Disequilibrium of uranium isotopes in some Syrian groundwater

Disequilibrium of uranium isotopes in some Syrian groundwater

Applied Radiation and Isotopes 55 (2001) 109–113 Disequilibrium of uranium isotopes in some Syrian groundwater A. Abdul-Hadi*, O. Alhassanieh, M. Gha...

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Applied Radiation and Isotopes 55 (2001) 109–113

Disequilibrium of uranium isotopes in some Syrian groundwater A. Abdul-Hadi*, O. Alhassanieh, M. Ghafar Chemistry Department, Atomic Energy Commission of Syria, P.O. Box 6091, Damascus, Syria

Abstract Uranium concentration in groundwater samples from three areas of Syria was determined using a-spectrometry and INAA. It was in the range of 0–6.13 mg/l in the phosphate areas, and lower than 1 ppb in the volcanic areas. The activity ratio of 234U/238U was investigated, and disequilibrium of uranium isotopes was found to occur (234U/238U=0.522.02). The excess of 234U was calculated. This excess can be interpreted by higher mobility of 234 U, which more readily forms the soluble (UO2)2+ ion in comparison with 238U, most of which remains in the insoluble 4+ state. This excess increases with increase in uranium concentration. Thorium concentration was measured using INAA, it was found to be in the rang 0–1.15 mg/l. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Uranium; Thorium; Concentrations; Disequilibrium; Activity ratio;

1. Introduction Radioactive disequilibrium between elements of a natural series provides useful information in many fields. The disequilibrium in sediments can test the absence of geochemical distributing processes (weathering and/or leaching) in natural systems. Disequilibrium studies allow definition of the time, at which distributing processes occurred and to assess, how long the system remained undistributed (Gascoyne, 1987; Ivanovich and Harmon, 1982; Rosholt, 1983). The 234U/238U-activity ratio can be used to test the opening of a system in the last 106 years. Disequilibrium in sediments and groundwater also gives information for migration studies: (1) the recent history of radionuclide migration, (2) factors, which affect radionuclide mobility, and (3) geochemical conditions of the water-rock systems, in which they migrated (Gascoyne, 1987; Schwarz et al., 1982; Smeille and Rosholt, 1984). Cherdestev (1954) first discovered differences in the isotopic abundance of 234U and 238U. These differences

234

U/238U; Solubility; Mobility.

are due to the greater solubility of the (UO2)2+ ion in comparison with 238U. This behavior has two explanations; nuclear recoil (Rosholt et al., 1963; Dooly et al., 1966; Fleischer and Raabe, 1978), and/ or electron stripping (Kolodony and Kaplan, 1970). Uranium concentrations and the activity ratios of 234 U/238U in the Syrian phosphate areas were measured previously (Takriti and Abdul-Hadi, 1998; Asfahani and Abdul-Hadi, 2001). It was found that 234U and 238U are in secular equilibrium for geological large-time conditions. The aim of this work is to determine uranium and thorium concentrations and 234U/238U activity ratio in groundwater and surface water in contact with phosphate and volcanic rocks proposed previously as sites for deposition of low and medium radioactive wastes in Syria (Abaas et al., 1995), and to investigate the relationship between the uranium concentration in groundwater, and uranium concentration and 234 U/238U activity ratios

2. Experimental *Corresponding author. Tel.: +963-11-6111926; +963-11-6112289. E-mail address: [email protected] (A. Abdul-Hadi).

fax:

Groundwater samples were collected from wells and springs that are used for irrigation and potable water

0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 6 9 - 9

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from three different areas of Syria. Fig. 1 shows the position of the areas on the map of Syria. Samples of the eastern area (Samples 1–9) were collected from wells in the region of about 100 km2 around AL-Sharquieh phosphate mine, which contains 40–70 ppm of uranium.

Fig. 1. Map of Syria with sample position.

