1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane

Bioaccumulation: aquatic / sediment

Description of key information

Bioaccumulation: aquatic: BCFss 1011 l/kg (0.80 µg a.i./l); 384 (4.4 µg a.i./l). BCFk 2992 l/kg (0.80 µg a.i./l); 1208 (4.4 µg a.i./l)
Lipid normalised (to 5%) values are: BCFss 934 l/kg (0.80 µg a.i./l); 255 l/kg (4.4 µg a.i./l) and BCFk 2765 l/kg (0.80 µg a.i./l); 803 l/kg (4.4 µg a.i./l). A BCF value of 2765 is used as a worst case.
Depuration rate constants: 0.161 d-1(0.8 µg a.i./l); 0.0125 d-1(4.4 µg a.i./l). 

Key value for chemical safety assessment

BCF (aquatic species):

2 765 L/kg ww

Additional information

There are no reliable bioaccumulation data available for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2), therefore good quality data for the structurally-related substance, 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl) oxy]trisiloxane (PhM3T, CAS 2116-84-9) have been read across.

1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) and PhM3T (CAS 2116-84-9) are within the Siloxanes Category. Substances in this category have similar properties with regard to bioaccumulation.

A review of the data available for substances in this Category indicates that BCF is dependent on log K_owas well as on chemical structure.

1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) and 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl) oxy]trisiloxane (PhM3T, CAS 2116-84-9) are structurally-similar substances, both are siloxanes with phenyl functionality. The target substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) is a linear siloxane containing three Si atoms linked by oxygen. The terminal silicon atoms are each substituted with one methyl group and two phenyl groups, and the central silicon atom is substituted with one methyl and one phenyl group. The source substance 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl) oxy]trisiloxane (PhM3T, CAS 2116-84-9) is a branched structure of four Si atoms, the longest siloxane chain contains three silicon atoms and two oxygen atoms, with a Si-O branch on the central silicon atom in the chain. The terminal silicon atoms are each fully methyl substituted, whilst the central silicon atom has one phenyl group attached.

1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) and 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl) oxy]trisiloxane (PhM3T, CAS 2116-84-9) both have predicted log Kow values of 9.

It is therefore considered valid to read-across the results for PhM3T to fill the data gap for the registered substance.

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

Steady-state BCF values of 1011 l/kg (0.80 µg a.i./l) and 384 (4.4 µg a.i./l) and kinetic BCF values of 2992 l/kg (0.80 µg a.i./l) and 1208 (4.4 µg a.i./l) were determined for PhM3T in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Lipid normalised (to 5%) values are: BCF_ss= 934 l/kg (0.80 µg a.i./l) and 255 l/kg (4.4 µg a.i./l) and BCF_k= 2765 l/kg (0.80 µg a.i./l) and 803 l/kg (4.4 µg a.i./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, 2017), 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 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.

Gosset 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 0.161 d^-1(0.8 µg a.i./l) and 0.0125 d^-1(4.4 µg a.i./l) obtained from the BCF study with PhM3T are considered to be valid and to carry most weight for bioaccumulation assessment of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. 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 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_lwis 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. 

The table below presents fugacity ratios calculated from the BCF data for PhM3T, using K_owfor K_lw.

Table: Calculated biota-water fugacity ratios for read-across substances PhM3T

Substance	Endpoint	Exposure concentration	BCF Value	Fbiota-water*
PhM3T	BCFss	0.80 µg a. i. /l	1011	2.35E-05
PhM3T	BCFss	4.4 µg a. i. /l	384	6.43E-06
PhM3T	BCFk	0.80 µg a. i. /l	2992	6.97E-05
PhM3T	BCFk	4.4 µg a. i. /l	1208	2.02E-05

*Using log K_ow9 for PhM3T

 

The fugacity-based BCFs 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 tends to be at a lower fugacity (or chemical activity) than in the water. It should be noted however, that the BCF studies may not have reached true steady-state in the timescale of the laboratory studies. The fugacity ratios 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.