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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Koc/log Koc (mean)

#1 (pH 7.3, organic carbon: 4.7%): 28.12/1.45

#2 (pH 5.7, organic carbon: 1.2%): 207.8/2.32

#3 (pH 7.1, organic carbon: 1.7%): 21.56/1.33

#4 (pH 6.1, organic carbon: 1.1%): 42.68/1.63

#5 (pH 4.9, organic carbon: 1.5%): 283.4/2.45

#6 (pH 5.5, organic carbon: 4.6%): 714.6/2.85

Overall mean Koc: 216.36

Overall mean log Koc: 2.34

Key value for chemical safety assessment

Koc at 20 °C:
216.36

Additional information

The soil adsorption and desorption of the substance was investigated on five soils of US and European origin and on one aquatic sediment. Radiolabelled test item was used in a concentration of approximately 0.45 to 0.006 mg/L, covering about two orders of magnitude. The soils were pre-equilibrated for approximately 24 h with 0.01 M CaCl2 solution, and then applied with test item, and subjected to one adsorption and two consecutive desorption cycles including complete supernatant exchange for each. Optimal soil to solution ratios were derived from a pre-test, and ranged from 1:1 to 1:20 (w/w) for the different soils. Equilibration times were 24 h per cycle, based on pre-test results. The experiments were carried out in the dark at ambient temperature. For the tested soils, Koc of the test item is high in acidic soils and decreases with increasing pH but seems to level off at neutral pH values. This is in agreement with the acidic nature of the test item. Freundlich adsorption exponents (1/n) are below 1.0 for all tested soils and indicate favored soil sorption of the test item at lower concentration. Koc for the two consecutive desorption cycles are significantly (about 1.4 to 2.9 times) higher than those for adsorption. Therefore, desorption of the substance from soil and sediment seems to be hindered, and once adsorbed, the compound is not readily released back into the aqueous phase. This effect was further characterized employing a two-step Accelerated Solvent Extraction with acetonitrile/water on the test item pre-adsorbed soil samples. For soils Pikeville, Carlyle, Nidda sediment and HCB (i.e. the soils of acidic pH or high in organic carbon content), solvent extraction conducted under mild conditions (i.e. 40 °C) were not able to completely mobilize the adsorbed test item. A certain fraction of unchanged test item was released by 'aggravated' extraction only, conducted at 100 °C.
Log Koc values in the range of 1.38 - 2.95 were derived (overall mean log Koc: 2.34). According to the classification system of Briggs, the adsorption Koc values would classify the substance as 'mobile', 'intermediate mobile' or 'low mobile' in soil, mostly depending on soil pH.

[LogKoc: 2.34]