Sources and behavior of technetium in the environment

The Science of the Total Environment, 64 (1987) 163~179

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Elsevier Science Publishers B.V., Amsterdam-- Printed in The Netherlands

S O U R C E S A N D B E H A V I O R OF T E C H N E T I U M IN THE ENVIRONMENT

E.H. SCHULTEand P. SCOPPA Commission of the European Communities, c/o E N E A - - Centro Ricerche Energia Ambiente, La Spezia (Italy)

ABSTRACT

Technetium is a man-made element produced in increasing amounts during the last decades. The chemical and physical properties of some technetium compounds are considered, and a discussion of possible source terms is included. Literature on the environmental behavior of technetium is reviewed to evaluate its transfer and equilibrium distribution in aquatic and terrestrial ecosystems. Considerable effort has been expended in the last years in order to understand the biogeochemical processes responsible for the long-term behavior of technetium in the environment and its transfer through food chains as well as to identify critical pathways of the long-lived radioisotope Tc-99 from the environment to man. INTRODUCTION

Over the last three decades technetium (Tc), principally as the long-lived isotope Tc-99, has been introduced into the biosphere mainly by nuclear weapon testing, operation of nuclear fuel cycle facilities, and discharges of low-level radioactive wastes [1-4]. Because of its long physical half-life of 2.15 x 10~ years, Tc-99 contributes significantly to the long-term radioactivity of highlevel waste derived from spent nuclear fuel [5]. In recent assessments of disposal operations of high-level radioactive waste into deep ocean sediments, Tc-99 was considered to be of primary importance [6]. In the past, predictions and evaluations of the long-term behavior of Tc in the environment were difficult, because knowledge about the accumulations of Tc and about the transfer of Tc in environmental compartments was very fragmentary. Moreover, this man-made element only recently introduced into the environment cannot be considered to be in equilibrium with the biosphere. Recent review articles summarize the available data on the environmental behavior of Tc [2, 7-9]. The latest results on this subject were discussed at the International Scientific Seminar held at Cadarache, France, in October 1984. Although knowledge about the behavior of Tc in the environment is still incomplete, the biogeochemical cycle of Tc can be outlined using data from laboratory studies and in-situ measurements gathered during the last decade in terrestrial, freshwater and marine environments. This paper provides an updated description and evaluation of the environmental behavior of Tc.

164 SOURCES OF TECHNETIUM Primordial technetium does not exist on earth, because all isotopes of technetium are radioactive and the longest-lived isotope (Tc-98) has a half-life of only 4.2 × 108 years. This half-life is very short compared with the age of the solar system. Technetium found in the environment today is mainly Tc-99, a beta-emitter with a half-life of 2.1 × 105 years. Tc-99 if formed by induced fission of uranium in reactors and during atomic weapon tests, and by spontaneous fission of uranium present in rocks, soils and water. Radioactive decay of the short-lived isomer Tc-99m utilized in radiopharmaceuticals results in negligible amounts of Tc-99. Technetium in ores Naturally occurring Tc-99 was first isolated from the uranium ore pitchblende, which contains 0.25-0.31 ng Tc per kg ore. At radioactive equilibrium 1 kg of pitchblende should contain 0.25 ng of Tc-99, equivalent to 0.175 Bq [10]. Technetium from nuclear weapon tests Detonation of nuclear explosives produces Tc-99 in three different ways: fission of U-235 and Pu-239 with a yield of ~ 6 % , and activation of Mo-99 present in the vicinity of the test site followed by decay. The fission reactions produce elemental Tc-99 which is oxidized rapidly to Tc207 in the presence of atmospheric oxygen at the high temperatures prevalent during the detonation. As the air cools, Tc207 may react with water vapour to give pertechnetic acid, HTcO4. Pertechnetic acid, pertechnates, and Tc207 become adsorbed on particulate matter present in the atmosphere and contribute to the radioactive fallout. The concentrations of Tc-99 in air close to the earth's surface have decreased since 1966 [11]. This decrease is probably caused by the continuing transfer o f Tc-99 containing particles - - produced during the most intense period of nuclear weapon testing - - from the air to the ground. Concentrations of Tc-99 in rain water, currently at 0.08pgl-', have decreased since 1962 [12]. The highest concentration (1 pg l - ' ) measured in 1962 [12] identifies nuclear tests in the atmosphere as the main source of Tc-99 in the atmosphere. Technetium from the nuclear fuel cycle Although the fission product Tc-99 is not released during normal operation of power reactors, technetium isotopes may be introduced into the environment at other stages of the nuclear fuel cycle. Most of the Tc-99 is released with the liquid effluents from the reprocessing plants. The releases by the facilities enriching recycled uranium are < 0.1% of the releases of the reprocessing plants.

