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EC number: 267-956-0
CAS number: 67953-76-8
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
Values / results for HEDP acid form
Values / results for HEDP potassium and sodium salts (HEDP-xK, HEDP (1 -2Na), HEDP (2 -3Na), HEDP-4Na).
Reference / comment
1.7 x 10-08 Pa (estimated)
MPBPVP (v1.43; EpiWeb4.0, 2009, Syracuse Research Corporation)
690 g/L: 60% w/w produced commercially
230 g/L (for HEDP-xK)
Expected to be <-3.5
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
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.
Steber and Wierich, 1986, and others.
Highly adsorbing in a process which is largely irreversible
Highly adsorbing in a process which is largely irreversible
BCF 71 and 31 at different concentration levels
BCF 17.9 (steady state, for HEDP (2-3Na))
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, Ksolids-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 Kow
<-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 Ksolids-water (soil) of
~790 l/kg is derived from the above Ksolids-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
Table: Ionisation behaviour of HEDP-H and its salts and impact
on environmental fate
Relevant information for HEDP
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, HEDP2- 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)
strong complexing agent
calcium complex (88%) and magnesium complex (12%) predominate in natural waters in presence of natural ligands
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 pKa 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
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). 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.
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