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plant interaction in a recultivated mining area
Microchemical Journal 90 (2008) 44–49
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Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Redistribution of uranium and thorium by soil/plant interaction in a recultivated mining area Victor G. Mihucz a,b, Zsolt Varga c, Enikő Tatár a,b, István Virág a, René van Grieken d, Zsuzsanna Koleszár e, Gyula Záray a,b,⁎ a
Department of Analytical Chemistry, Institute of Chemistry, L. Eötvös University, H-1518 Budapest, P.O. Box 32, Hungary Hungarian Satellite Centre of Trace Elements Institute to UNESCO, H-1518 Budapest, P. O. Box 32, Hungary Department of Radiation Safety, Institute of Isotopes, Hungarian Academy of Sciences, Konkoly-Thege Miklós 29-33, H-1121 Budapest, Hungary d Micro- and Trace Analysis Centre (MiTAC), Department of Chemistry, University of Antwerp, Campus Drie Eiken, Universiteitplein 1, BE-2610 Antwerp-Wilrijk, Belgium e Mecsek Öko Ltd., Esztergár Lajos 19, H-7633 Pécs, Hungary b c
A R T I C L E
I N F O
Article history: Received 4 March 2008 Accepted 12 March 2008 Available online 20 March 2008 Keywords: Uranium Tree Soil Mining ICP-MS HPLC
A B S T R A C T During the recultivation of the uranium mining area of Kővágószőlős (Hungary), the tailings were covered with clay and loess soil layers having a thickness of 30 cm and 100 cm, respectively. In the loess covering layer, acacia (Robinia pseudoacacia), poplars (Populus × albus, Populus × canescens), oak (Quercus pubescens), silver tree (Eleagnus angustifolia) were planted between 1996 and 2004. In order to establish the extent of the uranium and thorium transport from the sludge to the leaves by uptake and translocation processes through roots with a length higher than 1.3 m results in a remarkable redistribution of these pollutants, a gray poplar tree, growing spontaneously in the last uncovered tailing, being selected as reference tree. The U and Th concentrations in the leaves of the above-mentioned trees, in the covering layers as well as in the original sludge were determined by inductively coupled plasma sector field mass spectrometry (ICP-SF-MS). Generally, the Th concentration of the soils was about 4 times higher than that of uranium, while uranium concentration was about 10–130 times higher than that of thorium in the leaf samples and its concentration ranged from 28 to 1045 ng g− 1, the last value belonging to the poplar tree growing on the last uncovered tailing. In order to assume the mobility and bioavailability of uranium if the dry leaves fall down, the uranium species in the leaves of the poplar tree growing in the uncovered reservoir were determined applying ultrasound-assisted extraction with distilled water and ammonium acetate as well as high performance liquid chromatographic (HPLC)-ICP-SF-MS technique. About 20% of total uranium could be extracted in form of uranyl cations and a presumably negatively charged uranium compound. Estimations revealed that the annual increment of U in the soil surface layer due to the dead fallen leaves in case of the investigated gray poplar (Populus × canescens) is about 1.2%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Though the abundance of uranium and thorium in the environment is as high as that of lead, their homogeneous distribution in the earth's crust encumbers the exploitation for industrial purposes. Due to uranium mining radon-related lung cancer risk cannot be excluded [1], as radon and thoron are an intermediate decay product of the 238U and 232 Th, respectively. Moreover, radium-226 daughter may result in an increased risk of lymphoma and bone cancer as a consequence of high specific activity and radiotoxicity. The average uranium concentration in the earth's crust is about 1.7 mg/kg [2], meanwhile that of thorium is
⁎ Corresponding author. Department of Analytical Chemistry, Institute of Chemistry, L. Eötvös University, H-1518 Budapest, P.O. Box 32, Hungary. Tel.: +36 1 372 2607; fax: +36 1 372 2608. E-mail address: [email protected] (G. Záray). 0026-265X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2008.03.004
about five times higher [3]. Uranium mining and processing involve the removal of large amount of accompanying materials and its decay products, which has the potential to increase exposure of members of the public to natural radiation through different pathways from the ore containing uranium. The main uranium extraction/mining techniques are underground and opened-pit mining and can lead to water contamination [4]. Milling operations involve the processing of the ore to extract uranium in a partially refined form, known as yellowcake. The uranium dioxide powder used in nuclear reactors is prepared either by dry process or by wet chemical route. Of the several available wet processes, the ammonium diuranate route has been the most intensively followed [5]. At the Instituto de Pesquisas Energéticas e Nucleares (Brazil), for example, yellowcake is purified to ammonium diuranate by solvent extraction. Ammonium diuranate is converted to uranium dioxide, uranium tetrafluoride and thereafter to uranium hexafluoride. Finally, this latter compound is isotopically enriched,
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after which it is converted to ammonium uranyl carbonate and then to uranium dioxide for the manufacture of reactor fuel elements [6]. In Hungary, the search for natural sources of nuclear raw materials started in 1953. One year later, a significant accumulation of uranium ore was discovered in the Mecsek Mountain, situated in the Southwestern part of Hungary, below the village of Kővágószőlős. Extensive mining was conducted in this region between 1954 and 1997. The mine was closed in 1997 and recultivation of the abandoned mining area started right after the closure. For uranium and thorium uptake studies, knowledge of uranium and thorium species distribution is inevitable. The chemistry of uranium in aqueous systems is mainly controlled by the pH, the redox potential and complexing agents [7]. The (hydrated) uranyl ion is the dominant aqueous species in most soils below a pH of approximately 5. At higher pH, the uranyl ion hydrolyzes, forming a number of aqueous hydroxide complexes [8,9]. For example, under oxidizing conditions and carbon dioxide, UO2+ 2 hydrolyze forming a series of negatively charged strong hydroxocarbonate complexes [10], which facilitates separation of uranyl cations on an anionic resin [6]. For example, theoretical calculations demonstrated that six different hydroxocarbonate complexes can be formed depending on the pH of the aqueous system [11] if the total ligand concentration of uranyl and carbonate ions are 20 mmol dm− 3 and 15 mmol dm− 3, respectively, In case of ryegrass grown 28 days on soils artificially contaminated with uranyl hexahidrate, it was established that uranyl carbonate complexes and