Pentapotassium ({[(hydroxyphosphinato)methyl](phosphonatomethyl)nitroryl}methyl)phosphonate

Endpoint summary

Description of key information

In the natural environment the fate and behaviour of ATMP-N-oxide-5K is 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-N-oxide-5K does not show significant biodegradation in the short term and, together with data available on analogous substances, indicates it is not readily or inherently biodegradable, based on several reliable studies (OECD 301E Handley and Mead 1993; OECD 301E, Douglas and Pell, 1984; OECD 301D, Cremers and Hamwijk, 2006; SCAS test, Saeger, 1978; anaerobic screening test, BEL, 2005; OECD 306, Drake, 2005, Rowlands, 2005 and Hamwijk and Cremers, 2005; for further details, please refer to IUCLID Section 5.2.1 and 5.2). However, photodegradation in the presence of common metal ions has been observed (Lesueur et al., 2005 and Monsanto, 1980 and Monsanto 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. Oxidation may also play a role in the longer term environmental fate of ATMP, based on evidence of oxidation of structurally analogous phosphonates in the form of manganese complexes (Nowack and Stone, 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 aminomethylene phosphonate group presented in IUCLID Section 5.4. For the analogous substance 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 K_ow <-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-N-oxide-5K

Parameter	Values / results	Reliability	Reference/ Discussion
Vapour pressure	Acid form: 2.7E-09 Pa (estimated).	2	MPBPVP (v1.43; EpiWeb4.0, 2009, Syracuse Research Corporation)
Solubility	>450 g/L	2	Spini (2000)
Log Kow	-4.3	2	Spini (2000)
Biodegradability	Not rapidly degradable	2	Handley and Mead (1993)
Abiotic degradability	Amine analogue ATMP data: Significantly susceptible to photodegradation; the product is more susceptible to biodegradation than the parent structure	2	Gledhill and Feijtel (1992)
Adsorption	Highly adsorbing in a process which is largely irreversible	2	Spini (2000)
Bioaccumulation	Very low (BCF <4 and 22 L/Kg at two test concentrations)	2	EG&G (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	·        6 possible ionisations ·        pKa values in literature (<2, <2, 3.28, 5.27, 6.99, 12.09) ·        at pH7, ATMP-N-oxide-5K predominates, based on the pKa values.	Spini (2000)
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-N-oxide-5K 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 (Spini 2000), six pKa values of ATMP-N-oxide are reported, of <2, <2, 3.28, 5.27, 6.99, 12.09 (at 1 M ionic strength potassium nitrate).

 

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-N-oxide in water will be ionised four to five times; ATMP-N-oxide 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-N-oxide are (log values): Ca 5.69 and Mg 8.29 (Carter et al. 1967). 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.

Potassium counter-ions 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 EDTA, 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.