Hydrophobicity and Bioaccumulation

Bioaccumulation is the critical process by which xenobiotics can influence the living species in the aquatic ecosystems. This process can be divided into two types one involving bioconcentration, and the other biomagnification, which is more complex. Bioconcentration refers to an increase in the concentration of a chemical in the organism over a period of time compared with the chemical s concentration in the aqueous phase and is a net result of uptake, storage and elimination. The bioconcentration factor (BCF) is defined as the ratio of the concentration of a chemical in an aquatic organism to that in the aqueous 

To circumvent this problem, Bintein, Devillers and Karcher developed one of the most comprehensive models for a wide range of chemicals including many pesticides for which bioconcentration data were acquired in five families of freshwater fish. They utilized Kubinyi s bilinear model the log P values of the compounds in the dataset ranged from 1 to 9. 

Biphasic functions with a linear ascending (slope = 0.91) and descending slope (-1.975 + 0.91 =-1.065), and a rounded apex are represented by this model. Highly hydrophobic compounds with log P values 6 cover the apical and descending part of the curve. The uptake of chemicals from food and water has been compared and tissue concentrations are similar for water and food. Thus the uptake of pesticides from food is less critical than its uptake from water and only a fraction of the residue present in the lower level biota is transferred to the higher level of the food chain. 

The use of log Poet in predicting environmental toxicity provides some useful insights into its value in CADD. It is the principal parameter that predicts bioaccumulation of PCBs and DDT analogs in the food chain, but of even greater importance, log P establishes a base line toxicity of environmental hazards in aquatic and other systems as illustrated in QSAR 7.30.  

This 4-day toxicity study conducted in juvenile fat-head minnows by the USEPA, encompassed a large series of stable, unreactive and non-ionizable compounds such as hydrocarbons, alcohols, esters, ketones and herbicides whose partition coefficients ranged from -1.30 to 6. 

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An exception to the above rule of the thumb is the alkylphenol ethoxylates. Biodegradation of these compounds yields the relatively stable C9APEOm with 0 < m<3 ethoxylate units or the respective carboxylates (Ahel, 1989). These ethoxylated products are more hydrophobic and bioaccumulative (Ekelund et al., 1990 Ahel et al., 1993) than the precursor surfactant molecules and are also of toxicological concern (Granmo et al. 1989, Jobling et al., 1996). 

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