Slow desorption

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Again, this can be attributed to the slow desorption rate of VAM when KOAc is not present. 

By considering the limits of 1/y as zero and infinity corresponding to infinitely rapid desorption or slow reaction and very slow desorption or rapid reaction, respectively, show that... 

Besides methane and hydrogen, other products observed were mainly ethane, ethylene (probably due to propane cracking), propene which are shown in figure 3. Minor amounts of benzene and toluene were also found but these products could not be quantified because their slow desorption from the zeolite s channels. 

The slow desorption of oxygen will also suggest that the desorption of oxygen from the catalyst could be the slowest step during the steady state reaction. 

When the pH is readjusted to a higher value, the splitting increases and a spectrum which is essentially the same as the one obtained originally at the higher pH is observed. However, when the pH of a sample solution is lowered from 12.7 down to 3.0, slow desorption of Co-57 ions was observed, and the Mossbauer spectrum virtually remained unchanged (Figure 3(F)). These observations suggest that the chemical form of divalent Co-57 adsorbed from an alkaline solution is retained when the pH of the solution is lowered down to an acidic pH value. 

In order to achieve complete remediation, it is important that slow desorption has to be overcome [187]. Pignatello and Xing [107] considered the following... 

Unfortunately, there are also some disadvantages i) the decreased ionization efficiency at 12-15 eV also means a significant loss of sensitivity (Fig. 5.4), ii) low ion source temperatures cause long-lasting memory of previous samples due to slow desorption from the surfaces that have been in contact with the sample vapor, and iii) a weak molecular ion peak may well be enhanced, however, a spectrum showing no molecular ion peak at 70 eV will not turn into a spectrum exhibiting a strong molecular ion peak at 12 eV. 

For the sake of clarity, it must be mentioned that Huot et al. [24, 35] reported that at 350°C the absorption/desorption PCT plateau pressure hysteresis of the unmilled MgH was quite substantial, while the hysteresis of the same material milled for 20 h was very small. They argued that the plateau pressure difference observed for the uiimilled material was due to a very slow desorption kinetics which did not allow reaching equilibrium. In turn, ball milling increased desorption kinetics, which allowed reaching equilibrium and eliminated hysteresis. However, it must be noticed that the situation reported by Huot et al. is completely opposite to what is observed in Fig. 2.43, in which MgH synthesized by reactive mechanical milling shows a pressure hysteresis. 

Two seqnential interfacial reactions the first one being the reaction with the anion of the extractant that saturates the interface the second one being the slow desorption of the interfacial complex from the interface [see Eq. (5.56)]. 

For these entrapped contaminant spheres, the diffusion is rapid, on the order of seconds rather than days. Kahn et al. (2000) suggested a diffusion model for xenobiotics with a slow desorption phase, with a half-life of years rather than seconds, assuming that diffusion is hindered by the natural organic matter matrix and occurs when the dimensions of diffusing molecules approach those of the pores. Under these conditions, hindrance from the wall becomes significant (Renkin 1954) and the drag factor F can be expressed as... 

Reaction (13.47) accounts for more than 95% of the overall H2 consumption. Notably, the H2 uptake in Figure 13.18a is seen before the evolution of the reaction products, which suggests that H2 is first adsorbed and activated on the catalyst surface and then participates in the reduction of nitrates. However, a time delay in the detection of ammonia due to its slow desorption from the catalyst surface cannot be excluded. 

The above analyses show the qualitative agreement of the adsorption model with recent conductivity experiments, and indicate how the trapping energy in adsorption may be indirectly obtained by conductivity measurements in the temperature range where slow desorption occurs. The energy can be obtained accurately, however, only with more careful examination of the theory and experiment than is presented here. 

Farrell, J., D. Grassian, and M. Jones, Investigation of mechanisms contributing to slow desorption of hydrophobic organic compounds from mineral solids , Environ. Sci. Technol., 33,1237-1243 (1999). 

Changes in the electrical conductivity of a thin-semiconductive film when an eluate is adsorbed on the surface was used by Seiyama and co-workers (45). The response on a ZnO film (20-1000 A) depended upon the nature of the interaction. For electron acceptors, such as 02, a decrease in the conductivity was observed, while for electron donors such as ethyl alcohol and C02, an increase in the conductivity was measured. Temperatures of 200 °C or greater were necessary to avoid slow desorption rates and concomittant loss in resolution. Sensitivities were poor, but the phenomena are worth further scrutiny in hopes of obtaining materials exhibiting stable characteristics as well as... 

Tailing sometimes has a kinetic (rather than equilibrium) origin, stemming from slow desorption processes [28]. Thus the observation of tailing does not prove that the column is overloaded and consequently behaving nonlinearly (see Section 5.9). When kinetic factors are involved, the reduction of solute load will not eliminate tailing. 

Predicting fast and slow rates of sorption and desorption in natural solids is a subject of much research and debate. Often times fast sorption and desorption are approximated by assuming equilibrium portioning between the solid and the pore water, and slow sorption and desorption are approximated with a diffusion equation. Such models are often referred to as dual-mode models and several different variants are possible [35-39]. Other times two diffusion equations were used to approximate fast and slow rates of sorption and desorption [31,36]. For example, foraVOCWerth and Reinhard [31] used the pore diffusion model to predict fast desorption, and a separate diffusion equation to fit slow desorption. Fast and slow rates of sorption and desorption have also been modeled using one or more distributions of diffusion rates (i.e., a superposition of solutions from many diffusion equations, each with a different diffusion coefficient) [40-42]. 

Depending on the distribution chosen, as few as three fitting parameters may be required to define a distribution of diffusion rates. In some cases, a single distribution was used to describe both fast and slow rates of sorption and desorption, and in other cases fast and slow mass transfer were captured with separate distributions of diffusion rates. For example, Werth et al. [42] used the pore diffusion model with nonlinear sorption to predict fast desorption, and a gamma distribution of diffusion rate constants to describe slow desorption. 

Several researchers have confirmed that biodegradation can be limited by the slow desorption of organic compounds [22-25]. Though significant research has been conducted to study the sorption and desorption kinetics of organic compounds and their bioavailability, few studies have focused on the bioavailability of contaminants in soils containing only the desorption resistant fraction and how the degradation rates compare to those for freshly contaminated soils. 

Adsorption Studies on Cobalt Molybdate Catalyst. Thiophene and C4 Hydrocarbons. The peak delay due to slow desorption of thiophene and its C4 product from the catalyst in single-shot experiments was investigated further, as it seemed probable that the desorption was slow enough to have a considerable effect on the over-all rate of reaction. The delay in appearance of the peak maximum... 

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