1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane

Biodegradation in water and sediment: simulation tests

Description of key information

Sediment degradation half-life [M3T]: 174 days at 12°C (OECD 308)

Key value for chemical safety assessment

Half-life in freshwater sediment:

174 d

Additional information

Sediment degradation rates were determined in a reliable study conducted according to an appropriate test method. Preliminary testing identified the potential for volatile losses from the test system, and therefore adaptation of the test guideline and customisation of the test system was required. Further modifications to the OECD TG 308 include the selection of a spiking solvent (diethylene glycol methyl ether) and method to ensure distribution of the test material mainly to the sediment phase initially.

The study was conducted under aerobic conditions at a nominal temperature of 12°C, with two aquatic freshwater sediment systems which meet the requirements of the OECD TG 308. The sediments and overlying waters were collected from Calwich Abbey, Staffordshire, UK and Emperor Lake, Chatsworth, Derbyshire, UK. The Calwich Abbey sediment system is a silt loam (9% w/w sand / 73% w/w silt / 18% w/w clay) with an organic carbon content of 5.16 %. The pH of the overlying water as 7.5 at the start of the acclimation phase. The Emperor Lake sediment system is a sandy clay loam sediment (63% w/w sand / 15% w/w silt / 22% w/w clay) with an organic carbon content of 2.15 %. The pH of the overlying water as 6.3 at the start of the acclimation phase. 

At each sampling interval, extractable radioactivity in the overlying water and sediment samples was quantified by LSC. Non-extractable ¹⁴C-residues were quantified by combustion analysis. Radioactive components in the surface waters and sediment extracts were quantified separately by reversed phase high performance liquid chromatography with radiochemical detection with fraction collection and LSC. 

The mass balance for Calwich Abbey was generally in the range 92.1 – 105.7%, and the mass balance for Emperor Lake was generally in the range 90.0 – 103.8%. A small number of samples had lower recoveries (>80%), which were thought to be due to loss of volatilised test item or volatile metabolites during incubation or sampling. 

For the Calwich Abbey samples, the mean amount of radioactivity in the surface water decreased from a maximum of 55.0% associated radioactivity (AR) at day 0 sampling point 0 to 0.7% AR at the 100 day sampling point. The mean amount of radioactivity which was extracted from sediment using THF solvent was 31.5% AR at the day 0 sampling point, increasing to a maximum of 91.8% AR at day 3 and decreasing generally to 72.7% AR at the 100 day sampling point. Non-extractable radioactivity reached a maximum mean of 9.3% AR at the 45 day sampling point.

For Emperor Lake samples, the mean amount of radioactivity in the surface water decreased from a maximum of 23.8% AR at day 0 to 1.1% AR at the 100 day sampling point. The mean amount of radioactivity which was extracted from sediment using THF solvent was 67.9% AR at day 0, increasing to a maximum of 78.2% AR at day 7 and decreasing gradually to 44.5% AR at the 100 day sampling point. Non-extractable radioactivity reached a maximum mean of 7.3% AR at the 62 day sampling point.

Cumulative radioactivity recovered in individual NaOH traps was a minor component in Calwich Abbey samples reaching a maximum mean of 3.4% AR at the 100 day sampling point. In Emperor Lake individual NaOH trap samples, cumulative radioactivity increased to a maximum of 7.6 % AR at the 100 day sampling point.

Levels of radioactivity recovered in bulk NaOH traps situated after the tube furnace increased throughout incubation and reached a maximum cumulative value of 17.0% AR in Calwich Abbey samples and 42.6% AR in Emperor Lake samples at the 100 day sampling point. The radioactivity was confirmed to be ¹⁴CO₂ after precipitation with saturated BaCl_2,following conversion through the catalytic convertor.

M3T was the only significant component present in the surface water and sediment extracts analysed by HPLC. The radioactivity present in Emperor Lake individual NaOH traps was observed as an unknown component after analysis by HPLC. The individual NaOH traps were pooled and prepared by solid phase extraction in order to be suitable for LC-MS analysis. This unknown component was later confirmed to be trimethylsilanol. The transformation products are therefore consistent with the products of abiotic degradation via hydrolysis, trimethylsilanol and methylsilanetriol. It is considered that catalytic conversion of the abiotic transformation product, trimethylsilanol, to ¹⁴CO₂ is also likely to have been the origin of the radioactivity observed in the bulk NaOH traps.

The DegT50 of M3T in the water phase for Calwich Abbey was 1.45 days and for Emperor Lake was 3.07 days. The DegT50 of M3T in the total system (sum of surface water and sediment extracts) was 174 days and 76.2 days for Calwich Abbey and Emperor Lake, respectively.

The chemical safety assessment according to REACH Annex I indicates that it is not necessary to conduct the simulation test on ultimate degradation in surface water, because the risk characterisation ratios (RCRs) for the aquatic compartment are <1, even without taking into account any degradation of the parent material via hydrolysis at the regional and continental scales.