Determination of uranium isotopes in environmental samples by anion exchange in sulfuric and hydrochloric acid media

Applied Radiation and Isotopes 115 (2016) 274–279

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Determination of uranium isotopes in environmental samples by anion exchange in sulfuric and hydrochloric acid media$ L. Popov Kozloduy Nuclear Power Plant, Safety Department, Environmental Monitoring Division, 3321 Kozloduy, Bulgaria

H I G H L I G H T S







The method allows cost-effective determination of U isotopes. High amounts of environmental samples can be analyzed. High chemical yields, energy resolution and decontamination factors were achieved. Uranium isotope concentrations in mineral waters from Bulgaria are presented.

art ic l e i nf o

a b s t r a c t

Article history: Received 17 March 2016 Received in revised form 26 June 2016 Accepted 14 July 2016 Available online 18 July 2016

Method for determination of uranium isotopes in various environmental samples is presented. The major advantages of the method are the low cost of the analysis, high radiochemical yields and good decontamination factors from the matrix elements, natural and man-made radionuclides. The separation and purification of uranium is attained by adsorption with strong base anion exchange resin in sulfuric and hydrochloric acid media. Uranium is electrodeposited on a stainless steel disk and measured by alpha spectrometry. The analytical method has been applied for the determination of concentrations of uranium isotopes in mineral, spring and tap waters from Bulgaria. The analytical quality was checked by analyzing reference materials. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Uranium Anion exchange Water Bulgaria

1. Introduction Uranium is radioactive element with more than 27 characterized isotopes with mass numbers ranging from 215 to 242. It is one of the most important naturally occurring radioactive elements with two radioactive decay chains: most abundant 238U (half-life of 4468.109 years, 99.27456% by mass in natural uranium, 12.2369 Bq mg  1 in natural uranium; with its decay product 234U 2455.105 years, 0.00555% mass, 12.5624 Bq mg  1) and fissile 235 U (7038.108 years, 0.720012% mass, 0.5685 Bq mg  1) used for nuclear fuel (directly or after enrichment) (Laboratoire National Henry Becquerel, 2016). After the enrichment of 235U the remaining 238U (called depleted uranium - DU) is used mainly as counterweights in aircraft. Most important man-made uranium isotopes are: 236U (2342.107 years, found in spent or reprocessed nuclear fuel due to decay of 24°Pu), fissile 233U (1592.105 years, produced by ☆ This work was presented at 17th Radiochemical Conference ‘RadChem 2014′ (11–16 May 2014, Mariánské Lázně, Czech Republic). E-mail address: [email protected]

http://dx.doi.org/10.1016/j.apradiso.2016.07.013 0969-8043/& 2016 Elsevier Ltd. All rights reserved.

irradiation of 232Th and possibly reasonable alternative to 235U nuclear fuel) and 232U (70.6 years) used as radioactive yield tracer in radiochemical determinations of uranium isotopes (Laboratoire National Henry Becquerel, 2016). Natural uranium is found at trace levels in all major environmental samples (water, soil, organic materials, etc.). Direct measurement of uranium (238U and 235U only) in environmental samples is possible by gamma spectrometry only when its concentrations are higher than its natural levels in the environment. Usually uranium is measured always after radiochemical separation procedures. Although mass-spectrometry (Avivar et al., 2012; Boulyga and Becker, 2002; Maxwell et al., 2014) has been gathering momentum in the recent years for measurements of longlived alpha and beta radionuclides, alpha spectrometry is still the preferred instrument for measurements of alpha radionuclides due to the relatively lower price and maintenance costs of the equipment (Bojanowski et al., 2002; Boryło, 2013; Chen, 2001; Jia, 2016; Popov, 2013). Nowadays separation of uranium is widely conducted by the use of extraction chromatography resins:


UTEVA Resin (Horwitz et al., 1992), s

L. Popov / Applied Radiation and Isotopes 115 (2016) 274–279

275


TRU resin (Maxwell and Culligan, 2006; Maxwell et al., 2014),
TOPO-Microthene resin (Jia et al., 2004; Jia, 2016) s

as well as anion exchange resins (Bojanowski et al., 2002; Boryło, 2013; Chen et al., 2001;) and solvent extraction (Popov, 2012; 2013). In dilute sulfuric acid (or sulfate) media U is quantitatively extracted by strong base anion exchange resins (Bojanowski et al., 2002; Boryło, 2013; Korkisch, 1969). Additional study on anion exchange behavior in sulfuric acid media of U, Pu, Np, Th, etc. was performed and based on the results a cost-effective radioanalytical method for determination of U in environmental samples containing high amounts of matrix elements has been developed.

