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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Based on the pH-corrected log Koc and the ready biodegradability of the substance significant long-term adsorption to solid soil phase is not expected.

Key value for chemical safety assessment

Additional information

No study on adsorptivity of the test substance to soil particles is available. Because of the multi-constituent nature of the substance each of the substance’s components has been examined separately with help of model calculations for logKoc. Subsequently, the adsorption ability of the test substance in organisms was derived from the outcomes of the evaluation of its components.

An EpiWin calculation resulting in a logKoc of 2.65 for the uncharged molecule is available for CAS 104 -75 -6. Due to the pKa of 10.41, the substance will primarily exist as a cation in the environment which generally adsorb stronger to soils containing organic carbon and clay than their neutral counterparts. Hence, the Koc-model may underestimate adsorption to organic carbon since it does not consider the ionic structure of the molecule. The logKoc-value of the uncharged molecule therefore had to be corrected by Franco & Trapp’s calculation method to consider the pH-dependent molecule speciation. This resulted in logKoc-values of 3.81 - 3.83 for pH 5 to 9. However, as the substance was found to be readily biodegradable and no evidence of adsorption to sludge particles was given in the biodegradation tests, it can be expected that CAS 104-75-6 has only low adsorptive properties.

For CAS 6975-33-3 the pKa value of 3.76 indicates that the molecule will exist primarily in its charged anionic form under environmentally relevant conditions. Model calculations reveal that neither the charged nor the uncharged molecule has a high adsorptive potential (logKoc = 1.69; pH-corrected logKoc = 1.89 – 2.29). Similar results are derived from model calculations for CAS 102-71-6 which will just partly exist as cation in pH ranges relevant to aquatic and terrestrial organisms (pKa = 7.91). The molecule’s low adsorptive potential is indicated by the logKoc = 1 for the uncharged molecule and the pH-corrected logKoc = 0.84 – 1.27, respectively.

The following pKa values were calculated using SPARC On-Line Calculator v4.6, the logKoc values of the uncharged molecules were derived from EpiWin calculations (SRC KOCWIN v2.00) and the pH-corrected logKoc values were derived by Franco & Trapp's calculation method:

 

pKa

logKoc

pH-corrected logKoc

 

 

uncharged molecule

pH5

pH7

pH9

CAS 104-75-6

10.41

2.65

3.83

3.83

3.81

CAS 102-71-6

7.91

1

1.27

1.23

0.84

CAS 6975-33-3

3.76

1.69

2.29

1.90

1.89

 

These results demonstrate that under environmental conditions, where the three test substance components are mostly present in their charged form, the adsorption potential is low for each one of them, regardless of their charge. Since the main test substance is composed mainly of these three constituents, adsorption to soil particles is not expected for the test substance as well.