Subject Area 6.3
Depleted Uranium
Subject Area 6.3: Methodologies, modeling, monitoring of chemicals in multi-compartmental environments Research Article
Depleted Uranium Mobility and Fractionation in Contaminated Soil (Southern Serbia)* Mirjana B. Radenkoviƒ1**, Svjetlana A. Cupaƒ2, Jasminka D. Joksiƒ1 and Dragana J. Todoroviƒ1 1Institute of Nuclear Sciences 'Vin…a',Radiation and Environmental Protection Laboratory, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro 2Institute
for Soil Management, Faculty of Agriculture, University of Belgrade, Nemanjina 6, 11081 Belgrade, Serbia and Montenegro
** Corresponding author (
[email protected]) DOI: http://dx.doi.org/10.1065/espr2007.03.399 Please cite this paper as: Radenkoviƒ MB, Cupaƒ SA, Joksiƒ JD, Todoroviƒ DJ (2008): Depleted Uranium Mobility and Fractionation in Contaminated Soil (Southern Serbia). Env Sci Pollut Res 15 (1) 61–67 Abstract
Goal, Scope and Background. During the Balkan conflict in 1999, soil in contaminated areas was enriched in depleted uranium (DU) isotopic signature, relative to the in-situ natural uranium present. After the military activities, most of kinetic DU penetrators or their fragments remained buried in the ground in certain geomorphological and geochemical environments exposed to local weathering conditions. The contamination distribution, mobility and/or fixation of DU in the contaminated soil profile at one hot spot were the subject of our study. The results should disclose what happened with released DU corrosion products in three years elapsed, given the scope of their geochemical fractionation, and mark out the most probable host substrates in investigated soil type. Methods. Gamma-spectrometric analysis of soil samples taken in the DU penetrator impact-zone was done to obtain present contamination levels. Set of samples is subjected to five-step and threestep sequential extraction procedures, specifically selective to different physical/chemical associations in soil. The stable elements are determined in extracts by the atomic absorption spectroscopy. After the ion-exchange based uranium separation procedure, alpha-spectrometric analysis of obtained fractions was done and DU distribution in five extraction phases found from 235U/238U and 234U/238U isotopic ratios. Results. Depleted uranium concentration falls down to the 1% of the initial value, at approximately 150 mm distance to the source. Carbonates and iron/manganese hydrous oxides are indicated as the most probable substrates for depleted uranium in the characterized soil type. Therefore, in the highly contaminated soil samples, depleted uranium is still weakly bonded and easy exchangeable. The significant levels of organic-bonded depleted uranium are found in surface soil only. Discussion. Dependence of the fractionation on the contamination levels is evident. Samples with higher DU contents have shown a longer maintenance in the exchangeable phases, probably because adsorption/desorption mass transfer through the medium was not very fast. Organic-bonded, depleted uranium is present in surface soil samples due to its higher humus content. Considering geochemical composition of investigated soil, the indicating chemical associations as substrates are in agreement with some considerations based on the results for low-level waste unsaturated zones.
Conclusions. The soil contamination with depleted uranium in investigated area is still 'spot' type and not widespread. Dependence of the fractionation on the contamination levels and presence of weakly bonded, depleted uranium in the hot spots areas is evident. Recommendations and Perspectives. A detailed study may be undertaken with suitable extractive reagents to define a bio-available fraction of depleted uranium in soil. The comparison of results for different soil types investigated by the same methodology may be useful. An applied combination of physical/chemical procedures and analysis may help in the decision making on the remediation strategy for sites contaminated with depleted uranium used in military operations. Keywords: Alpha spectrometry; depleted uranium; fractionation; sequential extraction; soil; Southern Serbia; uranium isotopic ratios
Introduction
Depleted uranium (DU) differs from naturally occurring uranium by virtue of having most of its 235U and 234U isotopes content removed in the 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. Due to convenient physical and mechanical properties, materials made of depleted uranium are common both in civilian and military applications (Fetter and Hipple 1999, WHO 2001, Bleise et al. 2003). The ammunition with depleted uranium designed to penetrate the armour plate was used in air-attacks in Serbia, in 1999. After the actions, most of the kinetic DU penetrators or their fragments remained buried in the ground in local geomorphological and geochemical environments. The postimpact environmental assessment revealed significant contamination in the immediate weapons impact-zones only (UNEP 2002, Jia et al. 2005). The contamination distribution, mobility and/or fixation of DU in the contaminated soil profile at one hot spot were the subject of our study. During the radiation survey of terrain, a soil was sampled in the environment of the penetrator found about 50cm deep * ESS-Submission Editor: Dr. John Holder (
