Speciation and partitioning of cadmium and zinc in two contrasting estuaries: The role of hydrophobic organic matter
Limnol. Oceanogr., 49(1), 2004, 11-19 | DOI: 10.4319/lo.2004.49.1.0011
ABSTRACT: The hydrophobicity and particle-water interactions of cadmium (Cd) and zinc (Zn) in an organic-poor estuary (Plym, United Kingdom) and an organic-rich estuary (Beaulieu, United Kingdom) have been investigated using radiotracers. The hydrophobic fraction of dissolved Cd (retained by a reverse-phase C18 column) was about 50% in the River Beaulieu and less than 10% in the River Plym, and in both cases this fraction decreased rapidly with increasing salinity, such that only a few percent was hydrophobic at salinities above 10. Hydrophobicity of Cd was unaffected by the addition of suspended sediment particles, indicating that hydrophobic and hydrophilic forms adsorb in accordance with their relative abundance or that rapid reequilibration of dissolved Cd species occurs after specific forms have undergone adsorption. In the Plym, a logarithmic, salinity-dependent reduction in the sediment-water distribution coefficient, KD, and a high proportion of adsorbed Cd bound exchangeably indicate that particle-water interactions of Cd in environments of moderate concentrations of organic matter are governed by inorganic processes. In the Beaulieu, more uniform KDs and a significantly lower exchangeability of adsorbed Cd indicate that organic interactions might also be important. In both estuaries, the hydrophobic fraction of dissolved Zn was between about 80% and 100% and displayed an increase with increasing salinity and a reduction in the presence of suspended sediment particles. Throughout the Beaulieu, and in the Plym above salinities of about 4, KD also increased with increasing salinity. These observations were interpreted and modeled in terms of the preferential uptake and salting out of hydrophobic complexes of Zn and indicate that, regardless of the concentration of organic matter, the speciation and uptake of Zn in many estuaries might be strongly influenced by hydrophobic interactions.