Parameters affecting the adsorption of hexachlorobenzene to natural sediments

Many of the toxic contaminants introduced into aquatic systems are hydrophobic and, because of this, they are transported with the suspended sediments and eventually incorporated into the bottom sediments. Understanding the dynamics of contaminant sorption to sediments is therefore important in understanding and predicting the transport and fate of contaminants. In the present study, the adsorption and the partitioning of hexachlorobenzene (a hydrophobic organic chemical) to natural sediments from the Detroit River were investigated by means of batch mixing experiments. Experiments were conducted over a long period (2 to 5 months) to reach a steady-state equilibrium and at different sediment concentrations (10, 100, 500, 2,000, and 10,000 mg/L). It was found that the adsorption rate was greatest for small time and decreased to zero as time increased. For all concentrations, a steady-state equilibrium was always attained. Experiments quantitatively demonstrate the time-dependent effects of hexachlorobenzene dissolution, of the flocculation of colloidal matter in the water, and of the flocculation of the suspended sediments. Although a solids concentration effect on the partition coefficient was observed, it is shown to be due to the above processes. From the data, a true equilibrium partition coefficient as well as the approximate times to equilibrium can be determined.