of water adsorbed, normalized to full saturation, is given in figure 3 (right part) as a fiinction of pressure normalized to the saturating pressure for SPC model (0.044 bar). The isotherm is of type IV in lUPAC classification, showing a rapid increase at low... 

The forms of the isotherms and hysteresis loops have been subject to a classification initially proposed by Brunauer and taken up by the lUPAC. This classification shows the relationship between the form of the isotherms, the average radius of the pores and the intensity of the adsorbate-adsorbant interactions. Four types of isotherms out of the six proposed by the lUPAC are commonly encountered (Fig. 1.1). Similarly, the hysteresis loops correspond-... 

Differences in the shapes of the isotherms are found (Figure 1). The OS-char shows a flat plateau which characterises type I-isotherms according to lUPAC classification, pointing to a predominantly microporous structure. volumes adsorbed on the AA-char are considerably larger compared to those determined on the other wastes. Results indicate... 

The adsorption isotherms from GCMC simulation are shown in figure 1 as plots of absolute adsorbate density versus fiigacity. They are all of type I in the Brunauer classification, showing a... 

For adsorbents with sufficiently laig e porosities (often referred to as mesoporous systems) isotherms can exhibit hysteresis between the adsorption and desorption branches as illustrated schematically in figure 1. A classification of the kinds of hysteresis loops has also been made. It is generally accepted that such behavior is related to the occurrence of capillary condensation - a phenomenon whereby the low density adsorbate condenses to a liquid like phase at a chemical potential (or bulk pressure) lower than that corresponding to bulk saturation. However, the exact relationship between the hysteresis loops and the capillary phase transition is not fully understood - especially for materials where adsorption cannot be described in terms of single pore behavior. 

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