(1-hydroxyethylidene)bisphosphonic acid, potassium salt

Endpoint summary

Additional information

In the natural environment the fate and behaviour of HEDP and its ions are dominated by abiotic dissociation / complexing, irreversible adsorption to surfaces, and to a lesser extent by degradation processes. The most important properties are summarised in the table below.

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

Parameter	Values / results for HEDP acid form	Values / results for HEDP potassium and sodium salts (HEDP-xK, HEDP (1 -2Na), HEDP (2 -3Na), HEDP-4Na).	Reliability	Reference / comment
Vapour pressure	1.7 x 10^-08Pa (estimated)		2	MPBPVP (v1.43; EpiWeb4.0, 2009, Syracuse Research Corporation)
Water solubility	690 g/L: 60% w/w produced commercially	774 g/L 200 g/L 230 g/L (for HEDP-xK)	2 4 1	Merck (2006) Z&S (2018) Henkel (2010)
Log Kow	-3.5	Expected to be <-3.5	2	Michael, P.R. (undated, believed to be 1979a)
Biodegradability (see IUCLID Section 5.2)	Not readily degradable. Little biodegradation seen in simulation tests under various conditions	Not readily degradable. Little biodegradation seen in simulation tests under various conditions	2     2	Handley and Mead, 1992, Henkel 1979, Saeger 1977 and 1978, and others.
Abiotic degradability (see IUCLID Section 5.1.1, 5.1.3)	Not susceptible to hydrolytic degradation. Significantly susceptible to photodegradation in water. The product is more susceptible to biodegradation than the parent structure	Not susceptible to hydrolytic degradation. Significantly susceptible to photodegradation in water. The product is more susceptible to biodegradation than the parent structure	2	Saeger, 1979 Steber and Wierich, 1986, and others.
Adsorption	Highly adsorbing in a process which is largely irreversible	Highly adsorbing in a process which is largely irreversible	2	Michael, 1979
Bioaccumulation	BCF 71 and 31 at different concentration levels	BCF 17.9 (steady state, for HEDP (2-3Na))	2 4	EG&G, 1976 Steber and Wierich, 1986

While some biodegradation has been observed, the results for HEDP-H 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 301D, Handley and Mead, 1992; inherent test, Henkel, 1979b; anaerobic test, Henkel, 1981; for further details, please refer to IUCLID Section 5.2). However, photodegradation in water in the presence of common metal ions has been observed (Steber and Wierich, 1986 and Saeger, 1979; for further details, please refer to IUCLID Section 5.1.3). Based on evidence from the data summarised in this section, members of this group are considered to be partially degradable over short time periods, and with evidence of mineralisation, particularly in the light, over longer periods. Oxidation may also play a role in the longer term environmental fate of HEDP-H and its salts, based on evidence of oxidation of structurally analogous phosphonates in the form of manganese complexes (Nowack, 2003).

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 analogue group presented in IUCLID Section 5.4. For HEDP-H, K_{solids-water (sediment)} = 830-7900 l/kg (soft water), 680-2700 l/kg (hard water) is reported in the key study (Michael, 1979). Bioavailability from solution is extremely low due to the highly unfavourable hydrophilicity (reliable measured BCF 71 for HEDP-H, supported by log K_ow <-3.5 for HEDP (2-3Na) and -3.0 for HEDP-4Na).

In soil and sediments, removal is expected to occur by the same partitioning mechanisms. A consistent value of K_{solids-water (soil)} of ~790 l/kg is derived from the above K_{solids-water (sediment)} values. 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.

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

Table: Ionisation behaviour of HEDP-H and its salts and impact on environmental fate

Property	Relevant information for HEDP	Reference / comment
Multiple ionisations	5 possible ionisations Two sets of pKa values in literature (1.7, 2.47, 7.28 and 10.29); (1.6, 2.7, 6.9, 11.0) At pH7, HEDP^2-predominates, based on the pKa values	Martell and Sillen, 1968; Lacour et al (1999)
Implication for partitioning and environmental fate	very hydrophilic with very high solubility limit in water (several hundred grams per litre) highly adsorbing (please refer to section describing adsorption evidence- IUCLID Section 5.4)
Complexation	strong complexing agent calcium complex (88%) and magnesium complex (12%) predominate in natural waters in presence of natural ligands	Nowack (2003)

 

HEDP can ionise by loss of a hydrogen ion up to five times. The fifth ionisation (of the hydroxy group) cannot be attained under normal aqueous conditions. As a consequence of these properties and the molecular shape, it is a strong complexing agent, for metal ions 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. In a primary data source for information on pK_a values and stability constants (Martell and Sillen, 1968), pKa values of HEDP are reported, of 1.7, 2.47, 7.28 and 10.29. These were measured in 0.1 M potassium chloride. Also, four pKa values of HEDP (at 0.1 M ionic strength potassium nitrate) of 1.6, 2.7, 6.9, 11.0 are reported by Lacour et al (1999). Refer to IUCLID Section 4.21 for further information.

Ionisation state of a particular functionality changes most significantly at the pKa value (50% ionisation at the pK_a value), but at one pH unit lower than the pKa there is still 10% ionisation (of the acidic functional groups). In the present case, this means that at pH 7, HEDP in water will have two almost fully ionised groups, with a smaller proportion of the molecules ionised three times; HEDP-H 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 HEDP in natural river water sample from Switzerland with typical composition of metals, anthropogenic and natural ligands. The other ligands compete with HEDP and must be taken into account for a truly realistic assessment. In the presence of no other ligands, the mass balance is 88% as calcium complex, 12% as magnesium complex and 0.1% as copper complex. In the presence of ETDA, NTA and natural ligands, HEDP is present as calcium complex (88%) and magnesium complex (12%) only.

In this context, for the purpose of this assessment, read-across of data within the HEDP Category is considered to be valid.

Further information on the category and the validity of read-across are presented in Annex 3 of the CSR and Section 13 of IUCLID.

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