2. Experimental 2.1. Reagents and equipment All chemicals (Merck, Fluka) used were of analytical grade. Anion exchange resin AG1  4 (100–200 mesh) and borosilicate s glass Econo-Column chromatography columns (0.7  20 cm and 1.0  20 cm) were purchased from BioRad (USA). A Mars 5 (CEM, USA) microwave system with closed PTFE-vessels (EasyPrep™) was used for acid decomposition of the solid samples. A Hermle Z513 (Germany) centrifuge (with 100, 250 and 500 ml tubes) was used for separation of the precipitates. Uranium-232 (0.3633 Bq/ g71.6%, k ¼2) and plutonium-242 (0.1765 Bq/g 72%, k ¼2) tracer solutions were purchased from the Czech Metrological Institute (Czech Republic). After their radiochemical separation / purification and electrodeposition on stainless steel disks (ؼ20 mm), isotopes of uranium were measured on the alpha spectrometer “Alpha Analyst” (Canberra, USA) with 8 vacuum chambers (PIPS detectors with a 450 mm2 active area, 20 keV FWHM and 22% efficiency for a distance of 5 mm between planchet and detector surface). 2.2. Samples Mineral, tap and spring water samples from Western and Southern Bulgaria were collected. Sampling locations are shown on Fig. 1. 2.2.1. A. Uranium, plutonium and thorium behavior on anion exchange resin in sulfuric acid media 2.2.1.1. Elution curves of U, Pu and Th with anion exchange resin in 0.2 M to 4 M H2SO4 acid media (without reducing/oxidizing

Fig. 1. Map of the sampling locations.

Fig. 2. Elution curve of U from Biorad AG1  4 with H2SO4.

agents). Ten portions of 14 ml (VR, resin volume) Biorad anion exchange resin were transferred in 10 glass columns (i.d. x column length ¼10  200 mm, 14 mm resin bed length). The resin in each column was preconditioned with 84 ml (6.VR) solutions of 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M, 1.2 M, 1.4 M, 1.6 M, 2.0 M and 4.0 M H2SO4 correspondingly. Ten beakers containing 14 ml 0.2 M to 4 M H2SO4 were spiked with 232U and 242Pu tracers. The solution in each beaker was passed through the corresponding column. The columns were washed with 17.VR (  240 ml) 0.2 M to 4 M H2SO4 solutions by portions of 14 ml (VR). Each portion was evaporated to dryness. It was continued with Section A.3 – Electrodeposition. The results are presented in Figs. 2–4. In dilute sulfuric acid media U is quantitatively extracted by strong base anion exchange resins (Korkisch, 1969). Depending of the concentration of H2SO4 different elution curves for BioRad AG1  4 anion exchange resin were built for U, Pu and Th (Figs. 2– 4). Elution curves of U widen with decreasing concentrations of H2SO4 and its quantitative adsorption is possible only at concentrations of 0.2 M H2SO4 and lower (for very large samples / elution volumes concentrations even lower than 0.1 M H2SO4 will be required). At 0.2 M H2SO4 except U, Pu, Th, Po and some rare earth elements like Zr, Nb, Mo, Pa, etc. are partly (Pu, Th, Po, etc.) or fully (Pa, Zr, Nb, Mo) co-adsorbed by the resin and additional techniques should be applied for their separation from U.

Fig. 3. Elution curve of Pu from Biorad AG1  4 with H2SO4.

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L. Popov / Applied Radiation and Isotopes 115 (2016) 274–279

Fig. 4. Elution curve of Th from Biorad AG1  4 with H2SO4.

