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Environmental fate & pathways

Adsorption / desorption

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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
1. HYPOTHESIS FOR THE ANALOGUE APPROACH
The source substance is the racemic form of the target substance d-tetramethrin. Tetramethrin does consist of 50% d- and l-form, and both do exist as cis and trans isomers. Hence, the target substance does consist of d-cis-tetramethrin, d-trans-tetramethrin, l-cis-tetramethrin and l-trans-tetramethrin, whereas the target substance only contains the first two (i.e. d-cis-tetramethrin, d-trans-tetramethrin).
Both, the source as well as the target substance, do have a cis/trans ratio of approximately 1/4 and obviously do share same molecular formula and mass, functional groups and other properties. Thus, the source substance by definition does contain ~50% of the d-form and the l-form is not expected to be significantly different with respect to its adsorption/desorption properties. Adsorption/desorption behaviour of the l-form is expected being identical to the d-form, that makes up half of the substance tested (racemic form) and thus, the results from the racemic form is expected being not different from that of the d-tetramethrin. Accordingly, data do indicate that racemic tetramethrin and thus also d-tetramethrin is immobile with a high adsorption to soil and results available are sufficient for hazard and risk assessment.
2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
The d-tetramethrin with a purity > 80 % does contain its corresponding l-form as an impurity in the range of < 7%, other impurities from the manufacturing process are individually below 1% (w/w) each. The source substance that has been tested was having a purity of approx. 98% as tetramethrin with a cis/trans ratio of 1/4 and a d-form/l-form ratio of ~50/50.
3. ANALOGUE APPROACH JUSTIFICATION
Source and Target substance do share identical structure and molecular weight, only differentiating by the fact that the source substance is a racemic mixture, whereas the target substance represents almost pure d-form. Thus, behaviour regarding adsorption/desorption is expected not being affected significantly by stereochemistry, and being fully comparable, thus justifying using available data on adsorption/desorption properties on the racemic form for supportive read-across to the d-enantiomer. Even difference seen between the cis and trans isomers have been rather low and thus no significant differences for enantiomers are expected.
4. DATA MATRIX
Composition comparison
D-tetramethrin (target) tetramethrin (source)
D-trans tetramethrin 70 - 80% 40 – 50%
D-cis tetramethrin 10 - 20% 7 – 11 %
L-trans tetramethrin 0 – 5% 35 – 40%
L-cis tetramethrin 0 – 2% 7 – 11%
Reason / purpose:
read-across source
Sample No.:
#1
Type:
log Koc
Value:
3.3 dimensionless
pH:
6.7
Temp.:
25 °C
Sample No.:
#2
Type:
log Koc
Value:
3.4 dimensionless
pH:
6.7
Temp.:
25 °C
Conclusions:
Tetramethrin is immobile and remains preferably in soil, supported by a Koc determined being 2045 (log Koc 3.3) and 2754 (log Koc 3.4) at 25 °C and pH 6.7. The two values were attributed to cis- and trans-isomers, but d- and l-enantiomers could not be differentiated, in that the tetramethrin data are considered representative for d-tetramethrin too.

Description of key information

The determination of the adsorption coefficient (Koc) of TETRAMETHRIN was performed according to the OECD Guideline for the Testing of Chemicals, No. 121, Estimation of the Adsorption Coefficient (Koc) on Soil and Sewage Sludge using High Performance Liquid Chromatography (HPLC).

The log of the adsorption coefficient (Koc) of TETRAMETHRIN was estimated to be

1. Peak:

log Koc = 3.3 which is equal to

a Koc value of 2045 (1. Peak)

and

log Koc = 3.4 which is equal to

a Koc value of 2754 (2. Peak),

using the HPLC method. This values indicates that tetramethrin is immobile and remains preferably in soil.

Key value for chemical safety assessment

Koc at 20 °C:
3.3

Additional information