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Reference
Endpoint:
adsorption / desorption
Remarks:
adsorption
Type of information:
(Q)SAR
Adequacy of study:
key study
Study period:
2018
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
Please see attached information!
Qualifier:
equivalent or similar to
Guideline:
other: ECHA guidance R.6 QSARs and grouping of chemicals
Version / remarks:
2008
Qualifier:
according to
Guideline:
other: ECHA Practical guide: How to use and report (Q)SARs
Version / remarks:
Version 3.1 – July 2016
Principles of method if other than guideline:
Calculation method determining the Koc for monovalent organic bases of pKa between 2 to >12
GLP compliance:
no
Type of method:
other: calculation of log Koc for monovalent organic base
Media:
soil
Specific details on test material used for the study:
The following parameters were used as input for the model:
1. pKa 11.0 as experimentally determined in the reliable key study for the temperature of 20 °C (Damsgaaard-Sorensen and Unmack, 1935)
2. log Pn (tributylamine): 4.16, Pn being the partitioning coefficient n-octanol-water for the neutral compound.

Experimentally determined values for log Kow are only available for pH 9.0 (2.4) and pH 9.35 (3.34). For calculation of Koc according to Franco and Trapp (2008) log Kow for the neutral compound (log Pn) is required. Therefore, log Pn was calculated using ChemAxon Software. The documentation for the calculator plug-in is freely available . Calculator Plugin version 17.27.0 (23rd October 2017) was used. Definition logP as given by ChemAxon: “The partition coefficient is a ratio of concentrations of an un-ionized compound in the two phases of immiscible solvents (water and n-octanol) at equilibrium. logP is the 10-base logarithmic measure of the coefficient.” Result: log Pn (tributylamine): 4.16

To corroborate relevance of the ChemAxon tool, using the same software (ChemAxon) log D at pH 9 was calculated to check the fit with available experimental data :
log D (pH 9, ChemAxon): 2.34
log D being the apparent log Kow of an ionisable compound at the given pH.
Compared to the experimentally determined value of log D (pH 9.0) 2.4 this is very well in agreement and corroborates reliability of the ChemAxon calculation tool with regard to this compound.

- documentation log P calculator https://docs.chemaxon.com/display/docs/logP+Plugin
- documentation log D calculator: https://docs.chemaxon.com/display/docs/logD+Plugin

Test temperature:
25 °C
Details on study design: HPLC method:
QSAR, not applicable
Analytical monitoring:
not required
Remarks:
QSAR, not applicable
Details on sampling:
QSAR, not applicable
Details on matrix:
QSAR, not applicable
Details on test conditions:
As advised by the authors of the QSAR model, the optimum pH of 4.5 was used for estimation because this produced the best fit to the experimental data - for details see attached QMRF/QPRF documentation.
Computational methods:
The model by Franco and Trapp (2008) delivers an estimate for the sorption of organic monovalent bases at a pH characteristic for the “average soil”, considering explicitly the contribution of ionic interaction to overall sorption when the cationic fraction of the organic base binds to the net negatively charged soil surface. Details are given in the attached QMRF/QPRF documentation.
Key result
Type:
log Koc
Value:
4.65 dimensionless
pH:
4.5
Temp.:
20 °C
Matrix:
Soil
Remarks on result:
other: Optimum pH of 4.5 refers to the immediate surface pH of negatively charged organic colloids rather than to the bulk pH of soil, which usually is considerably higher due to the Nernst potential of H+ ions.
Type:
Koc
Value:
44 781 L/kg
pH:
4.5
Temp.:
20 °C
Matrix:
Soil
Remarks on result:
other: Optimum pH of 4.5 refers to the immediate surface pH of negatively charged organic colloids rather than to the bulk pH of soil, which usually is considerably higher due to the Nernst potential of H+ ions.
Details on results (HPLC method):
QSAR, not applicable
Adsorption and desorption constants:
QSAR, not applicable
Recovery of test material:
QSAR, not applicable
Concentration of test substance at end of adsorption equilibration period:
QSAR, not applicable
Concentration of test substance at end of desorption equilibration period:
QSAR, not applicable
Details on results (Batch equilibrium method):
QSAR, not applicable
Statistics:
For the applied model (equation 24 in the publication of Franco and Trapp (2008)), the best fit was obtained at soil pH 4.5 (see attached QMRF/QPRF documentation for mechanistic interpretation as well as details on statistical evaluation of model results):
- coefficient of determination (r^2) training set: 0.76 (n= 43)
- coefficient of determination (r^2) validation set: 0.55 (n= 22)
- RMSD training set: 0.38 (n= 43)
- RMSD validation set: 0.51 (n=22)