2.2.1.2. Influence of oxidizing and reducing agents on absorption of U, Th, Pu, Np in 0.2 M H2SO4. Eight beakers, containing 30 ml (  2VR) solutions of: 0.2 M H2SO4 – 0.04 M K2S2O8, 0.2 M H2SO4 – 0.2% H2O2, 0.2 M H2SO4 – 0.4% H2O2, 0.2 M H2SO4 – 4% H2O2, 0.2 M H2SO4 – 10% H2O2, 0.2 M H2SO4 – 0.07 M Na2SO3, 0.2 M H2SO4 – 0.02 M K2S2O5 and 0.2 M H2SO4 – 0.06 M ascorbic acid, were spiked with 232U (in equilibrium with 228Th) and 242Pu tracers. Eight columns (i.d. ¼1.0 cm) were prepared with 14 ml Biorad AG1  4 anion exchange resin. The resin in each column was reconditioned with 80 ml (  6VR) of the corresponding oxidizing or reducing media (in 0.2 M H2SO4). The solutions in each beaker were passed through the corresponding column and the resin was washed additionally with 100 ml (  7VR) of the initial solution. All absorbed by the resin radionuclides were stripped by addition of 40 ml (  3VR) 4 M H2SO4 to each column. The stripped solution was evaporated to dryness. It was continued with the next section A.3 – Electrodeposition. The results are presented in Table 1. 2.2.1.3. Electrodeposition. Finally, in order to form sulfate salts in the evaporated beakers (from sections A.1 and A.2), 5 ml 14 M HNO3, 1 ml 6 M NaOH and 0.5 ml 18 M H2SO4 were added to the dry residue in each beaker. Salts were evaporated to dryness and burned in an oven for 2 h at 550 °C in order to destroy any traces left by the resin and to remove excess sulfuric acid as SO3. Uranium (and Th, Pu, Np) was electrodeposited on a stainless steel disk using sulfate media (0.05 M H2SO4) under the following conditions: pH ¼ 2.2  2.5, duration of electrodeposition of 4 h (under cooling), current 1.1–1.2 A. Before the end of electrodeposition 1.0 ml 25% NH3 were added. Electrodeposited disks were measured on the “Alpha Analyst” alpha spectrometer.

2.2.2. B. Recommended analytical procedure for separation and determination of 234,235,236,238U in environmental samples 2.2.2.1. Water (tap, river, sea, etc.). Between 0.5 and 3 L of water (depending on the expected U concentrations) are analyzed. After filtration through “blue band” filter paper, the water samples are acidified to pH 1–1.5 with 10 M HCl or 14 M HNO3. About 0.3–0.4 g FeCl3  6H2O (  60–80 mg Fe3 þ ) are added as a stable carrier and 232 U with known specific activity as the yield determinant. The solution is heated (near boiling; for the dissociation of the stable neutral uranium-calcium-carbonate complexes) with occasional stirring for evaporation of up to 80–90% of its initial volume (for prevention of crystallization the final volume should not be below: 200 ml for seawater, 100 ml for low salt content waters). It is left to cool and the solution is diluted with distilled water to 0.2–0.5 L (for dissolution of any possibly formed crystals). Carefully 25% NH3 is added to pH 7–8 and the formed precipitate is transferred into centrifuging tubes or bottles (250–500 ml). The tubes/bottles are centrifuged for 10 min at 3000 rpm. It is continued with section B.3). 2.2.2.2. Aerosol filters, bottom sediments, soil, biological samples (grass, bone, etc.). Between 0.3 g (up to 0.5 g for mineral samples expected to contain refractory uranium) and up to 20 g (for biological samples) of the previously dried and sieved (0.1 mm) material are used for analysis. The sample is spiked with 232U tracer and burned for 24 h (or more) at 550 °C in order to destroy the organic content. 2.2.2.3. Samples containing refractory uranium (mineral samples – soil, bottom sediments, etc.). The ashed mineral samples are decomposed using a microwave digestion system with closed PTFEvessels after addition of 12:6:4:2 ml concentrated HCl: HNO3:HF: H2O2. The temperature and pressure are gradually increased to 230 °C for 2 h. The vessels are cooled. About 50 – 70 mg Fe3 þ are added as carrier. The decomposed sample is transferred into a PTFE-beaker, The PTFE-vessel is washed 3 times with 5 ml 0.2 M HNO3 and the washings are combined into the PTFE-baker. The solution is carefully evaporated on a sand band to dryness. About 10–15 ml 14 M HNO3 are added and it is evaporated to dryness again. The addition of 14 M HNO3 and evaporation to dryness is repeated 3 more times. The dry residue is dissolved with 20 ml 6 M HCl and the dissolved sample is transferred into a 250 ml centrifuge tube. The PTFE-baker is washed 3 times with 5 ml 0.2 M HCl and the washings are combined into the centrifuge tube. The solution is diluted with 60 ml distilled water. The solution is alkalized to pH 7–8 with 25% NH3. The tube is centrifuged for 10 min at 3000 rpm. It is continued with section B.3). 2.2.2.4. Samples without refractory uranium (biological samples – grass, carbonates, bones, etc.). The ash is dissolved in 20–50 ml aqua

