DETERMINATION OF NATURAL RADIONUCLIDES IN WATER FROM SLOVAKIA USING LSC

DETERMINATION OF NATURAL RADIONUCLIDES IN WATER FROM SLOVAKIA USING LSC Alena Belanová 1 Jana Merešová Marta Vršková Water Research Institute, Nábr. L. Svobodu 5, 812 49 Bratislava, Slovakia. ABSTRACT. Freshwater sources must be monitored from a radiological point of view. Usually, for the routine analysis of radioactivity contamination, screening procedures for gross alpha and gross beta activity determination are conducted along with activity concentration measurements of 222 Rn. In Slovakia, the drinking water originates mostly from underground sources; therefore, increased levels of natural radionuclides derived from uranium and thorium decay chains are expected. In the radiochemical laboratory of the Water Research Institute in Bratislava, liquid scintillation counting (LSC) is used for the determination of 226 Ra, 210 Po, 210 Pb, and 222 Rn activity concentrations in water samples. Moreover, since 2 nuclear power plants are operating in Slovakia, routine control of 3 H in surface water is required. This paper describes separation techniques and sample treatment procedures preceding the LSC measurement. INTRODUCTION Most of the drinking water in Slovakia originates from underground sources. Furthermore, the trend of consumption of natural mineral waters, which serve as substitutes for tap water, has increased (Vršková et al. 2004). The chemistry of mineral waters, relationships with geological structure, and the CO 2 and H 2 S contents control the volume activities of radionuclides (Daniel et al. 1996). The underground and mineral waters (total mineralization ranging from 1000 to 4000 mg/dm 3 ) are characterized by elevated mineralization; hence, higher concentrations of terrestrial radionuclides are expected. For this reason, it is necessary to monitor radioactivity of underground water, mineral water, and thermal springs. Gross alpha and gross beta activities are recommended as screening methods in regulation 528/2007 of the Ministry of Public Health of Slovak Republic (2007). If the gross alpha or gross beta activity in drinking water exceeds 0.2 Bq/L and 0.5 Bq/L, respectively, further analysis for specific radionuclides is required. Particularly, if the gross alpha activity is exceeded, then the activity concentration of 226 Ra has to be measured. In the case of alpha activity, if after subtraction of 226 Ra it still exceeds the limit, the concentration of U nat then has to be determined. Other radionuclides from the uranium and thorium decay chains must also be considered. Particularly, isotopes of radium, 210 Pb, and 210 Po pose potential risks to humans for internal exposure by ingestion or inhalation. 210 Pb and 210 Po have carcinogenic effects with respect to lung cancer (UNSCEAR 1988). According to the recommendation of the International Commission on Radiological Protection (ICRP 1980), the highest permissible level of annual intake of 210 Pb is 2 10 4 Bq. The ICRP in 1966 reevaluated the doses from ingestion and inhalation to the public and according to this assessment the radiotoxicity of 228 Ra increased. 228 Ra belongs to a group of the most toxic radionuclides; thus, in the Regulation for Drinking-water Quality of the World Health Organization (WHO 2004) the recommended reference value for 228 Ra was lowered to 0.1 Bq/L. Liquid scintillation counting (LSC) is used in the radiochemical laboratory of the Water Research Institute in Bratislava for the determination of several radionuclides in water samples. The main advantage of the LSC method is full automation of the measurement. After adjusting the measurement parameters and putting the sample vials into the machine, there is no need of other interven- 1 Corresponding author. Email: belanova@vuvh.sk. 2009 by the Arizona Board of Regents on behalf of the University of Arizona LSC 2008, Advances in Liquid Scintillation Spectrometry edited by J Eikenberg, M Jäggi, H Beer, H Baehrle, p 71 76 71

72 A Belanová et al. tion. Another advantage of LSC is the simultaneous determination of alpha and beta particles (Cha upnik and Lebecka 1993; Wallner 2001). METHODS All measurements were conducted on a Tri-Carb 2900TR liquid scintillation spectrometer using the analytical software QuantaSmart for spectra analysis. Glass vials were used throughout. 222 Rn Analysis Since radon is an inert gas, special precautions have to be made during sampling to avoid losses. Water for analysis of 222 Rn is sampled into 250-mL glass bottles at a slow flow. It is important to fill the bottle fully with water to avoid creating air bubbles where the 222 Rn could diffuse from the sample. The Slovak Technical Standard 75 7615 is used for 222 Rn determination. Seventeen ml of water is directly mixed with 5 ml of the scintillation cocktail Opti-Fluor O in a measuring vial. After 3 hr, 222 Rn and its short-lived daughter products in the vial reach radioactive equilibrium and the sample is measured. Alpha particles of decay products are detected with the same probability as the alpha particles emitted from 222 Rn; therefore, the detection efficiency is about 300%. The energy window for the analysis is set to 18.6 1710 kev and individual samples are measured for 30 min. A certified standard solution of 226 Ra is used for determination of the detection efficiency; the detection limit is 1.5 Bq/L. 226 Ra Analysis The method for separation of radium is