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Environmental fate & pathways

Adsorption / desorption

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Key value for chemical safety assessment

Koc at 20 °C:

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The highest degree of adsorption (67%) was obtained in eurosoil 5 (KD 8 mL/g. KOC 137 mL/g). This soil is characterized by the highest organic carbon content with 5.96% and sand with 82% as well as the lowest pH value of all tested eurosoils. But the pH value does not have an influence when comparing the results in the used soils because only the monophoshonate is present in the observed pH range of the soils. Adsorption in eurosoil 1 was 56% in equilibrium leading to a KD of 5 mL/g and a KOC of 162 mL/g. This soil is characterised by the highest amount of clay (75%) of all tested eurosoils. The organic carbon content of this soil is 3.27%. Only weak adsorption (approx. 15%) was determined in eurosoil 3. This degree of adsorption resulted in a KD of 0.7 mL/g and a KOC of 23 mL/g. This soil is characterised by an organic carbon content of 3.01%, 17% clay, 37% silt and 46% sand. A decline of adsorption was observed in this soil after 24h of agitation. Therefore values are based on the highest observed adsorption of 15% after 24h of agitation.No adsorption was found in eurosoil 2 and 4.

The results indicate that the amount of organic carbon is of high importance for the adsorption behaviour of the test item when exceeding a certain value. In contrast, no adsorption was observed in eurosoil 2 with an organic carbon content of 3.72%. Furthermore, the amount of clay can be relevant for the adsorption as indicated by high adsorption observed in eurosoil 1. The impact of the other fractions, silt and sand, is only of minor importance. The fact that adsorption is higher in eurosoil 3 (46% sand, 37% silt, 17% clay, 3.01% OC) than in eurosoil 2 (13% sand, 64% silt, 23% clay, 3.72% OC) let assume a more important impact of the sand fraction for the adsorption of the test item in soils. The highest carbon normalized distribution coefficients KOC was obtained for eurosoil 1 though not causing the highest adsorption of the test item. This result demonstrates again that not only a high amount of organic carbon is important for the adsorption behaviour but also a high amount of clay can lead to high adsorption.