[nitrilotris(methylene)]trisphosphonic acid, ammonium salt

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption, other

Remarks:

Batch equilibrium method

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study well documented, includes study protocol; meets generally accepted scientific principles, acceptable for assessment.

Principles of method if other than guideline:

TEST DETAILS: 10 g of sediment were placed in a sample bottle, along with 800 ml test solution at concentrations of 0.05, 0.10, 1.0 and 5.0 ppm in purified water or synthetic hard water. The sample bottles were then shaken by hand and allowed to settle for 30 minutes. The solution temperature and pH was then measured and the pH adjusted to 6-8 where necessary. Aliquots were removed for Day 0 analysis and the bottles then placed on a shaker at 100 cycles/minute. The pH of the solutions was readjusted on sample days where necessary.

Test concentrations are equivalent to 40, 80, 800 and 4000 µg test substance (active acid) respectively.

GLP compliance:

no

Type of method:

batch equilibrium method

Media:

sediment

Radiolabelling:

yes

Analytical monitoring:

yes

Details on matrix:

TEST MEDIA: Purified water or synthetic hard water (hardness 211 ± 5 ppm as CaCO3), plus river sediment obtained from the National Bureau of Standards. Sediment pH = 7.3; Organic carbon content: 11.8%

Details on test conditions:

pH 6-8

Sample No.:

#1

Phase system:

solids-water in sediment

Type:

Kp

Value:

1 270 L/kg

pH:

7.3

Matrix:

Sediment with overlying soft water

% Org. carbon:

11.8

Remarks on result:

other: Mean from values across a range of test substance concentrations after 1-8 days equilibration.

Key result

Sample No.:

#2

Phase system:

solids-water in sediment

Type:

Kp

Value:

1 500 L/kg

pH:

7.3

Matrix:

Sediment with overlying hard water

% Org. carbon:

11.8

Remarks on result:

other: Mean from values across a range of test substance concentrations after 1-8 days equilibration.

Adsorption and desorption constants:

Desorption: test substance could not be analysed in the solid phase due to effectively irreversible binding.

Transformation products:

not measured

ANALYSIS: Analysis was performed on days 0, 1, 2, 4 and 8. The concentration of the test substance in water was determined by liquid scintillation counting. The concentration in sediment was then calculated by difference, based on the assumption that any reduction in water concentration was due to adsorption to sediment. Due to the high level of adsorption significant change in aqueous concentration occurred, and therefore analysis of the soil was not essential.

K(sediment-water) values are expressed in litres/kilogram for soft water

	Day 0	Day 1	Day 2	Day 4	Day 8
0.05 ppm	360	1100	720	1300	1200
0.10 ppm	180	1200	1000	1200	1500
1.0 ppm	39	360	450	810	1100
5.0 ppm	13	170	52	170	340

K(sediment-water) values are expressed in litres/kilogram for hard water

	Day 0	Day 1	Day 2	Day 4	Day 8
0.05 ppm	590	1300	1200	1200	1600
0.10 ppm	590	1300	1500	1500	1500
1.0 ppm	87	1000	1400	1600	1400
5.0 ppm	140	230	650	930	1000

The 5.0 ppm test concentration may not have reached equilibrium over the test period due to saturation of some
of the sediment adsorption sites. Therefore, a mean value applicable to soft water is 1270 l/kg, and to hard water,
1500 l/kg.

Conclusions:

Rapid and high adsorption of ATMP to natural sediment with both soft and hard overlying waters was determined in a reliable study conducted according to generally accepted scientific principles.

Reference 2

Endpoint:

adsorption / desorption, other

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

Please refer to Annex 3 of the CSR and IUCLID Section 13 for justification of read-across within the ATMP category.

Reason / purpose for cross-reference:

read-across source

Sample No.:

#1

Phase system:

solids-water in sediment

Type:

Kp

Value:

1 270 L/kg

pH:

7.3

Matrix:

sediment - soft water

% Org. carbon:

11.8

Sample No.:

#2

Phase system:

solids-water in sediment

Type:

Kp

Value:

1 500 L/kg

pH:

7.3

Matrix:

sediment - hard water

% Org. carbon:

11.8

Description of key information

The substance adsorbs significantly to sediment, soil and sludge substrates based on the available study data. It is believed that the binding to organic carbon is not predominant, however it is useful for general context to note that Kd values appear consistent with a log K_oc (equivalent) value of approximately 4

Key value for chemical safety assessment

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

2.78

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in sediment)

Value in L/kg:

3.18

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

3.18

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in raw sewage sludge)

Value in L/kg:

3.95

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in settled sewage sludge)

Value in L/kg:

3.95

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in activated sewage sludge)

Value in L/kg:

4.04

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in effluent sewage sludge)

Value in L/kg:

4.04

at the temperature of:

12 °C

Additional information

This substance is a mineral-binding and complexing agent, with unusual chemical properties. ATMP and its salts adsorb strongly to inorganic surfaces, soils and sediments, in model systems and mesocosms, despite the very low log Kow; this has implications for the approach to environmental fate modelling. High adsorption is consistent with similar behaviour seen for structural analogues, and other common complexing agents such as EDTA.

