Organic compounds Partition isotherms

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. 

Soil Organic Matter. A variety of soils and sediments sorb organic compounds in patterns that can be approximated by linear isotherm models, at least over relatively narrow (e.g., 1 decade) concentration ranges. Such isotherms are generally, although often imprecisely, expressed in terms of a partitioning coefficient, fy such that... 

The C-type (constant-partitioning) isotherm, which suggests a constant relative aflSnity of the adsorbate molecules for the adsorbent, is usually observed only at the low range of adsorption. Deviation from the linear isotherm is likely at high adsorption levels. Nevertheless, because many nonpolar organic compounds of interest in soils are adsorbed at quite low concentrations, the linear C-type isotherm is often a reasonable description of adsorption behavior. 

It has been demonstrated that a partitioning process from water into soil organic matter is the major mechanism by which nonpolar organic compounds sorb in soil. This conclusion is based on the observation of linear isotherms, the magnitude... 

Figure 3.22 Representative isotherms for the sorption of volatile organic compounds in plant cuticular matrix at 25°C. Plant matrix-air partition coefficients derived from slopes of the isotherms. [Reproduced with permission from B. Welke, K. Ettlinger, and M. Riederer, Environ. Sci. Technol. 32, 1099 (1998). Copyright 1998, American Chemical Society.]...

The sorption and desorption of chlorinated pesticides limits the potential success of electro-osmotic transport. The sorption of hydrophobic organic compounds (HOCs) is a partitioning type of process and can be represented by linear isotherms until aqueous phase solubility is approached (Chiou, Peters, and Freed, 1979). Karickhoff (1981) reported that the sorption isotherm was linear for HOCs up to half of the aqueous phase solubility level of the solute. Although HOCs adsorb to mineral surfaces mainly due to van der Waals forces (Schwarzenbach, Gschwend,... 

This relationship resulted from the spontaneous tendency for the analyte, initially solubilized in an aqueous matrix, to partition into the surface monolayer of hydrophobic octadecyl- or octyl-bonded silica. Organic compounds as analytes will have a unique value for the thermodynamic distribution constant. One fundamental difference between LEE and SPE stands out Partition or adsorption of solute molecules onto a solid surface follows the principles of the Eangmuir adsorption isotherm, whereas EEE does not. We will briefly develop the principles below. This model assumes that an analyte A combines with a site of adsorption, S, in which there is a finite number of such sites according to... 

Sorption. Capture of neutral organics by non-living particulates depends on the organic carbon content of the solids (9). Equilibrium sorption of such "hydrophobic" compounds can be described by a carbon-normalized partition coefficient on both a whole-sediment basis and by particle size classes. The success of the whole-sediment approach derives from the fact that most natural sediment organic matter falls in the "silt" or "fine" particle size fractions. So long as dissolved concentrations do not exceed 0.01 mM, linear isotherms (partition coefficients) can be used. At higher concentrations, the sorptive capacity of the solid can be exceeded, and a nonlinear Freundlich or Langmuir isotherm must be invoked. 

In addition to the dependence of sorption on the organic fraction of the sorbent, and the KQw of the sorbate, Chiou et al. (13) cite the following observations as support for the hypothesis that the sorptive mechanism is hydrophobic partitioning into the organic (humic) fraction of the sediments (1) the linearity of the isotherms as the concentration approaches solubility, (2) the small effect of temperature on sorption, and (3) the lack of competition between sorbates for the sorbent. These arguments also illustrate the applicability of the approach for modeling sorption on hydro-phobic compounds an approach which has been criticized when used in the context of adsorption of trace metals onto oxides (17). 

For the following discussions, we will primarily use Kioc values from compilations published by Sabljic et al. (1995) and Poole and Poole (1999). According to these authors, the values should be representative for POM-water absorption (i.e., they have been derived from the linear part of the isotherms). Furthermore, many of the reported Kioc s are average values derived from data reported by different authors. Distinction between different sources of sorbents (e.g., soils, aquifer materials, freshwater, or marine sediments) has not been made. Nevertheless, at least for the apolar and weakly monopolar compounds, these values should be reasonably representative for partitioning to soil and sediment organic matter. 

Adsorption of purely hydrophobic compounds onto soils and sediments has been successfully modeled and predicted by treating the organic matter fraction as a separate phase in equilibrium with the aqueous phase (see Karick-hoff et al., 1979). Morel (1983) succinctly summarizes findings from the considerable amount of work that has been completed in this area (i) within a typical range of concentrations, sorption isotherms are linear, (ii) the extent of partitioning is proportional to the sediment organic C content, and... 

Vapor-phase sorption of carbon tetrachloride and benzene onto a peat soil (fom = 0.864) give linear isotherms indicating partitioning into the organic matter (Fig. 3.18). The sorption of these two compounds on the same soil from water is only 40-50% less than that observed from the vapor phase. This effect is much less than that observed when the mineral fraction is the primary absorbent. 

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