Reaction mass of [[(2-hydroxyethyl)imino]bis(methylene)]bisphosphonic acid and Phosphonic acid, P-[(tetrahydro-2-hydroxy-2-oxido-4H-1,4,2-oxazaphosphorin-4-yl)methyl]-

Endpoint summary

Additional information

In solution, HEBMP-H undergoes a reversible internal condensation reaction to form a pH-dependent equilibrium mixture with the cyclised structure 4-(phosphonomethyl)-2-hydroxy-2oxo-1,4,2-oxaza-phosphorinane (c-HEBMP) and water. The precise proportion of the two forms is dependent on pH, temperature and time, and under acidic or neutral conditions, the proportion of c-HEBMP in the multi-constituent substance may reach proportions as high as 60% w/ w. Spectroscopic evidence from 31P-NMR analysis suggests that in the pH range ca. 1 – 7 a high proportion (50-75% w/w) is present as the cyclised form. The two forms exist in equilibrium therefore it is extremely difficult to separate and purify both forms.

 

In the natural environment the fate and behaviour of HEBMP-H and its salts are dominated by abiotic dissociation / complexing, irreversible adsorption to surfaces, and less by degradation processes. The most important properties are summarised in the table below.

 

Removal from the aqueous phase occurs principally by irreversible adsorption to substrates present (minerals), and to a lesser extent removal via biodegradation. While some biodegradation has been observed, the results for HEBMP-H and its salts do not show significant biodegradation in the short term, and they are not considered readily or inherently biodegradable, based on two reliable studies (OECD 306 Ofjord 1997, OECD 302B Mead 2002; for further details, please refer to IUCLID Section 5.2).

 

The significant role of adsorption is discussed later in this section with relevant data across the HEBMP category presented in IUCLID Section 5.4. For HEBMP, Kp soil-water 1500 l/kg (for the linear constituent), 110 l/kg (for the cyclic constituent) are extrapolated from the adsorption coefficients reported in the key studies (Goller, 2014).

 

Adsorption and desorption behaviour of the cyclic and linear constituents differ in that the cyclic form appears to sorb reversibly with a much lower organic carbon-water adsorption coefficient, however the equilibrium between the two forms would quickly re-establish in the aqueous fraction, and under the negatively-charged, alkaline conditions at the surface of mineral substrate, it would be thermodynamically favourable for the ring structure present in the cyclic constituent molecule to open, and behaviour like the linear constituent is then to be expected. The linear constituent (like other aminomethylenephosphonates) shows negligible desorption and effectively irreversible binding.

 

Bioavailability from solution is extremely low due to the highly unfavourable hydrophilicity (based on very low log Kow values of -3.4 and -4 under environmental conditions for the cyclic and linear constituents respectively (QSAR predictions)).

 

In soil and sediments, removal is expected to occur by the same partitioning mechanisms. Bioavailability from interstitial water present in soils and sediments is extremely low due to both the very strong adsorption and unfavourable bioconcentration properties, even if the phosphonate were to be ingested in an adsorbed state in the soil or sediment constituents.

Table: Summary of significant properties affecting environmental fate of HEBMP-H and its salts

Parameter	Properties of HEBMP (acid)	Properties of HEBMP (sodium salt under environmental conditions)	Reliability	Reference/discussion
Vapour pressure	Linear: 2.7E-9 Pa at 25°C Cyclic: 5.5E-8 Pa at 25°C	<0.0038 Pa at 25°C	1	PFA (2011) Harlan (2012)
Solubility	620 - ≥840 g/l (reliability 4 supporting studies)	750 g/l	1	Harlan (2012) Rhodia (2010) Bozetto (2011) Zschimmer & Schwarz (2011)
Log Kow	Linear: -4 Cyclic: -3.4	No data	2	Peter Fisk Associates (2011)
Biodegradability	Not readily biodegradable	Not readily biodegradable	1	Ofjord (1997)
Abiotic degradability	No data	No data
Adsorption	Linear: Kp soil-water 1500 l/kg Cyclic: Kp soil-water 110 l/kg	Linear: Kp soil-water 1500 l/kg Cyclic: Kp soil-water 110 l/kg	1 1	Goller (2014)
Bioaccumulation	Very low, based on log Kow (endpoint waived)	Very low, based on log Kow (endpoint waived)

The properties of HEBMP-H and its salts are profoundly directed by their ionisation behaviour, as discussed in the table and paragraphs below.

