Organic compounds sorption processes

Note that such a conceptualization still requires a corresponding desorption of some number, n, of water molecules from the vicinal region to make a cavity near the solid surface in which i must fit. This solvent desorption process contributes an additional free energy term to the overall organic compound sorption process due to breaking of watenwater interactions in this layer. 

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. 

The fate of trace organic compounds applied to soil is controlled by several processes volatilization, degradation, sorption, leaching and bioaccumulation. The significance of each process is affected by the physicochemical properties of the organic compound, sludge and soil properties, and environmental conditions. 

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. 

Besides liquid and solid (snow, ice) water surfaces, mineral surfaces play the most important role with respect to adsorption processes involving gaseous organic compounds in the environment. Thus, a brief description of the chemical nature of mineral surfaces is helpful to understand our subsequent discussion of such sorption phenomena. 

Soil sorption of most hydrophobic organic compounds (e.g., nonpolar pesticides) is directly related to SOM content. HS are the major SOM components (Ferreira et al., 2002). FTIR spectroscopy makes possible the observation of how some chemical functions, present in humic structures, are involved in the sorption process. 

Although different processes may dominate in different situations, it can be assumed that multiple steps occur during sorption of an organic compound from liquids into or onto a solid phase. Any of the steps may... 

PFR models are limited, however, because of the slow velocities encountered in groundwater aquifers and the tendency for many contaminants (particularly hydrophobic organic compounds) to sorb. More appropriate but more complex models based on various forms of the advection-dispersion equation (ADE) have been used by several researchers to incorporate more processes, such as dispersion, sorption, mass transfer, sequential degradation, and coupled chemical reactions. 

Sorption of organic contaminants onto aquifer solids is frequently described as a partitioning process, where the hydrophobic organic compound partitions into natural organic material associated with the aquifer solids [8]. Sorption can be characterized as either an equilibrium or rate-limited phenomenon. Equilibrium sorption can be modeled as either a linear or non-linear process. Equilibrium sorption may be assumed when the flow of groundwater and other processes affecting contaminant transport are slow compared to the rate of sorption. In this event the sorption of the contaminant can be considered instantaneous. If we assume equilibrium sorption, the relationship between sorbed and aqueous contaminant concentrations may be described by a sorption isotherm. 

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