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Environmental fate & pathways

Bioaccumulation: aquatic / sediment

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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 a structurally-related substance. 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 H-L3, therefore good quality data for the structurally-related substance octamethyltrisiloxane (L3, CAS 107-51-7), have been read across.

H-L3 and L3 are members of the Reconsile Siloxanes Category. 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 Kow>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 Kow as well as on chemical structure.

The log Kow values of H-L3 and L3 are similar (6.2 and 6.60, respectively). H-L3and the source substance L3 are linear siloxanes with three silicon atoms, alternated by oxygen atoms. In L3, the Si atoms are fully methyl substituted, whereas in H-L3 the central silicon atom is substituted with one hydrogen atom and one methyl group. A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

Table: Key physicochemical properties of H-L3 and surrogate substance L3

Property

H-L3 (1873-88-7)

L3 (107 -51-7)

Molecular weight

222.51

236.54

Log Kow

6.2

6.60

Log Koc

3.8

4.34

Water solubility (mg/l)

0.02 (at 22°C)

0.034 (at 23°C)

Vapour pressure at 25°C (Pa)

8.5E+02

5.3E+02

Hydrolysis half- life at pH 7 (d)

2.2

13.7

It is therefore considered valid to read-across the results for L3 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 6 dossier.

The BCF values determined for L3 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).

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, 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 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 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 keliminationof >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 0.336 d-1(1.7 µg/l) and 0.186 d-1(21 µg/l) obtained from the BCF study with the read-across substance L3 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 (Fbiota-water) is:

Fbiota-water= BCFWD/LW/ Klwx ρl/ ρB

where BCFWD/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), Klw is the lipid-water partition coefficient and ρlis the density of lipid and ρBis 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 process. 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 L3, using Kow for Klw.

Calculated biota-water fugacity ratios for read-across substance L3

Endpoint

Exposure concentration

BCF Value

Fbiota-water*

BCFss

1.7 µg/l

5030

8.1E-02

BCFss

21 µg/l

7730

1.3E-01

BCFk

1.7 µg/l

3610

5.8E-02

BCFk

21 µg/l

5600

9.1E-02

Using log Kow6.6

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 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.