H-Type isotherms

In soils, phosphate sorption data usually take the shape of a high-affinity (H-type) isotherm, like that of Figure 9.6. Because of this and the nonreversible behavior of sorption, the downward movement of phosphate and other strongly bonded elements can be approximated by the tipping bucket model. This model visualizes the layers... 

The H-type isotherm, indicative of very strong adsorbate-adsorbent interaction (i.e., chemisorption), is really an extreme case of the L-type. This isotherm is not often encountered with organic molecules because few of them form strong ionic or covalent bonds with soil colloids. 

Hinzu studied the adsorption of nonionic surfectants on a C18 bonded phase (Resolve C18, Table 4.1) [16]. H-type isotherms similar to the ones obtained with ionic surfactants (Figures 4.2 and 4.4) were establish for two polyoxyethylene dodecyl ether surfactants (Brij 22 and Brij 35). The surfactant adsorption increased beyond the surfactant cmc. The adsorbed Brij 22 amount increased from 1.4 pmol/m at 20 cmc (0.0002 M, cmc=10 M) to 2 pmol/m at 1700 cmc (0.16 M or 100 g/L), a 40% increase. The increase of the Brij 35 adsorbed amount was almost continuous. It was about 0.3 pmol/m at 2 cmc (0.0002 M, cmc=10 M) and 0.9 pmol/m at 850 cmc (0.08 M or 100 g/L), a 200% increase. The log Cg versus log C , Freundlich plot of the Brij 22 ackorption data showed a bilinear curve similar to the plots found in Figures 4.3 and 4.5. The log Cg versus log C Freundlich plot of the Brij 35 adsorption data was a straight line without a break at the cmc concentration. Brij 35 is a polar and highly water soluble surfactant, it may have a low affinity for the apolar bonded Resolve C18 stationary ph e [16]. 

In many cases lateral interactions—that is, interactions between neighboring adsorbates—are present, causing those adsorbates that adsorb to anonanpty surface to hnd different conditions compared with those arriving to a clean surface (in the sense of absence of adsorbates other than the solvent). The interactions can be attractive, sometimes giving rise to S-type isotherms as discussed in Section 4.2.2, or repulsive, which can result in H-type isotherms, as the incoming adsorbates find it more difficult to adsorb as the surface becomes more covered. 

The effect of these factors on the adsorption isotherm may be elucidated by reference to specific examples. In the case of the isotherm of Fig. 5.17(a), the nonporous silica had not been re-heated after preparation, but had been exposed to near-saturated water vapour to ensure complete hydroxylation. The isotherm is of Type II and is completely reversible. On the sample outgassed at 1000°C (Fig. 5.17(h)) the isotherm is quite different the adsorption branch is very close to Type III, and there is extrensive hysteresis extending over the whole isotherm, with considerable retention of adsorbate on outgassing at 25°C at the end of the run. 

The H-type (high affinity) isotherm is indicative of strong adsorbent-adsorbate interactions such as inner sphere complexes. 

Fig. 5.1 Examples of adsorption isotherms. S-type aldrin on oven dry kaolinite from aqueous solution. L-type parathion on oven-dry attapulgite from hexane solution. H-type methylene blue at pH = 6 on montmorillonite from aqueous solution. C-type parathion on clay soil from hexane solution (Yaron et al. 1996)...

An adsorbate A is adsorbed in a fixed-bed adsorber that is of 25 cm height and packed with active charcoal particles of 0.6 mm diameter. The concentration of A in a feed solution is 1.1 mol m , and the feed is supplied to the adsorber at an interstitial velocity of 1.6 m h L The adsorption equilibrium of A is given by the following Freundlich-type isotherm. 

J. Haberman T.C. Castorina, The Surface Chemistry of HMX , PATR 3089 (1963) [A study was made of the solution-adsorption properties of HMX using a 14C labeled quarternary ammonium salt, stearyl tri-methyl ammonium bromide (STAB). The solvent system used consisted of 10% ethanol and 90% w. The solution-adsorption system isotherms for 10-micron HMX were measured. The authors conclude that an H-type Langmuir adsorption isotherm is obtd, the adsorption of STAB on HMX is reversible and therefore physi-... 