UO2PO−4 seemed the U species being preferentially taken up by the roots and transferred to the shoots [12]. However, U speciation calculations were performed for the average soil solution composition (considering soil solution U, anionic and cationic composition and pH). The distribution of the aqueous U species was calculated using a geochemical computer code. By investigating the effects of different concentrations (25, 50, 75 and 100%) of uranium tailings conditioned with garden soil on growth and biochemical parameters in sunflower, it was established that survival of sunflower plants over 100 days on higher tailing concentrations was up to 75%, which demonstrated that sunflower may be helpful in revitalization of uranium mining waste [13]. In other similar Chinese study involving nine plant species – lupine (Lupinus albus), Chinese mustard (Brassica chinensis), clover (Trifolium pratense), ryegrass (Lolium perenne), tobacco (Nicotiana tobacum), amongst them local vegetables like corn (Zea mays), chickpea (Cicer arietinum) and broad bean (Vicia faba) – it was established by gamma-spectroscopic measurements that plants took up 238 U and 226Ra, but 232Th was less bioavailable [14] from the uranium tailings sampled between 0 and 20 cm depth. Up to now, algal biomass has also been used for U removal from waste waters [15]. As it can be seen from the above-mentioned reports, over the years only few works dealt with transfer pathways of U to plants. The aim of the present work was to establish the potential risk of uranium redistribution in the environment through the tree roots and leaves. For this purpose, the concentration of uranium and thorium was determined in the sludge, clay and loess soil samples collected from the abandoned U mine. Moreover, U and Th were determined in the leaves of tree species planted in the recultivated area for three years as well as in the leaves of a poplar tree spontaneously growing on a tailing uncovered for ten years. In order to determine the bioavailability of uranium in the dried leaves, speciation study was aimed by HPLC-ICP-SF-MS hyphenated technique. 2. Experimental 2.1. Site description and soil and leaf sampling Recently, an excellent hydrogeological characterization of this mining area has been given by Somlai et al. [16] by reporting the results of an indoor Rn determination survey conducted among the population living in the vicinity of the abandoned mine [17] as well as of the Rn concentration in the former mining tunnel as well [18]. Successful rehabilitation of the mining area (Fig. 1) is a priority issue given the vicinity of the city of Pécs, inhabited from the Roman times. The assessment of post-mining
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landscapes as case studies is an important part of the evaluation of current rehabilitation practices. In the abandoned mining area of the present study, the rehabilitation started with filling and covering of the tailings of the processing plant with a clay and then with a loess layer having about 30 cm and 100 cm depth, respectively. In the next step, the covered tailings were sawn with weeds and three years after this successful operation, bushes and trees were planted on the grasslands in order to prevent runoffs and erosion. This procedure was similar to that reported by Hancock et al. [18]. The main criteria in the selection of the tree species to be cultivated were fast growing, resistance against the soil salt concentration and aridity of the region. After preliminary planting investigations and after a throughout revision one year after the phytoremediation processes, massive plantation of the rehabilitated tailings was conducted between 2002 and 2004 with about 20 species. However, the most successfully employed tree species were: poplars (Populus×alba, Populus×nigra, Populus×canescens), acacia (Robinia pseudoacacia), oak (Quercus pubescens) and silver tree (Eleagnus angustifolia). Poplars were planted in a 250×150 cm grid, the other species in groups in a proportion of 2–10%. Sludge and soil samples from a recultivated site were taken in the summer of 2006 with an Ejkelkamp corer. In case of the soil samples, five samples were taken from different points of the surface layer (0–10 cm) and from the 60–90 cm depth. After removing the organic debris, average samples were made from the cores. The sludge and soil samples were dried on air and passed through a 2-mm nylon sieve before analysis. Seven group of leaf samples were taken belonging to white poplar (Populus × albus), gray poplar (Populus × canescens), acacia (R. pseudoacacia), silver tree (E. angustifolia) and oak (Q. pubescens) growing in the recultivation area or in an uncovered tailing. Leaves were taken on a randomized basis and transported to the laboratory where 10–20 pieces were selected from each sample and dried at 70 °C for 72 h. 2.2. Materials, reagents and standards Throughout the experiments deionised Milli-Q water was used. SPEX U and Th stock solutions in concentration of 1 g dm− 3 were used. For the determination of total U and Th as well as of the extracted plant material, Tl internal standard stock solution (Merck, Germany) in concentration of 1 g dm− 3 was diluted with Milli-Q water and Suprapur® HNO3 solution to give 5% acid concentration. Daily dilutions of the stock solutions in the working range of 0.1 to 1 ng cm− 3 were made with Milli-Q water. All dilutions were carried out gravimetrically. For the microwave-assisted digestions, hydrogen peroxide and hydrogen fluoride were purchased from Merck. In order to validate the U and Th results for soil and leaf samples, an IAEA-385 certified reference material and a candidate moss reference material both supplied by the International Atomic Energy Agency were used, respectively. Ammonium acetate used for the extraction of leaves was purchased from Scharlau (Catalonia, Spain). Pyridine and formic acid used for preparation of the mobile phase for cation-exchange chromatography were purchased from Merck (Germany). 2.3. Microwave-assisted digestion of soil and leaf samples Uranium and thorium concentrations in soil and sludge samples were determined after total dissolution using HF/HNO3 mixture. Known amount of sample (approximately 0.1 g) was weighed into a PFA beaker and 3 cm3 of HF and 1 cm3 of HNO3 were added carefully. The sample was evaporated to almost dryness. The dissolution step was repeated once again. After evaporation, traces of HF were removed by the successively addition of 2 cm3 HNO3 followed by evaporation to almost dryness. The residue was dissolved in 15 cm3 5% HNO3 and after 3-fold dilution the uranium and thorium content was measured by ICP-SF-MS. For analysis of leaf samples a microwave-assisted digestion procedure was applied. Approximately 0.5 g of sample was weighed into
46 V.G. Mihucz et al. / Microchemical Journal 90 (2008) 44–49
Fig. 1. Location of the former uranium mine of Kővágószőlős (Hungary).
