Diffusion, bioconcentration

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],... 

The ability to predict the behavior of a chemical substance in a biological or environmental system largely depends on knowledge of the physical-chemical properties and reactivity of that compound or closely related compounds. Chemical properties frequently used in environmental assessment include melting/boiling temperature, vapor pressure, various partition coefficients, water solubility, Henry s Law constant, sorption coefficient, bioconcentration factor, and diffusion properties. Reactivities by processes such as biodegradation, hydrolysis, photolysis, and oxidation/reduction are also critical determinants of environmental fate and such information may be needed for modeling. Unfortunately, measured values often are not available and, even if they are, the reported values may be inconsistent or of doubtful validity. In this situation it may be appropriate or even essential to use estimation methods. 

Sijm DTHM, Van der Linde A. 1995. Size-dependent bioconcentration kinetics of hydrophobic organic chemicals in fish based on diffusive mass transfer and allometric relationships. Environ Sci Technol 29 2769-2777. 

Ionized substances do not readily penetrate membranes as aqueous pH can influence the substance uptake. Loss of membrane permeability is expected for substances with a considerable cross-sectional area (Opperhuizen et al., 1985 Anliker et al., 1988) or long chain length (> 4.3 nm) (Opperhuizen, 1986). Loss of membrane permeability due to the size of the molecules will thus result in total loss of uptake. The effect of molecular 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). 

A well-known subacute effect is the growth reduction in algae. Hitherto, only external effect concentrations have been reported for this type of subacute effect, since experimental problems make it difficult to determine those internal effect concentrations, and existing bioaccumulation models for, e. g., fish, do not apply to algae, e.g. [78]. It must be noted that algae and other small organisms are prone to diffusive uptake for contaminants from the ambient environment for which the link between bioconcentration and the internal effect concentration concept would be very promising. 

The consideration of bioconcentration as an interphase distribution between aqueous and organic phases governed by diffusion processes anticipates a potential parallelism with partitioning processes between other non-miscible phases. Direct relationships between the partition coefficients in different systems (water/organic phases A and B, respectively) were recorded by Collander (1951) ... 

Earthworms were used in the severely hit target Djakovica garrison to evaluate the amount and diffusion of DU added to the natural pool and to assess the repercussions on organisms. The total uranium concenttation (0.142-0.656 mg kg" , with an overall median value of 0.319 mg kg ) in earthworms sampled up to 20 cm depth was comparable with results (0.202-0.843 mg kg ) measured in 38 earthworm individuals of different variety and age collected in a large (about 500 km ) uranium-uncontami-nated area. The upsu ratio in the earthworms was variable (0.(X)5241-0.(X)7266) and generally independent of both the total uranium contents in soils and the absolute uranium levels in the animals [113]. Nevertheless, bioconcentration seemed greater at lower soil concentrations indicating that the rate of elimination of uranium by the animals increased as the soil uranium levels increased. 

Based on the observed lack of bioconcentration of hexabromobenzene, hepta- and octachloronaphthalene and octachlorodioxin, it has been suggested that molecules with certain spatial dimensions are not able to cross the membrane-water interface and enter the membrane (Opperhuizen et al. 1984, 1985). These molecules, which were suggested to have minimal internal cross sections (MICS) of 0.96 nm or larger, are thus believed to have a low or zero bioconcentration factor. From a mechanistic point of view it seems conceivable that a cut-off diameter exists for passive membrane diffusion. However, what this cut-off diameter exactly is remains unclear since in a recent unpublished bioconcentration experiment, brominated biphenyls with the same MICS as hexabromobenzene were accumulated in guppies. The obvious test of this effect is to perform bioconcentration experiments with conservative chemicals varying in their spatial dimensions. 

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