Sorption-desorption kinetic methods

Hinz C., Selim H.M. Kinetics of Zn sorption-desorption using a thin disk flow method. Soil Sci 1999 164 92-100. 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

Tien (1987) studied the kinetics of heavy metal sorption-desorption on sludge using the stirred-flow reactor method of Carski and Sparks (1985). Sorption-desorption reactions were rapid with an equilibrium reached in 30 min. The sorption-desorption reactions were reversible. The sorption rate coefficients were of the order Hg > Pb > Cd > Cu > Zn > Co > Ni, while the desorption rate coefficients were of the order Cd > Cu > Hg >... 

There are several advantages to the dynamic vapor sorption device. First, any humidity value can be dialed in, whereas salt solutions are not available for every humidity value and some are quite toxic. Second, since the weight is monitored as a function of time, it is clear when equilibrium is reached. The dynamic devices also give the sorption/desorption rates, although these can easily be misused (see the drying kinetics section later). The salt solution method, on... 

The conventional inverse gas chromatography (IGC) is based on equations that assume equilibrium is established during the course o the chromatograph. Consequently, those stationary phases that exhibit marked hysteresis in sorption/desorption give IGC sorption data at considerable variance with long-term gravimetric methods. A modified frontal procedure was developed that avoids the assumption of equilibrium to enable studies of interaction kinetics of gas phase components with a stationary phase, such as a biopolymer, having entropic as well as enthalpic relations affected by concentration shifts and time dependent parameters. 

H2O. It should be noted that carbosil HTT improves the kinetics of sorption-desorption of organic snbstances from the aqueous solution which is important e.g. in separation and preconcentration of substance trace amount by means of the solid-pha e-extraction method [25],... 

The majority of sorption kinetic stndies have ntilized either batch or flow-through methods coupled with aqueous measurements for determination of the concentrations of species of interest. More recent work has focused on molecular-scale approaches, including spectroscopic and microscopic techniques that allow for observations at increased spatial and temporal resolution to be made, often in situ and in real time. Complementary to both macroscopic and molecular-scale observations has been the utilization of theoretical techniques, such as molecular mechanics and quantum mechanics, to model surface complexes computationally. It has been through the integration of macroscopic, molecular-scale, and theoretical approaches that some of the most profound observations of sorption-desorption phenomena over the past decades have been made. 

The methods described above can be used to study both sorption and desorption kinetics although the batch method does not readily lend itself to the study of desorption kinetics unless dilution or infinite sink techniques are used. Because desorption studies are approached differently from sorption studies, special techniques are often required, as discussed next. 

Dilution of the equilibrium or steady-state solution in contact with the soil has been used to determine the reversibility of sorption reactions (El-rashidi and O Connor, 1982a,b Peek and Volk, 1985). The method is also applicable to kinetic studies (Amacher et al., 1986,1988). The method is particularly suited to sorption and kinetic experiments conducted in centrifuge tubes. When the sorption reaction has reached apparent equilibrium or a steady state, a portion (or all) of the solution in contact with the soil is replaced by a solution of identical composition to the equilibrating solution except that the replacement solution does not contain the solute of interest. This results in dilution of the solute concentration in remaining equilibrating solution and initiates the reverse desorption reaction. Further step-wise dilutions can be done. 

An intriguing application of these Pd nanoparticles in basic research concerns the question of the solubility of H2 in such materials relative to bulk palladium [47]. Hydrogen concentration-pressure isotherms of surfactant-stabilized palladium clusters and polymer-embedded palladium clusters with diameters of 2, 3 and 5 nm were measured with the gas sorption method at room temperature. The results show that, compared to bulk palladium, the hydrogen solubility in the a-phase of the clusters is enhanced fivefold to tenfold, and the miscibility gap is narrowed. Both results can be explained by assuming that hydrogen occupies the subsurface sites of the palladium clusters. The Pd-H isotherms of all clusters show the existence of hysteresis, even though the formation of misfit dislocations is unfavorable in small clusters. Compared to surfactant-stabilized clusters, the polymer-embedded clusters show slow absorption and desorption kinetics. Moreover, evidence for a cubic-to-icosahedral transition of quasi-free Pd-H clusters by the hydrogen content was reported [47c]. 

Nearly all of the data are collected at room temperature, and there is no accepted method for correcting them to other temperatures. Far fewer data have been collected for sorption of anions than for cations. The theory does not account for the kinetics of sorption reactions nor the hysteresis commonly observed between the adsorption and desorption of a strongly bound ion. Finally, much work remains to be done before the results of laboratory experiments performed on simple mineral-water systems can be applied to the study of complex soils. 

In this work the FR technique proved to be a powerful method for the investigation of adsorbed species on different chemisorption sites. The FR method has been shown to be capable of giving information on the dynamic behaviom of sorption in zeolites. Adspecies detected by the FR technique surely play a role in this dynamic phenomenoa This new technique is proposed for measuring and interpreting data on kinetics of ammonia ad-/desorption processes characterising acidic sites in zeolite catalysts. 

In the preformulation study, the comprehension of physicochemical properties regarding water-solid surface interaction is beneficial to the handling, formulation, and manufacture of the finished products. Data on sorption/de-sorption isotherm, hydration of salts of drug product, water sorption of pharmaceutical excipients, and kinetics of water adsorption or desorption of a substance can be obtained effectively by the dynamic vapor sorption method. The knowledge may be utilized for dosage form design and supports the understanding of the mechanism of action. 

The results of numerous investigations on the kinetics of sorption of pure substances in zeolites have since then appeared in the literature and the field has been reviewed recently by Walker et al. 42). The total uptake or loss of sorbate in a large number of crystallites is commonly observed, and it is generally assumed that the rate of these processes is controlled by diffusion in the solid. Variable diffusion coefficients were sometimes observed by this method, and it appears possible that other processes than diffusion in the solid had some influence on the rate in these cases. The apparent diffusivity will depend only on concentration (besides temperature) if the migration of sorbate particles in the solid is rate controlling. A simple criterion whether this condition exists can be obtained by measuring sorption or desorption rates repeatedly for various initial concentrations and boundary conditions, as described by Diinwald and Wagner 43). 

Statistical mechanical Monte Carlo as well as classical molecular dynamic methods can be used to simulate structure, sorption, and, in some cases, even diffusion in heterogeneous systems. Kinetic Monte Carlo simulation is characteristically different in that the simulations follow elementary kinetic surface processes which include adsorption, desorption, surface diffusion, and reactivity . The elementary rate constants for each of the elementary steps can be calculated from ab initio methods. Simulations then proceed event by event. The surface structure as well as the time are updated after each event. As such, the simulations map out the temporal changes in the atomic structure that occur over time or with respect to processing conditions. 

The success of GC as a separation method is primarily dependent on maximizing the differences in retention times of the individual mixture components. An additional variable of such a separation process is the width of the corresponding chromatographic peak. Whereas the retention times are primarily dependent on the thermodynamic properties of the separaton column, the peak width is largely a function of the efficiency of the solute mass transport from one phase to the other and of the kinetics of sorption and desorption processes. Figure 3 is important to understanding the relative importance of both types of processes. 

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