MODELING THE CHEMICAL SPECIATION OF TRACE-METALS IN THE SURFACE WATERS OF THE HUMBER SYSTEM

Calculations have been performed to estimate the chemical speciation a t equilibrium of six divalent trace metals (Co, Ni, Cu, Zn, Cd, Pb) in riverine, estuarine and marine surface waters of the Humber system. T he Windermere Humic Aqueous Model (WHAM) was used to compute distribut ions of dissolved metals. In the rivers, the free aquo ion (M2+) is a major part of dissolved Co, Ni, Zn and Cd, but accounts for less than 1% Of Cu and Pb. The main complexes are formed with carbonate ligands and dissolved natural organic matter, represented by fulvic acid. In t he low-salinity region of the estuary and in seawater, complexation wi th fulvic acid is less significant, although most of the Cu is still i n this form, while the speciation of Cd is dominated by chloride compl exes. Adsorption of metals by suspended particulate matter was calcula ted with a simple model involving the concentrations of the free aquo ions (M2+) and H+, together with a constant for each metal estimated f rom laboratory adsorption data. Calculated adsorbed concentrations wer e used to predict the partition coefficient (K-D) for each metal under different circumstances. The values can vary by an order of magnitude or more, depending upon solution conditions. Typical values for river s, low-salinity water and seawater are within one order of magnitude o f observations. However, there is a general tendency to underestimate K-D, possible reasons being (1) neglect of electrostatic enhancement o f adsorption at low ionic strengths; and (2) analytical overestimation of particulate metal in equilibrium with the solution phase. There is a strong case for the development of a more sophisticated adsorption model. (C) 1998 Elsevier Science B.V.