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Teflon bombs. Then, 10 cm3 of HNO3 was added and the samples were digested according to the EPA 3052 method. After digestion, samples were transferred quantitatively into polypropylene tubes and filled up to 10 cm3. Followed by 3-fold dilution the U and Th concentrations were measured by ICP-SF-MS with external calibration using Tl as an internal standard. 2.4. Extraction of leaves by ultrasonic procession For the extraction of U and Th from the leaves, an ultrasound-assisted extraction method was used similar to the method published by Filgueiras et al. [19]. Briefly, about 0.1 g of the samples, previously powdered in an agate mortar, was placed into polypropylene tubes. Then 5 cm3 of extractant was added (distilled water or ammonium acetate) and sonicated for 3 min at 30% amplitude. Then, the samples were centrifuged at 4500 rpm for 10 min. This procedure was repeated two times. The samples were filtered through a 0.22 µm membrane filter prior to injection onto the HPLC column. 2.5. Instrumentation For microwave-assisted digestion of soil and plant materials, a CEM (USA) equipment was used with Teflon bombs. Extraction of U from the plant materials was carried out with a Hielscher ultrasonic processor was used. The HPLC system (GBC, Australia), employed for speciation study, consisted of a solvent delivery unit (model LC 1140) and an HPLC pump (model LC 1150). Rheodyne (California, USA) injector was used. For U and Th measurements as well as for the U speciation, an Element2 inductively coupled plasma sector field mass spectrometer (ICP-SF-MS) (Thermo Finnigan, Germany) was used. The operating conditions of the ICP-SF-MS measurements for the determination of total U and Th concentrations as well as those of the hyphenated technique are summarized in Table 1. 3. Results and discussion For U and Th determination of the samples ICP-SF-MS was used in low resolution mode as significant molecular interference could not be observed. If, there is no either molecular or isobar interference, it is preferable to use this resolution mode the sensitivity is the highest, the detection limit for U being 0.8 pg cm− 3 and for Th, 0.1 pg cm− 3. The U and Th concentrations of the clay and loess samples used for covering of the last tailing to be covered in 2005, of the sludge itself of the
Table 1 Instrumental parameters for U and Th determination as well as U speciation analysis ICP-SF-MS Power (W) Ar carrier flow (dm3/min) Ar auxiliary flow (dm3/min) Ar nebuliser flow (dm3/min) Measurement mode Acquisition mode No. scans Points per peak Isotopes Integration/search window Nebulisator Spray chamber Sampler cone Skimmer cone
1200 16.0 0.8 1.1 Low resolution (R = 300); external calibration E-scan 15 (5 runs, 3 passes) 5 205 a 232 Tl , Th, 238U 60%/60% Meinhard Scott Ni, ∅ 1.0 mm orifice Ni, ∅ 0.4 mm orifice
HPLC Cation-exchange (pre)column Mobile phase Flow rate (cm3 min− 1) Injection volume (µl) a
Supelcosil 10 µm LC SCX-100 250 (4.6) × 4.1 mm 25 mmol dm− 3 pyridine (pH = 2.7 with formic acid) 1.5 (isocratic mode) 20
Tl used as internal standard in concentration of 0.5 ng cm− 3.
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Table 2 Uranium and thorium concentration of soil samples collected at the tailing and one recultivated site of the former uranium mining area of Kővágószőlős Sample
Concentration
Tailing
cU ± SD/µg g− 1
cTh ± SD/µg g− 1
Sludge Loess Clay
83.0 ± 1.2 2.52 ± 0.04 4.69 ± 0.05
3.48 ± 0.05 10.50 ± 0.18 9.20 ± 0.13
Recultivated site Loess at 0–10 cm depth Loess at 60–90 cm depth IAEA-385 IAEA-385 (reference values)
2.70 ± 0.03 2.69 ± 0.05 2.47 ± 0.02 2.25 – 2.45
8.72 ± 0.16 11.76 ± 0.38 8.34 ± 0.13 8.08 – 8.43
former uranium mining area of Kővágószőlős are tabulated in Table 2. Moreover, results of loess samples collected at the surface (0–10 cm) as well as in the lower extreme of the layer (60–90 cm) from an already recultivated area are also presented. The results showed that there was no change in the uranium and thorium concentration ratios in the soil and in the sludge samples. From Table 2 it can also be seen that the thorium concentration of the samples are about 2–5 times higher than those of U, except in the case of the sludge, where the concentration is about one order of magnitude higher than that of the Th. The results obtained for the soil sample is in good agreement with the fact that the solubility of U is higher than that of Th. Also, the uranium and thorium concentration in the recultivated area did not show significant change with the sampling depth. The accuracy of the results was checked by the analysis of IAEA reference material sample (IAEA-385) and the results were in agreement with the reference value. In case of the proposed moss reference material alpha spectrometry was used as an independent technique as no reference or certified value is available yet. The uranium concentration in the moss sample was 251±15 ng g− 1 and 260±23 ng− 1 obtained by ICP-SF-MS analysis and alpha spectrometry, respectively. The results obtained for the leaf samples collected from the recultivated area reflect the uranium mobility for all tree species investigated. Except for an oak (Q. pubescens) leaf sample, the concentration of uranium in the samples is about one order of magnitude higher than those for Th. Moreover, leaves were collected from two acacia trees (R. pseudoacacia) in the same area to see how the determined U and Th concentrations would differ. The differences in the U concentration of the two acacia samples were less than 2 times and for Th, less than 2 times. This deviation in the results is not significant in case of biological samples. By relating the uranium concentrations of the leaves to the average uranium concentration of the loess soil sample determined, it could be establish that the uranium concentration in the leaves of the acacia, poplar and silver tree were roughly between 7 and 14 times lower than the average concentration value of the loess layer used for recultivation. Meanwhile, this ratio calculated for the oak tree is about 90. This is in good agreement with the fact that the growth of oak trees is a slower process then that of the poplar and acacia tree species, for example, thus consequently, the roots of this group of trees were longer. Therefore, a possible risk for the future regarding uranium accumulation in this tree species can not be unequivocally excluded. During the soil sampling at the last tailing to be rehabilitated, leaves could be collected from a gray poplar, growing spontaneously at the margin of the uncovered tailing, apparently without any visible phytotoxic symptoms. The uranium concentration of the leaf sample collected showed a spectacularly high concentration in comparison with the uranium concentrations of the leaf samples belonging to trees growing in the recultivated area (Table 3). However, the U concentrations of the leaf samples compared with those of the loess in the recultivated area about 10–130 times higher. Calculating with a 30 m2 as surface area surrounding this tree where the leaves could fall and with a 1 cm thick soil surface layer, estimating the number of
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Table 3 Uranium and thorium concentration of the leaf samples collected at the tailing and one recultivated site of the former uranium mining area of Kővágószőlős Tree species Oak (Quercus pubescens) Silver tree (Eleagnus angustifolia) White poplar (Populus × albus) Gray poplara (Populus × canescens) Gray poplarb (Populus × canescens) Acaciac (Robinia pseudoacacia) Acaciac (Robinia pseudoacacia) Moss QC a b c
cU ± SD/ng g− 1
cTh ± SD/ng g− 1
28.7 ± 1.4 182 ± 5 319 ± 10 258 ± 6 1045 ± 22 407 ± 18 243 ± 6 251 ± 15
10.4 ± 1.4 24.8 ± 1.0 73.0 ± 2.5 4.12 ± 0.21 37.9 ± 1.5 2.62 ± 0.32 8.53 ± 0.20 762 ± 53
in only 1 h), elution of uranyl cations could not observed. Due to the fast U hydrolysis, the idea of further chromatographic separations involving anionic columns for these model solutions was abandoned.
