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Description of key information

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

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater sediment:
68 mg/kg sediment dw

Additional information

There are no sediment data available for tetradecamethylhexasiloxane. Although sediment testing is not an Annex VIII requirement, Reach Guidance (REACH R7b, ECHA 2008b) indicates that substances considered to be highly hydrophobic (log Kow >5) require sediment assessment even at tonnages below 1000 tpa.

A total of twenty-one sediment toxicity studies for siloxanes are available and sixteen 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 sound studies show relatively minimal effects across the dataset. In the use of the data set for hazard assessment and derivation of predicted no-effect concentration (PNEC), the following approaches are used:

- Where the hydrolysis half-life is >48 hours, the chemical safety assessment will focus on the parent form.

- Where data are available for a substance with natural sediment and with artificial sediment for the same species, the natural sediment data will be given preference over data obtained with artificial sediment.

- PNEC will be determined on a weight-of-evidence basis for each substance, including use of read-across. Equilibrium partitioning calculations will be used if necessary.

Dataset for L6

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.

READ-ACROSS JUSTIFICATION

Read-across from L4 to L6:

L6 and the surrogate substance L4 are linear siloxanes with 6 and 4 silicon atoms, respectively, with methyl branches and alternated by oxygen. L6 and L4 possess similar physicochemical properties: high molecular weight (459 and 311, respectively), low water solubility (both insoluble, at 2.3E-06 mg/l and 6.7E-04 mg/l, respectively), high log Kow (>9.41 and 8.21, respectively) and high log Koc (6.0 and 5.16, respectively). Both substances have negligible biodegradability and similar slow 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 L4 to L6.

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

Table: Key physico-chemical properties of L4 and L6

Property

L6 (107-52-8)

L4 (141-62-8)

Molecular weight

459

311

Log Kow (at 25°C)

>9.41 (read-across from L5)

8.21

Log Koc (at 23°C)

6.0

5.2

Water solubility (mg/l)

2.3E-06

6.7E-03

Hydrolysis half- life at pH 7 (h)

6300

728

 

 

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 d 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.

 

A NOEC value of ≥68 mg/kg sediment dry weight (≥92 mg/kg sediment dry weight, normalised to 5% organic carbon) for effects on Hyalella azteca is used as the key value for the CSA.