Anal. Chem. 1998, 70, 2353-2359
Determination of 226Ra in Mineral Drinking Waters by r Liquid Scintillation with Rejection of β-γ Emitters Jean Aupiais,* Corinne Fayolle, Patricia Gilbert, and Nicolas Dacheux
De´ partement Analyse Surveillance Environnement, CEA, BP 12, 91680 Bruye` res-le-Chaˆ tel, France
The radiotoxicity of radium isotopes (especially the longhalf-life 226Ra) requires their monitoring in drinking waters or nuclear wastes. We studied the applicability of the PERALS method of detection (photon electron rejecting r liquid scintillation) for radium measurement. This method combines r liquid scintillation with pulse shape analysis for β rejection and specific chemical extractants included in the scintillating cocktail. Radium is separated by an extractive-scintillator cocktail called RADAEX containing 2-methyl-2-heptylnonanoic acid (HMHN) and dicyclohexano-21-crown-7 (Cy221C7) as extractant molecules. The variation of the radium extraction has been studied relative to pH, salt concentrations, anion and cation effects, and the volume ratio between aqueous and organic phases. The main parameter affecting the radium extraction in mineral drinking water is its complexation by inorganic anions, especially sulfate. Due to the lack of thermodynamic data, some complexation constants had to be determined. For instance, the value reported in this paper for radium sulfate (log β ) 2.58 ( 0.22) is in good agreement with that from the literature. The knowledge of complexation constants allows the determination of radium extraction recovery for any solution when the inorganic anion concentrations had been measured by capillary zone electrophoresis. The detection limit for this technique is found to be equal to 0.006 Bq‚L-1 using only 6 mL of sample solution for analysis. Several French mineral waters have been studied and the results compared with determinations of uranium and thorium concentrations by ICPMS and time-resolved laser induced fluorescence (TRLIF). Radium has been a major concern in the environment since its discovery in 1898. The first paper dealing with radium in drinking water appeared in France in 1904. Laborde, an assistant of Curie, thought that the curative powers of mineral waters could be explained by the presence of radium in the sources.1 Indeed, the concentrations of major ions had already been measured, and there was no agreed upon explanation for the benefit of hydrotherapy. Nowadays, the effects of radium isotopes are known to be harmful and this element is considered as the most radiotoxic (1) Laborde, A. Radium 1904, 1, 1-6. S0003-2700(97)01246-8 CCC: $15.00 Published on Web 04/28/1998
© 1998 American Chemical Society
nuclide. Its measurement in drinking water is mandatory and leads to careful monitoring.2,3 Many countries require very low permissible concentrations for ingestion in drinking water.2-5 In France, the annual limit of incorporation by ingestion is 7 × 103 Bq‚year-1.2,3 Due to its low concentration in drinking water, methods required for the activity measurements have to be sensitive and reliable. Many authors have used (or use) welltested analytical methods such as R spectrometry,6-11 emanation methods involving 222Rn daughter measurements,12-15 thermal ionization mass spectrometry (TIMS),16,17 liquid scintillation counting,18-21 γ spectrometry,22 Cerenkov radiation measurements,23 and of course R liquid scintillation with rejection of β emitters.24,25 Whatever the detection method used, two major difficulties occur in the radium determination: the sample preparation and the actual measurement. The latter has been extensively studied, and all techniques described above are now very sensitive. But at low radium concentrations, a preconcentration technique is often required. Coprecipitation with barium sulfate (or derived (2) Remy, M. L.; Lemaitre, N. Hydroge´ ologie 1990, 4, 267-278. (3) Delacroix, D.; Guerre, J. P.; Leblanc, P. Radionucle´ ides & Radioprotection; Centres d’e´tudes de Saclay: Saclay, France, 1994. (4) Milvy, P.; Cothern, R. Environ. Geochem. Health 1989, 11 (2), 63-72. (5) Shabana, E. I.; Al-Hobaib, A. S.; Farouk, M. A. Radiochim. Acta 1996, 75, 33-35. (6) Surbeck, H. Sci. Total Environ. 1995, 173/174, 91-99. (7) Erickson, M. D.; Aldsdadt, J. H.; Alvarado, J. S.; Crain, J. S.; Orlandini, K. A.; Smith, L. L. J. Hazard. Mater. 1995, 41, 351-358. (8) Sill, C. W. Nucl. Chem. Waste Manage. 1987, 7, 239-256. (9) Lim, T. P.; Dave, N. K. CIM Bull. 1981, 74, 97-108. (10) Percival, R. D.; Martin, D. B. Anal. Chem. 1974, 46, 1472-1479. (11) Koide, M.; Brulan, K. W. Anal. Chim. Acta 1975, 75, 1-19. (12) Moore, W. S.; Arnold, R. J. Geophys. Res. 1996, 101, 1321-1329. (13) Smith, M. R.; Lautensleger, A. W.; Laul, J. C. J. Radioanal. Nucl. Chem. 1988, 123, 107-119. (14) Gascoyne, M. Appl. Geochem. 