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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: >7.5 days at pH 5, 6 days at pH 7 and 11 min at pH 9 and 25°C (Modified OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:
6 d
at the temperature of:
25 °C

Additional information

Hydrolysis half-times of >7.5 days at pH 5, 6 days at pH 7 and 11 minutes at pH 9 and 25°C were determined for the submission substance (F-D3) in a modified study conducted in accordance with OECD 111. Extra co-solvent (20% acetonitrile) was added because the substance has a low solubility in water (1.3E-06 mg/l at 20°C, predicted). A higher concentration of co-solvent was used to enhance solubility and minimise losses by sorption and volatilisation. The hydrolysis half-lives of a reference substance, hexamethylcyclotrisiloxane (D3, CAS No: 541-05-9) for which a standard (100% aqueous buffer) OECD 111 study is available were also measured. The half-lives for D3 (reference substance) under the modified OECD 111 were 2 h at pH 5, 5.4 h at pH 7 and 1.1 minutes at pH 9 and 25°C. Under standard OECD 111 study conditions, the half-lives of D3 were 2 minutes at pH 5, 23 minutes at pH 7 and 0.44 minutes at pH 9 and 25°C. The results for the half-lives of D3 under standard conditions suggest that the half-lives for the submission substance, F-D3 under standard conditions could be shorter than those measured in the non-standard study. However, the low solubility and high adsorption capacity of F-D3 are expected to limit the hydrolysis observed in practise. The measured half-times are used for assessment purposes.

 

The half-times reported are for the hydrolysis (ring-opening) of the parent substance. The initial product of the hydrolysis of F-D3 is 1,5-dihydroxy-1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)trisiloxane (the trimerdiol), which is formed by ring-opening of the parent substance. F-D3 did not show irreversible first-order behaviour. Instead, the kinetic profiles showed initial decline followed by attainment of a plateau. This was shown to be consistent with reversible hydrolytic ring-opening of the cyclic trimer followed by slower hydrolysis of the trimerdiol. The final hydrolysis product was demonstrated to be 3,3,3-trifluoropropylmethysilanediol.

Stoichiometric amounts of this final hydrolysis product were observed by LC-MS after 15 days at pH 5, 23 days at pH 7 and 6 days at pH 9, thus confirming hydrolysis of F-D3 rather than other possible loss mechanisms (such as volatilisation).

 

The attached figure shows the proposed hydrolysis reaction mechanism (water omitted) where A is F-D3 (parent), B is 1,3,5-tris[(3,3,3-trifluroproyl)methyl]trisiloxane-1,5-diol (the trimerdiol), C is 1,3-tris[(3,3,3-trifluroproyl)methyl]disiloxane-1,3-diol (the dimerdiol) and D and E are methyl(3,3,3-trifluoropropyl)silanediol (the silanol hydrolysis product). Based on numerous studies of hydrolytic degradation of cyclic and linear permethylsiloxanes in water, soil and sediment under environmentally relevant conditions of pH and temperature, siloxanes degrade by hydrolytic cleavage of the Si-O bond (Xu 1999, Lehmann et al 1994, Durham 2005, Xu 2009). However, the Si-C bond is not susceptible to abiotic hydrolysis. Therefore, the mechanism presented in Figure 4.1 is the only plausible one to explain the observed kinetic profiles from the study; it is consistent with the established chemistries of the silicon moieties. The observation that the rate of disappearance of the parent substance was much greater at the beginning and then slowed as the reaction proceeded, suggested that the hydrolytic ring opening was sufficiently faster than the hydrolysis of the intermediate trimerdiol such that the effect of the ring-chain hydrolysis-condensation equilibrium was observed as attainment of a (pseudo)plateau in the kinetic profiles. Although direct evidence of the formation of the proposed hydrolysis intermediates was not obtained, stoichiometrically quantitative formation of the final silanol product supported this scheme.

 

Figure 4.1 Proposed hydrolysis reaction mechanism

Formation of the cyclic tetramer, F-D4 (CAS# 429-67-4, EC 207-060-9, 2,4,6,8-tetramethyl-2,4,6,8-tetrakis(3,3,3-trifluoropropyl)cyclotetrasiloxane) was also demonstrated during the study. Up to 5% at pH 5, 8% at pH 7 and 7% at pH 9 F-D4 was observed by GC-FID. F-D4 can be formed by condensation reactions of the intermediate and final silanol hydrolysis products. Higher concentrations of the cyclic tetramer were observed during method development when using higher initial test substance concentration.

 

References

Xu S, Fate of Cyclic Methylsiloxanes in Soils. 1. The Degradation Pathway.Environ. Sci. Technol.1999, 33, 603-608.

Lehmann RG, Varaprath S, Frye CL, Degradation of Silicon Polymers in soil.Environ. Toxicol. Chem. 1994, 13, 1061-1064.

Durham J, Hydrolysis of Octamethylcyclotetrasiloxane (D4). Final report to the Silicones Environmental, Health and Safety Council (SEHSC) on HES Study No. 10000; Dow Corning Corporation: Auburn, MI, 2005.

Xu S, Anaerobic Transformation of Octamethylcyclotetrasiloxane (14C-D4) in Aquatic Sediment Systems. Final report to the Centre Européen des Silicones (CES) on HES Study No. 11101; Dow Corning Corporation: Auburn, MI, 2009.