Isolation of lead from water samples and determination of 210Pb

Analytica Chimica Acta 560 (2006) 84–93

Isolation of lead from water samples and determination of 210Pb ˇ Zeljko Grahek ∗ , Martina Roˇzmari´c Maˇcefat, Stipe Luli´c Centre for Marine and Environmental Research, Rudjer Boˇskovi´c Institute, Bijeniˇcka 54, 10000 Zagreb, Croatia Received 21 September 2005; received in revised form 7 December 2005; accepted 26 December 2005 Available online 7 February 2006

Abstract This paper describes the procedures of isolating lead and strontium from the larger volume of seawater and drinking water samples that enable the determination of 210 Pb on gamma spectrometer and 89,90 Sr on liquid scintillation counter. In one procedure, lead is directly isolated from water sample on the column filled with Sr resin by binding of lead on the Sr resin column from 0.2 M HCl in water sample, and successive elution with 0.2 and 8 M HCl. In others, lead and strontium are precipitated from sample with (NH4 )2 CO3 , followed by isolation on an anion exchange column. Lead, strontium and yttrium are bound onto anion exchange column (filled with Amberlite CG-400 in nitrate form) from alcoholic solutions of nitric acid. Lead, Sr and Y are separated from Mg, Ca, K, and other elements by elution with 0.25 M HNO3 in the mixture of ethanol and methanol. After that, strontium and yttrium are separated from lead by elution with 0.25 M HNO3 in the mixture of ethanol and water. The procedure with the Sr resin (direct isolation) is simpler and faster in the phase of isolation on the column in comparison with the procedure with the anion exchanger. The procedure with the anion exchanger, however, makes possible the simultaneous isolation of lead, yttrium and strontium and rapid determination of 89,90 Sr. These procedures were tested by determination of 210 Pb and 89,90 Sr in real sample. Obtained results showed that Pb can be efficiently isolated (with high recovery) from sample and activity of 6 mBq l−1 of 210 Pb and higher can be determined. © 2006 Elsevier B.V. All rights reserved. Keywords: Seawater; Lead; 89,90 Sr; 210 Pb; Rapid determination; Gamma spectrometry

1. Introduction Lead-210 is a naturally occurring radionuclide of the 238 U series. 210 Pb has an important role in human radiation exposure (because 210 Pb deposit in the skeleton long enough and thus highly contributes to skeletal dose [1]). 210 Pb can also be used for studying different environmental and marine processes (tracing atmospheric processes and analyzing the behavior of aerosols in troposphere, determining average erosion rates in soils, understanding sediment chronology, etc. [2–4]). These facts make the determination of this nuclide interesting. 210 Pb emits low energy beta particles (E max = 20 keV 81%, 61 keV 19%) and gamma rays (E = 46.5 keV). Nevertheless 210 Pb can be directly determined by gamma spectrometry [5–8]. In this case there is an important drawback: the high self absorption of soft gamma rays in the sample and the detector and low absolute transition probability of gamma decay (4.05%). Self ∗

Corresponding author. Tel.: +385 1 4561060; fax: +385 1 4680205. ˇ Grahek). E-mail address: [email protected] (Z.

0003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.12.057

absorption depends on sample quantity, composition, density, and distance from the detector. In indirect quantification, 210 Pb is usually determined through its grand-daughter 210 Po by alpha spectrometry or, to a lesser extent, through its beta daughter 210 Bi. This type of 210 Pb determination requires a pre-concentration, chemical separation from the sample and a sufficient in-growth period of 210 Po (or 210 Bi). Due to various possible ways of pre-concentration and separation of lead, bismuth, and polonium from the environmental sample, numerous determination methods have been developed, but they are all based on same basic procedure; after pre-concentration and separation, polonium is deposited onto silver foil and activity is determined by alpha spectrometry or bismuth and/or lead are separated and measured on low level liquid or gas flow counter [9–18]. If the radiochemical equilibrium of 210 Pb-210 Bi-210 Po is attained in the sample, 210 Pb can be determined through bismuth or polonium immediately after its separation from the sample. In contrast, if disequilibrium is expected, determination procedures become time consuming because of waiting for the in-growth period (radioactive

