18 The Potential Of I-129 as an Environmental Tracer

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.

. Atmospheric releases are not plotted, but they are considered in the total amount.Annual atmospheric releases ranged from 1.19 to 9.58 kg 129 I with a total amount of 235.5 kg in the period from 1952 to 2000.Anthropogenic 129 I predominates in marine environment in biosphere and upper layers of the oceans and in terrestrial environment in soil, therefore it can be expected that the isotopic ratio 129 I/ 127 I is increasing in these compartments of the ecosystem.Precipitation and seawater are probably the main carriers for 129 I exchange among different compartments in marine and terrestrial environment.Data from literature clearly show that 129 I levels in marine sediment, marine algae and soil are several times higher than in seawater or precipitation.Meaning that 129 I is most probably chemically or biologically transformed to species which accumulate in those compartments (Tables 3 and 4).To summarize, different values of 129 I/ 127 I isotopic ratios in environment are today envisaged as 10 -12 for pre-nuclear era, 10 -9 in slightly contaminated regions and 10 -9 -10 -6 in regions affected by the releases from NFRP.The highest ratios were found in the close vicinity of NFRP with values from 10 -6 to 10 -4 (Hou, 2009).

Factors affecting biogeochemical cycling of iodine
Iodine is a trace element present in the hydrosphere, lithosphere, atmosphere and biosphere at different concentrations and as different iodine species (Table 5).Speciation analysis of iodine was mainly done on stable 127 I (Hou et al., 1997;dela Veija et al., 1997;Sanchez & Szpunar, 1999;Hou et al., 2000c;Leiterer et al., 2001;Schwehr & Santschi, 2003;Shah et al., 2005;Gilfedder et al., 2008), with some studies on 129 I (Hou et al., 2001; Hou et al., 2003b;  Schwehr et al., 2005; Englund et al., 2010b).Majority of researches performed on 127 I and 129 I are limited to fractionations of iodine -water soluble, exchangeable, bound to oxides, organic-inorganic fraction, etc.In general just the most abundant chemical forms of iodineiodide (I -) and iodate (IO 3 -) are determined and the rest of total iodine content is associated with organic iodine.It is well known that organic iodine fraction mainly consist of iodine  et al., 2000aet al., Greenland, 1999 (n = 5) (n = 5) 0.07−0.24Hou, 2004England, Irish Sea, near Sellafield 2004-05 (n = 4) 89−820 Atarashi-Andoh et al., 2007Scotland, Scottish Sea, influence of Sellafield, 2003-2005 (n = 14) 7.  , 1995-1998(Fucus vesiculosus, n = 8) Klint, 1986-1999 (Fucus vesiculosus, n = 39) 2.50−9.123.54−37.5France (vicinity of La Hague) Goury, 1998-1999(Fucus vesiculosus, n = 3) Goury, 1998-1999     bound to proteins -but these are still not identified for most environmental and biological samples, not for 127 I and certainly not for 129 I.The main problem is lack of appropriate standards for speciation analysis and very small amounts of 129 I in environmental and biological samples.Iodine is released from marine environment to the atmosphere partly as aerosols formed from the sea spray -inorganic iodide and iodate -and mainly as volatile organic iodine compounds (VOIC) such as iodomethane (Baker et al., 2000;Leblanc et al., 2006, Chance et al., 2009).Bacteria, phytoplankton and brown algae present in marine environment are capable to reduce