Influence of nonlinear sorption kinetics on the slow-desorbing organic contaminant fraction in soil

Release rates of hydrophobic organic compounds (HOCs) from the soil matrix influence the availability of HOCs in soils or sediments for microbial degr adation or removal by physical means (e.g., soil washing or soil venting). In this study it was shown that the initial contaminant concentration influ ences the desorption rate. This was attributed to the presence of a limited number of high affinity sites that cause nonlinear sorption behavior. The experimental results could be described with a kinetic model composed of tw o separate compartments. One compartment was described with a Freundlich is otherm and corresponding kinetics and was assumed to represent sorption to high affinity sites. The second compartment was described with a linear sor ption isotherm and first-order kinetics. The model was used to simulate the influence of purging strategies on removal of QCB. The simulations showed that after removal of a fast-desorbing fraction, the slow-desorbing fractio n could he efficiently removed at very low purging rates. Intermittent purg ing reduced the total purging time but the simulations showed large fluctua tions in the aqueous pentachlorobenzene concentration. For each subsequent purging interval, the purging efficiency decreased due to the nonlinear des orption kinetics of the slow-desorbing fraction of pentachlorobenzene.