From the recultivated site. From the last uncovered tailing. Samples originating from two different acacia trees sampled from the recultivated area.
leaves of the investigated tree as well as the U concentration in the leaves, the effect of the U biological pump represented by this tree can result in an annual increment of about 1.2%. Previous results obtained by XRD demonstrated that the tailings contain two predominant uranium species as expected: uranyl sulphate and calcium diuranate (results not shown here). However, in order to perform uranium speciation study from the plant material by HPLC-ICPMS, uranium has to be extracted from the samples. As the aim of this study was to obtain an answer for the bioavailability of uranium from the tree leaves, two extractants were used: distilled water (pH = 4.4) and ammonium acetate (pH=6.6), the latter in concentration 0.2 mol dm− 3, to cover the physiological pH range of about 5.6. As above pH=5, hydrolysis of uranyl ions can readily occur, ultrasound-assisted extraction was preferred as being an effective tool for arsenic speciation. However, the percentage of the extractable amount of U in case of the leaves of the poplar grown spontaneously at the side of the tailing as well as of a poplar from the recultivated site was between 8% and 23% and it was independent of the type of extractant used. Thorium was practically unextractable with distilled water or ammonium acetate. As the samples resulted with extraction were diluted 10 times before the ICP-SF-MS analyses, there was no need of the application of the matrix matching technique for the external calibration procedure with Tl as internal standard. The low extraction recovery value is understandable taking into consideration of the hard texture of the leaves. The best results for the extraction of heavy metals with this method [19] were obtained with HCl as extractant. In spite of this fact, uranium speciation performed with a cationexchange HPLC column revealed that two peaks could be eluted under the separation condition employed either distilled water or ammonium acetate was used as extractant for the leaves of the poplar tree growing spontaneously at the side of the last uncovered tailing. At the same time, in the leaf extract of the poplar growing on the recultivated area, peaks could not be eluted from the cation-exchange column. The chromatogram of a 1 ng cm− 3 uranyl standard containing nitric acid in 5% can be seen in Fig. 2. By spiking the leaf extract of the poplar tree growing spontaneously on the tailing to be covered with this acidic uranyl nitrate solution, the second peak increased, which lead to the identification of this peak as being uranyl. The first peak might be a) diuranate as plants usually take up non-essential elements by passive absorption as our previous results showed [20] b) negatively hydroxocarbonate uranyl complexes likely formed during the extraction procedure. In the lack of a diuranate standard, modeling of the uranium hydroxocarbonate complex formation was performed as follows: freshly prepared 25 mmol dm− 3, 50 mmol dm− 3 and 100 mmol dm− 3 ammonium carbonate solutions containing 200 ng cm− 3 uranium as uranyl were prepared and subjected to cation-exchange HPLC-ICP-SF-MS measurements. The U concentration was chosen to roughly coincide with the concentration of the extractable uranium from the leaf of the spontaneously growing poplar tree in the uncovered tailing. However, due to the rapid hydrolysis of the uranyl containing ammonium carbonate solutions checked by monitoring with the ICP-SF-MS instrument (about 10-fold decrease of the U concentration
Fig. 2. Chromatogram obtained on a cation-exchange column for a 1 ng cm− 3 uranyl solution in 5% HNO3 (a), leaf extract of a poplar grown spontaneously on the tailing of Kővágószőlős (b) and of the leaf extract spiked with uranyl cations (c). Chromatographic conditions: Supelcosil SCX-100 10 µm 250 × 4.1 mm column equipped with a 10 µm 4.6 × 4.1 mm Supelcosil SCX-100 precolumn; mobile phase: 25 mmol dm− 3 pyridine (pH = 2.7; HCOOH); flow rate: 1.5 cm3 min− 1; t = 21 °C; injection volume: 20 µl. Detection at m/z = 238 in low resolution.
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4. Conclusions Uranium and thorium were determined in soil, sludge and leaf samples taken from an abandoned uranium mining site under recultivation. According to its higher mobility, uranium concentration in the plant material was about one order of magnitude higher in the samples compared with the Th concentration values. The amounts of uranium taken up even by a poplar tree spontaneously grown in the last tailing waiting for recultivation were low. The redistribution of uranium demonstrated to be a slow process and will not result in critical change in the uranium concentration of the surface soil layer even after the fall of the leaves. Acknowledgements The authors express their gratitude to the Scientific and Technological Foundation and to the Hungarian National Scientific Foundation for the grants B27/04 and T047174, respectively.