One sufficiently similar compound with relevant experimental data could be determined (based on OECD QSAR Toolbox 4.1), which was also in the test set used by Franco and Trapp (2008; supplementary data file):

Name: Butylamine

CAS: 109-73-9

Test guideline: OECD 106, batch equilibrium test

Source: von Oepen et al, 1991

Result:

Neutral (pH 6.7) agricultural soil (Alfisol): Koc = 105 (log Koc: 2.02)

Lake Constance sediment (pH 7.1; sublimnic soil): Koc = 107 (log Koc: 2.03)

Calculation according to Equation (7) as given in QMRF&QPRF (equ. 24 of the publication by Franco and Trapp, 2008):

Input data:

-       pKa ChemAxon: 10.21

-       log Pn ChemAxon: 0.698

Model output:

-       log Kow (pH 7) for calculation of “f”: -2.33

-       log D (pH 7) ChemAxon: -2.112 [control calculation, only – no model input)

-       log Koc from equation (7), at optimum pH (4.5): 2.13 (Koc: 136)

Conclusion:

The calculated value for log Koc (2.13) is in very good agreement with the experimental values determined for agricultural soil (Alfilsol, pH 6.7; log Koc 2.02) and lake sediment (pH 7.1; log Koc 2.03). The similar compound butylamine therefore corroborates data calculated for tributylamine.

Reference:

von Oepen, B.; Kördel, W.; Klein, W. (1991)

Sorption of nonpolar and polar compounds to soils: processes, measurements and experience with the applicability of the modified OECD-Guideline 106

Chemosphere, 22, 285-304

Validity criteria fulfilled:
not applicable
Conclusions:
The relevant log Koc for soils was calculated for tributylamine using a method specifically applicable to monovalent bases, considering as input values pKa and the log Kow value for the neutral molecule: log Koc = 4.65.
Executive summary:

The relevant log Koc for soils was calculated for the submission substance tributylamine using a method published by Franco and Trapp (2008) specifically applicable to monovalent bases, considering as input values pKa and the log Kow value for the neutral molecule (log Pn). Calculation was performed based on a MS Excel spreadsheet implementation of the model.

The submission substance is a monovalent base with a high pKa (11.0 at 20°C). As such, it will largely be dissociated at environmentally relevant pH (i.e. positively charged). Other than for neutral compounds where adsorption is largely governed by lipophilicity, in case of positively charged compounds (cations) sorption is caused by both, a) the sorption of the non-charged hydrocarbon part of the molecule mainly due to hydrophobic interaction as well as b) the ionic interaction of the positively charged residue with negatively charged residues of the soil colloidal fraction; the latter consisting of organic matter as well as inorganic clay minerals and usually displaying a net negative surface charge.

The strength and relevance of the ionic interaction will be dependent on the fraction of the ionisable compound which is positively charged at a given pH relevant for soil. This fraction can be estimated based on the Henderson-Hasselbach equation from soil pH and pKa of the monovalent base.

As such, the basic assumption of the model is that for ionisable compounds, sorption of the neutral fraction is governed by Koc_n, while sorption of the ionized fraction is governed by Koc_i. As such, overall Koc can be estimated as:

Koc = Psi_n * Koc_n + Psi_i * Koc_i                                                                                    

Psi_n = neutral fraction of the compound at prevailing pH

Psi_i = ionized fraction of the compound at prevailing pH

Based on this background, the model for Koc estimation was developed and optimized by fitting the neutral and ionic interactions (Koc_n; Koc_i) separately or in combination (direct regression) to the experimental data for bases using regression procedures.

The best fit was obtained at the pH of 4.5 (optimum pH). According to the authors' interpretation this refers to the immediate surface pH of negatively charged organic colloids rather than to the bulk pH of soil, which usually is considerably higher due to the Nernst potential of H+ ions. Therefore, the model delivers an estimate for the sorption of organic monovalent bases at a pH characteristic for the “average soil” (average bulk soil pH from experimental data set: ca. pH 6).

