Sorption-desorption kinetics

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). 

In a similar study (Comans et al., 1990), the reversibility of Cs+ sorption on illite was studied by examining the hysteresis between adsorption and desorption isotherms and the isotopic exchangeability of sorbed Cs+. Apparent reversibility was found to be influenced by slow sorption kinetics and by the nature of the competing cation. Cs+ migrates slowly to energetically favorable interlayer sites from which it is not easily released. 

Over the last decade, some research has indicated that (1) partition coefficients (i. e.,Kd) between solid and solution phase are not measured at true equilibrium [51,59-61], (2) the use of equilibrium rather than kinetic expressions for sorption in fate and effects models is questionable [22-24,60,61], and (3) sorption kinetics for some organic compounds are complex and poorly predictable [22 - 24,26]. This is mainly due to what has recently been discussed as slow sorp-tion/desorption of organic compounds to natural solid phase particles [107, 162-164,166-182]. The following is a summary of some important points supporting this hypothesis [1,66,67,170-183] ... 

Though this system is perhaps an extreme example of slow sorption kinetics, it illustrates that the assumption of rapid equilibrium between the sediment and aqueous phases is questionable. The importance of such an observation to the investigation of hydrolysis kinetics in sediment/water systems must be emphasized. Certainly, any model of hydrolysis kinetics in sediment/water systems must include explicit expressions for the kinetics of the sorption/desorption process. 

Unfortunately, our present understanding of sorption kinetics is inadequate to allow unambiguous representation of the sorption-desorption process. Clearly the states of sorbed pesticides include fractions which vary in their lability with respect to desorption (9. 10, 21). The fraction of the sorbed molecules in relatively labile and non-labile states is a function of the nature of the pesticide and sediment and the time of contact between the sediment and pesticide solution. 

The rate of adsorption and the rate of desorption are assumed to be dependent on the geometric relationship of the rock and solution (i.e., dependent upon the volume of solution and on the shape and area of the solid medium in contact with the solution). Because the dependence of sorption kinetics on the geometric relationship is not known, the rates for sorption are determined by experiment for the particular geometry (surface area of rock to volume of solution) for which the prediction of nuclide migration is desired. 

Sorption kinetics Sorption and desorption may not always be fast compared to other processes, such as advection and dispersion. In addition, the sorption equilibrium does not necessarily follow a linear relationship. 

The integral permeability coefficient P may be determined directly from permeation steady-state flux measurements or indirectly from sorption kinetic measurements 27 521 activity is usually replaced by gas concentration or pressure (unless the gas deviates substantially from ideal behaviour and it is desired to allow for this) and a<>, ax (p0, Pi) are the boundary high and low activities (pressures) respectively in a permeation experiment, or the final (initial) and initial (final) activities (pressures) respectively in an absorption (desorption) experiment. 

Rounds, S.A., B.A. Tiffany, and J.F. Pankow. 1993. Description of gas-particle sorption kinetics with an intraparticle diffusion model desorption experiments. Environ. Sci. Technol. 27 366-377. 

Karickhoff (1980) and Karickhoff et al. (1979) have studied sorption and desorption kinetics of hydrophobic pollutants on sediments. Sorption kinetics of pyrene, phenanthrene, and naphthalene on sediments showed an initial rapid increase in sorption with time (5-15 min) followed by a slow approach to equilibrium (Fig. 6.7). This same type of behavior was observed for pesticide sorption on soils and soil constituents and suggests rapid sorption on readily available sites followed by tortuous diffusion-controlled reactions. Karickhoff et al. (1979) modeled sorption of the hydrophobic aromatic hydrocarbons on the sediments using a two-stage kinetic process. The chemicals were fractionated into a labile state (equilibrium occurring in 1 h) and a nonlabile state. 

Z/mc plate height due to sorption/desorption micellar kinetics... 

Diffusion and sorption kinetics are critical for maintenance of sharp concentration profiles in fhe liquid and the beds. Complexation of metals to supported ligands is extremely sensitive to a variety of parameters, and the predominant presence of reactants at the upstream end and products at the downstream end may lead to imdesired variations in adsorption and desorption behavior. 

After selective imprinted polymers for a range of metals became available, slow sorption kinetics proved to be of considerable concern and indeed have remained so. Both sorption and desorption are presently too slow for industrial application. To try to overcome this a modified 2-step imprinting procedure has been developed to prepare porous polymer beads where the metal ligands occupy only the bead surfaces [134]. In another attempt the original imprinting strategy by Nishide has been used to synthesise the polymer on a silica gel [135]. In both cases sorption kinetics have improved significantly. 