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Redistribution of uranium and thorium by soil/plant interaction in a recultivated mining area Victor G. Mihucz a,b, Zsolt Varga c, Enikő Tatár a,b, István Virág a, René van Grieken d, Zsuzsanna Koleszár e, Gyula Záray a,b,⁎ a
Department of Analytical Chemistry, Institute of Chemistry, L. Eötvös University, H-1518 Budapest, P.O. Box 32, Hungary Hungarian Satellite Centre of Trace Elements Institute to UNESCO, H-1518 Budapest, P. O. Box 32, Hungary Department of Radiation Safety, Institute of Isotopes, Hungarian Academy of Sciences, Konkoly-Thege Miklós 29-33, H-1121 Budapest, Hungary d Micro- and Trace Analysis Centre (MiTAC), Department of Chemistry, University of Antwerp, Campus Drie Eiken, Universiteitplein 1, BE-2610 Antwerp-Wilrijk, Belgium e Mecsek Öko Ltd., Esztergár Lajos 19, H-7633 Pécs, Hungary b c
A R T I C L E
I N F O
Article history: Received 4 March 2008 Accepted 12 March 2008 Available online 20 March 2008 Keywords: Uranium Tree Soil Mining ICP-MS HPLC
A B S T R A C T During the recultivation of the uranium mining area of Kővágószőlős (Hungary), the tailings were covered with clay and loess soil layers having a thickness of 30 cm and 100 cm, respectively. In the loess covering layer, acacia (Robinia pseudoacacia), poplars (Populus × albus, Populus × canescens), oak (Quercus pubescens), silver tree (Eleagnus angustifolia) were planted between 1996 and 2004. In order to establish the extent of the uranium and thorium transport from the sludge to the leaves by uptake and translocation processes through roots with a length higher than 1.3 m results in a remarkable redistribution of these pollutants, a gray poplar tree, growing spontaneously in the last uncovered tailing, being selected as reference tree. The U and Th concentrations in the leaves of the above-mentioned trees, in the covering layers as well as in the original sludge were determined by inductively coupled plasma sector field mass spectrometry (ICP-SF-MS). Generally, the Th concentration of the soils was about 4 times higher than that of uranium, while uranium concentration was about 10–130 times higher than that of thorium in the leaf samples and its concentration ranged from 28 to 1045 ng g− 1, the last value belonging to the poplar tree growing on the last uncovered tailing. In order to assume the mobility and bioavailability of uranium if the dry leaves fall down, the uranium species in the leaves of the poplar tree growing in the uncovered reservoir were determined applying ultrasound-assisted extraction with distilled water and ammonium acetate as well as high performance liquid chromatographic (HPLC)-ICP-SF-MS technique. About 20% of total uranium could be extracted in form of uranyl cations and a presumably negatively charged uranium compound. Estimations revealed that the annual increment of U in the soil surface layer due to the dead fallen leaves in case of the investigated gray poplar (Populus × canescens) is about 1.2%. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Though the abundance of uranium and thorium in the environment is as high as that of lead, their homogeneous distribution in the earth's crust encumbers the exploitation for industrial purposes. Due to uranium mining radon-related lung cancer risk cannot be excluded [1], as radon and thoron are an intermediate decay product of the 238U and 232 Th, respectively. Moreover, radium-226 daughter may result in an increased risk of lymphoma and bone cancer as a consequence of high specific activity and radiotoxicity. The average uranium concentration in the earth's crust is about 1.7 mg/kg [2], meanwhile that of thorium is
⁎ Corresponding author. Department of Analytical Chemistry, Institute of Chemistry, L. Eötvös University, H-1518 Budapest, P.O. Box 32, Hungary. Tel.: +36 1 372 2607; fax: +36 1 372 2608. E-mail address: [email protected] (G. Záray). 0026-265X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2008.03.004
about five times higher [3]. Uranium mining and processing involve the removal of large amount of accompanying materials and its decay products, which has the potential to increase exposure of members of the public to natural radiation through different pathways from the ore containing uranium. The main uranium extraction/mining techniques are underground and opened-pit mining and can lead to water contamination [4]. Milling operations involve the processing of the ore to extract uranium in a partially refined form, known as yellowcake. The uranium dioxide powder used in nuclear reactors is prepared either by dry process or by wet chemical route. Of the several available wet processes, the ammonium diuranate route has been the most intensively followed [5]. At the Instituto de Pesquisas Energéticas e Nucleares (Brazil), for example, yellowcake is purified to ammonium diuranate by solvent extraction. Ammonium diuranate is converted to uranium dioxide, uranium tetrafluoride and thereafter to uranium hexafluoride. Finally, this latter compound is isotopically enriched,
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after which it is converted to ammonium uranyl carbonate and then to uranium dioxide for the manufacture of reactor fuel elements [6]. In Hungary, the search for natural sources of nuclear raw materials started in 1953. One year later, a significant accumulation of uranium ore was discovered in the Mecsek Mountain, situated in the Southwestern part of Hungary, below the village of Kővágószőlős. Extensive mining was conducted in this region between 1954 and 1997. The mine was closed in 1997 and recultivation of the abandoned mining area started right after the closure. For uranium and thorium uptake studies, knowledge of uranium and thorium species distribution is inevitable. The chemistry of uranium in aqueous systems is mainly controlled by the pH, the redox potential and complexing agents [7]. The (hydrated) uranyl ion is the dominant aqueous species in most soils below a pH of approximately 5. At higher pH, the uranyl ion hydrolyzes, forming a number of aqueous hydroxide complexes [8,9]. For example, under oxidizing conditions and carbon dioxide, UO2+ 2 hydrolyze forming a series of negatively charged strong hydroxocarbonate complexes [10], which facilitates separation of uranyl cations on an anionic resin [6]. For example, theoretical calculations demonstrated that six different hydroxocarbonate complexes can be formed depending on the pH of the aqueous system [11] if the total ligand concentration of uranyl and carbonate ions are 20 mmol dm− 3 and 15 mmol dm− 3, respectively, In case of ryegrass grown 28 days on soils artificially contaminated with uranyl hexahidrate, it was established that uranyl carbonate complexes and UO2PO−4 seemed the U species being preferentially taken up by the roots and transferred to the