# Bioconcentration

Bioconcentration is the accumulation of a substance in an aquatic organism through direct absorption from the surrounding water. In contrast to the concept of bioaccumulation , the intake pathway via food is not taken into account. The bioconcentration and the bioconcentration factor (BCF) derived from it are important test parameters in ecotoxicology . The uptake into the body of a fish or another aquatic organism takes place via the gills or other permeable body surfaces, which are particularly permeable to many lipophilic substances in soft-skinned organisms .

The term bioconcentration is used both for the dynamic process of gradual concentration of the substance in the organism and for the characterization of the respective instantaneous or final state ( state of equilibrium ). If the concentration in the water decreases again, the bioconcentration in the organism usually decreases according to an exponential curve that starts out more strongly and then later less . Depending on the fat solubility of the compound and the size of the organism, the decrease will take place at different speeds and sometimes only partially. Depending on the fat solubility of a compound, it can take a longer time to reach equilibrium. The time required can be estimated using the following equation:

${\ displaystyle t_ {balance} = 0.00654 \ cdot K_ {OW} +55.31}$ (in hours)

Consequently, with a compound with an octanol-water partition coefficient (log K OW ) of 4, it takes around five days to reach equilibrium, whereas with a log K OW of 6, it takes nine months.

## Bioconcentration Factor (BCF)

The bioconcentration compared to the water can be characterized as a bioconcentration factor. This represents a quantity that represents the ratio of the concentrations in the organism in comparison to the water. The unit of the BCF is l / kg, but is partially omitted. The kinetics of the rate of uptake or decrease of the bioconcentration and thus also of the BCF corresponds to the kinetics of the one-, two- or multi- compartment models .

In ecological fish tests, the BCF is defined as the ratio between the concentration of the test substance in the test fish (c f ) and the concentration in test water (c w ):

${\ displaystyle BCF = {\ frac {c_ {f}} {c_ {w}}}}$,

where the value is measured under equilibrium conditions, i.e. when the absorption rate through diffusion over body surfaces corresponds to the elimination rate (excretion, outward diffusion or degradation). It must be noted that the BCF differs between different species and may a. depends on the development phase.

In addition to animal experiments, the BCF of a substance can also be approximately calculated from the K OW value.

Compounds with a BCF of> 2000 l / kg or> 5000 l / kg for aquatic species meet the sub-criterion for PBT or vPvB substances under REACH .

## Bioconcentration versus bioaccumulation and biomagnification

In studies with aquatic organisms, the bioconcentration is often seen as just one of two possible absorption and accumulation paths that contribute to the overall bioaccumulation , because in addition to the direct absorption from the water, the absorption through food must also be taken into account ( biomagnification ). However, this conceptual subdivision is often difficult to prove in practice, since the various absorption routes into the body often run simultaneously and bioaccumulation is in a complex dynamic equilibrium between this absorption and the various forms of elimination (breakdown or excretion of the substance from the body) . In addition, especially in the case of small, soft-skinned aquatic organisms (as they are often used for test purposes) and in the case of lipophilic substances, the bioconcentration is the faster and therefore more important absorption mechanism compared to biomagnification.

## literature

• Bernd Beek: Bioaccumulation. New Aspects and Developments . The Handbook of Environmental Chemistry Vol. 2. Springer, Berlin 2000, doi: 10.1007 / 10503050 , ISBN 978-3-540-62575-9 .
• Thomas Braunbeck, David E. Hinton, Bruno Streit : Fish Ecotoxicology . Birkhäuser, Basel 1998, ISBN 978-3-0348-8853-0 .
• Karl Fent: Ecotoxicology . Georg Thieme, Stuttgart 2003, ISBN 9783131099921 .
• Jon Arnot, Frank Gobas: A review of bioconcentration factor (BCF) and bioaccumulation factor (BAF) assessments for organic chemicals in aquatic organisms. In: Environmental Reviews. 14, 2006, pp. 257-297, doi: 10.1139 / a06-005 .

## Individual evidence

1. OECD Guidelines for the Testing of Chemicals: Test No. 305: Bioaccumulation in Fish: Aqueous and Dietary Exposure, p. 56, doi: 10.1787 / 9789264185296-en .
2. Hawker DW and Connell DW (1988), Influence of partition coefficient of lipophilic compounds on bioconcentration kinetics with fish. Wat. Res. 22: 701-707, doi: 10.1016 / 0043-1354 (88) 90181-9 .
3. OECD Guidelines for the Testing of Chemicals: Test No. 305: Bioaccumulation in Fish: Aqueous and Dietary Exposure, p. 42, doi: 10.1787 / 9789264185296-en .
4. ^ Pim NH Wassenaar, Eric MJ Verbruggen, Ellen Cieraad, Willie JGM Peijnenburg, Martina G. Vijver: Variability in fish bioconcentration factors: Influences of study design and consequences for regulation. In: Chemosphere . 239, 2020, p. 124731, doi : 10.1016 / j.chemosphere.2019.124731 .