Kinetic desorption coefficient

Kommalapati RR,Valsaraj KT, Constant WD (2000) Soil-water partition coefficients, adsorption/desorption hysteresis, desorption kinetics and bioavailability of chlorinated organic compounds at the PPI site. Submitted to Hazardous Substance Research Center, Louisiana State University, Baton Rouge, LA... 

The adsorption of oxygen is assumed as irreversible and dissociative with a rate proportional to the fraction of vacant sites [1]. In contrast to Sant et al. [2] adsorption of both ethene and ethyne is also assumed to be first order in the vacant sites. The rate of the surface reactions between adsorbed ethene and adsorbed oxygen, and between adsorbed ethyne and adsorbed oxygen, is considered as proportional to the product of the involved surface coverages. The adsorption rate coefficients are obtained fi om the kinetic gas theory, while Arrhenius-type expressions are used for the rate coefficients of desorption,dissociation and the surface reactions. The kinetic parameter values used in this study are shown in Table 2. 

From the results of this kinetic study and from the values of the adsorption coefficients listed in Table IX, it can be judged that both reactions of crotonaldehyde as well as the reaction of butyraldehyde proceed on identical sites of the catalytic surface. The hydrogenation of crotyl alcohol and its isomerization, which follow different kinetics, most likely proceed on other sites of the surface. From the form of the integral experimental dependences in Fig. 9 it may be assumed, for similar reasons as in the hy-drodemethylation of xylenes (p. 31) or in the hydrogenation of phenol, that the adsorption or desorption of the reaction components are most likely faster processes than surface reactions. 

The reaction of Si02 with SiC [1229] approximately obeyed the zero-order rate equation with E = 548—405 kJ mole 1 between 1543 and 1703 K. The proposed mechanism involved volatilized SiO and CO and the rate-limiting step was identified as product desorption from the SiC surface. The interaction of U02 + SiC above 1650 K [1230] obeyed the contracting area rate equation [eqn. (7), n = 2] with E = 525 and 350 kJ mole 1 for the evolution of CO and SiO, respectively. Kinetic control is identified as gas phase diffusion from the reaction site but E values were largely determined by equilibrium thermodynamics rather than by diffusion coefficients. 

Unraveling catalytic mechanisms in terms of elementary reactions and determining the kinetic parameters of such steps is at the heart of understanding catalytic reactions at the molecular level. As explained in Chapters 1 and 2, catalysis is a cyclic event that consists of elementary reaction steps. Hence, to determine the kinetics of a catalytic reaction mechanism, we need the kinetic parameters of these individual reaction steps. Unfortunately, these are rarely available. Here we discuss how sticking coefficients, activation energies and pre-exponential factors can be determined for elementary steps as adsorption, desorption, dissociation and recombination. 

Having estimated the sticking coefficient of nitrogen on the Fe(lll) surface above, we now consider the desorption of nitrogen, for which the kinetic parameters are readily derived from a TPD experiment. Combining adsorption and desorption enables us to calculate the equilibrium constant of dissociative nitrogen adsorption from... 

Figure 36 shows that efflux of PNU-78,517 from the apical membrane is facilitated by BSA. Its permeability coefficients increase with BS A concentration the magnitudes of the values (10 6—10 5 cm/min) and trends indicate that the desorption kinetics are membrane-controlled and that BSA acts as a drug acceptor at the membrane interface via protein binding. The initial mass fraction readily... 

Over the last decade, some research has indicated that (1) partition coefficients (i. e.,Kd) between solid and solution phase are not measured at true equilibrium [51,59-61], (2) the use of equilibrium rather than kinetic expressions for sorption in fate and effects models is questionable [22-24,60,61], and (3) sorption kinetics for some organic compounds are complex and poorly predictable [22 - 24,26]. This is mainly due to what has recently been discussed as slow sorp-tion/desorption of organic compounds to natural solid phase particles [107, 162-164,166-182]. The following is a summary of some important points supporting this hypothesis [1,66,67,170-183] ... 

Girvin DC, Sklarew DS, Scott AJ, Zipperer JP (1990) Release and attenuation of PCB congeners, measurements of desorption kinetics and equilibrium sorption partition coefficients. GS6875, Electric Power Research Institute, Palo Alto, CA 94304... 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

Fig. 2.6 (a) Desorption kinetic curves at various temperatures under initial hydrogen pressure of 0.1 MPa of the as-received, nonactivated, commercial MgH powder Tego Magnan and (b) the Arrhenius plot of the desorption rate for the estimate of the apparent activation energy, fi, using kinetics data for four temperatures 350, 375, 400, and 420°C (fi -120 kJ/mol). Coefficient of fit = 0.996... 

Powder Apparent activation energy of desorption, (kJ/mol) Coefficient of fit in the Arrhenius equation Kinetic curves at temperatures taken for calculation (°C) Activation... 

Adsorption-desorption coefficients are determined by various experimental techniques related to the status of a contaminant (solute or gas) under static or continuous conditions. Solute adsorption-desorption is determined mainly by batch or column equilibration procedures. A comprehensive description of various experimental techniques for determining the kinetics of soil chemical processes, including adsorption-desorption, may be found in the book by Sparks (1989) and in many papers (e.g., Nielsen and Biggar 1961 Bowman 1979 Boyd and King 1984 Peterson et al. 1988 Podoll et al. 1989 Abdul et al. 1990 Brusseau et al. 1990 Hermosin and Camejo 1992 Farrell and Reinhard 1994 Schrap et al. 1994 Petersen et al. 1995). 

In a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

Irrespective of the sources of phenolic compounds in soil, adsorption and desorption from soil colloids will determine their solution-phase concentration. Both processes are described by the same mathematical models, but they are not necessarily completely reversible. Complete reversibility refers to singular adsorption-desorption, an equilibrium in which the adsorbate is fully desorbed, with release as easy as retention. In non-singular adsorption-desorption equilibria, the release of the adsorbate may involve a different mechanism requiring a higher activation energy, resulting in different reaction kinetics and desorption coefficients. This phenomenon is commonly observed with pesticides (41, 42). An acute need exists for experimental data on the adsorption, desorption, and equilibria for phenolic compounds to properly assess their environmental chemistry in soil. 

The first attempt to account for surface contamination in creeping flow of bubbles and drops was made by Frumkin and Levich (FI, L3) who assumed that the contaminant was soluble in the continuous phase and distributed over the interface. The form of the concentration distribution was controlled by one of three rate limiting steps (a) adsorption-desorption kinetics, (b) diffusion in the continuous phase, (c) surface diffusion in the interface. In all cases the terminal velocity was given by an equation identical to Eq. (3-20) where C, now called the retardation coefficient , is different for the three cases. The analysis has been extended by others (D6, D7, N2). 

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