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equilibrium), especially in the determination through 210 Po. Also, the additional step of 210 Bi (or 210 Po) separation from 210 Pb is required. Since analyses of environmental samples usually require the determination of low 210 Pb activities, it is necessary for the analysis, as a rule, to take a large quantity of the sample to provide an activity that can be reliably measured with the available instrument. From that aspect, the determination of 210 Pb lead in seawater and drinking water samples is especially interesting. In fact, the determination of low activities of 210 Pb (but also other ␣- and ␤-emitters) usually requires a large volume of samples. Considering the features of gamma spectrometric determination of low activities of 210 Pb and the impossibility (inefficiency) of direct measurement of large volumes of water, the first aim of this paper is to develop a novel procedure for the simpler and faster determination of 210 Pb in water samples. On the other hand, in the analysis in which treatment of large amount of sample is required the principle of economy (in sense of time and chemicals savings) imposes analyzing as many species (isotopes) in the same sample as possible. Therefore, the second aim of the paper is to show how the same water sample can be used for simultaneous isolation and mutual separations of certain other isotopes, especially for simultaneous separation and rapid determination of 89,90 Sr. Radioactive strontium isotopes are highly radiotoxic fission products (especially 90 Sr with long half life), and as such they were an interesting subject for various kinds of investigations, from their distribution and behavior in natural systems to their influence on human health. Both isotopes are pure ␤-emitters, and this fact places restrictions on the determination procedure, which involves chemical separation of strontium from all other elements and the subsequent detection on gas proportional or liquid scintillation counter. It is well known that Horwitz developed the Sr resin (prepared by sorbing of crown ether, 4,4 (5 )-bis(tert-butylcyclohexano)18-crown-6 dissolved in 1-octanol on inert polymeric support) for the separation of strontium from calcium and many other elements [19]. He also showed how Sr resin can be used for lead separation. The method for the 210 Pb determination in soil samples was developed on the base of his results [18]. Sr resin is very efficient for the separation of strontium from a wide variety of metal ions in nitric acid media. Under appropriate conditions, the resin can be extremely lead-selective. Thus, it is shown how this selectivity makes possible direct isolation of lead from seawater and drinking water samples and subsequently rapid 210 Pb determination on gamma spectrometer. However, due to high selectivity, Sr resin cannot bind yttrium which has a key role in the development of the rapid method for the 89,90 Sr determination. In the rapid method 90 Sr is usually determined through 90 Y and in this case yttrium should be isolated (with strontium) from the sample. Sr resin cannot ensure this isolation from real sample. But in several earlier papers it was shown how a strong base anion exchanger in combination with alcohol solutions of nitric acid can be used to isolate strontium and yttrium from natural samples [20–22]. Therefore, in this paper it is shown how lead can be separated from strontium and yttrium and other elements on the column filled with the anion exchanger and alcoholic solutions of nitric acid, how this method can be

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used for their rapid determination in seawater and drinking water samples, and how this method can be used for isolation of some other isotopes of interest. 2. Experimental On the basis of literature data [18,19,22,23], obtained results and experience, procedures for the isolation of Pb, (Sr and Y also) from drinking and seawater samples and subsequent determination of 210 Pb (and 89,90 Sr) were created. Before that, the possibility of separation of lead from Sr, Y, U, Th, alkaline elements, etc. by mixed solvent anion exchange and extraction chromatography has been examined. In the first step, distribution coefficients of the mentioned elements on strong base anion exchanger Amberlite CG-400 in nitrate form and various alcohol solutions of nitric acid, and distribution coefficients of Pb on Sr resin and acidic solution of seawater were determined. In the second step, optimal conditions for the separation of lead from other elements, and a procedure for its isolation from the real sample were established. In the third step, 210 Pb was quantitatively determined by gamma spectrometry. 2.1. Determination of distribution coefficients and elution curves The distribution coefficients of Pb and other elements and their elution curves for the Amberlite CG-400 (100–200 mesh) anion exchanger in the nitrate form and alcohol solution of nitric acid were determined as explained in prior work [19,23]. Ten milliliters of alcohol solution were equilibrated with 0.2 g of Amberlite CG-400 in nitrate form for 24 h. Before that, Amberlite CG-400 (100–200 mesh) anion exchanger was transferred from chloride to nitrate form by washing of 5 M HNO3 and dried at 80 ◦ C. The distribution coefficient was calculated from the relation: D = [(C0 − C)/C](V/m), where C0 is the initial concentration of cations, C the concentration after equilibrating, V the solution volume, and m is the mass of resin. The concentration of elements in the solution after equilibrating (except U, Th, Pu, Am) was determined by atomic absorption spectrometer (AAS). The concentration of U and Th was determined by spectrophotometer in the presence of Arsenazo III and Thorin. Distribution coefficient of Am was determined by measuring its isotope 241 Am activity before and after equilibrating on the liquid scintillation counter. A glass column with an inner diameter of 1 cm (I.D. = 1 cm), containing 3 or 5 g of Amberlite CG-400 in nitrate form (bed height 8 or 11 cm), was used for the lead separation from other elements. Model systems of the cation solution were obtained by mixing the alcohol solution and the solution of nitrate salts of cations. From this solution, cations were bound to the column. After that, cations were eluted from the column with different alcohol solutions of nitric acid. The cation elution was monitored by AAS and spectrophotometer. A 10 ml fractions were collected. The Sr resin (particles size 100–150 ␮m), HCl and HNO3 solutions were used for the determination of Pb distribution coefficients. For the preparation of seawater solutions of HCl

