ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2007, Vol. 81, No. 9, pp. 1448–1451. © Pleiades Publishing, Ltd., 2007.
PHYSICAL CHEMISTRY OF SEPARATION PROCESSES, CHROMATOGRAPHY
Chemical and Radiochemical Characterization of Depleted Uranium in Contaminated Soils1 M. B. Radenkovi c´a, A. B. Kandi c´a, I. S. Vukana c´a, J. D. Joksi c´a, and D. S. Djordjevi c´b a Institute
of Nuclear Sciences Vinca, œ P.O. Box 522, 11001 Belgrade, Serbia for Chemistry, Njegoseva 12, 11001 Belgrade, Serbia e-mail:
[email protected];
[email protected]
b ICTM-Centre
Abstract—The main results of chemical and radiochemical characterization and fractionation of depleted uranium in soils contaminated during the Balkan conflict in 1999 are presented in the paper. Alpha-spectrometric analysis of used depleted uranium material has shown the presence of man-made radioisotopes 236U, 237Np, and 239, 240Pu traces. The fractionation in different soil types was examined by the application of a modified Tessier’s five-step sequential chemical extraction procedure, specifically selective to certain physical/chemical associations. After ion-exchange-based radiochemical separation of uranium, depleted uranium is distinguished from naturally occurring uranium in extracts on the basis of the isotopic activity ratios 234U/238U and 235U/238U and particular substrates for recently present uranium material in soils are indicated. DOI: 10.1134/S0036024407090191
INTRODUCTION Depleted uranium (DU) differs from naturally occurring uranium by virtue of having most of its 235U and 234U isotopes removed during enrichment or fuel reprocessing for the nuclear-energy industry. Referred to as a low radioactive material, it typically contains 99.7990% of 238U, 0.0010% 234U, and 0.2000% 235U by mass, but may also contain traces of transuranic elements indicating irradiated fuel origin [1–3]. The depleted uranium used in the Balkans will be characterized in this study by the radiochemical separation of uranium, plutonium, and neptunium fractions from DU projectile material and alpha-spectrometric measurements of electrodeposited alpha-sources. To assess the environmental impact of depleted uranium ammunition used during 1999 in the Balkans, a study was done of the physical and chemical behavior of depleted uranium and its status in some types of contaminated soils some years after its appearance in the environment. For this purpose, the contamination levels are determined and selected soil samples are subjected to a series of successive solid/liquid chemical extractions in a modified Tessier’s sequential extraction procedure [4, 5]. Various sequential extraction procedures have been developed for metal specification in different soil types, but if reliable radiometric measurements are available, the method may be useful for analysis of radionuclide fractionation in soil as well [6, 7]. In this work, the extractive reagents, targeted to a specific physical/chemical association, such as ion-exchange, carbonate, iron/manganese oxide, organic, or acid-soluble methods, are applied in five phases to soil samples 1 The
text was submitted by the authors in English.
contaminated with depleted uranium. The distribution of depleted and naturally occurring uranium in the extracts obtained should outline their geochemical fractionation and indicate the mobility and bioavailability of depleted uranium in the soil of the investigated environment under local meteorological conditions. The determination of the uranium-specific activities and isotopic activity ratios 234U/238U and 235U/238U will be done by high-resolution alpha-spectrometry with relatively high sensitivity. Applied combined physical/chemical procedures and analysis should enable insight into the specific characteristics of depleted uranium behavior in soil and, from the practical standpoint, they may help in decision-making on the clean-up and remediation strategy for sites contaminated in military actions. MATERIALS AND METHODS Depleted uranium from a projectile collected in the contaminated area in Southern Serbia was characterized by alpha spectrometric analysis. After dissolution of the material, the relevant standard radiochemical procedures were used to separate the uranium fraction as well as small quantities of plutonium and neptunium isotopes in excess of uranium [8, 9]. The radiochemical procedures involved micro-coprecipitation of actinides on Fe(III) hydroxide and ion-exchange based acidic extraction using DOWEX 1 × 8 (100–200 mesh) anion resin and diisopropil ether liquid/liquid extractions for iron removal. For radiochemical yield recoveries, aliquots of the basic solution were spiked with 0.1 Bq 232U and 0.05 Bq 236Pu diluted tracer solutions, standardized previously by absolute activity measurements in the 2π
1448
CHEMICAL AND RADIOCHEMICAL CHARACTERIZATION Counts 30
1449
236Pu
20 237Np 239Pu
10
232U 238Pu
238U
0 500
600
700
800
900 Channel
Fig. 1. The spectra of the alpha source with 237Np, 239, 240Pu, 238Pu from projectile sample spiked with 236Pu and 232U and with 238U traces.
