Bioconcentration modeling

Although the simple bioconcentration model assumes relatively unhindered movement of a contaminant across the barriers between water and lipid tissue, such is often not the case. The uptake of an organic species can be a relatively complex process in which the chemical must traverse membranes in the gills and skin to reach a final lipid sink. A physiological component of the process by which a chemical species moves across membranes tends to cause bioconcentration to deviate from predictions based on hydrophobicity alone. 

Although a simple bioconcentration model assumes rapid movement of a hydrophobic contaminant through an organism, distribution may be relatively slow. The predominant limiting factor in this case is the blood flow. Slow transport to lipid tissue sinks can result in lower apparent BCF values than would be the case if true equilibrium were attained. 

Thomann, R. V. (1989) Bioconcentration model of organic chemical distribution in aquatic food chains. Environ. Sci. Technol. 23, 699-707. 

Last, even if bioconcentration, the process of accumulation of waterborne chemicals by aquatic organisms through nondietary routes, is a property and not an activity, it is important to stress that there exist a huge number of QSPR models allowing one to predict the accumulation of chemicals in biota. For further information on this important topic, the reader is referred to Devillers et al. [Ill], Dearden [112], and Dimitrov et al. [113] who review all the existing bioconcentration models and their advantages and limitations. 

Devillers J, Domine D, Bintein S, Karcher W. Comparison of fish bioconcentration models. In Devillers J, editor, Comparative QSAR. Philadelphia Taylor and Francis, 1998. p. 1-50. 

Some models for predicting bioconcentration and biomagnification are presented in Box 4.1. 

In general, it is easier to use models such as these to predict the distribution of chemicals (i.e., relationship between exposure and tissue concentration) than it is to predict their toxic action. The relationship between tissue concentration and toxicity is not straightforward for a diverse group of compounds, and depends on their mode of action. Even with distribution models, however, the picture can be complicated by species differences in metabolism, as in the case of models for bioconcentration and bioaccumulation (see Chapter 4). Rapid metabolism can lead to lower tissue concentrations than would be predicted from a simple model based on values. Thus, such models need to be used with caution when dealing with different species. 

Zaroogian, G.E., Heltshe, J. F., Johnson, M. (1985) Estimation bioconcentration in marine species using structure-activity models. 

Noegrohati, S., Hammers, W.E. (1992) Regression models for octanol-water partition coefficients, and for bioconcentration in fish. Toxicol. Environ. Chem. 34, 155-173. 

Huckins, J.N., M.W. Tubergen, and G.K. Manuweera. 1990b. Semipermeable membrane devices containing model lipid a new approach to monitoring the bioavailability of lipophilic contaminants and estimating their bioconcentration potential. Chemosphere 20 533-552. 

Chiou chose glyceryl trioleate (triolein) as model lipid because of its similarity to triglycerides which are abundant in organisms [109], Triolein is also a bulk lipid and the good correlation with the bioconcentration factor is restricted to neutral compounds of moderate hydrophobicity. No attempts were made to measure partitioning of ionogenic compounds with the glyceryl trioleate-water partition system. 

The dominant transport process from water is volatilization. Based on mathematical models developed by the EPA, the half-life for M-hexane in bodies of water with any degree of turbulent mixing (e.g., rivers) would be less than 3 hours. For standing bodies of water (e.g., small ponds), a half-life no longer than one week (6.8 days) is estimated (ASTER 1995 EPA 1987a). Based on the log octanol/water partition coefficient (i.e., log[Kow]) and the estimated log sorption coefficient (i.e., log[Koc]) (see Table 3-2), ii-hexane is not expected to become concentrated in biota (Swann et al. 1983). A calculated bioconcentration factor (BCF) of 453 for a fathead minnow (ASTER 1995) further suggests a low potential for -hcxanc to bioconcentrate or bioaccumulate in trophic food chains. 

The first-order one-compartment model [43,58], which considers the organism as one homogeneous compartment surrounded by a homogeneous medium, provides an acceptable estimation of surfactants bioconcentration, and has been adopted by the OECD and EPA in their guidelines [3-5]. The BCF can be determined as a ratio of the concentrations of the chemical in the organism (Ca) and the medium (Cw) under equilibrium state or as a ratio of the uptake and elimination constants (ki and, respectively). 

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