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Diss Factsheets

Environmental fate & pathways

Adsorption / desorption

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Description of key information

After exposure to soil, significant adsorption to solid soil phase is not expected.

Key value for chemical safety assessment

Koc at 20 °C:

Additional information

The adsorption potential of the substance was determined according to OECD guideline 106 (Batch Equilibrium Method) and in compliance with GLP criteria (BASF, 2009). In this study, five soils were used with ranging ratios of sand, silt and clay, and different pH and organic carbon content. The different soils were equilibrated for 4 hours at room temperature in the dark with five different concentrations of imidazole always with a soil-solution ratio of 1:1. After the equilibration period soil-solution samples were separated by centrifugation. As the substance is unstable and little adsorption was expected, soil pellets were extracted 2 times with a solvent (direct method) and the soil-solvent phases also separated by centrifugation. Imidazole present in the solution samples and the extraction solvent were determined by LC/MS/MS. Desorption experiments were not performed as soil pellets were extracted with solvent. To study adsorption kinetics, soil-solution ratios of 1:1 and 1:5 were equilibrated for 0.5, 2, 4 and 6 hours. These tests were performed with only two soils, one with the lowest pH value and organic carbon content and one with the highest pH and organic carbon content. The adsorption data were used to calculate percentage adsorption at equilibrium, the distribution coefficient Kd and the (total) organic carbon normalized adsorption coefficient Koc. Freundlich adsorption isotherm equations were also obtained considering all 5 concentrations.

The percentage of adsorption of imidazole on both soils and both soil-solution ratios increased slightly with prolonged equilibration times, reaching a plateau after about 2 hours, thus indicating that adsorption equilibrium wasdefinitely reached after 4 hours. Considering all concentration levels, adsorption of imidazole on soils at equilibrium depends on the soil type used and is ranging from 17% for LUFA 2.1 (sand) and Fiorentini (silt loam) to a maximum of 36% for LUFA 2.3 (sandy loam). Kd values range from 0.23 (sand) to 3.37 mL/g (silt loam), Koc values range from 23 to 207 mL/g. The Freundlich adsorption coefficients (dimension µg1/1n·mL1/n/g) range from 0.337 for LUFA 2.1 (sand) to 1.19 for Nierswalde Wildacker (silt loam). The organic carbon normalized Freundlich adsorption coefficients range from 41 for Fiorentini (silt loam) to 101 for LUFA 2.3 (sandy loam).

In addition, the Koc of Imidazole was calculated using SRC PCKOC v1.66. The calculated Koc of Imidazole is 9.7 (corrected log Koc = 0.99) (BASF 2002). Hence, Imidazole is not expected to adsorb to suspended solids and sediment based upon the log Koc as calculated for the uncharged molecule by use of SRC PCKOC v1.66 (at 25°C). However, a pka value of ca.7 indicates that the molecule will partly exist in its cationic form at lower environmental pH-values and cations generally adsorb more strongly to the soils clay than their neutral counterparts. According to a calculation based on the formula as given by Franco & Trapp (2008) the fully charged molecule has a Koc value of 1943.

[LogKoc: 2.32]