Sorption-desorption process

Sorption and desorption are usually modeled as one fully reversible process, although hystersis is sometimes observed. Four types of equations are commonly used to describe sorption/desorption processes Langmuir, Freundlich, overall and ion or cation exchange. The Langmuir isotherm model was developed for single layer adsorption and is based on the assumption that maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface, that the energy of adsorption is constant, and that there is no transmigration of adsorbate on the surface phase. 

An increase of the pH in the aqueous medium, and capture of SO42- by the mineral surface, could lead to the liberation of Pi. In addition, there seem to be selfregulation mechanisms for the Pi sorption-desorption process, depending on the S042 concentration at the interface. Processes such as enrichment and liberation... 

Various forms of macro- and microelements differ in their ability to migrate and redistribute among the soil profile. The elements contained in clastic minerals are practically immobile. The elements, bound to finely dispersed clay minerals, are either co-transported with clay particles, or are involved in sorption-desorption processes. Part of the elements are found in concretions and also in very thin coating films of hydrated iron oxides some elements make a part of specially edaphic organic compounds. 

Sorption/desorption is one of the most important processes influencing movemement of organic pollutants in natural systems. Sorption with reference to a pollutant is its transfer from the aqueous phase to the solid phase on the other hand, desorption is its transfer from the solid phase to the aqueous phase. Similar to all interphase mass-transfers, the sorption/ desorption process can be defined by the final-phase equilibrium of the pollutant at the aqueous-solid phase interface and the time required to approach final equilibrium. 

The main goal of this chapter is to review the most widely used modeling techniques to analyze sorption/desorption data generated for environmental systems. Since the definition of sorption/desorption (i.e., a mass-transfer mechanism) process requires the determination of the rate at which equilibrium is approached, some important aspects of chemical kinetics and modeling of sorption/desorption mechanisms for solid phase systems are discussed. In addition, the background theory and experimental techniques for the different sorption/ desorption processes are considered. Estimations of transport parameters for organic pollutants from laboratory studies are also presented and evaluated. 

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... 

The main reasons for investigating the rates of solid phase sorption/desorption processes are to (1) determine how rapidly reactions attain equilibrium, and (2) infer information on sorption/desorption reaction mechanisms. One of the important aspects of chemical kinetics is the establishment of a rate law. By definition, a rate law is a differential equation [108] as shown in Eq. (32) ... 

There are four types of rate laws that can be determined for solid phase sorption/desorption processes [109,110] mechanistic, apparent, transport with apparent, and transport with mechanistic rate laws, as follows ... 

Temperature has a marked effect on the kinetics of reaction rates of solid phase sorption/desorption processes [113-116]. Arrhenius noted the following relationship between k and T (Eq. 52) ... 

While first-order models have been used widely to describe the kinetics of solid phase sorption/desorption processes, a number of other models have been employed. These include various ordered equations such as zero-order, second-order, fractional-order, Elovich, power function or fractional power, and parabolic diffusion models. A brief discussion of these models will be provided the final forms of the equations are given in Table 2. 

In the next few sections, a case study of the environmental impact of highway construction and repair materials as well as hazardous solid waste materials is presented and discussed from the view points of sorption/desorption processes. 

In summary, the present case study involved sorption/desorption processes with distilled water of a variety of hazardous solid wastes and highway C R materials which are complex organic mixtures. The following are some of the findings ... 

There are a good number of sorption/desorption isotherm models which were developed in order to reflect the actual sorption/desorption processes occurring in the natural environment. Some models have a sound theoretical basis however, they may have only limited experimental utility because the assumptions involved in the development of the relationship apply only to a limited number of sorption/desorption processes. Other models are more empirical in their derivation, but tend to be more generally applicable. 

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. 

Inadequate understanding of the kinetics of the sorption/desorption process detracts from our ability to completely understand the effects of sorption on hydrolytic rates, and more research is needed in this regard. 