counting geometry. The neptunium fraction was additionally washed and separated by the acetate-based procedure [10] and without tracers added. Thin-layer alpha sources were prepared for each actinide element by Talvitie’s electrodeposition procedure [11]. Measurements were performed using a Canberra 2004 vacuum chamber (20 mbar) with a PIPS detector with 15.3% efficiency. The background was 1.3 × 10–4 imp s–1 in the energy range 3.5–9.5 MeV; energy calibration 9.2 keV/channel was done with a thin-layer source of 229Th at equilibrium, with resolution 24 keV for the 241Am line. The counting time was (3–6) × 105 s. The soil samples contaminated with depleted uranium were collected in Serbia and Montenegro. Gamma-spectrometric determination of uranium concentrations in soil samples was done using a HP Ge detector with 23% efficiency, in 100-ml cylinder geometry, without radioactive equilibrium reached due to the interruption after the fourth 238U-series member at the moment of DU-material production. The selected soil samples (10 g of each) were subjected to a set of five successive solid/liquid extractions with highly selective extractive reagents simulating mild to severe environmental conditions. The reagent used in the first phase was 1 M CH3COONH4 (pH 7); the residue was treated with 0.6 M HCl and 0.1 M NH2OH in 0.01 M HCl (pH 4). In the third phase, the extractant was a 0.2 M (COOH)2/0.2 M NH4H(COO)2 mixture (pH 3); in the forth, it was 30% H2O2 in 0.01 M HNO3 at 85°C (pH 2); and 6 M HCl at 85°C in the fifth phase. Extractions were performed in a mechanical rotational shaker at 20°C and with the unusually high solid/liquid ratio 1 : 45 to provide efficient rescue of RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
metals from the solid matrix. Repeat measurements were not done. RESULTS AND DISCUSSION The specific activities of 238U in top-soil samples taken at the projectile entrance spot and path through the soil were of the 104 Bq/kg order of magnitude, and in the nearest soil layer it was 105 Bq/kg. The contamination decreases with distance to 1% of the initial value at 120–160 mm to the source (DU kinetic penetrator). The naturally occurring uranium concentration determined by gamma spectrometry was within 20– 60 Bq/kg in the soils of the investigated areas. Radiochemical characterization of the projectile has shown specific activity 76 Bq/kg 239, 240Pu and 7.70 × 104 Bq/kg 236U and 237Np traces, indicating irradiated fuel origin of the depleted uranium material, which is in accordance with other reported results [12, 13]. Their relations are shown in Fig. 1. All three elements (U, Np, and Pu) have very similar chemical behaviors, and separation of neptunium and plutonium traces in the medium with uranium in excess was done by the application of a number of repeated radiochemical treatments. In addition, the alpha-energy line E = 4.787 MeV (42%) of 237Np interferes with the 234U line, meaning that almost no uranium should be present to claim that this line (665 channel) belongs to 237Np. As can be seen in Fig. 1, this is achieved because only 238U traces are visible in the spectra and because of the very low 234U/238U ratio in elemental uranium. Radionuclides 237Np, 239, 240Pu, and 236U were not detectable in contaminated soil samples taken in the nearest vicinity of the investigated projectile using the
