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Environmental fate & pathways

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The registered substance dichloro(diphenyl)silane (CAS 80-10-4) hydrolyses rapidly under environmental relevant conditions (DT50 = 10 sec at pH 7 and 1.5 °C; OECD 111) to the hydrolysis products diphenylsilanediol (CAS No. 947-42-2) and hydrochloric acid (CAS No. 7647-01-0). 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 cases where 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.


The water solubility of the parent substance is negligible as the parent substance hydrolyses very rapidly in the aquatic environment (DT50 = 10 sec at pH 7 and 1.5 °C) and the hydrolysis products are not readily biodegradable (OECD 310, read across). The parent compound has a low predicted vapour pressure of 0.078 Pa (25°C, QSAR) and thus exhibits low volatility. Therefore, indirect photodegradation is not an important degradation pathway and a long-range transport via the atmosphere is not expected for this compound. Due to it’s very rapid hydrolysis also the log Kow is negligible for the parent compound. Considering the fast hydrolysis rate and the low vapour pressure, the main compartment for environmental distribution will be water.


Silanol hydrolysis product diphenylsilanediol (CAS No. 947-42-2)

The silanol hydrolysis product is characterized by a predicted water solubility of 580 mg/L (20 °C, QSAR) and a low predicted vapour pressure of 9.9E-06 Pa (25°C, QSAR), indicating low volatility. Therefore, indirect photodegradation is not an important degradation pathway. There is no evidence of any significant biodegradation. However, its potential for bioaccumulation and its affinity to soil and sediment is low based on its low, predicted log Kow of 2.0 (QSAR).


Hydrochloric acid (CAS No. 7647-01-0)

Hydrochloric acid is well characterized in the public domain. It has a high vapour pressure of 42200 hPa at 20 °C and a high solubility in water (673 g/L at 30 °C).In the environment, it is expected to distribute into the air and water. In water, it forms the aqueous solution hydrochloric acid.

In air, hydrogen chloride reacts with hydroxyl radicals into chloride free radicals and water (calculated half-life of 11 d). In water, hydrochloric acid readily dissociates into hydrated protons and chloride anions. Thus, it is ionised and neutralisation depends on the buffer capacity of the receiving water. Because hydrochloric acid is an inorganic substance, biodegradation is not a relevant degradation pathway. However, due to its high solubility and readily dissociation in water, it is not expected to accumulate in organisms. The aquatic hazard is mediated by the proton in a concentration-dependent manner (i.e. pH effect) and its potency strongly depends on the buffer capacity of the respective aquatic system. Toxicity only occurs when the buffering capacity of the receiving water is exceeded and pH values fall below pH 6. The pH in rivers and lakes fluctuates within a natural range. The natural pH range in aquatic systems is generally not expected to be perturbed to a relevant extent by anthropogenic emissions when appropriate risk control measures are in place. Variations in effect values of experimental studies can largely be explained by variations in the buffer capacity of the test media (OECD, 2002).



OECD, 2002. Hydrogen Chloride - SIDS Initial Assessment Report for SIAM 15, Boston, USA: UNEP Publications.