165 Until 1980, considerable amounts of Tc-99 were released with the liquid effluents from the reprocessing facilities. In Europe, where fuel from light water reactors has been recycled for many years, the quantities of Tc-99 released into the environment were so high that radiological investigations of the mobility [15] and biomagnification of Tc-99 [16] became feasible. These Tc-99 discharges have been largely eliminated. Most of the Tc-99 present in spent fuel is now recovered and stored with radioactive wastes [13]. For instance, the Sellafield reprocessing plant at Seascale, U.K., released 179TBq Tc-99 in 1978, 44 TBq in 1979, and 57 TBq in 1980. After 1980 annual discharges dropped to an average of 4.6 TBq, representing 7% of the Tc-99 present in the spent fuel. A recent theoretical study [14] estimated the quantities of Tc-99 expected to be associated with gaseous and liquid effluents from the closed uranium fuel cycle for a 1000 MWe light water reactor operating with a capacity factor of 0.8 and 33000 MW d/t burnup. This reactor would produce 35 tons of spent fuel containing 18.9TBq of Tc-99. During reprocessing with the Purex process 20% of the Tc-99 would follow the uranium stream. Therefore, the recovered uranium would have 176mgTc per kg uranium for a total of 3.7 TBq Tc-99. Restrictions on the activity of fission products in uranium recycled in the U.S.A. would limit the Tc-99 concentration to 4 m g k g 1, corresponding to 85 GBq. The reactor would release 8.2 GBq Tc-99 in the liquid effluents. In such a fuel cycle 99.5% of the 18.9 TBq of Tc-99 produced per year would have to be separated and stored as nuclear waste. Only 0.45% (85 GBq) would be allowed to remain in the recycled uranium, and 0.04% (8.2 GBq) would be released into the environment.

Technetium from nuclear medicine The use of technetium in radiopharmaceuticals has continuously increased during the past 20 years. Several technetium complexes with selectivity for human organs and tissues are commercially available. These radiopharmaceuticals are always prepared with Tc-99m, which minimizes the radiation dose to the patient [17]. This isotope, with a half-life of 6.1h, decays to Tc-99 primarily by emission of a single 140 keV photon. The amounts of Tc-99m used in nuclear medicine and partially released to the environment after metabolism and excretion are important in terms of radioactivity but insignificant in terms of total Tc in the environment. The ratio between the specific activities of Tc-99m and Tc-99 is ~ 3 × 10~. Therefore, the decay of 197TBq of Tc-99m generates 630 kBq of Tc-99, corresponding to only 1 mg of technetium.

Inventory of technetium in the environment Information available in the open literature is sufficient to estimate the quantities of Tc-99 released into the environment. Nuclear weapon tests in the atmosphere produced between 100 TBq [1] and 140 TBq [18] Tc-99. Radioactive

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fallout is presently very low as a consequence of international agreements banning tests of nuclear weapons in the atmosphere. Nuclear reactors generated 2500 gigawatt years (thermal) (GWY) until the end of 1983. Six TBq Tc-99 are produced per GWY. Therefore, nuclear power stations all over the world generated 15 PBq, equivalent to 24 tons of Tc-99. If 10% of the fuel was reprocessed, if all of the technetium in the fuel reprocessed before the end of 1980 was released, and if only 10% of the technetium in the fuel reprocessed since 1981 was discharged, the total quantity of Tc-99 introduced into the environment would be ~ 1 PBq. This quantity is one order of magnitude higher than that produced by nuclear weapon tests. Thus, the liquid effluents from reprocessing plants are responsible for most of the Tc-99 in the marine environment. Approximately 93% of the Tc-99 produced is stored with the radioactive wastes waiting for disposal. BEHAVIOR IN THE ENVIRONMENT

Studies of the behavior of Tc in the environment gained momentum only during the last decade. The behavior of Tc in the terrestrial and freshwater environments has been reviewed [2, 7, 8] covering the literature through 1980. The distribution and fate of Tc in the marine environment (water, sediments, biota) has been intensively studied only during the past 5 years [16, 19, 20].

