1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane

Bioaccumulation: aquatic / sediment

Description of key information

Bioaccumulation: aquatic: BCFss 5030 l/kg (1.7 µg/l); 7730 l/kg (21 µg/l). BCFk 3610 l/kg (1.7 µg/l); 5600 l/kg (21 µg/l), read-across from L3. Lipid normalised (to 5%) values are: BCFss 18000 l/kg (1.7 µg/l); 27600 l/kg (21 µg/l) and BCFk 12900 l/kg (1.7 µg/l); 20000 l/kg (21 µg/l). A BCF value of 27600 is used in the exposure assessment as a worst case.

Depuration rate constants from BCF study: 0.336 d^-1 (1.7 µg/l); 0.186 d^-1 (21 µg/l).

Key value for chemical safety assessment

BCF (aquatic species):

27 600 L/kg ww

Additional information

There are no reliable bioaccumulation data available for 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane, therefore good quality data for the structurally-related substances, octamethyltrisiloxane, L3, (CAS 107-51-7) and dodecamethylpentasiloxane (L5, CAS No. 141 -63 -9) have been read-across.

1,1,1,3,5,5,5-Heptamethyl-3-octyltrisiloxane, L3 and L5 are within the Reconsile Siloxane Category of substances 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.

Partitioning between the lipid-rich fish tissues and water may be considered to be analogous to partitioning between octanol and water. A review of the data available for substances in this analogue group indicates that BCF is dependent on log K_ow as well as on chemical structure. 

The registration substance, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) and the surrogate substance octamethyltrisiloxane (L3, CAS 107-51-7) are linear siloxanes with 3 silicon atoms linked by two oxygen atoms. In L3, each silicon atom is fully methylated, whereas in 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane the central silicon atom is substituted by an octyl carbon chain in place of a methyl group. 1,1,1,3,5,5,5-Heptamethyl-3-octyltrisiloxane and L3 possess similar physico-chemical properties: high molecular weight (334.73 and 236.54 g/mol respectively), low water solubility (both insoluble, at 2.8E-05 mg/l and 0.034 mg/l respectively), high log K_ow(9.0 at 20°C and 6.6 at 25°C respectively) and high log K_oc(6.0 and 4.34 respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates at environmentally relevant temperature and pH.

The registration substance, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) and the surrogate substance dodecamethylpentasiloxane (L5, CAS 141-63-9) are linear siloxanes with 3 and 5 silicon atoms respectively. L5 is a linear chain with five silicon atoms linked by four oxygen atoms and each silicon atom is fully methylated, whereas 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane is a linear chain with three silicon atoms linked by two oxygen atoms. Each silicon atom is substituted by methyl groups, except for the central silicon atom which is substituted by an octyl- carbon chain in place of a methyl group.

1,1,1,3,5,5,5 -Heptamethyl-3-octyltrisiloxane and L5 possess similar physico-chemical properties: high molecular weight (334.73 and 384.85 g/mol respectively), low water solubility (both insoluble, at 2.8E-05 mg/l and 7.0E-05 mg/l respectively), high log K_ow(9.0 at 20°C and 9.41 at 25°C respectively) and high log K_oc(6.0 and 5.16 respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates at environmentally relevant temperature and pH.

In the context of structural similarity, L3 is more similar to the target substance than L5 is. However, the target substance and L5 may be more similar in terms of partitioning behaviour (log Kow). Since the Category hypothesis is dependent on log Kow and structural similarity, it is considered valid to read-across the results for L3 and L5 for the registered substance.

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

The BCF values determined for L3 in a reliable study were: Steady-state BCF values of 5030 l/kg (1.7 µg/l) and 7730 l/kg (21 µg/l) and kinetic BCF values of 3610 l/kg (1.7 µg/l) and 5600 l/kg (21 µg/l). Lipid normalised (to 5%) values are: BCFss = 18000 l/kg (1.7 µg/l) and 27600 l/kg (21 µg/l) and BCFk = 12900 l/kg (1.7 µg/l) and 20000 l/kg (21 µg/l). For L5, 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. 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). The studies were conducted according to an appropriate test protocol and in compliance with GLP.

Fish bioconcentration (BCF) studies are most validly applied to substances with log K_ow values 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 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_elimination of >0.01 d^-1 (half-life 70 d) 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 0.336 d^-1(1.7 µg/l) and 0.186 d^-1 (21 µg/l) for L3 and 0.0949 d-1 (4 ng/l) and 0.121 d^-1 (39 ng/l) for L5 obtained from the BCF study with the read-across substances 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/LW is the 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.

A study to determine storage lipid-air partition coefficients of cVMS has been carried out (Dow Corning Corporation, 2015c). The conclusion from that study is that partitioning of cVMS compounds between storage lipids and air or water is reasonably similar, but not identical, to octanol. K_{storage lipid-air} values for cVMS were systematically lower than K_octanol-air by 0.2 to 0.4 log units depending on temperature. K_{octanol-water} values may be expected to be similar.

The table below presents fugacity ratios calculated from the BCF data for L3 and L5, using both K_ow for K_lw and K_ow-0.4 as a worst-case approximation.

Table: Calculated biota-water fugacity ratios

Endpoint	Exposure concentration	BCF Value	Fbiota-water L3 using logKstorage lipid-water =log Kow(6.6)	Fbiota-water L3 using logKstorage lipid-water =log Kow-0.4 (6.2)	Fbiota-water L5 using logKstorage lipid-water =log Kow(9.4)	Fbiota-water L5 using logKstorage lipid-water =log Kow-0.4 (9.0)
BCFss	1.7 µg/l	5030	8.1E-02	2.0E-01	6.0E-05	1.1E-04
BCFss	21 µg/l	7730	1.3E-01	3.1E-01	5.2E-05	9.2E-05
BCFk	1.7 µg/l	3610	5.8E-02	1.5E-01	6.1E-05	1.1E-04
BCFk	21 µg/l	5600	9.1E-02	2.3E-01	5.2E-05	9.2E-05

The fugacity-based BCF directly reflects 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 tends to be 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 shows 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.