A review on 129I analysis in air

A review on 129I analysis in air

Journal of Environmental Radioactivity 126 (2013) 45e54 Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal hom...

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Journal of Environmental Radioactivity 126 (2013) 45e54

Contents lists available at ScienceDirect

Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad

Review

A review on

129

I analysis in air

Tania Jabbar a, *, Gabriele Wallner a, Peter Steier b a b

Department of Inorganic Chemistry, University of Vienna, Währingerstr. 42, A-1090 Vienna, Austria VERA Laboratory, Faculty of Physics e Isotope Research, University of Vienna, Währingerstr. 17, A-1090 Vienna, Austria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 June 2012 Received in revised form 18 July 2013 Accepted 19 July 2013 Available online 13 August 2013

A review of literature focused on 129I determination in air is provided. 129I analysis in the environment represents a vital tool for tracing transport mechanisms, distribution pathways, safety assessment and its application as environmental tracer. To achieve that, specific chemical extraction methods and high sensitivity analytical techniques have been developed. This paper is intended to give an overview about the sample collection, extraction and distribution of 129I in the air. Sensitivity of available measurement techniques for the determination of 129I is compared. The article also provides the summary of current worldwide distribution of 129I in air and respective radiation exposure of man. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Iodine-129 Air Sample collection Analytical techniques Radiological impact

1. Introduction Iodine is a biophilic element with one stable isotope, 127I and several radioactive isotopes. The specific activity of 4.59  1015 Bq g1 of 131I (T1/2 8 d) and long half life of 129I (T1/2 15.7 My) make these radioactive isotopes of considerable concern. Iodine-129 is derived from several sources and decays by emitting b-particle with a maximum energy of 154.4 keV and g-ray of 39.6 keV as well as X-rays (29e30 keV). Production of natural 129I takes place in stratosphere primarily by interaction of cosmic rays with Xe isotopes, and in the lithosphere by fission of uranium isotopes and neutron reactions on 128 Te and 130Te. The degassing of the 129I from the subterranean sources in the lithosphere either through volcanism or venting may be the greatest natural source of fission-related 129I released into the biosphere (Kilius et al.,1992). Thus total reservoir of earth is estimated to be approximately 50,000 kg. The main part of it is bound to the lithosphere and only 263 kg is available in the free inventory of environmental compartments atmosphere, hydrosphere and biosphere. The major sources to free inventory are the releases from volcanoes and cosmic ray interaction each of which contribute approximately 45% (Fabryka-Martin, 1984). Together these sources

* Corresponding author. Pakistan Institute of Nuclear Science and Technology, Nilore, Islamabad, Pakistan. Tel.: þ92 336 4331180. E-mail addresses: [email protected], [email protected] (T. Jabbar). 0265-931X/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvrad.2013.07.013

produce a steady state isotopic ratios 129I/127I between 1013 and 1012 (Fabryka-Martin et al., 1985; Daraoui et al., 2012; Hou and Hou, 2012; Reithmeier et al., 2005; Schmidt et al., 1998 and references therein). Since 1945, concentration of 129I in the environment has changed drastically. Airborne 129I releases from atmospheric nuclear weapon tests, nuclear accidents and nuclear fuel reprocessing plants (Fig. 1) exhibit major anthropogenic input. Until the year 2007, the European nuclear fuel reprocessing plants at La Hague and Sellafield have discharged 3800 and 1400 kg 129I to English Channel and Irish Sea, respectively. Part of marine iodine is emitted to the atmosphere (estimated value 27 kg till 2004), which together with direct gaseous emissions from the reprocessing facilities become the isotope source for the terrestrial environment (Englund et al., 2010a,b; Reithmeier et al., 2010). The total atmospheric emissions by these two reprocessing plants were 75 and 180 kg of 129 I to the air. Comparable amount of gaseous 129I, 145 kg was released from Marcoule during its operation 1956e1997 (Hou et al., 2009a). Besides emissions from the European and USA (especially Hanford site) facilities, northern Europe might have received an unquantified amount of 129I from reprocessing facilities in Russia, particularly from Mayak (estimated gaseous releases 160 kg during its operation 1948e1986) (Englund et al., 2008; Reithmeier et al., 2010). Measurements near Savannah River reprocessing plants have shown that approximately half (42%) of gaseous 129I emissions become attached to aerosols and settle down in the vicinity and remaining (58%) is available for long-distance transport

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Fig. 1. Atmospheric source of

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129

I. NFRP stands for nuclear fuel reprocessing plant.

(Reithmeier et al., 2010). Thus two orders of magnitude higher deposition was calculated on the northern hemisphere due to the airborne releases from the major European, former Soviet and US reprocessing facilities compared to the deposition from global background. The estimated residence time of atmospheric iodine (10e18 d) allow for extended global transport and substantial mixing (Moran et al., 1999a). In fact, anthropogenic 129I from reprocessing releases is currently found in rainwater (Lopez-Guiterrez et al., 2000; Michel et al., 2012; Moran et al., 1999a, 1999b; Szidat et al., 2000) and river water of the Northern hemisphere (Moran et al., 2002; Oktay et al., 2001; Schink et al., 1995), not only in Western Europe, but also in the USA, where known atmospheric releases currently are negligible, as well as in the Southern Hemisphere (Fehn and Snyder, 2000). Given its long half-life, 129I has become well distributed through biosphere and can be expected to behave in similar ways to stable iodine over long time periods (Muramatsu et al., 2004). However, compared to 129I stable iodine preferentially occupies the thermodynamically favorable sorption sites and therefore trace 129I has higher mobility than stable iodine (Hu et al., 2009). With its long half life, high-abundance fission yield, and presumably high mobility in the environment, 129I has been recognized as one of the most important radionuclides in studies of environmental protection and nuclear nonproliferation. Iodine-129 has also been shown a very useful isotope for age dating, determination of sources for old crustal fluids, tracing of marine sediments in subduction zone, estimating weathering rates, a suitable oceanographic tracer for studying transport and exchange of water masses as well as a useful environmental tracer for investigating the geochemical cycle of stable iodine (Fehn et al., 2000, 2003, 2007; Hou et al., 2009b; Moran et al., 1995; Nimz, 1998; Wallner et al., 2007). From the radiation protection viewpoint, 129I can be very useful in reconstructing the 131I doses to population years after a release event has occurred (Michel et al., 2005; Mironov et al., 2002). In order to assess short and long-term consequences of radioactive contamination in the environment, information on the distribution of radionuclide species, mobility and biological uptake is needed. Such information can be obtained by means of identification and quantification of a radionuclide species in a sample. Presently, most of the monitoring is focused on 129I concentrations and 129I/127I atomic ratios in a variety of media other than air, including inland water systems, vegetation and animal thyroids. Enhanced levels of 129I have been reported in the vicinities of nuclear spent fuel reprocessing plants in USA, Europe, Japan, and India