[email protected])
Env Sci Pollut Res 15 (1) 61 – 67 (2008) © Springer-Verlag 2008
61
Depleted Uranium
Subject Area 6.3
in the ground. After contamination levels, determination by high-resolution gamma spectrometry, a few of the samples were further subjected to a series of successive solid/liquid chemical extractions in a modified Tessier's sequential extraction procedure (Tessier et al. 1979). The extractive reagents, targeted to a specific physical/chemical association such as: water soluble, ion-exchangeable, carbonates, iron/ manganese oxides, organic and acid soluble are applied in five (three) phases. Thereby, the unusually high sample/reagent ratio was used to increase the efficiency of uranium rescue from the soil samples. Depleted uranium contribution to the total uranium contents in the sequential extraction fractions is determined on the basis of uranium isotope ratios. The isotopic analysis is performed by alpha spectrometry, following the ion-exchange based radiochemical procedure of uranium separation and thin-layer radioactive source preparation (Radenkovic et al. 1996, ASTM 1999). Uranium contents in extracts will outline its geochemical fractionation and indicate the mobility in the soil type of this region characterized by standard pedologic methods. The stable elements are determined in extracts by atomic absorption spectroscopy. 1
Materials and Methods
1.1
Sampling and characterization of soil
A hot-spot was detected by the increase of radiation dose rate measured during the radiation survey of terrain in the weapons impact area in Bratoselce (EM 625882), Southern Serbia. Depleted uranium penetrator was found in the soil at 0.5 m depth, covered with oxides and without the aluminium jacket crumbled in the projectile entrance corridor path at 0.2–0.3 m depth. The standard soil sampling procedures could not be applied in this case. About 1 cm thick soil slices were collected in the surrounding impact zone at the position of the projectile entrance into ground, the path through the subsurface ground, soil layer just next to DU penetrator, and different distances down along the depth profile and a few laterally. All collected soil samples are analysed by gamma spectrometry, and five of them are subjected to sequential extraction procedures: surface soil (0– 5cm) from non-contaminated area (sample S1), surface soil left after another projectile removal, two years before this sampling (sample S2), soil at 2 cm, 3–5 cm and 10 cm distance down along the projectile's DU penetrator (samples P2, P4, P10 respectively) as shown in the Fig 1.
Fig 1: Sampling scheme: P – DU penetrator; S1 – surface soil in noncontaminated area; S2 – contaminated surface soil: P2 – soil slice at 2 cm distance; P4 – soil at 3–5 cm distance; P10 – soil at 10 cm distance
The soil type characterization was performed using standard pedologic techniques. The content of extractable stable elements: Fe, Cr, Zn, Cd, Ni, Mn, K, Ca, Pb, Al and Mg in soil is determined as a sum of concentrations in the extracts of applied five-step sequential extraction procedure by atomic absorption spectroscopy. 1.2
Sequential extraction procedures
The most utilized and valuable technique for geochemical fractionation and speciation of metals and radionuclides in soils and sediments is a sequential extraction procedure (IUPAC 2000). Extraction steps and working conditions selected from modifications (Blanko et al. 2004, 2005, Poliƒ 1991, Schultz et al. 1996, 1998) of a commonly used scheme (Tessier et al. 1979) are applied here to insure optimal selectiveness and avoid artefacts associated with incomplete dissolution of phases or internal contamination. The 10 g of soil samples S1, S2, P2, P4 and P10 was leached by a series of extractive reagents according to a five-step scheme, modified as presented in Table 1.
Table 1: The five-step sequential extraction scheme applied to soil samples: S1, S2, P2, P4 and P10
Extraction Fraction Ion-exchangeable Carbonates, Fe/Mn hydrous oxides Fe/Mn oxides Organic matter
Acid soluble
62
Extractive Reagents
T (°C)
1 mol dm–3 NH4Ac , pH 7
20
Reagent/ Sample (w/w) 1/45
0.6M HCl and 0.1 mol dm–3 NH2OH.HCl in 0.01 mol dm–3 HCl , pH 4
20
1/45
0.2 mol dm–3 (COOH)2 and 0.2 mol dm–3 NH4H (COO)2 , pH 3 30% H2O2 in 0.01 mol dm–3 HNO3 in two portions, pH 2 3.2 mol dm–3 NH4Ac 6 mol dm–3 HCl in two portions
20
1/45
85
1/45
20 85
1/45 1/45
Reagent/ Sample Contact Time 2 hours in mechanical rotational shaker (MRS) 9 hours in MRS 3 hours manually 4 hours in MRS 2 hours and 3 hours in MRS ½ hour in MRS 3 hours and 6 hours in MRS
Env Sci Pollut Res 15 (1) 2008
Subject Area 6.3
Depleted Uranium