Table 1 Results (n ¼2, k¼ 2) for absorption of U, Th, Pu and Np after addition of oxidizing and reducing agents to anion exchange resin Biorad AG1  4 in 0.2 M H2SO4. Mediaa

0.2 M H2SO4 0.2 M H2SO4 0.2 M H2SO4 0.2 M H2SO4 0.2 M H2SO4 0.2 M H2SO4 0.2 M H2SO4 0.2 M H2SO4 acid a

– 0.04 M K2S2O8  0.2% H2O2  0.4% H2O2  4% H2O2  10% H2O2  0.07 M Na2SO3  0.02 M K2S2O5 – 0.06 M ascorbic

Absorbed U, % (of initial content)

Absorbed Th, % (of initial content)

Absorbed Pu, % (of initial content)

Absorbed Np, % (of initial content)

91.5 74.4 101.2 7 4.4 101.17 5.8 100.7 7 3.0 101.3 7 3.0 101.3 7 5.4 100.5 7 4.8 98.7 7 5.6

9.5 71.0 21.2 7 1.2 21.3 7 1.4 27.17 1.2 14.1 7 0.8 85.7 75.0 64.57 5.2 95.5 75.6

40.6 7 2.4 3.4 7 0.6 4.17 0.6 14.3 70.8 14.6 70.6 6.5 7 0.4 3.5 7 0.2 o 0.1

– – – – – – – 101.2 7 6.4

The resin was initially converted from chloride form to the corresponding media.

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277

regia after boiling for 2 h at 150–250 °C. In small portions, 5–10 ml 30% H2O2 is added meanwhile. The sample is cooled and filtered through a Whatman GF-A filter. The solution is transferred into a 250 ml centrifuging tube(s). If the iron content is low (lack of yellow color, as for biological samples) then iron carrier ( 60 80 mg Fe3 þ ) is added. The solution is diluted with distilled water. After alkalization to pH 7–8 with 25% NH3 the sample is centrifuged at 3000 rpm for 10 min. It is continued with section B.3).

which is preconditioned with 1.VR 2 M H2SO4 and 6.VR 0.2 M H2SO4. After passing the sample, the resin is washed with 7.VR 0.2 M H2SO4 (if the sample doesn’t contain Pu) or (in case of expected Pu in the sample) consecutively with 4.VR 0.2 M H2SO4, 10.VR 0.2 M H2SO4 þ 1 g ascorbic acid/100 ml solution and 3.VR 0.2 M H2SO4 (without any ascorbic acid for its full elution). Th is eluted with 4. VR 10 M HCl. U is stripped with 10.VR 0.1 M HCl and it is evaporated to dryness.

2.2.2.5. Purification of uranium from plutonium, neptunium, thorium and matrix elements. The supernatant is discarded and the iron hydroxide precipitate is dissolved in minimal amount of 12 M HCl. After addition of distilled water iron is precipitated again with 25% NH3 to pH 7–8 and the sample is centrifuged at 3000 rpm for 10 min (if the sample contains high amounts of Ca then one more Fe precipitation at pH 7–8 might be required). The supernatant is discarded and the precipitate is dissolved in minimal amount of 0.2 M H2SO4 after occasional stirring. It takes 20–40 min for complete dissolution of the precipitate (if after that time there is still an undissolved precipitate more amount of 0.2 M H2SO4 would be required; if white clouds or precipitate of CaSO4 is formed then with addition of distilled water the clouds/precipitate should disappear. In order to prevent formation of CaSO4 the solution should not be left overnight). The solution is passed through glass column (i.d. ¼0.7 or 1.0 cm, resin bed length ¼14 cm) with 5–6 ml (VR, i.d. ¼0.7 cm, for samples with low/medium content of matrix elements) or 14 ml (VR, i.d. ¼1.0 cm, for samples with high amounts of matrix elements) Biorad AG1  4 (100–200 mesh) anion exchange resin

Uranium is electrodeposited according to the previously described section A.3). Graphical illustration of the recommended procedure is presented on Fig. 5. 2.3. Quality assurance In order to ensure the high quality of the analytical results, blank, duplicate and spiked samples were analyzed periodically according to an internal quality assurance program. Yearly checks of the background and the efficiency were done on the measurement system. The detectors were calibrated every 2 years. The results from the analyses of reference samples for 234,235,238 U are presented in Table 2.

