Octaphenylcyclotetrasiloxane

Bioaccumulation: aquatic / sediment

Description of key information

Bioaccumulation: aquatic / sediment: Steady-state BCF 172 l/kg (0.12 µg/l) and 39.3 l/kg (1.1 µg/l) and kinetic BCF 209 l/kg (0.12 µg/l) and 47.2 l/kg (1.1 µg/l), based on read-across from a structurally-related substance, tetradecamethylcycloheptasiloxane CAS 107-50-6. Lipid normalised (to 5%) values are: BCFss= 234 l/kg (0.12 µg/l) and 53.4 l/kg (1.1 µg/l) and BCFk= 284 l/kg (0.12 µg/l) and 64.1 l/kg (1.1 µg/l).

Key value for chemical safety assessment

BCF (aquatic species):

284 L/kg ww

Additional information

There are no reliable bioaccumulation data available for the registration substance octaphenylcyclotetrasiloxane. A weight-of-evidence approach is applied to this endpoint.

Based on REACH R11 (ECHA, 2017), the substance is considered to meet the physicochemical indicators for hindered uptake. The bioavailability of the substance is expected to be limited by the large molecular size (molecular weight 793.19 , molecular diameter D_max16.9 Å) inhibiting the passage of the molecule through the cell membrane. The predicted log K_ow for this substance is subject to some uncertainty, since the prediction falls outside of the applicability domain of the model. For the purposes of assessment, the log K_ow is limited to 9.0, however it is highly likely that the log K_ow >10. There was no evidence of oral absorption in the acute oral toxicity or repeated dose toxicity study in rats for this substance.

In addition, good quality data for the structurally-related substance, tetradecamethylcycloheptasiloxane (CAS 107-50-6), have been read across.

Octaphenylcyclotetrasiloxaneandtetradecamethylcycloheptasiloxane (D7) are within the Reconsile Siloxane Category which have similar properties with regard to bioaccumulation. This Category consists of linear/branched and cyclic siloxanes which have a low functionality and a hydrolysis half-life at pH 7 and 25°C >1 hour and log K_ow>4. The Category hypothesis is that the bioaccumulation of a substance in fish (aquatic bioconcentration) is dependent on the octanol-water partition coefficient and chemical structure. In the context of the RAAF, Scenario 4 is applied.

 

Octaphenylcyclotetrasiloxane and tetradecamethylcycloheptasiloxane (D7) are large, high molecular weight cyclic siloxanes (molecular weight of 793.19 and 519.09 respectively; molecular diameter (D_max) of 16.9 Å and 14.1 Å respectively), and both have very high predicted log K_ow values of 9.0. The target substance octaphenylcyclotetrasiloxane is a cyclic siloxane consisting of four silicon atoms alternated by oxygen atoms, each silicon atoms is substituted by two phenyl groups.The source substance, tetradecamethylcycloheptasiloxane (D7) is a cyclic siloxane consisting of seven silicon atoms alternated by oxygen atoms, where each silicon atom is substituted by two methyl groups. A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and hydrolyse very slowly.

Table: Key physicochemical properties ofoctaphenylcyclotetrasiloxaneandtetradecamethylcycloheptasiloxane

Property	octaphenylcyclotetrasiloxane	tetradecamethylcycloheptasiloxane
Molecular weight	793.19	519.09
Log Kow	9.0 (QSAR)	9.0 (QSAR)
Log Koc	6.0 (QSAR)	6.0 (QSAR)
Water solubility (mg/l)	<1E-05 mg/l (QSAR)	<1E-05 mg/l (QSAR)
Vapour pressure at 25°C (Pa)	<1E-06 Pa at 25°C (QSAR)	1.3 Pa (QSAR)
Hydrolysis half- life at pH 7	Not available	63000 h (QSAR)
Hydrolysis products	Diphenylsilanediol	dimethylsilanediol
Ready biodegradability	Not readily biodegradable	Not readily biodegradable

 

It is therefore considered valid to read-across the result for D7 as a worst-case for the registered substance.

Additional information is given in a supporting report (PFA, 2017a) attached in Section 13.

The BCF values determined for tetradecamethylcycloheptasiloxane (CAS 107-50-6) were as follows:

Steady-state BCF values of 172 l/kg (0.12 µg/l) and 39.3 l/kg (1.1 µg/l) and kinetic BCF values of 209 l/kg (0.12 µg/l) and 47.2 l/kg (1.1 µg/l). Lipid normalised (to 5%) values are: BCFss= 234 l/kg (0.12 µg/l) and 53.4 l/kg (1.1 µg/l) and BCFk= 284 l/kg (0.12 µg/l) and 64.1 l/kg (1.1 µg/l).

Fish bioconcentration (BCF) studies are most validly applied to substances with log K_owvalues between 1.5 and 6. Practical experience suggests that if the aqueous solubility of the substance is low (i.e. below ~0.01 to 0.1 mg/l) (REACH Guidance R.11; ECHA, 2014), fish bioconcentration studies might not provide a reliable BCF value because it is very difficult to maintain exposure concentrations. Dietary bioaccumulation (BMF) tests are practically much easier to conduct for poorly water-soluble substances, because a higher and more constant exposure to the substance can be administered via the diet than via water. In addition, potential bioaccumulation for such substances may be expected to be predominantly from uptake via feed, as substances with low water solubility and high K_oc will usually partition from water to organic matter.

