Effect of organic complexation on the behaviour of dissolved Cd, Cu and Zn in the Scheldt estuary

Abstract

Samples from the Scheldt estuary have been assayed for dissolved Cd, Cu and Zn using differential pulse anodic stripping voltammetry, either as total (after UV irradiation) or labile concentrations. Under these experimental conditions, labile concentrations ranged between 51–65% of total Cu, 16–66% of total Zn and 53–91% of total Cd. The metal-organic interactions were assessed by evaluating (a) the distribution coefficient K_d for the distribution of the metals between the liquid phase (complexation) and their binding to particulate matter, and (b) the competitive effect exerted by inorganic complexing ligands using a multi-element interaction model. The proportion of organically bound metals (strong and labile) was estimated, in this speciation scheme, to range from 86 to 99% for Cu, from 90 to 96% for Zn, and from 10 to 35% for Cd. From the dissolved organic carbon distribution in the Scheldt (≤ 10 mg C 1⁻¹) and taking into account competition from major cations Ca and Mg, free ligand concentrations available for heavy metal complexation were estimated to be ≤ 0.15 mg C 1⁻¹. With these values, conditional stability constants for the chelation of Cu, Zn and Cd were calculated assuming either a single-step or a two-step complexation in the dissolved phase. Given the assumptions made in these models, stability constants in the range of 10^7.8–10^10.6 for Cu, 10^7.0–10^9.1 for Zn and 10^6.9–10^8.9 for Cd were obtained. The relevance of these data to previous in vitro and in situ studies is discussed taking into consideration current concepts of metal binding affinity for organic ligands.