Environmental aqueous chemistry Several reviews have been written about the chemistry of Tc [2, 10, 21-25]. Technetium may exist in the valence states + 7, + 4 and 0. The pertechnetate ion (TcO4) is probably the most stable chemical form of Tc in aqueous environments over a broad range of pH and Eh, including those characteristic of natural waters [2, 26]. However, Tc(+ 7) is reducible to lower valence states by Zn, HC1, hydrazine, hydroxylamine, ascorbic acid, tin(II) chloride and dilute sulfuric acid [21]. Inorganic or organic substances and a change in redox conditions in the environment may determine which Tc species are present. For instance, Eh values of 60 mV in sediments from the Irish Sea [27] are sufficiently low to reduce Tc(+ 7) [28]. Once reduced, Tc has a strong tendency to coordinate with ligands containing highly polar groups and positively charged ligands. Technetium (Tc + 4) humic acid complexes have been shown to be quite stable and to exchange ligands rather slowly [29]. However, Scoppa et al. [26] reported that Tc(+ 7) was not absorbed by or bound to commercial humic acids. Under reducing conditions in soils and sediments, organic matter is likely to be responsible for stabilizing reduced Tc in complexes with amine, carboxyl, sulfhydryl and hydroxyl as donor groups [30]. Technetium in soils and sediments may be immobilized and made biologically less available by precipitation or association with humic acid. The remobilization of Tc probably requires the oxidation of lower valent to heptavalent Tc. In welloxygenated seawater reduced Tc is quite unstable. More than 90% of reduced Tc is oxidized to pertechnetate in hours [26] or a day [31] depending on the

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experimental conditions. Approximately 10% of the Tc remains in reduced form even after 14 days [32]. Other experiments succeeded in completely oxidizing reduced Tc in seawater within 2h, probably via TcC12 or TcOCI~ as intermediates to pertechnetates [33, 34]. Therefore, the principal form of Tc in well-oxygenated seawater is pertechnetate. Pertechnetate is probably also the form of Tc in low-level liquid effluents from fuel reprocessing plants [3, 4]. Sediments and soils

Pertechnetate appears to be the predominant technetium species in aerobic aqueous solutions. Pertechnetate will not be sorbed significantly by anionic colloids and suspended sediments and is, therefore, highly mobile in aerobic sediments and soils [2, 26]. However, physical, chemical and biological processes may strongly influence the fate of Tc. Sediments

Most of the long-lived radionuclides released to the marine environment are scavenged by fine-grained particulate matter. The radioactivity of sediment fractions increases with decreasing [4, 35] grain-size. The partitioning of technetium between solid and liquid phases is governed by many factors, including porosity, permeability, chemical changes such as alterations of the redox conditions mediated by microbial activities, and physical or biological reworking of the sediments. All these factors control the ability of the sediments to act as permanent sinks or temporary reservoirs and future sources of Tc. The interactions of different chemical forms of Tc with sediments are described by distribution coefficients (Eqn (1)). The K D values are determined K D

-

[Tc]particulate [Tc]dissolved

(1)

by the nature of the sediment and are higher for fine particles in muddy sediments than for coarse particles in sandy sediments. Distribution coefficients for pertechnetate partitioning between aerobic sediments low in organic matter and seawater are generally very low [16,36,37]. Technetium has been shown to be taken up slowly and poorly absorbed by aerobic deep-sea sediments [38, 41]. The KD values for sediments from the Northeast Atlantic abyssal plane never exceed 3.5 [38]. The distribution coefficients and the uptake rates were not significantly different for Tc(IV) and Tc(VII). The similarity of K D values suggests that Tc(IV) was oxidized to pertechnetate during these experiments in aerobic seawater [41]. Pertechnetate is therefore highly mobile under oxic conditions that are prevalent in pelagic environments and upper layers of many sediments. Redox conditions are clearly responsible for the immobilization of Tc in marine sediments. In aerobic sediments diffusion of pertechnetate to reducing environments in deeper sediment strata is the rate limiting step of Tc fixation [26].

