Equilibrium speciation models were established for Hg (II), Cd (II) and Pb (II) in bog-, river- and sea water having a composition corresponding to that of the Gulf of Bothnia (6 ‰ salinity). The calculations were performed using the computer program SOLGASWATER. The models were based on available thermodynamic data at 25°C and included the inorganic ligands OH−, Cl−, Br−, HPO42− and CO2 (aq). Furthermore, complex formation with organic substances originating from a bog water was considered, Mg (II) and Ca (II) were also included as competing cations. All water types were assumed to be oxic and in equilibrium with atmospheric carbon dioxide, 10−3.5 atm (32 Pa).
The calculations showed that the speciation of Hg (II) and Pb (II) in bog and river waters is dominated by complexation with the organic substance, except for lead (II), where the Pb2+ -ion dominates at pH⩽5. In the Gulf of Bothnia, the chloro complexes HgCl2 and HgCl3− becomes prevailing as the salinity increases. Besides Pb2+, the species PbCl+ and PbCO3 become important upon increasing the salinity and pH. The dominating fraction of Cd (II) is Cd2+ in bog water, in river water and in waters of salinity lower than 4.5 ‰.
With increasing salinity CdCl+ and CdCl2predominate.