Absorption and Bioconcentration

This topic will be developed by first outlining the stmctural features of a biological membrane and discussing the different mechanisms involved in transporting compounds across this barrier. The extent to which absorption is influenced by the properties of the compound will be illustrated by considering both animal and plant systems. Some compounds can absorb and accumulate, particularly in aquatic organisms, and the requirements for this to occur will be considered. 

1 Structure Our understanding of the composition, structure, and function of biological membranes has developed rapidly in recent years. Not only do these cellular components influence uptake of nutrients, but they also play significant roles in communication between cells. In this discussion, however, the focus will be on the involvement of the membrane in the uptake of xenobiotics. The 

Chemical Concepts in Pollutant Behavior, Second Edition, by Ian Tinsley ISBN 0-471-09525-7 2004 John Wiley Sons, Inc. 

3 Pick s Laws and Membrane Flux Since compounds of environmental significance cross biomembranes only by passive diffusion, it is of interest to define those factors that control the flux through the membrane in order to assess which properties of the chemical may be significant. Pick s first law states that the flux, J, in mass per unit area per unit time is proportional to the concentration differential. 

In membrane studies, one measures or controls the concentration, C, in the aqueous phase adjacent to the membrane and it is assumed that a rapid partitioning determines the concentration, Co, on the donor side of the membrane and 

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Values of Henry s law constant k =plc, where p is the partial pressure of the solute in the gas above the solution and c is the concentration of the solute) is a quantity frequently apphed in the thermodynamic description of dilute aqueous solutions, which is used in environmental chemistry and atmospheric physics as a major criterion for describing air-water partitioning of solutes at near ambient conditions. It plays amajor role in evaluating the transport of pollutants between atmosphere and aquatic systems, rainwater and aerosols. The octanol-water partition coefficient is a dimensionless number defined as the ratio of the compound s concentration in a known volume of octan-l-ol (Cq) to its concentration in a known volume of water (c ) after the octan-l-ol and water have reached equihbrium. It has been found to be related to water solubility, soil/sediment absorption coefficients and bioconcentration factors of pollutants for aquatic life. The adsorption coefficient normalised to the organic carbon content of the soil (sediment) is a useful indicator of the binding capacity of... 

Figure 8. The bioconcentration ratios of chlorophenols in goldfish at 1 h exposure to their media as a function of the difference between pH and pKa. 4-CP 4-chlorophenol, 2,5-DCP 2,5-dichlorophenol, 3,5-DCP 3,5-dichlorophenol, 2,4,6-TCP 2,4,6-trichlorophenol, PCP pentachlorophenol, 2-CP 2-chlorophenol, 3-CP 3-chlorophenol, 2,4-DCP 2,4-dichlorophenol, 2,3-DCP 2,3-dichlorophenol, 2,4,5-TCP 2,4,5-trichlorophenol, 2,3,4,6-TCP 2,3,4,6-tetrachlorophenol, 2,6-DCP 2,6-dichlorophenol. Reprinted from [185] Water Res., 29, Kishino, T. and Kobayashi, K. Relation between toxicity and accumulation of chlorophenols at various pH, and their absorption mechanism in fish , pp. 431-442. Copyright (1995), with permission from Elsevier...

Absorption is the process for which most is known about the influence of specific chemical properties. Nichols et al. [56] reviewed the literature and summarized the chemical properties and approaches to their assessment that appear to dominate adsorption (and metabolism) processes in fish. These properties and predictive tools could be used as guidance in the design of compounds with the potential for reduced likelihood of ecological effects, specifically bioconcentration and bioaccumulation. 

PBDEs are transported from water to aquatic organisms, which results in bioconcentration in aquatic organisms. No data on the bioconcentration of PBDEs in fish were located. However, due to abundance of monitoring data illustrating the accumulation of PBDEs in fish (see Section 6.4.4), bioconcentration of PBDEs is expected to occur. 6i a laboratory study of Baltic blue mussels, BCFs (from water absorption) were found to be 1,300,000 for BDE-47, 1,400,000 for BDE-99, and 1,300,000 for BDE-153 (Gustafsson et al. 1999). At several sites along the coast and in the Schelde estuary (the Netherlands), BCFs for blue mussels were determined (Booij et al. 2000). The maximum BCFs were log BCF of 9 for BDE-99 and BDE-100, and approximate log BCFs of 7.4 for BDE-28, 8.4 for BDE-47, and8.2 for BDE-153. 

Most studies show that bioaccumulation predominantly results from chemical absorption directly from water via the respiratory surface (e.g., gills and/or skin) of the organism (i.e., bioconcentration) and from diet via the gastro-intestinal tract (i.e., biomagnification). Exceptions occur e.g., phytoplankton species generally do not absorb chemical by dietary ingestion, organisms obtain chemical via maternal transfer, and some species contain unique mechanisms for chemical uptake and elimination. 

Regarding human health risks the Deca risk assessors note that scientific information on bioconcentration in human adipose tissues and subsequent elimination via for example breast milk is insufficient, but that other brominated diphenyl ethers (i.e. Hexa, Penta and Tetra) are excreted with breast milk. Based on the low rate of oral absorption in rats and the low bioaccumulation potential, the Deca risk assessors state that they might anticipate a rather low excretion of this compound in the breast milk. ... 

Acrylamide is well absorbed via the gastrointestinal and respiratory tracts. It is also well absorbed through the skin but less rapidly than through the gastrointestinal tract a significant portion of the dermally applied dose remains in the skin. Upon absorption into the blood, acrylamide is rapidly distributed throughout the body with an apparent volume of distribution equal to total body water. With the exception of plasma, erythrocytes, and testes, acrylamide and glycidamide do not exhibit preferential bioconcentration in any body tissue. 

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