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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The registered substance is an Alkyl phosphate and stearyl amine salts (EC = 952-252-4) are mainly composed by primary alkylamines (CAS 90640-32-7, UVCB) and Phosphoric acid, octadecyl ester (CAS 39471-52-8, Multi-Constituent). Both elements present a long hydrogenated (saturated) carbon chains (16 to 18 carbon atoms). As mentioned in the read-across justification document, it is relevant to use the available environmental fate data of the primary alkl amine by a read-across approach for the salt. Therefore, the data of adsoption/desorption available for the primary alkyl amine were used to complete this endpoint for the registered substance.

Key value for chemical safety assessment

Koc at 20 °C:
34 850

Additional information

Adsorption/desorption of the primary alkyl amines

Due to the surface-active properties, long-chained alkyl amines adsorb strongly onto the solid phase of soil and sediments. The determination of a Koc from log Kow is not opportune, because the common equations for Koc derivation are not valid for both ionic and surface active substances.

Slangen (2000) studied the adsorption behaviour of 1-14C-labelled n-octadecylamine in a batch equilibrium experiment according OECD 106. Two soils collected in UK (Cranfield 164 soil, 21.8% clay, 6.6% organic matter, silt loam; Cranfield 266 soil, 50.2% clay, 2.6% organic matter, clay), one sediment collected in The Netherlands (18.7% clay, 4.1% organic matter, silt loam) and a sewage sludge (45.9% clay, 51.9% organic matter, silty clay) were used, encompassing a range of % clay and organic material. The test substance adsorbed partially onto the container walls which was considered for the determination of the adsorption coefficients. Adsorption kinetics was determined by measurements at different sampling times (up to 24 h), an equilibrium was reached after 3 hours. Desorption occurred to a lesser extent than adsorption: for Cranfield 164 soil 24.4% desorption after 47 hours and 24.2% after 166 hours were determined, while desorption for Cranfield 266 soil was 13.7% after 47 hours and 19.1% after 166 hours.The Freundlich adsorption isotherms were determined to:


Table: Freundlich adsorption isotherms determined by Slangen (2000):





Soil: Cranfield 164 silt loam



Soil: Cranfield 266 clay



Sediment: Oostvaardersplassen silt loam



Sewage sludge: DB1 silty clay




Apparently, the sorption onto Cranfield 266 soil is much higher than to Cranfield 164 despite of the higher organic matter content in Cranfield 164 soil. This can be explained that ionic interactions play a more important role than hydrophobic partitioning with organic matter. Alkyl ammonium ions can interact with the surface of mineral particles or with negative charges of humic substances. The influence of the chain length on the sorption behaviour is therefore expected to be low, and the experimental results obtained in the test with octadecyl amine can be taken as representative for the other products. As well, an influence of the double bond (in octadecenylamine) onto sorption is not expected.

The adsorption isotherms determined by Slangen (2000) are non-linear. The distribution constants for soils and sediment decrease dramatically as the concentrations decrease. The lowest aquatic equilibrium concentration in the experiment (5 µg/l) is more than one order of magnitude higher than the calculated PEC values. For example, with the isotherm determined for the sediment and an aquatic concentration of 10 ng/L, a Kp value of 37 L/kg is calculated, which is far below the constants determined in the experiment (707 – 3140 l/kg). Apparently, extrapolation to low concentrations would lead to unrealistic results.

According to the Danish EPA (2004) a more reliable method of extrapolation is to use the data originating from the lowest measured concentrations and to assume that the coefficient remains constant at lower concentrations. At the 2 lowest concentrations, values of 707 and 687 L/kg were experimentally determined, the mean value (697 L/kg) is used for the exposure assessment.

The mean values for the two soils are 252 and 342 l/kg, respectively. Because there is no principal difference between soil and sediments on respect to the sorption properties, as a worst case approach the value for sediment is also used for soils and suspended particles.

For the adsorption onto sludge, values of 687 and 674 L/kg were determined for the 2 lowest concentrations. The mean value (680 L/kg) is used for the exposure calculation.


In the table below, the distribution constants used in this assessment are summarized:


                                           Table: Distribution constants for primary alkyl amines




1050 m3.m-3




175 m3.m-3




349 m3.m-3






With a Kpsuspof 697 l/kg and a concentration of 15 mg/l suspended matter in surface waters, the adsorbed fraction is calculated as 1.0%.

[LogKoc: 4.54]