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Sediment toxicity

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Sediment testing is not an Annex VIII requirement for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. However, the low water solubility, high log Kow and high log Koc of the substance indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant. As aquatic toxicity testing is not feasible, it may be more appropriate to attempt sediment tests in order to look into potential toxicity of the substance in the environment.

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 are available 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.

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. Currently, except for one 28-day toxicity study with Lumbriculus variegatus, no sediment data are available with L5. Testing proposals for a Sediment-Water Chironomid Toxicity Test (OECD TG 218) and a 42-day survival, growth and reproduction test using the amphipod Hyalella Azteca are in place for L5. These proposals are also being read across to CAS 3390-61-2.

In the interim, data from Chironomus riparius and Hyalella azteca are read-across from dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) and decamethyltetrasiloxane (L4, CAS 141-6-8) respectively.

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 6.0, Ecotoxicological Information discussion.

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

Table: Key physico-chemical properties of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) and surrogate substances

Property

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

Dodecamethylpentasiloxane, L5 (141-63-9)

Decamethyltetrasiloxane, L4 (141-62-8)

Dodecamethylcyclohexasiloxane, D6 (540-97-6)

Molecular weight

547

385

311

445

Log Kow (at 25°C)

9.0

9.4

8.21

8.9

Log Koc (at 23°C)

6.0

6.3

5.16

5.9

Water solubility (mg/l)

9.0E-11

7.0E-05

6.7E-03

5.1E-03

Hydrolysis half- life at pH 7 (h)

>329

728

728

1 year

Sediment toxicity data

A result for effects on survival and biomass at a loading rate of 19 mg/kg dwt sediment with Lumbriculus variegatus have ben read-across from L5. 28-day NOEC and LC50 values of ≥19 and >19 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 >38 and ≥38 mg/kg dry weight respectively.

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 d NOEC of ≥68 mg/kg sediment dry weight for growth rate (≥92 mg/kg sediment dry weight, normalised to 5% organic carbon). 

The following results are read-across from the structural analogue D6: A 28-d NOEC of ≥620 mg/kg dry weight have been determined for effects on emergence and development rate of Chironomus riparius (≥969 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.