Potassium salts of [hexane-1,6-diylbis[nitrilobis(methylene)]]tetrakisphosphonic acid (4-7 K:1)

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption, other

Remarks:

adsorption

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 ug Dequest 2050 respectively, and hence 20, 40, 400 and 2000 ug active acid respectively.

GLP compliance:

no

Type of method:

batch equilibrium method

Media:

sediment

Radiolabelling:

yes

Analytical monitoring:

yes

Details on test conditions:

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%

Key result

Sample No.:

#1

Phase system:

sediment-water

Type:

Kp

Value:

>= 2 400 - <= 3 900 L/kg

Remarks on result:

other: K values after 8 d in a sediment / soft-water system.

Key result

Sample No.:

#2

Phase system:

sediment-water

Type:

Kp

Value:

>= 3 100 - <= 3 900 L/kg

Remarks on result:

other: K values after 8 d in a sediment / hard-water system.

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	1900	3900	1900	3900	3900
0.10 ppm	920	3900	1900	3900	3900
1.0 ppm	180	2600	1600	3600	3600
5.0 ppm	12	80	1400	690	2400

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	3900	3900	3900	3900	3900
0.10 ppm	1500	7900	3900	7900	3900
1.0 ppm	650	3700	3700	4400	3300
5.0 ppm	57	220	520	1200	3100

log K(sediment-water): 2.25 - 3.59 (soft water), 2.81 - 3.89 (hard water)

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 any water is 3700 l/kg.

Conclusions:

An adsorption K(sediment-water) 3700 l/kg was determined in a reliable study conducted according to generally accepted scientific principles.

Description of key information

The substance adsorbs significantly to sediment, soil and sludge substrates based on the available study data. While the binding is not necessarily to organic carbon, Kd values appear consistent with a log Koc(equivalent) value of approximately 5.

Key value for chemical safety assessment

Koc at 20 °C:

74 000

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

3.17

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in sediment)

Value in L/kg:

3.57

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

3.57

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in activated sewage sludge)

Value in L/kg:

4.44

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in raw sewage sludge)

Value in L/kg:

4.35

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in settled sewage sludge)

Value in L/kg:

4.35

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in effluent sewage sludge)

Value in L/kg:

4.44

at the temperature of:

12 °C

Additional information

This substance is a mineral-binding and complexing agent, with unusual chemical properties. HMDTMP 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 HMDTMP 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 Regulation Annex VIII, are an extremely important part of the data set for HMDTMP 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 HMDTMP 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 5 was obtained by evaluating Kp(sediment-water) data in a reliable study conducted with HMDTMP-H according to generally accepted scientific principles (Michael, 1979). River sediments were placed with a range of concentrations of HMDTMP-H in purified and synthetic hard water and the water was analysed by using liquid scintillation on days 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.

The key log Kp values used for chemical safety assessment have been derived using EUSES (v 2.2.0).

The presence of calcium in solution tends to significantly increase the adsorption of ATMP. Similar effects are expected for HMDTMP. In natural waters this will play a part in the fate of HMDTMP, 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 for each substance.

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. After approximately 40-50 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 and lake water microcosms with HMDTMP-H (Saeger, 1979). 66-80% removal (binding) is seen after 11 days in the same test.

These findings are consistent with removal from the aqueous phase in a sediment microcosm studies that were found not to be by degradation (Saeger, 1979).

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.

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 and potassium salts in the HMDTMP category are freely soluble in water. The HMDTMP anion can be considered fully dissociated from its sodium or potassium cations when in dilute solution. Under any given conditions, the degree of ionisation of the HMDTMP 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 HMDTMP-H, HMDTMP.4Na, HMDTMP.7K or another salt of HMDTMP.

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

1. HMDTMP is present as HMDTMP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the media and not whether HMDTMP (4-7K) salt, HMDTMP (4-7Na) salt, HMDTMP-H (acid form), or another salt was used for dosing.

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

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 HMDTMP than potassium and sodium. This could result in HMDTMP-dication (e.g. HMDTMP-Ca, HMDTMP-Mg) and HMDTMP-trication (e.g. HMDTMP-Fe) complexes being present in solution.