| 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. The adsorption and partitioning of hexachlorobenzene and three polychlorobiphenyl congeners (all hydrophobic organic chemicals) to natural sediments from the Detroit River were investigated by means of batch mixing experiments. Experiments were conducted for long periods (two to six months) in order to reach a steady-state and at different sediment concentrations (from 2 mg/L to 10000 mg/L). It was found that the adsorption rate was greatest for short times and decreased to zero as time increased. For all concentrations, a steady-state was always attained. Experiments quantitatively demonstrate the time-dependent effects of contaminant dissolution, colloids in the water and from the sediments, and the flocculation of colloids and the suspended sediments. Although a concentration effect of solids on the partition coefficient was observed, it is shown to be due to the above processes. Effects of physical characteristics such as particle size and sediment flocculation have also been determined and shown to be important rate controlling parameters. Also, organic content has been shown to be both a rate and equilibrium controlling parameter. From the data, a true equilibrium partition coefficient as well as the approximate times to equilibrium can be determined for each organic chemical. |
