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The toxicity of trisodium HEDTA to aquatic organisms was investigated in studies on the substance itself, as well as on the structural analogues disodium EDTA, tetrasodium EDTA, calcium disodium EDTA, manganese disodium EDTA and EDTA. A justification for the read across can be found in Section 13 of the IUCLID file.

Both HEDTA and EDTA are strong complexing agents. In the aquatic environment, the ionised substances will rapidly form complexes with available metals or trace nutrients. The complex stability constants with heavy metals are several orders of magnitude higher than those of Ca or Mg complexes. After addition of HEDTA or EDTA (as acid, Na-salt or Ca-salt) to water, the concentration of uncomplexed trace metals present in the water therefore decreases drastically. If enough HEDTA or EDTA is available, Ca/Mg complexation will also then occur. Uncomplexed HEDTA or EDTA is only found when it is present in over-stoichiometric concentrations (EU Risk Assessment EDTA, 2004).

The toxicity of the chelants HEDTA and EDTA to aquatic organisms is thought to be based on their disturbance of metal metabolism. Due to the complex nature of the EDTA-metal interactions, the impact may vary from one environment to the other, depending on metal concentration, pH, nature of the sediment, concentration of organics, etc. (EU Risk Assessment EDTA, 2004). In laboratory settings, the ability of HEDTA or EDTA to bind metals and nutrients in the test media causes nutrient deficiency by reducing the essential concentration of different ions.

Overall, the acute toxicity of trisodium HEDTA and its structural analogues to fish and Daphnia appear to be in the same order or magnitude, mainly influenced in laboratory settings by water hardness and pH. The lowest value, retained for risk assessment purposes, is a 21 d NOEC of 25.6 mg Na3-HEDTA from a study in Daphnia magna (recalculated value from the original 21 d NOEC of 25 mg/L expressed as Trilon BD, containing Na2-EDTA).

In algae, toxicity could clearly be demonstrated to be secondary effects linked to iron complexation in the medium. The EC50 values from studies in algae were therefore not retained for risk assessment purposes.

The toxicity of HEDTA on microorganisms is considered likely to be low based on read-across to the structural analogue EDTA and one available study of low quality on HEDTA. The toxicity of Na2-EDTA, a structural analogue of the substance, on microorganisms in an activated sludge resulted in an EC10 value after 30 minutes of >500 mg/L. Similar low toxicity was observed in additional studies on activated sludge. Thus, inhibition of degradation activity of activated sludge is not anticipated.