Samples 10–12 were collected from a spring and two wells (35 and 45 m depth) around Aen-Lelon village in the coastal region (about 2 km2), which contains small anomalies in deposits of phosphate with 100–200 ppm of uranium. Samples 13–24 were collected from wells and springs in the southern volcanic area of Al-Souidaa County (about 200 km2). Table 1 shows the description of the collected samples. Polyethylene bottles were used; they were washed successively with distilled water, diluted acid, acetone, diluted acid and distilled water before taking the samples, two samples of 5 l were taken from each position, then HNO3 was added to the samples. The bottles were washed with dilute nitric acid and the solutions were filtered before evaporation. The 51 l volume of solution was evaporated to dryness 40– 608C. The reside was weighed and 0.5 g was taken and dissolved in a Teflon beaker by an excess of a mixture of conc. HClO4, HNO3 and HF (2/2/1), and spiked by tracer 232U (about 1.5 Bq) for efficiency measurement. The solution was evaporated to near dryness and dissolved in 8 M HCl. This solution was passed through an anion exchange resin column Dowex 1X8 (50– 100 mesh, 1.6 meq/dry g). Uranium is adsorbed onto the resin and then washed with 0.1 M HCl (Alhassanieh et al., 1999). After the evaporation of this solution,

Table 1 Samples description and uranium concentrations Sample

Depth (m)

Residual (g)

A234 (Bq/l)

A238 (Bq/l)

A234 =A238

C (mg/l)

Ue

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

50 35 70 10 85 75 35 80 70 Spring 45 35 Spring 100 380 140 140 110 150 Spring 90 Spring Spring 170

6.25 3.45 5.2 2.7 5.85 5.6 6.2 5.16 10.62 2.42 1.48 2.11 1.36 0.86 0.69 2.64 0.61 1.42 1.66 2.48 1.13 0.63 1.0 2.57

0.040  0.006 0.043  0.005 0.033  0.007 0.028  0.009 0.073  0.006 0.138  0.009 0.087  0.005 0.062  0.007 0.105  0.006 0.0093  0.0017 0.0364  0.0045 0.072  0.008 * * * * 0.006  0.002 0.0022  0.0010 * * * 0.0058  0.0010 * 0.022  0.006

0.026  0.003 0.028  0.004 0.033  0.003 0.016  0.003 0.048  0.004 0.076  0.006 0.066  0.008 0.037  0.005 0.052  0.007 0.0086  0.0019 0.0364  0.0065 0.056  0.006 * * * * 0.0042  0.0010 0.0042  0.0011 * * * 0.0076  0.0015 * 0.0123  0.0008

1.54  0.30 1.54  0.28 1.00  0.23 1.75  0.65 1.52  0.18 1.81  0.12 1.32  0.18 1.68  0.30 2.02  0.30 1.08  0.31 1.00  0.22 1.28  0.15 * * * * 1.43  0.59 0.52  0.27 * } * 0.77  0.20 * 1.79  0.50

2.10  0.41 2.26  0.41 2.66  0.61 1.29  0.48 3.87  0.46 6.13  0.41 5.32  0.73 2.98  0.53 4.19  0.62 0.69  0.20 2.94  0.65 4.52  0.81 0.18  0.04a * 0.17  0.02a 0.06  0.03a 0.34  0.14 0.34  0.18 * 0.04  0.02a * 0.61  0.16 0.15  0.05a 0.99  0.28

1.13 1.22 0 0.97 2.01 4.97 1.70 2.03 4.27 0.06 0 1.27 * * * * 0.15 0.16 * * * 0.14 * 0.78

A. Abdul-Hadi et al. / Applied Radiation and Isotopes 55 (2001) 109–113

uranium was electroplated from a buffer solution (1% H2SO4, 10% NH4OH, pH=2) onto a stainless-steel disk using a Teflon cell with a platinum cathode. The current was about 1.2 A, and the voltage about 8–10 V. The disk was washed with distilled water and acetone before being counted by a-spectroscopy (SBD-detector, 19% Eff., multiplexer and ND-MCA) for about 48 h. The yield of the chemical separation was 60–95%. The electrodeposition was repeated three times for each sample. Uranium concentration and 234U/238U activity ratio were calculated as the median of 6 analysis of each sample, the errors were calculated as the standard deviation s. Table 1 shows the activity of 234U, 238U and 234U/238U activity ratio for all samples. 0.2 g of the reside of samples from the eastern and southern areas was analyzed by INAA in the MNSR reactor with 5  1011 n/cm2 s thermal neutron flux to determine U and Th concentrations in the samples. It can be noted that the concentrations of U measured by a- spectrometry and INAA were in good agreement.

