Adsorption concentrated solutions

A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

Equation XI-27 shows that F can be viewed as related to the difference between the individual adsorption isotherms of components 1 and 2. Figure XI-9 [140] shows the composite isotherms resulting from various combinations of individual ones. Note in particular Fig. XI-9a, which shows that even in the absence of adsorption of component 1, that of component 2 must go through a maximum (due to the N[ factor in Eq. XI-27), and that in all other cases the apparent adsorption of component 2 will be negative in concentrated solution. 

Dye adsorption from solution may be used to estimate the surface area of a powdered solid. Suppose that if 3.0 g of a bone charcoal is equilibrated with 100 ml of initially 10 Af methylene blue, the final dye concentration is 0.3 x 10 Af, while if 6.0 g of bone charcoal had been used, the final concentration would have been 0.1 x Qr M. Assuming that the dye adsorption obeys the Langmuir equation, calculate the specific surface area of the bone charcoal in square meters per gram. Assume that the molecular area of methylene blue is 197 A. ... 

Carbon adsorption Aqueous solutions typical concentration < 1 % SS < 50 ppm Adsorbate on carbon usually regenerated thermally or chemically... 

According to experimental data,208,209 the SNIFTIR technique can be used to probe the electrical properties of the electrical double layer even in more concentrated solutions where cyclic voltammetry (cv), impedance, chronocoulometry, and other techniques are not applicable. Iwasita and Xia210 have used FTIR reflection-adsorption spectra to identify the potential at which the orientation of water molecules changes from hydrogen down to oxygen down. 

In view of the fundamental importance of the Gibbs-Thomson formula, and the magnitude of the discrepancies between the figures calculated from it and the experimental results, it is of obvious interest to inquire to What causes the deviations may be due. The first point to be noticed is that the complex substances which exhibit them most markedly form, at least at higher concentrations, colloidal and not true solutions. It is, therefore, very probable that they may form gelatinous or semi-solid skins on the adsorbent surface, in which the concentration may be very great. There is a considerable amount of evidence to support this view. Thus Lewis finds that, if the thickness of the surface layer be taken as equal to the radius of molecular attraction, say 2 X io 7 cms., and the concentration calculated from the observed adsorption, it is found, for instance, for methyl orange, to be about 39%, whereas the solubility of the substance is only about 078%. The surface layer, therefore, cannot possibly consist of a more concentrated solution of the dye, which is the only case that can be dealt with theoretically, but must be formed of a semi-solid deposit. 

Figure 10.2 Schematic isotherm for the simplest cases of chemical adsorption from solution onto a solid substrate. The amount of adsorbate available to adsorb is best gauged by the concentration c...

In the following example, a solid consisting of a soluble constituent A and an insoluble constituent B is considered. Leaching is carried out with a pure solvent S and a solution is produced containing a mass a of A, per unit mass of S and the total mass of A in solution is P. It will be assumed that the quantity of solvent removed in the underflow from each of the thickeners is the same, and that this is independent of the concentration of the solution in that thickener. It will be assumed that unit mass of the insoluble material B removes a mass s of solvent S in association with it. Perfect mixing in each thickener will be assumed and any adsorption of solute on the surface of the insoluble solid will be neglected. In a given thickener, therefore, the ratio of solute to solvent will be the same in the underflow as in the overflow. 

Adsorption of Ag on the surface of PdO is also an interesting option offered by colloidal oxide synthesis. Silver is a well-known promoter for the improvement of catalytic properties, primarily selectivity, in various reactions such as hydrogenation of polyunsaturated compounds." The more stable oxidation state of silver is -F1 Aquo soluble precursors are silver nitrate (halide precursors are aU insoluble), and some organics such as acetate or oxalate with limited solubility may also be used." Ag" " is a d ° ion and can easily form linear AgL2 type complexes according to crystal field theory. Nevertheless, even for a concentrated solution of AgNOs, Ag+ does not form aquo complexes." Although a solvation sphere surrounds the cation, no metal-water chemical bonds have been observed. 

The concentration of the polymer solution added to the particle suspension has also been shown to affect process performance. The use of more dilute solutions appears to enhance floe growth. In concentrated solutions, intermolecular repulsion enhances coiling of the molecules, reducing their effective size. Because of the very high adsorption rates, there is insufficient time for relaxation of the molecules, and the surface covered by a molecule and its extension into solution are both reduced. 

