Kinetics of sorption and desorption

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. 

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... 

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. 

Moisture sorption of microcrystalline cellulose has been studied extensively. Fig. 6A includes the sorption and desorption studies for microcrystalline cellulose. The inserts are plots of moisture content versus time, which approximately represent the kinetics of sorption and desorption at each humidity. The equilibrium adsorption isotherm (Type II) has been fit to the BET equation (C = 16.48, = 0.033 g/g solid) and this curve is... 

It must be noted that a retardation coefficient is strictly applicable only when a linear relationship exists between Cs and Caq and the partitioning equilibrium is rapidly established. In some nonlinear cases, or when the kinetics of sorption and desorption are slow, mixing is increased as chemicals are released from aquifer solids after some time has passed, resulting in a smearing or tailing of a pollutant peak (Fig. 3-28). 

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. 

Rates of sorption and desorption of phosphate. Eur. J. Soil Sd. 48 101-114 Strens, R.G.S. Wood, B.J. (1979) Diffuse reflectance spectra and optical properties of some iron and titanium oxides and oxyhydr-oxides. Min. Mag. 43 347—354 Stumm, W. Eurrer, G. (1987) The dissolution of oxides and aluminum silicates Examples of surface-coordination-controlled kinetics. 

The rate of sorption and desorption of pesticides on soils and soil constituents has been investigated by a number of workers (see, e.g., Hance, 1967) and is dependent on the type of sorbent, pesticide, and rate of mixing. For example, sorption seems much slower on humic substances (Khan, 1973). Other factors that may affect the kinetics are swelling of the sorbent and temperature (Hance, 1967). 

Crosslinked poly(2-hydroxyethylmethacrylate), PHEMA, has been prepared by radical polymerization in presence of different amounts of water and water-diacetine solutions. While homogeneous samples can be prepared at all water-diacetine content, amounts of water higher than 40% causes the formation of macroporous opaque sponges. Mechanical properties of water swollen samples have been measured and related to the crosslink formation during the polymerization process. From water diffusion kinetics in sorption and desorption experiments at 37 C the diffusion coefficients have been calculated. Measurements of the swelling at equilibrium in water-diacetine mix-res indicate the presence of "cosolvency" phenomenon for the samples studied. 

Peak diffuseness may be a result of the kinetics of the sorption-desorption process (i.e., slow mass transfer or exchange at sorbent surfaces). Peak diffusion in this case is usually nonsymmetric because the rates of sorption and desorption are not the same. Band spreading due to the final rate of mass exchange is closely related to the diffusion phenomenon. Physical adsorption, for all practical purposes, is instantaneous. The overall process of sorption, however, consists of several parts (a) the movement of sorbate molecules toward the sorbent surface, resulting fi om intergrain diffusion (outer diffusion), (b) movement of sorbate molecules to the inside of pores (i.e., internal diffusion of the sorbate molecules in the pores and surface diffusion in the pores), and (c) the sorption process in general. 

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. 

In order to test the reversibility of metal-bacteria interactions, Fowle and Fein (2000) compared the extent of desorption estimated from surface complexation modeling with that obtained from sorption-desorption experiments. Using B. subtilis these workers found that both sorption and desorption of Cd occurred rapidly, and the desorption kinetics were independent of sorption contact time. Steady-state conditions were attained within 2 h for all sorption reactions, and within 1 h for all desorption reactions. The extent of sorption or desorption remained constant for at least 24 h and up to 80 h for Cd. The observed extent of desorption in the experimental systems was in accordance with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. 

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 reaction rate Rj in these equations is a catch-all for the many types of reactions by which a component can be added to or removed from solution in a geochemical model. It is the sum of the effects of equilibrium reactions, such as dissolution and precipitation of buffer minerals and the sorption and desorption of species on mineral surfaces, as well as the kinetics of mineral dissolution and precipitation reactions, redox reactions, and microbial activity. 

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 a sediment system, the hydrolysis rate constant of an organic contaminant is affected by its retention and release with the sohd phase. Wolfe (1989) proposed the hydrolysis mechanism shown in Fig. 13.4, where P is the organic compound, S is the sediment, P S is the compound in the sorbed phase, k and k" are the sorption and desorption rate constants, respectively, and k and k are the hydrolysis rate constants. In this proposed model, sorption of the compound to the sediment organic carbon is by a hydrophobic mechanism, described by a partition coefficient. The organic matrix can be a reactive or nonreactive sink, as a function of the hydrolytic process. Laboratory studies of kinetics (e.g., Macalady and Wolfe 1983, 1985 Burkhard and Guth 1981), using different organic compounds, show that hydrolysis is retarded in the sohd-associated phase, while alkaline and neutral hydrolysis is unaffected and acid hydrolysis is accelerated. 

In recent years the Coal Research Laboratory has been investigating the kinetics and isotherm behavior of methanol sorption on coal (6, 7, 10) along with the sorption of other vapors on coal (6) and of polar vapors on swelling gels (9, 10). Methanol sorption was shown to be reversible on coal, and its sorption behavior supports the model of coal as a gel or mixture of gels in its physical structure. All indications (I, 6, 7) are that its interaction is with specific and a fixed number of sites for a particular coal sample. Although the sorption of methanol is reversible, coal exhibits sorption behavior which is interpreted in terms of an irreversible swelling of the coal gel upon initial exposure to methanol vapor. As a result of these studies, an isotherm and experimental rate equation for the sorption and desorption were derived that fit the observed data. The isotherm derived for methanol sorption on coal was ... 

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,...

Since the fraction of empty sites in a zeolite channel determines the correlation factor (Section 5.2.2), as is well known from single-file diffusion in the pores of a membrane, the strong dependence of the diffusion coefficients on concentration can be understood. This is why a simple Nernst-Planck type coupling of the diffusive fluxes (see, for example, [H, Schmalzried (1981)]) is also not adequate. Therefore, we should not expect that sorption and desorption are symmetric processes having identical kinetics. Surveys on zeolite kinetics can be found in [A. Dyer (1988) J. Karger, D.M. Ruthven (1992)]. 

Many of the early studies on kinetics of soil chemical processes were obviously concerned with diffusion-controlled exchange phenomena that had half-lives (r1/2) of 1 s or greater. However, we know that time scales for soil chemical processes range from days to years for some weathering processes, to milliseconds for degradation, sorption, and desorption of certain pesticides and organic pollutants, and to microseconds for surface-catalyzed like reactions. Examples of the latter include metal sorption-desorption reactions on oxides. 

Kuo and Lotse (1973) used a two-constant rate equation, derived below, which is adapted from the Freundlich equation to study the kinetics of P04 sorption and desorption on hematite and gibbsite. 

The Elovich equation has been used to describe the kinetics of P04 sorption and desorption on soils and soil minerals (Atkinson et al., 1970 Chien and Clayton, 1980 Chien et al., 1980 Sharpley, 1983), potassium reactions in soils (Sparks et al., 1980b Martin and Sparks, 1983 Sparks and Jardine, 1984 Havlin and Westfall, 1985), borate dissolution from... 

Recently, Hodges and Johnson (1987) used five different kinetic equations to describe sulfur sorption and desorption on soils. Coefficients of determination showed that shell progressive particle diffusion, Elovich,... 

Pesticide Sorption and Desorption Kinetics 129 Classes of Pesticides 129 Reaction Rates 130... 

PESTICIDE SORPTION AND DESORPTION KINETICS Classes of Pesticides... 

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. 

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