the most thermodynamically stable form of iodine, the iodate to iodide.On the other hand microalgae and macroalgae-seaweed accumulate iodide and transform it into VOIC -the most important are CH 3 I, CH 2 I 2 , CH 2 BrI and CH 2 ClI (Leblanc et al., 2006).The emitted organic iodine is decomposed by sunlight into inorganic iodine compounds.The photolytic lifetimes of VOIC differ; CH 2 I 2 has a lifetime of 5 minutes, followed by CH 2 BrI with a lifetime of 45 minutes and CH 2 ClI with a lifetime of 10 h (Stutz, 2000).The longest photolytic lifetime of 14-18 days has CH 3 I (Stutz, 2000).During this process of photolization reactive iodine oxides such as HOI, I 2 O 2 and IO 2 form, which either form condensable vapours as nuclei for aerosols or react with ozone.From the atmosphere iodine enters the marine and terrestrial environment by processes of wet and dry deposition.In the iodine terrestrial cycle interactions between water and soil are most important (Santschi & Schwehr, 2004).Beside physical and chemical factors, biological processes especially promoted by microorganism influence the cycling of iodine.Microorganisms are involved in environmental processes as primary producers and also as consumers and decomposers.
They have bioremedial and biotransformable potential and in this way affect the mobility of elements.Oxidation and reduction mechanisms contribute to transformations between soluble and insoluble forms.Experiments with 125 I tracer showed the importance of microbial participation in iodine accumulation -sorption and desorption processes -in soil.Muramatsu et al. (1996) observed desorption of iodine from flooded soil during cultivation of rice plants.Microorganisms created reducing conditions in the flooded soil and iodine once adsorbed on the soils was desorbed (Muramatsu et al., 1996).Amachi et al. (2001) reported a wide variety of terrestrial and marine bacteria that are capable to produce CH 3 I under oligotrophic conditions.Aerobic bacteria showed significant production of CH 3 I, whereas anaerobic did not produce it.The methylation of iodide was catalysed enzymatically with S-adenosyl-L -methionine as the methyl donor.The biding of iodine by organic matter and/or iron and aluminium oxides has the potential to modify the transport, bioavailability and transfer of iodine isotopes to man (Santschi & Schwehr, 2004).Because of the same chemical properties 129 I and 127 I should behave similar in environmental processes.Major pathways are the volatilization of organic iodine compounds into the atmosphere, accumulation of iodine in living organisms, oxidation and reduction of inorganic iodine species, and sorption of iodine by soils and sediments.These processes are influenced or even controlled by microbial activities (Amachi, 2008). 129I is gradually released in trace quantities into the atmosphere and aquatic environment from reprocessing plants.It is then physically transported in the air or water media under the influence of chemical and biological processes.Newly introduced 129 I from NFRP is in volatile form and as such more mobile compared to 127 I.By taking this aspect into account one cannot be sure that biogeochemical behaviour of 129 I and 127 I is the same.Even more, Santschi & Schwehr (2004) discussed that biogeochemical behaviour of iodine and its isotopes appears to be different in North American and European waters.

