The Partition Mechanism
For many persistent, xenobiotic, hydrophobic compounds bioconcentration can be satisfactorily treated as a physical-chemical partitioning process between the lipid phases of an aquatic organism and the surrounding aqueous medium. For example, bioconcentration factors for fish have generally been shown to be related to the n-octanol/water partition coefficient KOW. Biodegradation is a complicating factor, and kinetic models based on diffusion are useful as tools for understanding the process of bioconcentration only in the absence of biodegradation. The possible significance of lipid solubility in bioconcentration has been investigated by A. J. Dobbs and N. Williams, who found decreasing lipid solubilities with increasing solute hydrophobicity. Certainly for many large chemicals, such as polychlorinated naphthalenes, dibenzodioxins,perchlorobiphenyl, and perchloro-p-terphenyl, bioconcentration factors are less than anticipated on the basis of hydrophobicity, as measured by KOW.