However, there are limitations with laboratory studies such as BCF and BMF studies with highly lipophilic and adsorbing substances. Such studies assess the partitioning from water or food to an organism within a certain timescale. The studies aim to achieve steady-state conditions, although for highly lipophilic and adsorbing substances such steady-state conditions are difficult to achieve. In addition, the nature of BCF and BMF values as ratio values, means that they are dependent on the concentration in the exposure media (water, food), which adds to uncertainty in the values obtained.

For highly lipophilic and adsorbing substances, both routes of uptake are likely to be significant in a BCF study, because the substance can be adsorbed to food from the water. 

Dual uptake routes can also occur in a BMF study, with exposure occurring via water due to desorption from food, and potential egestion of substance in the faeces and subsequent desorption to the water phase. Although such concentrations in water are likely to be low, they may result in significant uptake via water for highly lipophilic substances.

Goss et al. (2013) put forward the use of elimination half-life as a metric for the bioaccumulation potential of chemicals. Using the commonly accepted BMF and TMF threshold of 1, the authors derive a threshold value for k_eliminationof >0.01 d^-1(half-life 70 days) as indicative of a substance that does not bioaccumulate.

Depuration rates from BCF and BMF studies, being independent of exposure concentration and route of exposure, are considered to be a more reliable metric to assess bioaccumulation potential than the ratio BCF and BMF values obtained from such studies.

The depuration rate constants of 0.0442 d^-1 (0.12 µg/l) and 0.0625 d^-1 (1.1 µg/l) obtained from the BCF study with D7 are considered to be valid and to carry most weight for bioaccumulation assessment. These rates are indicative of a substance which does not bioaccumulate.

Burkhard et al. (2012) has described fugacity ratios as a method to compare laboratory and field measured bioaccumulation endpoints. By converting data such as BCF and BSAF (biota-sediment accumulation factor) to dimensionless fugacity ratios, differences in numerical scales and unit are eliminated.

Fugacity is an equilibrium criterion and can be used to assess the relative thermodynamic status (chemical activity or chemical potential) of a system comprised of multiple phases or compartments (Burkhard et al., 2012). At thermodynamic equilibrium, the chemical fugacities in the different phases are equal. A fugacity ratio between an organism and a reference phase (e.g. water) that is greater than 1, indicates that the chemical in the organism is at a higher fugacity (or chemical activity) than the reference phase. The fugacity of a chemical in a specific medium can be calculated from the measured chemical concentration by the following equation:

f = C/Z

Where f is the fugacity (Pa), C is concentration (mol/m³) and Z is the fugacity capacity (mol(m³.Pa)).

The relevant equation for calculating the biota-water fugacity ratio (F_biota-water) is:

F_biota-water= BCF_WD/LW/ K_lwx ρ_l/ ρ_B

where BCF_WD/LW is ratio of the steady-state lipid-normalised chemical concentration in biota (µg-chemical/kg-lipid) to freely dissolved chemical concentration in water (µg-dissolved chemical/l-water), K_lw is the lipid-water partition coefficient and ρ_l is the density of lipid and ρ_B is the density of biota.

It can be assumed that n-octanol and lipid are equivalent with respect to their capacity to store organic chemicals, i.e. K_lw= K_ow. For some substances with specific interactions with the organic phase, this assumption is not sufficiently accurate. Measurement of K_lw values for siloxane substances is in process. Initial laboratory work with olive oil as lipid substitute indicates that the assumption that K_lw= K_ow is appropriate (Reference: Dow Corning Corporation, personal communication). However, the calculated fugacity ratios presented here should be used with caution at this stage.

The table below presents fugacity ratios calculated from the BCF data for the read-across substance D7, using K_owfor K_lw. BMF values do not require adjustments because these values are already equivalent to fugacity-based values.

Table4.3.3 Calculated biota-water fugacity ratios

Endpoint	Exposure concentration	BCF Value	Fbiota-water*
BCFsteady-state	0.12 µg/l	172.0	5.86E-06
BCFsteady-state	1.1 µg/l	39.3	1.34E-06
BCFkinetic	0.12 µg/l	209	7.12E-06
BCFkinetic	1.1 µg/l	47.2	1.61E-06

*Using log K_ow 9

The fugacity-based BCF directly reflect the thermodynamic equilibrium status of the chemical between the two media included in the ratio calculations. The fugacity ratios calculated are all below 1, indicating that the chemical in the organism is at a lower fugacity (or chemical activity) than in the water. It should be noted however, that the BCF study may not have reached true steady-state in the timescale of the laboratory studies. The fugacity ratio indicates that uptake may be less than expected on thermodynamic grounds, suggesting that elimination is faster than might be expected on grounds of lipophilicity alone.

Reference:

Burkhard, L. P., Arnot, J. A., Embry, M. R., Farley, K. J., Hoke, R. A., Kitano, M., Leslie, H. A., Lotufo, G. R., Parkerton, T. F., Sappington, K. G., Tomy, G. T. & Woodburn, K. B. (2012). Comparing Laboratory and Field Measured Bioaccumulation Endpoints. Integrated Environmental Assessment and Management 8, 17-31.

ECHA (2017). REACH Guidance on Information Requirements and Chemical Safety Assessment. Chapter R.11 PBT/vPvB assessment. Version 3.0, June 2017.

Goss, K-U., Brown, T. N. & Endo, S. (2013). Elimination half-life as a metric for the bioaccumulation potential of chemicals in aquatic and terrestrial food chains.Environmental Toxicology and Chemistry32, 1663-1671