Measurement of 129 I
129 I decays by emitting beta particles (E max = 154.4keV), gamma rays (E = 39.6 keV) and Xrays (29−30 keV) to stable 129 Xe (Tendow, 1996).Therefore it can be measured by gamma and X-ray spectrometry and by beta counting using liquid scintillation counters (LSC).Another method for determination of 129 I is neutron activation analysis (NAA) that is based on neutron activation of 129 I(n, ) 130 I, which is measured by gamma spectrometry (E = 536 keV (99 %).In recent year's mass spectrometry -such as accelerator mass spectrometry (AMS) and inductively coupled plasma mass spectrometry (ICP-MS) are also used.For determination of 129 I levels in environmental samples only two analytical methods are available, radiochemical neutron activation analysis (RNAA) and AMS.The main advantage of the AMS is the detection limit that is close to 10 -14 expressed as 129 I/ 127 I ratio.RNAA can only measure 129 I at elevated levels -nuclear era.AMS enables measurement of 129 I in all environmental samples, also the natural, pre-nuclear levels, and the needed amount of sample is 10-100 times smaller than in the case of RNAA.Detection limits for 129 I using different analytical methods are compared in Table 6.

Direct gamma and X-ray spectrometry
Direct gamma-X spectrometry (E = 39.6 keV; X-rays, 29−30 keV) is a non-destructive technique that is rapid and can be applied to different matrices.It is used for monitoring of environmental samples collected in vicinity of NFRP such as thyroid, urine, seaweed, and for nuclear waste by using high purity Ge or plenary Si detector (Suarez et al., 1996;Bouisset et al., 1999;Frechou et al., 2001;Lefevre et al., 2003;Frechou & Calmet, 2003;Barker et al., 2005).To lower the detection limits normally big samples (50−500 g) are used, which induces considerable attenuation at low energies.The attenuation depends on the matrix composition of the sample and geometric parameters of the container.Therefor the mass energyattenuation coefficient (self-absorption correction) at a given energy must be measured for all sample matrices with respect to that of the standard source.Experimentally obtained selfabsorption correction factors are used to obtain accurate results (Bouisset et al., 1999;Lefevre et al., 2003, Barker et al., 2005).To quantify self-absorption correction factors 210 Pb (46.5 keV) and 241 Am (59.6 keV), with gamma lines close to 129 I are used.Detection limits as low as 2 Bq kg -1 dry mass can be reached for Fucus sp.samples (Bouisset et al., 1999).Chemical separation of 129 I from the sample matrix and interfering radionuclidesdestructive method -improves the detection limit when using direct gamma-X spectrometry (Suarez et al., 1996).By using direct gamma-X spectrometry 129 I was determined in seaweed sample FC-98 Seaweed, which was prepared by Frechou et al. (2001), by using direct gamma -X spectrometry (Osterc & Stibilj, 2008).

Liquid Scintillation Counting (LSC)
Liquid scintillation counting is based on emissions of beta particles from radionuclides -beta decay (E max = 154.4keV). 129I has to be separated from the sample matrix and other radionuclides and dissolved or suspended in a scintillation cocktail containing an organic solvent and a scintillator.Beta particles emitted from the sample transfer energy to the solvent molecules, which in turn transfer their energy to the scintillator which relaxes by emitting light -photons.In a liquid scintillation counter each beta emission (ideally) results in a pulse of light, which is amplified in a photomultiplier and detected.
Recently extraction chromatographic resins for the separation and determination of 36 Cl and 129 I have been developed.First results show a promising potential to use the resins within the context of the monitoring of nuclear installations -during operation and especially during decommissioning (Zulauf et al., 2010).

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
ICP-MS has been used to determine 129 I in contaminated environmental samples with high level 129 I content such as sediments, groundwater samples, soil and seaweed (Izmer et al., 2003;Izmer et al., 2004;Becker, 2005;Brown et al., 2007;Li et al., 2009).The lowest detection limit of the method reported as 129 I/ 127 I isotopic ratio is 10 -7 .
The method is based on iodine separation and injection to the machine as solution or gaseous iodine, I 2 .Iodine is decomposed into iodine atom and ionized to positive iodine ion at a temperature ~6000−8000 K.It is then extracted from the plasma into a high vacuum of the mass spectrometer via an interface.The extracted ions are separated by mass filters of either quadropole type time-of-flight or combination of magnetic and electrostatic sector and measured by an ion decetor (Hou et al., 2009).Difficulties encountered when determining 129 I with ICP-MS are low 129 I quantities present with high 127 I concentrations, isobaric and molecular ions interferences ( 129 Xe + , 127 IH 2 + ), memory effects and tailing of 127 I. To improve 129 I/ 127 I determination it was found that introduction of helium gas into collision cell reduces peak tail of a high-abundant isotope, 127 I by up to three orders of magnitude.Detection limits have been improved by applying oxygen as collision gas for selective reduction of 129 Xe (Izmer et al., 2003, Hou et al., 2009).

