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 col lected from Indian Creek, Alabama, were measured with a gas purge-indu ced desorption technique and compared to a model developed in this wor k. DDT entered this sediment via effluent discharged upstream from the collection site from 1947 to 1970 by a DDT manufacturing facility. Th e purge technique used vessels constructed with 70-100 100 mu m fritte d glass bottoms, through which air enters, distributing gas bubbles ev enly to a sediment-water suspension. Purging compound from the water p hase with the gas bubbles induced desorption of compound from the sedi ment particles. The purged chemicals were captured on tenax traps. Pur ge experiments were performed with sediment masses ranging from 0.37 t o 3.7 g in 200 mL water at an air flow rate of 1 L/min. The total perc entage removal of each compound after 46 days averaged 22, 58, and 75% for DDT, DDD, and DDE, respectively, indicating the extreme resistanc e of DDT to desorption from this sediment. The time to reach 25% remov al was approximately 50 days for DDT and 4 days for DDD. Results are c ompared to a simple Fickian diffusion model in which diffusion is assu med to occur in one direction from a fixed depth to a plane surface. T he concentration at the exposed surface is assumed to be in equilibriu m with the aqueous concentration, which in turn is depleted by transfe r to the gas phase through first-order decay. Simulations indicate tha t DDD transfer across the liquid-gas interface is limiting at high sed iment concentrations.