shoots [12]. However, U speciation calculations were performed for the average soil solution composition (considering soil solution U, anionic and cationic composition and pH). The distribution of the aqueous U species was calculated using a geochemical computer code. By investigating the effects of different concentrations (25, 50, 75 and 100%) of uranium tailings conditioned with garden soil on growth and biochemical parameters in sunflower, it was established that survival of sunflower plants over 100 days on higher tailing concentrations was up to 75%, which demonstrated that sunflower may be helpful in revitalization of uranium mining waste [13]. In other similar Chinese study involving nine plant species – lupine (Lupinus albus), Chinese mustard (Brassica chinensis), clover (Trifolium pratense), ryegrass (Lolium perenne), tobacco (Nicotiana tobacum), amongst them local vegetables like corn (Zea mays), chickpea (Cicer arietinum) and broad bean (Vicia faba) – it was established by gamma-spectroscopic measurements that plants took up 238 U and 226Ra, but 232Th was less bioavailable [14] from the uranium tailings sampled between 0 and 20 cm depth. Up to now, algal biomass has also been used for U removal from waste waters [15]. As it can be seen from the above-mentioned reports, over the years only few works dealt with transfer pathways of U to plants. The aim of the present work was to establish the potential risk of uranium redistribution in the environment through the tree roots and leaves. For this purpose, the concentration of uranium and thorium was determined in the sludge, clay and loess soil samples collected from the abandoned U mine. Moreover, U and Th were determined in the leaves of tree species planted in the recultivated area for three years as well as in the leaves of a poplar tree spontaneously growing on a tailing uncovered for ten years. In order to determine the bioavailability of uranium in the dried leaves, speciation study was aimed by HPLC-ICP-SF-MS hyphenated technique. 2. Experimental 2.1. Site description and soil and leaf sampling Recently, an excellent hydrogeological characterization of this mining area has been given by Somlai et al. [16] by reporting the results of an indoor Rn determination survey conducted among the population living in the vicinity of the abandoned mine [17] as well as of the Rn concentration in the former mining tunnel as well [18]. Successful rehabilitation of the mining area (Fig. 1) is a priority issue given the vicinity of the city of Pécs, inhabited from the Roman times. The assessment of post-mining
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landscapes as case studies is an important part of the evaluation of current rehabilitation practices. In the abandoned mining area of the present study, the rehabilitation started with filling and covering of the tailings of the processing plant with a clay and then with a loess layer having about 30 cm and 100 cm depth, respectively. In the next step, the covered tailings were sawn with weeds and three years after this successful operation, bushes and trees were planted on the grasslands in order to prevent runoffs and erosion. This procedure was similar to that reported by Hancock et al. [18]. The main criteria in the selection of the tree species to be cultivated were fast growing, resistance against the soil salt concentration and aridity of the region. After preliminary planting investigations and after a throughout revision one year after the phytoremediation processes, massive plantation of the rehabilitated tailings was conducted between 2002 and 2004 with about 20 species. However, the most successfully employed tree species were: poplars (Populus×alba, Populus×nigra, Populus×canescens), acacia (Robinia pseudoacacia), oak (Quercus pubescens) and silver tree (Eleagnus angustifolia). Poplars were planted in a 250×150 cm grid, the other species in groups in a proportion of 2–10%. Sludge and soil samples from a recultivated site were taken in the summer of 2006 with an Ejkelkamp corer. In case of the soil samples, five samples were taken from different points of the surface layer (0–10 cm) and from the 60–90 cm depth. After removing the organic debris, average samples were made from the cores. The sludge and soil samples were dried on air and passed through a 2-mm nylon sieve before analysis. Seven group of leaf samples were taken belonging to white poplar (Populus × albus), gray poplar (Populus × canescens), acacia (R. pseudoacacia), silver tree (E. angustifolia) and oak (Q. pubescens) growing in the recultivation area or in an uncovered tailing. Leaves were taken on a randomized basis and transported to the laboratory where 10–20 pieces were selected from each sample and dried at 70 °C for 72 h. 2.2. Materials, reagents and standards Throughout the experiments deionised Milli-Q water was used. SPEX U and Th stock solutions in concentration of 1 g dm− 3 were used. For the determination of total U and Th as well as of the extracted plant material, Tl internal standard stock solution (Merck, Germany) in concentration of 1 g dm− 3 was diluted with Milli-Q water and Suprapur® HNO3 solution to give 5% acid concentration. Daily dilutions of the stock solutions in the working range of 0.1 to 1 ng cm− 3 were made with Milli-Q water. All dilutions were carried out gravimetrically. For the microwave-assisted digestions, hydrogen peroxide and hydrogen fluoride were purchased from Merck. In order to validate the U and Th results for soil and leaf samples, an IAEA-385 certified reference material and a candidate moss reference material both supplied by the International Atomic Energy Agency were used, respectively. Ammonium acetate used for the extraction of leaves was purchased from Scharlau (Catalonia, Spain). Pyridine and formic acid used for preparation of the mobile phase for cation-exchange chromatography were purchased from Merck (Germany). 2.3. Microwave-assisted digestion of soil and leaf samples Uranium and thorium concentrations in soil and sludge samples were determined after total dissolution using HF/HNO3 mixture. Known amount of sample (approximately 0.1 g) was weighed into a PFA beaker and 3 cm3 of HF and 1 cm3 of HNO3 were added carefully. The sample was evaporated to almost dryness. The dissolution step was repeated once again. After evaporation, traces of HF were removed by the successively addition of 2 cm3 HNO3 followed by evaporation to almost dryness. The residue was dissolved in 15 cm3 5% HNO3 and after 3-fold dilution the uranium and thorium content was measured by ICP-SF-MS. For analysis of leaf samples a microwave-assisted digestion procedure was applied. Approximately 0.5 g of sample was weighed into
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Fig. 1. Location of the former uranium mine of Kővágószőlős (Hungary).
