Speciation and partitioning of cadmium and zinc in two contrasting estuaries:The role of hydrophobic organic matter

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 C 18 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, K D , 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 K D s 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, K D 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.It is becoming increasingly evident that one of the principal controls on the adsorption of transition metals to mineral surfaces is the concentration and nature of organic ligands. Davis and Leckie (1978) were among the first to study the effects of a range of different synthetic organic ligands on the adsorption of trace metals to hydrous oxides, and they demonstrated that metal adsorption was either enhanced or reduced depending on whether or not coordinating groups were involved in binding with the mineral surface. A general observation of the many studies undertaken since is that in the presence of most (but not all) organic ligands, metal adsorption is enhanced at low pH and reduced at high pH (Warren and Haack 2001).Far less often studied are the effects of natural organic ligands on the adsorption of metals to natural, heterogeneous (and often organic-rich) suspended particles. Such studies rely on the addition of a radiotracer analogue of the metal of interest to a natural particle suspension and on controlling the concentration (or presence) of dissolved organic ligands. Accordingly, Paulson et al. (199...

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

“…(Note that the two types of associations are not distinguished in future discussion and the terms complex and complexation are all-embracing.) Our operational definition of trace metal hydrophobicity is based on the net retention of metal by a C18 column (Garnier et al 1997;Turner et al 2004):…”

“…Significantly, under circumneutral conditions and for both metals, f H is enhanced in brackish water compared with freshwater. It is possible that, because of their size and conformation, complexes of these metals are salted out as water attains a more ordered structure in the presence of dissolved ions, an effect that appears to be significant for complexes of a number of metals in other estuaries (Turner et al 2001. Trace metal distributions and reactivity in the upper estuary-The upper reaches of the Tamar estuary afford an ideal natural reactor to examine the combined effects of pH, salinity, and concentration of SPM on dissolved trace metal fractionation and reactivity.…”

“…Octanol extraction-For selected river water samples collected on a number of occasions, we undertook an independent assessment of trace metal hydrophobicity by determining the extent of metal extraction in n-octanol according to the technique described in Turner and Mawji (2004). Briefly, six 5-ml aliquots of sample were pipetted into 50-ml polyethylene centrifuge tubes that had been cleaned successively in detergent, acid, and water, and to three aliquots 5 ml of n-octanol (HPLC-grade, Aldrich) were added and the contents shaken on a horizontal shaker at 300 rpm for 16 h at room temperature.…”

“…Recently, we distinguished hydrophobic and hydrophilic species of radiolabeled trace metals, added to filtered estuarine samples, using C18 columns (Turner et al 2001. Experimental results revealed that, after the addition of estuarine particles, the more hydrophobic forms, or in some cases the more hydrophilic forms, were preferentially removed from solution.…”

Seasonal variation of dissolved heavy metals in the reservoir of Shahid Rajaei dam, Sari, Iran

“…This phenomenon was also partly seen in S9 station located near the Shirin Rud River entrance. However, a uniform distribution of Co concentration was observed in [2,19], salinity [8,[20][21][22], redox condition [23][24][25] and pH [25][26][27][28]. Temperature influences on the metabolism and growth rates of the aquatic organisms, plant photosynthesis, oxygen solubility in water, and sensitivity of the organisms to disease, parasites and toxic materials.…”