Decamethyltetrasiloxane

Sediment toxicity

Description of key information

Long-term studies are available for two organisms (Hyalella azteca and Lumbriculus variegatus) with the registration substance. The data show no effects at the limit of solubility in organic carbon. A true PNEC cannot be calculated from the test data because the NOEC values that have been determined are limit values. Data for Chironomus riparius with the closely related substance L3 has been read-across. A 28-d NOEC of 39 mg/kg dry weight has been determined for effects of L3 on emergence and development rate of Chironomus riparius (89 mg/kg dry weight, normalised to 5% organic carbon).

Additional information

A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017). The hypothesis for read across of sediment ecotoxicity evidence within the Siloxanes Category is that no structure-based or property-based pattern is evident from the category dataset of existing studies, although patterns are identifiable associated with extrinsic aspects of test design to which effects may be attributed. The approach will be revisited in the event that reliable new data become available. With this in mind, a single overall interpretation is made across the category. To fulfil the requirements of REACH, a conservative approach is made by reading across on a nearest-neighbour basis the reliable data within the category.

In the context of the RAAF, Scenario 6 is expected to apply to this endpoint. It is considered that effects observed in benthic organisms are associated primarily with extrinsic factors associated with test design and not to structural similarities as such.

Over twenty-five sediment toxicity studies for siloxanes are available and have been reviewed in detail. The majority of these studies are of standard duration in standard test species. There is a general trend for studies using natural sediment, which all have pH <~8, to show no effects, or higher NOECs than those with artificial sediment. No significant toxicity (NOEC <100 mg/kg) in any organism is found at pH near 7 with natural sediment. The data suggest that it is possible to read across sediment toxicity data between different siloxane structures, especially where natural sediment data are available, given that the studies which are not suspected to be confounded by extrinsic factors show relatively minimal effects across the dataset.

Dataset for L4

 

A 28-day study has been conducted with L4 using Hyalella azteca and Lumbriculus variegatus. Data for Chironomus riparius are read across from L3. The selection of read-across substance is based on structural similarity and key physico-chemical properties (log K_ow, log K_oc, degradation).

Read-across justification

Read-across from L3 to L4:

L4 and the source substance L3 are linear siloxanes with four silicon atoms and three silicon atoms respectively, alternated by oxygen atoms. In both L3 and L4, the Si atoms are fully methyl substituted. L3 and L4 possess similar physicochemical properties.  A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

No effects at the limit of solubility have been reported in short-term and long-term studies in other trophic levels conducted with both substances.

Given the similar properties and structural similarities, it is considered valid to read across sediment toxicity data from L3 to L4.

 

A comparison of the key physico-chemical properties is presented in the table below:

 

Table: Key physico-chemical properties of L4 and surrogate substance L3

Property	L3 (107-51-7)	L4 (141-62-8)
Molecular weight	237	311
Log Kow (at 25°C)	6.60	8.21
Log Koc (at 23°C)	4.34	5.16
Water solubility (mg/l)	3.4E-02	6.7E-03
Hydrolysis half- life at pH 7 (h)	329	728

 

Sediment toxicity data

A 28-day study has been conducted with the registered substance and Hyalella azteca. 28-day NOEC and LC50 values of ≥68 and >68 mg/kg have been determined for the effects of the registered substance on mortality and growth of the freshwater amphipod Hyalella azteca in sediment containing 3.7% organic carbon (OC). The results normalised for a standard sediment of 5% organic carbon are equivalent to LC50 and NOEC values of >92 and ≥92 mg/kg dry weight respectively.

It is possible to calculate the organic carbon (OC) solubility according to the following formula:

OC solubility mg/kg = Koc* water solubility

Which for L4 is 1.4E+05 * 6.7E-03 mg/l = 970 mg/kg OC.

The sediment in this long-term toxicity study had an OC content of 3.7%, therefore the calculated solubility in this sediment is 36 mg/kg. This indicates that the substance is not toxic to freshwater sediment organisms in excess of the OC solubility of the registered substance.

No effects on survival or biomass have been reported when testing the registered substance at a loading rate of 17 mg/kg dwt sediment with Lumbriculus variegatus. Therefore 28-day NOEC and LC50 values of ≥17 and >17 mg/kg have been determined respectively in a sediment containing 2.5% organic carbon. The results normalised for a standard sediment of 5% organic carbon are equivalent to LC50 and NOEC values of >34 and ≥34 mg/kg dry weight respectively.

A 28-d NOEC result for Chironomus riparius is read across from L3. A 28-d NOEC of 39 mg/kg dry weight has been determined for effects on emergence and development rate of Chironomus riparius (89 mg/kg dry weight, normalised to 5% organic carbon).

The results of all tests are expressed relative to mean measured exposure concentrations in the treated sediment.