Reversibility of adsorption

Although adsorption and desorption together establish an adsorption equilibrium, desorption has received relatively little experimental attention. 

If adsorption occurs as a bimolecular reaction, as suggested by the Langmuir equation 

If one of the kinetic constants has been determined and Kads is known, the other rate constant can be calculated. 

The principle we have applied here is called microscopic reversibility or principle of detailed balancing. It shows that there is a link between kinetic rate constants and thermodynamic equilibrium constants. Obviously, equilibrium is not characterized by the cessation of processes at equilibrium the rates of forward and reverse microscopic processes are equal for every elementary reaction step. The microscopic reversibility (which is routinely used in homogeneous solution kinetics) applies also to heterogeneous reactions (adsorption, desorption dissolution, precipitation). 

In the application of the principle of microscopic reversibility we have to be careful. We cannot apply this concept to overall reactions. Even Eqs. (4.43) - (4.45) cannot be applied unless we know that other reaction steps (e.g., diffusional transport) are not rate controlling. In a given chemical system there are many elementary reactions going on simultaneously. Rate constants are path-dependent (which is not the case for equilibrium constants)and may be changed by catalysts. For equilibrium to be reached, all elementary processes must have equal forward and reverse rates 

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There are various parameters and assumptions defining radionuclide behavior that are frequently part of model descriptions that require constraints. While these must generally be determined for each particular site, laboratory experiments must also be conducted to further define the range of possibilities and the operation of particular mechanisms. These include the reversibility of adsorption, the relative rates of radionuclide leaching, the rates of irreversible incorporation of sorbed nuclides, and the rates of precipitation when concentrations are above Th or U mineral solubility limits. A key issue is whether the recoil rates of radionuclides can be clearly related to the release rates of Rn the models are most useful for providing precise values for parameters such as retardation factors, and many values rely on a reliable value for the recoil fluxes, and this is always obtained from Rn groundwater activities. These values are only as well constrained as this assumption, which therefore must be bolstered by clearer evidence. 

The primary process initiating dust surface chemistry is the collision of a molecule from the ISM with the surface. The sticking probability is a measure of how often molecules will stick to the dust surface but this depends on the collision energy, the temperature of the grain surface and the nature of the chemical surface itself. The silicate surface is highly polar, at least for a grain of sand on Earth, and should attract polar molecules as well as atoms. The adsorption process can also be reversed, resulting in thermal desorption, both as the reverse of adsorption and by new molecules as the product of surface reactions. 

From the slope and intercept of these plots the saturation coverage and adsorption coefficient were obtained by application of the previously given equation. The results are listed in Table I, which also includes the area per dye molecule at saturation coverage and the standard free energy of adsorption, AG°. Although the latter parameter was calculated as before (23), its thermodynamic validity is questionable since reversibility of adsorption of these dyes was not demonstrated. The molecular... 

Desorption. The reverse of Adsorption which is described in Vol 1, p A105-L... 

Reversibility of adsorption steps Kinetic order of steps ... 

The effect of step reversibility. A suggestion about the reversibility of adsorption steps for A2 in the impact mechanism (19) does not lead to any essentially new results. Kinetic curves WL(PAz) and fKm(Pn) have the same type as those with saturation. 

Steady-state kinetic curves W(PB) for the adsorption mechanism (8), taking into account the reversibility of adsorption steps, are illustrated in Fig. 4(a), (b). At a given value of k x with increasing k 2 [Fig. 4(a)], the region of multiplicity for steady states diminishes and at some value of k 2 it vanishes completely. With increasing k 2, the kinetic curve can achieve a maximum and finally take the form of that with saturation. A similar effect is observed with the reversibility of the first step at a given value oik 2 [Fig. 4(b)]. But the parametric sensitivity here is lower than in the former case. 

Removal of suspended solids from a liquid may be required, either to recover valuable products or to remove obnoxious particulates. Clarification of dilute aqueous suspensions is frequently performed in deep-bed granular filters, where the grains provide a surface on which the particles may adsorb. If two types of particles differ in their adsorption rate, separation of them may be accomplished by selectively adsorbing one species on the grains. Furthermore, reversibility of adsorption would... 

Explain the reversibility of adsorption of organics by soil surfaces. Explain the practical significance of your answer. 

Thus, the data obtained show that chloroform and acetylene and its derivatives are suitable IR-spectroscopic probe-molecules for basic centers in zeolites. These probes exhibit the following advantages as compared to the conventionally used molecules, like CO2 and pyrrole (1) the wride ranges of the frequency shifts, which allows one to differentiate the centers of different nature and strength, (2) the easiness and reversibility of adsorption/desorption of these molecules, and (3) the favorable spectral range where the spectral features attributed to adsorbed probes appear. The use of such an approach allows us to shed some light on the nature and properties of basic sites in zeolites. The similar technique will be applied in our future studies devoted to other solid superbases. 

Mollmann, S.H., Jorgensen, L., Bukrinsky, J.T., Elofsson, U., Norde, W., and Frokjaer, S. (2006) Interfacial adsorption of insulin-conformational changes and reversibility of adsorption. European Journal of Pharmaceutical Sciences, 27,194—204. 

Mollmann SH, Jorgensen L, BukrinskyJT, Elofsson U, Norde W, and Frokjaer S. Interfacial Adsorption of Insulin Conformational Changes and Reversibility of Adsorption. Eur JPharm Sci 2006 27(2-3) 194-204. 

Desorption is the reverse of adsorption for a simple adsorption process and is the final step of surface reactions in which products are evolved,... 

Reversibility of Adsorption. Apparently, the data in Figure 10.13 imply that the Gibbs equation (10.2) does not hold for the protein. As we have seen, it is valid for the amphiphile. However, the slopes dll/d In c given in the figure differ only by a factor 2 between the two surfactants, whereas the values of Fm differ by two orders of magnitude. The explanation is not fully clear. Application of the Gibbs equation to polymers is anyway questionable, because it is generally not known what the relation is between concentration (c) and activity (a) of the surfactant. Moreover, proteins and other polymers are virtually always mixtures. 

Investigations on the reversibility of adsorption equilibria of heavy metals by Lion et al. (1982) indicate that their behaviour is strongly different in natural sediments compared to the experience on well-characterized solid surfaces. It is suggested that reduced reversibility for metals such as lead and copper is mainly the effect of hydrolysis and specific adsorption (Forstner, 1986). 

Adsorption is a binding process of a gas or liquid solute to accumulate on the surface of a solid or a liquid (adsorbent) with the formation of a molecular or atomic film (the adsorbate). Different from adsorption, absorption requires a substance to permeate or dissolve into a liquid or solid. A combination of adsorption and absorption is called sorption and the reverse of adsorption is called desorption. A schematic representation is depicted in Figure 11.1. 

Some researchers have reported reversible adsorption of OH at low coverages, i.e., at Eu < 0.9 At the same time, however, even at these low coverages, aging effects were observed, which may render the conclusion as to the reversibility of adsorption open to question. 

Reversibility of adsorption of ammonia from water solutions onto silicas and zeolites... 

Simple adsorption isotherm studies are widely used to assess the amount of coating that is needed to fully treat a filler surface. The isotherms may be determined by a number of techniques for example following either the reduction in solution concentration (gravimetrically, spectroscopically, titrimetrically, etc.), or the bnild np on the filler after isolation and washing. Reversibility of adsorption may also be stndied. 

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