Application of multiple source preparation procedures on environmental samples for determination of Uranium in Alpha Spectrometry

. Different source preparation procedures for determination of uranium radioisotopes in environmental samples by alpha spectrometry are presented. Addition of 232 U tracer was followed by the preconcentration of radionuclides of interest from the samples by co-precipitation on Fe(OH) 3 at pH 9 using ammonia solution. The uranium extracts were obtained by column separation using UTEVA resin after which the uranium sources from the samples were prepared using either electrodeposition or micro-coprecipitation with LaF 3 before counting. The procedures were tested on spinach, soil, tap water and waste water from the Libanon Gold mine and the results obtained were compared.


Introduction
Alpha-particle spectrometry is a measurement technique that has found many practical applications in such diverse fields as nuclear decay data measurements, geological studies, or the measurement of low levels of activity in the environment. 1 The samples used in this study are water, spinach and soil. Complete decomposition of the spinach and soil sample matrix is necessary for an accurate radiochemical analysis of alpha-emitting radionuclides. Therefore, conventional wet digestion was used to achieve this. The aim of the work presented is to investigate two different source preparation approaches for determination of low-level uranium radioisotopes in water, spinach and soil samples by alpha-particle spectrometry. The preconcentration method for uranium was used by coprecipitation with Fe(OH)3 at pH 9 using an ammonia solution in the water samples. 2 Micro-coprecipitation with LaF3 method was used for the tap water and waste water from the mine while electrodeposition was used for the soil and spinach samples after uranium separation using a UTEVA column. These methods allow the radionuclides of interest and the tracers to be made into a thin, flat and uniform source, which allows adequate transmission of the alpha-particles to the surface of the detector. 2 * Corresponding author: mcrufguy@yahoo.com

Spinach
A triplicate of 2 g of ashed spinach sample was weighed into a glass beaker and 10 mL of concentrated HNO3 was added to make a slurry. 0.5 g of 232 U tracer of concentration 0.3163 Bq/g was added and heated for three hours at 150°C. 1 mL H2O2 was added into the sample and evaporated to dryness. 6 mL of concentrated HNO3 and 1 mL of H2O2 (6% w/v) were added into the sample and evaporated to dryness at 250 °C 1 . The sample was dissolved in 3 M HNO3 and loaded on a chromatographic column of length 10 cm containing UTEVA resin (Uranium and TEtraValents Actinides) of size 100 µg. The extractant for the anion exchange chromatography in the UTEVA Resin is diamyl, amylphosphonate (DAAP), which extracts nitrato complexes of the actinide elements at room temperature. The container is rinsed once with 5 mL 3 M HNO3 and the washing solution is also loaded on the column. The column was washed with 20 mL 3 M HNO3, 5 mL 9 M HCl, and 25 mL 5 M HCl after which uranium was eluted with 15 mL 1 M HCl into a clean tube. 5 The eluted solution was evaporated to dryness. 1 mL of concentrated HNO3 and 1 mL of H2O2 were added and evaporated to dryness. 4 The residue was dissolved in 1 mL of 6 M HCl and then transferred into the electrodeposition cell shown in figure 1. The electrodeposition method was a modified Talvitie's and Hallstadius's method whereby the chemically separated uranium was further dissolved in 10 mL of electrolytic solution made up of 5.7% Ammonium oxalate in 0.3M HCl. 6 The deposition planchet which served as the anode in the electrodeposition setup is a stainless steel disk of diameter 25 mm. The electrodeposition was started with an electrical current of 0.6 A with an initial voltage of 9.26 V fluctuating to a maximum voltage of 9.59 V for 2 hours at room temperature. 1 mL of ammonia was added a minute before putting off the power supply. The deposition diameter was 20 mm. The disc with electrodeposited radionuclides was dried and counted.

Soil:
A triplicate of 2 g of oven-dried soil sample was weighed into a glass beaker and 10 mL of concentrated HNO3 was added to make a slurry. 0.5 g of 232 U tracer of concentration 0.3163Bq/g was added and heated for three hours at 150°C. 10 ml each of 70% w/w concentrations of HNO3, HF, HClO4 were added to each of the samples and evaporated to dryness on a hot plate. 2 mL of concentrated HNO3 was added into the sample and evaporate to dryness. The sample was dissolved in 3 M HNO3 and loaded on the column containing UTEVA resin. The container is rinsed once with 5 mL 3 M HNO3 and loaded on a column. The column is washed with 20 mL 3 M HNO3, 5 mL 9 M HCl, and 25 mL 5 M HCl after which uranium was eluted with 15 mL 1 M HCl into a clean tube. The eluted solution was evaporated to dryness. 1 mL of concentrated HNO3 and 1 mL of H2O2 were added and evaporated to dryness. The residue was dissolved in 1 mL of 6 M HCl and then transferred into the electrodeposition cell. The electrodeposition was started with an electrical current of 0.6 A for 2 hours. 1 mL of ammonia was added a minute before putting off the power supply. The disc with electrodeposited radionuclides was dried and counted.