1989, 4, 577-591. (15) Noguchi, M.; Wakita, H. Geochem. J. 1973, 7, 81-88. (16) Cohen, A. S.; O’Nions, R. K. Anal. Chem. 1991, 63, 2705-2708. (17) Volpe, A. M.; Olivares, J. A.; Murrel, M. T. Anal. Chem. 1991, 63, 913916. (18) Zhu, Y. J.; Yang, D. Z. J. Radioanal. Nucl. Chem. 1995, 194, 173-175. (19) Salonen, L. Sci. Total Environ. 1993, 130/131, 23-35. (20) Higuchi, H.; Vesugi, M.; Satoh, K.; Ghashi, N.; Noguchi, M. Anal. Chem. 1984, 56, 761-763. (21) Escobar, V. G.; Tome´, F. V.; Lozano, J. C.; Sanchez, A. M. Appl. Radiat. Isot. 1996, 47, 861-867. (22) Von Gunten, H. R.; Surbeck, H.; Ro¨ssler, E. Environ. Sci. Technol. 1996, 30, 1268-1274. (23) Al-Masri, M. S.; Blackburn, R. Sci. Total Environ. 1995, 173/174, 53-59. (24) Case, G. N.; McDowell, W. J. Radioact. Radiochem. 1990, 1, 58-69. (25) Burnett, W. C.; Tai, W. C. Anal. Chem. 1992, 64, 1691-1697.
Analytical Chemistry, Vol. 70, No. 11, June 1, 1998 2353
methods) and adsorption on manganese dioxide are the main methods used for concentrating the radium isotopes. The first one leads to poor resolution in R spectrometry. Lim et al.9 optimized the resolution as a function of the barium carrier. Other ways such as electrodeposition after radiochemical separation11 or metal adsorption onto thin layers (disk impregnated with MnO2)6 have also been studied. The methods of radium adsorption onto manganese dioxide are very efficient and have been optimized.26,27 But all require time-consuming radiochemical separations prior measurement. The radon emanation method is sensitive, but prior equilibration between 226Ra and 222Rn is necessary, which requires 20 days. Some authors have proposed original techniques such as the use of thermal ionization mass spectrometry (TIMS) to obtain a higher sensitivity (in the femtogram range). But this also demands very efficient radiochemical separations. In particular, barium has to be removed because its ionization potential is similar to that of radium and tends to suppress its thermal ionization.17 Furthermore, TIMS is expensive and requires considerable maintenance, which increases the analysis cost. And, last, the works of McDowell et al.28 have led to the application of radium-selective crown ethers for direct measurement by R liquid scintillation with rejection of β emitters. The main advantage of this method is its combination of a specific extractant for a given cation with the scintillant cocktail. But this technique as it is often applied with the usual preconcentration methods (such as coprecipitation, adsorption, ion exchange, and so on) is time consuming.24,25 It is important to keep in mind the advantages of such a technique like the high selectivity and low detection limit.25,29,30 So we have studied radium extraction with no prior chemical separation other than a pH adjustment, and we have determined its performance with this constraint. In conclusion, all methods previously described require sample preparation time and are not as convenient for effective monitoring. We propose to show the utility of the PERALS method of detection in combining fast sample preparation with good sensitivity. This technique is able to discriminate β scintillation from R scintillation with a β rejection rate equal to 99.95%. The very low background and R efficiency near 100% allow the measurement of R emitters at ultratrace levels (up to 3.7 × 10-5 Bq‚L-1).29,30 As described elsewhere,29 the R liquid scintillation associates an organic scintillant cocktail and a specific extractant. In the case of radium, the extractive-scintillator cocktail called RADAEX contains two molecules, 2-methyl-2-heptylnonanoic acid (HMHN) and dicyclohexano-21-crown-7 (Cy221C7), as extractants. This cocktail has been described in detail by McDowell.28,31 We have developed a method for the determination of 226Ra in 14 mineral drinking waters, and we have compared these results with the uranium concentrations measured by several methods (ICPMS and TRLIF). It appears that activity measurements down to a few (26) Moore, S. W.; Reid, D. F. J. Geophys. Res. 1973, 78, 8880-8886. (27) Crespo, M. T.; Gasco´n, J. L.; Acen ˜a, M. L. Sci. Total Environ. 1993, 130/ 131, 383-391. (28) McDowell, W. J.; Moyer, B. A.; Case, G. N.; Case, F. I. Solvent Extr. Ion Exch. 1986, 4, 217-236. (29) Dacheux, N.; Aupiais, J. Anal. Chem. 1997, 69, 2275-2282. (30) Aupiais, J. J. Radioanal. Nucl. Chem. 1997, 218, 201-207. (31) McDowell, W. J.; Arndsten, B. A.; Case, G. N. Solvent Extr. Ion Exch. 1989, 7, 377-393.
