Exciting Elements #1 - Iodine

This is the first installment of a new series title Exciting Elements. Exciting Elements will be a profile of an element on the Periodic Table. However, since this is not a chemistry blog, but a geology blog, the posts will concern the geochemical behaviour of the subject element. For example, each post will contain info about how it moves in the environment, are there environment toxicology risks associated with the element, does it form minerals, how do we detect it and can it be used to answer questions about the Earth.

Interested yet??? Well, you should be since there are 118 elements in the Periodic Table (although not all relate to geology very well) and I aim to blog about them. So hold on to your pipettes!

I have decided to proceed in no particular order, at least at the start, so we will be jumping around all over the table and the first spot that we will land is on element number 53 - iodine.

I figured that iodine would be a good place to start since I am doing my Ph. D. on the the geochemical behaviour of this element and it is always good to start off with what you know....of course this means that iodine has officially taken over every aspect of my life.

Discovery


Iodine was initially discovered by French chemist Bernard Courtois in 1811. Courtois was a gunpowder manufacturer during the Napoleonic Wars and was making sodium carbonate, required for the production of saltpetre by adding sulphuric acid to seaweed. One day he added too much acid and a purple vapour rose from his sample and then crystallized on cold surfaces. This was the first synthesis of iodine.

The name iodine originates from the purple vapour that emanates from native iodine and is derived from the Greek ἰοειδής which means purple.

Pure iodine. Even though it looks like a metal, it is not. It is actually a halogen. (http://images-of-elements.com/iodine.php)

Isotopes


The most common isotope of iodine is iodine-127, which is stable and makes up 100% of the natural inventory of iodine. However, there are also a few radioactive isotopes of iodine that are of interest either as tracers of natural processes or because they pose health risks.

The most dangerous of the iodine isotopes in terms of the health risks it poses is iodine-131. 131I has a half life of 8 days and has received substantial media attention since it was one of the major isotopes released by the Fukushima disaster. It is even more dangerous because iodine is readily absorbed into plant and animal tissue. In humans iodine is absorbed into our thyroid gland and this means that if 131I is present it can be absorbed by our bodies and hence is more dangerous due to internal exposure.

Iodine-129 has half life of15.7 million years, is the subject of my PhD. thesis, and is not nearly as dangerous from a health perspective as 131I, however it is still of interest (otherwise I'd be in big trouble!). 129I is produced naturally in the atmosphere by cosmic ray interaction with xenon gas, in rocks by the spontaneous fission of uranium-238 and by humans, in nuclear fuel reprocessing plants or nuclear disasters and atomic bomb testing. In fact, before humans the amount of 129I on Earth was about 250kg, now it is about 6000kg.

Iodine-125, which has a half-life of 59 days is a minor radioactive isotope of iodine, although it is used in many laboratories as a tracer of lab methods and can also be used in medicine. There are many other isotopes of iodine that are extremely rare. Some of these can be useful in medical imaging, but most simply are produced naturally and decay rapidly without anyone really noticing/caring.


Behaviour


Iodine is a member of the halogen group found on the right side of the Periodic Table. In nature it behaves similarly to chlorine. The most important thing to know about iodine in nature is that it is always in motion. What I mean by this somewhat cryptic sentence is that it can be found in the hydrosphere, atmosphere or biosphere and transfers between them with ease; either as part of a volatile or soluble organic compound or inorganic compound depending on the local environmental conditions, such as pH, oxygen rich or oxygen poor conditions, and organic content. My thesis is on the movement and sources of iodine in the Canadian Arctic so trying to understand this more fully is part of what I hope to accomplish with my research.

Dr. Udo Fehn (http://www.earth.rochester.edu/fehnlab/index.htm) This figure shows where iodine-129 is produced and where it can be found in the environment. It does not include anthropogenic production. The t is the residence time of iodine in each place. e.g. 18 days for the atmosphere. 

One of the reasons it is really important to understand the behaviour of iodine well is in the field of nuclear waste storage. When we store nuclear waste it is essential that the waste be isolated from the environment for long enough that all the dangerous isotopes decay. However, when we store reactor fuel there is lots of iodine-129 present that has a long, long half life. This means that any repository for nuclear waste has to actually be designed to keep 129I secure, even after everything else has decayed away. However, should the worst occur and the waste leak or another Fukushima occur it is pretty important that we have a good idea of how iodine behaves in the environment!!


Minerals 
Iodine minerals are few and far between. The most common are: elemental iodine (I2), iodoargyrite (AgI), and marshite (CuI). There are several others, but they are generally quite rare. For a full list see: http://www.mindat.org/chemsearch.php?inc=I%2C&exc=&sub=Search+for+Minerals

 Iodoargyrite. (http://www.mindat.org) Locality: Schone Aussicht Mine, Germany (4mm)

Some iodoargyrite that I synthesized in my lab for accelerator mass spectrometry analysis of 129I in river water from the Yukon Territory, Canada (Photo: M. Herod)

Most of the iodine mined today is found in Chile, but some is also mined in Japan. The deposits in Chile occur as caliche. Caliche is a bit of a mystery to me, so the information that I am distilling here comes from Sirocco Mining Inc. which is a major producer of iodine. Caliche is a sedimentary rock usually composed of calcium carbonate that forms in arid soils. It forms when minerals in the upper layer of a soil are dissolved by rain and then re-precipitate below in a deeper soil horizon. This usually takes place in arid to semi-arid environments. The caliche of Chile is unusual in that it contains high concentrations of unusual elements such as iodine. Most caliches are composed of calcium carbonate, but oddly the Chilean caliche is mainly composed of nitrates. The reason for this is not clearly understood, but the overall formation is still similar. It is believed that the nitrates, iodine and other salts were dissolved in highly saline ephemeral lakes in the desert that would evaporate depositing their salts, which would then slowly get leached and form caliche deposits.

Uses


Over the years iodine has been used in many different applications. Perhaps the most well known is as a disinfectant. It was not that long ago that if you cut yourself in order to disinfect the would all you had to do was slap a little iodine on it. Indeed, it is still possible to buy iodine as drops that can be used to purify water in the field (although I am not fond of the taste).

Radioactive isotopes of iodine are used commonly in medical imaging as well as in some cases as radiation therapy for cancer. In fact, 131I can cause thyroid cancer but can also be used to treat it.

Many countries use iodized table salt to help make sure that the population has enough iodine in its diet since iodine is a very essential micronutrient and iodine deficiency is actually a very real health concern in many places.

Let me know what you think about iodine or if you have any questions. Also, what should my next element be? First suggestion wins!

Matt

References: 


MinDat: http://www.mindat.org/

Sirroco Mining Inc. Aguas Blancas Project: http://www.siroccomining.com/s/AguasBlancas.asp?ReportID=109021


Argonne National Lab: http://www.evs.anl.gov/pub/doc/iodine.pdf