1 research outputs found
Bioconcentration and Aquatic Toxicity of Superhydrophobic Chemicals: A Modeling Case Study of Cyclic Volatile Methyl Siloxanes
Many
chemicals in commerce are classified as “superhydrophobic”,
having log octanol–water partition coefficients (log <i>K</i><sub>OW</sub>) 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