2354 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
mBq‚L-1 can be performed and provide effective monitoring of this element in mineral drinking waters. EXPERIMENTAL SECTION The R liquid scintillation spectrometer was supplied by Ordela Inc. The extractive-scintillator cocktail RADAEX is marketed by ETRAC. All other reagents such as NaOH, NH4OH, and ammonium salts (NH4Cl, NH4NO3, (NH4)2CO3, (NH4)2HPO4, (NH4)2SO4) were of analytical grade and were purchased from Prolabo, Aldrich, and Merck. The radium solution came from Curie’s stock. It was stored in 1 M HCl and was standardized by its γ-ray emission at 186 keV with an accuracy better than 3%. Activity: 760 Bq/mL. For each experiment in which it was added, only 50 µL of radium solution was used in order to minimize the effects of the acidity and the chloride concentration. All extractions were carried out according to the following procedure: The pH of the aqueous solution (6 mL) was first adjusted to the desired value. The extractive-scintillator cocktail (1.2 mL) was then mixed gently for 5 min in a rotative mixer (Turbula). Separation of the organic and aqueous phases was performed by centrifugation at 2000 rpm in a Jouan G 4.11 centrifuge for 10 min. The organic phase was taken off and 1 mL transferred into a 10 × 75 mm culture tube. The aliquot was sparged for 5 min with toluene-saturated argon (purity 99.9999%), and finally the tube was hermetically sealed before counting in the PERALS spectrometer. THEORETICAL SECTION Detection Limit. The detection limit is defined by the following equation,32 taking into account R and β risks equal to 2.5%:
4 γd ) 2(∆c)s ) (1 + x1 + 2Bt) t
(1)
where (∆c)s is the threshold of detection, t the counting time, and B the number of background counts per unit time. The detection limit has been found to be equal to 6 mBq‚L-1 for the pure radium 226Ra solution. Radium Extraction. The equation linking the recovery yield P, the distribution coefficient D, and the volume ratio V h /V (V h) volume of organic phase, V ) volume of aqueous solution)29,30 is given by
P)
D(V h /V) × 100 1 + D(V h /V)
(2)
This equation only takes into account extraction of a cation by a soluble organic extractant. But, in fact, many complexant ions in aqueous solution can prevent extraction of radium. So let us consider a solution containing radium and a complexant anion such as SO42- or NO3- etc. The reaction of complexation can be described as follows: (32) Neuilly, M. Statistique applique´ e a` l’exploitation des mesures; Masson: Paris, 1986; pp 113-117.