The submission substance is in the applicability domain of the model. Results for the similar compound butylamine with experimental data from batch equilibrium test confirm the reliability of results obtained for the submission substance (exp. data butylamine for soil and sediment, respectively: log Koc 2.02 and 2.03; calculated value for log Koc: 2.13).

For the submission substance, the following results were obtained, which are reliable and adequate for environmental risk assessment:

log Koc (20 °C): 4.65

Koc (20 °C): 44781 L/kg.

Description of key information

The relevant log Koc for soils was calculated for tributylamine using a method specifically applicable to monovalent bases, considering as input values pKa and the log Kow-value for the neutral molecule: log Koc = 4.65.

Key value for chemical safety assessment

Koc at 20 °C:
44 781

Additional information

The relevant log Koc for soils was calculated for the submission substance tributylamine using a method published by Franco and Trapp (2008) specifically applicable to monovalent bases, considering as input values pKa and the log Kow-value for the neutral molecule (log Pn). Calculation was performed based on a MS Excel spreadsheet implementation of the model.

The submission substance is a monovalent base with a high pKa (11.0 at 20°C). As such, it will largely be dissociated at environmentally relevant pH (i.e. positively charged). Other than for neutral compounds where adsorption is largely governed by lipophilicity, in case of positively charged compounds (cations) sorption is caused by both, a) the sorption of the non-charged hydro-carbon part of the molecule mainly due to hydrophobic interaction as well as b) the ionic interaction of the positively charged residue with negatively charged residues of the soil colloidal fraction; the latter consisting of organic matter as well as inorganic clay minerals and usually displaying a net negative surface charge.

The strength and relevance of the ionic interaction part will be dependent on the fraction of the ionisable compound which is positively charged at a given pH relevant for soil. This fraction can be estimated based on the Henderson-Hasselbach equation from soil pH and pKa of the monovalent base.

As such, the basic assumption of the model is that for ionisable compounds, sorption of the neutral fraction is governed by Koc_n, while sorption of the ionized fraction is governed by Koc_i. Therefore, overall Koc can be estimated as:

Koc = Psi_n * Koc_n + Psi_i * Koc_i                                                                                    

Psi_n = neutral fraction of the compound at prevailing pH

Psi_i = ionized fraction of the compound at prevailing pH

Based on this background, the model for Koc estimation was developed and optimized by fitting the neutral and ionic interaction part (Koc_n; Koc_i) separately or in combination (direct regression) to the experimental data for bases using regression procedures.

The best fit was obtained at the pH of 4.5 (optimum pH). According to the authors' interpretation this refers to the immediate surface pH of negatively charged organic colloids rather than to the bulk pH of soil, which usually is considerably higher due to the Nernst potential of H+ ions. Therefore, the model delivers an estimate for the sorption of organic monovalent bases at a pH characteristic for the “average soil” (average bulk soil pH from experimental data set: ca. pH 6).

The submission substance is in the applicability domain of the model. Results for the similar compound butylamine with experimental data from batch equilibrium test confirm the reliability of results obtained for the submission substance (exp. data butylamine for soil and sediment, respectively: log Koc 2.02 and 2.03; calculated value for log Koc: 2.13).

For the submission substance, the following results were obtained, which are reliable and adequate for environmental risk assessment:

log Koc (20 °C): 4.65

Koc (20 °C): 44781 L/kg.

Supporting information is available from accepted models for KOC estimation implemented in KOCWIN v2.00 (EPI Suite v4.11). Calculation is possible via two methods, based on log Kow and based on first oder molecular conntectivity index (MCI). However, Koc is calculated for the neutral molecule fraction, only. As such, ionic interaction of cationic tributylammonium with the prevalent negative soil net charge is not considered. The following results were obtained by the two different models:

Based on experimentally determined log Kow of 3.34 (pH 9.35): log Koc = 2.71;

Based on the MCI-method: log Koc = 3.27.

In conclusion, it is obvious that ionic interaction between cationic tributylammonium and the prevalent negative soil net charge significantly contributes to the organic carbon adsorption coefficient Koc. Therefore, log Koc of 4.65 as calculated according to the method of Franco and Trapp (2008) will be used for environmental risk assessment.

[LogKoc: 4.65]