(Brauer et al., 1974; Doshi et al., 1994; Fritz and Paton, 2006; Parry, 2001; Tsukada et al., 1991; Wershofen and Aumann, 1989) but information on atmospheric deposition of 129I far away from installations of nuclear industry is limited making it difficult to model atmospheric transport of possible future releases. Some review articles on the global distribution of anthropogenic 129 I have already been published in recent years in which sources, transport processes, analytical methods and speciation analysis were discussed (Fehn, 2012; Hou et al., 2009b; Hu and Moran, 2010; Reithmeier et al., 2010). However, a comprehensive review article on 129I analysis in air is still missing. This article aims to review the analytical procedures that have been developed and used for the determination of 129I in air including solvent extraction and the ion exchange. An outlook to future developments in sample collection and separation chemistry is presented. Nuclear measuring techniques, e.g. gamma-ray spectrometry and radiochemical neutron activation analysis (RNAA) are discussed. Capabilities of these techniques are compared with those of mass spectrometric (MS) techniques like accelerator mass spectrometry (AMS), inductively coupled plasma mass spectrometry (ICP-MS), gas chromatography mass spectrometry (GCeMS) and laser induced fluorescence. An attempt is made to summarize the environmental levels of 129 I measured in air by different workers to evaluate transport pathways over the world. The survey shows a gradient in the airborne 129I concentration with the distance from the reprocessing plants and geographical location. Higher value at latitude 56 N (Southern Sweden) as compared to 37 N (Seville, Spain) may partly be related to the proximity to source and marine waters. However, exponentially decreasing trend with increasing altitude has been observed because of scavenging by clouds and precipitation in the lower troposphere. 1.1. Atmospheric iodine chemistry Iodine, as one of the elements of the VIIA group in the periodic table, is occurring in six oxidation states: 1, 0, þ1, þ3, þ5 and þ7 (Hu and Moran, 2010). Due to the widely prevalent disease goiter, which is caused by iodine deficiency, major efforts were devoted to study the distribution, concentrations and environmental cycle of iodine. Iodine is transferred between the environmental compartments on different time scales and in different chemical forms (species). However, the dominant transfer pathway is either through the atmosphere as reactive gases (e.g. I2, CH2I2, CH2ICl, CH3I) or in the aqueous phase (i.e. rain, rivers, lakes and oceans). In the atmosphere, iodine may appear as elemental iodine or iodine compounds adsorbed on particles (aerosols) or in the form of vapor of molecular iodine, organic iodine compounds (iodo-carbons) and as inorganic compounds (iodide oxides and acids etc.); their concentrations vary with location, season and climate (Hou et al., 2009b). Atmospheric iodine chemistry is of increasing interest due to the nucleation of iodine gases to form new aerosols and its potential role in tropospheric ozone depletion (Kolb, 2002). It is believed that atmospheric new particle formation containing iodine on a large scale can have a significant effect on climate. Iodine occurs as a trace element in the Earth’s crust with an average abundance of 0.45 mg kg1 (Hu and Moran, 2010 and references therein). Low levels of iodine are found in soil, river, lakes and terrestrial plants. By contrast, main reservoirs of iodine in the earth’s surface environment are ocean and sea with concentration in the range 40e65 mg L1. Even higher concentration of iodine is found in springs and subsurface brine due to decomposition of organic matter. From oceans and seas, iodine seems to enter the atmosphere as CH3I by iodovolatilization (Whitehead, 1984). Another suggested mechanism for iodine to enter the atmosphere is by transformation of iodide into elemental iodine

T. Jabbar et al. / Journal of Environmental Radioactivity 126 (2013) 45e54

through photochemical oxidation (Miyake and Tsunogai, 1963) or through reaction with atmospheric ozone (Garland and Curtis, 1981). The latter mechanisms are assumed to be considerably less important in comparison to the methyl iodide path (Whitehead, 1984). The flow of iodine in the form of methyl iodide from the sea is reported to be 1.2  109 kg y1 (Rasmussen et al., 1982). However, recent data suggest that releases from the terrestrial pool (seaweed and brown algae), vegetation and soil can also add significant amounts to the atmosphere (Leblanc et al., 2006). The photochemistry of iodine in the troposphere is not yet fully understood. However, once transferred into the atmosphere, I2 and CH3I undergo photolytic dissociation leading to the formation of I atoms (Vogt, 1999), thereby significantly influencing ozone cycling, especially in the polar lower stratosphere.

I2 þ hv/I þ I CH3 I þ hv/CH3 þ I The predominant fate of I atoms is following reaction with ozone.