Neutron Activation Analysis (NAA)
NAA enables determination of 129 I in environmental samples at 10 -10 129 I/ 127 I isotopic ratios.
The concentration levels of 129   (Osterc & Stibilj, 2005;Osterc et al., 2007).In first step pre-concentration of iodine from large amounts of sample is performed.Solid samples, such as soil, sediment, vegetation, biological samples can be decomposed by alkaline fusion (Hou et al., 1999, Osterc et al., 2007).The sample is mixed with potassium hydroxide/alkali solution and then gradually heated to 600 °C.Iodine is leached from the decomposed sample with hot water, isolated with solvent extraction and precipitated as PdI 2 or MgI 2 or trapped on activated charcoal (Fig. 2) (Hou et al., 1999, Osterc et al., 2007).Another method to separate iodine from solid samples is combustion at high temperature, ~1100 °C (Muramatsu & Yoshida, 1995).Released iodine is trapped in an alkaline solution or adsorbed on activated charcoal.The pre-concentrated iodine is than irradiated for up to 12 hours simultaneously with a 129 I/ 127 I standard.After radiochemical separation the 130 I induced from 129 I (see nuclear reaction 1) is counted on a high purity Ge detector and compared to standard of known activity and corrected for chemical yield (Osterc et al., 2007).
Fig. 2. The scheme for pre-concentration of iodine from solid samples (Osterc et al., 2007) For liquid samples, such as milk, urine and water samples anion exchange method using anion exchange resins can be applied.Adsorbed iodide is eluted and isolated from the eluate with solvent extraction and precipitated as PdI 2 or MgI 2 (Parry et al., 1995;Hou et al., 2001;Hou et al., 2003a).

Accelerator Mass Spectrometry (AMS)
An AMS facility is set up off injector and analyser linked with a tandem accelerator.The detector is either a combination of time-of-flight and silicon charged particle detector or gas ionization energy detector.Iodine has to be separated from the sample with same techniques as used for NAA, such as pyrohydrolysis at 1000 °C, and prepared as AgI targets (Muramatsu et al., 2008).Negative iodine ions are produced from AgI targets by Cs sputter ion source and injected into the tandem accelerator.The formed 129 I -and 127 I -ions are accelerated to positive high-voltage terminal converting negative ions to I 3+ , I 5+ or I 7+ .The positively charged ions pass through a magnetic analyser where ions of 129 I and 127 I based on charge state and energy are selected and directed to a detector.AMS measures the 129 I/ 127 I isotopic ratio and the 129 I absolute concentration is calculated by the 127 I content determined in the sample and the chemical yield for separation of iodine from sample -preparation of AgI targets (Hou et al., 2009).AMS is the only technique that enables measurement of pre-nuclear age samples and samples with low 129 I content, below 10 -10 129 I/ 127 I isotopic ratio (Moran et al., 1998;Fehn et al., 2000a;Buraglio et al., 2001;Alfimov et al., 2004;Santschi & Schwehr, 2004;Snyder & Fehn, 2004;Michel et al., 2005;Fehn et al., 2007;Hou et al., 2007;Keogh, et al., 2007;Muramatsu et al., 2008;Gomez-Guzman et al., 2011).Instrumental background of 10 -14 129 I/ 127 I has been obtained (Buraglio et al., 2000).But the detection limit depends on the chemical separation before measurement and especially on addition of iodine carrier.When carrier and chemical processing are included the typical reported blank 129 I/ 127 I isotopic ratio is 1 • 10 -13 (Buraglio et al., 2000).For environmental samples with a very low 129 I/ 127 I isotopic ratio Hou et al. (2010) reported a method for preparation of carrier free AgI targets based on co-precipitation of AgI with AgCl to exclude the influence of interferences from 129 I and 127 I in the carrier.They calculated a detection limit of 10 5 atoms, which corresponds to 2 • 10 -16 g of 129 I.

