научная статья по теме CHEMICAL AND RADIOCHEMICAL CHARACTERIZATION OF DEPLETED URANIUM IN CONTAMINATED SOILS Химия

Текст научной статьи на тему «CHEMICAL AND RADIOCHEMICAL CHARACTERIZATION OF DEPLETED URANIUM IN CONTAMINATED SOILS»

ЖУРНАЛ ФИЗИЧЕСКОМ ХИМИИ, 2007, том 81, № 9, с. 1631-1634

== PHYSICAL CHEMISTRY ^^^^^^

OF SEPARATION PROCESSES, CHROMATOGRAPHY

УДК 543.544

CHEMICAL AND RADIOCHEMICAL CHARACTERIZATION OF DEPLETED URANIUM IN CONTAMINATED SOILS

© 2007 M. B. Radenkovic*, A. B. Kandic*, I. S. Vukanac*, J. D. Joksic*, D. S. Djordjevic* *

*Institute of Nuclear Sciences "Vinca", P.O. Box 522,11001 Belgrade, Serbia **ICTM-Centre for Chemistry, Njegoseva 12,11001 Belgrade, Serbia E-mail: mirar@vin.bg.ac.yu; jasnaj@vin.bg.ac.yu

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 modified Tessier's five-step sequential chemical extraction procedure, specifically selective to certain physical/chemical associations. After ion-exchanged based radiochemical separation of uranium depleted uranium is distinguished from naturally occurring uranium in extracts on the basis of isotopic activities ratios 234U/238U and 235u/238U and particular substrates for recently present uranium material in soils are indicated.

INTRODUCTION

Depleted uranium (DU) differs from naturally occurring uranium by virtue of having most of its 235U and 234U isotopes removed in the enrichment or fuel reprocessing for the nuclear energy industry. Referred as a low radioactive material, it typically contains 99.7990% of 238U, 0.0010% 234U and 0.2000% 235U by mass, but also may contain traces of transuranic elements indicating irradiated fuel origin [1-3]. Depleted uranium used in Balkans will be characterized in this study by the ra-diochemical separations 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 Balkans, a study was done on depleted uranium physical/chemical behavior and its status in some types of contaminated soils some years after the appearance in the environment. For that 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 extractions procedures are developed for metal specification in different soil types but if reliable radiometric measurements are available, the method may be useful for analysis of radionuclides 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 and acid soluble methods are applied in five phases to the soil samples contaminated with depleted uranium. The distribution of depleted and naturally occurring uranium in obtained extracts should outline their geochemical fractionation

and indicate the mobility and bioavailability of depleted uranium in the soil of investigated environment in local meteorological conditions. Determination of 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 practical aspect it may help in decision making on the clean-up and remediation strategy for sites contaminated in military actions.

MATERIALS AND METHODS

Depleted uranium of the projectile collected at contaminated area in Southern Serbia, was characterized by the alpha spectrometric analysis. After dissolution of material, relevant standard radiochemical procedures were used to separate uranium fraction as well as a small quantities of plutonium and neptunium isotopes in excess of uranium [8, 9]. The radiochemical procedures involved micro-coprecipitation of actinides on the Fe(III)hydroxide and ion-exchange based acidic extraction using DOWEX 1 x 8 (100-200 mesh) anion resin and di-isopropil ether liquid/liquid extractions for iron removal. For radiochemical yield recoveries, the aliquots of basic solution were spiked with 0.1 Bq 232U and 0.05 Bq 236Pu diluted tracer solutions, standardized previously by absolute activity measurements in 2n counting geometry. Neptunium fraction was additionally washed and separated by the acetate-based procedure [10] and without tracer added. A thin-layer alpha-sources were prepared for each actinide element by the Talvitie's electrodeposition procedure [11]. Measurements were performed using Canberra 2004 vacuum

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Counts 30 r

20

10

236

237-

Np

238U

J_il*

L uni 11

Pu

ii_l_ш Hilll

500

600

700

800

900 Channel

Fig. 1. The spectrum of the alpha-source with 237Np, 239, 240Pu, 238Pu from projectile sample, spiked with 236Pu and 232U, and with 238U traces.

chamber (20 mbar) with PIPS detector with 15.3% efficiency. The background was 1.3 x 10-4 imp s-1 in the energy range 3.5-9.5 MeV, energy calibration 9.2 keV/channel was done with the thin-layer source of 229Th in equilibrium, with resolution 24 keV for 241Am line. The counting time was (3-6) x 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 HP Ge detector with 23% efficiency, in 100 ml cylinder geometry, without radioactive equilibrium reached due to the interruption after a fourth 238U-series member at the moment of DU material production.

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 seviere 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 0.2 M (COOH)2/0.2 M NH4H(COO)2 mixture (pH 3); in the forth: 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 mechanical rotational shaker at 20°C and the unusually high solid/liquid ratio 1 : 45 to provide efficient rescue of metals from solid matrix. Replicate 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 104 Bq/kg order of magnitude, and in

the nearest soil layer it was 105 Bq/kg. The contamination decreases with the distance to 1% of 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 investigated areas.

Radiochemical characterization of the projectile has shown specific activity 76 Bq/kg 239, 240Pu, 7.70 x 104 Bq/kg 236U and 237Np traces, indicating irradiated fuel origin of depleted uranium material that is in accordance with other reported results [12, 13]. Their relations are shown in the Fig. 1.

All three elements (U, Np and Pu) have a very similar chemical behavior, 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. Besides, the alpha-energy line E = 4.787 MeV (42%) of 237Np interfere with the 234U line, meaning that almost no uranium should be present to claim that this line (665 channel) belongs to 237Np. As it can be seen in the Fig. 1, this is achieved, since only the 238U traces are visible in spectrum and 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 same methodology. Determined 234U/238U, 235U/238U and 236U/238U ratios are given in the Table 1.

The 238U distribution in five extracted phases obtained for selected soil samples taken at different contaminated areas are presented in the Table 2. The share of uranium quantity of each of five extraction phases in

ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ том 81 № 9 2007

CHEMICAL AND RADIOCHEMICAL CHARACTERIZATION

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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 x 10-7

235U/238U 0.10 1.54 x 10-3

236u/238U 0.053 3.33 x 10-5

the total uranium extracted within whole sequential procedure is given in percentiles.

In the first "exchangable" phase of the five-step Tessier's procedure, a non-selectively 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 ura-nyl-ion (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 related to contamination. Low clay and humus content in the most of the samples indicated hydrous (crystalline) oxides of the 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, uranium shares are significant in the forth phase extracts. The crystalline iron oxides and partly the silicate matrix are attacked in the fifth phase where acid soluble uranium associati

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