O3 þ I/IO þ O2 Most of IO radicals are rapidly photo-dissociated to I and O atoms or react with HO2, NOx and IO leading to the production of several unstable iodine reservoir HOI, INO2, IONO2 and I2O2 that recycle back to I e.g. following reaction,

INO2 /I þ NO2 for detailed set of reactions see Vogt et al. (1999). In this way, a variety of short and long lived oxidized iodine species are formed, some as aerosol particles which eventually initiate cloud condensation and subsequent iodine fallout redistribution as shown in Fig. 2 modified from Leblanc et al. (2006). The

47

chemical reactions of iodine and its radioisotopes are the same, but the ratios of the isotopes in these reactions may differ, depending on the relative concentrations of the isotopes in a particular environment. The residence time of iodine in the atmosphere is 14, 10 and 18 d for particulate, inorganic and organic gas (mainly CH3I) respectively. Total concentration of iodine in the atmosphere ranges from 1 to 100 ng m3 (Wershofen and Aumann, 1989; Whitehead, 1984) with an average of 20 n m3. High iodine concentration was observed in urban area due to the combustion of oil and coal, while in coastal area it was due to emission of gaseous iodine from algae, seawater, as well as sea spray (Hou, 2009). So out of 8.6  1012 t of total amount of iodine in crust, flux from ocean into atmosphere is 2.4  105 t/y, 70% of it is expected to return by dry and wet deposition into ocean and remaining 30% redistributed on land according to proportion of the surface areas (Fuge, 1990; Fuge and Johnson, 1986; Muramatsu et al., 2001). Water-soluble iodine species (elemental and particulate iodine, and highly polar HI or HOI) are incorporated into rain and snow or adsorbed to aerosols and removed by precipitation. Thus inorganic iodine compounds have higher tendency to be deposited because of their higher solubility in rain water than that of organic compounds (Muramatsu and Wedepohl, 1998). Iodine from precipitation is accumulated in soils, transported by surface water, infiltrating ground water and due to its biophilic nature makes its way through the biosphere (Michel et al., 2012). In summary, both iodine isotopes follow similar chemical pathways, although the concentration of total 129I is much lower than that of 127 I (Schwehr et al., 2006). The total iodine concentrations and the relative contribution of the iodine species measured in atmosphere are shown in Table 1. As can be seen, the fractions of the investigated iodine species are in a wide and uncertain range, although the gaseous organic fractions are dominant in all the measurements (Michel et al., 2005). But there is a discrepancy found between most model calculations, which suggest that iodate should be the dominant iodine species in

Fig. 2. Iodine biogeochemical cycle in the ocean, atmosphere and terrestrial compartment (modified from Leblanc et al., 2006).

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T. Jabbar et al. / Journal of Environmental Radioactivity 126 (2013) 45e54

Table 1 The relative contribution of the iodine species measured in the atmosphere. Location

Regensburg, Germany North Sea Fohr, Northern Germany Fohr, Northern Germanya Karlsruhe reprocessing plant Karlsruhe reprocessing planta Antarctica Japan Japanese coast a 129

Total Iodine concentration (ng m3)

Species fractions (%) Particulate

HI/I2

11.80 11.16 12.8 4  105 e e 1.42e2.58 e 9.7e25.6

26 25 15 18 12e28 2e30 15e17 8e27 8e12

3.5 6.5 12 16 45 (as inorganic) 43 (as inorganic) 1.5e27 17e35 24e49 13e22 3e5 (as I2) 3.4e10 13e15 (as inorganic)

Reference HOI

Organic 64 47 40 40 46e74 34e98 24e38 59e82 73e77

Gaebler and Heumann, 1993 Gaebler and Heumann, 1993 Michel et al., 2005, 2012 Wershofen and Aumann, 1989 Gaebler and Heumann, 1993 Gaebler and Heumann, 1993 Gaebler and Heumann, 1993

I.

the particulate phase (McFiggans et al., 2000; Vogt et al., 1999) and field observations show that organically bound iodine and iodide are the dominant iodine species (Baker et al., 2001; Baker, 2004, 2005; Gilfedder et al., 2007). Measurement of 129I species in the air over the North Sea indicated particle associated, inorganic and organic gaseous species were in the range of 18%, 43% and 40%, respectively with similar distribution for 127I (Michel et al., 2012). In contrast, Wershofen and Aumann observed a different distribution of iodine isotopes (mainly gaseous organic iodine) near Karlsruhe Reprocessing Plant WAK (Wershofen and Aumann, 1989). As expected they noticed that the closer the location to the reprocessing plant, the higher the percentage of gaseous 129I, while no such trend was observed for 127I. Gaseous iodine supplied to the atmosphere becomes gaseous and particulate forms as the result of gas to particle conversion and most of aerosol iodine exists in fine particles, only soil derived iodine exists in the coarse ones (Tsukada et al., 1991). The particle size distribution of atmospheric 129I and 127I in Tukai-mura, Japan showed similar pattern (rich in the fine particles <1 mm), although the sources of both isotopes were different. 2. Sample collection In principle, methods of air sampling of 129I are the same as for the stable iodine. But relatively low concentration of 129I compared to stable iodine requires larger sampling time. A variety of sampling methods have been used depending on objectives. For filtering iodine adsorbed on particles, high efficiency particulate filers (HEPA) and for trapping iodine vapors, an impregnate on the surface of solid adsorbents are used. These materials are used either alone or in combination for speciation analysis. In any case, air is pumped through the sampling material. The sampling of particulate iodine is simpler than the sampling of the different gaseous iodine species. Filtration is most common method used to capture aerosol particles. It is considered the most simple, versatile, convenient and economical way of aerosol sampling. The most important types of filters for aerosol sampling are fibrous and porous membrane filters. Several investigations have been carried out to measure particulate 129I in atmosphere using nuclepore polycarbonate, fluoropore, polypropylene, glass fiber and cellulose nitrate filter (Englund et al., 2010a,b; Jabbar et al., 2011; Lopez-Guiterrez et al., 1999, 2004; Santos et al., 2006; Szidat et al., 2000; Tsukada et al., 1991; Wershofen and Aumann, 1989; Winchester and Duce, 1967). For size distribution studies, aerosols have also been sampled with aerodynamic diameter larger than 0.06 mm using a Berner-impactor distinguishing eight classes of diameters (Ernst et al., 2003). Since gaseous iodine is easily adsorbed on surfaces, it is collected by ventilating air through a tube filled with adsorbent material. At the moment, for organic compounds of iodine such as