168 Reducing sediments rich in organic matter retain Tc strongly and may immobilize it as insoluble TcO2"2H20 or Tc(IV) sulfides, or coprecipitate Tc(IV) with iron sulfides [2]. In such environments the distribution coefficient may reach values as high as 1000. For instance, pertechnetate disappeared almost completely from the liquid phase of reducing sediments. Less than 1% of the Tc activity was detected in the interstitial and overlaying waters after 1 week [26, 39, 40]. The fixation rate is dependent on the physicochemical characteristics of the sediment, the concentration of organic matter in the sediment, the mass ratio between sediment and water, the area of the water-sediment interface, and the pertechnetate concentration. The amount of Tc sorbed by 1 g of sediment was found to be approximately proportional to the concentration of pertechnetate in seawater. Saturation was observed only at relatively high concentrations (~> 10 4M) of pertechnetate [26]. Bacterial activity in the sediments has been claimed to be the cause of the slow fixation rates [37]. In other experiments the fixation rates of pertechnetate by untreated sediments and by sediments treated with antibiotics were observed to be the same [26]. Bacterial activity and organic matter probably promote the reduction of Tc(VII) and the fixation of Tc by consumption of oxygen and maintenance of reducing conditions in the sediment.

Soils

Terrestrial environments receive Tc mainly from atmospheric fallout derived from nuclear explosions and release of gases from fuel reprocessing plants [12]. Ditechnetium heptoxide and pertechnetic acid are very likely the predominant compounds of Tc in fallout. The solubility and mobility of Tc in soils will be determined by its oxidation state, by the concentration of organic matter in the soil, and by biologically mediated processes [2, 8, 14, 16]. Technetium is probably immobilized by ion exchange, reduction to less soluble forms, and complexation with humic acid [53]. Complexation with low molecular weight organic ligands may increase the solubility of reduced forms of Tc [30, 31, 47]. Under aerobic conditions present in well-drained surface soils, the highly water-soluble pertechnetate will be the predominant Tc compound in soil. Average concentrations of 10 19g Tc-99/g soil have been reported for agricultural soils [7]. Hardy [42] estimated Tc-99 levels of 10 14g g 1 in surface strata of soils. In soil sorption studies essentially no Tc was retained by sands after a 5-week equilibration period [43]. Similar results have been reported for subsoils with low ion-exchange capacities [44]. The mobility of Tc in the transition zone between unsaturated, usually oxic and water-saturated, anoxic strata in both mineral and organic soils is markedly influenced by redox gradients [45]. Technetium in anoxic strata diffused to oxic environments and became available to plants. Other experiments have shown that total Tc in the rooting zone of field soils and the fraction of Tc remaining available for uptake by plants decreased with time after a single application of pertechnetate [46].

169 In surface soils with a broad range of physicochemical properties the sorption of Tc varied from < 1 to 31% of the initially added Tc. Eighty percent of the variability in Tc sorption among soils can be attributed to differences in concentrations of organic matter, iron oxides, aluminum oxides, and clays in the soils. Some of the pertechnetate was reduced to the much less soluble Tc(IV) and some was taken up and transformed by microbes. Chemical reduction of pertechnetate caused by changes in redox conditions is much more important for the removal of Tc from solution than microbial uptake and sorption [47]. The distribution of Tc in brown chalky soil depends on the initial concentrations of added Tc [48, 49]. At low concentrations Tc was sorbed at the soil surface, whereas at high concentration water-soluble Tc remained available for leaching to lower soil strata. Calcareous brown soils sorb little Tc [50]. Approximately 40% of the initial amount was leached within 2 months. Brown acid soils retained Tc so strongly that only 2% was leached in 2 months. Biochemical reduction of Tc(VII) was also claimed as a mechanism for the fixation of Tc. Microbially mediated formation of volatile Tc compounds has been observed [51]. Pertechnetate at sufficiently high concentrations may serve as an oxidant at the soil redox potential, a behavior analogous to that of permanganate. Under aerobic field conditions, Tc will move in soils with or near the infiltrating water front [52].

