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Adsorption / desorption

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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
(Q)SAR
Adequacy of study:
key study
Study period:
2015
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:
QSAR prediction: migrated from IUCLID 5.6
Qualifier:
no guideline followed
Principles of method if other than guideline:
The batch equilibrium method (OECD Method 106) and HPLC estimation method (OECD Method 121) were concluded to be either unfeasible or invalid, due to the nature of this test item. Limitations of the stability of the test item in water at an environmentally relevant pH and potential non-partitioning interactions prevent experimental determination of adsorption coefficient by either of these methods.

The water solubility of 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione was determined to be approximately 167 mg/L, which results in a moderately acidic solution, and rapid degradation/transformation was observed at an approximately neutral pH during hydrolysis testing. Absolute quantification of a degradation/transformation rate was not possible, however, due to the observed second order transformation kinetics. The magnitude of degradation/transformation observed precludes meaningful monitoring and quantification of the test item in a soil and solution mixture test system over the time period required by the OECD Guideline 106.

Based on available dissociation constant data for the test item, it was determined that the test item would be present predominantly as an anionic form over the environmentally relevant pH range (pH 5.5 to 7.5). The absence of suitable anionic reference compounds with which to generate a valid calibration curve, as well as potential non-partitioning interactions originating from the presence of residual anionic silanol groups on the column stationary phase, preclude the use of the HPLC estimation method.

The batch equilibrium method guideline, OECD Method 106, specifies that where it is not possible to undertake experimental work, an alternative estimation method should be employed. Such estimations are typically derived from the partition coefficient of the test item, directly correlating partitioning between an organic phase (n octanol) and an aqueous phase (water) with the partitioning between the organic carbon content of soil and the interstitial soil water (i.e. a Quantitative Structure Activity Relationship, or QSAR). Estimation of adsorption coefficient was possible using the Quantitative Structure Activity Relationship (QSAR) method by taking into account the anionic form of the test item present over the environmentally relevant pH range.
Computational methods:
The batch equilibrium method guideline, OECD Method 106, specifies that where it is not possible to undertake experimental work, an alternative estimation method should be employed. Such estimations are typically derived from the partition coefficient of the test item, directly correlating partitioning between an organic phase (n-octanol) and an aqueous phase (water) with the partitioning between the organic carbon content of soil and the interstitial soil water (i.e. a Quantitative Structure Activity Relationship, or QSAR).

The adsorption coefficient, therefore, was subsequently estimated using the specialized predictive estimation software KOCWIN, version 2.00, September 2010 (©2000 to 2008, U.S. EPA). Of the two methods employed by this software, only the Quantitative Structure Activity Relationship (QSAR) method was considered suitable for use. It was not possible to input an anionic form into the alternative molecular connectivity index (MCI) method.

Estimation of adsorption coefficient was possible using the Quantitative Structure Activity Relationship (QSAR) method by taking into account the anionic form of the test item present over the environmentally relevant pH range (pH 5.5 to 7.5). Although a partition coefficient value, estimated from the saturation concentration of the test item in water and n-octanol, is available for 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione, it was considered more appropriate in this case to consistently use computer modeled partition coefficient values for all forms, derived from the integrated KOWWIN module of EPI Suite, for the estimation of an adsorption coefficient. By inputting the structures as an equivalent sodium salt form, the initial partition coefficient estimate recognized the structure as an anion, correcting the initial partition coefficient estimate for the increased hydrophilic character of the structure, which was then also reflected in the final adsorption coefficient estimation value. It is important to note that the SMILES representation of the sodium salt as a covalently bound substance is required by the EPI Suite software, but does not accurately reflect the salt structure in aqueous solution. To account for the different estimated properties of substance when the negative charge is on the sulfur or nitrogen, the QSAR was performed on the covalent representation of each.

