Induced desorption of DDT, DDD, and DDE from a contaminated sediment

Release profiles of p,p'-DDT, p,p'-DDD, and p,p'-DDE from sediment collected from Indian Creek, Alabama, were measured with a gas purge-induced desorption technique and compared to a model developed in this work. DDT entered this sediment via effluent discharged upstream from the collection site from 1947 to 1970 by a DDT manufacturing facility. The purge technique used vessels constructed with 70-100 100 mu m fritted glass bottoms, through which air enters, distributing gas bubbles evenly to a sediment-water suspension. Purging compound from the water phase with the gas bubbles induced desorption of compound from the sediment particles. The purged chemicals were captured on tenax traps. Purge experiments were performed with sediment masses ranging from 0.37 to 3.7 g in 200 mL water at an air flow rate of 1 L/min. The total percentage removal of each compound after 46 days averaged 22, 58, and 75% for DDT, DDD, and DDE, respectively, indicating the extreme resistance of DDT to desorption from this sediment. The time to reach 25% removal was approximately 50 days for DDT and 4 days for DDD. Results are compared to a simple Fickian diffusion model in which diffusion is assumed to occur in one direction from a fixed depth to a plane surface. The concentration at the exposed surface is assumed to be in equilibrium with the aqueous concentration, which in turn is depleted by transfer to the gas phase through first-order decay. Simulations indicate that DDD transfer across the liquid-gas interface is limiting at high sediment concentrations.