methyl iodide, the most widely used material is activated charcoal due to high adsorption capabilities. The charcoal used is impregnated with substances which react chemically with iodine compounds. Low volatility of the impregnate and the final product, humidity, good chemical and radiolytic stability are important factors for selection of impregnate (IAEA, 1973). For example, Wershofen and Aumann used activated coconut shell charcoal whereas Sakurai used cartridge filters packed with silver impregnated silica gel to monitor radioiodine (129I) in reprocessing plants (Wershofen and Aumann, 1989; Sakurai et al., 1997). Vapor phase iodine was also collected by drawing air through a glass fiber prefilter to remove particulates, then through triethylene di-amine (TEDA) activated charcoal or a chemically treated, low background petroleum-based charcoal cartridge which adsorbs organic iodine compounds only (Fritz and Paton, 2006). Similarly, for selective adsorption of inorganic gaseous iodine, cellulose filters impregnated with LiOH in glycerol or tetrabutyl ammonium hydroxide (TBAH) have been used (Ernst et al., 2003). Considering problems of collection and subsequent recovery of iodine species onto and from solid sorbents, there has been another approach using water based collection both by water cocondensation and cryogenic freeze out for trapping iodine. To ensure efficient collection, hydroxides or thiosulphates of alkali metals are added to the water. By this method it was not possible to estimate the amount of air actually sampled (Farmer et al., 1998). Also, such a system did not collect methyl iodide efficiently (IAEA, 1973). Recently, a cylindrical diffusion denuder has been used with starch/amylase or a cyclodextrin as chemisorption material for the collection of elemental/gaseous iodine at the inner surface of cylindrical denuder sampler (Chen et al., 2006; Huang, 2009; Huang et al., 2010). However, low sample flow rates require extremely long sampling time. This problem can be partially reduced using annular or parallel plate denuder. The speciation studies of 129I provide useful information about sources and transfer pathways. For instance, a multistage sampler has been developed for speciation of 129I in atmosphere by preparing glass microfiber filters which are arranged in consecutive order. The particulate iodine is collected by a particle filter, HI and I2 by a NaOH impregnated filter, HOI is adsorbed on a TBAH impregnated filter and finally organoiodine is adsorbed on a filter loaded with activated charcoal or TEDA impregnated active charcoal (Gaebler and Heumann, 1993; Hou et al., 2009b).

3. Extraction and separation techniques Various pre-concentration and extraction methods have been employed for separation of 129I prior to measurement; a brief summary is given below.

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A wet oxidation coupled with distillation at 50e60  C in presence of hydrogen peroxide, hydroxylamine hydrochloride and sodium nitrite has been used. The distillate is collected in alkaline solution (Doshi et al., 1994; Nedveckaite and Filistowicz, 1993). The chemical yield of procedure was found to be less than 70%. Alternatively, filter paper is reduced to pulp by stirring with 1.5 M HNO3 after addition of carrier. The separation of liquid phase is carried out by centrifugation. The solution is then mixed with NaClO at pH 10 to ensure isotopic exchange between carrier and sample by converting all iodine to periodate (Paul et al., 1987). Acid decomposition method cannot be used together with ICP-MS due to formation of iodine volatile compounds during aerosol formation in pneumatic nebulizer which causes loss of analyte and memory effects (Oliveira et al., 2010) by following reaction;

Air Sample 600 ºC in stream of Oxygen

Mixture of NaOH & NaHSO3

Mixture of H2O2 & hydroxylamine

Combustion

Acid leaching

Alkali leaching

Leachate/Trapping solution

IO3  þ 5I þ 6Hþ /3I2 þ 3H2 O For particulate iodine fraction, two extraction methods have been employed to release water soluble iodine (e.g. iodide, iodate) from the filter papers. In the more vigorous, hot method, pieces of filter are suspended in Milli-Q water in sealed polyethylene tubes and continuously stirred at 95  C for 3 h. The tubes are then cooled and the extract is immediately filtered (0.45 mm, cellulose acetate). By cold method, iodine is extracted from each filter using ultrasonication for 5e20 min at 20  C followed by filtration (Baker et al., 2000; Xu et al., 2010). For the extraction of non-water soluble iodine species, TMAH has been applied (Chen et al., 2006). Extraction of iodine species in ultrapure water using ultrasonic method and pressurized decomposition with dilute ammonia has also been reported (Gilfedder et al., 2010) to improve efficiency. Another approach to increasing the extractable yield of iodine was sulphite leaching using Na2SO3 or NaHSO3. Aerosols are extracted in mixture of 0.05 M NaOH and 0.025 M NaHSO3 through stirring for 2 d or mixture is heated for few hours at boiling point to extract organic and inorganic iodine. The iodine is then reduced to iodide under acidic conditions. This method has been widely used for determination of total particulate iodine (Englund et al., 2010a,b; Gaebler and Heumann, 1993; Jabbar et al., 2011; Lopez-Guiterrez et al., 1999, 2004; Santos et al., 2006; Szidat et al., 2000). A combustion method has also been applied for extracting iodine (Fig. 3). In this method, sample is combusted at higher temperature (>600  C), in a stream of oxygen using tracer (125I) to provide an estimate of yield. The exhausted iodine, mainly I2, is trapped within an alkali solution (KOH) or active charcoal (Tsukada et al., 1991; Wershofen and Aumann, 1989). Extraction of molecular iodine trapped in the diffusion denuder has also been potentially performed via 4-iodo-N, N dimethylaniline derivatization at room temperature by shaking about 120 min (Huang et al., 2010). The solution is then extracted into cyclohexane. Once iodine has been trapped it needs to be purified for analysis. Iodine in the leachate or trapping solution is usually separated by solvent extraction. Solution is acidified and iodide or iodate is oxidized to I2 by addition of NaNO2 or NH2OHeHCl. Molecular iodine formed is then extracted into CCl4 or CHCl3. After reduction by 0.1 M NaHSO3 and 0.1 M H2SO4, iodide is back-extracted into water. Alternatively, a portion of silver wool is placed in the chloroform for overnight to get AgI directly on the surface of wool (Delmore et al., 2010). Attempts have been made to replace solvent extraction by anion exchange method. Pre-concentration of 129I using anion exchange resin (Dowex or AG1 x8) in addition to purify the sample of interfering materials has been used successfully for large volume sea water samples. The adsorbed iodide is eluted by nitrate solution (Wimschneider and Heumann, 1995). This method with some modifications has been used recently for air filters (Jabbar et al., 2011).