Note, that on inputting the Na-S covalent approximation of the salt, the software automatically converts the relevant thiadiazole ring structure to an aromatic, delocalized system. Although a hydrogen is represented attached to the sulfur within this functional group, on evaluation of the associated molecular formula, it is confirmed that no such hydrogen is considered during the calculation procedure.
Type:
log Koc
Value:
2.71
Remarks on result:
other: Molecular form
Type:
log Koc
Value:
1.03 - 1.97
Remarks on result:
other: Single anionic charge
Type:
log Koc
Value:
-4.55 - 1.24
Remarks on result:
other: Double anionic charge

QSAR Estimations of Adsorption Coefficient

Form

Comments

Estimated Adsorption Coefficient (log10Koc)

Molecular form

Theoretically present under moderately acidic pH conditions, dominating (>50%) at pH values less than 6.

2.71

Single anionic charge

Dominant species expected over a majority of the environmentally relevant pH range of 5.5 to 7.5.

1.03 to 1.97

Double anionic charge

-4.55 to 1.24

 

On evaluation of QSAR estimated adsorption coefficient values for this substance, and considering the degree of dissociation within the environmentally relevant pH range (pH 5.5 to 7.5) and the expected transformations (i.e., tautomerization and disulfide cleavage), the overall conclusion was that the test item is expected to display predominantly high to very high mobility in soils. This conclusion is based on the mobility classification system according to McCallet al.(1980) 

Adsorption Coefficient Mobility Classification (McCallet al., 1980) *

Range of Koc

Range of Log Koc

Mobility Class

0 – 50

Up to 1.7

Very High

50 – 150

1.7 to 2.2

High

150 – 500

2.2 to 2.7

Medium

500 – 2000

2.7 to 3.3

Low

2000 – 5000

3.3 to 3.7

Slightly

> 5000

>3.7

Immobile

 

*McCall, P.J., R.L. Swann, D.A. Laskowski, S.M. Unger, S.A. Vrona and H.J. Dishburger (1980) Estimation of Chemical Mobility in Soil from Liquid Chromatography Retention Times. Bull. Environ. Contam. Toxicol. 24:190-195.

Validity criteria fulfilled:
yes
Conclusions:
Limitations of the stability of the test item in water at an environmentally relevant pH and potential non-partitioning interactions prevented experimental determination of adsorption coefficient by either the batch equilibrium method (OECD Method 106) or the HPLC estimation method (OECD Method 121), respectively. Estimation of adsorption coefficient, taking into account the predominant anionic forms of the test item present within the environmentally relevant pH range (pH 5.5 to 7.5), was possible using the Quantitative Structure Activity Relationship (QSAR) method. The resulting estimated adsorption coefficient (log10 Koc) values for the predominant anionic forms ranged from -4.55 to 1.97. Based on the estimated adsorption coefficients for 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione, and considering the degree of dissociation expected within the environmentally relevant pH range and the expected transformations (i.e., tautomerization and disulfide cleavage), the overall conclusion is 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione is expected to display high to very high mobility in soils.
Executive summary:

Using KOCWIN, version 2.00, September 2010 (©2000 to 2008, U.S. EPA), and taking into account the predominant anionic forms expected to be present within the environmentally relevant pH range (pH 5.5 to 7.5), the estimated adsorption coefficient (log Koc) values for 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione ranges from -4.55 to 1.97. 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione is expected to display high to very high mobility in soils.

Description of key information

5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione is expected to display high to very high mobility in soils (Harlan Laboratories Ltd, 2015a).

Key value for chemical safety assessment

Koc at 20 °C:
93.3

Additional information

Key Study:

Using KOCWIN, version 2.00, September 2010 (©2000 to 2008, U.S. EPA), and taking into account the predominant anionic forms expected to be present within the environmentally relevant pH range (pH 5.5 to 7.5), the estimated adsorption coefficient (log Koc) values for 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione ranges from -4.55 to 1.97. 5,5'-Dithiodi-1,3,4-thiadiazole-2(3H)-thione is expected to display high to very high mobility in soils (Harlan Laboratories Ltd, 2015a).

[LogKoc: 1.97]