User Guide to Thresholds and Classification - Environmental ...

379User Guide for Thresholds and Classifications19F.3 Influence of external and internal factors on the bioconcentrationpotential of organic substancesFactors influencing the uptakeThe uptake rate for lipophilic compounds is mainly a function of the size of the organism (Sijm and Linde,1995). External factors such as the molecular size, factors influencing the bioavailability, and differentenvironmental factors are of great importance to the uptake rate as well.Size of organismSince larger fish have a relatively lower gill surface to weight ratio, a lower uptake rate constant (k1) is to beexpected for large fish compared to small fish (Opperhuizen and Sijm, 1990; Sijm and Linde, 1995). Theuptake of substances in fish is further controlled by the water flow through the gills; the diffusion throughaqueous diffusion layers at the gill epithelium; the permeation through the gill epithelium; the rate of bloodflow through the gills, and the binding capacity of blood constituents (ECETOC, 1995).Molecular sizeIonised substances do not readily penetrate membranes; as aqueous pH can influence the substanceuptake. Loss of membrane permeability is expected for substances with a considerable cross sectional area(Anliker et al, 1988; Opperhuizen et al, 1985) or long chain length (> 4.3 nm) (Opperhuizen, 1986). Loss ofmembrane permeability due to the size of the molecules will thus result in total loss of uptake. The effect ofmolecular weight on bioconcentration is due to an influence on the diffusion coefficient of the substance,which reduces the uptake rate constants (Gobas et al, 1986).AvailabilityBefore a substance is able to bioconcentrate in an organism it needs to be present in water and available fortransfer across fish gills. Factors, which affect this availability under both natural and test conditions, will alterthe actual bioconcentration in comparison to the estimated value for BCF. As fish are fed duringbioconcentration studies, relatively high concentrations of dissolved and particulate organic matter may beexpected, thus reducing the fraction of chemical that is actually available for direct uptake via the gills.McCarthy and Jimenez (1985) have shown that adsorption of lipophilic substances to dissolved humicmaterials reduces the availability of the substance, the more lipophilic the substance the larger reduction inavailability (Schrap and Opperhuizen, 1990). Furthermore, adsorption to dissolved or particulate organicmatter or surfaces in general may interfere during the measurement of BCF (and other physical/chemicalproperties) and thus make the determination of BCF or appropriate descriptors difficult. As bioconcentrationin fish is directly correlated with the available fraction of the chemical in water, it is necessary for highlylipophilic substances to keep the available concentration of the test chemical within relatively narrow limitsduring the uptake period. Substances, which are readily biodegradable, may only be present in the test waterfor a short period, and bioconcentration of these substances may thus be insignificant. Similarly, volatilityand hydrolysis will reduce the concentration and time in which the substance is available forbioconcentration.January 2012 EPA0109