The piscine bioconcentration characteristics of cyclic and linear oligomeric permethylsiloxanes

Using a conventional "resaturation" method whereby aquarium water was continuously passed through a column containing sand or fine glass beads coated with cyclic and linear permethylsiloxanes, their uptake levels by rainbow trout and fathead minnows have been compared. Because of the uncertainty associated with defining the actual aqueous concentrations of such poorly soluble substances, this study was focused on defining the "attainable uptake" levels from saturated solutions rather than precise definition of actual bioconcentration factor values. Although cyclic Me2SiO-oligomers accumulated to a greater extent in fish than did comparable linear oligomers, uptake decreased sharply with increasing molecular weight. Thus, in the cyclic series (Dx), order of magnitude decreases were observed for each incremental molecular weight increase; i.e., for the compounds D4, D5, and D6 uptake levels of approximately 200, 20 and 2 ppm, respectively, were observed. Uptake of D8 was below our detection limit of 300 ppb. In the linear series, uptake of the tetramer MD2M was an order of magnitude less than observed for D4 and little or no uptake (i.e., less than 0.5 ppm) was observed for MD3M, MD4M and MD7M. The branched oligomer M3T exhibited levels comparable to its unbranched isomer MD2M, while M4Q was more comparable to the D6 uptake of 1-2 ppm. Very similar uptake levels of D5 resulted with and without a surfactant, even though the surfactant afforded a 20-fold increase in the D5 content of the water. This suggests that bio-availability is defined by the amount present in true solution as individual molecules and is not affected by the presence of aggregates or micelles. The highly inverse relationship observed in this study between uptake and molecular weight is strongly supportive of earlier estimates of a limiting molecular weight of about 600. These findings also strongly contradict a recent Japanese study, which concluded that bioconcentration not only occurred but actually increased with molecular weight in a series of commercial polydimethylsiloxane fluids. Also contrary to a recently published inference of biotransformation in fish, no evidence for such phenomena was observed in this study.