Hydrophobicity and reactivity of trace metals in the low‐salinity zone of a turbid estuary

Abstract

We chemically fractionated water samples from the low‐salinity zone of a turbid estuary (Tamar, UK) using C18 columns to examine the controls on the complexation, hydrophobicity, and reactivity of dissolved trace metals (Co, Cu, Mn, and Pb). The hydrophobic (C18‐retained) fraction of metal was related to the solubility of metal in the amphiphilic solvent, n‐octanol, and in acid‐base titrations and for all metals this fraction exhibited a peak centered around pH 7–8 in both freshwater and brackish water. In freshwater, hydrophobicity decreased in the order Pb > Cu > Mn > Co, and in brackish water, hydrophobicity was either reduced (Cu and Pb) or enhanced (Co and Mn) compared to freshwater, but in all cases peaks were more distinct. Salinity distributions of dissolved trace metals in the estuary indicated significant removal of Co, Cu, and Mn, and a source of Pb that was coincident with the onset of the turbidity maximum, and all distributions could be accounted for empirically by end‐member water mixing and sediment resuspension. Salinity distributions of the hydrophobic fraction of metal were not consistent with any variation in water composition (including pH), but revealed that this fraction was preferentially removed from (Co, Cu, and Mn) or added to (Pb) the aqueous phase in the vicinity of the turbidity maximum. Removal implies that hydrophobic species are more reactive than hydrophilic species, and that either re‐equilibration between these fractions is not attained within the hydrodynamic timescales of the upper estuary, or new equilibria are established by the buffering effects of additional anions and ligands in the water column. Addition of hydrophobic species of Pb coincided with minimum total and hydrophobic concentrations of Co, Cu, and Mn, and could, therefore, be explained in terms of reduced competition for nonspecific hydrophobic ligands from these and other (e.g., Al) metals in this region of the estuary.