Solution adsorption isotherms

Giles C H, Smith D and Huitson A 1974 General treatment and olassifioation of the solute adsorption isotherm J. Colloid Interface Sci. 47 755-65... 

Both kinetic and equilibrium experimental methods are used to characterize and compare adsorption of aqueous pollutants in active carbons. In the simplest kinetic method, the uptake of a pollutant from a static, isothermal solution is measured as a function of time. This approach may also yield equilibrium adsorption data, i.e., amounts adsorbed for different solution concentrations in the limit t —> qo. A more practical kinetic method is a continuous flow reactor, as illustrated in Fig. 5. 

C. H. Giles, T. H. MacEwan, S. N. Natchwa and D. Smith, Studies in Adsorption, Part XI A System of Classification of Solution Adsorption Isotherms and its Use in Diagnosis of Adsorption Mechanism and its Measurement of Specific Surface Area of Solids, J. Chem. Soc., p. 3973,1960. 

Adsorption Isotherms. The adsorption isotherms were determined using the serum-replacement adsorption or desorption methods (7). For the adsorption method, the latex samples (50 or 100 cm 2% solids) containing varying amounts of PVA were equilibrated for 36 hours at 25°C, placed in the serum replacement cell equipped with a Nuclepore membrane of the appropriate pore size, and pressurized to separate a small sample of the serum from the latex. For the desorption method, the latex samples (250 cm 2.5% solids) were equilibrated for 36 hours at 25°C and subjected to serum replacement with DDI water at a constant 9-10 cm /hour. The exit stream was monitored using a differential refractometer. The mean residence time of the feed stream was ca. 25 hours. It was assumed that equilibrium between the adsorbed and solute PVA was maintained throughout the serum replacement. For both methods, the PVA concentration was determined using a An-C calibration curve. 

Fig. 4.18 shows the result of Cd2+ adsorption on illite in presence of Ca2+ (Comans, 1987). The data are fitted by Freundlich isotherms after an equilibration time of 54 days. It was shown in the experiments leading to these isotherms that adsorption approaches equilibrium faster than desorption. Comans has also used 109Cd to assess the isotope exchange he showed that at equilibrium (7-8 weeks equilibration time) the isotopic exchangeabilities are approximately 100 % i.e., all adsorbed Cd2+ is apparently in kinetic equilibrium with the solution. The available data do not allow a definite conclusion on the specific sorption mechanism. 

The relationship between qf and Cf is given by the single solute adsorption isotherm ... 

According to another system of classification of solution adsorption isotherms by Giles et al. [18] the main classes are S, L, H and C isotherms. These workers indicated that L curves occur in probably the majority of cases of adsorption from... 

Adsorption kinetic and adsorption isotherm of pesticide metobromuron at the high area ACC were investigated in relation to water treatment. The ACC used in this study seems to be quite effective in adsorption of metobromnron from aqueous solutions. Adsorption of that pesticide was found to follow second-order kinetic model and the adsorption isotherm is well represented by Frenndlich model. 

Recently, 500 MHz i fluorine NMR was used to study adsorption of Nafion ionomer on PEFC catalysts and the supporting carbons in aqueous solution. It was observed that Nafion adsorbs strongly on carbon as well as on Pt and PtRu. The adsorption was classified into primary and secondary adsorption. At low concentration of Nafion ionomer, the adsorption was found to follow a Langmuir isotherm (primary adsorption). Although there was uncertainty in the types of adsorption isotherms at high concentration of Nafion ionomer, the secondary adsorption isotherms were fitted to a Langmuir isotherm as well. 

The data which are plotted as isotherms in the case of adsorption from liquid solutions on solid adsorbents are different in nature from those of gas (or vapor) adsorption on the same adsorbents. In fact, while the isotherm for adsorption of a single gas by a solid represents directly the quantity (weight or volume under standard conditions) of gas adsorbed per unit weight of the solid, the experimental measurement in adsorption from solution is the change in concentration of the solution which results from adsorption. The fact that a change in concentration is measured emphasizes that there are at least two components in the solution [13]. 

A number of related non-ideal adsorption isotherms have been proposed in order to fit experimental data of adsorption at electrodes and a review is presented in ref. 107. Recently, a simplified molecular theory of solute adsorption at polarizable electrode interface has been presented by Fawcet and Nobriga [108]. 

