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

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.52 mg/L

Assessment factor:

50

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.052 mg/L

Assessment factor:

500

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

20 mg/L

Assessment factor:

10

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

496 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

49.6 mg/kg sediment dw

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

174 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

18 mg/kg food

Assessment factor:

90

Additional information

DTPMP-H and its salts are phosphonic acid substances of very high water solubility, and low octanol-water partition coefficient. The phosphonic acid groups are multiply ionised at pH values relevant to biological and environmental systems. Ionisation gives them the ability to form stable complexes with metal ions, particularly polyvalent ones. Phosphonates are found to adsorb strongly to inorganic matrices, and hence they adsorb strongly to sewage sludge and to soil. They will be removed to a high extent in biological waste water treatment by adsorption.

 

The toxicity of DTPMP-H and its salts to environmental species is presented and interpreted in terms of the concentration of active DTPMP acid in the test media. As such the results of tests conducted on DTPMP-H and its salts are directly comparable, because the ionisation state will depend only on the pH of the test medium. Section 1 describes the pKa values for the ionisation of DTPMP-H. Ten pKa values, of 1.03, 2.08, 3.11, 4.15, 5.19, 6.23, 7.23, 8.30, 11.18 and 12.58 are reported. At environmentally-relevant pH values DTPMP will be ionised typically around seven times, and will form stable complexes with metal ions.

 

The substances have the potential to cause effects on aquatic plants as a consequence of nutrient limitation caused by complexation of trace metals. As complexing agents, these substances could remobilise metals in the environment; however, their high degree of adsorption to sediments suggests that this is unlikely to occur. The substances are acids and when present at high concentration they have the potential to cause local effects on aquatic organisms as a consequence of lowered pH.

 

Effects on aquatic organisms arising from exposure to the acid form of the substance are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H+concentration) to a level below their tolerable range. It is not considered appropriate or useful to derive a PNEC with studies in which pH deviations may have been attributable to the cause of effects seen because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment.

 

Read-across between the DTPMP salts and the parent acid substance is considered appropriate because:

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 profoundly affected by their concentration in water, 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 ~pH7 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.

Read-across between ATMP and DTPMP is considered appropriate because:

The registration substance and the ATMP read-across substance are aminomethylenephosphonic acids. They are part of the analogue group described in the Category Hypothesis, which can be found in Section 1.4 of the CSR. The analogue group consists of several phosphonates that share a common chemistry incorporating alkyl backbones with one or more tertiary amine centres and multiple methylphosphonate groups present. In the course of their intended use, they form complexes with polyvalent metal ions. Binding to solid mineral substrates is observed with both phosphonates in the environmental context which is also consistent with the toxicokinetic evidence that in vivo the substances are found in bone. The binding behaviour is particularly important and is seen to be a dominant effect both in the environment and in vivo.

DTPMP acid (CAS 15287-60-8) has three amine centres (five methylphosphonate groups), connected by ethyl chains. ATMP acid (CAS 6419-19-8) has three methylphosphonate groups connected by a central amine nitrogen. As well as being structural analogues, the phosphonates have consistent chemical properties including high MW (573 and 299 respectively), very low log K_ow (<-3 for all substances) and are highly soluble in water. DTPMP and ATMP are very strong chelators with the ability to bind to inorganic surfaces. The substances in question generally possess similar physicochemical properties and are not readily biodegradable and not bioaccumulative. In the natural environment the fate and behaviour of these substances and their ions are dominated by abiotic dissociation / complexing, irreversible adsorption to surfaces, and less by degradation processes, and they will partition strongly to the solid phase of the soil. The K_soil-water values for ATMP and DTPMP have been calculated at 600 l/kg and 380 l/kg respectively, based on measured K_{sediment-water} in a study from Michael 1979.

Based on the binding properties of the substance described in the previous section, the use of supporting read-across soil data from ATMP to DTPMP is considered to be valid and conservative since ATMP has a stronger binding capacity to soil than DTPMP.

See the long-term toxicity to aquatic invertebrates discussion for further details.

Reference: SIAR Phosphonates (2004), SIDS Initial Assessment Report for SIAM 18, Phosphonic Acid Compounds Group 3, Diethylene triamine penta(methylene phosphonic acid) and alkali metal salts. Sponsor Country: United Kingdom, Shared partnership with: American Chemistry Council, Phosphonic Acids Compounds Panel. Paris, 20-23 April 2004l.

 

 

 

Open sea PNEC

DTPMP-H and its salts are used in some scenarios (offshore oilfield) in open sea. The methodology of CHARM (2005 and van der Wal, 2003) has been used to derive the PNEC for risk characterisation in this setting.

Reliable, relevant and adequate ecotoxicological data exist for fish, crustaceans, molluscs, and algae. For fish and crustaceans, data show that marine species are more than 10 times less sensitive to the substances than freshwater species. Seawater is an around pH = 8 buffered solution of dissolved salts such as chlorine and sodium, but also sulphate, calcium, magnesium, bicarbonate, potassium, etc. thus once in seawater solution, the substances will first partition according to their dissociation constant and then will form complexes. It is therefore likely the substances will be much less bioavailable to draw toxic effects in the seawater compartment than in the freshwater compartment. The substances will also cause less disturbance to the ionic balance of the seawater solutions than the freshwater solutions. This is probably the main reason why seawater species show less sensitivity to the substances than freshwater species. Testing the substances on an algae marine species such as Skeletonema costatum would probably be more possible than in freshwater regarding nutrient limitations as explained in the paragraph related to effects on algae.

Besides the algae data which must be considered with extreme caution, short term data exist for fish, crustaceans and molluscs. One long term study is available, for Fish, with DTPMP acid showing a 60d-NOEC = 25.6 mg/L (ABC 1980). The 96-hour study on the oyster cannot be considered to be a chronic study since the lifespan of this oyster is greater than 3 years. A long term study has been read-across from the related substance ATMP acid (CAS 6419-19-8) deriving a 21d-NOEC = ≥25.6 mg/L (Monsanto 1976).

For open sea, CHARM guidance can be followed to derive a PNEC. According to table 2 of this guidance, when two short term studies and two long term studies are available on two biota groups (here Fish and Crustacean) then an assessment factor of 10 or 1000 apply respectively either to the NOEC or the LC50, whichever gives the lowest value. Here, the lowest acute LC50is the fish study on Oncorhynchus mykiss showing a 96h-LC50 of 216 mg/L (ABC 1980). Using this LC50 and an assessment factor of 1000 a PNECmarine-CHARM of 0.216 mg/L is calculated. Using the NOEC value from the Fish study (ABC 1980) and an assessment factor of 10, a PNECmarine-CHARM of 2.56 mg/L is calculated. Thus the lowest value is retained and the final PNECmarine-CHARM = 0.216 mg/L.

Note that a lower assessment factor of 500 may be justifiable for the same reasons as the freshwater data set. Therefore this PNEC can be considered a worst case.

Conclusion on classification

The substance is not classified according to Regulation (EC) No 1272/2008 (as amended) since the EC and LC50 values are >1 mg/L (180 mg/L and above), while the long term NOECs are >1 mg/L (60 day with fish is >25.6 mg/L).