Soil plant interactions and translocations in plants Recent results from laboratory and field studies indicate that plants may play an important role in the biogeochemical cycling of Tc in the terrestrial environment. In general, Tc-99 transfer factors from soil to plant vary considerably within and among species depending on the mode of contamination of the soil or nutrient solution and the concentration of Tc [2,8]. The rate of Tc-99 uptake increases with increasing concentrations of Tc [54]. Plants growing on sands took up a constant fraction of the added Tc-95m. This result suggests that neither soil fixation nor uptake mechanisms can regulate the uptake of Tc [45]. The absorption of pertechnetate by roots of soybeans increased linearly with time [55]. When Tc and Mo are present in the soil, Tc is preferentially assimilated by lettuce root [56]. No relationship was found between the amount of Tc supplied to the soil by irrigation water and transfer rates of Tc from the roots to aerial parts of the plants. Actinides may enhance the uptake of Tc, especially from soils that were contaminated with Tc some time ago. However, a proportionality between transfer factors and concentrations of actinides was not observed [57]. Sulfate, phosphate, selenate, and molybdate, but not perrhenate, competitively inhibit the uptake of pertechnetate by soybeans [55]. The uptake of Tc by plants from soils probably requires the expenditure of energy [54, 58]. Once absorbed, Tc is rapidly transported, principally as pertechnetate, to the aerial parts of the plants [55, 58-61], where Tc will become associated with proteins of low molecular weight or will remain as pertech-

170 netate [62-64], which is highly xylem-mobile in shoots because of the negative charge of the cell walls. In pea plants, more than 80% of the Tc is associated with leaves. Technetium concentrations in pea plants were found to be proportional to concentrations of Tc in the nutrient solutions [60,61]. Chronic contamination of various soils with Tc has been shown to cause increased concentrations of Tc in plants and enhanced translocation to the aerial parts [65], but the transfer factor was not linearly proportional to the Tc concentrations in the soil [66]. In contrast, the Tc concentrations in leaves of spinach have been reported to increase linearly with the Tc concentrations in the nutrient solutions in which the spinach grew [67]. Plant roots assimilated pertechnetate preferentially from mixtures of Tc cysteine complexes and pertechnetate. The pertechnetate is not translocated to the shoots but only to the leaves [67]. Similar results have been obtained with spinach plants grown on soils containing Tc compounds different from pertechnetate. Complexes of Tc with amino acids may also be absorbed by the roots [59]. Leaf-aging markedly influences the distribution and transformation of Tc-99 in spinach plants. The concentration of pertechnetate decreases with time [68]. Pertechnetate is the predominant Tc compound only in young growing leaves. The uptake and transformation rates decrease with increasing age of the leaves [65]. A summary [8] of Tc-99 soil plant transfer factors for a variety of plants lists maximal values of 180 on a dry weight basis and 140 on a wet weight basis. Values ranging from 10 to 30 have been reported for lettuce [7] and for tobacco [651. Considerable decreases of transfer factors with increasing age of plants have been observed [61,65,69] in many soils and at various Tc concentrations. Formation of complexes of Tc with organic matter and progressive reduction of pertechnetate by roots and soil microflora might be responsible for the observed decrease of Tc transfer factors with time. High concentrations of Tc may cause toxicoses in terresial plants. The growth of soybeans was inhibited by 0.1 gg Tc/g of soil [2]. Chemical toxicity was observed in lettuce at a Tc concentration o f 0 . 2 p g g 1. However, Tc levels of 0.1 pg g 1 that are toxic to soybean did not influence the growth of Swiss chard [43]. The toxicity of Tc is mainly caused by biochemical rather than radiation effects. Technetium probably competes with essential trace elements [2,64,70]. The toxicity of Tc may vary considerably with the nature of the plants and their metabolic activities [71].

