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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Adsorption coefficients have been derived in a reliable OECD 106 test in soil-water systems under a range of conditions. At low concentrations of the range predicted to occur in the environment, the average Kd has been extrapolated to 1500 l/kg (for the linear constituent structure). There is some evidence that binding in high-clay substrates is likely to be higher still and in hard waters (calcium concentration equivalent to 40 mg/l or higher) the Kd would be significantly higher. For the cyclic constituent, the equivalent value was 110 l/kg based on a preliminary test at higher concentration. Adsorption and desorption behaviour of the cyclic constituent is not yet fully understood and until/unless definitively proven otherwise, the chemical safety assessment assumes at this stage that some desorption of cyclic constituent from the substrate is possible. This is a conservative interpretation and is not the case for the linear constituent (and other aminomethylenephosphonates) which show negligible desorption and effectively irreversible binding. Under the negatively-charged, alkaline conditions at the surface of mineral substrate, it would be thermodynamically favourable for the ring structure present in the cyclic constituent molecule to open, and behaviour like the linear constituent is then to be expected.

Key value for chemical safety assessment

Other adsorption coefficients

Type:
other: log Kp (soil water) for linear constituent
Value in L/kg:
1 500
at the temperature of:
22 °C

Other adsorption coefficients

Type:
other: log Kp (soil water) for cyclic constituent
Value in L/kg:
110
at the temperature of:
22 °C

Additional information

No existing adsorption / desorption data were available for HEBMP-H and its salts in advance of the REACH programme. This substance is a mineral-binding and complexing agent, with unusual chemical properties. Experience with close structural analogues made clear that adsorption / binding, particularly to mineral substrates, is almost certainly extremely high, despite the very low log Kow. Furthermore, the property of binding to substrates is fundamental to understanding and modelling of environmental exposure, for substances like this. Therefore, adsorption / desorption (screening) data, required in Section 9.3.1 of Annex VIII, is an extremely important part of the data set for HEBMP-H and its salts.

 

The normal data requirement in Section 9.3.1 of Annex VIII is a screening study, conventionally either an HPLC method (OECD 121) to estimate the value of Koc (organic carbon-water partition coefficient), a sludge adsorption test, or a soil test. In the case of HEBMP-H and its salts, the binding to substrates is not mediated by organic carbon, so Koc as such is not a useful parameter. Adsorption behaviour onto the normal aminopropyl column used in OECD 121 would not necessarily follow the pattern of adsorption onto inorganic surfaces of importance environmentally. Understanding of sludge binding is informative, but much less significant in the chemical safety assessment than binding to matrices with a higher inorganic content. It is important to understand Kd directly, and as a function of variables such as solid-phase composition and characteristics, water hardness, dilutions, and phase ratios.

 

In summary, this is a case in which estimation methods are not appropriate. Therefore the only appropriate course to deriving the data that are both necessary to fulfil the REACH Annex VIII data requirement, and also appropriate for use in the chemical safety assessment of HEBMP-H and its salts, was an adapted OECD 106 test. The test was conducted in 2012 -13.