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Environmental fate & pathways

Adsorption / desorption

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Description of key information

log Koc = 4.07

Key value for chemical safety assessment

Additional information

The normal approach to modelling binding behaviour in environmental exposure assessment takes its approach from an inherent assuption that the substance is binding only to the organic components of substrates (i.e. organic carbon present in soils, sediments, and WWTP sludges). This assumption does not apply to ATMP and its salts.

ATMP and its salts adsorb strongly to inorganic surfaces, soils and sediments, in model systems and mesocosms; 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.

The nature of the adsorption is believed to be primarily due to interaction with inorganic substrate.  While Koc is the conventional indicator for adsorption, there is not necessarily any interaction with organic carbon present in the substrate in the case of these substances."

A Log Koc value of 4.07 was obtained by evaluating Ksediment-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, an adsorption coefficient value of 0.92-0.95 is reported in reviewed literature paper (Steber and Wierich, 1987). The Freundlich isotherm constants values of ca. 1300-4000 for ATMP were determined by following the EPA test guide method in an activated sludge. A paper (Jaworska, 2002) supported the measured data for water sediment and water active sludge studies.

Studies were also carried out on two studies (Nowack and Stone1999). Goethite was used as a testing medium in both cases. Adsorptions were reported in both cases. Another study (Nowack, 2002) has been reported the same phenomena using active sludge as testing medium. However, no quantitative result has been reported and therefore they all are treated as supporting studies.

This substance is a member of a category of various ionised forms of the acid ATMP (CAS 6419-19-8). The main assumption is that ammonium is not significant in respect of all the properties under consideration. In dilute aqueous conditions of defined pH a phosphonate ion 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. Exposure of the soil would only be expected to occur via some form of aqueous processing (e.g. from spreading of WWTP sewage sludge). Thus it is acceptable to additionally read across between the parent acid and salts for the soil compartment. In the present context the effect of the counter-ion (potassium) will not be significant. In biological systems and the environment, polyvalent metal ions will be present, and the phosphonate ions show very strong affinity to them.