Sorption processes

Persistence of pesticides in the environment is controlled by retention, degradation, and transport processes and their interaction. Retention refers to the abihty of the soil to bind a pesticide, preventing its movement either within or outside of the soil matrix. Retention primarily refers to the sorption process, but also includes absorption into the soil matrix and soil organisms, both plants and microorganisms. In contrast to degradation that decreases the absolute amount of the pesticide in the environment, sorption processes do not affect the total amount of pesticide present in the soil but can decrease the amount available for transformation or transport. 

Desorption is the reverse of the sorption process. If the pesticide is removed from solution that is in equdibrium with the sorbed pesticide, pesticide desorbs from the sod surface to reestabUsh the initial equdibrium. Desorption replenishes pesticide in the sod solution as it dissipates by degradation or transport processes. Sorption/desorption therefore is the process that controls the overall fate of a pesticide in the environment. It accomplishes this by controlling the amount of pesticide in solution at any one time that is avadable for plant uptake, degradation or decomposition, volatilization, and leaching. A number of reviews are avadable that describe in detad the sorption process (31—33) desorption, however, has been much less studied. 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

Fair, James R., Sorption Processes for Gas Separation, Chemical Engineering, July 14, 1969. 

Such vessels can also be baked at a temperature of several hundred degrees, to drive off any gas adsorbed on metal surfaces. The pumping function of an ion gauge was developed into efficient ionic pumps and turbomolecular pumps , supplemented by low-temperature traps and cryopumps. Finally, sputter-ion pumps, which rely on sorption processes initiated by ionised gas, were introduced. A vacuum of 10 "-10 Torr, true UHV, became routinely accessible in the late 1950s, and surface science could be launched. 

The authors repeated the experiment with two, more strongly retained, solutes m-dimethoxy benzene and benzyl acetate. These solutes were found to elute at (k ) values of 10.5 and 27.0 respectively on a silica column operated with the same mobile phase. The results obtained are shown as similar curves in Figure 13. The m dimethoxy benzene, which eluted at a (k ) of 10.5, also failed to displace any ethyl acetate from the silica gel even when more than 0.5 g of solute resided on the silica surface. Consequently, the m-dimethoxy benzene must have also interacted with the surface by a sorption process. 

An absorbent material is one which changes either chemically, physically, or both during the sorption process. Certain chemicals, in absorbing moisture during this process, will dissolve into the water from the initial crystalline structure. Further added water results in a phase change from solid to liquid. An adsorbent is another material in which there are no chemical, phase, or physical changes during the sorption process. 

Sorbent Any agent that is used in a sorption process. 

Broughton D. B., Gerhold C. G. (1961) Continuous Sorption Process Employing Fixed Bed of Sorbent and Moving Inlets and Outlets, U.S. Patent No. 2, 985, 589. 

In preparative selective chromatography, the formation of broad zones of the substances is determined by the formation of sharp boundaries of each zone. The formation of these sharp boundaries of substance zones in column sorption processes for systems in which the interphase transfer is limited by substance diffusion in sorbent grains [104, 122, 123] is determined by the dimensionless criterion X ... 

Nikolaev NI, Zolotarev PP, Popkov YuM, Ulin VI (1981) Theory and Practice of Sorption Processes, (in Russian) Voronezh University, Voronezh 14 12... 

Fischer-Tropsch synthesis products contain also high quantities of n-a-olefins that can be recovered by selective sorption processes with suitable molecular sieves [19]. A large-scale Fischer-Tropsch synthesis plant operates in South Africa [20]. Another plant was started in Indonesia in 1993 [21]. 

BCR CRM 402 Trace Elements in White Clover 0, = 0.456 at 2i.r°C w.c. = 7.52 0.04%. Despite its relatively high value, the water activity is still in the range for long-term stability and major differences in the water content as a consequence of sorption processes are not to be expected. This is supported by the fact that no spoilage has been observed, even though the production was in 1987. 

M. Streat and D. Naden, eds., Ion Exchange and Sorption Processes in Hydrometallurgy, John Wiley Sons, 1987. 

A sorption process on the surface of a porous material, like Zeolite and other solid adsorbents, or within a concentrated salt solution, like LiCl and others, are examples for such chemical reactions for thermal energy storage. 

The methods described in this chapter can be transferred to liquid sorption processes with slight modifications. The example of concentrated salt solutions and water absorption is described by Kessling [6],... 

Regarding submerged plants, sorption of Cu(II) by Myriophyllum spicatum L. (Eurasian water milfoil) has been shown to be fast and fits isotherm models such as Langmuir, Temkin, and Redlich-Peterson. The maximum sorption capacity (c/lll l j ) of copper onto M. spicatum L. was 10.80 mg/g, while the overall sorption process was best described by the pseudo-second-order equation.115 Likewise, Hydrilla verticillata has been described as an excellent biosorbent for Cd(II). In batch conditions, the qmsx calculated was 15.0 mg/g. Additionally, II. verticillata biomass was capable of decreasing Cd(II) concentration from 10 to a value below the detection limit of 0.02 mg/L in continuous flow studies (fixed-bed column). It was also found that the Zn ions affected Cd(II) biosorption.116... 

For a classical diffusion process, Fickian is often the term used to describe the kinetics of transport. In polymer-penetrant systems where the diffusion is concentration-dependent, the term Fickian warrants clarification. The result of a sorption experiment is usually presented on a normalized time scale, i.e., by plotting M,/M versus tll2/L. This is called the reduced sorption curve. The features of the Fickian sorption process, based on Crank s extensive mathematical analysis of Eq. (3) with various functional dependencies of D(c0, are discussed in detail by Crank [5], The major characteristics are... 

Pure PHEMA gel is sufficiently physically cross-linked by entanglements that it swells in water without dissolving, even without covalent cross-links. Its water sorption kinetics are Fickian over a broad temperature range. As the temperature increases, the diffusion coefficient of the sorption process rises from a value of 3.2 X 10 8 cm2/s at 4°C to 5.6 x 10 7 cm2/s at 88°C according to an Arrhenius rate law with an activation energy of 6.1 kcal/mol. At 5°C, the sample becomes completely rubbery at 60% of the equilibrium solvent uptake (q = 1.67). This transition drops steadily as Tg is approached ( 90°C), so that at 88°C the sample becomes entirely rubbery with less than 30% of the equilibrium uptake (q = 1.51) (data cited here are from Ref. 138). 

The primary active surface that interacts with the chemical in the sorption process has been shown to be the organic fraction of the soil(6-10). Therefore, the sorption characteristics of a chemical can be normalized to obtain sorption constant based on organic carbon (K ) which is essentially independent of any soil. 

Finally, the third major input information required is external (i.e., extrinsic to the compound itself) the environmental physical conditions (see Fig. 2). Temperature and water regimes are often the most determinant factors which affect the mobility of chemicals in the environment by accelerating volatilization or sorption processes. Solar radiation is also crucial in the chemicals fate since it is strongly related to photodegradation and volatilization processes as well. 

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