Influence of metal complex formation on heavy metal and free EDTA and DTPA acute toxicity determined by Daphnia magna

The EC50 values (24h, Daphnia magna) of Fe3+, Cu2+, Mn2+, Zn2+, Cd2+ and Hg2+ complexed by EDTA or DTPA were compared with those of the respective uncomplexed metals and free EDTA and DTPA. The experiments gave the following toxicity order: Hg2+ > Cu2+ > Cd2+ > Zn2+ > Fe3+ > Mn2+. After complexation, the toxicities were 1.1 – 1700 (EDTA) and 7.0 – 3400 (DTPA) times lower. With Hg2+ however, complexation did not decrease the toxicity. Excluding Cd-DTPA, the toxicity reductions had a high correlation with the conditional stability constants of the respective metal chelates. Compared with the toxicity of free EDTA and DTPA, metal complexation increased the toxicity of EDTA and DTPA considerably. The results of the previous Microtox® study were well in accordance with this investigation.