49

Solvent extraction CCl4 or CHCl3

Ion exchange Dowex 1 x 8 Cl(100-200 mesh)

Derivatization 4-iodo-N, N dimethylaniline

Source preparation for measurement

Mass spectrometry / radiometry/ flouresence Fig. 3. Summary of analytical procedures of

129

I.

Finally iodine is either measured directly by ICP-MS or precipitated3 as AgI/MgI2 for AMS measurement or gamma-ray spectrometry after neutron irradiation. An extract of cyclohexane after derivatization is concentrated with stream of nitrogen and is injected into GCeMS (Fig. 3). 4. Measurement techniques Iodine-129 is one of the hard-to-measure radionuclides because of the long half-life and low energies of beta and gamma-rays emitted from it. Gamma spectrometry and low level liquid scintillation counting do not allow to carry out state of the art environmental research due to lack of sensitivity. Measurement techniques that have been proven to be effective in determining atmospheric 129 I include RNAA, AMS, ICP-MS, GCeMS and laser-induced fluorescence. The present knowledge about the radioecology of 129I is strongly affected by the analytical capabilities of neutron activation analysis (NAA). NAA was firstly proposed and applied in 1962 by Studier according to Schmidt et al. (1998) for the determination of 129I, based on the following nuclear reaction: (Edward, 1962).  129 ðn;gÞ;s ¼ 30b130 b ;12:3h130

I

!

130

I

!

Xe

Since I can also be produced by the reactions of 235U (n, f) 129I 130 133 (n, g) I, Cs (n, a) 130I and 128Te (n, g) 129Te/ 129I (n, g) 130I (Muramatsu et al., 1984). It is necessary to purify iodine before neutron irradiation in order to reduce production of 130I trough the reactions. An anion exchanger retaining 129I in a polyethylene capsule can be used as target for the irradiation. Activated charcoal adsorbing 129I can also be irradiated in reactor but uranium impurities present in charcoal will interfere with 129I determination. When iodine is purified by solvent extraction, either solid residue left after evaporation of H2SO3 solution containing MgO or LiOH in quartz ampoule or iodine precipitated as PdI2 are used as target (Lin and Gostomski, 2012). After irradiation, 24Na, 42K, and 82Br, which can interfere with the 130I measurement, were removed by further purification (Purkayastha and Martin, 1956). The NAA offers an

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advantage for simultaneous determination of stable iodine by following thermal neutron reaction (Seren et al., 1947).  127 ðn;gÞ;s ¼ 6:2b;128 b ;24:99min128

I

!

I

!

Xe

The activities are generally measured with gamma-ray spectrometry. The lower limit of detection of 129I was reported to be 1  1010 and 2.1  108 atoms by beg and slow sum coincidence methods, respectively (Nedveckaite and Filistowicz, 1993). However, this technique was not found sensitive enough to detect 129I at pre-anthropogenic levels. Therefore, the natural abundances and their transition to high contamination levels cannot be quantified. In addition, the complexity of the method, radiation exposure during analysis and the need for relatively larger sample size have impeded more systematic studies. On the contrary, AMS is capable of providing the missing information due to its extreme sensitivity. Since 1980, accelerator mass spectrometry has been used for analysis of 129I (Elmore et al., 1980; Finkel and Suter, 1993; Proctor et al., 1994). The radionuclide of interest (129I) is first prepared as a solid target, mixed with Ag or Nb powder and then injected to the system as a negative ion by ion sputtering (e.g. using a Csþ primary ion source). The sputtered negative ions from the sample is pre-accelerated, mass analyzed by a magnet, stripped to I3þ or greater oxidation state in the stripper tank, and separated from the interferences such as 128TeH and 127 IH  . Since 129Xe does not form a stable negative ion, mea2 surement of 129I is not troubled by the stable isobar (Fifield, 1999). As 129I is always accompanied in nature by ubiquitous stable iodine therefore it is advantageous to measure isotopic ratio. This is also important with respect to the modeling of the radiological consequences of 129I in the environment. Although 129I/I theoretical detection limit of AMS is 2  1015 but lack of the blank material with ratios below 1014 establishes a practical detection limit of 1014 (Fehn, 2012). The detection limit of 129I very much depends on the level of the procedure blank (Roberts et al., 1996; Roberts and Caffee, 2000). Blank isotope ratios of 3  1013 and 4  1014 have been reported after chemical separation and of best chemical blank materials (Woodward iodine from Woodward Corp. Oklahoma, USA) (Schmidt et al., 1998). Actually, AMS is the only method for the determination of 129I in the pre-nuclear age samples of 129 127 I/ I < 1010 (Hou and Roos, 2008). Since AMS only determines the isotopic ratio, second technique (IC or ICP-MS) is required to determine stable iodine. The most reliable method for total iodine analysis is currently ICP-MS which decomposes all iodine species to Iþ (plasma temperatures ca. 7000e8000 K) before quantification with the mass spectrometer. Other spectrometry techniques which allow 129I determination are ICP-MS and GCeMS. Over past two decades, ICP-MS has been widely used because of short analysis time and relatively easy operation. The positively charged ions are extracted from the inductively coupled plasma (at atmospheric pressure) into a high vacuum of the mass spectrometer via an interface. The extracted ions are then separated by mass filters of either quadrupole type time of flight or combination of magnetic and electrostatic sector, and finally measured by an ion detector. The calculated detection limit in ambient air based on standards is about 1.4  108 atoms of 129I using thermal decomposition corresponding to 129I/127I of 107 (Farmer et al., 1998). More recently, Fujiwara et al., have reported 129I/127I of 108 using dynamic reaction cell (DRC-ICP-MS) (Fujiwara et al., 2011). A potentially attractive feature of the technique is ability to measure 129I and stable iodine (127I) simultaneously. The measurement of 129I by ICP-MS suffer from the isobaric interference of 129Xeþ due to xenon impurity in the plasma gas (i.e. Ar), the polyatomic in127 terferences of 127IHþ I peak) 2 , low sensitivity (tailing from the

