Trichloro(hexadecyl)silane

Bioaccumulation: aquatic / sediment

Link to relevant study record(s)

Reference

Endpoint:

bioaccumulation in aquatic species, other

Type of information:

(Q)SAR

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification

Principles of method if other than guideline:

Calculation based on BCFBAF v3.01, Estimation Programs Interface Suite™ for Microsoft® Windows v 4.10. US EPA, United States Environmental Protection Agency, Washington, DC, USA.

The BCFBAF Program is an update and expansion of the previous BCFWIN Program that was part of the EPI Suite version 3.20. The update pertains to estimation of Bioconcentration Factor (BCF). The BCFBAF program estimates BCF of an organic compound using the compound's log octanol-water partition coefficient (Kow). For the update, a more recent and better evaluated database of BCF values was used for both training and validation. The BCF data were re-regressed using the same methodology as in the original BCFWIN program.
 
The original estimation methodology used by the original BCFWIN program is described in a document prepared for the Environmental Protection Agency (Meylan et al., 1997). The estimation methodology was then published in journal article (Meylan et al, 1999). Meylan, WM, Howard, PH, Boethling, RS et al. 1999. Improved Method for Estimating Bioconcentration / Bioaccumulation Factor from Octanol/Water Partition Coefficient. Environ. Toxicol. Chem. 18(4): 664-672 (1999).
 
The BCFBAF method classifies a compound as either ionic or non-ionic. Ionic compounds include carboxylic acids, sulfonic acids and salts of sulfonic acids, and charged nitrogen compounds (nitrogen with a +5 valence such as quaternary ammonium compounds). All other compounds are classified as non-ionic.
 
Validity of the model according to the OECD principles
 
1. Defined endpoint: log BCF (bioconcentration factor)
2. Unambiguos algorithm:
Non-Ionic compounds:
Log Kow  <  1.0: All compounds with a log Kow of less than 1.0 are assigned an estimated log BCF of 0.50 (same as in BCFWIN).
Log Kow  1.0  to 7.0: Log BCF  =  0.6598 Log Kow  -  0.333  +Σcorrection factors
Log Kow  > 7.0: Log BCF  =  -0.49 Log Kow  +  7.554  +Σcorrection factors
Ionic compounds:
log BCF  =  0.50    (log Kow < 5.0)
log BCF  =  0.75    (log Kow 5.0 to 6.0)
log BCF  =  1.75    (log Kow 6.0 to 7.0)
log BCF  =  1.00    (log Kow 7.0 to 9.0)
log BCF  =  0.50    (log Kow > 9.0)
3. Applicability domain: Currently, there is no universally accepted definition of model domains. The training sets of the models contain diverse molecules, so that the fragment libraries are abundant. It is however possible that a compound has functional groups or other structural features that are not represented in the training set and for which no fragment coefficients were developed. Additionally, there can be more instances of a given fragment than the maximum for all training set compounds. These points should be taken into consideration while interpreting test results.
Molecular weight limits of the training set: 68-959 g/mol
Log Kow limits: 1 (validity range of the QSAR equation) - 11.26 (maximum value in the training set).
4. Appropriate measures of goodness of fit, robustness and predictivity.
a) 1.0 < log Kow < 7.0: N = 396 compounds, correlation coefficient R2= 0.792, Q2= 0.78, standard deviation sd = 0.511, average deviation ad = 0.395.
b) log Kow > 7.0: N = 35 compounds, correlation coefficient R2= 0.634, Q2= 0.57, standard deviation sd = 0.538, average deviation ad = 0.396.
Overall statistics including all Non-Ionic and Ionic compounds: training set N = 537, R2= 0.833, sd = 0.502, ad = 0.382.
Validation set: N = 158, R2= 0.82, sd = 0.59, ad = 0.46.
For the training set, 72.7% predictions are within 0.5 log unit and 93.5% within 1.0 log unit.
5. Mechanistic interpretation if possible: Bioaccumulation of stable substances is determined by partitioning between aqueous and lipid phases. Estimating bioconcentration factors from octanol-water partition coefficients is well established and essentially valid for neutral organics of intermediate lipophilicity (0 < log Kow < 6). For higher log Kow values, a maximum range in log BCF of approx. 6-7 (log Kow between ca. 6-8) is observed, followed by a plateau or a gradual decrease. (Source: M. Müller and M. Nendza, Literature Study: Effects of Molecular Size and Lipid Solubility on Bioaccumulation Potential).
 

