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EC number: 451-620-7
CAS number: 352230-22-9
Table 1: Bioconcentration factors at
different time points and concentrations in water
Duration of exposure (days)
Low Concentration level
Concentration in the water (μg a.i./l)
Mean measured test concentration
0.80 μg a.i./l
Concentration in fish (μg a.i./kg)
Mean measured steady-state concentration
(Days 28, 35,
42) 809 μg
Steady-State BCF 1011 l/kg
High Concentration level
4.4 μg a.i./l
(Days 21, 28,
35, 42) 1691 μg
Steady-State BCF 384 l/kg
Table 2: Depuration
Concentration in the water(µg a.i./l)
Concentration in fish (µg a.i./kg)
High concentration level
1011 l/kg (0.80 µg a.i./l); 384 (4.4 µg a.i./l)
2992 l/kg (0.80 µg a.i./l); 1208 (4.4 µg a.i./l)
normalised (to 5%) values are:
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).
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).
Reaction Mass of
3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9) is a
multi-constituent substance containing two main constituents;
3,3-diphenylhexamethyltrisiloxane (Constituent 1) and
3,3,5,5-tetraphenylhexamethyltetrasiloxane (Constituent 2).
The constituents of the submission
substance have log Kow values >4.5. The reported values are
>9.0 for each constituent, based on a validated QSAR estimation method
(See IUCLID Section 4.7). Log Kow values of 9.1 for
Constituent 1 and 13.0 for Constituent 2 were obtained from KOWWIN v1.68
(U.S. EPA, Sept. 2000). At such high log Kow of 13 for
Constituent 2, absorption is expected to be limited due to the very
limited solubility. Therefore, Constituent 2 is not expected to be
bioaccumulative. Bioaccumulation (BCF) for Constituent 1 is therefore
There are no reliable
bioaccumulation data available for 3,3-diphenylhexamethyltrisiloxane
(Constituent 1), therefore good quality data for the
(PhM3T, CAS 2116-84-9) have been read across.
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 Kow as well as on chemical structure.
(Constituent 1) 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 3,3-diphenylhexamethyltrisiloxane is a linear siloxane
containing three Si atoms linked by oxygen. The terminal silicon atoms
are each substituted with three methyl groups, and the central silicon
atom is substituted with two phenyl groups. The source substance
(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.
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 Constituent 1 of the submission substance.
Additional information is given in a supporting report (PFA 2017at)
attached in Section 13.
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: BCFss= 934 l/kg (0.80 µg a.i./l) and 255 l/kg
(4.4 µg a.i./l) and BCFk= 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 Kow 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 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 Koc
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
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 kelimination
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 0.161 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 3,3-diphenylhexamethyltrisiloxane.
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
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 (Fbiota-water) is:
Where BCFWD/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), Klw is the
lipid-water partition coefficient and ρl is the density
of lipid and ρB 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. Klw = Kow. For some substances
with specific interactions with the organic phase, this assumption is
not sufficiently accurate. Measurement of Klw values for
siloxane substances is in progress. Initial laboratory work with olive
oil as lipid substitute indicates that the assumption that Klw =
Kow 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 PhM3T, using Kow for
Table: Calculated biota-water
fugacity ratios for read-across substances PhM3T
0.80 µg a. i. /l
4.4 µg a. i. /l
*Using log Kow 9
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 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.
ECHA (2017). Guidance on
Information Requirements and Chemical Safety Assessment. Chapter R.11:
PBT/vPvB assessment, Version 3.0. June 2017
PFA (2017at). Siloxane Category
Report for Environmental Endpoints. PFA.404.114.001
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