Fate of Tc in marine and freshwater ecosystems Sufficient data are now available to evaluate the overall fate of Tc in marine and freshwater systems. Technetium released to the environment is highly water-soluble and undoubtedly exists as pertechnetate. Evidence from environmental studies suggests that Tc persists in oxic water columns, becomes widely dispersed in water bodies, and remains available for uptake by organisms. In

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oxic water Tc is not sorbed readily by suspended inorganic particles, but may be sorbed by organic particles. In anoxic waters Tc(VII) may become reduced to lower-valent compounds that coordinate easily with organic and inorganic ligands. The complexes might be taken up preferentially by biota. In freshwater samples Tc remains dissolved and is not sorbed significantly by suspended particles with diameters < 45 pm [72]. In seawater samples that did not contain particles > 0.22 pm Tc was not associated with particulate matter and was not sorbed by container walls or filters [26].

Concentration and distribution of Tc in water bodies The distribution of some man-made radionuclides in European coastal waters is influenced by the controlled discharges of low-level nuclear waste from fuel reprocessing plants [73-75]. Technetium, present in the effluents of Sellafield and Cap le Hague plants in soluble form, enters the North Sea [76] and will be transported by the Gulf stream to the Norwegian coast and into arctic waters. Recently, Beasley and Lorz [31] calculated Tc-99 activities from the Sr-90 activities in the mixed surface layer of ocean waters at 4(~60°N latitude. Assuming that Tc-99 behaves similarly to Sr-90, values of 3.5-7.0 pBq 1 1 were calculated for the Tc-99 activities. In-situ measurements of Tc-99 in seawater just above the sediments in the Irish Sea [27] gave 49.3 + 2.5mBql-l.At a distance of 5-10 km north and south of the point at which the effluent of the Cap le Hague reprocessing plant enters the sea, 18-20 mBq 1 1 Tc-99 were found. At a distance of 100kin the activity had decreased to 1.5~3.0mBq 1 1. Seasonal variations of the Tc-99 activities of between 5 and 20mBql 1 were observed 5 km north of the release point [77]. In 1982 a Tc activity of 5mBql i was measured in surface waters of the northeast Scottish coast. The technetium probably came from the Sellafield reprocessing plant [78]. Much lower Tc-99 concentrations were found for bottom waters (68 + 11pBq 1 1) and surface waters (95 + 11pBq 1 1 ) of the Baltic Sea [15]. The higher activities in the bottom waters may be caused by the inflow of contaminated, more saline waters from the North Sea as a bottom current. Macrophytic brown algae of the genera Fucus and Ascophyllum used as bioindicators for Tc-99 showed that Tc discharged by the reprocessing plants at Sellafield and Cap le Hague has reached the Scandinavian coasts and the artic waters off Spitzbergen and Greenland. Holm et al. [79] reported that the decrease of the Tc-99 activity with distance from the source depends more on physical dilution than on biological removal and sedimentation. In Norwegian waters, the Tc activities in Fucus vesiculosus and Ascophyllum nodosum decreased to one-third over a distance of 2000 km. From Tc concentrations measured in Fucus near the release point at Sellafield [4] and Cap le Hague [80], the times required for Tc-99 to reach the Scottish coast (1 year), the North Sea (3 years), the Baltic Sea (4 years), the central Norwegian coast (3 4 years), Spitzbergen (4-6 years), and the east coast of Greenland (6-8 years) were

172 estimated [78,81]. These results demonstrate the high mobility of Tc in the marine environment and suggest that Tc-99 may serve as a long distance tracer for marine waters using Fucus as a bioindicator. Data on releases into and concentrations and distribution of Tc-99 in ground and surface freshwater bodies have been summarized by Luyckx [1], Wildung et al. [2], and Ehrhardt and Attrep [12].