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3. Results and discussion As shown in Table 1 and Fig. 2, the uranium concentration in the samples of the eastern phosphate area lay in the range 0.69–6.13 mg/l and the uranium concentration in the phosphate itself was 36–168 ppm (Takriti and Abdul-Hadi, 1998; Asfahani and AbdulHadi, 2001). Samples from the southern volcanic area of Syria have very low uranium concentrations, 7 of 12 samples collected from this area had concentrations lower than the detection limit of our a-spectrometry system (LLD=0.2 mg/l). Therefore, they were only measured by INAA to be lower than 0.2 mg/l (see Table 1). Lower concentrations of uranium in this area can be explained by the very weak concentration in volcanic deposits. As Table 2 shows, the natural thorium concentration in the samples (measured by INAA) was generally very low due to its very low concentration in the phosphate and volcanic rocks themselves.

Fig. 2. Uranium concentrations in the investigated samples (error has indicate  1 Standard deviation).

Table 2 Mean values, standard deviations and ranges of concentration of U and Th in the investigated samples Area

Eastern Coastal Southern

U [mg/l]

Th [ng/l]

Mean value

Standard deviation

Range

Mean value

Standard deviation

Range

3.42 2.57 0.24

1.50 1.56 0.30

1.29–6.13 0.69–4.52 0.00–0.99

229 } 126

420 } 319

0.0–1153 } 0.0–1120

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Fig. 3 shows the 234U/238U activity ratio as a function of uranium concentration. All samples with a uranium concentration above 1 mg/l show disequilibrium with 234 U/238U activity ratios in the range 1–2. The 234U-excess (Fig. 4), which is defined as (Cowart and Osmond, 1980): 234

Ue ¼ ½234 U=238 U  1 * U

increases with the increase in U-concentration.

Fig. 3.

The following theories account for such disequilibrium: (1) Escape and subsequent decay of 234Th from crystal boundaries by a-recoil leads to 234U enrichment in fluid phase (Kigoshi, 1971). The radioactive decay of 238U to 234U involves one alpha (4.20 MeV) and two b-particle emissions. (2) Nuclear damage in the crystal lattice and changes in the electronic structure (caused by b-decay), corre-

234

U/238U activity ratio as a function of uranium concentration samples (error has indicated  1 Standard deviation).

Fig. 4.

234

U excess as a function of uranium concentration samples (error has indicate 1 Standard deviation).

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sponding to the ‘‘Szilard–Chamers effect’’, and leading to increased solubility and mobility of 234 U (Rosholt et al., 1963; Fleischer et. al., 1978). Cowart and Osmon (Cowart and Osmon, 1980; Osmon and Cowart, 1983; Toulhcat et al., 1988) proposed a classification of waters based on two parameters, uranium concentration and 234U/238U activity ratio. High activity ratios (>2) can be due to a higher than normal ratio of leachable uranium in aquifer or amorphous uraninite, enhancing a-recoil. An activity ratio, lower than 1 implies intense dissolution. Uranium concentration levels reflect both redox conditions and uranium concentrations in surrounding rocks. In oxidizing conditions, uranium is very soluble as U(VI)-complexes, mainly carbonates. In reducing conditions, uranium is unsoluble as U(IV)-oxides. Low uranium concentration in groundwater in comparison with the phosphate itself is an indication of the general low solubility of uranium complexes in groundwater in this area. Experiments about the distribution of some actinides and fission products between the phosphate and groundwater from this area, shows that more than 99% of these nuclides are adsorbed in the solid phase (Ghafar et al., 2000). According to the classification of groundwater after Cowart and Osmon (1980) and Osmon and Cowart (1983) our samples were mainly normal oxidized or normal reduced.

Acknowledgements The authors would like to thank Prof. Dr. I. Othman, the director general of the Atomic Energy Commission of Syria, for his encouragement. The assistance of H. Arsan, N. Salman, H. Kourdi, H.Isaa and M. Doueer during this work is also acknowledged.

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