Elsevier

Applied Radiation and Isotopes

Volume 61, Issue 6, December 2004, Pages 1203-1210
Applied Radiation and Isotopes

Comparison of transfer and distribution of technetium and rhenium in radish plants from nutrient solution

https://doi.org/10.1016/j.apradiso.2004.05.074Get rights and content

Abstract

Tracer experiments were carried out to compare the plant uptake behavior of Tc and Re from nutrient solutions. Radish plants, transplanted to nutrient solution including various tracers, showed the same uptake and distribution of 95mTc and 183Re. The trend was the same when the 99Tc and stable-Re concentrations were increased in nutrient solution. The behavior of other elements was different from that of Tc and Re. These findings suggest the possible use of Re as the chemical analogue of Tc in soil solution to plant uptake experiments.

Introduction

Technetium has no stable isotopes. Among its isotopes, 99Tc is of potential long-term importance in the environment because it has a long half-life of 2.11×105 yr and it is produced in the fissions of 235U and 239Pu at relatively high ratios (ca. 6%). The radionuclide has been widely distributed in the environment as a result of fallout from nuclear weapons testing (Attrep et al., 1971; Holm et al., 1988; Garcı́a-León et al., 1990; Tagami and Uchida, 2002) and discharges from nuclear facilities (Dahlgaard, 1994; Aarkrog et al., 1997; Smith et al., 2001).

The most stable chemical form of Tc in the surface environment is TcO4 (Brookins, 1988), which is considered to be highly mobile in biogeochemical cycles; indeed, a transfer factor (TF) of 5 (on a wet weight basis) for Tc in the edible parts of common plants was recommended as allowable by the IAEA (1982). The fact that Tc has the highest TF in plants among non-nutrient elements (Wildung et al., 1977; Hoffman et al., 1982; IAEA, 1994) makes Tc unique. Thus, knowledge of Tc behavior in the soil to plant systems is of special interest because of potential long-term radiological consequences.

The chemical forms of Tc in the environment presumably play an important role in determining the fate of Tc in the soil to plant systems. The TFs of Tc obtained from recent field observations (Green and Wilkins, 1995; Uchida et al., 2000) are lower than those obtained from laboratory studies (Wildung et al., 1977; Hoffman et al., 1982; Yanagisawa and Muramatsu, 1993). Determination of the physico-chemical forms of 99Tc in soil and plant samples would advance our understanding of its behavior in the environment, but the amount of 99Tc in the natural environment is at ultra-trace levels even in soil (Tagami and Uchida, 2002), therefore, the chemical species formed by 99Tc in the environment are unclear.

Rhenium, which lies just above Tc in the periodic table, may be considered as a potential chemical analogue for Tc. These two elements behave similarly in the environment (Brookins, 1988). Although the amount of Re in the environment is greater than that of 99Tc, no data are available for terrestrial plant samples. This lack is primarily due to Re being one of the rarest elements in the earth's crust (Wedepohl, 1995). Also, Re is not considered an essential element for animals or plants, so there is little practical interest in it. Radiochemical neutron activation analysis has been used to measure Re and some data on the concentrations of the element in seaweeds were obtained (Fukai and Meinke, 1962; Scadden, 1969). Other more sensitive methods such as flameless atomic absorption spectrophotometry (Yang, 1991) and inductively coupled plasma mass spectrometry, ICP-MS (Mas et al., 2004) have recently been applied for Re determination in seaweeds. The concentration ratios (CFs) of Re (Re [g] per gram of dried seaweed divided by Re [g] per gram of seawater) were calculated to range from 380 to 8900 in brown seaweeds (Fukai and Meinke, 1962; Scadden, 1969; Yang, 1991); the average Re content in seawater is 8.24 pg g−1 (Colodner et al., 1993). High CFs for Tc were also found in brown seaweeds (Hurtgen et al., 1988). The dominant chemical forms of Re and Tc in open seawater are thought to be ReO4 and TcO4, respectively, and these highly soluble chemical forms would be readily absorbable by seaweeds.

We believe that a terrestrial plant might absorb Re at as high rate as that reported for Tc. In which case, Re could be used as a chemical analogue for Tc in soil solution to plant systems if their uptake behavior was the same, although no study from that viewpoint has been carried out yet. Therefore, we carried out experiments using a multi-tracer technique (Ambe, 2000; Ambe et al., 2002) and a stable multi-element technique with the addition of 99Tc. The multi-element technique, which employs 10–30 stable elements, has recently become available without using radioisotopes even at the low concentrations through the application of ICP-MS. The uptake of Tc and Re by plants, and their distribution in different plant parts, were studied using nutrient solutions. It was assumed that if uptake ratios and distribution ratios of Tc and Re were the same in the plants, then their TFs from nutrient solutions to the plants would also be very close.

Section snippets

Plant cultivation

Radish seedlings, 3 days after germination, were grown in a nutrient solution prepared from a commercially available nutrient powder, HYPONeX®, by dissolving it in deionized water (1 : 1000 in weight). The mjor anion concentrations in the nutrient solution were 2 mm for Cl, 1.8 mm for SO42−, 0.4 mm for H2PO4 and 3.4 mm for NO3. The plants were placed in a greenhouse at 21°C and exposed to normal daylight conditions for about 1 month. The average fresh weight of the plants was 5.5 g.

Each plant was

Uptake of TcO4 and ReO4 by radish plants

The physicochemical forms of Re in plants are not known, but there are several reports on those of Tc (Krijger et al., 1999; Bennet and Willey, 2003). Technetium is known to be absorbed by plants through their roots as TcO4, which is the most stable chemical form in water under aerobic conditions. The TcO4, is passed through the xylem, and finally Tc is translocated to the leaves. No evaporation of Tc from the leaves has been reported. We assume that Re would also be taken up as ReO4, which

Acknowledgements

We are grateful to Dr. S. Enomoto, Dr. R. Hirunuma and Dr. H. Haba, The Institute of Physical and Chemical Research (RIKEN), Japan, for kindly providing us with the multitracer solution. We would lie to express our deep gratitude to Dr. T. Sekine, Tohoku University, Japan, for his cooperation with making 95mTc. We thank Ms. N. Ogiu and Mr. K. Tabei for their assistance with the experiments. This work has been partially supported by the Agency for Natural Resources and Energy, the Ministry of

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