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Environmental fate & pathways

Biodegradation in soil

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Reference
Endpoint:
biodegradation in soil: simulation testing
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
other:
Transformation products:
yes
No.:
#1
No.:
#2
No.:
#3
Details on transformation products:
Degradation of the registration substance is expected to be principally via abiotic transformation under aqueous conditions (such as in a degradation simulation study). Therefore, the transformation products expected in the environment are those identified in the abiotic degradation study (reported in Section 5.1.2 of the IUCLID). Environmental hazard assessment is based on the properties of the hydrolysis products.

Description of key information

Key value for chemical safety assessment

Additional information

Testing is waived on the basis that the chemical safety assessment according to Annex I indicates that it is not necessary to conduct the soil simulation test. Simulation test (soil) is not considered necessary because:

 

· The constituents of the submission substance are expected to hydrolyse rapidly and the silanol hydrolysis products have low log Kow values and, therefore, exposure of the soil compartment is expected to be low. 

· The risk characterisation ratios (RCRs) for the soil compartment, based on the assumption that the silanol hydrolysis products are not biodegradable, are <1. 

· The non-silanol hydrolysis products, ethanol and methanol are readily biodegradable.

Soil simulation studies, which are considered reliable, are available for dimethylsilanediol and methylsilanetriol (Silanol HP-X) (Lehmann et al. (1994), Lehmann et al. (1998), Sabourin et al. (1996a and b)).

These studies were conducted according to generally accepted scientific principles, and are used as weight of evidence that degradation in soil does occur but not to a significant extent. Initial rates of soil degradation from these studies are considered to be the data of most use, and most representative of field conditions, as rates tended to slow over the course of the experiments. Initial rates of degradation ranged from 0.16 to 2.1% per month. The decrease in rates of degradation over the course of the experiments may be due to some form of binding of organosilane compounds to soil, as indicated by HPLC analysis of HCl-extractable fractions from the soil in some of the studies.

It is considered relevant to read-across the soil degradation study for dimethylsilanediol to methylsilanetriol because both silanol hydrolysis products are structural analogues. Dimethylsilanediol contained two hydroxy groups and two methyl (alkyl) groups attached to silicon while methylsilanetriol has three hydroxy groups and one methyl (alkyl) group attached to silicon. Both dimethylsilanetriol and methylsilanetriol contained the same function groups, Si-OH. Both are expected to have similar physicochemical properties such as high water solubility, low log Kow and low vapour pressure.