Quality assurance of 129 I analyses
To be able to determine 129 I by RNAA in environmental samples from nuclear era preconcentration of iodine from large amounts of sample (up to 150 g) is needed.In this preconcentration step contamination of sample with 129 I is possible.It is important to make a blank control when establishing a new method and verify the method by reference materials to evaluate possible contamination during the entire analytical process; including preconcentration, irradiation, radiochemical separation and gamma activity measurement.Also analysis of 129 I by AMS requires intensive and continuous control -control charts of the analytical blank and verification of accuracy by analysis of reference materials, which has to be continued periodically also during routine operation (Szidat et al., 2000a).Influence of sample mass -AgI targets on accuracy of 129 I determination was studied by Lu et al. (2007).They found that samples with masses above 0.3 mg did not show an influence on accuracy -ion current of the sample was constant, but it fell strongly for samples with masses below 0.3 mg.Samples wit masses below 0.1 mg did not produced sustainable currents for 129 I determination.Presence of 5000 129 I atoms or 50 µg in the target is sufficient for a successful 129 I determination.To validate and or evaluate an analytical method, to run a laboratory inter-comparison, to check accuracy of analytical method, and ensure globally comparable and traceable results to stated references, as the SI units, certified reference materials are needed.Environmental samples represent a huge variety of different combinations of substances to be analysed and the matrices in which they are embedded.This countless combinations of substances -elements, radionuclides, contaminants -and matrices means that certified reference materials always lack.The only reference material with a recommended value for 129 I available on the market was the reference material IAEA-375 Soil -Radionuclides and Trace Elements in Soil.Top soil to a depth of 20 cm was obtained from the "Staryi Viskov" collective farm in Novozybkov, Brjansk, Russia in July 1990.Unfortunately this reference material is now out of stock.Only informative and not certified values for 129 I, determined in one laboratory, are reported for NIST SRM 4357 -Ocean Sediment Environmental Radioactivity Standard, which is a blend of ocean sediments collected off the coast of Sellafield, UK, and in the Chesapeake Bay, USA, and NIST SRM 4359 -Seaweed Radionuclide Standard, which is a blend of seaweed collected off the coast of Ireland and the White Sea.
Recently a new reference material, with a certified value for 129 I, IAEA-418: I-129 in Mediterranean Sea Water was characterised in an interlaboratory comparison exercise.The used method was AMS (accelerator mass spectrometry).Another new reference material for radionuclides in the mussel Mytilus galloprovincialis from Mediterranean Sea, IAEA-437 was characterised.They reported for the mussel sample collected in 2003 at Anse de Carteau, Port Saint Louis du Rhône, France an informative average massic activity of 0.8 ± 0.1 mBq kg -1 dry mass (Pham et al., 2010).

Applications of I-129 as an environmental tracer
Use of 129 I as an intrinsic tracer for natural iodine kinetics was discussed as early as 1962 (Edwards, 1962).Already at that time two reprocessing plants, one for military purposes in Marcoule, France (from 1958) and one for nuclear fuel in Thurso, United Kingdom (from 1958) existed.To be able to use 129 I as an environmental tracer certain conditions have to be met.These are: (1) 129 I must trace a single environmental process with a defined time scale; (2) 129 I must be equilibrated with 127 I; (3) The predominant chemical species of 129 I and their geochemical properties must be known (Santschi & Schwehr, 2004); (4) Conservative behaviour, meaning relatively constant concentration in a reservoir over time, is desirable.The natural 129 I/ 127 I ratio has been strongly shifted by continuous additions from anthropogenic sources, which still persists.To trace existing and future global changes in inventories of anthropogenic 129 I continuous monitoring and revised budget calculation are indispensable (Aldahan et al. 2007a).Recently also a prediction model system to better understand the dispersion of 129 I from point sources (Sellafield and La Hague) to the northern North Atlantic Ocean has been developed (Orre et al. 2010).United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR, 2000) identifies as globally dispersed radionuclides 3 14 C and 129 I.Because of its very long half live is 129 I one of the most important radionuclides in long-term radiological assessment of its discharges from nuclear fuel reprocessing plants. 129I is present in the environment in low quantities (in traces) and its increase in a particular compartment of the ecosystem can be instantly recognized.

