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

Toxicity to soil macroorganisms except arthropods

Additional information

A 7 day LC₅₀value of >1000 mg/kg has been determined for the effects of the test substance on the mortality of the earthworm E. foetida. The test material has a Henry's Law Constant higher than 100 Pa m³mol^-1, and no precautionary measures were taken to prevent the material evaporating from the test medium during test media preparation and exposure, and no analysis took place. It cannot thus be excluded that most of the test material evaporated out of the test media by day 7 when the soil was inspected for earthworms survival and the reliability of the study cannot be established. There are no reliable data describing the long-term toxicity of the registered substance to soil macroorganisms.

Stability study using the related substance octamethyltrisiloxane (L3) under OECD TG 222 conditions without test organisms:

A stability/recovery test was conducted in preparation for terrestrial ecotoxicology studies with the related substance L3. M3T and L3 are members of the Siloxane Category of compounds and have high vapour pressures (210 Pa and 530 Pa), high log Kow (8.2 and 6.60), high log Koc (5.3 and 4.3) and low water solubility (1.9E-03 mg/l and 0.034 mg/l). In addition, the substances have a slow hydrolysis rate relative to the time-scale of ecotoxicity testing (t_1/2= 630 h and 329 h at pH 7 and 25°C). In the context of terrestrial toxicity, both L3 and M3T are expected to have similar stability in soil and are likely to remain in the soil as the parent substance during the terrestrial toxicity studies. Therefore, it is considered valid to read-across the results of the soil stability study with L3.

The study demonstrated a method of introducing neat¹⁴C-octamethyltrisiloxane (¹⁴C-L3) into natural soil with subsequent mixing to distribute the test article throughout the soil uniformly. 

The second phase of the study investigated the stability of¹⁴C-L3 in the same soil under conditions representative of those used for the OECD TG 222 Earthworm Acute Toxicity and Reproduction Test. The system was partially open to allow for respiration during a planned future toxicity experiment. 

Effect of headspace on loss of test material during dosing of soil was examined. The main experiment test vessel was selected in order to minimise headspace as much as possible, while still allowing for enough tumbling to ensure homogeneity. However, even the container with the best recovery only observed an average recovery of 69.7% (or 22.9 mg/kg soil (d.w.)).

Measures were taken to avoid loss of test substance through volatilisation during the homogeneity experiment. These included:

- fitting the test vessel jar with a polytetrafluoroethylene (PTFE) lined lid and covering the threads of the jar with PTFE tape;

- spiking the soil with a calibrated gastight syringe;

- spiking with a concentration 34% above the saturation concentration, to attempt to account for test substance losses;

- taping the lid closed to avoid inadvertent opening.

The stability experiment was then carried out using the spiked soil from the homogeneity experiment, which was divided into five beakers. Each beaker had 10 mL of headspace and was covered with plastic film having five small holes (approximately 3 mm in diameter) to allow for air exchange. The plastic film was secured with rubber bands. 

By day 3 in the stability experiment, only 6.3% of the initially observed radioactivity was detected, and sampling was stopped. However, the average of the homogeneity determination was taken as the initial concentration for each beaker in the stability test. The soil was not analysed again after dividing into the beakers, therefore the potential loss from moving the test soil from the container used in the homogeneity experiment to the 5 test beakers used in the stability study was not determined.

A single peak was observed consistent with¹⁴C-L3, showing that the primary mechanism of loss in the initial recovery experiment was volatilisation.

The absence of degradation products in the vast majority of samples, coupled with the rapid loss of¹⁴C activity, shows that the primary mechanism of test article loss was volatilisation of¹⁴C-L3 from the simulated OECD TG 222 test setup.