Test organisms (species):

other: Fish

Route of exposure:

aqueous

Test type:

other: calculation

Water / sediment media type:

natural water: freshwater

Type:

BCF

Value:

39.3 L/kg

Basis:

whole body w.w.

Remarks on result:

other: Regression based estimate

Conclusions:

The BCF of the hydrolysis product of the test substance was calculated to be 39.3 l/kg wet weight. This was calculated using an appropriate QSAR and the result is considered reliable.

Description of key information

No conclusion can be reached on bioaccumulation based on the available data

Additional information

Trichloro(hexadecyl)silane (CAS No. 5894-60-0) hydrolyses very rapidly in contact with water to the corresponding hexadecylsilanetriol and hydrochloric acid. The hydrolysis half-life is < 2 min under environmental conditions. Therefore, the hazard assessment for the environment, including sediment and soil compartments, needs to be based on the properties of the hydrolysis products in accordance with REACH guidance for Chemical Safety Assessment R.16 (ECHA, 2016). Due to the fast hydrolysis of the substance, the bioaccumulation of the hydrolysis products is more relevant for the environmental assessment.

The hydrolysis product hydrochloric acid is an inorganic substance and does not bioaccumulate in organisms (OECD SIDS, 2002).

Experimental data on bioaccumulation are not available for the silanol hydrolysis product hexadecylsilanetriol. The estimated log Kow of 5 (QSAR) and the estimated log Koc of 5.6 indicate a potential for bioaccumulation. Additionally, an estimated BCF of 39.3 L/kg (QSAR) is available. Therefore, based on the BCF value the silanol hydrolysis product does not meet the screening criteria for bioaccumulation (BCF < 2000 L/kg). However, this result needs to be considered with caution since the training data set of the calculation model contains only one substance with a C-Si-O bond.

Hexadecylsilanetriol is moderately soluble in water (4.7 mg/L at 25 °C), has a low volatility (< 1E-05 Pa at 25 °C) and is inherently biodegradable based on read across to structurally analogue substances (biodegradation of 65% and 81% after 28 and 60 d, respectively). Therefore, it is not expected to be very bioavailable in the water phase. Studies with aquatic and terrestrial organisms are only available for a structurally similar read-across substance, which shares the same silanol hydrolysis product. The studies did not show any toxicity effects. Long-term aquatic toxicity studies for aquatic invertebrates and algae are also available. No long-term effects were observed in a 21 d study with Daphnia magna (NOELR of ≥ 10 mg/L) and no toxicity toward algae was observed up to the limit of water solubility indicating that no toxic bioaccumulation takes place. Long-term terrestrial studies are available for soil macroorgansism (except arthropods) and plants. The NOEC (56 d) for earthworms was 500 mg/kg soil dw and the NOEC (21 d) for terrestrial plants was 250 mg/kg soil dw. A long-term sediment toxicity study is in progress, which will provide further information for the endpoint. Nevertheless, no reliable conclusion can be reached at the moment with the above mentioned information. Concerning the PBT assessment the substance is inherently biodegradable and according to the ECHA Guidance on Information Requirements and Chemical Safety Assessment Chapter R.11: PBT/vPvB assessment (2017) if the P criterion is not fulfilled for the substance, further generation of information on bioaccumulation is not needed in order to prevent unnecessary testing of animals.