An electrochemical study of the speciation of copper,zinc and iron in two estuaries in England

The organic speciation of copper, iron and zinc in estuarine waters is studied using electrochemical techniques. Complexing capacities for copper and zinc were determined by cathodic stripping voltammetry (CSV) of their complexes with respectively catechol and amino pyrrolidine dithiocarbamate (APDC). Iron speciation was studied by CSV measurements of dissolved iron before (‘free iron’) and after acidification and UV-irradiation (‘total iron’) of the filtered samples. Complexing capacities of copper were found to vary between 1·4 and 5 × 10⁻⁷m with conditional stability constants, logK′_CuL, between 9·2 and 10·3 in the Tamar estuary. Complexing capacities of zinc were less at between 0·4 and 1·6 × 10⁻⁷m with values for logK′_ZnL between 8·1 and 9·4. Copper complexing capacities generally decreased with increasing salinity, and variations in the results were related to high concentrations of suspended material. Similar variations in the dissolved vanadium concentrations suggested that part of this element was associated with colloidal material. The total dissolved iron concentration in samples from the River Ribble decreased from 10⁻⁶m at low salinity to 10⁻⁷m at high salinity, but the free iron concentration was found to decrease from 8 to 3 × 10⁻⁸m over the same salinity range, which may be compared with the calculated solubility of iron in seawater of 2 × 10⁻⁸m. Comparative experiments showed that on average about 24% of the non-labile iron fraction was stabilized by organic material, the rest being composed of inorganic colloidal material.