 

Studies on analogous phosphonate complexing agents have revealed that adsorption is correlated with concentration in the aqueous phase and also relates significantly to the type and nature of inorganic content in the substrate.

 

The normal approach to modelling binding behaviour in environmental exposure assessment assumes that the substance is binding only to the organic carbon present in soils, sediments, and WWTP sludges. This assumption does not apply to ATMP and its salts. The extent of binding to substrates is fundamental to understanding and modelling of environmental exposure, for substances like this. Therefore, adsorption / desorption data, required in Section 9.3.1 of REACH Annex VIII, are an extremely important part of the data set for ATMP and its salts.

 

The nature of the adsorption is believed to be primarily due to interaction with inorganic substrate or generalised surface interactions. While Koc is the conventional indicator for adsorption, the interaction with organic carbon present in the substrate may be exceeded by these other interactions in the case of ATMP and its salts, meaning that Koc as such is not a meaningful parameter. It is convenient for comparison purposes to determine the value of log Koc that is consistent/equivalent to the degree of sediment or soil binding exhibited by the substance. Thus, a log Koc(equivalent) value of approximately 4 was obtained by evaluating Kp(sediment-water) data in a reliable study (with ATMP-H) conducted according to generally accepted scientific principles (Michael, 1979). Water samples were analysed by using liquid scintillation on day 0, 1, 2, 4, 8. Methods and sample data were represented clearly and the test substance was being described adequately. The result is considered as reliable and has been assigned as key study.

 

From other various sources, adsorption to goethite (a common iron(III) oxyhydroxide mineral present in soils) has been studied and reported in three separate papers. Approximately 100% adsorption at pH8; approximately 50% at pH 10 and negligible adsorption at pH 12 was seen in the absence of metal ions; the presence of zinc(II), copper(II) and iron(III) ions has a negligible effect on adsorption to the goethite (Nowack and Stone, 1999a). In the presence of calcium, the adsorption of ATMP to goethite increases significantly (Nowack and Stone, 1999b). Adsorption of 30 μM/g at pH 7.2 is reported (Stone and Knight, 2002). These data are of non-assignable reliability.

 

 

The presence of calcium in solution tends to significantly increase the adsorption of ATMP. In natural waters this will play a part in the fate of ATMP, particularly in slightly alkaline waters.

 

The key data are in the study by Michael (1979). Given that the sediment was not analysed, it is necessary to review the conclusions drawn. It is reasonable to assume that removal from the water column would be due to adsorption to sediment, given that:

• the relatively high concentration makes it unlikely to be due to adsorption to glassware

• significant biodegradation can be ruled out

• there are no other likely explanations of removal from the water.

Adsorption proportions can vary across a relatively wide range with e.g. differing soil types/characteristics and loading concentration. Surface area may also have a role in the quantitative partitioning in any given case. No convincing, consistent explanations have been reached by the authors of the various studies/ papers as to a consistent means to predict Kd. Best use must therefore be made of the available results for sediments and soils.

The key Kp values have been calculated using the physical and chemical properties of ATMP acid and the log Koc value from the key study (Michael, 1979) using EUSES software (v 2.1.2).

 

There is no evidence for desorption occurring. Effectively irreversible binding is entirely consistent with the known behaviour of complexation and binding within crystal lattices. The high levels of adsorption which occur are therefore a form of removal from the environment. For analogous phosphonate complexing agents, after 38-50 days, the phosphonate is >95% bound to sediment with only 5% extractable by ultrasonication and use of 0.25N HCl xylene solvent (based on radiolabelling) in river microcosms (cited in Gledhill and Feijtel, 1992). In the same study, ATMP-H rapidly, and to a high degree was irreversibly removed from a natural water column. (Saeger, 1979, see also IUCLID section 5.2.2).

 

In the context of the exposure assessment, largely irreversible binding is interpreted as a removal process; 5% remaining after 40 - 50 days is equivalent to a half-life of 10 days which is significant for the environmental exposure assessment in the regional and continental scales. This abiotic removal rate is used in the chemical safety assessment of ATMP-H and its salts.

 

Note on relevance of the HPLC Screening Method for phosphonates

A screening study using the conventional HPLC method (OECD 121) to estimate the value of Koc (organic carbon-water partition coefficient) is considered not appropriate. Adsorption behaviour onto the normal aminopropyl column used in OECD 121 would not necessarily follow the pattern of adsorption onto substrates that are of importance in the environment. Understanding of sludge binding is informative, but much less significant in the chemical safety assessment than binding to matrices with a higher inorganic content or high surface area. It is important to understand Kd directly, and preferably as a function of variables such as solid phase composition and characteristics, water hardness, dilutions, and phase ratios.

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.