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Source: W. Mabey and T. Mill, 1978, Critical review of hydrolysis of organic compounds in water under environmental conditions, Jour. Phys. Chem. Ref. Data 17, no. 2:383-415.

Source: W. Mabey and T. Mill, 1978, Critical review of hydrolysis of organic compounds in water under environmental conditions, Jour. Phys. Chem. Ref. Data 17, no. 2:383-415.

eral surfaces). In addition, the adsorption of organic chemicals depends on the organic matter content of the soil. The relationship between the organic content of soil and the adsorption coefficient of organic chemicals is generally linear for soils with an organic carbon content greater than 0.1 (Hamaker and Thompson 1972).

The adsorption process is usually reversible. At equilibrium, the adsorption coefficient, which is the rate at which the dissolved organic chemical in water transfers into the soil, can be described with the linear Freundlich isotherm equation as

Cw where:

K,j = distribution coefficient

Cs = concentration adsorbed on soil surfaces, ug/g Cw = concentration in water, ug/ml

Other nonlinear isotherm equations are also used (Lyman 1982), such as:

Cw where n is a constant usually between 0.7 and 1.1.

As in Equations 9.12(18) and 9.12(19), the distribution or adsorption coefficient Kd is directly proportional to the organic carbon content of the soil; thus, Kd can be written as

oc where:

Koc = normalized adsorption coefficient foc = soil organic carbon content

The normalized adsorption coefficient can be estimated from the organic chemical's water solubility or octanol water partition coefficient with use of regression equations (Dragun 1988b), such as log(Koc) = a- log(S) + b 9.12(21)

where:

S = water solubility Kow = octanol water partition coefficient a,b,c,d = coefficients that depend on the organic chemical

Table 9.12.4 lists the adsorption coefficient Koc for several organic chemicals. The regression coefficients a, b, c, and d for several chemicals are in Brown and Flagg (1981), Briggs (1973), and Keneya and Goring (1980). Therefore, after the distribution coefficient Kd is estimated, the effect of adsorption on the mobility of a specific compound can be calculated with use of a form of the advection-disper-sion equation, Equations 9.13(1) and 9.13(3), that includes the retardation factor R.

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