3. Results and discussion Environmental waters (esp. mineral) often contain significant quantities of carbonates (and hydrogen carbonates) which form

Fig. 5. Graphical illustration of the method.

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L. Popov / Applied Radiation and Isotopes 115 (2016) 274–279

Table 2 Results from analyses of uranium isotopes in certified reference samples. Nuclide / Sample

Chemical yield, %

Measured value (n ¼3, k¼ 2)

Reference value

Deviation, %

14.60(14) 14.76(4)

 2.0  3.8

Water Alpha Low AL-2009 (NPL, UK), Bq/kg 238 U 93.7 17.2(5)

18.0(4)

 4.4

Water Alpha Low AL-2010 (NPL, UK), Bq/kg 238 U 98.2 7.27(42)

7.76(20)

 6.3

19.3(6) 19.3(6)

 3.6  3.1

Water Alpha Low AL-2008 (NPL, UK), Bq/kg 234 238

U U

99.6

14.3(4) 14.2(4)

Water Alpha Low AL-2013 (NPL, UK), Bq/kg 234 238

U U

99.9

18.6(6) 18.7(6)

IAEA-315, marine sediment (IAEA, Austria), Bq/kg d.w. 234

U

238

U

88.1

18.0 7 0.6 17.2 70.5

17.8 (16.6 – 20) 17.6 (16.1 – 18.5)

þ 1.1  2.3

IAEA-375, soil (IAEA, Austria), Bq/kg d.w. 234 238

U U

92.9

25.7 7 1.0 25.0 7 1.0

25 (17 – 32)a 24.4 (19 – 29.8)a

þ 2.8 þ 2.5

IAEA-384, lagoon sediment (IAEA, Austria), Bq/kg d.w. 234 238

U U

88.5

40.0 7 1.5 35.47 1.3

40 (35 – 43)a 35.5 (33.4 – 36.8)

0  0.3

1.22 (1.15 – 1.26) 1.11 (1.07 – 1.15)

 7.4

IAEA-414, mixed fish (IAEA, Austria), Bq/kg d.w. 234

U

238

U

a

89.7

1.137 0.06 1.08 70.06

 2.7

Information value.

with uranium and calcium stable neutral complexes even in low pH (Bernhard et al., 2009). These neutral uranium carbonate complexes are not possible to be retained or extracted by cation/ anion exchange resins and liquid extraction/extraction chromatography. An easy way completely to destroy the complexes is by evaporation of the water in relatively strong hydrochloric or nitric acidic media (then all carbonates will escape as carbon dioxide) (Popov, 2013). By this way the equilibrium between U tracer and U isotopes in the sample will be ensured (otherwise underestimated results might be expected) as well as the chemical recovery of uranium will be kept reproducible and high at that early stage of the analysis. Complete dissolution of the solid mineral samples (except biological samples: bone, grass, etc.) is required because uranium is incorporated into the crystal lattices of the oxides of Si, Zr, Al, etc. (Popov, 2012). It can be achieved in two ways: fusion with appropriate melting agents (hydroxides, carbonates, peroxides, fluorides, borates, sulfates, pyrosulfates, etc. or in combination) or decomposition by microwave closed vessels acid digestion systems. The latter was used in the present method as a more convenient and efficient technique for samples containing refractory uranium. Modern microwave acid decomposition systems can ensure safe operation even at temperatures over 250 °C. Higher temperatures and pressures as a rule lead to better sample decomposition. The mixture of concentrated HCl, HNO3, HF and H2O2 and the ratio between them (12:6:4:2) often leads to complete decomposition but for some cases the amount of HF should be increased or decreased according to the sample amount and matrix. Incomplete decomposition will lead to overestimated

