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Environmental fate & pathways

Adsorption / desorption

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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 Koc(equivalent) value of approximately 4.

Key value for chemical safety assessment

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 soil)
Value in L/kg:
2.78
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 suspended matter)
Value in L/kg:
3.18
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 Annex IX, 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 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 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). These data are of non-assignable reliability. Adsorption of 30 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: >90% adsorption within 24 hours was reported by Gledhill and Feijtel (1992).

 

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 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 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 and lake water microcosms. (Saeger, 1977, see 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.

Read-across of adsorption data between acid and salt forms in the ATMP Category

Under the pH conditions found in natural water-sediment systems, ATMP will be present as complexes and salts (refer to the Chapter 5 Endpoint Summary). Adsorption values measured with ATMP-H as the test material are therefore also relevant information for the chemical safety assessment of ATMP salts. Refer to the Chapter 5 Endpoint Summary, and CSR Section 1.4 and Chapter 4, for further information.

 

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.