Hydrophobic compounds, bioconcentration

Chessells, M., Hawker, D.W., Connell, D.W. (1992) Influence of solubility in lipid on bioconcentration of hydrophobic compounds. Ecotoxicol. Environ. Saf. 23, 260-273. 

Banerjee, S., Suggatt, R.H., and O Grady, D.P. A simple method for determining bioconcentration parameters of hydrophobic compounds. Environ. Sci. TechnoL., 18(1) 78-81, 1984a. 

For very hydrophobic compounds, when is very high (10 ), fhere is evidence from field studies involving fish, birds, and animals fhaf fhere is a bioaccumulation or bioconcentration up to food chain [114]. The pesticide DDT is an example of bioaccumulating chemical with a Kqw of —10 . The thermodynamic model for air/water and 1-ocfanol/wafer acfivify coefficients and the importance of the activity coefficient at infinite dilution measurements have been presented as well [114]. 

Chessells, M., D. W. Hawker, and D. W. Connell, Influence of Solubility in Lipid on Bioconcentration of Hydrophobic Compounds. Ecotoxicol. Environ. Safety, 1992 23, 260-273. 

Muller, J.F., D.W. Hawker, and D.W. Connell. 1994. Calculation of Bioconcentration Factors of Persistent Hydrophobic Compounds in the Air/Vegetation System. Chemosphere 29, 623-640. 

Water solubility is one of the major parameters which affect the fate and distribution in the environment. Hydrophobic compounds with high octanol-water partition coefficients tend to bio accumulate. Opperhuizen and Voors [63] have shown that hydrophobicity of PCDEs determines the bio concentration factor of PCDEs and that bioconcentration kinetics of PCDEs resemble those of PCBs. 

The measurement of bioconcentration is difficult because the water concentration must remain constant during the run and contact must be maintained until equilibrium is reached in the organism. Equilibration, signalled by a plateau in the concentration vs time plot, may take several days. This entails, particularly in the case of relatively hydrophobic compounds of low water solubility, dosing in a flow-through chamber at levels well below the toxic threshold. A complete experiment involves analysis of samples during the exposure, or "uptake phase", and also following transfer to a clean environment where release (depuration) occurs. Both the parent chemical, from which the bioconcentration factor is calculated, and known metabolites are analyzed (15). 

Glycerol trioleate has been used in an attempt to simulate lipid membranes and to take into account some of the solvent associations plausibly occurring in biota an impressive direct correlation was observed between log Phv (Phv is the partition coefficient between glycerol trioleate and water) and BCF values in rainbow trout expressed on a lipid basis, and these results were used to support the view that the bioconcentration of nonpolar hydrophobic xenobiotics is significantly determined by their lipid solubility (Chiou 1985). This conclusion is further supported by the results of an extensive examination of a series of highly hydrophobic compounds which do not demonstrate a high potential for bioconcentration (Chessells et al. 1992). 

Various predictive models have been proposed to estimate BCFs for organic compounds based on their hydrophobicity as represented by their n-octanol/water partition coefficient (Kow) [15], Explosive compounds are weakly hydrophobic (low Kow values) and therefore BCFs predicted using these models should be low [3,6,17]. The equation derived by Meylan et al. [15] for nonionic compounds in the log Kow range of 1 to 7 (log BCF = 0.86 log Kow - 0.39) was used to calculate predicted BCFs for explosives and related compounds (Table 6.2) for empirically derived BCF values reported in Table 6.1. Comparison of predicted and empirically derived BCFs revealed that most explosives and related compounds bioconcentrate in aquatic... 

Those compounds that have higher values have a higher tendency to bioconcentrate, also have lower tendencies to be excreted, and will take longer times to achieve steady state. So, compounds that bioconcentrate will not only need to show high values, but will also have to persist to achieve Cf values approaching those predicted from Tb-2. Dissolved Organic Matter Natural waters will always contain a certain amount of DOM. It has been demonstrated that hydrophobic compounds can partition into this compartment (Section 2.2.5.3, Chapter 2). The analysis of water samples does not usually discriminate between the amounts of compound in free solution and that associated with the DOM and, hence, the amount detected, Ctot would be... 

Correlations between the bioaccumulation tendency of organic chemicals in aquatic organisms and 1-octanol/water partition coefficients show a loss of linearity for very hydrophobic compounds. In order to establish the possible cause(s) of this phenomenon, the roles of metabolism, exposure time, membrane permeation, lipid solubility, and bioavailability on the bioconcentration potential of chemicals, are discussed. Data are presented showing their relative importance. It is concluded that, although insufficient experimental data presently exist which can conclusively establish the cause(s), reduced lipid solubility and reduced bioavailability are the most likely factors contributing to the loss of linear correlation for non-metabolizing chemicals. 

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 major route for bioaccumulation of hydrophobic organic compounds in aquatic animals is passive diffusion over cell membranes. In fish, the gill epithelia are the predominant port of entry, with less than 40% of uptake across the skin [181]. Since permeability of the membrane is a direct function of the membrane-water partition coefficient and the diffusion coefficient across the membrane interior [182], the bioconcentration factor (logBCF) can be directly correlated with log K0Vl. or log Km%v for compounds with intermediate hydro-phobicity [183,184],... 

Figure 3.3 shows that the log /fow-log sw plot for compounds with log Kow > 5 deviates from linearity. This phenomenon is also observed for plots of log bioconcentration factor (BCF) versus log Ko (Connell, 1990). Chiou (1985) has shown that a similar deviation occurs in a triolein-water system alone, at log Ko > 5.5, as a result of solute-triolein incompatibility. Similarly, Banerjee and Baughman (1991) argued that BCFs of large molecules are smaller than expected on the basis of their hydrophobicity as a result of their disrupting effect on the structure of the lipid phase. The curvilinearity is not likely to be caused by sorption to DOC in the experimental systems (Section 3.4.), because deviations from linearity already occur at log /fow 5, whereas the effect of DOC-bo mA contaminants can only be expected at log /fow values > 7. 

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