chemical recoveries and respectively to systematic underestimated U concentrations. Uranium in bioorganic samples is easily leached by aqua regia because only the dissolved uranium can pass through the plant/animal cell membranes (so bioorganic U is not incorporated into any refractory oxides). Environmental samples contain sometimes large amount of Ca (bone, sediment, grass, etc.) which should be removed by 2 (or 3) consecutive precipitations of Fe as hydroxide at pH ¼7–8 with ammonia in order to eliminate 4 95% of the initial Ca (Popov et al., 2006). If some Ca is omitted later in the analysis (prior the column load) it can possibly form precipitate of CaSO4. Addition of distilled water usually will dissolve that precipitate (KSp, CaSO4 ¼4.9  10  5). Adsorption of U by anion exchange resins in dilute sulfuric acid media has obvious advantages over hydrochloric or nitric acid media (Korkisch, 1969). The major matrix elements (Fe, Al, Ca, Mg, etc.) are not adsorbed and are fully eluted with additional 2–5 resin bed volumes of 0.2 M H2SO4. In 0.2 M H2SO4, except U, only small numbers of elements are co-adsorbed – Pu (Fig. 3), Th (Fig. 4), Po and some rare earth elements like Zr, Nb, Mo, Pa, etc. Adsorption by (and/or elution with) more concentrated sulfuric acid solutions might lead to partial or complete loss of U (Fig. 2). Another unique feature of the anion exchange of U in sulfuric acid media is that the more diluted is the sulfuric acid the higher are the distribution coefficients of U: Kd 4100 for 0.25 M H2SO4, Kd 41000 for 0.05 M H2SO4 and Kd 410,000 for 0.025 M H2SO4 (Korkisch, 1969). Therefore initial adsorption from more diluted sulfuric acid is favorable, although it should be accompanied by elution with 0.2 M H2SO4, otherwise the separation factors U/Pu and U/Th will diminish to just 1–10 (it was observed that even Th (IV) and Pu(IV,V, VI) are quantitatively co-adsorbed with U(VI) at o0.05 M H2SO4 which might be utilized for initial adsorption of U, Th and Pu and development of analytical method for their sequential determination). Addition of oxidizing (K2S2O8, H2O2) and reducing agents (Na2SO3, K2S2O5) doesn’t alter significantly the behavior of U, Th and Pu (Table 1), except only the ascorbic acid, because of the complete removal of Pu. Surprisingly Np(V, VI) is not adsorbed in diluted H2SO4, but the addition of ascorbic acid leads to its almost quantitative adsorption by the resin (as Np(IV)-ascorbate complex). For that reason the initial column load is performed in sulfuric acid without any ascorbic acid (Np is eluted) and later the adsorbed Pu is reduced and eluted in 0.2 M H2SO4 þ ascorbic acid. Ascorbic acid also slightly narrows the elution curves of U and it should be eluted from the resin (simply by adding pure 0.2 M H2SO4) prior to its conversion from sulfate to chloride form (otherwise partial loss of U, from 5% to 25%, can occur). Th is easily eluted from the resin with 10 M HCl. Lower concentrations of HCl will partly co-elute U. Uranium is stripped from the resin with 0.1 M HCl. For oxidation of any organics in the stripped solution (and for sublimation of any Po present) the dry residue is burned at 550 0C. For minimization of U losses (and achievement of almost quantitative U-recoveries) the addition of ascorbic acid can be safely avoided because of the fact that the levels of Pu in the environment are typically several magnitudes lower than U concentrations. Columns with different internal diameters (i.d. ¼0.7 or 1.0 cm) can be used depending on the expected levels of matrix elements in the sample. For most of the analyzed samples 5.5 ml anion exchange resin (3.5  4 g dry resin) and column with 0.7 cm i.d. (resin bed length ¼14 cm) provides at least 5.5 meq capacity ( 220 mg U(VI), 110 mg Fe(III), etc.) which is adequate for efficient (smaller amount of waste solutions) and economical U determination with reproducible and almost quantitative recoveries of U. Experiments with smaller columns (i.d. ¼ 0.5 cm and 2  2.5 ml resin) resulted in non-reproducible U recoveries. For samples with

L. Popov / Applied Radiation and Isotopes 115 (2016) 274–279

Table 3 Concentrations of

279

234,238

U (n ¼ 2, k ¼2) in mineral, spring and tap waters from Bulgaria.

