Abstract-A mechanistic mass balance bioconcentration model is developed and parameterized for ionogenic organic chemicals (IOCs) in fish and evaluated against a compilation of empirical bioconcentration factors (BCFs). The model is subsequently applied to a set of perfluoroalkyl acids. Key aspects of model development include revised methods to estimate the chemical absorption efficiency of IOCs at the respiratory surface (E W ) and the use of distribution ratios to characterize the overall sorption capacity of the organism. Membranewater distribution ratios (D MW ) are used to characterize sorption to phospholipids instead of only considering the octanol-water distribution ratio (D OW ). Modeled BCFs are well correlated with the observations (e.g., r 2 ¼ 0.68 and 0.75 for organic acids and bases, respectively) and accurate to within a factor of three on average. Model prediction errors appear to be largely the result of uncertainties in the biotransformation rate constant (k M ) estimates and the generic approaches for estimating sorption capacity (e.g., D MW ). Model performance for the set of perfluoroalkyl acids considered is highly dependent on the input parameters describing hydrophobicity (i.e., log K OW of the neutral form). The model applications broadly support the hypothesis that phospholipids contribute substantially to the sorption capacity of fish, particularly for compounds that exhibit a high degree of ionization at biologically relevant pH. Additional empirical data on biotransformation and sorption to phospholipids and subsequent incorporation into property estimation approaches (e.g., k M , D MW ) are priorities with respect to improving model performance. Environ. Toxicol. Chem. 2013;32:115-128. # 2012 SETAC
Novel methods are developed and tested for measuring the octanol-air partition coefficient (KOA), which is suggested to be a valuable descriptor of airvegetation and air-soil equilibrium. Data are reported for six chlorobenzenes (CBs), five polychlorinated biphenyls (PCBs), and DDT over the temperature range -10 to +20 "C with values approaching 10l2. KOA varies log-linearly with reciprocal absolute temperature and increases by a factor of approximately 30 over this temperature range, the temperature coefficient being approximately 62 kJ/mol for CBs and 70 kJ/ mol for PCBs: f o r PCBs, the values of KOA are within a factor of 4 of values calculated as the ratio of the octanol-water and the air-water partition coefficients, with a factor of 7.4 applying to DDT. It is suggested that for hydrophobic chemicals it is preferable to measure KOA directly since this avoids handling aqueous solutions. * Corresponding author. 0013-936)(/95/0929-1599$09.00/0 0 1995 American Chemical Society VOL. 29, NO. 6, 1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY rn 1599KOA is then determined from the slope of the plot of In CA or In CO versus time, since CO and CA are always related by KOA. This is the equation used in the dynamic stripping method for determining KAW (12). The expected concentration history in octanol and air are also shown in Figure 1A. The characteristic time of response of the system is
(ii) MTC Is Infinite, Equilibrium Conditions Edst, andNo Mixing Occurs. In this case, the air immediately achieves equilibrium with the immobile octanol, and a concentration front or "shock wave" moves through the column as shown in Figure 1B. As a result, the exit air KOA Vi / G.
The critical body residue (CBR), estimated from aquatic toxicity QSARs and bioconcentration‐log Kow relationships, appears to be relatively constant, at about 4 mmol L−1 of fish, for the acute toxicity of a variety of hydrophobic narcotic organic chemicals examined by the U.S. Environmental Protection Agency (Duluth, MN) in tests with the fathead minnow. However, for hydrophilic chemicals (log Kow < 1.5) the bulk of the toxicant is in the water phase rather than the organic/lipid phase of the organism, so the whole‐body residues in these cases should be similar to the LC50 water concentration. Over the log Kow range of – 1.5 to 6, acutely toxic whole‐body residues for narcotics can be approximated by the QSAR‐derived equation: CBR (mM) = 2.5 mM + 50/Kow. Estimates obtained by this method are in reasonable agreement with the limited literature data available for acutely toxic whole‐body residues of hydrophobic narcotic organic chemicals. Elimination half‐lives estimated from nonlinear curve fitting to time‐toxicity information were relatively constant for the Duluth bioassay data at approximately 3 h. Despite the relatively high variability of this type of kinetics data, the literature information for small aquatic organisms, from both toxicity‐and bioconcentration‐based tests, was in a similar range. It appears that QSARs created with raw aquatic bioassay data occur primarily as a result of the influence of chemical‐physical properties on the partitioning process. Log Kow appears to have little to do with the inherent potency of the neutral, narcotic organic chemicals examined.
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