The Potential Of I-129 as an Environmental Tracer

Abstract

Iodine has two natural isotopes – the only stable iodine isotope is 127I, whilst 129I is the only radioactive iodine isotope that is formed in nature (T1⁄2 = 1.57 · 107 years). However, the main sources of 129I in the environment are anthropogenic from nuclear fuel reprocessing plants (NFRP) and nuclear accidents. Current levels of 129I do not represent any radiological hazard to humans, but the liquid discharges of 129I from reprocessing plants into the ocean makes it a unique oceanographic tracer to study the movement of water masses, transfer of radionuclides and marine cycles of stable elements such as iodine. The gaseous releases of 129I from reprocessing plants can be used as an atmospheric and geochemical tracer (Hou, 2004). 129I and 127I have the same chemical properties and therefore it is expected that they also behave similar in environment. Lack of 129I and 127I speciation data makes it difficult to confirm or disprove this assumption. The main problem is the mobility – species of newly introduced and old − natural 129I. The old 129I is in equilibrium with 127I – natural 129I/127I ratio and this is disturbed with 129I from NFRP which is released to the environment in volatile form. As such it is rapidly transferred among surface compartments. Liquid discharges to oceans influence areas in accordance with marine currents. Wet and, to a lesser extent, dry depositions of atmospheric 129I are the main sources for 129I in terrestrial environment, which is distant from 129I sources such as NFRP. The biggest reservoir of iodine is the ocean with an average concentration of approximately 50-60 μg L-1 seawater. From marine environment is iodine transferred to the atmosphere by volatilization mainly as iodomethane (CH3I) and then washed out to terrestrial environment by wet and dry deposition. It is accumulated in soils where it is strongly bound-adsorb to organic matter, and iron and aluminium oxides in soil (Fuge, 2005). In the accumulation processes of iodine in soil besides various physico-chemical parameters including soil type, pH, Eh, salinity, and organic matter content, soil microorganism – especially bacteria were found to play an important role (Muramatsu & Yoshida, 1999, Amachi, 2008). In this way the biogeochemical cycling of 129I is strongly connected to processes in ocean and soil systems – the atmosphere being the bridge between them.