The Exchange Isotherm. For adsorption at constant pH the amount of selenite on the surface is related to the solution concentration by an equation of the Langmuir type (Equation 1). 

Similarly, when two or more adsorptives adsorb competitively on a surface, the adsorption isotherm for adsorptive i at a given temperature is a function of the equilibrium partial pressures of all of the adsorptives. In the case of adsorption from a liquid solution, an adsorption isotherm for any preferentially adsorbed solute may be similarly defined in terms of the equilibrium concentration of the respective solution component, but the isotherm usually... 

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-... 

Adsorption isotherm An equation or distribution on a graph representing the concentration of an adsorbed chemical species as a function of its concentration in an associated aqueous solution. Adsorption isotherms are measured in systems that are at equilibrium and where temperature, pressure, and possibly other conditions are held constant. Isotherms are often described with linear, Freundlich, or Langmuir equations. 

Solute adsorption is usually restricted to a mono molecular layer, since the solid-solute interactions, although strong enough to compete successfully with the solid-solvent interactions in the first adsorbed monolayer, do not do so in subsequent monolayers. Multilayer adsorption has, however, been observed in a number of cases, being evident from the shape of the adsorption isotherms and from the impossibly small areas per adsorbed molecule calculated on the basis of monomolecular adsorption. 

Giles, C. H, Smith, D., and Huitson, A. (1974). A general treatment and classification of the solute adsorption isotherms. I. Theoretical. J. Coll. Interface Sci. 47,755-765. 

While much has been learned about the role and selection of the operation parameters in displacement chrcmatography (60-66), little is known yet about the rules of displacer selection and the means available to control the selectivity of the separation. The paucity of well characterized displacers and the lack of knowledge of the solute adsorption isotherms hinder most strongly the wider acceptance and use of displacement chromatography. In most cases, displacer selection is still done by trial-and-error. In the majority of modem displacement chromatographic publications a reversed-phase system was used to separate small polar molecules, antibiotics, oligopeptides and small proteins (52-66). 

One more isotherm equation that could be helpful for the determination of the micropore volume is the osmotic isotherm of adsorption. Within the framework of the osmotic theory of adsorption, the adsorption process in a microporous adsorbent is regarded as the osmotic equilibrium between two solutions (vacancy plus molecules) of different concentrations. One of these solutions is generated in the micropores, and the other in the gas phase, and the function of the solvent is carried out by the vacancies that is, by vacuum [26], Subsequently, if we suppose that adsorption in a micropore system could be described as an osmotic process, where vacuum, that is, the vacancies are the solvent, and the adsorbed molecules the solute, it is possible then, by applying the methods of thermodynamics to the above described model, to obtain the so-called osmotic isotherm adsorption equation [55] ... 

The first recorded isotherms of adsorption from solution were probably those reported by van Bemmelen in 1881 (Forrester and Giles, 1972). In his investigations of the absorptive power of soils, van Bemmelen noted the importance of the colloidal structure and drew attention to the relevance of the final state (i.e. equilibrium concentration) of the solution in contact with the soil. 

Solid-solution adsorption systems have been found to give a variety of isotherm types, as classified by Giles et al. (1960). Many exhibit the characteristic Type I shape (see Figure 1.7), with the equilibrium concentration, c, replacingp[p°. In these cases, over a limited range of c, it is usually possible to apply an empirical equation of Langmuir form... 

The model has been further tested in terms of the behavior of solvent molecules isotherms for binary-solvent mixtures A/B adsorption energies (ch values) of the polar solvent B in such mixtures as a function of surface coverage 6b, etc. Again, good agreement of experimental data with calculated values is observed. An important requirement of the model and related experimental correlations is that the solvent molecule must be treated (thermodynamically) in the same manner as solute molecules. Thus, if. solute adsorption energies are measured for a molecule X, the behavior of X as a component (solvent) of the mobile phase should then be predictable. 

The adsorption capacity of activated carbon may be determined by the use of an adsorption isotherm. The adsorption isotherm is an equation relating the amount of solute adsorbed onto the solid and the equilibrium concentration of the solute in solution at a given temperature. The following are isotherms that have been developed Freundlich Langmuir and Brunauer, Emmet, and Teller (BET). The most commonly used isotherm for the application of activated carbon in water and wastewater treatment are the Ereundlich and Langmuir isotherms. The Freundlich isotherm is an empirical equation the Langmuir isotherm has a rational basis as will be shown below. The respective isotherms are ... 

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