Time Dependence of Chlorobenzene Sorption/Desorption by Soils

Soil contact time with organic chemicals (aging) has been found to cause increased sorption coefficients and reduced isotherm linearity as well as increased environmental sequestration and persistence. In this study, the sorption/desorption behavior of chlorobenzene (CB) on four soils was evaluated after aging periods of 24 h and 14 mo. The four soils ranged in organic matter content from 0.69 to 13.4%. Sorption isotherms were performed after each aging period to observe changes in CB uptake. Desorption kinetic profiles were generated after each aging period to observe changes in CB release from soil, with desorption followed for a period of four months. These data demonstrated no increase in CB uptake by three soils between the 24-h and 14-mo sorption periods and only a slight increase (~22%) by the fourth soil. The sorption isotherms were linear (r2 > 0.99) and did not display increased nonlinearity with aging. A three-site desorption model was used to evaluate desorption parameters for both aging periods. Sorption/desorption was not fully reversible; non-desorbable fractions (fnd) were observable after only 24 h of aging, and ranged from 0.17 to 0.27. Non-desorbable fraction increased for all four soils with longer aging time; after 14 mo aging fnd ranged between 0.28 and 0.45. The CB sorption data were difficult to reconcile with current dual mode models that have been proposed to account for the sorption of organic contaminants and pesticides by soil organic matter.