Kinetics of sorption

A wide variety of guest molecules may be trapped by the Wemer-type crystalline host lattice, ranging, eg, from noble gases to condensed aromatic hydrocarbons. These clathrates may be formed from solution or by sorption. Kinetics of sorption—desorption have been studied (83). 

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. 

Sorbed pesticides are not available for transport, but if water having lower pesticide concentration moves through the soil layer, pesticide is desorbed from the soil surface until a new equiUbrium is reached. Thus, the kinetics of sorption and desorption relative to the water conductivity rates determine the actual rate of pesticide transport. At high rates of water flow, chances are greater that sorption and desorption reactions may not reach equihbrium (64). NonequiUbrium models may describe sorption and desorption better under these circumstances. The prediction of herbicide concentration in the soil solution is further compHcated by hysteresis in the sorption—desorption isotherms. Both sorption and dispersion contribute to the substantial retention of herbicide found behind the initial front in typical breakthrough curves and to the depth distribution of residues. 

Reaction kinetics at phase houndaiies. Rates of adsorption and desorption in porous adsorbents are generally controlled by mass transfer within the pore network rather than by the kinetics of sorption at the surface. Exceptions are the cases of chemisorption and affinity-adsorption systems used for biological separations, where the kinetics of bond formation can be exceedingly slow. 

The kinetics of sorption of arsenite and arsenate in the presence of sorbed silicic acid have been only recently examined (Waltham and Eick 2002). These authors demonstrated that the sorption of silicic acid (added 60 h before arsenic) decreased the rate and the total amount of arsenic sorbed. The amount of arsenite sorbed decreased as the surface concentration of silicic acid increased. Furthermore, the inhibition of arsenite sorbed ranged from about 4% at a pH of 6 and 0.1 mM silicic acid up to 40% at a pH of 8 and 1 mol IT1 silicic acid. In contrast, silicic acid reduced the rate of arsenate sorption which decreased by increasing pH and silicic acid concentration, but the total quantity of arsenate sorbed remained nearly constant, indicating that arsenate was able to replace silicate. 

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. 

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. 

The Elovich model was originally developed to describe the kinetics of heterogeneous chemisorption of gases on solid surfaces [117]. It describes a number of reaction mechanisms including bulk and surface diffusion, as well as activation and deactivation of catalytic surfaces. In solid phase chemistry, the Elovich model has been used to describe the kinetics of sorption/desorption of various chemicals on solid phases [23]. It can be expressed as [118] ... 

In previous work (l.> > ) it was found that the kinetics of sorption was an important parameter affecting the migration of nuclides in geologic media. For example, in experiments designed to measure the kinetics of reaction for radionuclides in solution with tablets of rock, it was found that periods from several minutes to several hours were required for the radionuclides to reach steady state concentrations on the rock tablets and in the solutions. Figure 1 shows the reaction curves found for the sorption of plutonium and americium from solution by a tablet of granite. The reaction rates for the sorption of plutonium and americium from solution are not the same, and both require a number of hours to reach steady state concentrations. 

Because of the results obtained for the kinetics of sorption of methanol on both acetylated and unacetylated coals, the mechanism for sorption of methanol on coal must explain the following experimental observations sorption follows a second-order rate equation the experimental rate constants vary with pressure, and two different behaviors are noted the rate constants decrease upon partial acetylation at equilibrium one molecule of methanol is sorbed on one site. In addition, the mechanism must also account for the observations that the reverse desorption experimental rates are independent of the original pressure of sorption, and increased acetylation substantially decreases the rate of desorption. The following mechanism is postulated ... 

Figure 12. Isotherms and kinetics of sorption for Vroctane in ethylenediammonium forms of synthetic fluorhectorites of exchange capacities 90 (a and c) and 150 (h and d) milliequivalents per 100 grams ( ) sorption points, (O) desorption points (73). Qt, Qoo are amounts sorbed at time t and at equilibrium,...

Amirbahman, A., Kent, D.B., Curtis, G.P. and Davis, J.A. (2006) Kinetics of sorption and abiotic oxidation of arsenic(III) by aquifer materials. Geochimica et Cosmochimica Acta, 70(3), 533-47. 

The authors used all three models to study kinetics of sorption of 1,4-dichlorobenzene, 1,2,4-trichlorobenzene, 1,2,3,4-tetrachlorobenzene, and pentachlorobenzene on soils and sediments using a batch technique. The one-box model did not fit the data well, while the two-box model described the data better. However, there are three fitting parameters kx, k2, and ) in this model, which could explain the better conformity of the data to the model. Another disadvantage of this model is that it is not easy... 

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. 

Elzerman AW, Coates JT. 1987. Hydrophobic organic compound on sediments Equilibria and kinetics of sorption. In Hites RA, Eisenreich SJ, eds. Sources and Fates of Aquatic Pollutants. Advances in Chemistry Series 216. Washington, DC American Chemical Society, 263-317. 

In addition, the PEI-DVP polymer was modified with chloroacetic acid in order to study the complexing properties. This derivative showed a lower sorption capacity and slower kinetics of sorption for Cu2+, Ni2+, and Co2+ compared to the unmodified copolymers [54]. 

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 number of additional equations are often used to describe reaction kinetics in soil-water systems. These include the Elovich equation, the parabolic diffusion equation, and the fractional power equation. The Elovich equation was originally developed to describe the kinetics of gases on solid surfaces (Sparks, 1989, 1995 and references therein). More recently, the Elovich equation has been used to describe the kinetics of sorption and desorption of various inorganic materials in soils. According to Chien and Clayton (1980), the Elovich equation is given by... 

In the case that the fluid in contact with the polymeric surface is a liquid, the amount of that liquid sorbed to attain gel-saturation (or solution) after C becomes Cg is of course considerably greater than the amount sorbed to attain Cg. The kinetics of sorption thereafter, however, is dependent upon the history of the... 

When the composite membranes have been preconditioned and cleaned as described above, the kinetics of sorption as well as the sorption capacity at the asymptotic limit is highly reproducible [125] as shown in Fig. 8, which records as... 

Ho Y. S. and McKay G., The kinetics of sorption of divalent metal ions onto sphagnum moss peat. Water Res. 34 (2000), pp. 735-742... 

A gravimetric technique to study the sorption, desorption, and diffusion characteristics of uxiter vapor in excised skin under dynamic conditions is described. The technique features a continuously recording microbalance and a humiditygenerating apparatus which provides a stream of air with any given relative humidity. The diffusion coefficient is determined from the kinetics of sorption and desorption. The technique can be used to study other polymeric films, fibers, and powders. 

In batch systems, the distribution or sorption coefficient (Xliquid phases. The X(j is commonly measured under equilibrium or at least steady-state conditions, unless the goal of the experiment is to examine the kinetics of sorption. It is defined as follows ... 

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

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