Sorption of benzo[a]pyrene to sediment contaminated by acid mine drainage: contrasting particle concentration-dependencies in river water and seawater

The sorption of benzo[a]pyrene (BaP) to sediment contaminated by acid mine drainage from the River Carnon, SW England, has been studied as a function of particle concentration in river water and seawater. The sediment is iron-rich and has a high specific surface area, and previous studies have demonstrated that it exhibits positive surface charge when suspended in native river water (pH 5), and, through an increase in pH (to about 8) and the adsorption of organic matter, negative charge when suspended in seawater. Unit sorption of BaP exhibited an inverse dependency on particle concentration (particle concentration effect, PCE) in seawater, but was relatively invariant in river water. Consequently, BaP was apparently salted out at particle concentrations below about 100 mg l−1, and salted in at higher particle concentrations. The absence of a PCE (and inherent salting in) for such a hydrophobic compound is an unusual, and possibly unique observation, and is inconsistent with a third (e.g. colloidal) phase model. More likely, a PCE in seawater is caused, in part, by salt induced particle–particle or particle–organic matter interactions which result in increasing particle aggregation or sorption irreversibility with increasing particle concentration. Although the results have limited environmental application and do not conclusively resolve any causal mechanisms for the PCE, they suggest that, with respect to neutral organic compounds, adsorbed organic matter is a pre-requisite for a PCE in the natural environment, and the effect should not solely or generally be attributed to the existence of a third phase.