described in the Slovak Technical Standard 75 7622. Ba and Pb carriers are added to l-l water samples and the ph is adjusted using the ammonia to methylorange indicator. The sample is heated to boiling and H 2 SO 4 solution (1:1) is dropped in until the color changes. Thereafter, 2 more drops of H 2 SO 4 are added to completely precipitate radium with the Ba sulfate. After several hours (at least 4 hr), the supernatant is decanted and centrifuged. The precipitate is rinsed with water until the ph reaches 7, and ammonia solution of EDTA is added. The sample is then heated in a water bath, stirring until the precipitate is dissolved. After self-cooling of the solution, the Ba and Ra sulfates are precipitated using glacial acetic acid to bromine-cresol green indicator and the precipitate is centrifuged. The date and time of the precipitation must be noted. The sulfate precipitate is transferred into a scintillation vial with 10 ml of distilled water and mixed with the scintillation cocktail. The conditions for 226 Ra measurement by LSC were optimized. Three types of LSC cocktails (Ultima Gold LLT, Opti-Fluor O, and Insta-Gel ) were compared. Ultima Gold LLT showed the best performance values. The sample was mixed with the scintillation cocktail at a volume ratio of 10:10. Samples were measured after 18 days following Ra separation in order to eliminate the contribution of 224 Ra from the thorium decay chain. The counting time was 30 min, impulses were collected in the energy window 0 2000 kev, and the alpha and beta discrimination mode applied. The detection limit for 226 Ra is 0.006 Bq/L. For 226 Ra, the daughter isotopes 222 Rn, 218 Po, and 214 Po also emit alpha particles. Therefore, the counting efficiency of 226 Ra depends on the degree of equilibrium with its daughters and can reach up to 400% if secular equilibrium is attained. The effect of daughter products is usually eliminated by use of correction coefficients. In our procedure, the effect is eliminated by using a calibration curve prepared and measured at the same conditions as the sample (time between separation and

Determination of Natural Radionuclides in Water from Slovakia 73 measurement is the same). The comparison of results for 226 Ra measured by LSC and by the classical method STS 75 7622 showed good agreement. This method may be used for the simultaneous determination of all radioisotopes of radium ( 226 Ra, 228 Ra, and 224 Ra) as published by Cha upnik and Lebecka (1993). However, the measuring conditions (energy window, time of measurement) have to be optimized. 210 Pb Analysis LSC is rarely used for the determination of environmental 210 Pb because of the low environmental 210 Pb activities (Lebecka and Cha upnik 1990). Using a low-background LS spectrometer and an appropriate radiochemical sample preconcentration can solve this problem. The solid-phase extraction procedure in combination with LSC measurement is used in our laboratory (Merešová et al. 2006). Two L are acidified with 40 ml of buffer solution acetate to a ph of 5.5. Afterwards, 0.2 ml of 2 mg/l carrier Pb(NO 3 ) 2 solution are added. Six different types of cation exchangers and organic sorbents were compared for the ability to remove Pb 2+ ions from the complex sample matrices. The yield of Pb 2+ was found to be strongly dependent on the type of ion exchanger and markedly influenced by the ph of the sample and the sorbent volume/weight, as well. The ion-exchanger Dowex 50WX2-1003 was selected because of its high yield (94.5% for 3 ml of ion-exchanger volume) and its fast sample flow. Subsequently, the ion-exchanger with captured lead is rinsed with 3M HNO 3 to concentrate into a 25-mL volumetric flask. Finally, 10 ml of the concentrate is transferred into a vial and 10 ml of liquid scintillator Ultima Gold LLT is added. Two other types of commercial scintillators, Opti-Fluor O and Insta-Gel, were tested as well. Both Ultima Gold LLT and Insta-Gel showed good qualities for LSC measurement of 210 Pb, but the best performance values were obtained using Ultima Gold LLT. To calculate activity concentration of 210 Pb, a counting window of 3 20 kev was selected and the background counts were considered for proper activity concentration determination. By setting the measurement time to 600 min, a detection limit of 0.013 Bq/L could be obtained. 210 Po Analysis Prior to other treatment, the sample has to be deprived of 222 Rn by intensive aeration for about 60 min. After aeration, the ph is adjusted to 2.0 2.2. Then, 500 ml of the sample is mixed with 350 mg of ZnS(Ag) scintillator. The sample is consecutively filtrated through a blue ribbon filter. The procedure described above follows the Czech Technical Standard 75 7626. The process was further modified for the purpose of LSC measurement. The ZnS(Ag) scintillator with adsorbed polonium is transferred with 10 ml of HCl (1:1) into a 50-mL beaker, covered, and heated until the scintillator is completely dissolved. The sample is then evaporated to near dryness. The residue is transferred with 10 ml of distilled water into the measuring vial and 10 ml of scintillator Ultima Gold LLT is added. The cocktail Ultima Gold LLT was chosen from 6 available scintillators. The measurement has to be completed within 2 5 days after the separation. The energy window 100 300 kev is used for the activity calculations, and the time of measurement is 30 min. The detection limit is about 0.02 Bq/L.