129 I as an oceanographic tracer
Transport, circulation and exchange of water masses in the Northeast Atlantic and Arctic Oceans has long been studied by using radionuclides such as 137 Cs, 134 Cs, 90 Sr, 125 Sb and 99g Tc originating from reprocessing of spent nuclear fuel.In recent years 129 I became interesting as an oceanographic tracer, because the discharges from NFRP in La Hague and Sellafield increased since 1990 and highly sensitive analytical method, AMS, developed for analysis (Hou, 2004).Concentrations and species of 129 I and 129 I/ 127 I isotopic ratio were determined in many environmental and biological samples from marine environment, especially in areas influenced by NFRP.Results for Northeast Atlantic, Arctic and Baltic Seas indicate a strong influence of liquid discharges from NFRP in La Hague and Sellafield.Hou et al. (2000a) determined 129 I concentrations in archived time series seaweed Fucus vesiculosus samples from Danish, Norwegian and Northwest Greenland coast collected in a period from 1980 to 1997 (Table 3).They used the 129 I/ 99 Tc ratio to estimate the origin of and transit times of 129 I. Transit times were estimated to be 1−2 years from La Hague, 3−4 from Sellafield, to Denmark (Klint) and Norway (Utsira), and 9−14 years from La Hague, 11−16 from Sellafield, to NW Greenland.Iodine exists in seawater mainly as dissolved iodate and iodide, and a small amount of organic iodine (Wong, 1991).Chemical speciation of 129 I can be used to investigate the transport, dispersion, and circulation of the water masses -especially at the boundary of two or more sources.(Hou et al., 2001).

5.2
129 I as a geochemical tracer 129 I was used in geochemical studies as a tracer for determining ages and migration of brines (Muramatsu et al., 2001, Snyder et al., 2003a, Fehn et al., 2007).Isolated system contain lower or close to estimated pre-nuclear 129 I/ 127 I ratio, 1.5 • 10 -12 .For correct interpretation of results -age calculation based on 129 I one must consider the effect of possible fissiogenic production and initial concentration on isotopic ratios.The estimated pre-nuclear ratio can be disturbed along continental margins with lower isotopic ratios likely caused by releases of methanerich fluids with high stable iodine concentrations derived from old organic sources, where 129 I already partly decayed.The isotopic ratio of the open ocean is not disturbed, justifying the use of estimated pre-nuclear ratio (Fehn et al., 2007).

129 I in precipitation
Atmospheric releases of 129 I from European and Hanford NFRP were much higher than from nuclear weapons tests and Chernobyl accident together (Table 1).Measurement of 129 I in atmosphere and precipitation can be used to investigate the transport pathways of 129 I from point sources, such as NFRP.But it is important to be aware that 129 I levels in atmosphere and precipitation can originate either directly from atmospheric releases from NFRP, and from volatilization from seawater and terrestrial environment.To study transport pathways of 129 I all of this aspects have to be considered and obtained results for atmospheric and precipitation samples compared to reported releases from NFRP in particular timescale.Many precipitation and atmospheric samples have to be measured continuously to establish a pattern or trend.

129 I for reconstruction of I dose
The same chemical and physical properties of isotopes of particular element enable to use 129 I as a tool for the reconstruction of 131 I doses after a nuclear accident.This was done after the nuclear accident in Chernobyl.Levels of 129 I were determined in soils and from the measured 129 I/ 131 I ratio, 12−19 (Kutschera et al., 1988;Mironov et al., 2002), the long-lived 129 I can be used to reconstruct 131 I dose to thyroids.This method is limited only to areas that were relatively strong contaminated by fallout from Chernobyl like areas in Ukraine and Belarus (Michel et al., 2005;Straume et al., 2006).

Radiological hazard of 129 I for man
Transport pathways of iodine to human are ingestion and inhalation.Iodine present in food is adsorbed into blood in small intestine -inhaled iodine from the air is also transferred into blood.More than 80 % of iodine absorbed into the blood is concentrated in the thyroid gland, which is therefore the target organ of iodine -also radioactive 129 I. Due to low beta and gamma energy of 129 I and long half-life the radiation toxicity of 129 I is mainly related to long term and low dose internal exposure of the thyroid to the beta radiation of 129 I.An average iodine content in human thyroid is 10−15 mg. 129I and 127 I are taken up by thyroid indiscriminately.The highest reported 129 I/ 127 I ratio was 10 -4 in close vicinity of NFRP, which corresponds to 10 -6 g or 6.64 Bq at 10 mg stable iodine content in thyroid.The corresponding annual radiation dose to thyroid would be 0.1 mSv year -1 , which is 2.5 times higher than the dose regulation limit of 0.04 mSv year -1 set by the U.S. NRC for combined beta and photon emitting radionuclide to the whole body or any organ (Hou et. al., 2009).An annual thyroid equivalent dose of 1 mSv, which is comparable to the level of natural back-ground radiation, would only be reached by ratios exceeding 1.5 • 10 -3 (Michel, 1999).Current concentrations of 129 I in the environment do not represent any radiological hazard for man, even in the vicinity of nuclear fuel reprocessing plants.But to assess environmental impact and potential risk and consequences during long-term exposition information on the distribution and radionuclide species, speciation analysis, influencing the mobility, biological uptake and accumulation of radionuclides is needed (Salbu, 2007).Speciation analysis provides crucial information for evaluation of radionuclide transport mechanism in the environment and to the human body and accurate risk assessments (Hou et al., 2009).

