Nitrilotrimethylenetris(phosphonic acid)

Endpoint summary

Description of key information

In the natural environment the fate and behaviour of ATMP acid and its ions are dominated by abiotic dissociation / complexing, irreversible adsorption to surfaces, more than by degradation processes.

Additional information

The most important properties relevant for understanding environmental fate in the context of chemical safety assessment are summarised in the table below.

 

While some biodegradation has been observed, the results of aerobic and anaerobic biodegradation studies for ATMP acid 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 301D, Cremers and Hamwijk, 2006; Horstmann and Grohmann, 1988; SCAS test, Saeger, 1978; anaerobic screening test, Brixham Laboratory, 1995; OECD 306, Drake, 2005, Rowlands, 2005 and Hamwijk and Cremers, 2005; Muttzall and Hanstveit, 1996; for further details, please refer to IUCLID Section 5.2). However, photodegradation in the presence of common metal ions has been observed (Lesueur et al., 2005 and Monsanto, 1980 and 1979; for further details, please refer to IUCLID Section 5.1). 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.

 

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 ATMP Ksolids-water (sediment) values of 1270 l/kg (soft water), 1500 l/kg (hard water) are reported in the key study (Michael, 1979). Degradation processes operate most rapidly in combination as abiotic breakdown products are more susceptible to biodegradation than the parent material. Bioavailability from solution is extremely low due to the highly unfavourable hydrophilicity (reliable measured BCF <22, supported by log Kow <-3.5 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 600 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 substance were to be ingested in an adsorbed state in the soil or sediment constituents.

 

Table: Summary of significant properties affecting environmental fate of ATMP and its salts

Parameter	Values / results	Reliability	Reference/ Discussion
Vapour pressure	Acid form: 2.7E-09 Pa (estimated). For salts, <2.7E-09 Pa (estimated)	2	MPBPVP (v1.43; EpiWeb4.0, 2009, Syracuse Research Corporation)
Solubility	Acid form: 500 g/L Most salts: 500 g/L	2 4	Salt solubility for pH 7 (industry use information based on production materials)
LogKow	Acid form: -3.5 Under neutral conditions / for salts, <-3.5	2	Company data
Biodegradability (see section 4.1.2)	Not rapidly degradable	2	Cremers and Hamwijk (2006)
Abiotic degradability (see Section 4.1.1)	Significantly susceptible to photodegradation; the product is more susceptible to biodegradation than the parent structure	2	Monsanto (1979, 1980) Lesueur (2005)
Adsorption	Highly adsorbing in a process which is largely irreversible	2	Michael (1979)
Bioaccumulation	Very low (BCF <4 and 22 at two test concentrations)	2	Monsanto (1976)

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

 

Table: Ionisation behaviour of ATMP and impact on environmental fate

Property	Relevant information for ATMP	Reference/ Comment
Multiple ionisations	·        7 possible ionisations ·        pKa values in literature (<2, <2, 4.30, 5.46, 6.66, 12.3) ·        at pH7, ATMP-5 predominates, based on the pKa values.	Martell and Sillen 1968
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)
Complexation	·        strong complexing agent ·        calcium complex (55%), and magnesium complex (42%) predominate in natural waters in presence of natural ligands	Nowack (2003)

Each of the three phosphonic acid groups in ATMP can ionise by loss of one or two hydrogen ions; in addition, the amine nitrogen can be protonated. 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 will dissolve and dissociate 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), six pKa values of ATMP are reported, of <2, <2, 4.30, 5.46, 6.66, 12.3. These were measured in 1 M potassium nitrate. The original source (a 1967 paper) is cited in the data book.

 

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). In the present case, this means that at pH 7, ATMP in water will be almost fully ionised four times, with a majority of the molecules ionised five times; ATMP acid in its molecular state is not present under the normal conditions of the natural environment considered in the chemical safety assessment.

 

Stability constants of representative metals with ATMP are (log values): Hg 21.7; Co 18.4; Cu 17; Pb 16.4; Ni 15.5; Zn 14.1; Cd 12.7; Ca 7.6; Mg 6.7 (Gledhill and Feijtel, 1992, see IUCLID Section 4.27). The stability constants of phosphonates were critically reviewed for IUPAC (Popov et al., 2001). They reviewed techniques for determining stability constants for the complexation of metal ions by a number of phosphonates. Their paper presents and critically evaluates stability constants, and is quite complex as they consider the different protonation levels for the compounds separately. The data are consistent with the overall values reported above.

Sodium, potassium and ammonium 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 ATMP in natural river water sample from Switzerland with well-known composition of metals, anthropogenic and natural ligands. The other ligands compete with ATMP and must be taken into account for a truly realistic assessment. In the presence of no other ligands, ATMP is present as calcium complex (33%), copper complex (28%), magnesium complex (25%) and zinc complex (11%). In the presence of ETDA, NTA and natural ligands, ATMP is present only as calcium complex (55%), and magnesium complex (42%).

 

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.

 

The acid, sodium, potassium and ammonium salts in the ATMP category are freely soluble in water. The ATMP anion can be considered fully dissociated from its sodium, potassium or ammonium cations when in dilute solution. Under any given conditions, the degree of ionisation of the ATMP species is determined by the pH of the solution. At a specific pH, the degree of ionisation is the same regardless of whether the starting material was ATMP-H, ATMP.4Na, ATMP.7K or another salt of ATMP.

 

Therefore, when a salt of ATMP is introduced into test media or the environment, the following is present (separately):

1. ATMP is present as ATMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether ATMP (3-5K) salt, ATMP (3-5Na) salt, ATMP-H (acid form), or another salt was used for dosing.

2. Disassociated potassium, sodium or ammonium cations. The amount of potassium or sodium present depends on which salt was dosed.

3. It should also be noted that divalent and trivalent cations would preferentially replace the sodium or potassium ions. These would include calcium (Ca²⁺), magnesium (Mg²⁺) and iron (Fe³⁺). These cations are more strongly bound by ATMP than potassium, sodium and ammonium. This could result in ATMP-dication (e.g. ATMP-Ca, ATMP-Mg) and ATMP-trication (e.g. ATMP-Fe) complexes being present in solution.

 

In this context, for the purpose of this assessment, read-across of data within the ATMP 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 in IUCLID Section 13.

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

Popov, K., Ronkkomaki, H. and Lajunen L.H.J. (2001) Critical evaluation of stability constants of phosphonic acids. Pure Appl. Chem. (73), pp 1641 -1677