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Description of key information

WoE (read-across from L5): 28 day NOEC ≥38 mg/kg dry weight normalised to 5% organic carbon, based on mean measured concentrations, for the effects of L5 on survival and biomass ofLumbriculus variegatus.

WoE (read-across from L5): 28 day NOEC ≥27 mg/kg dry weight normalised to 5% organic carbon, based on mean measured concentrations, for the effects of L5 on emergence and combined developmental rate ofChironomus riparius.

WoE (read-across from L5: 28 day NOEC ≥52 mg/kg dry weight normalised to 5% organic carbon, based on mean measured concentrations, for the effects of L5 on survival and growth ofHyalella azteca.

Key value for chemical safety assessment

Additional information

There are no sediment data available for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. Although sediment testing is not an Annex VIII requirement, Reach Guidance (REACH R7b, ECHA 2017) indicates that substances considered to be highly hydrophobic (log Kow>5) require sediment assessment even at tonnages below 1000 tpa.

A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017at). 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. 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.

More than twenty-five sediment toxicity studies for siloxanes are available and 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.

In the use of the dataset for hazard assessment and derivation of predicted no-effect concentration (PNEC), the following approaches are used:

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No sediment data are available with the registration substance, 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2). Data can be read across from dodecamethylpentasiloxane (L5, CAS 141-63-9) on the basis that both are within the siloxanes category and share similar properties. The selection of read-across substance is based on key physico-chemical properties (log Kow, log Koc, degradation). Further details on justification of read-across can be found in Section 7.0, Ecotoxicological Information discussion.

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

Table7.1.3. Key physico-chemical properties of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) and surrogate substancedodecamethylpentasiloxane, L5 (141-63-9)

Property

1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (3390-61-2)

Dodecamethylpentasiloxane, L5 (141-63-9)

Molecular weight

547

385

Log Kow(at 25°C)

9.0

9.4

Log Koc(at 23°C)

6.0

6.3

Water solubility (mg/l)

9.0E-11

7.0E-05

Hydrolysis half- life at pH 7 (h)

>329

728

 

Sediment toxicity data for L5

Measured long-term sediment toxicity data are available with L5 for three sediment dwelling species;Lumbriculus variegatus,Chironomus ripariusandHyalella azteca. The studies were all conducted in natural sediment and show no effects at the highest concentrations tested. The maximum test concentrations were selected for each sediment test based on the limit of test substance solubility in organic carbon. In each test, the sediment was dosed just above the limit of organic carbon solubility to allow for potential volatilisation losses of the test substance.

Lumbriculus variegatus

No effects on survival or biomass have been reported when testing the registered substance at a loading rate of 19 mg/kg dwt sediment withLumbriculus variegatusunder test guideline OECD 225, in compliance with GLP.

The study was conducted at nominal test substance concentrations of 1.9, 3.8, 7.5, 15 and 30 mg/kg sediment dry weight. The equivalent mean measured concentrations were 1.3, 2.7, 6.3, 12 and 19 mg/kg sediment dry weight.

28 day NOEC and LC50values of ≥19 and >19 mg/kg, respectively, have been determined in a sediment containing 2.5% organic carbon. The results normalised for a standard sediment of 5% organic carbon are equivalent to LC50and NOEC values of >38 and ≥38 mg/kg sediment dry weight, respectively (Smithers Viscient, 2016).

Chironomus riparius

A 28 day sediment toxicity study with L5 was conducted according to OECD TG 218 and in compliance with GLP. No effects on the emergence and combined developmental rate ofChironomus ripariuswere observed in a natural sediment containing 3.1% organic carbon.

The study was conducted at nominal test substance concentrations of 2.2, 4.4, 8.8, 18, and 35 mg/kg sediment dry weight. The equivalent mean measured concentrations were 1.3, 3.7, 6.2, 11, and 17 mg/kg sediment dry weight.

A NOEC value of ≥12 mg/kg was determined based on initial measured concentrations and a NOEC value of ≥17 mg/kg was determined based on mean measured concentrations. The results normalised for a standard sediment of 5% organic carbon are equivalent to NOEC values of ≥19 mg/kg sediment dry weight for initial measured concentrations, and ≥27 mg/kg sediment dry weight for mean measured concentrations (Smithers, 2020a).

Hyalella azteca

A 28 day sediment toxicity study with L5 was conducted according to ASTM Standard E1706-19 and in compliance with GLP. No effects on the survival and growth ofHyalella aztecawere observed in a natural sediment containing 3.2% organic carbon.

The study was conducted at nominal test substance concentrations of 2.2, 4.4, 8.8, 18, and 35 mg/kg sediment dry weight. The equivalent mean measured concentrations were 2.2, 4.4, 9.1, 19, 33 mg/kg sediment dry weight.

A NOEC value of ≥34 mg/kg was determined based on initial measured concentrations and a NOEC value of ≥33 mg/kg was determined based on mean measured concentrations. The results normalised for a standard sediment of 5% organic carbon are equivalent to NOEC values of ≥53 mg/kg sediment dry weight for initial measured concentrations, and ≥52 mg/kg sediment dry weight for mean measured concentrations (Smithers, 2020b).