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Review
, 36 (4), 882-897

Assessing the Bioaccumulation Potential of Ionizable Organic Compounds: Current Knowledge and Research Priorities

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Review

Assessing the Bioaccumulation Potential of Ionizable Organic Compounds: Current Knowledge and Research Priorities

James M Armitage et al. Environ Toxicol Chem.

Abstract

The objective of the present study was to review the current knowledge regarding the bioaccumulation potential of ionizable organic compounds (IOCs), with a focus on the availability of empirical data for fish. Aspects of the bioaccumulation potential of IOCs in fish that can be characterized relatively well include the pH dependence of gill uptake and elimination, uptake in the gut, and sorption to phospholipids (membrane-water partitioning). Key challenges include the lack of empirical data for biotransformation and binding in plasma. Fish possess a diverse array of proteins that may transport IOCs across cell membranes. Except in a few cases, however, the significance of this transport for uptake and accumulation of environmental contaminants is unknown. Two case studies are presented. The first describes modeled effects of pH and biotransformation on the bioconcentration of organic acids and bases, while the second employs an updated model to investigate factors responsible for accumulation of perfluorinated alkyl acids. The perfluorinated alkyl acid case study is notable insofar as it illustrates the likely importance of membrane transporters in the kidney and highlights the potential value of read-across approaches. Recognizing the current need to perform bioaccumulation hazard assessments and ecological and exposure risk assessment for IOCs, the authors provide a tiered strategy that progresses (as needed) from conservative assumptions (models and associated data) to more sophisticated models requiring chemical-specific information. Environ Toxicol Chem 2017;36:882-897. © 2016 SETAC.

Keywords: Bioaccumulation; Ecological risk assessment; Ionizable organic chemical (IOC); Toxicokinetics.

Figures

Figure 1.
Figure 1.
Model [30] predictions and observed values for the gill uptake rate constant (k1, upright triangles), gill elimination rate constant (k2, inverted triangles) and steady-state blood/water concentration factor (BCFBW, stars) as a function of pH, for the weak acids 2,3,4-trichlorophenol (log KOW,N = 4.0) and 2,3,4,5-tetrachlorophenol (log KOW,N =4.2) and the weak base diphenhydramine (log KOW,N = 3.6). Solid, long-dashed, and short-dashed lines show predicted values for BCFBW, k1, and k2 respectively, where the modeled BCFBW is the ratio of k1 and k2. The dotted line denotes what the model-calculated BCFBW would be in the absence of any pH reduction in the gills.
Figure 2.
Figure 2.
Modeled [30] effects of biotransformation on steady-state blood/water bioconcentration factors (BCFBW) for hypothetical weak acids (left panel) and weak bases (right panel) with pKa and log KOW,N values as indicated. No biotransformation - solid lines; moderate (0.02/h) biotransformation - dashed lines; fast (0.1/h) biotransformation - dotted lines. Figure 3: Proposed interim workflow for screening and prioritization of IOCs for bioaccumulation hazard assessment and ecological exposure and risk assessment. For the latter, the ―B question‖ would be replaced by the results and decision-contexts for the exposure and risk estimates.
Figure 3:
Figure 3:
Proposed interim workflow for screening and prioritization of IOCs for bioaccumulation hazard assessment and ecological exposure and risk assessment. For the latter, the ―B question‖ would be replaced by the results and decision-contexts for the exposure and risk estimates.

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