Modelling the chemical speciation of trace metals in the surface waters of the Humber system
Calculations have been performed to estimate the chemical speciation at equilibrium of six divalent trace metals (Co, Ni, Cu, Zn, Cd, Pb) in riverine, estuarine and marine surface waters of the Humber system. The Windermere Humic Aqueous Model (WHAM) was used to compute distributions 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 the low-salinity region of the estuary and in seawater, complexation with fulvic acid is less significant, although most of the Cu is still in this form, while the speciation of Cd is dominated by chloride complexes. Adsorption of metals by suspended particulate matter was calculated with a simple model involving the concentrations of the free aquo ions (M2+) and H+, together with a constant for each metal estimated from laboratory adsorption data. Calculated adsorbed concentrations were used to predict the partition coefficient (KD) for each metal under different circumstances. The values can vary by an order of magnitude or more, depending upon solution conditions. Typical values for rivers, low-salinity water and seawater are within one order of magnitude of observations. However, there is a general tendency to underestimate KD, possible reasons being (1) neglect of electrostatic enhancement of 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.