and memory effects (Hou and Roos, 2008; Li et al., 2009). For example, iodine has a high ionization potential (10.45 eV) and its ionization efficiency in argon plasma is only 30% (Hu and Moran, 2010). In addition, it has been reported that signal of iodine is less stable in acidic medium than in alkaline medium (MoredaPiñeiro et al., 2011). Other technique like GCeMS has been applied recently for quantification of gaseous radioactive iodine. The MS is operated in the electron impact ionization mode with an acceleration energy of 70 eV. The reported Limit of detection for 129I (sampling time 46 d at 14.4 L min1) is 3.2  107 atoms m3 (Huang et al., 2010). A laser-induced fluorescence detector has been developed for online detection of molecular 129I in the atmosphere. The detection limit was 2.1  1010 atoms using Ar laser (Goles et al., 1981). Later, 3 Hee20Ne laser was found to have better sensitivity of 129I detection as compared to Ar (Kireev et al., 1994). The laser beam excites the 129I molecules to a metastable state which results in fluorescence emission from the 129I molecule. The fluorescence from the sample is received by a pair of photomultipliers which convert the fluorescence emission to electrical signals. The main problem encountered is the high background of 127I2 with the overlapping absorbance and fluorescence spectra to that of 127I129I (Kireev et al., 1994; Baronavski and Mcdonald, 1977). 5. Distribution of

129

I in air

Air is one of the most important media for iodine cycling in terms of iodine species transformations and transport of iodine from the ocean to the terrestrial ecosystems. Therefore, efforts have been made to obtain data on the atmospheric deposition of 129I due to nuclear weapon testing, Chernobyl accident and releases from reprocessing plants. In order to examine the present worldwide distribution of 129I, atmospheric levels of 129I that have been determined by NAA and AMS in areas near source and remote from nuclear facilities are shown in Table 2. The first analysis of 129I in air was performed by Brauer et al., during 1965e1970 on samples taken from normal background areas of USA by RNAA. The background 129I concentrations were defined as levels in air resulting from natural production, as well as fallout from atmospheric weapons tests in the 1950s and 1960s. But high uncertainty reported might be attributed to lack of sensitivity of RNAA. At Richland, however, two order of magnitude higher concentrations were reported that exhibit the influence of Hanford site (Brauer et al., 1974). Since the 1963 test ban treaty, the concentration of most of radioisotopes has essentially returned to prebomb levels (e.g. 36Cl or 14C), however, the situation is different for 129 I because of ongoing input to the environment. Additional studies of 129I in the atmosphere focused on regional emissions from known point sources, such as the reactor accident at Chernobyl and fuel reprocessing plants in USA and Europe. The contribution of accidental release of 129I from Chernobyl can be seen in air of Yavne, Israel (Paul et al., 1987). The concentration of 107 atoms m3 was one order of magnitude higher than normal background areas of USA. An indication of Chernobyl contribution can be assessed by comparing 129I concentrations in rain collected at Beit-Dagan, Israel in 1982 (8  107 atoms L1) with the sample collected after the Chernobyl accident in May, 1986 (9.1  108 atoms L1). Nedveckaite and Filistowicz also reported that after Chernobyl accident the increase of 129I air concentration did not exceed normal values (14  105 atoms m3 in 1982e1984) by more than one order of magnitude in the area of Vilnius, Lithuania (Nedveckaite and Filistowicz, 1993). Because of the short residence time of iodine in the atmosphere (approximately 15e18 d compared to 10 y for 14C) global dispersal of 129I and its equilibrium with stable iodine have not been

T. Jabbar et al. / Journal of Environmental Radioactivity 126 (2013) 45e54 Table 2 Atmospheric levels of measured

129

51

I.

Year of sampling

Sample type

Analytical technique

129

Richland, WA Several background places in the USA Vilnius, Lithuania

1965e68 1965e70

Filter paper

RNAA

1040  240 27  12

1982e1984

FPA-15, Active carbon impregnated with HMTA Nucleopore polycarbonate filter, TBAH impregnated cellulose filter, activated coconut shell

e

14

e

RNAA

79 8496 1779 1513 53684 6115 2007 1032

39  10

May, 1986 1986e2003

Filter paper Petroleum-based charcoal cartridge

AMS RNAA

260  80 30571 978 471 35

0.02a

Paul et al., 1987 Fritz and Paton, 2006

1987

Glass fiber filter

RNAA

1021

e

Doshi et al., 1994

1987e1989 1993e1995 1998 2001e2002 2001 Jan.eMar. 2001 Jan.eJul. 2001 2002

Fluoropore filter TEDA activated charcoal filter Polypropylene filter

RNAA AMS

66 14  11 0.85

8e2.75 1.44

Tsukada et al., 1991 Lopez-Guiterrez et al., 1999, 2004 Santos et al., 2006

Glass microfibre filter Glass microfibre filter Cellulose nitrate filter Borosilicate glass fiber filter