Conclusion
Anthropogenic 129 I considerable enriched pre-nuclear environmental levels.Presently the main sources of 129 I in the environment are nuclear fuel reprocessing plants (NFRP).Global distribution of 129 I is not uniform -concentrations are elevated near NFRP -but anthropogenic 129 I was detected in remote areas such as Antarctic.Before the onset of nuclear age 129 I and 127 I were in equilibrium.Analysis of pre-nuclear material and deep layer of marine sediment gave the best estimated value for natural 129 I/ 127 I ratio in surface reservoirs to be (1.5 ± 0.15) • 10 -12 .In transport and exchange of 129 I among different compartments marine and soil ecosystems influenced by present biota -microorganisms play major role.Biogeochemical cycling of iodine is influenced by its strong association with organic material -ocean is the main reservoir of mobile iodine, where it is rapidly exchanged between biota, hydrosphere and atmosphere.Aldahan, A., Alfimov, V., Possnert, G. (2007a). 129I anthropogenic budget: Major sources and sinks.Applied Geochemistry, Vol. 22,No. 3,, ISSN 0883-2927

Fig. 1 .
Fig. 1.Liquid and atmospheric releases of 129 I from NFRP in La Hague and Sellafield for period from 1952 to 2000 (compiled by Lopez-Gutierrez et al., 2004).

Table 2 .
129I/ 127 I isotopic ratios in pre-nuclear age environmental and biological samples

Table 3 .
129I/ 127 I isotopic ratios in nuclear age environmental and biological samples from marine compartments Table 4. 129 I/ 127 I isotopic ratios in nuclear age environmental and biological samples from terrestrial compartments www.intechopen.com

Table 5 .
Concentrations of stable iodine in environmental compartments

Table 6 .
Limits of detection for 129 I in various samples using different analytical methods www.intechopen.com I in environmental samples are very low and chemical separation/pre-concentration procedures have to be developed which can be used for a wide variety of matrices.Neutron activation analysis is based on induction of 129 I with thermal neutrons -irradiation in a nuclear reactor via following nuclear reaction: (Hou et al., 1999) by measuring of 130 I activity on a high purity Ge detector.Interfering nuclear reactions induced during irradiation of sample from other nuclides resulting in 130 I production can influence the correct determination of 129 I.These undesired nuclides are 235 U,128Te and 133 Cs and nuclear reactions: 235 U(n, f) 129 I(n,γ) 130 I, 235 U(n,f) 130 I, 128 Te(n, ) 129m Te( - ) 129 I(n, ) 130 I and 133 Cs(n, ) 130 I(Hou et al., 1999).They have to be removed from the sample before irradiation to avoid nuclear interferences. Durng irradiation radioactivity in sample is produced mainly due to the radioisotopes 23 Na(n, ) 24 Na (T ½ = 14.96 hours), 41 K(n, ) 42 K (T ½ = 12.36 hours) and 81 Br(n, ) 82 Br (T ½ = 35.30hours) present in sample, which renders the direct measurement of 130 I after irradiation and radiochemical separation of induced 130 I after irradiation is necessary.Solvent extraction with CCl 4 or CHCl 3 are normally used to extract iodine