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Environmental fate & pathways

Adsorption / desorption

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Description of key information

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

Key value for chemical safety assessment

Koc at 20 °C:
31.7

Additional information

Assessment

In accordance with column 2 of REACH Annexes VIII and IX, the study 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. N,N,N',N'-Tetramethylhexamethylenediamine (TMHDA, CAS 111-18-2) has a log Kow of 1.99 (BASF SE 2013, report no. 11L00324; @23°C, pH 12) and is not readily but inherently biodegradable (see IUCLID chapter 5.2.1).

In the GLP key study according to OECD guideline 121, the log Koc was determined to be 1.5 at pH 7.0 (BASF SE, 2013; study no. 11L00324). At pH 9 no reliable Koc could be determined as the retention time on the HPLC system was larger than the retention time for the reference substance (DDT; 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 substance's environmental fate. (Reuter et al.: Continental-scale digital Soil Mapping using European Soil Profile Data: Soil pH. Pp. 91-102. In Böhner et al. (2008): SAGA – Seconds Out. Hamburger Beiträge zur Physischen Geographie und Landschaftsökologie, Vol.19, 113 pp.). In addition, the environmentally relevant pH range is 4 to 9 in fresh surface waters, whereas marine environments have a stable pH of about 8. In agricultural soils and sewage treatment plants the pH normally varies between 5.5 and 7.5 (REACH Guidance document R7a, R.7.1.17.1: Comment 21, p. 145, ECHA, 2015, v4.1).

The Koc value was also estimated using QSAR models. According to the MCI method of the KOCWIN v2.00 module of EPI Suite v4.11, N,N,N',N'-Tetramethylhexamethylenediamine has a Koc of 180 L/kg and a log Koc of 2.3. 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 (log Kow of 1.99 @23°C, pH 12; BASF SE 2013, report no. 11L00324; @23°C, pH 12). The latter method resulted in a Koc of 78.5 L/kg and a log Koc of 1.9. These estimates are representative for uncharged molecules.

The pKa of 10.1 (measured; BASF SE, 2012, rep. no.: 11L00324) indicates that the molecule will exist primarily as a cation under environmentally relevant conditions. 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 molecule. Under environmentally relevant conditions (pH from 5 to 8) the substance is almost completely present in its charged form (as calculated by the formula ionised = 100 / (1+10(pKa - pH)): 100% at pH 5 to 8). Following the method of Franco & Trapp, the log Koc was estimated to be 3.25 at pH 7 (pH 5: log Koc = 3.25; pH 9: log Koc = 3.24) using the measured pKa of 10.1 and the measured log Kow for the uncharged molecule of 1.99. The correction was performed for pH 5, 7 and 8, which represents 98% of the European soils.

The key result - an experimentally determined log Koc for pH 7 - demonstrates that adsorption of TMHDA to the solid soil phase is not expected under entvironmentally relevant conditions (log Koc = 1.5 at pH 7). This result is supported by the estimated data for the uncharged molecules of TMHDA which indicate that adsorption is not to be exptected (KOCWIN v2.00: log Koc < 3). Only the estimated adsorption coefficients according to Franco & Trapp indicate that the substance will adsorb to the solid soil phase (log Koc > 3 at pH 5 to 8). In conclusion, TMHDA is not expected to significantly adsorb to the solid soil phase.

[LogKoc: 1.5]