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Teflon bombs. Then, 10 cm3 of HNO3 was added and the samples were digested according to the EPA 3052 method. After digestion, samples were transferred quantitatively into polypropylene tubes and filled up to 10 cm3. Followed by 3-fold dilution the U and Th concentrations were measured by ICP-SF-MS with external calibration using Tl as an internal standard. 2.4. Extraction of leaves by ultrasonic procession For the extraction of U and Th from the leaves, an ultrasound-assisted extraction method was used similar to the method published by Filgueiras et al. [19]. Briefly, about 0.1 g of the samples, previously powdered in an agate mortar, was placed into polypropylene tubes. Then 5 cm3 of extractant was added (distilled water or ammonium acetate) and sonicated for 3 min at 30% amplitude. Then, the samples were centrifuged at 4500 rpm for 10 min. This procedure was repeated two times. The samples were filtered through a 0.22 µm membrane filter prior to injection onto the HPLC column. 2.5. Instrumentation For microwave-assisted digestion of soil and plant materials, a CEM (USA) equipment was used with Teflon bombs. Extraction of U from the plant materials was carried out with a Hielscher ultrasonic processor was used. The HPLC system (GBC, Australia), employed for speciation study, consisted of a solvent delivery unit (model LC 1140) and an HPLC pump (model LC 1150). Rheodyne (California, USA) injector was used. For U and Th measurements as well as for the U speciation, an Element2 inductively coupled plasma sector field mass spectrometer (ICP-SF-MS) (Thermo Finnigan, Germany) was used. The operating conditions of the ICP-SF-MS measurements for the determination of total U and Th concentrations as well as those of the hyphenated technique are summarized in Table 1. 3. Results and discussion For U and Th determination of the samples ICP-SF-MS was used in low resolution mode as significant molecular interference could not be observed. If, there is no either molecular or isobar interference, it is preferable to use this resolution mode the sensitivity is the highest, the detection limit for U being 0.8 pg cm− 3 and for Th, 0.1 pg cm− 3. The U and Th concentrations of the clay and loess samples used for covering of the last tailing to be covered in 2005, of the sludge itself of the
Table 1 Instrumental parameters for U and Th determination as well as U speciation analysis ICP-SF-MS Power (W) Ar carrier flow (dm3/min) Ar auxiliary flow (dm3/min) Ar nebuliser flow (dm3/min) Measurement mode Acquisition mode No. scans Points per peak Isotopes Integration/search window Nebulisator Spray chamber Sampler cone Skimmer cone
1200 16.0 0.8 1.1 Low resolution (R = 300); external calibration E-scan 15 (5 runs, 3 passes) 5 205 a 232 Tl , Th, 238U 60%/60% Meinhard Scott Ni, ∅ 1.0 mm orifice Ni, ∅ 0.4 mm orifice
HPLC Cation-exchange (pre)column Mobile phase Flow rate (cm3 min− 1) Injection volume (µl) a
Supelcosil 10 µm LC SCX-100 250 (4.6) × 4.1 mm 25 mmol dm− 3 pyridine (pH = 2.7 with formic acid) 1.5 (isocratic mode) 20
Tl used as internal standard in concentration of 0.5 ng cm− 3.
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Table 2 Uranium and thorium concentration of soil samples collected at the tailing and one recultivated site of the former uranium mining area of Kővágószőlős Sample
Concentration
Tailing
cU ± SD/µg g− 1
cTh ± SD/µg g− 1
Sludge Loess Clay
83.0 ± 1.2 2.52 ± 0.04 4.69 ± 0.05
3.48 ± 0.05 10.50 ± 0.18 9.20 ± 0.13
Recultivated site Loess at 0–10 cm depth Loess at 60–90 cm depth IAEA-385 IAEA-385 (reference values)
2.70 ± 0.03 2.69 ± 0.05 2.47 ± 0.02 2.25 – 2.45
8.72 ± 0.16 11.76 ± 0.38 8.34 ± 0.13 8.08 – 8.43
former uranium mining area of Kővágószőlős are tabulated in Table 2. Moreover, results of loess samples collected at the surface (0–10 cm) as well as in the lower extreme of the layer (60–90 cm) from an already recultivated area are also presented. The results showed that there was no change in the uranium and thorium concentration ratios in the soil and in the sludge samples. From Table 2 it can also be seen that the thorium concentration of the samples are about 2–5 times higher than those of U, except in the case of the sludge, where the concentration is about one order of magnitude higher than that of the Th. The results obtained for the soil sample is in good agreement with the fact that the solubility of U is higher than that of Th. Also, the uranium and thorium concentration in the recultivated area did not show significant change with the sampling depth. The accuracy of the results was checked by the analysis of IAEA reference material sample (IAEA-385) and the results were in agreement with the reference value. In case of the proposed moss reference material alpha spectrometry was used as an independent technique as no reference or certified value is available yet. The uranium concentration in the moss sample was 251±15 ng g− 1 and 260±23 ng− 1 obtained by ICP-SF-MS analysis and alpha spectrometry, respectively. The results obtained for the leaf samples collected from the recultivated area reflect the uranium mobility for all tree species investigated. Except for an oak (Q. pubescens) leaf sample, the concentration of uranium in the samples is about one order of magnitude higher than those for Th. Moreover, leaves were collected from two acacia trees (R. pseudoacacia) in the same area to see how the determined U and Th concentrations would differ. The differences in the U concentration of the two acacia samples were less than 2 times and for Th, less than 2 times. This deviation in the results is not significant in case of biological samples. By relating the uranium concentrations of the leaves to the average uranium concentration of the loess soil sample determined, it could be establish that the uranium concentration in the leaves of the acacia, poplar and silver tree were roughly between 7 and 14 times lower than the average concentration value of the loess layer used for recultivation. Meanwhile, this ratio calculated for the oak tree is about 90. This is in good agreement with the fact that the growth of oak trees is a slower process then that of the poplar and acacia tree species, for example, thus consequently, the roots of this group of trees were longer. Therefore, a possible risk for the future regarding uranium accumulation in this tree species can not be unequivocally excluded. During the soil sampling at the last tailing to be rehabilitated, leaves could be collected from a gray poplar, growing spontaneously at the margin of the uncovered tailing, apparently without any visible phytotoxic symptoms. The uranium concentration of the leaf sample collected showed a spectacularly high concentration in comparison with the uranium concentrations of the leaf samples belonging to trees growing in the recultivated area (Table 3). However, the U concentrations of the leaf samples compared with those of the loess in the recultivated area about 10–130 times higher. Calculating with a 30 m2 as surface area surrounding this tree where the leaves could fall and with a 1 cm thick soil surface layer, estimating the number of
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Table 3 Uranium and thorium concentration of the leaf samples collected at the tailing and one recultivated site of the former uranium mining area of Kővágószőlős Tree species Oak (Quercus pubescens) Silver tree (Eleagnus angustifolia) White poplar (Populus × albus) Gray poplara (Populus × canescens) Gray poplarb (Populus × canescens) Acaciac (Robinia pseudoacacia) Acaciac (Robinia pseudoacacia) Moss QC a b c
cU ± SD/ng g− 1
cTh ± SD/ng g− 1
28.7 ± 1.4 182 ± 5 319 ± 10 258 ± 6 1045 ± 22 407 ± 18 243 ± 6 251 ± 15
10.4 ± 1.4 24.8 ± 1.0 73.0 ± 2.5 4.12 ± 0.21 37.9 ± 1.5 2.62 ± 0.32 8.53 ± 0.20 762 ± 53
in only 1 h), elution of uranyl cations could not observed. Due to the fast U hydrolysis, the idea of further chromatographic separations involving anionic columns for these model solutions was abandoned.
