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Description of key information

Due to its high adsorption and low toxicity to aquatic organisms, HEDP was assigned to soil hazard category 3 (R.7c Section R.7.11.6, ECHA 2017) and a screening assessment was conducted, based on the equilibrium partitioning method (PEC * 10 / PNECscreen). In accordance with Table R.7.11 -2 of R.7c Section R.7.11.6, confirmatory testing for long-term toxicity to terrestrial organisms has been carried out with one trophic level, earthworms (consumers) (see IUCLID section 6.3.1). 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 in the chemical safety assessment. 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

A long-term toxicity to soil macroorganisms except arthropods study is available, which reported a 56-day NOEC value of 472.5 mg active acid/kg soil dry weight, based on reproduction, for Eisenia fetida (Noack, 2014). This is supported by a 14-day LC50 value of >960 mg active acid/kg soil dry weight for the effects of HEDP (2-3Na) on Eisenia fetida (Henkel, 1984).

Additionally, two supporting studies are available for HEDP (2-3Na) that indicate low toxicity to plants. Minimum toxicity was observed in a range of weeds and crops at application rates ranging from 3 -30 kg/ha and 1 -100 mg/L for tomato, cucumber and radish plants (Gledhill and Feijtel, 1992). A 14-day NOEC value of ≥960 mg active acid/kg soil dry weight was determined for the effects of HEDP (2-3Na) on Avena sativa (Henkel, 1984).

HEDP 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 across other phosphonates, 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, sodium and potassium salts in the HEDP category are freely soluble in water and, therefore, the HEDP anion is fully dissociated from its sodium or potassium cations when in solution. Under any given conditions, the degree of ionisation of the HEDP 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 HEDP-H, HEDP (1-2Na), HEDP (2-3Na), HEDP-4Na, HEDP-xK or another salt of HEDP.


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

  1. HEDP is present as HEDP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the system and not whether HEDP (1-2Na), HEDP (2-3Na), HEDP-4Na, HEDP-xK salts, HEDP-H or another salt was added.
  2. Disassociated sodium/potassium cations. The amount of sodium/potassium present depends on which salt was added.
  3. Divalent and trivalent cations have much higher stability constants for binding with HEDP than the sodium or potassium 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 HEDP-dication (e.g. HEDP-Ca, HEDP-Mg) and HEDP-trication (e.g. HEDP-Fe) complexes in solution, irrespective of the starting substance/test material.

In this context, for the purpose of this assessment, read-across of data within the HEDP Category is considered to be valid.