Chemical Partitioning to Solids

Chemical partitioning also occurs between water and solid phases and between air and solid phases, in a process most generally termed sorption. Types of sorption include adsorption, in which a chemical sticks to the two-dimensional surface of a solid, and absorption, in which a chemical diffuses into a three-dimensional solid. Chemical sorption in the environment is much more difficult to predict than is chemical partitioning between air and water, partly because the types of sorptive solid phases (sorbents) vary enormously, and partly because there are many different mechanisms by which sorption can occur. In this section, only partitioning between water and solid phases is considered. 

Use of a partition coefficient without reference to the conditions under which it was measured suggests that the coefficient is a constant. A constant coefficient implies a linear relationship between the amount of dissolved chemical and the amount sorbed. In actuality, the relationship between dissolved and sorbed chemical concentrations is often nonlinear and may be expressed as a sorption isotherm. (Use of the term isotherm indicates that sorption measurements are being made at a constant temperature.) Laboratory measurements of sorption sometimes fit a relationship known as the Freund-lich isotherm, 

FIGURE 1-10 Freundlich isotherms having exponents less than, equal to, and greater than one. An exponent of one corresponds to a linear isotherm, in which case the relationship between the aqueous concentration of a chemical and the chemical s concentration on a solid phase at equilibrium with the water can be described by a single partition coefficient. If the exponent is less than one, the ratio of sorbed concentration to aqueous concentration decreases as the chemical concentration in the system increases. Such a decrease would happen, for example, if the solid contained a finite number of sites that became filled (saturated) at higher aqueous chemical concentrations. An exponent greater than one might occur if the sorbed chemical modified the solid phase to favor further sorption. 

FIGURE 1-11 Some examples of polar and nonpolar chemical species. Note that unbalanced electrical charge, asymmetry, and presence of oxygen all tend to make chemicals more polar. 

Kow has been measured and tabulated for many chemicals and also lends itself to estimation based on the chemical s structure (Lyman et al., 1990 Howard, 1989-1991 Schwarzenbach et al, 1993 Yalkowsky and Valvani, 1979 Syracuse Research Corporation s Environmental Fate Data Base and Hansch and Leo, 1985). See Table 1-3 for Kow values for some chemicals commonly found in the environment. 

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