Kinetics of sorption processes

Fokin, A. D., and Chistova, Y. D. (1967). Possibility of using internal diffusion equations to describe the kinetics of sorption processes in soils. Sov. Soil Sci. (Engl. Transl.) pp. 776-780. 

Kinetics of Sorption Processes as a Basis for Estimating Cation Distribution in Unit Cells of Zeolites... 

The kinetics of the sorption process of catiorric blue dye has been studied at the following phase ratio V= 50 rrrl, = 0.01 g/1, arrdpH = 3.41. The mass of the cel-Mosic sorbent is 0.5 g, the rrrass of the carbortate rock is 0.3 g. The resrrlts obtained in studying the kinetics of sorption process allow calculating the degree of the dye extraction. In addition, the pH of the medirrm has been measttred. The results are presented in Table 1. 

Many factors affect the mechanisms and kinetics of sorption and transport processes. For instance, differences in the chemical stmcture and properties, ie, ionizahility, solubiUty in water, vapor pressure, and polarity, between pesticides affect their behavior in the environment through effects on sorption and transport processes. Differences in soil properties, ie, pH and percentage of organic carbon and clay contents, and soil conditions, ie, moisture content and landscape position climatic conditions, ie, temperature, precipitation, and radiation and cultural practices, ie, crop and tillage, can all modify the behavior of the pesticide in soils. Persistence of a pesticide in soil is a consequence of a complex interaction of processes. Because the persistence of a pesticide can govern its availabiUty and efficacy for pest control, as weU as its potential for adverse environmental impacts, knowledge of the basic processes is necessary if the benefits of the pesticide ate to be maximized. 

Mechanisms of Sorption Processes. Kinetic studies are valuable for hypothesizing mechanisms of reactions in homogeneous solution, but the interpretation of kinetic data for sorption processes is more difficult. Recently it has been shown that the mechanisms of very fast adsorption reactions may be interpreted from the results of chemical relaxation studies (25-27). Yasunaga and Ikeda (Chapter 12) summarize recent studies that have utilized relaxation techniques to examine the adsorption of cations and anions on hydrous oxide and aluminosilicate surfaces. Hayes and Leckie (Chapter 7) present new interpretations for the mechanism of lead ion adsorption by goethite. In both papers it is concluded that the kinetic and equilibrium adsorption data are consistent with the rate relationships derived from an interfacial model in which metal ions are located nearer to the surface than adsorbed counterions. 

The kinetics of sorption can be considered as the sum of two processes 1) rapid sorption by labile sites which are in equilibrium with solutes dissolved in bulk solution, and 2) hindered sorption by sites which are accessible only by slow diffusion. Alternatively, sorption kinetics can be modeled by a radial diffu-sional process into spherical sorbents. The slow sorption process prevents complete equilibration within one day, the time used in typical batch experiments. Because the apparent rate of diffusion decreases with increasing hydrophobicity, time to equilibrium is longer for highly hydrophobic compounds. 

Reaction kinetics. The time-development of sorption processes often has been studied in connection with models of adsorption despite the well-known injunction that kinetics data, like thermodynamic data, cannot be used to infer molecular mechanisms (19). Experience with both cationic and anionic adsorptives has shown that sorption reactions typically are rapid initially, operating on time scales of minutes or hours, then diminish in rate gradually, on time scales of days or weeks (16,20-25). This decline in rate usually is not interpreted to be homogeneous The rapid stage of sorption kinetics is described by one rate law (e.g., the Elovich equation), whereas the slow stage is described by another (e.g., an expression of first order in the adsorptive concentration). There is, however, no profound significance to be attached to this observation, since a consensus does not exist as to which rate laws should be used to model either fast or slow sorption processes (16,21,22,24). If a sorption process is initiated from a state of supersaturation with respect to one or more possible solid phases involving an adsorptive, or if the... 

The main objectives of this chapter are to (1) review the different modeling techniques used for sorption/desorption processes of organic pollutants with various solid phases, (2) discuss the kinetics of such processes with some insight into the interpretation of kinetic data, (3) describe the different sorption/ desorption experimental techniques, with estimates of the transport parameters from the data of laboratory tests, (4) discuss a recently reported issue regarding slow sorption/desorption behavior of organic pollutants, and finally (5) present a case study about the environmental impact of solid waste materials/complex... 

