Hexachlorobenzene

A concentration of 100 µg hexachlorobenzene (HCB) per kg wet weight has been measured in fish that live and in the sediment/water interface. The average fat content of the fish is about 5 %  (i.e. 0.05 kg lipid per kg wet weight). The aqueous concentration of HCB in water was determined to be 0.1 ng L^-1, in the sediment (f_oc = 0.05 kg_ockg^-1 in dry sediment) a concentration of 2 µg kg^-1 sediment (dry) was analysed.

Estimate (roughly) which concentration one would expect in the fish (in µg per kg wet weight) if they were in equilibrium with
a) the water phase or
b) the sediment phase.

What assumptions do you make?

Use the following partition constants:

log K_lipid/water = 5.8 (L_water/L_lipid)

log K_oc/water = 5.3 (L_water/kg_oc)

Answer:

a) 100 µg per kg wet weight correspond to 2·10⁶ ng per kg lipid. In equilibrium with water one would expect 6.3·10⁴ ng/kg_lipid if HCB only partitions into fish lipids.
On basis of the total wet weight of the fish one would expect a concentration of 3.15 µg/kg_{wet weight} instead of the 100 µg/kg_{wet weight} that have been measured.

b) The K_oc is 2 ·10⁵ L_water/kg_oc. The sediment/water partition constant thus is 1·10⁴ L_water/kg_sediment because only 5% of the sediment is organic carbon. The equilibrium concentration in the pore water then was about 2·10^-4 µg /L_water oder 0.2 ng/L_water based on the measured 2 µg kg^-1 in wet sediment. In equilibrium with the sediment one would expect 1.26·10⁵ ng/kg_lipid if HCB only partitions into the fish lipids.

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