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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Adsorption to solid soil phase is not expected.

Key value for chemical safety assessment

Additional information

QSAR-disclaimer:

In Article 13 of Regulation (EC) No 1907/2006, it is laid down that information on intrinsic properties of substances may be generated by means other than tests, provided that the conditions set out in Annex XI (of the same Regulation) are met.

 

According to Annex XI of Regulation (EC) No 1907/2006 (Q)SAR results can be used if (1) the scientific validity of the (Q)SAR model has been established, (2) the substance falls within the applicability domain of the (Q)SAR model, (3) the results are adequate for the purpose of classification and labeling and/or risk assessment and (4) adequate and reliable documentation of the applied method is provided.

 

For the assessment of the substance, (Q)SAR results were used for the estimation of the adsorption potential. The criteria listed in Annex XI of Regulation (EC) No 1907/2006 are considered to be adequately fulfilled and therefore the endpoint(s) sufficiently covered and suitable for risk assessment.

 

Therefore, further experimental studies on the adsorption potential are not provided.

Assessment:

In order to assess the adsorption potential of the substance, the Koc value was estimated using QSAR models. According to the MCI method of the KOCWIN v2.00 module of EPI Suite v4.11, propylenediamine has a Koc of 22.8. The MCI module is more reliable than the log Kow method of KOCWIN v2.00, which estimates the KOC based on the n-octanol/water partition coefficient (estimated log Kow -1.2; KOWWIN v1.68; BASF SE, 2014). The latter method resulted in a Koc of 1.7. These estimates are representative for uncharged molecules; the substance is within the applicability domain of the estimation models.

 

However, the measured pKa of 9.82 indicates that the substance will primarily exist as a cation in the environment. Cations generally adsorb stronger to soils containing organic carbon and clay than their neutral counterparts. Franco & Trapp (2008, 2009, 2010) have developed a method to take this effect into consideration when assessing the adsorption potential. 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. The method is based on the dissociation constant pKa and the log Kow for the uncharged moelcule. The pKa of propylenediamine is 9.82 (measured; see IUCLID chapter 4.21). For the log Kow of the uncharged molecule, the calculated value of -1.2 (KOWWIN v1.68; BASF SE, 2020) was used.

Regarding the charged molecule, at pH 7 the log Koc was estimated to be 1.24 (Koc = 17 L/kg) following the method of Franco & Trapp (2008, 2009, 2010) based on these data. The correction was performed for pH 5, 7, and 8, which represents 98% of the European soils. The value at pH 7 will be used as key value, i.e. for PEC and PNEC calculations.

 

In conclusion, the log Koc of the charged molecule is expected to be < 3 (pH 7, 25 °C). Therefore, adsorption of the substance to the solid soil phase is not to be expected.

 

However, in accordance with Regulation (EC) No 1907/2006, Annex VIII, Section 9.3.1, Column 2, the study on adsorption/desorption screening does not need to be conducted if based on the physicochemical properties the substance can be expected to have a low potential for adsorption (e.g. the substance has a low octanol water partition coefficient), or the substance and its relevant degradation products decompose rapidly.

The substance is readily biodegradable according to OECD criteria (weight-of-evidence, see IUCLID Ch. 5.2.1). In addition, the substance has a log Kow of -1.2 (estimanted, KOWWIN v1.68, substance in AD, see IUCLID Ch. 4.7). Therefore, a study does not need to be conducted.