pentapotassium 2,2’,2’’,2’’’,2’’’’-(ethane-1,2-diylnitrilo)pentaacetate

Endpoint summary

Additional information

Several acute and chronic aquatic toxicity studies with either the pentapotassium or pentasodium salt or the free acid form of Diethylenetriaminepentaacetic acid (DTPA) were conducted with fish, invertebrates, and algal species. Upon contact with water, the pentapotassium and pentasodium salts of DTPA are expected to dissociate, thus, these chemicals are considered essentially equivalent to the free acid form of DTPA in aquatic toxicity tests. Stoichiometric conversions were used to interconvert toxicity endpoint values (LC50, NOEC, etc.) among DTPA (free acid) and its pentapotassium and pentasodium salts. 

 

A consideration for DTPA and its salts in the aquatic environment is the effect of water hardness on toxicity. The toxicity of sodium salts and the free acid form of DTPA increases as water hardness decreases (Schmidt and Brauch, 2004). In addition, in non-buffered systems, the addition of the free acid form of DTPA can make the pH more acidic, while the addition of the pentasodium salt of DTPA can make the pH more alkaline.

 

In cases where the free acid form or the alkali salt form is present, toxic effects on aquatic organisms are most often related to metal deficiencies caused by the complexation of essential metals in the test media (Schmidt and Brauch, 2004).  For instance, studies revealed that the observed reproductive toxicity in Daphnia carinata was mainly caused by complexation of manganese, zinc, and iron (van Dam et al., 1998; van Dam et al., 1999). Also, iron deficiency can lead to toxicity in algae. Thus, in toxicity studies with algae, fish and invertebrates (particularly the chronic studies), the observed toxicity of pentasodium DTPA will likely reflect the limitation of essential metals in the test media, and not the inherant toxicity of the molecule. OECD 23 (Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures) recommends that when testing chelants, compensatory adjustment to water quality parameters or the testing of an appropriate salt of the test substance help achieve a valid test result (OECD, 2000).

 

For fish, two valid acute studies with rainbow trout and bluegill sunfish and one valid chronic study with the crimson-spotted rainbowfish were available for assessment. For pentapotassium DTPA, the lowest LC50 of >1000 mg/L was based on a 96 -hour exposure of rainbow trout. Based on a 28 -day exposure with the crimson spotted rainbowfish, the NOEC for pentapotassium DTPA was 148 mg/L based on a lack of effects on reproduction. 

 

For invertebrates, three valid acute studies with Daphnia magna, Daphnia carinata,and Crangon crangon and one valid chronic study with D. carinata were available for assessment. For pentasodium DTPA, the lowest LC50 for freshwater invertebrates was 363 mg/L based on a 48 -hour exposure to D. carinata.   In an 18 -day exposure with D. carinata, the NOEC of 74 mg/L pentapotassium DTPA (based on reproduction) was calculated via stoichiometric conversion from Fe(III)-DTPA (disodium salt dihydrate).

 

The algal study available for DTPA was deemed not reliable mainly due to the fact that the observed effects were due to nutrient limitation and not inherent toxicity of the test substance. OECD 23 (Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures) states that “Data from tests in which complexation has been judged to have had a significant bearing on the result are likely to be of questionable value for classifying substances and for extrapolating to a predicted no effect concentration for risk assessment” (OECD, 2000). In OECD 23, compensatory adjustment of the algal media with essential ions is the suggested method for obtaining a valid toxicity study with chelating agents. Compensatory adjustment of the algal media was performed in an algal test with Na₂H₂EDTA 2*H₂O (Dufkova, 1984). Dufková (1984) demonstrated with Scenedesmus quadricauda that not the absolute EDTA concentration, but rather the ratio of the EDTA to the bivalent cations is crucial to algae growth. Higher concentrations (400 mg/l Na₂H₂EDTA 2*H₂O), when in surplus over trace elements in the nutrient solution, inhibited cell division, chlorophyll synthesis and the production of algal biomass, especially in the earlier phase of algae growth. No negative influence was observed when the concentration of trace elements in the nutrient solution was increased corresponding to the increased EDTA concentration. Thus, the NOEC and EC50 values based on growth inhibition of Scenedesmus quadricauda were greater than 400 mg/l Na₂H₂EDTA 2*H₂O.  Due to the similarity in chemical structure and mechanism of action, it is assumed that the NOEC and EC50 values for a valid algal study with pentapotassium DTPA would be in the 620 mg/L range (based on stoichiometric conversion), similar to that of Na₂H₂EDTA 2*H₂O.

 

Taken together, the lowest aquatic toxicity value for pentapotassium DTPA is the 18 -day NOEC of 74 mg/L based on D. carinata reproduction.

 

References:

 

Dufkova V. 1984. EDTA in algal culture media. Arch. Hydrobiol. Suppl. 67:479-492.

OECD (Organisation for Economic Co-operation and Development) 2000. Guidance Document on Aquatic Toxicity Testing of Difficult Substances and Mixtures. OECD No. 23. ENV/JM/MONO(2000)6.

 

Schmidt CK, Brauch H-J. 2004. Impact of aminopolycarboxylates on aquatic organisms and eutrophication: Overview of available data. Environ Toxicol.19(6):620-37.

 

van Dam RA, Barry MJ, Ahokas JT, Holdway DA. 1996. Comparative acute and chronic toxicity of diethlenetriamine pentaacetic acid (DTPA) and ferric-complexed DTPA to Daphniacarinata. Arch Environ Contam Toxicol 31:433– 443.

 

van Dam RA, Barry MJ, Ahokas JT, Holdway DA. 1999. Investigating mechanisms of diethylenetriamine pentaacetic acid toxicityto the cladoceran Daphnia carinata. Aquat Toxicol 46:191–210.