Interactions of Tc with marine and freshwater biota Little information about the behavior of Tc in aquatic biota was available prior to 1979. A comprehensive review of the environmental behavior of Tc published in 1979 had only two references to the interaction of Tc with biota [2]. During the past years the fate of Tc in marine and freshwater environments has been extensively studied using Tc-95m. Concentrations of Tc-99 have been measured in water and biota and bioaccumulation factors calculated. Recent reviews summarize the concentration factors and biological halflives of Tc in marine [16, 19, 20, 31] and freshwater organisms [82]. Most of the marine organisms studied had very small concentration factors for Tc. The concentration factors never exceeded 20 for marine phytoplankton [19, 83]. Similar values were found for green and red algae [19, 33, 37, 84, 85], annelida [37], mollusca [32, 38, 86-88], crustacea [88-92], echinodermata [85], and pisces [37, 85, 89]. Some species of brown macroalgae [16, 33, 37, 84], polychaetes [19, 32, 37], and crustaceans [37, 89] had bioaccumulation factors two orders of magnitude higher than those normally found. Biologically-based explanations for these differences in concentration factors among biota belonging to the same families or genera are not available. Experiments conducted with the aim of identifying specific binding sites for Tc showed that the soluble fractions of plant [96] and animal tissues [90, 97, 98] contained most of the Tc in the form of pertechnetate and some of it associated with low-molecular-weight proteins [87, 93-95]. No evidence for molecules specifically binding Tc was found. The Tc-99 concentration factors were similar for similar marine and freshwater organisms [72, 82, 96] despite the differences in experimental and environmental conditions. Freshwater molluscs and macrophytes have been shown to have the highest concentration factors (60-120) among freshwater biota [82]. The elimination of Tc from marine organisms is quite rapid, regardless of whether the Tc comes from water or food. In most cases more than 60%, and sometimes 80-90% of the Tc is eliminated with biological half-lives of days or weeks [16, 19, 20, 32, 84, 86-92, 94, 97]. Only in two crustaceans has Tc been shown to have biological half-lives of almost 1 year [32, 89]. Bioaccumulation factors based on Tc-99 activities measured in the field in various marine biota and waters exceed those obtained in the laboratory by 1000-fold [77-80, 89]. Bioaccumulation factors for macrophytes and molluscs in the field have been determined as 90000 and 7000, respectively [77, 78, 87].

173 These large differences may be caused by chemical and experimental factors. Concentration factors derived from field studies are preferred for the assessment of radiological impacts.

Transfer of Tc in food chains Recent studies on the transfer of Tc in the aquatic food chain [20, 82, 94] showed that Tc, with only a few exceptions, is not efficiently passed to higher trophic levels. Bottom-dwelling organisms had only a slight effect on the remobilization of Tc bound to sediments [98]. Low concentration factors in phytoplankton, organisms of the first trophic level [19, 31, 83], will hardly disturb the distribution of Tc within the water column. Higher organisms also have low concentration factors [8, 16, 20]. Biota rarely derive more than 50% of their body burden of Tc from spiked water or from contaminated food [16, 32, 87-89, 92, 94]. Approximately 70% of the Tc in the body is present in the digestive system (hepatopancreas), in the liver or the gut [32, 87, 92, 94]. The turnover rates at these locations is high. Little Tc is transferred from the digestive system to muscles or other tissues with low metabolism that could retain the radioisotope for longer times [92]. Most species studied eliminated Tc rapidly [16, 20, 31]. Most of the Tc ingested by fish was found to be excreted in soluble form, either with the urine or via the gills. Only 20% of the Tc was eliminated with the feces. Crustaceans excrete only a minimal amount of Tc via feces [92]. The low accumulation of Tc from water and food, and the short biological half-lives of Tc in marine organisms do not allow much Tc to pass to higher trophic levels. In contrast, the accumulation through the food chain is an important source of Tc for higher freshwater organisms such as snails, fish, and frog larvae [72, 82]. The accumulation of Tc by these organisms is related to their feeding habits and the concentrations of Tc in the food sources. The transfer of Tc in terrestrial food chains is governed largely by the physicochemical forms of Tc in plants and their bioavailability to animals. Pertechnetate, applied in aqueous solutions to herbaceous and grass pastures, was rapidly immobilized by the vegetation, probably via foliar absorption and translocation of Tc [99]. The low transfer factors from pasture to milk suggest that plant-incorporated Tc was not very available to ruminants. Biological incorporation of Tc into plant tissues appears to reduce the transport of Tc through the food chain. Plant-Tc is only poorly resorbed in the gastrointestinal tract [100, 101]. Rats absorbed intravenously injected pertechnetate efficiently and excreted most of the Tc in the urine. When Tc was administered mixed with commercial food, 50% of the Tc was eliminated with feces. When plant-Tc was administered, 90% of the Tc was excreted with the feces. Pertechnetate injected into the rumen of sheep or fed as plant-associated Tc was absorbed in the gastrointestinal tract only to a small degree. Ninety percent of the administered doses were excreted via the feces [101]. Guinea pigs and rats absorb plant-associated Tc in the intestines less efficiently than inorganic Tc [2].