Ra2+ + An- T RaA(2-n)+ K ) [RaA(2-n)+]/[Ra2+][An-]
P) (3)
D(V h /V) D(V h /V) + 1 +
∑
× 100
(10)
Ki[Ain-]
i
The recovery yield P is defined by
P)
A h Ra2+ (ARa2+)initial
× 100 )
A h × 100 A h + Afree + Acomplex
RESULTS AND DISCUSSION
(4)
where A h is the absolute activity in the organic phase, Afree is the absolute activity of noncomplexed radium in the aqueous phase, and Acomplex is the absolute activity of radium in complexed species in the aqueous phase. Introducing the distribution coefficient D and the volume ratio V h /V, eq 4 is modified as follows:
P)
D(V h /V) × 100 D(V h /V) + 1 + Acomplex/Afree
(5)
Combining eqs 3 and 5 and taking into account that the volume activity is proportional to the concentration, we can write
P)
D(V h /V) D(V h /V) + 1 + K[An-]
× 100
(6)
Note that the radium extraction, keeping D and K constant, is determined by the concentration of the complexing anion. But the behavior of P is not as easy to predict since K depends on the ionic strength. Moreover, several complexant anions can be simultaneously present in solution. In this case, we apply the ionic specific interaction theory. According to this theory, the variation of K can be written as33,34
log K ) log K0 + log γRa2+ + log γAn- - log γRaA(2-n)+ (7) The activity coefficient γj of an ion j of charge zj in a solution of ionic strength Im may be described as follows:
log γj ) -zj2DDH +
∑ k
j,k,Immk
(8)
With j,k,Im the ion interaction coefficient, mk the molality of all ions k present in solution, and DDH, according to the nomenclature, the Debye-Hu¨ckel term, which is expressed at 20 °C (temperature of experiments) as
DDH )
0.5050xIm 1 + 1.5xIm
(9)
In case of i simple complexant anions in solution, eq 6 is altered as follows: (33) Ciavatta, L. Ann. Chim. (Rome) 1980, 70, 551-567. (34) Grenthe, I.; Wanner, H. Guidelines for the extrapolation to zero ionic strength; OECD Nuclear Energy Agency Data Bank: Gif-sur-Yvette, France, 1992, pp 2-24.
Optimum pH Extraction. We have studied the optimum pH range for the radium extraction. Knowing that radium is an alkali earth, it was assumed that a competition for the extraction would probably occur between radium and alkali metals. So two common reagents (in an inorganic analytical laboratory) have been used: sodium hydroxide and ammonium hydroxide. Results are shown in Figure 1. It clearly appears that optimum pH is above 10 and that sodium ions strongly compete for complexant ions with radium. The extraction yield (eq 2) dramatically decreases if sodium hydoxide (12%) is used instead of ammonia (84%), with the same pH behavior observed for both curves. This behavior is not due to the Cy221C7 because its selectivity for sodium is low.35 Actually the sodium cationic radius (rNa ) 1.02 Å) is too small for the Cy221C7 cavity (r ) 1.70 Å). So even at a maximum concentration of 10-3 M in Na+ at pH 10, the constant for the reaction with the crown ether and sodium is assumed to be very low35 and cannot explain the poor extraction. We suspect that the second extractant (HMHN), which is a carboxylic acid, has a higher affinity for sodium ions. Reactions discussed have been explained in terms of H+/Na+ cation exchange on HMHN31 if RADAEX is used without sodium salt conversion. Otherwise, Ra2+/Na+ cation exchange occurs. Ammonium Salt Effects. The effects of the counterion, e.g. ammonium salts with anions such as chloride, phosphate, sulfate, nitrate, and carbonate, have been also studied. These common ions, found at different concentrations in drinking water, can form complexes with radium and thus alter its extraction by RADAEX. All extraction curves are shown in Figure 2. A decrease in the percentage of recovery yield is observed at high concentration due to complexation. But, depending on the corresponding complexation constant, the decrease in yield can be greater for carbonate, phosphate, or sulfate. The strong decrease with sulfate salts is easily understandable when one takes into account the complexation constant related to the reaction Ra2+ + SO42- T RaSO4, e.g. log K0 ) 2.75.36 But, as shown in the eq 6, the recovery yield can fit to the Figure 2 only if the complexation constant value is calculated with respect to the ionic strength. For instance, the radium extraction in sulfate medium in Figure 2 shows an optimum recovery yield up to 0.1 M, which is not easily explainable without an ionic strength correction. It can be explained by a strong reduction of complexation constant K with respect to the ionic strength. As example, let us consider the complexation of radium by sulfate anions. The activity coefficients are (35) Izatt, R. M.; Bradshaw, J. S.; Nielsen, S. A.; Lamb, J. D.; Christensen, J. J. Chem. Rev. 1985, 85, 271-339. (36) Langmuir, D.; Riese, A. C. Geochim. Cosmochim. Acta 1985, 49, 15931601.
Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
2355
Table 1. Determination of K by the Specific Interaction Theory [An-] (m) An- ) SO424.127 1.784 0.842 0.410 An- ) NO35.005 3.531 2.223 1.054 0.101
Figure 1. Variation of the recovery yield for radium by adjustment of pH with NH4OH (2) and NaOH (b).
