Salting out of chemicals in estuaries: implications for contaminant partitioning and modelling

Many neutral chemicals are salted out of aqueous solution via electrostriction and exhibit increased sorption with increasing concentration of dissolved salt. Salting out has significant implications for the reactivity, transport and fate of chemicals discharged to estuaries, but attempts to define or model the effect in such environments have been limited. This paper examines new and existing data on the sorption of neutral chemicals (specifically, a tetrachlorinated biphenyl, a phthalic acid ester, and neutral species of tributyltin) to estuarine suspended particles in order to evaluate the potential application and limitations of salting theory to estuarine chemical modelling. It is shown that the salinity dependence of sorption may be empirically modelled using a salting equation, but salting constants derived from data-fitting are often significantly greater than those derived by calculation or from conventional aqueous solubility studies. This suggests that the hydrophobicity of sediment organic matter is modified by interactions with dissolved seawater ions, and (or) chemical solubility is enhanced in river water via hydrophobic interactions with dissolved organic matter. In some estuaries, trace metals also appear to be salted out, suggesting that stable neutral complexes are formed between transition metals and a specific, but undefined pool of dissolved organic ligands. Despite successful empirical modelling of the effect in estuaries, predictive modelling of salting out is currently hampered by a lack of understanding or definition of the precise interactions between (i) neutral solute (or trace metal) and dissolved and sediment organic matter, and (ii) sediment organic matter and dissolved seawater ions.