Partitioning of hydrochlorinated pesticides to chitin in seawater: Use of a radial-diffusion model to describe apparent desorption hysteresis

Desorption hysteresis has been reported frequently in the literature, with several theories advanced to explain the cause of this phenomenon. Batch experiments were performed to observe sorption-desorption behaviour of endrin and heptachlor epoxide onto chitin in seawater as a function of chitin concentration, temperature and salinity. For full range of solids concentrations, sorption was described by a single linear partition coefficient. Desorption hysteresis was found at low chitin concentrations for endrin, increasing both at low temperatures and high salinity values. Results of these experiments were interpreted with a radial-diffusion sorption rate model. Model predictions of the desorption experiments showed that slow desorption rates explained most of the apparent hysteresis. The “salting effect” in marine systems makes partition coefficients 0.2 log unit higher than comparable partition coefficents measured in freshwater.