From the recultivated site. From the last uncovered tailing. Samples originating from two different acacia trees sampled from the recultivated area.
leaves of the investigated tree as well as the U concentration in the leaves, the effect of the U biological pump represented by this tree can result in an annual increment of about 1.2%. Previous results obtained by XRD demonstrated that the tailings contain two predominant uranium species as expected: uranyl sulphate and calcium diuranate (results not shown here). However, in order to perform uranium speciation study from the plant material by HPLC-ICPMS, uranium has to be extracted from the samples. As the aim of this study was to obtain an answer for the bioavailability of uranium from the tree leaves, two extractants were used: distilled water (pH = 4.4) and ammonium acetate (pH=6.6), the latter in concentration 0.2 mol dm− 3, to cover the physiological pH range of about 5.6. As above pH=5, hydrolysis of uranyl ions can readily occur, ultrasound-assisted extraction was preferred as being an effective tool for arsenic speciation. However, the percentage of the extractable amount of U in case of the leaves of the poplar grown spontaneously at the side of the tailing as well as of a poplar from the recultivated site was between 8% and 23% and it was independent of the type of extractant used. Thorium was practically unextractable with distilled water or ammonium acetate. As the samples resulted with extraction were diluted 10 times before the ICP-SF-MS analyses, there was no need of the application of the matrix matching technique for the external calibration procedure with Tl as internal standard. The low extraction recovery value is understandable taking into consideration of the hard texture of the leaves. The best results for the extraction of heavy metals with this method [19] were obtained with HCl as extractant. In spite of this fact, uranium speciation performed with a cationexchange HPLC column revealed that two peaks could be eluted under the separation condition employed either distilled water or ammonium acetate was used as extractant for the leaves of the poplar tree growing spontaneously at the side of the last uncovered tailing. At the same time, in the leaf extract of the poplar growing on the recultivated area, peaks could not be eluted from the cation-exchange column. The chromatogram of a 1 ng cm− 3 uranyl standard containing nitric acid in 5% can be seen in Fig. 2. By spiking the leaf extract of the poplar tree growing spontaneously on the tailing to be covered with this acidic uranyl nitrate solution, the second peak increased, which lead to the identification of this peak as being uranyl. The first peak might be a) diuranate as plants usually take up non-essential elements by passive absorption as our previous results showed [20] b) negatively hydroxocarbonate uranyl complexes likely formed during the extraction procedure. In the lack of a diuranate standard, modeling of the uranium hydroxocarbonate complex formation was performed as follows: freshly prepared 25 mmol dm− 3, 50 mmol dm− 3 and 100 mmol dm− 3 ammonium carbonate solutions containing 200 ng cm− 3 uranium as uranyl were prepared and subjected to cation-exchange HPLC-ICP-SF-MS measurements. The U concentration was chosen to roughly coincide with the concentration of the extractable uranium from the leaf of the spontaneously growing poplar tree in the uncovered tailing. However, due to the rapid hydrolysis of the uranyl containing ammonium carbonate solutions checked by monitoring with the ICP-SF-MS instrument (about 10-fold decrease of the U concentration
Fig. 2. Chromatogram obtained on a cation-exchange column for a 1 ng cm− 3 uranyl solution in 5% HNO3 (a), leaf extract of a poplar grown spontaneously on the tailing of Kővágószőlős (b) and of the leaf extract spiked with uranyl cations (c). Chromatographic conditions: Supelcosil SCX-100 10 µm 250 × 4.1 mm column equipped with a 10 µm 4.6 × 4.1 mm Supelcosil SCX-100 precolumn; mobile phase: 25 mmol dm− 3 pyridine (pH = 2.7; HCOOH); flow rate: 1.5 cm3 min− 1; t = 21 °C; injection volume: 20 µl. Detection at m/z = 238 in low resolution.
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4. Conclusions Uranium and thorium were determined in soil, sludge and leaf samples taken from an abandoned uranium mining site under recultivation. According to its higher mobility, uranium concentration in the plant material was about one order of magnitude higher in the samples compared with the Th concentration values. The amounts of uranium taken up even by a poplar tree spontaneously grown in the last tailing waiting for recultivation were low. The redistribution of uranium demonstrated to be a slow process and will not result in critical change in the uranium concentration of the surface soil layer even after the fall of the leaves. Acknowledgements The authors express their gratitude to the Scientific and Technological Foundation and to the Hungarian National Scientific Foundation for the grants B27/04 and T047174, respectively.
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