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attacked in the fifth phase, where acid-soluble uranium associates are extracted. The first group of results concerns samples taken at Cape Arza on the Montenegrean coast. The A1, A2, and A3 samples belong to a sandy loam texture class, with 69% sand (2–0.02 mm), 15.8% silt, and 14.9% clay. The B sample is 99.1% sand, mostly 2–0.2 mm, with a negligible percentage of silt and clay. The difference between these two soil types from the same region is obvious. Most of the uranium in the A samples is extracted in the third and fifth phase, and for the B sample in the second and forth phases. Therefore, uranium is not very attendant in the first phase, mainly because the contamination levels are low and because of the harsh environmental conditions. Uranium distribution is very different for the C group of soil samples taken in the continental environment in southern Serbia. There is a strong dependence of fractionation on the contamination levels, as shown in Fig. 2, resulting in weakly bonded exchangeable uranium in highly contaminated soils regardless of the soil geochemical structure. Assuming the reduction of 235U content to 0.2% in depleted uranium, and prior 234U isotope leaching, the ratios 235U/238U and 234U/238U determined in the extracts obtained by alpha spectrometric analysis, between 0.1–0.5 and 0.5–1, respectively, may be attributed to the mixture of depleted and naturally occurring uranium in varying proportions. It appears that depleted uranium “covered” the naturally occurring uranium in highly contaminated samples and natural ratios were found as dominant in deeplayer soils and uncontaminated samples. Assuming that the sequential extraction procedure is sufficiently selective, these proportions are the result of the capacity of depleted uranium to be incorporated into exchangeable or structural physical/chemical associations and forms in soil under exposure to certain meteorological conditions and geochemical environment. In general, the obtained distribution in the sequential extraction phases
Table 1. The activity and mass ratios of uranium isotopes determined in the uranium fraction of projectile sample Activity ratios
Mass ratios
234U/ 238U
0.125
6.25 × 10–7
235U/ 238U
0.10
1.54 × 10–3
236U/ 238U
0.053
3.33 × 10–5
same
methodology. The determined 234U/238U, and 236U/238U ratios are given in Table 1.
235U/238U,
The 238U distribution in the five extracted phases obtained for the selected soil samples taken from different contaminated areas are presented in Table 2. The share of uranium quantity of each of the five extraction phases in the total uranium extracted within the whole sequential procedure is given in percentiles. In the first “exchangable” phase of the five-step Tessier’s procedure, nonselectively bonded uranium with both poorly soluble U(IV) and soluble U(VI) forms is extracted within various substrates in soil samples. In the second step, dissolution of carbonates and manganese hydroxides is provoked where the uranylion(VI) may be expected. The presence of uranium in a high excess in these two phases of extraction is a result of anthropogenic influence and is related to contamination. Low clay and humus content in most of the samples indicated hydrous (crystalline) oxides of iron and manganese as prevailing substrates for uranium extracted in the third phase [14]. In the forth step of treatment, the oxidative degradation of the organic matter and therefore mobilization of organic bonded (humic and fulvic acids) uranium is done [15, 16]. Consequently, only in the surface soil samples are uranium shares significant in the forth phase extracts. Crystalline iron oxides and partly the silicate matrix are Table 2.