74 A Belanová et al. U nat Analysis In our laboratory, mass concentration of U nat is usually determined spectrophotometrically according to the Slovak Technical Standard 75 7614; the LSC method is also applied. From a 0.5-L sample, uranium is preconcentrated and isolated from accompanying elements via sorption on silica gel with high porosity (10 ml). After elution with 40 ml of acetic acid, the aliquot part of the solution is transferred into a vial together with the scintillation cocktail Insta-Gel and measured. The conditions of measurement of U nat were optimized from Vršková et al. (2004). Three types of LSC cocktails were compared: Ultima Gold LLT, Opti-Fluor O, and Insta-Gel. Insta-Gel proved to be most suitable for the U nat measurement. In the 20-mL vials, a 10:10 ratio of sample to cocktail is taken and the samples are measured for 30 min each. LSC measurement of U nat is based on the detection of alpha particles with energies ranging from 4.15 to 4.78 MeV; therefore, a wider counting window from 160 to 450 kev is selected for analysis of uranium via LSC. The detection limit of this method is 0.07 Bq/L, corresponding to a mass concentration of 6.0 μg/l. The comparison of U nat mass concentrations determined by LSC and by spectrophotometry showed good agreement. 3 H Analysis No special sampling procedures for the determination of 3 H activity concentration in water are necessary. However, precautions should be taken to avoid contamination from the tritium source. Prior to measurement, the sample is distilled. Ten ml of distilled sample is transferred into a vial together with 10 ml of scintillation cocktail Ultima Gold LLT and mixed for about 1 2 min (ISO 9698). The energy window used for activity calculation is set from 0 to 18.6 kev and the background subtraction is considered. The water from a tritium-free underground thermal spring is used as a blank sample for the background evaluation. The detection limit varies from 1.8 to 3.1 Bq/L according to the time of measurement (750 270 min). The underground water samples are electrolytically enriched; hence, a detection limit of about 0.2 Bq/L is attained. Such a low detection level is not necessary for monitoring purposes, but is crucial for hydrological studies where 3 H is used as a valuable tracer. RESULTS We have presented the methods used in our laboratory for the determination of several radionuclides in water samples. The objects of our interest were particularly the mineral waters with elevated mineralization because of a potential higher level of radionuclide content, compared to those of drinking water. Eight bottled mineral waters widely accessible in the Slovak market were used for the study. The results of our research are presented in Table 1. Most of these mineral waters are carbonated; therefore, the 222 Rn concentrations were below the detection limit (1.5 Bq/L) in all samples. Also, the concentration of 3 H in all studied mineral waters was under the detection limit (3.1 Bq/L). This result was expected since the underground springs are not or very little impacted by the cosmogenic and anthropogenic sources of 3 H. Activity concentrations in the analyzed samples did not exceed the limit concentrations for natural mineral water, nor for tap water. In the mineral water of»erínska, the activity concentration of 226 Ra was slightly elevated. Regarding these results, one can assume that substituting tap water with mineral water may not cause potential health hazards. However, the highest permissible levels for these natural radionuclides in mineral water are calculated from the total effective dose of 0.1 msv/yr assuming that the daily intake of mineral water is only 0.2 L for an adult (CD 98/83/EC).

