The influence of speciation on the ecotoxic effects of heavy metals in aquatic systems

The fate, bioavailability and exposure of aquatic organisms to potentially toxic metals are strongly influenced by the speciation of the metal ions in the medium. Metal speciation is mainly controlled by pH, ionic strength, and presence of ligands ranging from small ions (e.g. citrate, EDTA) to highly complex compounds such as dissolved organic matter, DOM. In the concept of Life Cycle Impact Assessment (LCIA) the Characterisation Factor (CF) expresses the relative hazard of a chemical as the product of a Fate Factor (FF) and an Effect Factor (EF): CF = FF * EF. CFs has been developed for the total chemical mass emitted into the environment. In this study we show that by means of the metal speciation, EF can be corrected in such a way that the resulting CF becomes more accurate for each metal in the LCIA. The chemical speciation in various media was calculated by Visual Minteq ver. 2.56. Calculations showed that the speciation is very dependent on the metal concentration and the composition of the media. A large variation in heavy metal toxicity for the same test organism was found for a given metal in different media. Our main hypothesis is that this is due to differences in speciation and that if we correct for this we will find the same EC50 for the free metal ion species for Daphnia magna in both natural waters and synthetic media. This should be applied to correct the EF influence on CF in LCIA when the speciation for the metal is known. The hypothesis has been tested by comparing studies carried out in natural waters and in synthetic test media and analysing correlations between the free metal concentration and acute as well as chronic toxicity reported for Cd, Cu and Zn.