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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Adsorption to solid soil phase is not to be expected under environmentally relevant conditions.

Key value for chemical safety assessment

Additional information

The adsorption coefficient was determined by QSAR as well as by a read-across approach to the experimental value for an analogue substance.

 

The Koc for the uncharged molecule of CAS 6864 -37 -5 was estimated using both models of KOCWIN v2.00 of EPI Suite v4.11 (BASF SE, 2015). Using the Molecular Connectivity Index (MCI) which is regarded as more reliable compared to the log Kow method, the log Koc for the uncharged molecule was estimated to be 3.08 (Koc = 1195 L/kg). Based on the measured log Kow of 2.3 (pH 10; BASF SE, 2010; 09L00312), the log Koc was estimated to be 1.48 (Koc = 30.4 L/kg) using the log Kow method. Both methods are based on fragment corrections. Since the identified fragment (Nitrogen-to-Cycloalkane, aliphatic) occurs at a higher number than the training data set of the models, the substance is not within the applicability domain of the estimation model.

 

The pKa of 10.3 (measured; BASF SE, 2010, 09L00312) indicates that the molecule will exist primarily as a cation in the environment. Cations generally adsorb stronger to soils containing organic carbon and clay than their neutral counterparts. Hence, the results from the KOCWIN models may underestimate adsorption to organic carbon since it does not consider the ionic structure of the molecule.

 

Regarding the charged molecule, the log Koc was estimated to be 4.49 (Koc = 31226 L/kg) following the method of Franco & Trapp (2008, 2009, 2010), using the pKa of 10.3 and the estimated log Kow for the uncharged molecule of 4.1 (KOWWIN v1.68; substance not within applicability domain). The correction was performed for pH 5, 7, and 8, which represents 98% of the European soils. The model is not yet validated; in addition, the applicability domain is not clearly defined. Nevertheless, the Koc values of the Franco & Trapp method give a good indication on the adsorption potential of a substance depending on the pH conditions of soil.

 

Experimental data on the adsorption coefficient of the charged molecule are only available for the structurally similar substance 4,4´-Diamino-3,3´,5,5´-tetramethyl-dicyclohexylmethan (CAS 65962 -45 -0). The log Koc was determined to be 2.6 at 23 °C and pH 7.0 following the OECD guideline 121 (BASF SE, 2013). At pH 9 no reliable Koc could be determined as the retention time on the HPLC system was larger than for the reference substances (log Koc > 5.63). As 98% of the European soils have a pH within a range of 5 to 8, the value determined at pH 9 is not relevant for the assessment of the substances environmental fate.

 

Due to the uncertainty of the estimated data (applicability domains) as well as higher environmental relevance of the experimentally determined Koc (log Koc = 2.6 at pH 7) for the analogue substance 4,4´-Diamino-3,3´,5,5´-tetramethyl-dicyclohexylmethan (CAS 65926 -45 -0), the latter value was used for the environmental risk assessment.