K
σK
log K + nDDH
σlogK+nDDH
44.95 21.30 7.96 8.82
3.23 1.63 0.83 1.19
3.92a 3.42a 2.80a 2.63a
0.12 0.10 0.09 0.13
log K0 ) 2.58 ( 0.22; ∆ ) 0.35 ( 0.11 41.06 12.57 5.77 2.13 0.91
4.62 1.44 0.70 0.41 2.52
2.65b 2.09b 1.69b 1.14b 0.39b
0.05 0.05 0.05 0.08 1.21
log K0 ) 0.83 ( 0.20; ∆ ) 0.37 ( 0.07 An- ) HPO423.896 28.05 3.17 3.70c 1.768 13.80 1.61 3.23c 0.839 11.64 1.46 2.96c 0.408 5.35 1.03 2.41c
0.05 0.05 0.05 0.08
log K0 ) 2.63 ( 0.75; ∆ ) 0.29 ( 0.14 a
n ) 8. b n ) 4. c n ) 8.
The complexation constant K0 has been determined using the specific interaction theory and eq 6. The error is calculated according to the following equation:
σK2 )
( )
( )
(
)
∂K 2 2 ∂K 2 2 ∂K 2 σK + σP + σ[A2-]2 ∂D ∂P ∂[A2-]
(16)
log γRaSO4 ) 0
(11)
We did not use data at low ionic strength (I < 0.1 m) because in the maximum extraction range the propagated error is too high. In fact, this method can be applied if P is lower than P - ∆P. Calculated data are reported in Table 1. We obtain the following results:
log γSO42- ) -4DDH + SO42-,NH4+mSO42-
(12)
log K ) 2.58 ( 0.22
log γRa2+ ) -4DDH + Ra2+,SO42-mSO42-
(13)
∆ ) 0.35 ( 0.11
Figure 2. Variation of P in NH4Cl (b), (NH4)2HPO4 ([), (NH4)2SO4 (1), NH4NO3 (2), (NH4)2CO3 (9) media at pH 10.
Equation 11 is justified because zj ) 0 and RaSO4 ) 0 as defined by the theory.34 No available data in sulfate media are mentioned. According to the results of Ciavatta,33 the coefficients for ammonium and potassium are equal. So we suppose that SO42-,NH4+ ) SO42-,K+ ) -0.06. Moreover, we have only data for hydrochloric and perchloric media. Therefore, we have used experimental data to adjust D, log K, and ∆ values. The ionic strength is
∑c z
Im ) 1/2
2
i i
(14)
In sulfate medium, the ammonium concentration is twice the sulfate. Equation 14 can be written
Im ) 3mSO422356 Analytical Chemistry, Vol. 70, No. 11, June 1, 1998
(15)
The value of the complexation constant is in good agreement with that calculated by Langmuir et al.36 If the SO42-,NH4+ value is valid, then for Ra2+,SO42- we can propose the following value:
Ra2+,SO42- ) 0.41 ( 0.15 The uncertainty of SO42-,NH4+ has been arbitrarily fixed to 0.1. The fitting process using the previous computed data is shown in Figure 3 and is relatively well correlated with experimental data. In contrast, the weak value related to the reaction with chloride, Ra2+ + Cl- T RaCl+ (log K0 ) -0.136), is in good agreement with the results gathered in Figure 2. The same calculation cannot be made because results are too inaccurate and there are only two points available. K and ∆ have been also determined in NH4NO3 and (NH4)2HPO4 media. All data are reported in Table 1. However, due to a lack of accurate data and control parameters, no value is proposed in carbonate medium.
Table 2. Radium Activity Measurements in 14 Drinking Watersa 238U
b
name
extraction yield by RADAEX (%)
Volvic Cristaline Montjoie Aulusb Chantereine Fontestorbes Alet Ax les thermes Evian Badoitb Vittel Plancoe¨tb Contrex La Bourboule-Choussy
74 ( 4 76 ( 4 81 ( 4 73 ( 3 83 ( 4 78 ( 4 79 ( 4 70 ( 4 67 ( 3 77 ( 4 75 ( 4 78 ( 4 65 ( 3 78 ( 4
226Ra
activity (mBq‚L-1) 28 ( 13
