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Bioaccumulation: terrestrial

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In accordance with column 2 of REACH Annex IX, the bioaccumulation in terrestrial species study does not need to be conducted as the substance has a low potential for bioaccumulation (the substance has a log octanol water partition coefficient less than 3).  The partitioning behaviour of Na3NTA was estimated using the speciation model ChemEQL based on the assumption that Na3NTA completely dissociates upon dissolution in water and that the presence of the sodium ions does not appreciably influence the equilibrium behaviour of NTA.  It was determined that the partitioning behaviour of Na3NTA is pH-dependent and the estimated Log Kow for the different NTA species is highest at pH 1.4, with a value of -4.14; at pH 7, the log Kow reduces to -13.2. At pH 14, the estimated log Kow is -31.2.  

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Additional information

In accordance with column 2 of REACH Annex IX, the bioaccumulation in aquatic species study does not need to be conducted as the substance has a low potential for bioaccumulation (the substance has a log octanol water partition coefficient less than 3). It is reasonable to assume that based on the evidence for aquatic organisms (see below) that bioaccumulation is unlikely to occur and the requirement to derive a BCF for terrestrial organisms is waived.

The partitioning behaviour of Na3NTA was estimated using the speciation model ChemEQL based on the assumption that Na3NTA completely dissociates upon dissolution in water and that the presence of the sodium ions does not appreciably influence the equilibrium behaviour of NTA. It was determined that the partitioning behaviour of Na3NTA is pH-dependent and the estimated Log Kow for the different NTA species is highest at pH 1.4, with a value of -4.14; at pH 7, the log Kow reduces to -13.2. At pH 14, the estimated log Kow is -31.2.

This assumption is supported by additional secondary references:

N.M. Brouwer and P.M.J. Terpstra prepared a report on the 'Ecological and Toxicological Properties of NTA As a Detergent Builder', indicating that the bioaccumulation of Nitrilotriacetic acid (NTA acid) and its intermediates is not likely to occur because NTA acid is not or not significantly enrichted in any organism.

Although these results are related to NTA acid, this study is considered in this assessment as NTA acid and trisodium nitrilotriacetate (Na3NTA) display the same behaviour in the environment: splitting of sodium ions or protons and uptake of multivalent metal ions with subsequent formation of 1:1 or 1:2 complexes.

Since sodium salts are generally considered to be completely dissociating, a solution of Na3NTA in water yields the tribasic anion nitrilotriacetate. Nitrilotriacetic acid is a weak acid, and in such a solution, the NTA will therefore exist as an equilibrium mixture of several species:

NTA- - -<-> HNTA- -<-> H2NTA-<-> H3NTA <-> H4NTA+

with the last species occurring when, in a very acidic environment, the central nitrogen atom is protonated. Due to pH differences, the NTA speciation equilibrium will be different for Na3NTA and for NTA acid, unless dissolved in a buffered solution (controlled pH). A solution of NTA acid will be (slightly) acidic, whereas a Na3NTA solution will be alkaline (‘basic’). Toxicologically, this is not assumed to be significant, since it can be presumed that ‘in vivo’ systems are buffered systems. The chelating behaviour of Na3NTA and NTA acid will be slightly different, but this is not a significant effect for the relevant endpoint under REACH with regard to environmental fate and behaviour, ecotoxicology and toxicology.Therefore, also results on NTA acid and nitrilotriacetate are considered for the assessment of trisodium nitrilotriacetate.

 

This is in line with the Canadian ‘Draft Screening Assessment for Nitrilotriacetic acid (CAS 139-13-9)’ from January 2010, which also considered information relating to Na3NTA and nitrilotriacetate in the assessment of NTA acid. This is due to the fact that the toxicological endpoints, as stated in the Canadian ‘Screening Assessment for Nitrilotriacetic acid’, of NTA acid and Na3NTA are similar. Moreover, the dissociation of NTA acid and Na3NTA leads to the common moiety nitrilotriacetate.

The findings of Brouwer and Terpstra are further supported by the results reported in the EU RAR (2005) on trisodium nitrilotriacetate. This document reports the following results for the bioaccumulation by a series of species (fish, snail, backswimmer, worm, frog, and crayfish). Trisodium nitrilotriacetatae was tested at a concentration of 400 µg/l. The following BCF values were obtained:

Species

BCF (l/kg)

Equilibrium after

Fish (Brachidanio rerio)

1-3

96 hours

Guppy (Lebistes reticulatis)

Male 1-2

Female 6

72-96 hours

Goldfish (Carassius auratus)

1-2

72-96 hours

Snail (Lymnaea stagnalis)

8

≥20

3-7 days

≤72 days

Notonecta sp.

2-4

48 hours

Tubificidae

5-10

5 days

Frog larvae (Rana temporaria)

5-10

96 hours

Frog (Rana temporaria)

< 1

 

Crayfish (Procambarus)

1

4 hours

 

The available data demonstrate that only a low accumulation of Na3NTA occurs in the hydrosphere. It is reasonable to assume that based on this evidence, the same trend will be observed in terrestrial organisms and bioaccumulation is unlikely to occur.

 

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