H-type describes strong chemisorption interactions, which is basically an extreme case of the L-type isotherms (e.g., phosphate-iron oxide interactions)... 

Adsorption isotherms, 178-190 Freundlich, 179 Langmuir, 183 S-type, 178-179 L-type, 178-179 C-type, 178-179 H-type, 178-179 Aerobic decomposition, 323 Alkalinity, 82—91 Definition, 88 Types of alkalinity, 82 Aluminosilicate clays, 102 Aluminum cation, 103, 160 Acidity, 160 Complexation, 160 Polymeric aluminum, 160 Exchangeable, 160,162 Hydrolysis, 69, 75 Solubility, 71 Soluble complexes, 69 Aluminum hydroxide, 78-80 Solubility, 78 pH effect, 79... 

The nitrogen gas adsorption-desorption isotherms of the metakaolins are classified as type II (BDDT classification [27]). They are almost reversible with a closed hysteresis cycle, indicating the absence of micropores. The samples obtained at room temperature show N2 isotherms similar to those from metakaolins. This treatment did not significantly modify the structure of the metakaolin and the porous properties of these samples are very close to those of the parent metakaolin. The samples obtained under reflux conditions for 6h show nitrogen gas adsorption-desorption isotherms different from those of the parent metakaolins. They show an increase of adsorption at low relative pressures and reach a plateau at intermediate values of P/Po. This kind of isotherm is classified as type I (BDDT classification, [27]) and it is characteristic of microporous materials. For treatment times higher than 6 h, the isotherms are analogous to those of metakaolins, classified as type II [27], which indicates the loss of the microporosity formed at lower times. 

See Chapter 10 for a description of this shape of adsorption isotherm, classified as L-type or H-type. 

Organic matter can also adsorb basic molecules, and s-triazines in particular, by cation exchange. Adsorption is sensitive to pH, which not only determines the BH /B ratio in solution, but also the fraction of carboxylic acid groups that dissociate and thereby become potential adsorption sites for the organic cation. Adsorption isotherms are L-type or H-type, depending on the acidity of the soil organic matter. The steps involved in adsorption are ... 

The pure gas adsorption properties of the adsorbents with respect to methane, were investigated at ambient temperature (300 K) up to 35 bars. This investigation was performed by coupling a Tian-Calvet type isothermal microcaloiimeter and a manometric device built in house [17]. The absolute isotherms of adsorption were obtained from the correction of the primary excess values using an appropriate expression for gas non ideality in this range of pressure. In this way, the GPR equation of state was taken into account [28]. A point by point introduction adsorptive procedure was used to evaluate a pseudo-differential enthalpy of adsorption noted A,, h via the measured exothermic thermal effect associated with each dose. These calculations have been already detailed in our previous publications [13]. For each sample, the values of A h were obtained with a maximum bare error of 0.6 % in the whole range of pressure. 

RDS and the SSA may have to be used to determine surface concentrations for some species however, all adsorption/desorption steps are still quasi-equilibrated and Langmuir isotherms can still be used to describe the surface concentrations of species associated with these steps, hence the term L-H-type reactions. These statements also apply to the bimolecular surface reactions discussed in the next section. 

Very few attempts have been made to investigate the influence of porosity on the course of adsorption from solution. Recent work in this laboratory has shown that in favourable cases it is possible to extend the application of the OCg-method for the analysis of solute isotherms. The analysis gave a clear indication of the effect of primary micropore filling in distorting the isotherm shape when applied to certain L-type isotherms (e.g. of iodine and salicylic acid adsorption by activated charcoals), but difficulties were encountered when the non-porous reference material gave H-shaped or S-shaped isotherms (i.e. either very high or very low affinity). 

It was noted earlier (p. 115) that the upward swing in the Type IV isotherm characteristic of capillary condensation not infrequently commences in the region prior to the lower closure point of the hysteresis loop. This feature can be detected by means of an a,-plot or a comparison plot (p. 100). Thus Fig. 3.25(a) shows the nitrogen isotherm and Fig. 3.25(h) the a,-plot for a particular silica gel the isotherm is clearly of Type IV and the closure point is situated around 0 4p° the a,-plot shows an upward swing commencing at a = 0-73, corresponding to relative pressures of 013 and therefore well below the closure point. 

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