Iodine in the air

Iodine, I imagine that this halogen has been the subject of considerable public interest and alarm in recent times.

The iodine in a used nuclear fuel can escape during an accident, it is important to understand that the form of the iodine which is released from the fuel pellets is not guaranteed to be the form of the iodine which is encountered by plants, animals and humans. Once the iodine escapes from the fuel it enters a complex web of chemical reactions which can convert it into new things.

A key part of the iodine is in the form of organic iodines, in a 1980s paper the different forms of iodine which were released by the Chernobyl accident were captured using a clever filter design by some Japanese workers. They showed that the majority of the radioactive iodine which was in their air was in the form of organic compounds such as methyl iodide.

H. Noguchi and M. Murata in Journal of Environmental Radioactivity, 1988, volume 7, pages 65 to 74 showed that the iodine in the air was split into four parts. 

70 % Organic iodine

19 % Particulate iodine

6 % HOI

5 % I2

It is interesting to note that in California that one organic iodine compound (methyl iodide) is used as a pesticide in strawberry farming. It is used to fumigate the soil, years ago the standard chemical was methyl bromide but the use of methyl bromide for this purpose has been outlawed as it harms the ozone layer.

In the case of methyl bromide the molecules could diffuse up into the ozone layer where they would then undergo a photochemical break down reaction which forms methyl radicals and bromine atoms. These bromine atoms would then catalyse the break down of ozone.

The reason why methyl iodide was chosen was that it is less stable against light, it is more quickly broken down to form methyl radicals and iodine atoms because the carbon iodine bond is weaker than a carbon bromine bond. The iodine atoms formed can then react with tropospheric ozone, nitrogen oxides and other things to form new substances such as iodine monoxide (IO^.) and nitrosyl iodide. The rate at which these reactions will occur will depend on how much light and how clean the air is.