Sorption Kinetics. The adsorption and desorption data were analyzed in terms of a model based on the following main assumptions. Micropore diffusion within the sieve crystals is the rate-controlling process. Diffusion may be described by Fick s law for spherical particle geometry with a constant micropore diffusivity. The helium present in the system is inert and plays no direct role in the sorption or diffusion process. Sorption occurs under isothermal conditions. Sorption equilibrium is maintained at the crystal surface, which is subjected to a step change in gas composition. These assumptions lead to the following relation for the amount of ethane adsorbed or desorbed by a single particle as a function of time (Crank, 4). 

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. 

Sorption equilibria and kinetics are influenced by the nature of the adsorbent and the adsorbate, by the mechanism of adsorption, and by environmental parameters such as temperature, relative humidity, concentration of the adsorbate, and air velocity and turbulence past the adsorbent surface. Air velocity and turbulence only affect sorption kinetics the other parameters also affect equilibria. In general, low adsorbate saturation vapor pressure, low temperature, and high adsorbate concentration in the air increase adsorption. Relative humidity does not always affect adsorption. Colombo et al. (1993) found a 35 % decrease in adsorbed mass when relative humidity was changed from <10 % to 35 %, but only an 8 % decrease when the humidity was increased from 35 % to 70 %. Building materials, which are exposed to indoor air in the normal humidity range of 35-70 %, will typically already be covered by at least one monolayer of adsorbed water, and the formation of multilayers will only have a limited influence on sorption properties for other airborne substances. Kirchner et al. (1997) found that an increase in air velocity increased the rate of desorption of a VOC mixture from painted gypsum, but not from carpet. The air velocity of air above the tuft may be insignificant for the desorption processes of carpet fibers deeper in the tuft. 

The input concentration and extent of sorption, as well as sorption kinetics, are seen to be correlated during iodine sorption and transport (Hu et al., 2005). In experiments on iodine transport in SRS surface soils at varying initial concentrations and residence times, iodate sorption was stronger, both at lower concentration and at longer residence time. The first-order desorption rate coefficients... 

Analysis of sorption kinetics. During adsorption a concentration profile C(x,t) of protein is established in an unstirred layer separating the adsorbing surface, situated at x=0, from the buffer solution. It IS assumed that initially no protein is present in the system and that at time t=0 the bulk concentration of protein in the buffer is changed to a fixed value It is also assumed that the adsorption rate is proportional to the number of free binding sites and to the protein concentration at the surface. The rate of desorption is assumed to be proportional to the surface concentration. For this binding model one hast... 

The main focus of this volume is on imderstanding the transport of molecules in microporous solids such as zeolites and carbon molecular sieves, and the kinetics of adsorption/desorption. This subject is of both practical and theoretical interest, since the performance of zeohte-based catalysts and adsorbents is strongly influenced by resistances to mass transfer and intracrystalline diffusion. However, at an even more basic level, the performance of microporous catalysts and adsorbents depends on favorable adsorption equilibria for the relevant species, so a general imderstanding of the fundamentals of adsorption equilibrium is a necessary prerequisite for understanding kinetic behavior. This chapter is intended to provide a concise summary of the general principles of adsorption equiHbriiun and of the main features of sorption kinetics in microporous solids, which generally depend on a combination of both equilibriiun and kinetic properties. 

This approach works also with adsorption/desorption-induced changes of reflectivity, provided the sorption kinetics are fast enough to follow the potential modulation. In this case absorption features in the obtained spectra are attributed to changes in coverage (no influence of the electrode potential is presumed). Precisely speaking, these spectra should not be called electroreflectance spectra (see also [88] concerning this classification). 

Most practical zeolitic adsorbents are used in a pellet form (with or without binders) where a network of meso-macro pores provide the access of the gases to the adsorption sites (inside the micropores of crystalline zeolites). The zeolite crystal and the pellet radii are typically in the range of 0.5-2.0 pm and 0.5-2.0 mm, respectively. Consequently, the kinetics of ad(de)sorption of Nz and Oz are often controlled by the transport of these gases through the mesoporous network, and the ad(de)sorption kinetic (Knudsen, molecular and Poiseuille flow) time constants are large (>0.5 seconds 1). Thus, the kinetics of ad(de)sorption processes may not be critical. The thermodynamic adsorptive properties (a,b) and the desorption characteristics (c) under local equilibrium conditions often determine the separation performance of a zeolite. 

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