The influence of salting out on the sorption of neutral organic compounds in estuaries

The relative (unsaturated) solubility and sorption of 2,2′,5,5′-tetrachlorobiphenyl have been studied along an estuarine salinity gradient. The aqueous compound was salted out with increasing salinity and an aqueous salting constant of about 0.002 l g−1 was derived. Sorption of the compound to estuarine particles increased with increasing salinity for a range of particle concentrations, but the magnitude of this effect (sorption salting constants of about 0.005–0.01 l g−1) indicated that salting out of the aqueous phase was not solely responsible. It is suggested that the hydrophobicity of sediment organic matter is enhanced by its interaction with seawater ions through a reduction in the charge of the particle surface and, possibly, modification of the structure of the organic matter. Examination of literature data on the sorption of neutral organic compounds to estuarine sediment indicates a general increase in sorption with increasing salinity which can be empirically defined by a salting equation. Although charge reduction of estuarine particles is a general observation, it is not possible to establish the general significance of this effect (or any other form of salting out of sediment organic matter) on the sorption of organic compounds in estuaries because appropriate site- and compound-specific aqueous salting constants are unavailable. Increased sorption at high salinities has obvious implications for the disposal and transport of organic chemicals in estuaries. However, the inverse dependency of sediment–water partitioning on particle concentration is likely to be of at least equal significance in macrotidal environments where sediment resuspension occurs. An empirical model, combining the effects of salinity and particle concentration, is proposed for deriving first-order estimates of the partitioning of neutral organic compounds in estuaries.