By Pablo Barillas May 3 rd 2011 Dr. Mason

 Both I-131 and I-125 are radioactive isotopes of naturally occurring I-127.  I-131 is highly radioactive and gives of energy in both beta and gamma radiation.  I-131 has a half-life of 8 days and decays into stable Xenon 131.  Created by nuclear fission of either uranium or plutonium.

 I-125 has electron capture radiation decay where a proton will capture an electron and give off a neutrino.  I-125 has a half-life of 59 days will decay into Tellurium 125.  I-125 is created by the electron capture decay of Xenon 125.

 Iodine as a molecule is nonpolar and is highly soluble in non polar organic solvents.  Due to its nonpolar nature, it does not dissolve well in water.  Solubility is aided by iodide ions already in the water, such as from HI, KI etc and will form the triiodide ion, I 3 −  Iodine is easily oxidized and easily reduced as well.

 I-131 was discovered by Glenn T. Seaborg and John Livingood at the University of California Berkeley in the late 1930's.  I-131 was a huge issue post- WWII both in Japan and nuclear test areas in the USA.  Currently I-131 is a source of major concern at the Fukishima nuclear plant.

 I-131, even though harmful in large exposures, has some medicinal uses.  I-131 is currently used as treatment for hyper- thyroidism by destroying tissue with radiation.  Sometimes also used as a radioactive label, but not often since it will destroy tissue in the process.

 I-125 which is less harmful has more medicinal uses than I-131.  I-125 is used in brachytherapy which aids in the destruction of cancerous tissues.  I-125 is also used in radiology by tagging antibodies in a radioimmunoassay.

 I-131 is a nuclear fission product so anywhere fission takes place, I-131 can be found.  Fuel rods in nuclear reactors can crack and release I-131 into the surrounding cooling water, which can be released into the environment.  Another major source of I-131 is nuclear waste, if not correctly sealed will leak into the environment.  A stated before, atomic bomb testing released a lot of I-131 into the atmosphere.  Industrial, medical and university waste can also be sources of I-131.

 I-131 is easily assimilated into the food web by uptake in primary producers such as seaweed.  Shellfish can then bioaccumilate I-131 by filter feeding.  Vertebrates can absorb I-131 readily by the thyroid gland.  Due to radiation, I-131 will lead to tissue destruction and cancer.

 As stated before, ingestion is the main mode of entry for most organisms.  If there is a recent explosition or emission of radioactive products, I-131 maybe inhaled as well.  Once in the body, the sodium-iodide symporter, located in the outer cells of the thyroid will actively transport iodide ions inside.

 Stimulated by TSH, the thyroid now begins production of Triiodothyronine (T 3 ) and Thyroxine (T 4 ).  To do this, the enzyme thyroperoxidase (TPO) oxidizes the iodide anion and incorporates it into thyroglobin which will later become T 3 or T 4.

 Most of I-131 can be eliminated in the urine.  If the organism stops the intake of radioactive iodine, it can let the short half-life of the isotope decay it away.  To prevent I-131 uptake entirely, it is possible to saturate the thyroid with stable I-127.  This can be done with KI pills.

 “Radioactive I-131 from Fallout”  “EPA: Radionucleotides, Iodine”  “Radiotoxicity of Iodine-123, Iodine-125, and Iodine- 131”  “Thyroid Disorders”  “Seaweed as a model for iodide uptake and retention in the thyroid”