Bioconcentration and Aquatic Toxicity of Superhydrophobic Chemicals: A Modeling Case Study of Cyclic Volatile Methyl Siloxanes

Abstract

Many chemicals in commerce are classified as "superhydrophobic", having log octanol-water partition coefficients (log KOW) approaching or exceeding 7. Examples include long-chain alkanes, halogenated aromatics, and cyclic volatile methylsiloxanes (cVMS). We show that superhydrophobic chemicals present unique assessment challenges because of their sparing solubility in water and difficulties in empirical determinations of bioconcentration factors (BCFs) and aquatic toxicity. Using cVMS as an example, BCFs are considerably lower than expected due to biotransformation. Reviewed aquatic toxicity test data for cVMS in a range of aquatic organisms show little or no toxic effects up to solubility limits in water and sediment. Explanations for this apparent lack of toxicity of cVMS, and by extension to other superhydrophobic chemicals, are explored using a conventional one-compartment uptake model to simulate bioconcentration and toxicity tests using an assumed baseline narcotic critical body residue (CBR) and a range of organism sizes. Because of the low aqueous concentrations, equilibration times are very long and BCFs are sensitive to even very slow rates of biotransformation. Most organisms fail to achieve the assumed CBR during feasible test durations even at the solubility limit. Regulatory evaluation of superhydrophobic substances requires specially designed test protocols addressing biotransformation and dietary uptake.

A re‐evaluation of PETROTOX for predicting acute and chronic toxicity of petroleum substances

“…Initially the limit identified in Equation 1 was parameterized at log K OW of 6 [9,[40][41][42][43], which effectively reduced the bioavailability of more hydrophobic components while allowing fractional contribution to overall toxicity of the substance to be included in P TU calculations [44][45][46]. This nonlinear partitioning behavior may reflect the impact of low solubility and slow toxicokinetics [47] or other competing processes that limit accumulation in the target lipid for these low-solubility constituents. The upper limit on the K LW in the TLM (Equation 1) was calibrated by minimizing the root mean square error (RMSE; the sum of log residuals) that was computed separately for acute and chronic toxicity data sets.…”

Quantitative weight-of-evidence analysis of the persistence, bioaccumulation, toxicity, and potential for long-range transport of the cyclic volatile methyl siloxanes

“…This has the advantage that the validity of the test to detect adverse effects is demonstrated and that values producing incipient responses, such as the lowest-adverse-effect level (LOAEL) and lowest-observed-effect concentration (LOEC), may be characterized for purposes of assessment of hazard or risk. However, these larger exposures may not be environmentally realistic or even thermodynamically attainable for substances, such as cVMS, that are poorly soluble in water, which results in slow uptake that might exceed feasible durations of tests (Fairbrother et al 2015;Mackay, Powell, and Woodburn 2015c).…”

“…organisms (Mackay, Powell, and Woodburn 2015c). In the QWoE, the greatest weighting among the relevant methodologies was given to high-quality field studies on trophic biomagnification (BMF, biomagnification factor).…”

“…The cVMSs have been the subject of several regulatory reviews in the United Kingdom (Environment Agency 2010a; 2010b; 2010c), Canada (Environment Canada [EC] and Health Canada [HC] 2008a;, and Nordic States (IVL 2005;Nordisk Ministerråd 2005), a judicial review (Giesy et al 2016; Siloxane D5 Board of Review 2011), and an evaluation for the European Chemicals Agency (ECHA) (Environment Agency 2014a; 2014b). In addition, several review papers have recently been published on the environmental and biological properties of D5 (Fairbrother et al 2015;Gobas et al 2015a;Mackay 2015;Mackay et al 2015a;2015c); as these papers were reviews and not original experimental studies they were not included in the quantitative weight of evidence (QWoE) but provided a separate opinion on one of the cVMS. Since and during the time when these reviews were conducted, new information has been published in reports and in the scientific literature, and this led us to undertake a QWoE method to assess the properties of the cVMSs as a whole weight of evidence (WoE).…”

Bioconcentration, bioaccumulation, biomagnification and trophic magnification: a modelling perspective

“…The columns on the right with log K OW of 8 represent an extreme condition of super-hydrophobicity 15,32 in which the diet is responsible for 99.8% of the inputs. The wet-weight concentrations are now nearly constant and the lipid-weight concentrations are highly variable.…”