[[(phosphonomethyl)imino]bis[ethane-2,1-diylnitrilobis(methylene)]]tetrakisphosphonic acid

Endpoint summary

Description of key information

DTPMP-H and its salts are of low short-term toxicity to fish and aquatic invertebrates. The lowest reliable short-term toxic concentrations determined for DTPMP-H are a 96-h LC50 for the rainbow trout, Oncorhynchus mykiss, that is in the range 180-252 mg/l and a 48-h EC50 for Daphnia magna of 252 mg/l (however the latter is thought to be due to pH and the study has been assigned reliability 4). DTPMP-H is of low long-term toxicity to fish (O. mykiss 60-day NOEC: 25.6 mg/l).

No data are available for long-term toxicity to aquatic invertebrates with DTPMP-H. However a long-term toxicity study with D. magna has been reliably read-across from ATMP acid (CAS 6419-19-8), where a 21-d NOEC ≥25 mg/l was determined.

Algae

The effects of DTPMP-H observed in tests with algae are likely to be a consequence of nutrient limitation caused by complexation and not true toxicity. Thus, a 95-hour ErC50 for Selenastrum capricornutum (now known as Pseudokirchneriella subcapitata) of 0.45 mg/l is likely to over-estimate the true toxicity. The toxicity of DTPMP-H and its salts to algae is best represented by the 95 hour ErC50 value of >10 mg/l. This value was obtained in the only test where steps were taken to counter the effects of nutrient complexation and it is therefore most likely to be indicative of true toxicity.

Microorganisms

There are no reliable data describing the short-term toxicity of DTPMP-H to sewage sludge micro-organisms. However data have been read-across from other phosphonic acids. The data indicate a low level of toxicity (approximately 200 mg/l).

Read-across between DTPMP salts and the parent acid substance is considered appropriate for the following reasons.

The category hypothesis is that all the members are various ionised forms of the acid CAS 15287-60-8. In dilute aqueous conditions of defined pH, a salt will behave no differently to the parent acid, at identical concentration of the particular speciated form present, and will be fully dissociated. Hence some properties (measured or expressed in aqueous media, e.g. ecotoxicity) for a salt can be directly read across (with suitable mass correction, see Section 1.4 of the CSR) to the parent acid and vice versa. In the present context the effect of the sodium and potassium counter-ions will not be significant. The ammonium salt does present a particular issue since ammonia drives the toxicity of the substance and will be assessed separately.

The behaviour of phosphonates in the environment will be affected by their concentration in water, pH, the concentration and identities of metal ions, and the solids content per unit volume of water. From surface water through to soil a wide range of these parameters will be exhibited. Very few data concerning background concentrations of phosphonates in the environment are published, possibly due to the difficulties in detecting these substances at low concentrations in environmental media. Almost all natural waters contain more ions than the usual PEC values of the phosphonates.

In addition, in the environment the salt form is immaterial and speciation will occur in natural media. Similarly, all environmental related endpoints, use of buffered test media results often reflect a salt speciation relevant for ~pH 7 only and it would be impossible to test specific salts associated with high and low pH. Detaching or attracting a proton does not change the chemical safety assessment of the molecule as long as no other part of the molecular skeleton is changed, because in studies or when there is exposure, the pH will control the identity of the form or forms present.

Therefore it is considered appropriate to read-across between the DTPMP category members and attribute the effects to the acid component of the phosphonate.

Additional information

Other environmental effects

DTPMP-H and its salts are not readily biodegradable in laboratory studies carried out under standard conditions. Although these data suggest the potential for persistence, there is, however, evidence of partial degradation by abiotic processes in natural waters, and biodegradation following acclimation, or under conditions of low inorganic phosphate. In the presence of commonly found metal ions possessing redox properties, such as iron and copper, metal-catalysed photodegradation can be rapid, which promotes further biodegradation. Therefore they are not considered to be persistent.

There is evidence from structurally analogous substances that they have the potential to disrupt bioavailable concentrations of metallic cations in the blood of fish and invertebrates (McLaughlin and Fisk, 2010). The likelihood of this occurring is highest in situations where concentrations of available cations in an organism are limiting. This is unlikely in test media for aquatic toxicity tests. The low bioaccumulation potential of the substances is a further factor to consider since the internal concentration of the phosphonic acid in the organism is not expected to be high.

It is not possible to draw from the available data definitive conclusions regarding the significance of this effect for the results of the toxicity tests. However even though short-term and long-term toxicity was observed at high test concentrations in reliable tests it is unlikely that such high concentrations of phosphonic acid substances would be encountered in the environment. Therefore, it appears that even if the occurrence of cationic imbalances in the blood of the test organisms were responsible for the observed effects, then these effects would be unlikely to occur under environmental conditions. It is not considered necessary to explore this issue further at this stage.