Equation 10.27 is generally known as Freundlich equation. Equation 10.27 with concentration replaced by pressure was also used to describe the adsorption isotherms of gases on solids, suggesting the incorrect idea that adsorption from solution by a solid could be paralleled with gas or vapor adsorption on the same adsorbents. Whereas in some cases the restriction to dilute solutions was imposed by the solubility of solids (e.g., benzoic acid in water or stearic acid in benzene) it was not imposed on the investigation of mixtures of completely miscible liquids, e.g., acetic acid in water. 

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]. 

Roy. SoG. A, Lxxxv. 557,1911), who employed a solution of nonylic acid. Their method of determining F was ingenious. A slow stream of air-bubbles was blown up through the solution, and the number and diameter of the bubbles was determined. Adsorption of solute occurred at the surface of each bubble, and the quantity so adsorbed was carried with the bubble into an upper part of the experimental vessel. Diffusion of solute back into the lower portion of the vessel was prevented by suitable baffles and the upper part gradually increased in concentration. If n was the number and r the radius of the bubbles, v the volume of the upper compartment of the vessel in c.c.s and Ac the concentration change in gram-molecules per square centimetre,... 

This is the important Gibbs adsorption isotherm. (Note that for concentrated solutions the activity should be used in this equation.) Experimental measurements of y over a range of concentrations allows us to plot y against Inci and hence obtain Ti, the adsorption density at the surface. The validity of this fundamental equation of adsorption has been proven by comparison with direct adsorption measurements. The method is best applied to liquid/vapour and liquid/liquid interfaces, where surface energies can easily be measured. However, care must be taken to allow equilibrium adsorption of the solute (which may be slow) during measurement. 

The Accurel EPIOO carrier, macro porous polypropylene granulates, was obtained from Akzo Nobel. Particle size was in the range of 200-1000 m. 1 gram of carrier was washed with 96% ethanol and water prior to the immobilization. Lipase solution (purified and concentrated solution of Humicola lanuginosa lipase (103 KLU/ml), pH 7.5) was added to the wet carrier. The lipase loadings were in the range of 125-500 KLU/g carrier. The amount of adsorbed lipase activity was calculated from the difference in lipase activity in the supernatant before subtracted the lipase activity after adsorption. 

Before concentration, acid hydrolyzates are neutralized, most commonly with barium carbonate, although such organic bases as methyldioctylamine has been used.81 This step normally causes little loss, except by adsorption on, for example, barium sulfate,82 but the following points are of interest. Neutralization with ammonia has been recommended,83 as the neutral solution may be evaporated directly to dryness without filtration, and the ammonium sulfate formed is insoluble in methyl sulfoxide, a solvent used for trimethyl-silylation. The authors83 also found that, when hydrolyzates are neutralized with ion-exchange resins, the pH of the concentrated solutions may differ by as much as 2 units of pH. D-Fructose has been found to be epimerized by barium carbonate or pyridine, and lead... 

Figure 1.5 shows a schematic representation of the double layer at a planar solid-liquid interface. The potential drop across the Helmholz layer is shown as linear (in the presence of specific adsorption, it will not be completely linear), followed by a tailing-off of the potential into the diffuse layer. For concentrated solutions (>0.1 M) the diffuse layer is typically a nanometer or less, while for dilute solutions it may be tens or even hundreds of nanometers. 

Fortunately, this is the case in many environmental applications where the gas species to be removed are in such low concentrations (large excess of inerts) that the expansion factor is practically zero. As pointed out in the introduction of this section, the basic principles of the analysis are also applicable in the case of adsorption of solutes from the gaseous phase. Again, for environmental applications, the concentration of solutes is so low that the pressure drop is only due to the flow of the gas. Here, the expansion factor has the same meaning, i.e. it measures the change of the volume of the gas phase, which is negligible in the case of low concentrations of the removed gas species. 

In applying these results to adsorption from solution, the activity equals the pressure or concentration multiplied by the activity coefficient /. Differentiation of Equation (47) at constant temperature yields... 

One isotherm that is both easy to understand theoretically and widely applicable to experimental data is due to Langmuir and is known as the Langmuir isotherm. In Chapter 9, we see that the same function often describes the adsorption of gases at low pressures, with pressure substituted for concentration as the independent variable. We discuss the derivation of Langmuir s equation again in Chapter 9 specifically as it applies to gas adsorption. Now, however, adsorption from solution is our concern. In this section we consider only adsorption from dilute solutions. In Section 7.9c.4 adsorption over the full range of binary solution concentrations is also mentioned. 

Next let us consider adsorption from solutions that are not infinitely dilute. Suppose, for example, that adsorption is studied over the full range of binary liquid concentrations. Figure 7.18 is an example of such results for the benzene-ethanol system adsorbed on carbon. At... 

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