[[(phosphonomethyl)imino]bis[ethane-2,1-diylnitrilobis(methylene)]]tetrakisphosphonic acid

Endpoint summary

Additional information

In the natural environment the fate and behaviour of DTPMP acid 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 DTPMP 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, Huntingdon, 1984; OECD 301D, TNO, 2006; SCAS test, Saeger, 1978; anaerobic screening test, Saeger, 1978; OECD 306, TNO, 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 Saeger, 1979a). 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 DTPMP acid and its salts, 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 in Section 5.4. For DTPMP K_{solids-water (sediment)} 1340 l/kg (soft water), 950 l/kg (hard water) is 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, supported by log K_ow<-3.4 under environmental conditions).

In soil and sediments, removal is expected to occur by the same partitioning mechanisms. A consistent value of K_{solids-water (soil)} is 380 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 DTPMP acid and its salts

Parameter	Value (DTPMP-H)	Value (DTPMP salts)	Reliability	Reference/ Discussion
Vapour pressure	2.7E-09 Pa (estimated)	<2.7E-09 Pa (estimated)	2	MPBPVP (v1.43;EpiWeb4.0, 2009, Syracuse Research Corporation)
Solubility	ca. 500 - 700 g/l, with no pH adjustment / miscible	≥ ca. 500 - 700 g/l / miscible	2     4	Salt solubility for pH 7 (industry use information based on production materials)
LogKow	-3.40	<-3.40	2	Michael, P.R. (undated, believed to be 1979a)
Biodegradability (see Section 4.1.2)	Not rapidly degradable	Not rapidly degradable	2	Saeger V.W.et al., Monsanto (1978)
Abiotic degradability (see Section 4.1.1)	Significantly susceptible to photodegradation; the product is more susceptible to biodegradation than the parent structure	As for the acid	2	Gledhill and Feijtel, 1992
Adsorption	Highly adsorbing in a process which is largely irreversible	As for the acid	2	Gledhill and Feijtel, 1992
Bioaccumulation	Very low (BCF <10 and <94 at two test concentrations)	As for the acid	1	Yokohama Laboratory, 2002

 

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

Table: Ionisation behaviour of DTPMP and impact on environmental fate

Property	Relevant information for DTPMP	Reference / comment
Multiple ionisations	·   13 possible ionisations ·   pKa values in literature (1.03, 2.08, 3.11, 4.15, 5.19, 6.23, 7.23, 8.30, 11.18, 12.58) ·   At pH7, DTPMP^6-predominates, based on the pKa values	Tomson et al. 1994
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 (41%), zinc complex (35%), magnesium complex (21%) and copper complex (2%) predominate in natural waters in presence of natural ligands	Nowack (2003)

 

DTPMP acid can ionise by loss of a hydrogen ion up to ten times and protonation of the amines up to three 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. DTPMP has thirteen possible ionisations. In a primary data source for information on pKa values and stability constants (Martell and Sillen), eight discernible pKa values of DTPMP are reported, of 2.8, 4.45, 5.5, 6.38, 7.17, 8.15, 10.1, 12.04. These were measured in 0.1 M potassium chloride. The original source (a 1967 paper in Russian) is cited in the data book. In a source giving no experimental details, DTPMP is described as having ten pK_a values, of 1.03, 2.08, 3.11, 4.15, 5.19, 6.23, 7.23, 8.30, 11.18, 12.58 (Tomson et al. 1994). These values are a more complete set of data, are consistent with the values from the standard source, and are the best available set.

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; the converse being true for the protonated amine groups). In the present case, this means that at pH 7, DTPMP in water will be almost fully ionised five times, with a majority of the molecules ionised six times, and some seven or eight times; DTPMP acid in its molecular state is not present under the normal conditions of the natural environment considered in the chemical safety assessment.

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 DTPMP in natural river water sample from Switzerland with well-known composition of metals, anthropogenic and natural ligands. The other ligands compete with DTPMP and must be taken into account for a truly realistic assessment. In the presence of no other ligands, the mass balance is 24% as zinc complex and 76% as copper complex. 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%).

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 DTPMP Category is considered to be valid.