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The European Risk Assessment for H4EDTA ( Institute for Health and Consumer Protection, Summary Risk Assessment, CAS 60 -00 -4, Special Publication I.04.279, 2004) provides a comprehensive summary of the impact of available trace metals on the aquatic toxicity of H4EDTA. The toxicological profile of EDTA is based on disturbances of metal metabolism. For the interpretation of toxicity tests, the complex formation properties of EDTA have to be taken into account. Beside Ca and Mg, test media contain a certain amount of heavy metal ions being necessary as trace nutrients. The complex formation constants of heavy metal complexes are by several orders of magnitude higher than of Ca/Mg-complexes, thus after addition of the test substance EDTA (as acid or Na-salt) the concentration of uncomplexed trace metals decreases drastically. The degree of Ca/Mg complexation is dependent on the amount of added EDTA. Uncomplexed EDTA is only available when it is present in over-stoichiometric concentrations.

 

Short-term tests reveal that EDTA and Na-EDTA are more toxic in an uncomplexed form. This can only occur if they are available in over-stoichiometric amounts to the chelants. Under these conditions the complexing agents can cause nutrient deficiency by reducing the essential concentration of different ions. The higher the water hardness the higher was the concentration of EDTA necessary to cause a toxic effect expressed as mortality.

 

The apparent effects of complexing agents to algal growth are related to essential trace metal bioavailability. It was demonstrated that not the absolute EDTA concentration, but rather the ratio of the EDTA concentration to the metal cations is crucial to algae growth. With sufficient trace metal amounts, H4EDTA concentrations up to 310 mg/L caused no effects. Similar results are obtained when Fe(III)EDTA is used as test substance, due to its slow metal exchange kinetics overchelation of the nutrient metal ions is avoided. Therefore direct effects caused by the intrinsic toxicity of EDTA are not expected in surface waters, where in nearly every case a stoichiometric surplus of metal ions is present.