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Environmental fate & pathways

Adsorption / desorption

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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:
A Log Koc value of 3.57 was determined in a reliable study conducted according to generally accepted scientific principles.
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:
HYPOTHESIS FOR THE CATEGORY APPROACH
HMDTMP acid is a member of the HMDTMP category, which is comprised of the acid form and various salts of the same acid. In the context of the RAAF, read-across of environmental, ecotox, human health and physicochemical data within the HMDTMP Category is considered as Scenario 6.

SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
This category covers various sodium and potassium salts of [hexane-1,6-diylbis-[nitrilobis(methylene)]tetrakisphosphonic acid. The different salts are prepared by neutralising the acid to a specific pH and accordingly the constituents proportions and degree of ionisation are comparable between substances under similar conditions (in vivo and in the environment). All category members are based on the HMDTMP structure.
Since the salts are prepared from the acid, the impurity profile for HMDTMP acid is also typical of the salts in this Category, although acidic impurities would also be present as salts. Typically, impurities include residual inorganic acids/salts and organic by-products from manufacturing.

CATEGORY APPROACH JUSTIFICATION
The category hypothesis is that all the members are various ionised forms of the same parent acid. This category covers various sodium and potassium salts of [hexane-1,6-diylbis-[nitrilobis(methylene)]tetrakisphosphonic acid. The different salts are prepared by neutralising the acid to a specific pH and accordingly the constituents proportions and degree of ionisation are comparable between substances under similar conditions (in vivo and in the environment). All category members are based on the HMDTMP structure. Data are available for the acid form and some salts. Only acid aqueous solutions are commercially available. The properties of the members of the category are consistent across all endpoints.
Contributing effect of counter-ions:
The approach assumes in general that alkaline metal (sodium and potassium) counter-ions are not significant in respect of all the properties under consideration.
In an environmental context, the speciation will be controlled by the prevailing conditions and ions present, and will be the same regardless of the starting form. In dilute aqueous conditions of defined pH and mineral composition, a salt or complex introduced into this medium will behave no differently to the parent acid, at identical concentration of the particular speciated form present and will be fully dissociated. Hence some properties (measured or expressed in aqueous media, e.g. ecotoxicity) for a salt can be directly read-across (with suitable mass correction) to the parent acid and vice versa, and from one salt to another.
Where data are available across different salt forms of HMDTMP, this approach is supported.

DATA MATRIX
A data matrix table of all the data available on the HMDTMP Category members is available in Section 13 of the technical dossier.
Reason / purpose:
read-across source
Sample No.:
#1
Phase system:
sediment-water
Type:
Kp
Value:
>= 2 400 - <= 3 900 L/kg
Matrix:
Sediment with overlying soft water
Sample No.:
#2
Phase system:
sediment-water
Type:
Kp
Value:
>= 3 100 - <= 3 900 L/kg
Matrix:
Sediment with overlying hard water

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 Annex IX, 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 according to generally accepted scientific principles (Michael, 1979). River sediments 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 considered as reliable and has been assigned as key study.

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. (Monsanto internal report, cited by Gledhill and Feijtel, 1992). 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 (Gledhill 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.