Water:
Three samples each of volume 4 L of tap water and three samples each of volume 100 mL of waste water from the mine were measured into different beakers. Each of the samples was acidified with 12 mL of concentrated HNO3 after which 0.5 g of 232 U tracer was added into each sample. 8 mL of Fe 3+ carrier (5 mg of Fe/mL) was added to every sample to initiate the co-precipitation of the radionuclides of interest with Fe(OH)3. 4 The pH of the samples was changed to 9.6 by gradually adding concentrated NH3. Each solution was stirred for 3 hours and allowed to settle down by leaving them overnight. The supernatant was then decanted, centrifuged at 3500 rpm for 5 minutes and rinsed with deionised water until neutral pH was obtained. The Fe(OH)3 precipitate of each was then dissolved in 5 mL of 3 M HNO3. Preconditioning of UTEVA column was done by rinsing the column with 10 mL H2O, 10 mL 1M HNO3 and 10 mL 3 M HNO3. Each sample was loaded on the column. The centrifuge tubes were rinsed twice with 5 mL 3 M HNO3 and loaded on every column after which the column was rinsed with 20 mL 3 M HNO3, 5 mL 9 M HCl, 25 mL 5 M HCl. Uranium was finally eluted with 15 mL of 1 M HCl into a clean tube. 0.1 mL of lanthanum tracer La 3+ (1 mg/mL), 1 mL TiCl3 and 1 mL concentrated HF were added in the uranium fraction. Each of the samples was left for a minimum of 30 minutes in an ice bath. An Eichrom polypropylene filter disk of diameter 25mm and size 0.1μ size was placed on a funnel for vacuum filtration after which it was rinsed with deionised water and 10 mL of LaF3 suspension of 0.2 mg/mL. 5 The sample was passed through the filter paper and filtrated. 2 The tube was rinsed twice with 5 mL 0.58 M HF, deionised water and filtrated. Each filter paper was then removed from the funnel with a pair of tweezers to avoid curling, dried under an IR lamp and stuck onto aluminium disk for counting.

Fig. 2. Separation column system using ion-exchange. 6
Counting: The measurements were done using 450 mm 2 active surface Passivated Implanted Planar Silicon (PIPS) semiconductor detectors installed in the 12-chamber Alpha Analyst System (Canberra). The measurements were carried out at a source to detector distance of about 5 cm. The accumulation and analysis of Alpha spectra was done using Genie 2000 software with measurement time of about 144,000s. The calibration of the detectors was made with a standard radionuclide source containing 238 U, 234 U, and 239 Pu, 241 Am. The yielded Alpha counting efficiencies are shown in Table 1 . 3. Samples prepared using microprecipitation procedure ready for counting. Fig. 4. Samples prepared using electrodeposition procedure ready for counting.

Results and discussion
After counting for about 144,000s to obtain enough counts for both the background and the samples, the activity concentrations of all the samples were obtained. The activity concentrations of the Tap Water, Mine Water, Spinach and Soil samples are shown in the graphs below: The results show the activity concentrations of the mine water samples being greater than the activity concentrations of the tap water samples as predicted. In the first set of samples (MW), 238 U and 234 U have similar activity concentrations with 235 U having the lowest as expected. In the second set of samples (TW) the activity concentration of 238 U is almost half of that of 234 U with 235 U having the lowest concentration.
The results of the second set of data show the activity concentrations of the soil samples being greater than the activity concentrations of the spinach samples as predicted. With regards to the activity concentrations of the soil samples, 238 U and 234 U have similar activity concentrations with 235 U having the lowest. However, the activity concentration of 234 U in the spinach samples is almost three times that of 238 U with 235 U having the lowest concentration.
The radiochemical recovery of the individual sample (see Table 1) is defined as where: Y(%) = radiochemical recovery P( 232 U) = net count for 232 U m( 232 U) = net count for 232 U A( 232 U) = current tracer activity = counting efficiency for tracer Good radiochemical recoveries were obtained for this particular method with average values of 74.4% (MW), 59.5% (TW), 64.7% (SP) and 77.4% (SL). It is well-known that soil samples are often considered to be one of the most difficult matrices to digest. The Soil average radiochemical recovery of 77.4% being the highest indicates that both the digestion and the source preparation procedures were very effective and points to the fact that this method also works for samples with matrices that are tough to digest.

Conclusion
This work presents two of the most effective ways of source preparation; micro co-precipitation technique with LaF3 following column separation using UTEVA resin for two types of water samples; tap water and waste water from the mine, and electrodeposition for spinach and soil samples. The results obtained show that the method allows for a fast and efficient determination of uranium activity concentrations in all of the samples analyzed.