Tetradecamethylhexasiloxane

Bioaccumulation: aquatic / sediment

Description of key information

Bioaccumulation: aquatic: BCFss 1430 l/kg (4 ng/l); 1240 l/kg (39 ng/l). BCFk 1450 l/kg (4 ng/l); 1240 l/kg (39 ng/l). Lipid normalised (to 5%) values are: BCFss = 4210 l/kg (4 ng/l) and 3650 l/kg (39 ng/l) and BCFk = 4260 l/kg (4 ng/l) and 3650 l/kg (39 ng/l). A BCF value of 4260 l/kg is used in the exposure assessment as a worst case.

Depuration rate constants from BCF study: 0.0949 d-1 (4 ng/l); 0.121 d-1 (39 ng/l).

Key value for chemical safety assessment

BCF (aquatic species):

4 260 L/kg ww

Additional information

There are no reliable bioaccumulation data for tetradecamethylhexasiloxane (L6); therefore good quality data for the structurally-related substance dodecamethylpentasiloxane (L5, CAS No. 141-63-9) have been read-across.

L5 and L6 are within a group of substances which have similar properties with regard to bioaccumulation.

A review of the data available for substances in this group indicates that BCF is dependent on log K_ow as well as on chemical structure. The log K_ow values and chemical structure of L5 and L6 are similar (log K_ow = >9).

It is therefore considered valid to read-across the results for L5 to fill the data gap for the registered substance.

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

The BCF values determined for L5 were: steady-state BCF values of 1430 l/kg (4 ng/l) and 1240 (39 ng/l) and kinetic BCF values of 1450 l/kg (4 ng/l) and 1240 l/kg (39 ng/l) were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Lipid normalised (to 5%) values are: BCF_ss= 4210 l/kg (4 ng/l) and 3650 l/kg (39 ng/l) and BCF_k= 4260 l/kg (4 ng/l) and 3650 l/kg (39 ng/l).

Growth correction was not applied, though based on fish weight data reported the growth of the fish during the study was minimal.

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_ocwill 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 highlylipophilic and adsorbingsubstances, both routes of uptake are likely to be significant in a BCF study, because the substance can be absorbed by 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 70d) 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 0.0949 d^-1(4 ng/l)and 0.121 d^-1(39 ng/l)obtained from the BCF study 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, L. P.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, L. P.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/LWis 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_lis the density of lipid and ρ_Bis 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_lwvalues for siloxane substances is in progress. Initial laboratory work with olive oil as lipid substitute indicates that the assumption that K_lw= K_owis appropriate (Reference: Dow Corning Corporation, personal communication). However, the calculated fugacity ratios presented here should be used with caution at this stage.

Calculated biota-water fugacity ratios

Endpoint	Exposure concentration	BCF Value	Fbiota-water*
BCFss	4.0 ng/l	1430	6.04E-05
BCFss	39 ng/l	1240	5.24E-05
BCFk	4.0 ng/l	1450	6.13E-05
BCFk	39 ng/l	1240	5.24E-05

*Using log K_ow9.4

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 ratios indicate that uptake may be less than expected on thermodynamic grounds, suggesting that elimination is faster than might be expected on grounds of lipophilicity alone.

References

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. and Woodburn, K. B. (2012). Comparing Laboratory and Field Measured Bioaccumulation Endpoints. Integrated Environmental Assessment and Management 8, 17-31.

Goss, K-U., Brown, T. N. and Endo, S. (2013). Elimination half-life as a metric for the bioaccumulation potential of chemicals in aquatic and terrestrial food chains. Environmental Toxicology and Chemistry 32, 1663-1671.