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

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption, other

Remarks:

Adsorption in batch equilibrium test

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

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 µg Dequest 2060 respectively, and hence 20, 40, 400 and 2000 µg 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:

>= 830 - <= 5 600 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.

Key result

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.

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

	Day 0	Day 1	Day 2	Day 4	Day 8
0.05 ppm	360	920	3900	1300	920
0.10 ppm	490	1100	2300	1500	1900
1.0 ppm	74	940	1000	1500	1200
5.0 ppm	460	2400	830	5600	330

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

	Day 0	Day 1	Day 2	Day 4	Day 8
0.05 ppm	360	1300	1300	1900	900
0.10 ppm	420	1100	1500	1900	1100
1.0 ppm	140	1200	1500	2700	850
5.0 ppm	57	680	1100	1800	660

log K(sediment-water): 2.52 - 3.28 (soft water), 2.82 - 3.04 (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 1150 l/kg.

Validity criteria fulfilled:

yes

Conclusions:

Very high adsorption coefficients for sediment-water systems (log K(sediment-water): 2.52 - 3.28 (soft water), and 2.82 - 3.04 (hard water)) were determined in a reliable study conducted according to generally accepted scientific principles. Mean value of Ksediment-water applicable to soft water is 1340 l/kg, and to hard water, 950 l/kg. 

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 K_d values appear consistent with a log K_{oc (equivalent)} value of approximately 4.

Key value for chemical safety assessment

Koc at 20 °C:

19 000

Other adsorption coefficients

Type:

log Kp (solids-water in sediment)

Value in L/kg:

2.98

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

2.58

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in activated sewage sludge)

Value in L/kg:

3.85

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

2.98

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in raw sewage sludge)

Value in L/kg:

3.76

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in settled sewage sludge)

Value in L/kg:

3.76

at the temperature of:

12 °C

Other adsorption coefficients

Type:

log Kp (solids-water in effluent sewage sludge)

Value in L/kg:

3.85

at the temperature of:

12 °C

Additional information

This substance is a mineral-binding and complexing agent, with unusual chemical properties. DTPMP 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.

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 DTPMP 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 DTPMP and its salts.

The nature of the adsorption 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 DTPMP 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/equivalent to the degree of sediment or soil binding exhibited by the substance.

Thus, a log K_{oc(equivalent)} value of approximately 4 was obtained by evaluating K_{p(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.

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 pH7; approximately 50% at pH 9 and negligible adsorption at pH 12 was seen in the absence of metal ions; the presence of zinc(II) and copper(II) ions has a negligible effect but the presence of added iron(III) causes a more significant decrease of adsorption to the goethite (Nowack and Stone, 1999a). No discernible effect is observed in the presence of calcium (Nowack and Stone, 1999b). These data are of non assignable reliability. Adsorption of 25 μM/g at pH 7.2 is reported (Stone and Knight, 2002).

Adsorption to wastewater treatment plant sludge has been reported in two further studies: 80-90% adsorption within 24 hours was reported by Gledhill and Feijtel (1992) and 85% removal in WWTP was reported by Nowack (1998).

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

The key data are in the study by Michael (undated). 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 K_d. 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.

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 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.

[LogKoc: 4.28]