Adsorption from Liquid Solutions

In this chapter, we have so far discussed the adsorption of gases in solids. This section gives a brief description of the adsorption process from liquid solutions. This adsorption process has its own peculiarities compared with gas-solid adsorption, since the fundamental principles and methodology are different in almost all aspects [2,4,5], In the simplest situation, that is, a binary solution, the composition of the adsorbed phase is generally unknown. Additionally, adsorption in the liquid phase is affected by numerous factors, such as pH, type of adsorbent, solubility of adsorbate in the solvent, temperature, as well as adsorptive concentration [2,4,5,84], This is why, independently of the industrial importance of adsorption from liquid phase, it is less studied than adsorption from the gas phase [2], 

Nevertheless, liquid-phase adsorption fundamentally onto activated carbon [2,85], silica [2,86,87], zeolites [2,88,89], and resins [90] provides a feasible technique, and is one of the most extensively used technologies for removal of organic pollutants from industrial wastewater [2,8,84-90], 

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The same relation has been observed in adsorption from liquid solutions, but is not absolute. 

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]. 

Adsorption from liquid solution is almost a new world in comparison with adsorption from the gas phase the fundamental principles and methodology are different in almost all respects (Gregg, 1961). 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

Three carbon samples showing differences in pore structure are chosen to study the effect of porous texture on adsorption from liquid solutions. The benzene adsorption/desorption isotherms are applied to determine the properties of geometrical surface structure of investigated carbons. The liquid adsorption data are analyzed in terms of the theory of adsorption on heterogeneous solids. The relation between parameters of porous structure of the activated carbon samples and parameters of adsorption from the liquid phase is discussed. 

Richardeau et al. investigated thiophene adsorption from liquid solutions containing hydrocarbons over HFAU zeolites in a stirred batch system at room temperature.144 They found that the maximum number of thiophene molecules adsorbed per gram of zeolites is equal to their concentration of acidic sites and considered that the acidic sites are the adsorption sites. They further found that the presence of toluene causes a large decrease in the removal of thiophene, and when the concentration of thiophene is high (27.7 wt%), an acid-catalyzed condensation of thiophene occurs to form dimers, trimers, and tetramers, which remain trapped on the zeolite. They concluded that thiophene removal by adsorption on acidic zeolites could only be carried out from diluted solutions containing no olefinic compounds. 

Radlce, C. J., Prausnitz, J. M. Thermodynamics of multi-solute adsorption from liquid solutions, AIChE J., 1972, 18, 1, 761-768. 

This is one of the oldest empirical adsorption isotherms, developed by Freundlich in 1907. It is useful for adsorption from liquid solutions and also for chemisorption isotherms. 

Larionov and co-workers (Institute of Physical Chemistry, the U.S.S.R. Academy of Sciences, Moscow) (308-312) carried out systematic theoretical and experimental investigations of the adsorption from liquid solutions of nonelectrolytes on silica adsorbents. They studied the adsorption of individual substances and binary liquid solutions (benzene/carbon tetrachloride, carbon tetrachloride/isooctane, benzene/isooctane, etc.) on Si02 samples with different degrees of porosity but identical surface chemical properties. The experimental results were compared with the theoretical calculations carried out by the Gibbs method. This procedure made it possible to calculate the dependence of the enthalpy, entropy, and free energy of wetting on the concentration and to obtain expressions describ-... 

The mathematical models that have been applied to the physical adsorption from liquid solutions are generally extensions of the theories that have been developed to describe the sorption of gases on solid surfaces with modifications to account for the competition between the solute and solvent for the adsorption sites. Two of these models have been applied to the adsorption isotherms of nonelectrolytes from solution they are the Langmuir model and the Brunauer, Emmett, and Teller (BET) model in addition the Freundlich empirical equation has also been used. In the Langmuir model it is assumed that the adsorbed species forms a monolayer on the surface of the adsorbent, that the adsorbed molecules... 

On the other hand, the difference will be accentuated in cases where there is a weak specific uptake of one component into the adsorbed phase because the contribution of n to n will be small and the second term, on the right-hand side of Equation (5), will contribute more to the amount adsorbed. Then approximation (6) cannot be used and Equation (5) must be utilized and therefore, a definition of the adsorbed phase, (n ), must be specified. This is commonly done by the acceptance of the monolayer concept of adsorption from liquid solutions. It is assumed that only one layer of molecules covering the solid surface is affected by the solid and hence only this monolayer differs from the bulk liquid phase. This concept allows the number of molecules present in the adsorbed phase to be calculated based on a known specific area of solids and known cross-sectional areas of molecules of the adsorbate and the solvent ... 

Adsorption from liquid solution is more complicated than that from the gas phase. In a two-component liquid solution, both the solvent and the solute will be adsorbed to different extents. Usually the adsorption of the solute is of interest. The experimental procedure and expression for the amount adsorbed both differ from those used for gas adsorption. The basis for expressing the amount adsorbed from liquids is the concept of surface excess given by Gibbs in 1878. Gibbs surface excess is the difference in the amount of a given component in the surface layer (per unit surface area) over that in the bnlk Uqnid. 

Adsorption from liquid solution is complicated by the presence of the solvent. Interactions between solute-surface, solvent-surface as well as solute-solvent are all involved. The effects of solubility on adsorption have long been known. 

In the seventies and eighties there were developed also the studies of adsorption from liquid solutions [14,19,20] and gas mixtures [325] on the solid heterogeneous surfaces. Then adsorption both fi-om diluted solutions [325] and... 

An interesting discussion of lAST, including references for its extension to adsorption from liquid solutions, heterogeneous LAST etc., is provided by Shapiro and Stenby (2002). 

The application of Langmuir and lAST/RAST to adsorption from liquid solutions is limited, thus we concentrate our comparative evaluation on the adsorption of gas mixtures on solids. 

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