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Prussian Blue

Well right now I imagine that farmers in Japan are unhappy and concerned about the the future.

I know that the Japanese government have introduced rules which ban the sale of some foods made in some part of their country. What I want to tell you about now is the idea of biological half life. The idea of biological half life is that a living animal is able to excrete a toxic element at a rate which is often proportional to the concentration of the toxic metal inside the animal.

So for a radioactive isotope if the animal has concentration A (an activity) then the rate is typically

Rate = dA/dt = k A

This leads to the toxic or radioactive isotope having a half life in the animal, if we consider for a moment an isotope with a very long half life which allows us to ignore the change in activity (number of radioactive events per second) due to the physical decay of the isotope. A good examples would be chlorine-36 and iodine-129 which have a very long half life.

For these isotopes the reduction in activity will be due to the rate at which the animal loses the radioactive atoms from its body. The half life for the disappearance of an isotope in an animal is given by the equation

1/Effective half life = (1/Biological half life) + (1/Radioactive or physical halflife)

For some elements such as cesium and tritium it is possible to alter the biological half life, in the case of tritium as long as it is in the form of water it is possible to reduce the biological half life by drinking vast volumes of water. I have been told by a former glovebox worker at Harwell that the post exposure treatment for exposure to tritium in the form of water was to drink 5 litres of water or beer after exposure.

In the case of cesium a less intoxicating treatment is known, it is prussian blue. This is a coordination polymer of iron and cyanide units. The cyanide is very tightly bonded to the iron so there is no danger of cyanide poisoning as a result of eating prussian blue, for humans who have internal cesium-137 contamination the normal dose of prussian blue is 3 grams per day while in extreme cases 30 grams a day can be used.

Prussian blue is a very nice drug for removing cesium it is a complex solid which consists of several different chemical forms, it works becuase it has lots of potassium ions there potassium ions can be exchanged for cesium. The gut of a person who has eaten prussian blue is where the ion exchange occurs, it works by making a thermodynamic sink for cesium. When the person or animal goes to the toilet the cesium loaded prussian blue comes out. The cesium will be very tightly bonded so it will not leach out from the prussian blue in things like cow pats.

If any farmers in Japan are reading, my advice would be to ask your farmers union to find out about about the use of prussian blue to lower the cesium contamination level in animals.



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