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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Reference
Endpoint:
biodegradation in water and sediment: simulation testing, other
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
Please refer to the justification for grouping of substances provided in IUCLID Section 13.
Reason / purpose for cross-reference:
read-across source
Remarks on result:
other: no degradation observed
Transformation products:
no
Details on results:
For each condition, the highest amount of radioactivity is recovered in the liquid phase, even at the end of the incubation. For the Aerobic Surface Water condition, the radioactivity recovered in the liquid phase does not significantly decrease with time. This can be expected since little microbial activity is assumed to be present in these reactors. For all other conditions, the radioactivity recovered from the liquid phase slowly decreases. Only under the Ferri-Reducing and Methanogenic conditions, the decrease seems to be limited to the first period of 90 days.
At the end of the 13-18 months of incubation, for all conditions, there is a very low but significant cumulated release of 14C marked molecules in the gas samples. The detected non-condensable radioactivity can be attributed to volatile 14C-DMSD that was not retained by the cold trap. Indeed, even at -70 °C, DMSD has a positive vapor pressure (unknown value), and some molecule can escape.
At this stage of the incubation, no clear trend emerges yet from the evolution of radioactivity recovered from the solid phase with time; neither for the total radioactivity nor for the ratio between 14C-DMSD by-products extractable and not extractable by THF.
The HPLC analysis aimed at detecting the presence of different 14C-metabolites of 14C-DMSD. However the results don’t show any evidence for the presence of any metabolite other than 14C-DMSD in the liquid phase, in the THF extract or in the acid extract of the residual solids.
Conclusions:
There is no evidence for any significant degradation or conversion of the source substance Dimethylsilanediol (DMSD) in aquatic or terrestrial conditions representative for a diversity of environmental conditions. As explained in the read across justification provided in section 13, no biodegradation in water or soil is expected for the target substance.

Description of key information

Key value for chemical safety assessment

Additional information

Dimethoxydimethylsilane (CAS 1112-39-6) hydrolyses rapidly in contact with water (DT50: <0.6 h), to dimethylsilanediol (CAS 1066-42-8) and methanol (CAS 67-56-1). Thus, the environmental fate assessment is based on the hydrolysis products i.e. dimethylsilanediol and methanol rather than the parent substance.

Dimethylsilanediol: In her PhD thesis, M. Fischer-Reinhard (2007) investigated the biodegradation of DMSD by microbial consortia (activated sludge from the wastewater treatment plant of a silicone oil producing plant, and a mix of several activated and digested industrial and municipal wastewater sludges) under aerobic, anoxic, sulfato-reducing and methanogenic conditions. While the experimental set-up and duration was not optimal to detect slow DMSD degradation rates, there was also no evidence for a significant conversion of DMSD to CO2 under these environmentally relevant conditions.

The biodegradation of DMSD (dimethylsilanediol) was further investigated under any of various water environmental matrices in contact with soil or sediments, in the presence of microorganisms that have high chance to degrade DMSD (CES, 2016). Different sites known or expected to be contaminated by silanols have been selected in order to maximize the probability to obtain samples with various organisms previously exposed and potentially adapted to silanols. The four types of matrices sampled (surface water, sludge, leachate and soil) from each site were mixed together to get four groups with an initial microbial diversity as broad as possible. Eight different conditions that cover a range of environmentally relevant redox conditions were tested in duplicate.

The experimental set-up and procedure was designed to ensure to close the 14C balance as much as possible, by minimizing and checking the 14C losses in order to unequivocally interpret the results. The reactors were incubated for more than 1 year and sacrifices were performed four times: directly after starting the incubation to characterize the initial situation (d0), after 3 months of incubation (d87-94), after 6 months of incubation (d180-196) and after a total incubation of 13 months to 18 months (d398-557) depending on the tested condition.

As a result of this extensive study, there was no evidence for any significant degradation or conversion of DMSD in any of the aquatic or soil conditions representative for a diversity of environmental conditions.

In consequence, there is no evidence for significant DMSD biodegradation in the aquatic or sediment compartment.

Methanol: The other hydrolysis product methanol is readily biodegradable based on information from the OECD SIDS (OECD, 2004).

In view of the overall evidence on the biodegradation of both hydrolysis products it is not necessary to conduct further simulation tests on ultimate degradation in surface water and the sediment, or to identify degradation products.

An exposure assessment was performed for the substance and it was clearly shown that the risk characterisation ratios (RCRs) for the aquatic and sediment compartment, based on the assumption that the silanol hydrolysis product is not biodegradable, are well below 1.