Determination of Natural Radionuclides in Water from Slovakia 75 Table 1 Activity concentrations of radionuclides in bottled mineral waters in Slovakia measured 2001 2007 using LSC ( means not measured). 226 Ra (Bq/L) 210 Pb (Bq/L) 210 Po (Bq/L) U nat (μg/l) Budiš 0.18 ± 0.06 0.035 ± 0.007 <0.02 <6.0»erínska 0.44 ± 0.13 8.1 ± 2.4 Fatra 0.01 ± 0.01 <6.0 Kláštorná 0.09 ± 0.04 0.021 ± 0.004 <0.02 6.6 ± 1.9 Korytnica 0.03 ± 0.01 0.038 ± 0.008 <6.0 Mitická 0.02 ± 0.01 <0.02 6.5 ± 1.9 Salvator 0.26 ± 0.08 7.0 ± 2.0 Slatina 0.14 ± 0.06 0.042 ± 0.008 <0.02 Mineral a 1.9 0.8 0.5 Tap a 0.6 0.3 0.2 a Calculated highest permissible levels for radionuclides in distributed natural mineral water and drinking tap water specified by the Ministry of Public Health of Slovak Republic regulation 528/2007. CONCLUSIONS In the future, we would like to integrate LSC measurement widely into the standard operational procedures of our laboratory. At present, we use a proportional counter for gross alpha and beta measurements, but for high mineralization, LSC measurement is more appropriate (ASTM D 7283-06; ISO/CD 11704). The described method for 226 Ra determination has high potential, since it may be used for the simultaneous determination of all radium isotopes ( 226 Ra, 228 Ra, and 224 Ra) as noted by Cha upnik and Lebecka (1993). However, the measuring conditions (energy window, time of measurement) have to be optimized. REFERENCES American Society for Testing and Materials (ASTM). D 7283 06 standard test method for alpha- and beta- activity in water by liquid scintillation counting. West Conshohocken, PA, USA: ASTM Intl. Cha upnik S, Lebecka JM. 1993. Determination of 226 Ra, 228 Ra and 224 Ra in water and aqueous solutions by liquid scintillation counting. In: Noakes JE, Schönhofer F, Polach HA. Liquid Scintillation Spectrometry 1992. Tucson: Radiocarbon. p 397 403. Council of the European Union. 1998. Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. Czech Technical Standard 75 7626. Water quality. Determination of polonium 210. Daniel J, LuËivjanský L, Stercz M. 1996. Natural rock radioactivity geochemical atlas of Slovakia. Geological Survey of Slovak Republic, Bratislava, Slovakia. Guidelines for Drinking-water Quality. 2004. 3rd Edition, Volume 1. Recommendations. Geneva: World Health Organization (WHO). Available at http:// www.who.int/water_ sanitation_health/dwq/gdwq3/ en/. International Commission on Radiological Protection (ICRP). 1980. Limits of Intakes of Radionuclides by Workers. Publication 30. Oxford. Pergamon Press. ISO/CD 11704 Water quality. Measurement of gross alpha and beta activity concentration in non-saline water. Liquid scintillation counting method. ISO 9698 Water quality. Determination of tritium activity concentration. Liquid scintillation counting method. Lebecka J, Cha upnik S. 1990. Method of determination on 210 Pb by liquid scintillation technique. In: Proceedings of the Rare Nuclear Processes Conference. Bratislava, Slovakia, 1990. Merešová J, Vršková M, SedláËková K, JakubËová Z. 2006. Pre-concentration and determination of 210 Pb in water by liquid scintillation spectrometry. Proceedings of International Conference on Isotopes in Environmental Studies Aquatic Forum. Monaco, 25 29 October 2004. Monaco: International Atomic Energy Agency. Regulation of the Ministry of Public Health of Slovak Republic. 2007. Num. 528/2007, which determines details on the requirements on limitation of irradiation from natural ionising radiation. Slovak Technical Standard 75 7614. Water quality. Determination of uranium. Slovak Technical Standard 75 7615. Water quality. Determination of radon 222. Slovak Technical Standard 75 7622. Water quality. Determination of radium 226. United Nations Scientific Committee on the Effects of

76 A Belanová et al. Atomic Radiation (UNSCEAR). 1988. Source, Effects and Risks of Ionizing Radiation. Report on General Assembly with Annexes. New York: United Nations. Vršková M, Merešová J, Kassai Z. 2004. Determination of 226 Ra and U nat in mineral waters using liquid scintillation spectrometry. Proceedings of Workshop LiquiScint 2004-LSC in Radiochemistry and Environmental Sciences [CD-ROM]. 17 19 May 2004, Prague, Czech Republic. Prague: Czech Chemical Society. Wallner G. 2001. Determination of 228 Ra, 226 Ra and 210 Pb in drinking water using liquid scintillation counting. In: Möbius S, Noakes J, Schönhofer F. Liquid Scintillation Spectrometry 2001. Tucson: Radiocarbon. p 269 74