The sorption/desorption processes were studied by a batchtype technique. Aqueous solutions were prepared by mixing rock powders with distilled-deionized water. For the sorption experiments, portions of these aqueous solutions were loaded with tracer quantities of a single radioactive nuclide and contacted with wafers of a given rock type. For the desorption experiments, wafers from the sorption experiments were contacted with the remaining portions of the aqueous solutions. 

In this section the distinction between dissolved and sorbed species is introduced into the box model concept in the simplest possible manner, that is, by assuming a reversible linear equilibrium relationship between the dissolved concentration, C (molm 3), and the species sorbed on solids, Cs(mol kg 1). (The units m3 and kgs refer to water volume and solid mass, respectively.) The sorption/ desorption process shall be fast compared to other processes which affect the chemical (e.g., mixing, chemical transformation). As discussed in Chapter 9 (Eq. 9-7), the (observed) solid-water distribution ratio Kd is defined by. 

If the rate constants for the sorption-desorption processes are small equilibrium between phases need not be achieved instantaneously. This effect is often called resistance-to-mass transfer, and thus transport of solute from one phase to another can be assumed diffusional in nature. As the solute migrates through the column it is sorbed from the mobile phase into the stationary phase. Flow is through the void volume of the solid particles with the result that the solute molecules diffuse through the interstices to reach surface of stationary phase. Likewise, the solute has to diffuse from the interior of the stationary phase to get back into the mobile phase. 

Detenbeck (37) and Detenbeck and Brezonik (38, 39) examined the effect of pH on phosphorus sorption for LRL sediments. Their results suggested that the flux of inorganic P from sediments could be diminished by as much as 90% if the pH of sediments decreased from 6.0 to 4.5. However, there was no observed treatment effect for TP and an apparent increase in SRP summer averages at pH 4.7 (Figure 4). Therefore, chemical sorption-desorption processes probably do not control phosphorus levels in LRL. The direction of response at lower pH implies that the balance between biotic uptake, deposition to sediments, and release from organic detritus by decomposition most likely controls SRP levels in the water column. 

Sorption/desorption processes involving the substrate and availability of active reaction sites on the iron surface... 

Assumptions inherent in the use of a Koc (or Kom) are that sorption is exclusively to the organic component of the soil all soil organic carbon has same sorption capacity per unit mass equilibrium is observed in the sorption-desorption process and the sorption and desorption isotherms are identical (Green and Karickhoff, 1990). Both Koc and Kd have units of L/kg or cm3/g. 

Selim et al. (1976b) developed a simplified two-site model to simulate sorption-desorption of reactive solutes applied to soil undergoing steady water flow. The sorption sites were assumed to support either instantaneous (equilibrium sites) or slow (kinetic sites) first-order reactions. As pore-water velocity increased, the residence time of the solute decreased and less time was allowed for kinetic sorption sites to interact (Selim et al., 1976b). The sorption-desorption process was dominated by the equilib-... 

This type of dehydration seems to be irreversible, whereas the sorption-desorption process at 40°C proceeds along a hysteresis-loop pattern (Fig. I).19... 

Figure 7.7b Surface waters in the Mississippi River showing U behavior in pre-flood, flood, and post-flood conditions (April, 1992, August, 1993, November, 1993). The U removal in the Mississippi River is not controlled by conventional sorption/desorption processes with carrier-phase oxides. It is generally conservative in behavior except for exceptionally high-discharge periods, such as the flood of 1993. (Modified from Swarzenski and McKee, 1998.)...

The latter point is especially well illustrated by the sorption-desorption process. When a molecule is sorbed, its velocity is zero when desorbed, it is on average equal to the fluid velocity v. Since a great number of molecules occupy a zone, at any given time many will be stationary and many in downstream motion. The zone as a whole, moving along as the blurred sum of these two parts, advances smoothly, assuming the extremes of neither the mobile nor stationary populations. 

Let us represent the sorption/desorption process in chromatog-raphy by A A2 where A, represents the sorbed state of the... 

The reason that the R/ data are so similar is that straight-phase TLC is principally based on sorption-desorption processes. The only variation between the barbiturates is the nature of the substituent at the C5 position. 

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