AMS AMS AMS AMS

4.2 0.19 0.19 79

10.3 1.0 1.0 e

1983e2008

Glass fiber filter

AMS

2008

Diffusion denuder

GCeMS

0.4e40.4 0.07e8.9 4660

20 12 e

Bonn, Germany Karlsruhe 0.7 km 5 km 14 km 0.7 km 5 km 14 km 23 km Yavne, Israel Hanford site, USA 1.5 km 22 km 29 km 78 km Trombay, India 1.8 km Tokaimura, Japan Seville, Spain

Vienna, Austria Sonnblick, Austria Zugspitze, Germany Fohr, Northern Germany Southern Sweden Northern Sweden Mainz, Germany a

1986 1986 1987

I  105 (Atoms m3)

129 127

I 108

Sampling location

I/

Reference Brauer et al., 1974

Nedveckaite and Filistowicz, 1993 Wershofen and Aumann 1989

Jabbar et al., 2011 Jabbar et al., 2012a,b Michel et al., 2012; Ernst et al., 2003 Englund et al., 2010a,b Huang et al., 2010

Ratio measured by AMS.

completed. Iodine-129 released from a point source will enter the global circulation slowly and is not expected to be uniformly distributed. Some 129I will preferentially re-deposit locally unless the pulse extends well into the stratosphere (Kilius et al., 1992). Thus exponentially decreasing 129I levels in atmosphere with distance to the reprocessing plants were observed. For example, level of atmospheric 129I around WAK was several orders of magnitude higher than the background level measured at the Bonn ‘control’ site in the same year. The most striking feature of data was the relatively high abundance of gaseous organic bound 129I that accounted 34e65% of total 129I for five sampling sites located between 23 and 0.7 km from WAK reprocessing plant (Wershofen and Aumann, 1989). For Hanford site in USA, 70e90% of total iodine was detected in the vapor phase (Fritz and Paton, 2006). The measurement made at a location 78 km upwind from the reprocessing plant was not completely out of influence but appeared to be representative of the regional background. Doshi et al., observed 1.02  108 atoms m3 of particulate 129I at about 1.8 km from Trombay reprocessing plant, India comparable to particulate concentration 0.62  108 atoms m3 observed by Wershofen and Aumann near WAK reprocessing plants (Doshi et al., 1994). Since 1990’s much attention was paid to the monitoring of 129I in background zones i.e. remote from 129I sources. This is apparent from measurements carried out in Europe (Spain, Sweden, Germany and Austria). However, reported results are difficult to interpret as they are obtained from different years in different locations. Various sample preparation procedures and analytical techniques make this comparison more complex. In Table 2, an average 129I concentration in ambient air samples collected from Seville (37.4 N, 6 W) during 1993e1995 and 2001e2002 are shown (Lopez-Guiterrez et al., 1999,

2004; Santos et al., 2006; Szidat et al., 2000). The time profile of 129I concentration showed the clear variation similar to the trend in stack emissions from Sellafield (54.42 N, 3.49 W), La Hague (49.67 N, 1.87 W) and Marcoule (44.14 N, 4.7 E) reprocessing plants but no short term correlation with releases from these plants was found. However, a tentative influence of 129I sources on measurement under certain meteorological conditions was identified. Rather comparable trends of 129I concentration in atmosphere of northern Sweden (Kiruna at 67 N, 20.3 E) and Vienna, Austria (48.25 N, 16.35 E) were observed. Contrary to northern Sweden, a strong correlation between atmospheric 129I concentration in Vienna and monthly gaseous discharges from Sellafield was evident (Jabbar et al., 2011). On the other hand, southern Sweden (Ljungbyhed at 56 N, 13.23 E) which is only 260 km from the coast of North Sea had six times higher concentration of 129I in aerosols than northern Sweden (Englund et al., 2010a,b). A measurement of airborne iodine species collected during 2002 at the island of Fohr (54.7 N, 8.5 E) in North Sea demonstrated a massive influence of local marine aerosols for the transport of 129I from sea to the atmosphere (Michel et al., 2012). Therefore, a higher concentration (one order of magnitude) of 129I (particle associated) in air samples collected at Fohr compared to northern Sweden and Austria was attributed to direct influence of liquid discharges by Sellafield and La Hague (Michel et al., 2012). Furthermore comparing 129I aerosol concentrations at Seville and Vienna during the same year we suggest that geographical location may explain the difference in isotope concentration, being higher at the middle latitude close to the reprocessing plants. Therefore, influence of gaseous emission in southwest Spain can not be considered as direct as in the case of other study (Vienna, Austria).

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T. Jabbar et al. / Journal of Environmental Radioactivity 126 (2013) 45e54

