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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The adsorption and desorption characteristics of 14C-labelled test substance (diflufenzopyr sodium salt) were examined in seven soils (5 US soils and 2 EU soils) by the batch equilibrium method. The adsorption Koc for the loam soil are generally higher than that of soils with higher sand content. However, the desorption Koc values of all soils are considerably greater than the corresponding adsorption Koc values indicating a stronger affinity to soil once adsorbed.
Average Koc value of all US soils for the adsorption based on the linear model is 60, without silt loam the Koc value is reduced to 40. The corresponding average Koc values from the Freundlich model are 68 and 47, respectively. For the EU soils Loamy Sand and Sandy Clay Loam the adsorption Koc values from the Freundlich model and the linear isotherm model are reported to be 30 and 53 and 24 and 41, respectively.

Key value for chemical safety assessment

Koc at 20 °C:
68

Additional information

The adsorption and desorption characteristics of 14C-labelled test substance (diflufenzopyr sodium salt) were examined in seven soils (5 US soils and 2 EU soils) by the batch equilibrium method. Five soils originated from the USA and are classified as loam soil (two soil samples), sandy loam, silt loam and clay loam. Two soils were sampled in Europe and are classified as loamy sand and sandy clay loam.

In a preliminary test, the soil-to-water ratio, the equilibrium time, the stability and the adsorption potential of the test substance to the vessel surface were established at an initial concentration of 5 µg test substance/mL. Equilibrium was reached after 20 hours (for sandy loam after 2h) in the adsorption step and in the desorption step. Since some degradation of the test substance was observed in the preliminary test, the stability tests were repeated in the definite test. Adsorption to the vessel surface was was < 1 % of the initila concentration.

In the definite test, the adsorption ranged from 3 % (sandy loam) to 80 % (silt loam) under the applied conditions. The mass balance ranged from 90.4 % to 101.8 % of the applied radiocarbon in all soils. The supernatants of the adsorption and desorption test and the extractable radioactivity from the soil were analysed by HPLC. The radioactivity in the supernatant after the adsorption step contained 94.5 % to 100.0 % of the a.i. for all soils except silt loam, where only 74.4 % of the radiocarbon was identified as a.i In the supernatant, after the desorption step 86.7 % to 100.0 % of the radioactivity was identified as a.i. The a.i. content in the soil extracts ranged from 79.6 % to 95.3 % of the radioactivity. The soil bound residues varied from 0.2 % (sandy loam, 75 % sand) to 16 2 % of the applied radioactivity (silt loam, 10 % sand).

The data of the definite study were evaluated by applying the Freundlich and the linear isotherm model. Both models fitted the experimental data adequately. In silt loam, K and Koc values of adsorption, respectively were significantly higher than those determined for the other soils. These values may be explained by the high degradation of the test substance (about 25 %) during the test period Nonetheless, K and Koc values for the silt loam soil have been calculated and average Koc values for the US-soils were determined with and without the silt loam values.

Average Koc value of all US soils for the adsorption based on the linear model is 59, without silt loam the Kd value is reduced to 40. The corresponding average Koc values from the Freundlich model are 68 and 47, respectively.

For the EU soils Loamy Sand and Sandy Clay Loam the adsorption Koc values from the Freundlich model are reported to be 30 and 53, respectively.

The adsorption Koc for the loam soil are generally higher than that of soils with higher sand content. The soil with higher sand content adsorbed weakly. However, the desorption Koc values of all soils are considerably greater than the corresponding adsorption Koc values indicating a stronger affinity to soil once adsorbed. The desorption Koc values, except for clay loam, indicate only a medium mobility based on McCall et al. (1981) classification. Any potential for the test substance (diflufenzopyr sodium salt) to leach is however greatly reduced, if not eliminated, by its short soil half-life (about 6 days field conditions, 8-10 days laboratory).