238U
distribution in five extracted phases obtained for investigated soil samples Phase I
Sample
238U,
Bq/kg (%)
Phase II 238U,
Bq/kg (%)
Phase III 238U,
Bq/kg (%)
Phase IV 238U,
Phase V 238U,
Bq/kg (%)
Bq/kg (%)
A1
1.37 ± 0.15 (8.8)
0.25 ± 0.04 (1.6)
9.82 ± 0.72 (62.9)
1.06 ± 0.16 (6.8)
3.12 ± 0.17 (19.9)
A2
0.76 ± 0.10 (7.4)
0.248 ± 0.037 (2.4)
4.37 ± 0.43 (42.7)
0.284 ± 0.035 (2.8)
4.56 ± 0.31 (44.6)
A3
1.29 ± 0.15 (15.5)
1.19 ± 0.12 (14.3)
4.69 ± 0.23 (56.3)
0.20 ± 0.05 (2.5)
0.96 ± 0.12 (11.5)
B
0.25 ± 0.04 (1.2)
10.8 ± 1.6 (52.0)
1.46 ± 0.10 (7.1)
8.12 ± 0.40 (39.1)
0.124 ± 0.019 (0.59)
C1
12.4 ± 1.8 (3.2)
28.2 ± 2.6 (7.4)
213 ± 29 (56.0)
41.9 ± 2.6 (11.0)
85.3 ± 6.2 (22.4)
C2
9.52 ± 0.49 (18.2)
8.12 ± 0.40 (15.5)
8.12 ± 0.40 (15.5)
17.53 ± 1.13 (33.5)
9.1 ± 0.6 (17.3)
C3
393.0 ± 1.1 (45.5)
1.48 ± 0.22 (0.17)
55.1 ± 8.3 (6.4)
135 ± 20 (15.6)
279 ± 59 (32.3)
C4
3110 ± 802 (83.0)
517 ± 56 (13.8)
86 ± 14 (2.3)
10.4 ± 1.6 (0.3)
23.02 ± 0.91 (0.61)
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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CHEMICAL AND RADIOCHEMICAL CHARACTERIZATION Bq/kg 20
1451
300
16 200
12 8
100
4 0
I
II
III IV Phase
0
V
I
II
III IV Phase
V
Fig. 2. Uranium distribution in five phases of sequential extraction for low and high contaminated soil samples, respectively, both taken at 50-cm depth.
indicated carbonates and iron and manganese oxides in soil as the most probable substrates for depleted uranium originating from weapons. We can conclude that specific chemical forms and types of physical/chemical associations in soil may have a greater influence on mobility, eco-toxicity, and bioavailability than the total amount, but there are limitations if the contamination is very high. ACKNOWLEDGMENTS This work was supported by the Serbian Ministry of Science and Environmental Protection, within the Project of Fundamental Research no. 102016. REFERENCES 1. M. Betti, J. Environ. Radioact. 64 (2–3), 113 (2003). 2. S. Fetter and F. N. Hippel, Sci. Glob. Security (1999). 3. Depleted Uranium: Sources, Exposure, and Health Effects (WHO-World Health Organization, Geneva, 2001), WHO/SDE/PHE/01.1. 4. A. Tessier, P. G. C. Campbell, and M. Bisson, Anal. Chem. 54, 844 (1979). 5. P. Polic œ, Ph.D. Thesis, (University of Belgrade, Belgrade, 1991).
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6. M. K. Schultz, K. G. W. Inn, Z. C. Lin, et al., Appl. Radiat. Isot. 49, 1289 (1998). 7. P. Blanko, F. Vera Tome, and J. C. Lozano, Appl. Radiat. Isot. 61, 345 (2004). 8. Annual Book of Standards (ASTM–American Standard Test Methods Committee, 1999), 12.01: C 999-90; C 1000-90. 9. M. Radenkovic œ, D. Vukovic œ, V. ⁄ipka, and D. Todorovic œ J. Radioanal. Nucl. Chem. 208 (2), 467 (1996). 10. C.-K. Kim, A. Takaku, M. Yamamoto, et al., J. Radioanal. Nucl. Chem. 132 (1), 132 (1989). 11. N. A. Talvitie, Anal. Chem. 44, 280 (1972). 12. V. ⁄ipka, M. Radenkovic œ, D. Paligoric œ, and J. Djuric œ, in Proceedings of XXI Conference of JDZZ (Kladovo, 2001), pp. 69–72 [in Serbian]. 13. UNEP–United Nations Environment Programme: Depleted Uranium in Serbia and Montenegro. PostConflict Environmental Assessment in the Federal Republic of Yugoslavia (United Nations Report, Geneva, 2002). 14. J. Slavek and W. F. Pickering, Water Air Soil Pollut. 28, 151 (1986). 15. L. Boruvka and O. Drabek, Plant Soil Environ. 50, 339 (2004). 16. J. Lenhart, S. Cabaniss, P. MacCarthy, and B. Honeyman, Radiochim. Acta 88, 345 (2000).
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