EFFECT OF ORGANIC COMPLEXATION ON THE BEHAVIOR OF DISSOLVED CD, CU AND ZN IN THE SCHELDT ESTUARY

Samples from the Scheldt estuary have been assayed for dissolved Cd, C u 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-orga nic interactions were assessed by evaluating (a) the distribution coef ficient K-d for the distribution of the metals between the liquid phas e (complexation) and their binding to particulate matter, and (b) the competitive effect exerted by inorganic complexing ligands using a mul ti-element interaction model. The proportion of organically bound meta ls (strong and labile) was estimated, in this speciation scheme, to ra nge from 86 to 99% for Cu, from 90 to 96% for Zn, and from 10 to 35% f or Cd. From the dissolved organic carbon distribution in the Scheldt ( less than or equal to 10 mg C l(-1)) and taking into account competiti on from major cations Ca and Mg, free ligand concentrations available for heavy metal complexation were estimated to be less than or equal t o 0.15 mg C l(-1). With these values, conditional stability constants for the chelation of Cu, Zn and Cd were calculated assuming either a s ingle-step or a two-step complexation in the dissolved phase. Given th e assumptions made in these models, stability constants in the range o f 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 co ncepts of metal binding affinity for organic ligands.