Potassium salts of [hexane-1,6-diylbis[nitrilobis(methylene)]]tetrakisphosphonic acid (4-7 K:1)

Endpoint summary

Additional information

In the natural environment the fate and behaviour of HMDTMP and its ions 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.

While some biodegradation has been observed, the results for HMDTMP and its salts do not show significant biodegradation in the short term, and they are not readily or inherently biodegradable, based on several reliable studies (OECD 301E, Douglas and Pell, 1984; OECD modified screening, Horstmann and Grohman, 1988; OECD 301E, Martienssen, 2010; modified SCAS, Horstmann and Grohman, 1988; modified SCAS, Saeger et al., 1978; for further details, please refer to IUCLID section 5.2). However, photodegradation in the presence of common metal ions has been observed (Leseur et al, 2005, Brandenburg University of Technology, 2010 and Saeger, 1979; for further details, please refer to IUCLID section 5.1). Based on evidence from the data summarised in this section, members of the HMDTMP category are considered to be partially degradable over short time periods, and with evidence of mineralisation, particularly in the light, over longer periods.

Removal from the aqueous phase occurs principally by irreversible adsorption to substrates present (minerals), and to a lesser extent removal by photodegradation, oxidation in the presence of iron (III) and limited biodegradation. The significant role of adsorption is discussed later in this section with relevant data across the HMDTMP category presented in IUCLID section 5.4. For HMDTMP-H log Ksolids-water (sediment) values of 2.25 -3.59 l/kg (soft water) and 2.81 -3.89 l/kg (hard water) are reported in the key study. The degradation processes operate most rapidly in combination as abiotic breakdown products are more susceptible to biodegradation than the starting material. Bioavailability from solution is extremely low due to the highly unfavourable hydrophilicity (reliable measured BCF <10 (concentration level 18.8 mg/l) and <94 (concentration level 2.03 mg/l) for an analogous aminomethylenephosphonate, supported by log Kow -4 under environmental conditions).

In soil and sediments, removal is expected to occur by the same partitioning mechanisms. A consistent value of Ksolids-water(soil) is 1480 l/kg. 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 HMDTMP and its salts

Parameter	Properties of HMDTMP (acid)	Properties of HMDTMP (salts under environmental conditions)	Reliability	Reference / discussion
Vapour pressure	< 2.7E-09 Pa	< 2.7E-09 Pa	2	MPBPVP (v1.43;EpiWeb4.0, 2009, Syracuse Research Corporation)
Solubility	15 g/l	410 g/l	4 2	Heidolph (1983) Company literature
Log Kow	-4.7	<-4.7	2	Michael (1979)
Biodegradability	Not readily biodegradable	Not readily biodegradable	1	Douglas and Pell (1984) Multiple studies for HMDTMP-H, see IUCLID Section 5.2
Abiotic degradability	Significantly susceptible to photodegradation in water	Significantly susceptible to photodegradation in water	2	Lesueur et al (2005) Brandenburg University of Technology (2010) Saeger (1979)
Adsorption	Highly adsorbing in a process which is largely irreversible		2	Michael, 1979
Bioaccumulation		Very low (BCF <10 and <94 at two test concentrations, based on read-across)	1	Yokohama Laboratory (2002)

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

 

Table: Ionisation behaviour of HMDTMP and impact on environmental fate

Property	Relevant information for HMDTMP	Reference / comment
Multiple ionisations	10 possible ionisations pKa values (predicted): (1.3, 2, 3.4, 4.1, 5.9, 6.6, 8, >10) at pH7, HMDTMP-5^-predominates, based on the pKa values.	PFA, 2011
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)	Heidolph, 1983 and product literature data; please refer to water solubility section. Michael, 1979; please refer to adsorption section.
Complexation	Strong complexing agent Complexes with calcium, zinc, magnesium and copper can be expected to predominate in natural waters in the presence of natural ligands	Company product literature Nowack (2003) (for an analogous phosphonate)

 

 

HMDTMP can ionise by loss of a hydrogen ion up to eight times and protonation of the amines up to two times. 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. HMDTMP has ten possible ionisations. Ionisation is understood based on predicted pKa values and pH determination of stoichiometrically adjusted solutions (Liebsch, 2012, see IUCLID Section 4.20). Eight pKa values of HMDTMP are reported, of 1.3, 2, 3.4, 4.1, 5.9, 6.6, 8, >10 (PFA, 2011). These values are consistent with the values from the stoichiometric determinations, and are the best available set.

Ionisation state of a particular functionality changes most significantly at the pKa value (50% ionisation at the pKa value), but at one pH unit lower than the pKa 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, HMDTMP in water will be almost fully ionised at least four times, with a majority of the molecules ionised five times, and some six or seven times; HMDTMP acid in its molecular state is not present under the normal conditions of the natural environment considered in the chemical safety assessment.

Sodium and potassium 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 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 DTPMP in natural river water sample from Switzerland with well-known composition of metals, anthropogenic and natural ligands. The other ligands compete with HMDTMP and must be taken into account for a truly realistic assessment. In the presence of ETDA, NTA and natural ligands, DTPMP is present as calcium complex (41%), zinc complex (35%), magnesium complex (21%) and copper complex (2%) and complexes of the same metals are likely to be significant for HMDTMP.

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. Specific deficiencies in the available studies of biodegradability are not significant compared to the other fate and distribution mechanisms. In this context, for the purpose of this assessment, read-across of data within the HMDTMP Category is considered to be valid. Further information on the validity of read-across within the HMDTMP category is presented in Annex 7 of the CSR, and in IUCLID Section 13.

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