Most of the sorption/desorption transformation processes of various solid phases are time-dependent. To understand the dynamic interactions of organic pollutants with solid phases and to predict their fate with time, knowledge of the kinetics of these processes is important [20,23]. 

The following sections represent the different solid phases used to study the leaching kinetics and sorption processes of different SWM leachates. 

A recent process development from Zurich by M. Bariska (20) has a number of important and interesting features. The method is specifically designed to test the applicability of ammonia forming to commercial practice. Secondly it constitutes a lower extremum, an investigation of conditions of minimum useful plasticity. Furthermore it is based on extensive investigations of kinetics, thermodynamics, sorption processes, and structural changes characteristics of the ammonia wood system. 

Duration of a cycle of HHP operation is defined as time required for reaction hydrogenation/dehydrogenation in pair hydride system. This time determines heat capacity of HHP. Duration of a cycle depends on kinetics of hydrogenation reactions, a heat transfer between the heated up and cooling environment, heat conductivities of hydride beds. Rates of reactions are proportional to a difference of dynamic pressure of hydrogen in sorbers of HHP and to constants of chemical reaction of hydrogenation. The relation of dynamic pressure is adjusted by characteristics of a heat emission in beds of metal hydride particles (the heat emission of a hydride bed depends on its effective specific heat conductivity) and connected to total factor of a heat transfer of system a sorber-heat exchanger. The modified constant of speed, as function of temperature in isobaric process [1], can characterize kinetics of sorption reactions. In HHP it is not sense to use hydrides with a low kinetics of reactions. The basic condition of an acceptability of hydride for HHP is a condition of forward rate of chemical reactions in relation to rate of a heat transmission. 

A fundamental understanding of sorption processes requires a detailed mechanistic knowledge of the equilibria, kinetics, and dynamics of the sorption process. The FSR is a cyclic batch process for which adsorption is carried out at a relatively higher pressure and desorption (regeneration) is accomplished at a lower pressure, generally using part of the product from the adsorption... 

A significant amount of work has been done on determining sorption capacity and the kinetics of sorption in zeolites because of their applications as adsorbents and as catalysts in the chemical process industry. However, most of this work has been done with single components, whereas all practical applications involve multicomponent mixtures. Hence, measurement of binary or multicomponent equilibria on zeolites is of considerable importance. 

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). 

In a study achieved by Memon et al. [16] the sorption of carbofuran and methyl parathion on treated and untreated chestnut shells has been studied using high performance liquid chromatography. In this study, the maximum sorption of methyl parathion and carbofuran onto chestnut shells was achieved at a concentration of 0.38.10 and 0.45.10" mol.dm respectively. Adsorption isotherms depicted a better fitting with the Langmuir isotherm. The results of sorption energy obtained from the Dubinin-Radushkevich isotherm pointed out that adsorption was driven by physical interactions. The kinetics of sorption follows a first-order rate equation. The thermodynamic parameters AS and AG indicate that the sorption process is thermodynamically favourable, and spontaneous, whereas the value of AH shows the exothermic nature of sorption process for methyl parathion and endothermic nature of carbofuran. The developed sorption method has been employed in methyl parathion and carbofuran in real surface and ground water samples. The sorbed amount of methyl parathion and carbofuran may be removed by methanol to the extent of 97-99% from the surface of chestnut shells. 

Rates of adsorption and desorption in porous adsorbents are generally controlled by transport within the pore network, rather than by the intrinsic kinetics of sorption at the surface. Since there is generally little, if any, bulk flow through the pores, it is convenient to consider intraparticle transport as a diffusive process and to correlate kinetic data in terms of a diffusivity defined in accordance with Pick s first equation ... 

Here H is the theoretical plate height, a parameter that characterizes the effectiveness of the chromatographic separation. The smaller the H the more powerful is the separation. A is the Eddy diffusion term (or multipath term), B relates to longitudinal diffusion, C represents the resistance of sorption processes (or kinetic term), and u is the linear flow rate. For a given chromatographic system. A, B, and C are constants, so the relationship between H and u can be plotted as shown in Fig. 6. 

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