The trend of generally lower 129I concentration with increasing altitude was also observed caused by clouds scavenging and precipitation. Such depletion was evident by less incorporation of 129I in high altitude snow from Fiescherhorn (w4000 m above sea level) (Reithmeier et al., 2006). The influence of elevation can also be seen as illustrated by comparison of particle associated 129I concentration measured at the summits of Eastern Alps Sonnblick and Zugspitze (w3106 and 2962 m above sea level) with concentration measured at inland area of Vienna (Jabbar et al., 2012a). The Alp air shows concentration lower by a factor of 10 than that from Vienna. The reason behind this depletion may also be related to the fact that the injection of 129 I into the atmosphere from the reprocessing facilities does not reach high altitude. The variation of atmospheric 129I concentration has been correlated with releases from European reprocessing plants. A historical record of dry deposition of 129I in Vienna, Austria, over four decades back to the 1960s followed a distinct European fingerprint of 129I gaseous emissions and decreased with passage of time from 1980 onward (Jabbar et al., 2012b). While at Southern and Northern Sweden, time series of 129I concentrations in aerosol samples reflected strong correlation with 129I liquid discharges from Sellafield and La Hague reprocessing facilities. A gradual increase in 129I concentration reported was in agreement with the increase of liquid discharges. An investigation of annual deposition of 129I in Tokyo (about 130 km from Tokai Reprocessing Plant) showed a close relationship of the deposition with atmospheric emissions of 129I from Tokai reprocessing plant (Toyama et al., 2012). Toyama et al., observed two times greater concentration of 129 I in spring than fall, before the Tokai reprocessing plants was put into operation. They further proposed that air masses come from Europe in spring while from Pacific Ocean in fall. To summarize, strongly elevated 129I concentration are observed in the vicinity of reprocessing plants. The differences in 129I concentration in air over globe might be due to proximity to the coast, geological location and meteorological conditions. Besides these differences, European atmosphere is strongly influenced by discharges from the reprocessing facilities. 6. Radiation exposure It is important to consider potential effect on humans since iodine and its compounds are volatile and highly mobile in the environment, easily entering humans via ingestion or inhalation, accumulating in the thyroid and creating an irradiation risk. The health effects of internal exposure to 129I derive from the emission of low energy beta and gamma radiation. In normal circumstances, the impact of 129I on health is minimal due to its low radiotoxicity. The potential risks of internal exposure to 129I are a result of possible inhalation in a workplace environment or to both inhalation and ingestion of contaminated food in the case of the public (Guen et al., 2000). Iodine (including 129I) from air and food is transferred into blood. The most of iodine (>80%) uptake concentrated into thyroid gland. As a result, thyroid gland and surrounding tissues (e.g., parathyroid gland) receive the highest radiation doses. Assuming the data of ICRP reference man for human iodine content (1e10 mg stable iodine in thyroid by ICRP, 1995) and isotopic ratio 129I/127I of 106 in thyroid, an annual dose equivalent to the thyroid can be 103 mSv for both 1-year old child and an adult. Such a high value of isotopic ratio has not yet been reported in the environment except for the close proximity to reprocessing plants. Michel et al., observed an annual equivalent dose of 6 nSv for adults from average 129I/127I ratio of 1.1  108 in human thyroid gland, in Germany (Michel et al., 2005), while Hou et al. estimated 0.1 mSv

(close to nuclear facility) to the thyroid from internal exposure of I for the 129I/127I ratio of 104 (Hou et al., 2009b). In another study, Hu and Moran calculated annual committed thyroid dose of 2.5  108 mSv from contaminated food and groundwater near Yucca Mountain (Hu and Moran, 2010). In any case, the annual committed thyroid dose is much less than dose for combined beta and photon emitting radionuclides to whole body or any special organ (US-NRC 0.04 mSv y1) except near reprocessing plants. Given the environmental 129I abundances, the relatively low 129 127 I/ I ratios in human thyroid glands, (109e108) can only be explained by additional iodine sources with low ratios in the diet. Though this exposure is surely not of radiological relevance but further increase of the natural 129I/127I isotopic ratios up to 103 in atmosphere, hydrosphere, pedosphere and biosphere may result in significant thyroid exposure (1 mSv y1). It is necessary to mention here that there is about 68,000 kg of 129I in still unprocessed spent fuel till 2005, which is 10 times more than 129I already released to the environment (Hou et al., 2009b). 129

7. Summary At present, human nuclear activities, especially gaseous emissions and liquid discharges from reprocessing plants mainly contribute to the airborne 129I in the atmosphere. 129I from these sources enters into the global atmosphere, transported by aerosols and makes its way to biosphere. Still today, the radioecology of 129I is unclear because of complex chemical behavior of iodine. Iodine occurs as multiple species (particle associated, inorganic and organic gaseous) in the atmosphere with varying percentage contribution. The speciation of 129I is based on trapping different species using different adsorbents. Two extraction methods are mostly used for extraction of 129I including alkaline leaching and combustion followed by purification either by solvent extraction or anion exchange with the recently developed derivatization. Since the naturally occurring abundances of 129I are not accessible by INAA, development of mass spectrometry methods, especially AMS, presented the most important breakthrough in analytical technologies for 129I. This technique has opened new possibilities in the environmental research which had been impossible in past because of absence of high analytical sensitivity. Available data shows that the natural concentration of 129I was enhanced by three orders of magnitude especially in Europe. Present global atmospheric 129I abundances do not give rise to significant radiation exposure. Only high concentration in the closest proximity of reprocessing plants is of radiological relevance, but future development should be carefully surveyed. The current practice of increasing discharges should be discontinued or minimized in order to prevent 129I from becoming significant. References Baker, A.R., 2004. Inorganic iodine speciation in tropical Atlantic aerosol. Geophys. Res. Lett.. http://dx.doi.org/10.1029/2004GL020144. Baker, A.R., 2005. Marine aerosol iodine chemistry: the importance of soluble organic iodine. Environ. Chem. 2, 295e298. Baker, A.R., Thompson, D., Campos, M.L.A.M., Parry, S.J., Jickells, T.D., 2000. Iodine concentration and availability in atmospheric aerosol. Atmos. Environ. 34, 4331e4336. Baker, A.R., Tunnicliffe, C., Jickells, T.D., 2001. Iodine speciation and deposition fluxes from the marine atmosphere. J. Geophys. Res. 106, 28743e28749. Baronavski, A.P., Mcdonald, J.R., 1977. A Radioiodine Detector Based on Laser Induced Fluorescence. NRL memorandum report 3415. Brauer, F.P., Rieck Jr., H.G., Hooper, R.L., 1974. Particulate and Gaseous Atmospheric Iodine Concentration. IAEA-SM-181/6, pp. 351e366. Chen, H., Brandt, R., Bandur, R., Hoffmann, T., 2006. Characterization of iodine species in the marine aerosol to understand their roles in particle formation processes. Front. Chem. China 2, 119e129. Daraoui, A., Michel, R., Gorny, M., Jakob, D., Sachse, R., Synal, H.-A., Alfimov, V., 2012. Iodine-129, Iodine-127 and Caesium-137 in the environment: soils from Germany. J. Environ. Radioact. 112, 8e22.

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