1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane

Sediment toxicity

Description of key information

NOEC ≥68 mg/kg dwt (≥ 92 mg/kg dwt, normalised to 5% organic carbon)

Additional information

There are no sediment data available for 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3). Although sediment testing is not an Annex IX requirement, Reach Guidance (REACH R7b, ECHA 2008b) indicates that substances considered to be highly hydrophobic (log K_ow >5) require sediment assessment even at tonnages below 10000 tpa.

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.

A total of twenty-four sediment toxicity studies for siloxanes and nineteen results from studies of standard duration in standard test species have been reviewed in detail. 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 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3)

No data are available for the effects of the registration substance on sediment organisms. Data are read-across from the structural analogue decamethyltetrasiloxane (L4; 141-6-8) for the purpose of hazard assessment.

Justification of read-across and comparison of physico-chemical properties is discussed in section 6 of IUCLID (Section 7.0 of the CSR).

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.

Sediment toxicity data

A result for effects in natural sediment on the invertebrate amphipod Hyalella azteca is read across from L4. The result from that test is a 28-day NOEC of ≥68 mg/kg sediment dry weight for growth rate (≥92 mg/kg sediment dry weight, normalised to 5% organic carbon). 

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

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 surrogate substance, L4, 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.

The sediment in this long-term toxicity study had an OC content of 2.5%, therefore the calculated solubility of L4 in this sediment is 24 mg/kg.

 

The data show no effects on Hyalella azteca at the limit of solubility in organic carbon, and no effects on Lumbriculus variagatus at the highest concentration tested, in sediment toxicity tests with the surrogate substance L4. A true PNEC cannot be calculated from the test data because the NOEC values that have been determined are limit values. Testing is proposed with the surrogate substance, L4, using a third organism Chironomus riparius. The approach will be reviewed when this study is available.