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Diss Factsheets

Ecotoxicological information

Toxicity to soil macroorganisms except arthropods

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toxicity to soil macroorganisms except arthropods: long-term
Data waiving:
study technically not feasible
Justification for data waiving:

Description of key information

Key value for chemical safety assessment

Additional information

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. H-L3 and L3 are members of the Siloxane Category of compounds and have high vapour pressures (850 Pa and 530 Pa), high log Kow (6.2 and 6.60), high log Koc (3.8 and 4.3) and low water solubility (0.02 mg/l and 0.034 mg/l). In addition, the substances have a slow hydrolysis rate relative to the time-scale of ecotoxicity testing (t1/2 = 53 h and 329 hours at pH 7 and 25°C). In the context of terrestrial toxicity, both L3 and H-L3 are expected to have similar stability in soil and prior to volatilisation, any exposure is likely to be to 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 neat 14C-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 of 14C-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 of 14C activity, shows that the primary mechanism of test article loss was volatilisation of 14C-L3 from the simulated OECD TG 222 test setup.

H-L3 is more volatile and also more rapidly degradable than L3, meaning that similar losses from the test system would be expected under equivalent test conditions. Additionally, in an OECD TG 216 (Soil Microorganisms: Nitrogen Transformation Test) study for the effects of the related substances HMDS and L3 on nitrate formation rate of soil microflora, analysis of the test substance concentrations show that test material was lost by day three of the test (see Section 6.3.4). Based on these experimental findings, the registrants believe it is not technically feasible to conduct an OECD TG 222 test for the registration substance on the basis that the test substance is too volatile to maintain adequate concentrations in the test system.