Location

Sampling

Chemical yield, %

234

Barzia (mineral, bulk) Bachkovo (spring, bottled) Kyustendil (mineral, bulk) Rudartsi (mineral, bulk) Sandanski (mineral, bulk) Sandanski (tap, bulk) Sofia central bath (mineral, bulk) Varshets (mineral, bulk) Varvara (mineral, bulk) Velingrad (mineral, bulk) Vetren dol (mineral, bulk)

Aug-2014 Aug-2014 Jun-2014 Aug-2014 Aug-2014 Sep-2014 Aug-2014 Aug-2014 Aug-2014 Aug-2014 Aug-2014

89.6 98.6 86.5 96.1 92.4 92.6 88.3 90.7 97.8 95.8 95.4

0.6770.07 61.3 7 3.7 0.23 7 0.03 9.917 0.59 0.56 7 0.06 6.62 7 0.33 1.107 0.08 0.85 7 0.08 0.277 0.05 3.58 7 0.25 24.971.5

high content of matrix elements columns with i.d. ¼1.0 cm (14 ml resin; resin bed length ¼14 cm) can be safely used. After the alpha spectrometric measurement corrections should be made to subtract 228Th from 232U sum peak (because of the overlapping peaks of 228Th (daughter of 232U) and 232U) otherwise systematic overestimated chemical recoveries for U might be expected (Popov, 2012). After all studies and improvements of the method, the chemical yields became reproducible in the range 85–99% and the interferences of the matrix elements and radioactive isotopes (Ca, Mg, Fe, Th, Pu, Np, Po, etc.) were eliminated with separation factors from 102 (Np) to over 103 (Th, Pu, Po). The analyses of the reference samples with different matrices (water, soil, sediment) confirmed the good quality of the results - Table 2. As a result the typical energy resolution of the measured alpha spectra is in the range of 20–30 keV. 3.1. Application of the method The method was applied for determination of uranium isotopes in mineral, spring and tap water samples from Western and Southern Bulgaria (Fig. 1, Table 3), because no data is available in the literature for uranium concentrations in these samples although they are used as daily drinking water from the local population or even nation-wide. In the analyzed mineral water samples U concentration is in the range 0.2  61 mBq/L which is below the guideline level of 3.0 Bq/L (radiological levels: 238U – 3.0 Bq/L, 234U – 2.8 Bq/L; chemical levels: U  30 mg/L) in Bulgarian national regulation N 9 for drinking water quality (Bulgarian Ministry of the Environment and Waters, 2014) and the Council Directive 2013/51/Euratom (The Council of the European Union, 2013). For all analyzed water samples (except the water sample from the village of Vetren dol) there were no equilibrium between 238 U and 234U with a ratio 234U/238U always above 1 and in the range 1.00–1.56, caused by the Szilard-Chalmers effect (increased solubility of the recoil daughter 234U after alpha-particle emission from 238U and partial destruction of the crystalline structure). From the measured 234U/238U ratios it can be concluded that the analyzed waters are relatively ‘young’.

4. Conclusions The presented method allows accurate determination of uranium isotopes in major types of environmental materials. The main advantages of the method are: low cost of the analysis, good separation factors from matrix, natural and man-made radionuclides, high chemical yields, good energy resolution of the measured electrodeposited disks, reproducibility and scalability. The method was applied successfully for determinations of uranium isotopes in reference materials with various matrices, as

U, mBq/L

238

U, mBq/L

0.43 7 0.05 52.7 7 3.2 0.177 0.03 8.25 7 0.49 0.38 7 0.05 5.92 7 0.30 0.89 7 0.06 0.7770.08 0.20 7 0.04 2.75 7 0.22 24.971.5

234

U /238U

1.56 1.16 1.35 1.20 1.47 1.12 1.24 1.10 1.33 1.30 1.00

well as in mineral, spring and tap waters from Western and Southern Bulgaria.

