Desorption coefficient

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. 

The desorption and termination constants were calculated for a copolymer from the corresponding homopolymer constants as discussed in Nomura and Fujita (12.) The homopolymer desorption coefficients were calculated from the appropriate chain transfer constants and radical diffusivities in the aqueous and polymer phases using an extension of the desorption theory developed by Nomura and Fujita (12.). The homopolymer termination constants were corrected for the Trommsdorff effect by using the Friis and Hamielec (12) correlation. 

In many early experiments, hysteresis was observed for highly hydrophobic compounds such as PCBs (79, 80). Since the time to reach equilibrium can be quite long for strongly hydrophobic compounds, a solute may have never reached equilibrium during the sorption isotherm experiment. Consequently, Kj would be underestimated, which leads to the discrepancy between the sorption and desorption coefficients that was attributed to hysteresis. The case for hysteresis being an artifact is supported by recent data for tetrachlorobenzene (log K = 4.7), illustrating that sorption and desorption require approximately two days to reach equilibrium with approximately equal time constants (78). Finally, the diffusion model is consistent with the observation that the extent of hysteresis was inversely related to particle size (81). 

When the rates of sorption or desorption processes are known, environmental fate modeling can provide an educated estimate and prediction on the accessibility and bioavailability of a target pollutant to a specific transport mechanism in the environment. Hence, the present chapter is an attempt to assess fate (i.e., in terms of pollutant mobility using predictive sorption or desorption coefficients) as well as effects (i. e., in terms of bioavailability) of various pollutants and to correlate these observations for development of predictive relationships. 

The second modeling approach discussed in this section presents an overview of the fundamentals of quantitative structure-activity relationships (i.e., QSARs [102-130]) and quantitative structure-property relationships (i.e., QSPRs [131-139]). It will show how such an approach can be used in order to estimate and predict sorption/desorption coefficients of various organic pollutants in environmental systems. 

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... 

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. 

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. 

A conditioning method which is very effective for reducing particle desorption coefficients without resorting to high temperature bake-out is discharge cleaning (1, 31, 37) The... 

Equations [13] to [15] are very similar to those already established by Ruckenstein and Prieve (3). Because the adsorption coefficient is defined here in terms of a concentration per unit area instead of a concentration per unit volume the expressions of the adsorption coefficient K" and of the desorption coefficient K are symmetrical. 

SAMPLE COLLECTION DESORPTION COEFFICIENT OF TIME (HRS.) TUBE EFFICIENCY (%) VARIATION CV... 

Here the pre-exponential factor, K, is equal to the ratio of the adsorption and desorption coefficients, a//. Alternatively, b may be regarded as a function of the enthalpy and entropy of adsorption (Everett, 1950 Barrer, 1978, p. 117). 

A useful expression for the radical desorption coefficient follows (Nomura et at., 197Q. 

In Stage 111, the reduction in >p and increase in d lead to a reduction in desorption coefficient with an increase in polymer-phase viscosity. For vinyl chloride emulsion polymerization the separate monomer phase disappears at about 70% conversion (AT,. = 0.7). Therefore, as soon as iVp reaches a constant value, the only parameter that changes for AT < is I. In fact, it is mainly the increase in that causes the acceleration in rate. For X > the situation is more complex, with both lc,p and falling as polymer concentration increases. For vinyl acetate, the separate monomer phase already disappears at 20% conversion. For X > X, is almost constant however [Mp], fe,p, and all decrease with conversion. These effects will he discussed in more detail later. 

The model has certainly many limitations, some related to the status of current knowledge and others due to the lack of appropriate information. It is clear nowadays that sorption/desorption effects play an important role in the actual levels of the concentrations of certain pollutants, depending on the different ambient conditions, but, above all, in the interaction of the mater-ial/substance. The fact is that the values for specific coefficients of adsorption or desorption of a particular chemical substance in a given material are generally unknown. Recent studies have been made in order to obtain the information needed on the sorption/desorption coefficients for different coupling material/substances [29]. So far, that work has only been done for a very limited number of cases. Given the hundreds of substances and materials that can be present, that limitation probably represents the major bottleneck to the wide application of the model. 

R varies with time, and there are experimental difficulties in defining t = 0 and A = 0 and in measuring when the rate of monolayer contraction is high (23). Ter Minassion-Saraga (24) and later Gershfeld and Patlak (25) found that log A of a contracting monolayers was linear with - /t in the initial temporal phase of contraction. The initial desorption coefficient, Ku was expressed by Equation 2 ... 

Kinetic coefficients. The kinetic adsorption and desorption coefficients can be estimated [82, 219, 412] if the form of the potential (Z) of interaction between a particle (a surfactant molecule) and the solution surface is known here Z is the coordinate measured from the surface into the bulk of liquid. If the function (Z) has the form of a potential barrier with a potential well, then the saddle-point method [261] implies... 

In addition to the conventional sorption distribution coefficients which were based on the addition of a solution of known DBCP concentration to soil, desorption coefficients were measured by desorbing DBCP residues into water with a 3-hour equilibration period (2.). When distribution coefficients determined by these two methods were found to be very different, estimates of were also obtained by calibrating the one-dimensional analytical model to field data. The various estimates of are compared in the Results section. 

If absorption measurements are made on samples, which initially contain no water, by immersing them in salt solutions of different concentrations, the effective absorption diffusion coefficient can be calculated. These measurements are followed by desorption experiments in which the samples are dried by blowing air across both major surfaces and the effective desorption coefficient is calculated. The diffusion coefficients obtained from such absorption and desorption measurements are shown in Table II. It is evident that the ratio of the diffusion coefficient obtained from desorption to that from absorption measurements increases as the concentration of water in the rubber, C, Increases. The diffusion coefficient obtained from a desorption experiment can only be expected to be greater than that from an absorption experiment if D decreases with increasing liquid concentration (3). 

Taking into account the limiting values of the logarithm of desorption coefficient instead of eq. (2.31) the normalization condition is... 

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