(1-hydroxyethylidene)bisphosphonic acid, potassium salt

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption, other

Remarks:

other: sediment water partition coefficient

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Remarks:

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 test substance (active acid) respectively.

GLP compliance:

no

Type of method:

other: other measurement

Media:

other: water - sediment

Radiolabelling:

yes

Analytical monitoring:

yes

Type:

Koc

Value:

16 610 L/kg

Type:

log Koc

Value:

4.22 dimensionless

Sample No.:

#1

Phase system:

sediment-water

Type:

Kp

Value:

>= 830 - <= 7 900 L/kg

Matrix:

Sediment with overlying soft water

Remarks on result:

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

Sample No.:

#2

Phase system:

sediment-water

Type:

Kp

Value:

>= 680 - <= 2 700 L/kg

Matrix:

Sediment with overlying hard water

Remarks on result:

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

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	920	3900	1300	1900
0.10 ppm	490	1100	2300	1500	1900
1.0 ppm	74	940	1000	1500	1900
5.0 ppm	460	2400	830	5600	7900

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	360	1300	1300	1900	1900
0.10 ppm	420	1100	1500	1900	2300
1.0 ppm	140	1200	1500	2700	2000
5.0 ppm	57	680	1100	1800	1800

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 1900 l/kg, and to hard water,
2000 l/kg.

Conclusions:

A Log Koc value of 4.22 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 between members of the HEDP category.

Reason / purpose for cross-reference:

read-across source

Type:

Koc

Value:

116 610 L/kg

Type:

log Koc

Value:

4.22 dimensionless

Sample No.:

#1

Phase system:

sediment-water

Type:

Kp

Value:

>= 830 - <= 7 900 L/kg

Matrix:

Sediment with overlying soft water

Remarks on result:

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

Sample No.:

#2

Phase system:

sediment-water

Type:

Kp

Value:

>= 680 - <= 2 700 L/kg

Matrix:

Sediment with overlying hard water

Remarks on result:

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

Description of key information

HEDP-H and its salts are expected to adsorb 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 context to note that the Kd values derived from the key study appear consistent with a log K_oc (equivalent) value of approximately 4.2.

Key value for chemical safety assessment

Other adsorption coefficients

Type:

log Kp (solids-water in sediment)

Value in L/kg:

3.3

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

2.9

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in activated sewage sludge)

Value in L/kg:

4.18

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in raw sewage sludge)

Value in L/kg:

4.08

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in settled sewage sludge)

Value in L/kg:

4.08

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in effluent sewage sludge)

Value in L/kg:

4.18

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

3.3

at the temperature of:

12 °C

Additional information

HEDP-H and its salts are mineral-binding and complexing agent, with unusual chemical properties. HEDP-H and its salts adsorb strongly to inorganic surfaces, soils and sediments, in model systems and mesocosms, despite the very low log K_ow; 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 (Gledhill and Feijtel, 1992).

 

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 HEDP-H 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 IX, is an extremely important part of the data set for HEDP-H and its salts.

 

The nature of the adsorption of HEDP-H and its salts is believed to be primarily due to interaction with inorganic substrate or generalised surface interactions. While K_oc 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 HEDP-H and its salts, meaning that K_oc as such is not a meaningful parameter. It is convenient for comparison purposes to determine the value of log K_oc that is consistent with/equivalent to the degree of sediment or soil binding exhibited by the substance.

 

Thus, a log K_oc (equivalent) value of 4.22 was obtained by evaluating K_{p (sediment-water)} data in a reliable study with HEDP-H conducted according to generally accepted scientific principles (Michael, 1979). In this study, 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 is considered as reliable and has been assigned as key study.

 

Adsorption coefficient values of 0.91-0.98 for the adsorption of HEDP (2-3Na) to activated sludge and primary sludge are reported in a literature paper (Steber and Wierich, 1986a). Freundlich isotherm constants values of ca. 2600-13000 for HEDP (2-3Na) were determined in this study however it was assigned reliability 4 due to insufficient documentation (Steber and Wierich, 1986a). A second literature paper (Jaworska, 2002) reports partitioning for water-sediment, water-active sludge and water-soil studies for HEDP-H. Kd ranges of ≥920 - ≤1300, ≥20 -<-190 and ≥2600 - ≤12700 for sediment, soil and activated sludge were reported, but assigned reliability 4 due to the information being secondary literature. Another study (Stone, Knight and Nowack, 2002) reported that iron, copper and zinc have no discernible effect on adsorption of HEDP-H using activated sludge as the testing medium, however this study was also secondary literature and was assigned reliability 4.

 

Adsorption data for soils (Kd (soil-water) values of 182 (silt loam), 6.15 (sand) and 129 (silty clay loam); and Koc values of 51600 (silt loam), 653 (sand) and 6260 (silty clay loam)) for HEDP-H were derived in an OECD 106 study using radiochemical analysis (Springborn Laboratories, 1991, reliability 2) and further values for adsorption to soil are reported in literature for HEDP (2-3Na) (Steber and Wierich, 1986b and c; reliability 4 - documentation insufficient for assessment). The Kd values from the Springborn Laboratories (1991) and Steber and Wierich (1986b and c) studies are consistent(Kd 6.15 -182 l/kg in both tests). The log Koc (equivalent values) of 3.8-4.7 from the silty soils and 2.8 from the sands reported in the Springborn Laboratories (1991) study are consistent with the log Koc (equivalent) value of 4.22 derived from the key sediment adsorption study (Michael, 1979). In further studies (Nowack and Stone, 1999b; reliability 4 - secondary literature), goethite (an iron-based mineral commonly found in soil) was used as a testing substrate. High levels of adsorption of HEDP-H were reported. A study by Fischer (1991) reports Freundlich constants of 46 -2378 for clay minerals for HEDP (2-3Na) and a Kd adsorption constant of 2042 (dimensionless) for Neckar sediments with overlying Neckar water for HEDP (2-3Na) (Fischer, 1993 - reliability 2).

 

A screening study using the conventional HPLC method (OECD 121) to estimate the value of K_oc (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 K_d 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 HEDP category are freely soluble in water and, therefore, the HEDP anion is fully dissociated from its sodium or potassium cations when in solution. Under any given conditions, the degree of ionisation of the HEDP 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 HEDP-H, HEDP (1-2Na), HEDP (2-3Na), HEDP-4Na, HEDP-xK or another salt of HEDP.

 

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

1. HEDP is present as HEDP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the system and not whether HEDP (1-2Na), HEDP (2-3Na), HEDP-4Na, HEDP-xK salts, HEDP-H or another salt was added.
2. Disassociated sodium/potassium cations. The amount of sodium/potassium present depends on which salt was added.
3. Divalent and trivalent cations have much higher stability constants for binding with HEDP than the sodium or potassium ions, so would preferentially replace them. These ions include calcium (Ca²⁺), magnesium (Mg²⁺) and iron (Fe³⁺). Therefore, the presence of these in the environment or in biological fluids or from dietary sources would result in the formation of HEDP-dication (e.g. HEDP-Ca, HEDP-Mg) and HEDP-trication (e.g. HEDP-Fe) complexes in solution, irrespective of the starting substance/test material.

In this context, for the purpose of this assessment, read-across of data within the HEDP Category is considered to be valid.

Gledhill and Feijtel (1992) Environmental properties and safety assessment of organic phosphonates used for detergent and water treatment applications. The Handbook of Environmental Chemistry, Vol. 3, Part F, (Ed.: Hutzinger, O.), Springer-Verlag, Berlin.