References Avivar, J., Ferrer, L., Montserrat, C., Cerda, V., 2012. Fully automated lab-on-valvemultisyringe flow injection analysis-ICP-MS system: an effective tool for fast, sensitive and selective determination of thorium and uranium at environmental levels exploiting solid phase extraction. J. Anal. Spectrom. 27, 327–334. Bernhard, G., Geipel, G., Reich, T., Brendler, V., Amayri, S., Nitsche, H., 2009. Uranyl (VI) carbonate complex formation: Validation of the Ca2UO2(CO3)3 (aq.) species. Radiochim. Acta 89, 511–519. Bojanowski, R., Radecki, Z., Pięckoś, R., 2002. Rapid determination of 226Ra and uranium isotopes in solid samples by fusion with lithium metaborate and alpha spectrometry. Sci. World J. 2, 1891–1905. Boryło, A., 2013. Determination of uranium isotopes in environmental samples. J. Radioanal. Nucl. Chem. 295, 621–631. Boulyga, S.F., Becker, J.S., 2002. Isotopic analysis of uranium and plutonium using ICP-MS and estimation of burn-up of spent uranium in contaminated environmental samples. J. Anal. . Spectrom. 17, 1143–1147. Bulgarian Ministry of Environment and Waters, 2014. Regulation N 9 for drinking Water Quality. 〈https://eea.government.bg/bg/legislation/water/NAREDBA___9_ ot_16.03.2001.pdf〉 (in Bulgarian) (accessed 21.06.16). Chen, Q., Aarkrog, A., Nielsen, S.P., Dahlgaard, H., Lind, B., Kolstad, A.K., Yu, Y., 2001. Procedures for Determination of 239,240Pu, 241Am, 237Np, 234,238U, 228,230,232Th, 99 Tc and 210Pb-210Po in Environmental Materials, Riso-R-1263(EN). Riso National Laboratory,, Roskilde, Denmark 〈http://orbit.dtu.dk/fedora/objects/or bit:90124/datastreams/file_7727225/content〉 (accessed 01.02.16). Horwitz, E.P., Dietz, M.L., Chiarizia, R., Diamond, H., 1992. Separation and preconcentration of uranium from acidic media and extraction chromatography. Anal. Chim. Acta 266, 25–37. Jia, G., Torri, G., Innocenzi, P., 2004. An improved method for the determination of uranium isotopes in environmental samples by alpha spectrometry. J. Radioanal. Nucl. Chem. 262, 433–441. Jia, G., 2016. Sequential separation and determination of uranium and thorium isotopes in soil samples with Microthene-TOPO chromatographic column alpha spectrometry. J. Radioanal. Nucl. Chem. http://dx.doi.org/10.1007/ s10967-016-4905-3. Korkisch, J., 1969. Modern Methods for The Separation of Rarer Metal Ions, first ed., Pergamon Press, Oxford. Laboratoire National Henry Becquerel, 2016. Recommended Data by the Decay Data Evaluation Project, 〈http://www.nucleide.org/DDEP_WG/DDEPdata.htm〉 (accessed 31.01.16). Maxwell, S.L., Culligan, B.K., 2006. Rapid column extraction method for actinides in soil. J. Radioanal. Nucl. Chem. 270, 699–704. Maxwell, S.L., Culligan, B.K., Hutchison, J.B., Utsey, R.C., McAlister, R.C., 2014. Rapid determination of actinides in seawater samples. J. Radioanal. Nucl. Chem. 300, 1175–1189. Popov, L., Hou, X., Nielsen, S.P., Yu, Y., Djingova, R., Kuleff, I., 2006. Determination of radiostrontium in environmental samples using sodium hydroxide for separation of strontium from calcium. J. Radioanal. Nucl. Chem. 269, 161–173. Popov, L., 2012. Method for determination of uranium isotopes in environmental samples by liquid-liquid extraction with triisooctylamine/xylene in hydrochloric media and alpha spectrometry. Appl. Radiat. Isot. 70, 2370–2376. Popov, L., 2013. Novel method for determination of uranium isotopes in environmental samples by liquid-liquid extraction with triisooctylamine in sulfuric and hydrochloric acid media. J. Radioanal. Nucl. Chem. 298, 555–562. The Council of the European Union, 2013. Council Directive 2013/51/Euratom, of 22 October 2013, laying Down Requirements for the protection of the Health of the General public with Regard to Radioactive Substances in Water Intended for Human Consumption. 〈http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?ur i¼ OJ:L:2013:296:0012:0021:EN:PDF〉 (accessed 21.06.16).