from 4.84 at pH 1.2 to 1.30 at pH 10.5 and pH 11.5. This

higher partition coefficient at lower pH suggests a greater

affinity for the organic part of the soil as the pH

decreases.

 A number of areas of research in the region of sorption

chemistry remain controversial, such as reversibility,

extent of reversibility, rate of attaining equilibrium, and

the effect of competitive solutes in sorption equilibria.

In many cases sorption is considered to be reversible

(Angley, 1987). However, Miller and Webber (1984) reported

that many researchers disagree about the reversibility of

sorption with various chemicals. Laquer and Manahan (1987)

reported that the sorption of phenol onto a siltstone showed

differences in adsorption and desorption isotherms, an

effect termed hysteresis. Rogers et al. (1980) found that

once sorbed, benzene tends to resist desorption. Van

Genuchten et al. (1977) suggested that values of the

equilibrium desorptive constant should be different from

that of adsorption while Equation (3-2) holds for both

cases. Nathwani and Phillips (1977) drew the same

conclusions on some hydrocarbons in crude oil, and found

that the percentage of hydrocarbon component desorbed varied

inversely with the amount of organic matter in the soil

matrix. Researchers have also shown diverse results about

the rate of attaining equilibrium. Miller and Webber (1986)

observed equilibrium occurring after several days for

nitrobenzene and lindane. Rao and Davidson (1980) found