Table: Summary of ionisation behaviour of HEBMP-H and its salts

Property	Relevant information for HEBMP	Reference/comment
Multiple ionisations	Linear form: four possible ionisations pKa values (predicted): (1.3, 2.7, 5.9, >8), cyclic form: three possible ionisations pKa values (predicted):(1.3, 2.7, 7.3). At pH7, linear HEBMP trivalent anion and cyclic HEBMP divalent anion predominates, based on the pKa values.	PFA, 2011, Zschimmer and Schwarz, 2012
Implication for partitioning and environmental fate	Very hydrophilic with very high water solubility limit in water (several hundred grams per litre) Highly adsorbing (please refer to IUCLID section 5.4)	Product literature data; please refer to water solubility section. Goller, 2014; please refer to adsorption section.
Complexation	Strong complexing agent Complexes with calcium, zinc, magnesium and copper can be expected to be significant in natural waters in the presence of natural ligands	Company product literature Nowack (2003) (for an analogous phosphonate)

The linear form of HEBMP-H can ionise by loss of a hydrogen ion up to four times and protonation of the amine nitrogen atom. As a consequence, it is a strong complexing agent, and is highly hydrophilic. Because ionisation is a rapid and reversible process, salts such as sodium and potassium salts will dissolve readily in water to give a speciation state dictated by the pH of the medium. HEBMP has five possible ionisations. Ionisation is understood based on predicted pK_a values and pH determination of stoichiometrically adjusted solutions (Liebsch, 2012). Four pK_a values, of 1.3, 2.7, 5.9 and >8 have been reported for the non-cyclic HEBMP constituent and three pK_a values have been reported for the cyclic constituent: 1.3, 2.7 and 7.3. At environmentally-relevant pH values, HEBMP will typically be ionised around 1-3 times and will form stable complexes with metal ions.

 

Ionisation state of a particular functionality changes most significantly at the pK_a value (50% ionisation at the pK_a value), but at one pH unit lower than the pK_a there is still 10% ionisation (of the acidic functional groups; the converse being true for the protonated amine groups). In the present case, this means that at pH 7, HEBMP in water will be almost fully ionised at least twice, with most of the linear form molecules ionised three times and twice for the cyclic form; HEBMP acid in its molecular state is not present under the normal conditions of the natural environment considered in the chemical safety assessment.

 

Sodium counter-ions, where present, are not significant in respect of the properties under consideration and have been assessed in depth in the public literature. Additionally, the sodium counterions are expected to fully dissociate when in contact with water, including atmospheric moisture, but the phosphonate will complex with polyvalent metal ions when they are present. Nowack (2003) presents calculated speciation of the analogous aminomethylenephosphonate ATMP in natural river water sample from Switzerland with well-known composition of metals, anthropogenic and natural ligands. The other ligands compete with the phosphonate and must be considered for a truly realistic assessment. In the presence of ETDA, NTA and natural ligands, ATMP is present only as calcium complex (55%), and magnesium complex (42%) and complexes of the same metals are likely to be significant for HEBMP.

The available weight of evidence shows that removal from solution to a non-bioavailable bound form, and abiotic mechanisms, are important in the environmental exposure and risk assessment. In this context, for the purpose of this assessment, read-across of data within the HEBMP Category is considered to be valid and the approach appropriate. Further information on the category and the validity of read-across are presented in Chapter 1 of the CSR, and IUCLID Section 13.

Nowack, B. (2003). Review: Environmental chemistry of phosphonates. Water research (37), pp 2533-2546.