Adsorption-Desorption Coefficients

It is seen, from equation (5), that a graph relating the reciprocal of the corrected retention volume to the concentration of the moderator can provide values for the adsorption/desorption coefficient and the surface area of the stationary phase. Scott and Simpson [1] used this technique to measure the surface area of a reversed phase and the curves relating the reciprocal of the corrected retention volume to moderator concentration are those shown in Figure 2. 

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). 

The spatial distributions of bromacil and terbuthylazine adsorption-desorption coefficients, both vertically and laterally, in the experimental field are given in... 

Adsorption/desorption coefficients in reiease medium (soii/siudge)... 

Local adsorption - desorption coefficients are related to the corresponding detailed probabilities via... 

It is worth noticing here that local adsorption - desorption coefficients used in (6.1.11) had been defined through detailed probabilities Uc c, dc>c in the following way... 

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. 

Fig.3.1.9 (a) The adsorption-desorption isotherm (circles, right axis) and the self-diffusion coefficients D (triangles, left axis) for cyclohexane in porous silicon with 3.6-nm pore diameter as a function of the relative vapor pressure z = P/PS1 where Ps is the saturated vapor pressure, (b) The self-diffusion coefficients D for acetone (squares) and cyclohexane (triangles) as a function of the concentration 0 of molecules in pores measured on the adsorption (open symbols) and the desorption (filled symbols) branches. 

Major problems are associated with using the linear distribution coefficient for describing adsorption-desorption reactions in groundwater systems. Some of these problems include the... 

The reaction is carried out in close-loop reactor connected to a mass spectrometer for 1S02, 180160 and 1602 analyses as a function of time [38], The gases should be in equilibrium with the metallic surface (fast adsorption/desorption steps 1 and f ) If the bulk diffusion is slow (step 6) and the direct exchange (step 5) does occur at a negligible rate, coefficients of surface diffusion Ds can be calculated from the simple relationship between the number of exchanged atoms Ne and given by the model of circular sources developed by Kramer and Andre [41] ... 

Here A(g) and B(g) denote reactant and product in the bulk gas at concentrations CA and Cg, respectively kAg and kBg are mass-transfer coefficients, s is an adsorption site, and A s is a surface-reaction intermediate. In this scheme, it is assumed that B is not adsorbed. In focusing on step (3) as the rate-determining step, we assume kAg and kBg are relatively large, and step (2) represents adsorption-desorption equilibrium. 

For this estimate, values for the surface diffusion coefficient (D) and the surface exchange coefficient (i) in eq 2 were obtained by linearizing Mitterdorfer s rate expressions for surface transport and adsorption/desorption (ref 84) and re-expressing in terms of the driving forces in eq 2. 

Bowman, B.T. and Sans, W.W. Adsorption, desorption, soil mobility, aqueous persistence and octanol-water partitioning coefficients of terbufos, terbufos sulfoxide and terbufos sulfone, J. Environ. Sol. Health, B17(5) 447-462, 1982. 

The nature of soil-phenolic acid interaction adsorption-desorption. Adsorption of a solute from solution onto a solid matrix results in a higher solute concentration at the fluid-solid interface than in the solution. Huang and coworkers (27) observed a high sorption capacity of the mineral fraction of four latosols for phenolic acids. On the basis of their results, distribution coefficients,... 

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). 

Kd is the desorption coefficient for product D. The first-order desorption term should be strictly Kd(Cdp — H Cdg), allowing for an equilibrium backpressure, where H is an equilibrium adsorption constant relating mole fractions in the gas and zeolite phases. H Cdg was shown empirically to be small compared with Cdp under our conditions. 

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