Adsorption sites number concentration

When Kpa 1 goes 0— 0. Normally, however, it is the condition KpA 1 (low gas concentration) that leads to 0 = KpA and 0 1. As a consequence Eq. (4.272) simplifies to Rads = k adsPa, the adsorption site number N we include in the adsorption coefficient. Note that the expression for the adsorption flux Fads = kadsfig (Eq- 4.266) looks similar but the dimensions of kads are different and it must be taken into account that p = nVT (Eq. 4.22 note that n is not the mol number but the gas phase molecule density or molecule number concentration, respectively) flux is the rate per unit of area (F = R q). 

At low values of the bulk concentration Bcy surface coverage is proportional to this concentration, but at high values it tends toward a limit of unity. This equation was derived by Irving Langmuir in 1918 with four basic assumptions (1) the adsorption is reversible (2) the number of adsorption sites is limited, and the value of adsorption cannot exceed A° (3) the surface is homogeneous aU adsorption sites have the same heat of adsorption and hence, the same coefficient B and (4) no interaction forces exist between the adsorbed particles. The rate of adsorption is proportional to the bulk concentration and to the fraction 1-9 of vacant sites on the surface = kjil - 9), while the rate of desorption is proportional to the fraction of sites occupied Vj = kjd. In the steady state these two rates are equal. With the notation kjk = B, we obtain Eq. (10.14). 

Sorption processes are influenced not just by the natures of the absorbate ion(s) and the mineral surface, but also by the solution pH and the concentrations of the various components in the solution. Even apparently simple absorption reactions may involve a series of chemical equilibria, especially in natural systems. Thus in only a comparatively small number of cases has an understanding been achieved of either the precise chemical form(s) of the adsorbed species or of the exact nature of the adsorption sites. The difficulties of such characterization arise from (i) the number of sites for adsorption on the mineral surface that are present because of the isomorphous substitutions and structural defects that commonly occur in aluminosilicate minerals, and (ii) the difference in the chemistry of solutions in contact with a solid surface as compound to bulk solution. Much of our present understanding is derived from experiments using spectroscopic techniques which are able to produce information at the molecular level. Although individual methods may often be applicable to only special situations, significant advances in our knowledge have been made... 

Stober (173) found also a close relation of the adsorption sites for ammonia and the number of surface silanol groups. Fused silica and crystalline quartz behaved in a similar manner. About the same concentration of adsorption sites was found in the SOj adsorption. 

Some components in a gas or liquid interact with sites, termed adsorption sites, on a solid surface by virtue of van der Waals forces, electrostatic interactions, or chemical binding forces. The interaction may be selective to specific components in the fluids, depending on the characteristics of both the solid and the components, and thus the specific components are concentrated on the solid surface. It is assumed that adsorbates are reversibly adsorbed at adsorption sites with homogeneous adsorption energy, and that adsorption is under equilibrium at the fluid- adsorbent interface. Let (m" ) be the number of adsorption sites and (m 2) the number of molecules of A adsorbed at equilibrium, both per unit surface area of the adsorbent. Then, the rate of adsorption r (kmol m s ) should be proportional to the concentration of adsorbate A in the fluid phase and the number of unoccupied adsorption sites. Moreover, the rate of desorption should be proportional to the number of occupied sites per unit surface area. Here, we need not consider the effects of mass transfer, as we are discussing equilibrium conditions at the interface. At equilibrium, these two rates should balance. Thus,... 

In considering photoactivity on metal oxide and metal chalcogenide semiconductor surfaces, we must be aware that multiple sites for adsorption are accessible. On titanium dioxide, for example, there exist acidic, basic, and surface defect sites for adsorption. Adsorption isotherms will differ at each site, so that selective activation on a particular material may indeed depend on photocatalyst preparation, since this may in turn Influence the relative fraction of each type of adsorption site. The number of basic sites can be determined by titration but the total number of acidic sites is difficult to establish because of competitive water adsorption. A rough ratio of acidic to basic binding sites on several commercially available titania samples has been shown by combined surface ir and chemical titration methods to be about 2.4, with a combined acid/base site concentration of about 0.5 mmol/g . 

All adsorbents have upper limits to the amount of arsenic that they can adsorb from air, water, or other fluids. That is, there is a finite number of adsorption sites on each gram of adsorbent. The maximum adsorption capacity, which is often measured in molal, represents the highest concentration of a solute (such as arsenic) that can be adsorbed by a given mass of a particular adsorbent. The maximum adsorption capacity is routinely obtained from laboratory experiments and measurements, and is closely related to the cation exchange capacity (cec) or anion exchange capacity (aec) of the materials. The cec or aec provide... 

The Langmiur isotherm—used by Guiochon and others in the study of preparative scale chromatography—is based on the concept that adsorption nonlinearity occurs when there are so many molecules that they compete with one another for a limited number of adsorption sites. It is obvious that when two concentrated solutes are present at the same time, they will interfere with one another s adsorption. The one that adsorbs most strongly will almost totally displace the weaker adsorber. This is the basis of displacement chromatography, a nonlinear form developed by Tiselius in 1943 (17) and revived recently by Horvath [18]. 

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