Desorption kinetics of neutral hydrophobic organic compounds from field-contaminated sediment

The chemical release rates from a field-contaminated sediment (Lake Charles , LA) using Tenax desorption were studied. Two dichlorobenzenes (m-, p-), h exachlorobutadiene, and hexachlorobenzene were investigated. Contrary to re ports that sorption rates are inversely related to K-OW, the slow desorptio n rates were found to be similar for the four compounds. The data were mode led by a two-compartment irreversible adsorption and radial diffusion model . Desorption kinetics from the first irreversible compartment can be modele d by radial diffusion and assume an irreversible adsorption constant and so il tortuosity of 4.3. The desorption half-life is approximately 2-7 days. D esorption from the second irreversible compartment is very slow (half-life of approximately 0.32-8.62 years) presumably caused by entrapment in soil o rganic matter that increases the constrictivity of the solid phase to chemi cal diffusion. From the kinetic data, it is deduced that the diffusion pore diameter of the second irreversible compartment is approximately equal to the critical molecular diameter. The mass of chemicals in this highly const rictive irreversible compartment is approximately one-fourth of the maximum irreversible, or resistant, compartment. The slow kinetics observed in thi s study add additional support to the notion that the irreversibly sorbed c hemicals are 'benign' to the environment. (C) 2000 Elsevier Science Ltd. Al l rights reserved.