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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Adsorption to the solid soil phase would be expected.

Key value for chemical safety assessment

Additional information

The test item is a multiconstituent substance with dicyclohexylamine as counterion for the dye complex. To estimate the potential for adsorption QSAR calculations have been conducted (BASF SE 2018). The dye complex of the main components consist of two different organic structures complexed by Cr(3+)-ions. Since metal complexes cannot be calculated using QSAR models, the two organic parts where calculated using EPISuite KOCWIN program (v2.00) as a worst case consideration. The KOCWIN v2.00 model comprises two methodologies to estimate the logKoc: (1) Estimation using first-order Molecular Connectivity Index (MCI) and (2) estimation using logKow. KOCWIN requires only a chemical structure to make predictions. However, dicyclohexylamine (CAS 101-83-7) has a logPow of -0.4 (ECHA Homepage) and has, therefore, no potential for adsorption. Further estimation is not relevant.

The calculated logKoc values of both organic parts was determined to be 4.948 (estimate from MCI) and 4.798 (estimate from logKow) for the first part and 5.035 (estimate from MCI) and 5.2941 (estimate from logKow) for the second part, respectively. However, these values were determined for the uncharged molecules. Since the pKa-values (calculated with SPARC) are 4.29 and 4.71 for the first and the second part, respectively, it can be assumed, that both molecules would be almost entirely present in their charged form at the environmentally relevant pH range of 4 to 9. Therefore, the adsorption behaviour changes. The estimation method of Franco and Trapp (2008) can be used to correct the logKoc values for the charged molecules. For the first part the corrected logKoc values range from 3.88 (pH 4) to 2.12 (pH 9) and for the second part from 4.39 (pH 4) to 2.23 (pH 9). Therefore, a definite logKoc value cannot be determined since the value depends on the environmental conditions. However, the estimated logKoc values for the uncharged molecules can definitely be considered as absolute worst-case and the adsorption to the solid soil phase would be expected.

Franco A. , Trapp S., Estimate of the soil-water partition coefficient normalized to organic carbon for ionizable organic chemicals. Environmental Toxicology and Chemistry, 27, 2008.