In an effort to preserve the soil samples characteristics closely to 'as found in the environment', and to avoid chemical transformations, oxidation processes and the lost of moisture, samples were not dried or homogenized. The content of 'hygroscopic water' indicating colloidal fraction and soil granulation was determined by 1 g of samples drying at 105°C. The sample/reagent ratio in all phases was 1/45. Extractions were conducted at room temperature (20°C) except for the fifth one at 85°C, when eventually re-adsorbed metals were desorbed by 30 min shaking in 3.2 mol/dm–3 ammonium acetate solutions. The solid/liquid mixtures in sealed onelitre bottles have been stirred continuously in a mechanical rotational shaker. About 12 hours of settling after agitation period, the liquid phase was separated with pipette. The precipitate was washed two times and all extracts gathered and normalized to 1,000 ml. Replicates were not done. In order to get more information on the uranium in the ionexchangeable phase, a substantive, three-step sequential extraction procedure was applied onto new probes from the same bulk of soil samples S2, P2, P4 and P10. In the first step, the samples were treated with de-ionized water, 1 hour in mechanical rotational shaker (MRS) at room temperature to obtain water soluble fraction. In the second step, a 1 mol/dm3 MgCl2 (pH 7) was used as the extractive reagent for 1 hour agitation at room temperature. The third extraction was done with 1 mol dm–3 NH4Ac, (pH 7) 2 hours in the MRS for ion-exchangeable ion separation. 1.3
Analytical determinations of uranium isotopes and stable elements
The gamma-spectrometry method was used to determine specific activities of uranium isotopes in different spots of the soil profile. Collected soil samples in polyethylene vials or Marinelli beakers cylindrical geometry were analysed by high resolution gamma-spectrometry using HPGe detector with 23% relative efficiency and 1.8 keV energy resolution for 1,332 keV 60Co line. The lines 1,000 keV and 768 keV of 234mPa are used to determine 238U isotope in high-level activity samples and line 63.3 keV of daughter nuclei 234Th for lower levels of activity, using Genie 2000 software. Alpha-spectrometry method was used to determine uranium isotopic composition in the extracts obtained in five-step and three-step sequential extraction procedures. After evaporation of extracts to dryness, the anion-exchange based radiochemical procedure of uranium separation from environmental samples (Radenkoviƒ et al. 1996) was applied to separate and purify the uranium fraction. Solutions were spiked with 0.1 Bq of 232U standard tracer solution for ra-
diochemical yield recovery. After co-precipitation on ferrichydroxide, the separation was done using Dowex 1x 8 (100– 200 mesh) resin in chloride form, followed by di-iso-propyl ether liquid/liquid extraction in purpose of iron removal. Thin-layer radioactive sources for alpha spectrometry measurements were obtained by electro-deposition procedure (Talvitie 1972). Uranium isotopes are deposited as a thin homogeneous film at stainless steel discs. Measurements are performed in vacuum controlled (20 mbar) alpha spectrometry chamber Canberra 2004 with PIPS detector (300 mm2 area) and appropriate electronic devices. The counting efficiency was 16% at 25 mm distance, multichannel energy scale 9.1 keV/ch and energy resolution 24 keV for 241Am line. The counting time was 1–3 days per each alpha-emitting source. Specific activities of 234U, 235U and 238U isotopes determined by gamma and alpha-spectrometry are used to calculate relevant activities ratios for 234U/238U and 235U/238U. Stable elements concentrations in the extract were determined by standard atomic absorption spectroscopy, flame and graphite techniques (Perkin Elmer AA600, AA200), using mixed standard solutions for calibration to simulate soil matrix composition. 2 2.1
Results and Discussion Soil characteristics
The soil type in the investigated area belongs to a granitediorite class with relatively high naturally occurring uranium content (4–6 mg/kg). Fine soil texture is characteristic for loamy send with low clay content (Table 2). Clay mineralogical composition has shown a main of feldspar aluminosilicates with laminiferous biotite and muscovite mica exponents, both expected to be very permeable for water flow. The 'exchangeable pH value' determined by the KCl titrations is 6.9–7.2, that is a very sensitive interval for changes in natural environment and consequently for uranium mobility or sorption. The humus content of 2.16% in surface soil sample is relatively low compared to that usual in noncultivated soils, indicating a low possibility of uranyl-ions incorporation into the complex compounds and DU immobilization. Although the samples were not pre-treated, the 'hygroscopic water' (evaporated at 105°C) was in the range of 1.2–1.9%, where the reductive processes caused by microorganisms action may not be favoured. The opposite effect is expected from the P2O5 content of 14.1 mg/100 g, considering the high stability of uranium phosphate compounds, especially in alkaline conditions. Some stable ele-
Table 2: The fine texture of surface and deep-layer soil in Bratoselce
Soil sample
Coarse fragments >2 mm (%, wt)
Fine soil texture (%, wt) Sand 2–0.02 mm
Silt 0.02–0.002 mm
Clay