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Because of low absorption of Tc by terrestrial animals and the high turnover rates in most of their tissues [100, 101] terrestrial biota are not important for the recycling of Tc in the biosphere. The major pathways of Tc from the terrestrial environment to man involve the consumption of plant material and poultry eggs [102] rather than milk and meat. CONCLUSIONS

Technetium is present in all environmental compartments as the long-lived isotope Tc-99 in the form of pertechnetate. Under oxic conditions pertechnetate is highly mobile in marine, freshwater and terrestrial ecosystems. Pertechnetate is hardly sorbed by oxic sediments and soils. Under anoxic conditions pertechnetate may be reduced and immobilized. Low concentration factors and high elimination rates in most biota, and the high mobility are responsible for the fast recycling of technetium in the biosphere. Technetium remains mobile and available for accumulation over long distances and times. Technetium, because of its high mobility and low concentration in the environment, is not a major hazard to man. However, the considerable amounts of Tc-99 accumulating in high-level radioactive wastes are of great concern with respect to potential biochemical and radiological effects for man and his environment. REFERENCES Note: Many references cite articles published in ~'Technetium in the Environment", G. Desmet and C. Myttenaere (Editors), Proceedings of the Seminar on the Behavior of Technetium in the Environment, organized by the Commission of the European Communities, Radiation Protection Programme and held in Cadarache, France, October 23-26, 1984. Sole distributor in the U.S.A. and Canada: Elsevier Science Publishing Co., 52 Vanderbilt Avenue, New York, NY 10017. The Proceedings (417 pp.) published in 1986 will be cited as "CADARACHE". 1 F. Luyckx, Technetium discharges into the environment, CADARACHE, 21 27. 2 R.E. Wildung, K.M. McFadden and T.R. Garland, Technetium sources and behavior in the environment, J. Environ. Qual., 8 (1979) 156-161. 3 J.E. Till, F.O. Hoffmann and D.E. Dunning Jr., A new look at Tc-99 releases to the atmosphere, Health Phys., 36 (1979) 21-30. 4 R.J. Pentreath, D.F. Jeffries, M.B. Lovett and D.M. Nelson, The behavior of t r a n s u r a n i c and other long-lived radionuclides in the Irish Sea and its relevance to the deep sea disposal of radioactive waste, in Proc. 3rd NEA Seminar on Radioecology (Tokyo), NEA, OECD, Paris, rue A. Pascal, 1980, pp. 203-220. 5 F.W. Wickler and V. Schultz, Radioecology: Nuclear Energy and the Environment, Vol. I, CRC Press Inc., Florida, 1982, p. 98. 6 D.R. Anderson (Ed.), Eighth International NEA/Seabed Working Group meeting Varese, Italy, 1983, Scandia Report SAND83.2122, 1984. 7 R. Bittel, Le technetium et l'environnment, Radioprotection, 15 (1980) 141-146. 8 P.J. Coughtrey, D. Jackson and M.C. Thorne, Radionuclide Distribution and Transport in Terrestrial and Aquatic Ecosystems: A Critical Review of Data, CEC, Vol. 3, A.A. Balkema, Rotterdam, 1983, pp. 210-228. 9 M.D.S. Turcotte, Environmental behavior of technetium-99, Report DP-1644, E.I. du Pont de

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