Sodium salts of [[(phosphonomethyl)imino]bis[ethane-2,1-diylnitrilobis(methylene)]]tetrakisphosphonic acid (5-7 Na:1)

Endpoint summary

Description of key information

Due to its high adsorption and low toxicity to aquatic organisms, DTPMP (5-7Na) was assigned to soil hazard category 3 (R.7c Section R.7.11.6, ECHA 2017) and a PNECscreen was conducted, based on the equilibrium partitioning method (PEC * 10 / PNECscreen). Testing for toxicity to terrestrial organisms has been carried out with one trophic level, earthworms (consumers). PNECsoil has been derived using both the equilibrium partitioning method and measured terrestrial ecotoxicity data and the most conservative value (PNECsoil based on measured terrestrial ecotoxicity data) has been used for PNECsoil. As the terrestrial RCRs based on this PNEC are < 1, no further toxicity testing of soil organisms is currently considered to be necessary.

Additional information

Two studies have been read-across to DTPMP(5-7Na) from structurally analogous substances as part of a weight of evidence approach to fulfil the soil macroorganisms except arthropods endpoint. The following data are selected for chemical safety assessment for this endpoint: HEDP-H: 56-day NOEC and EC₅₀ values of 472.5 and >945 mg active acid/kg soil dry weight, based on reproduction, for Eisenia fetida (Noack, 2014). A second reliable study is read-across from HMDTMP(4-7K), which reports 56-day NOEC, EC₁₀ and EC₅₀ values of 556, 543 and >1000 mg active acid/kg soil dry weight, based on reproduction for Eisenia fetida (Noack, 2020). Whilst a reliable study is available for the DTPMP category (DTPMP(5-7Na)), the effects are attributed to the sodium cation, instead of the DTPMP anion. This is further explained in a separate report, attached in IUCLID Section 13 (PFA, 2021).

 

DTPMP acid and its salts are highly adsorbing to soil and soil mineral substrates. The nature of the adsorption is believed to be primarily due to interaction with inorganic substrate or generalised surface interactions. High adsorption is consistent with similar behaviour seen for the analogues HMDTMP, ATMP-N-oxide and ATMP, and other common complexing agents such as EDTA. By reference to findings with other similar substances, the binding occurs very rapidly and is effectively irreversible. For environmental fate purposes in exposure assessment this is assessed as a removal process.

The acid and salts in the DTPMP category are freely soluble in water and, therefore, the DTPMP anion is fully dissociated from its cations when in solution. Under any given conditions, the degree of ionisation of the DTPMP species is determined by the pH of the solution. At a specific pH, the degree of ionisation is the same regardless of whether the starting material was DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP-xK, DTPMP (xNH4) or another salt of DTPMP.

 

Therefore, when a salt of DTPMP is introduced into test media or the environment, the following is present (separately):

1. DTPMP is present as DTPMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether DTPMP-H, DTPMP (1-3Na), DTPMP (5-7Na), DTPMP-xK, DTPMP (xNH4), or another salt was used for testing.

2. Disassociated ammonium, potassium or sodium cations. The amount of ammonium, potassium or sodium present depends on which salt was added.

3. Divalent and trivalent cations have much higher stability constants for binding with DTPMP than the sodium, potassium or ammonium ions so would preferentially replace them. These ions include calcium (Ca²⁺), magnesium (Mg²⁺) and iron (Fe³⁺). Therefore, the presence of these in the environment or in biological fluids or from dietary sources would result in the formation of DTPMP-dication (e.g. DTPMP-Ca, DTPMP-Mg) and DTPMP-trication (e.g. DTPMP-Fe) complexes in solution, irrespective of the starting substance/test material.