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and HNO3 , artificial seawater was prepared according to Bowen [24]. Ten milliliters of those solutions were equilibrated with 0.2 g of Sr resin for 24 h. The concentration of lead was determined by atomic absorption spectrometer. A glass column with an inner diameter of 1 cm (I.D. = 1 cm), containing 2 g Sr resin (100–150 ␮m, bed height 8 cm) was used for the separation of strontium from lead by 0.5 M HNO3 elution. 2.2. Lead isolation procedures 2.2.1. Procedure for direct isolation from drinking water and seawater samples by using Sr column Five milligrams of Pb2+ , known activity of 210 Pb, and 20 ml of conc. HCl (11 M) were added to 1 l of drinking water and/or seawater (0.2 M HCl). This solution was loaded on a column (I.D. = 1 cm) filled with 3 g of Sr resin of 100–150 ␮m (bed length 11 cm, 0.34 g resin ml−1 ). Before loading the sample, 30 ml of 0.2 M HCl were passed through the column. The sample was then passed through the column at a flow rate of 2.5 ml min−1 (the maximum for gravitational flow). After passing the sample, 50 ml of 0.2 M HCl were passed through the column. After that, Pb was stripped with 30 ml of 8 M HCl. This volume was transferred to the 100 g detection vessel. Detection was made on a gamma spectrometry system. After the detection, Pb recovery was determined by AAS. For the separation from a larger volume, the following procedure was used: 5 mg Pb2+ , known activity of 210 Pb and 200 ml of conc. HCl were added to 10 l of drinking water and/or seawater (0.2 M HCl). This solution was loaded on the column (I.D. = 1.5 cm) filled with 6.5 g of Sr resin of 100–150 ␮m (bed length 11 cm, 0.34 g resin ml−1 ). Before loading the sample, 50 ml of 0.2 M HCl were passed through the column. The sample was passed through the column at the flow rate of 10 ml min−1 (gravitational flow). After passing the sample, 100 ml of 0.2 M HCl were passed through the column. Pb was stripped with 60 ml of 8 M HCl. This volume was transferred to the 100 g detection vessel. Note 1: The higher flow rate can be obtained by applying low over-pressure in the column. Actually, a column was constructed with a reservoir of 1 l of volume on top of the column which was kept under low over-pressure. Note 2: After gamma counting, the sample was evaporated to dryness, dissolved in 0.1 M HCl, mixed with scintillation cocktail Ultima Gold LLT and counted on liquid scintillation counter [12]. 2.2.2. Isolation after precipitation The optimum conditions of precipitation were investigated in the following way: a specific mass of (NH4 )2 CO3 was added to 1 l of seawater. Previously, 5 mg of Pb were added to the water. After dissolution and precipitation, pH was measured. The precipitate was separated from the solution by centrifugation. By measuring the initial concentration and the final concentration of cations on AAS, the quantity of precipitated lead was calculated. From the obtained results, it follows that lead quantitatively precipitated in the pH range of 9–11. For that reason, a previously developed procedure for the strontium and yttrium isolation was

