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Dimethoxydimethylsilane (CAS 1112-39-6) hydrolyses rapidly in contact with water (DT50 <0.6 h, pH 7, 25 °C, OECD 111, GLP), to dimethylsilanediol (CAS 1066-42-8) and methanol (CAS 67-56-1). The ECHA guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). The ECHA guidance R.16 also suggests that in case the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity. Thus, the environmental fate assessment in this dossier is based on the hydrolysis products dimethylsilanediol and methanol rather than the parent substance.

Dimethylsilanediol: Dimethylsilanediol (DMSD) is characterized by a high water solubility, a vapour pressure of 7 Pa at 25 °C (QSAR) and a low log Pow of -0.41. In consequence, DMSD has a low volatilization potential from water (Henry´s Law constant = 1.65E-4 Pa m³/mole at 12 °C). On the other hand, DMSD volatilization from soil has been observed (Lehmann & Miller, 1996; Xu and Miller 2016), which is supported by the relatively high octanol/air partition coefficient (log Koa = 6.4; Xu and Kropscott, 2012). However, considering the physical properties DMSD is expected to accumulate in water, rather than the atmosphere. Experimental adsorption coefficients indicate that DMSD has a very weak affinity to soil particles. DMSD may be subject to degradation by hydroxyl radicals in air (half-life 20 days, Tuazon et al. 2000) or in the presence of nitrate/nitrite in water (Buch et al. 1984). Biodegradation was investigated extensively (e.g. Sabourin et al., 1996; Lehman et al., 1998, Fischer-Reinhard 2007) but only low or no biodegradation was observed. Owing to the low log Pow of -0.41, bioaccumulation is not relevant for DMSD.

Methanol: Like DMSD, methanol is water miscible at 20 °C, exhibits a low log Pow (-0.82 to -0.64) but a considerably higher vapour pressure (12790 Pa at 20 °C) (values taken from SIDS Initial Assessment Report for Methanol, OECD, 2004). In consequence, volatility of methanol from water is higher than for DMSD (Henry´s Law constant = 0.3827 Pa m³/mole at 12 °C). In contrast to DMSD, methanol is considered to be readily biodegradable (OECD, 2004). Experimental BCF values of < 10 in fish species, including Cyprinus carpio and Leuciscus idus, have been measured for methanol indicating negligible potential for bioaccumulation.