ADSORPTION-DESORPTION HYSTERESIS IN ORGANIC POLLUTANT AND SOIL SEDIMENT INTERACTION

Adsorption and desorption of pollutants to soil and sediment materials are major fate mechanisms. The hypothesis that adsorption and desorption are reversible processes has been tested. The organic pollutants naphthalene, phenanthrene, and p-dichlorobenzene have been studied in the laboratory using batch reactors at room temperature from a few hours to over 2 months. The adsorption experiments were at equilibrium within 1-4 days and could be modeled using simple linear isotherms with K(p) values consistent with published K(oc) and K(ow) relationships. Desorption experiments were conducted with the contaminated sediments by successive dilutions. Desorption experiments varied from 1 day to 5 months, and observed desorption rates were from 1 to 3 orders of magnitude smaller than previously measured or predicted. If equilibrium were obtained during the desorption, typically over 82-99% of the adsorbed pollutant would have been desorbed, but generally only 30-50% of the adsorbed pollutant could be desorbed. These desorption results could not be explained by commonly invoked kinetic models or artifacts of the procedure. The possibility and consequences of such adsorption and desorption behavior being the result of either hysteresis or irreversible adsorption is discussed.