[[(phosphonomethyl)imino]bis[hexamethylenenitrilobis(methylene)]]tetrakisphosphonic acid

Endpoint summary

Additional information

Short-term toxicity data are available for fish and invertebrates reporting a 96 h LC₅₀value of 130 mg active acid/L withScopthalamus maximus, and a 96 h EC₅₀value of 110 mg/L withAcartia tonsa.

Short-term toxicity data with freshwater organisms are available with the closely related substance HMDMTP acid (CAS 23605-74-5). An LC₅₀value of 440 mg active acid/L was reported in a 14 day study withOncorhynchus mykiss, while a 48 h EC₅₀value of 570 mg active acid/L was reported withDaphnia magna. It is not possible to exclude that the effects observed with the freshwater invertebrates are due to pH effects. However the results do indicate that the substance is of low short-term toxicity.

Only a disregarded study is available assessing the effects of the registered substance on algal inhibition, however the endpoint has been waived on the basis that the complexing properties of phosphonic acid limit algal nutrient availability.Further testing is also not recommended as the tests will be assessing the toxicity of a nutrient bound phosphonic acid and not the substance itself. For further details see section 7.1.4

Long-term toxicity endpoints have been waived on the basis of the risk characterisation outcome being <1.

Cationic imbalance in organisms

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 inappropriate to explore this issue further at this stage.

 

Justification for read-across between BHMT and HMDTMP:

Data from HMDTMP-H (CAS number 23605-74-5) have been used to fill in gaps where necessary for the registered substance.

BHMT-H (CAS 34690-00-1) hasthree amine centres (five methylphosphonate groups), connected by two hexyl chains.HMDTMP-H (CAS 23605-74-5) has two amine centres (four methylphosphonate groups), connected by a hexyl chain.

As well as being structural analogues, all these phosphonates have consistent chemical properties including high MW (685 and 492 respectively), very low log K_ow(≤-4 for BHMT-H and HMDTMP) and are highly soluble in water. The substances generally posses similar physico-chemical properties and are not readily biodegradable.