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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 (DMSD) and methanol rather than the parent substance.

Dimethylsilanediol: Biodegradability of dimethylsilanediol in water was evaluated on the basis of read-across to trimethylsilanol (CAS 1066-40-6) as source substance. The biodegradation rate was found to be negligible.

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. Reactors were incubated for more than one year and sacrifices were performed four times. 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 her PhD thesis, 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. There was also no evidence for a significant conversion of DMSD to CO2 under these environmentally relevant conditions.

Biodegradability of dimethylsilanediol in soil has also been subject to several earlier studies published in the scientific literature (Sabourin et al., 1996; Lehman et al., 1998; Sabourin et al., 1999). Various types of soil and duration of tests were applied; however biodegradation rate was no more than a few percent per month.

All available sources suggest that biodegradation of dimethylsilanediol in water, soil or sediment is negligible. Thus dimethylsilanediol can be regarded as persistent in the environment.

Methanol: In contrast, the second hydrolysis product methanol is readily biodegradable considering information from the OECD SIDS (OECD, 2004).

References:

Fischer-Reinhard, M. (2007). Mikrobieller Abbau von Siliconölen, Siloxanen und Silanolen?, Ph.D. Thesis

Lehmann RG, Miller JR, Collins HP (1998). Microbial degradation of dimethylsilanediol in soil. Water Air and Soil Pollution 106: 111-122

OECD (2004). SIDS Methanol.

Sabourin CL, Carpenter JC, Leib TK, Spivack JL (1996). Biodegradation of dimethylsilanediol in soils. Applied and Environmental Microbiology 62: 4352-4360

Sabourin CL, Carpenter JC, Leib TK, Spivack JL (1999). Mineralization of dimethylsilanediol by microorganisms isolated from soil. Environmental Toxicology and Chemistry 18: 1913-1919

CES - Silicones Europe (2016). Fate of silanols in various aquatic / soil environments: Investigation of dimethylsilanediol (bio)degradation in microcosms.