used in this work [23]. Therefore, the following procedure can be used for simultaneous isolation of 89,90 Sr and 210 Pb from liquid sample. 2.2.3. Lead, strontium and yttrium precipitation from seawater Five milligrams of Pb2+ and 20 mg of Y and known activity 89,90 of Sr and 210 Pb were added to 10 l of seawater (from the Adriatic sea, Sr conc. is 7.9 mg ml−1 ). After that, 600 g of solid (NH4 )2 CO3 were slowly added with intense stirring. (Concentration of (NH4 )2 CO3 should be at least 0.3 M.) pH was adjusted to 11 by adding NaOH (in granules or as a solution). The precipitated carbonates were separated from the solution by vacuum filtration through a G-4 sinter funnel. After filtration, the precipitate was dissolved in a minimum quantity of 5 M HNO3 (200 ml) and filtered through a G-4 sinter funnel. The obtained solution was transferred to a 400 ml glass and evaporated almost to dryness. The sample was then prepared for isolation on an anion exchange column by mixing an alcohol solution of nitric acid. The same procedure was used for the isolation from 10 l of drinking water, and 40 mg of Sr carrier was added to drinking water. Ca2+ was also added to the sample for the precipitation improvement. Note: If the solution above the precipitate is clear (the sample is left overnight after carbonate precipitation), it can be decanted, thus reducing the volume for filtration. This work used a homemade filtration system, consisting of a pump for the suspension transport to the vacuum container with a G-4 sinter funnel and a pump for draining extra fluid from the vacuum container. 2.2.4. Isolation on anion exchange column After the cooling of the residue (nitrate solution from the previous step), 200 ml of absolute methanol and 100 ml of absolute ethanol (min 96%) were added. The obtained solution was passed through a column (I.D. = 2.2 cm) filled with 30 g of Amberlite CG-400 anion exchanger (100–200 mesh) in the nitrate form (bed length 19 cm, 0.43 g ml−1 in the presence of 0.25 M HNO3 in methanol), at a flow rate of 2 ml min−1 . Before that, the column should be prepared for isolation and separation by passing 100 ml of 0.25 M HNO3 in a mixture of ethanol and methanol through the column. (The exchanger swells and changes volume.) After passing the sample, Mg and a part of calcium were separated by passing 150 ml of 0.25 M HNO3 in a mixture of ethanol and methanol (1:2) through the column. The residual Ca, Sr, Y, Pb and some actinides were stripped with 250 ml of de-ionised water. The volume was reduced (by evaporation) to 10 ml and 210 Pb was determined by gamma spectrometry. The volume of that fraction (after detection) was reduced by evaporating almost to dryness and then mixed with 100 ml of 0.25 M HNO3 in a mixture of ethanol and methanol (1:2). This sample was passed through the same anion exchange column again. (The separation in two cycles prevents greater loss of strontium and yttrium.) Before passing the sample, the column must be prepared for separation by passing 100 ml of 0.25 M HNO3 in a mixture of ethanol and methanol (1:2). After the sample was passed through the column, residual calcium was

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eluted by 150 ml of 0.25 M HNO3 in the ethanol and methanol (1:2) mixture at a flow rate of 2 ml min−1 . Yttrium and strontium remained bound on the column. After separating calcium, Sr and Y were eluted with 200 ml of 0.25 M HNO3 in ethanol–water mixture (10 ml 5 M HNO3 + 150 ml ethanol + 40 ml H2 O). After that lead was stripped with 200 ml H2 O. This volume can be reduced to 20 ml and 210 Pb can be once again counted on the gamma spectrometer. In the water–ethanol fraction, yttrium was precipitated as hydroxide with NH4 OH at pH 9 by heating at 80 ◦ C. The Y(OH)3 precipitate was separated from the solution by centrifugation, dissolved with a few drops of HCl and re-precipitated as a hydroxide. The precipitate was again separated from the solution by centrifugation. The solutions from the first and second centrifugation were coupled. The Y(OH)3 precipitate was dissolved in 20 ml of HNO3 , and the solution was transferred to the PTFE vessel for counting on LSC. After separating yttrium, strontium was precipitated as SrCO3 , dissolved by 20 ml of 5 M HNO3 and transferred to a PTFE vessel for counting on LSC. The method used for the determination of 89,90 Sr was described in a prior paper [23]. Note: Lead can be separated from Sr on the Sr column as follows: water fraction from the previous step should be evaporated almost to dryness and after that 0.5 M HNO3 solution (of dry residue) should be prepared. This solution is loaded on a column (I.D. = 1 cm) filled with 3 g of Sr resin of 100–150 ␮m (bed length 11 cm, 0.34 g resin ml−1 ). Before loading the sample, 30 ml of 0.5 M HNO3 should be passed through the column. The sample is then passed through the column. After passing the sample, 50 ml of 0.1 M HNO3 are passed through the column. After that, Pb is stripped with 60 ml of 8 M HCl. 2.3. Determination of 210 Pb by gamma spectrometry For the determination of 210 Pb it was necessary to know how detection efficiency depends on the mass (and density) of the sample in given geometry of the counting vessel. In this work a plastic vessel with a radius of 6 cm and height of 4.5 cm, i.e. with the volume of 120 cm3 was used. It was determined how the efficiency depends on mass (thickness) of liquid samples (solutions of 210 Pb in methanol, de-ionized water, 5 M HNO3 and 8 M HCl). The known activity of 210 Pb in the mentioned solutions was counted on a gamma detector (different masses in a vessel). The counting time was arranged so that the counting error was less than 0.3%. The range of 165–178 channels (45.6–48.8 keV) was used. Efficiency was calculated as the ratio of detected activity and known activity. The efficiency for the solid samples was determined in the same way. Two methods were used for preparing the samples. In the first method, known activity of 210 Pb was placed on a solid sample as a point source. Actually, 10 ␮l of highly active 210 Pb solution was placed right in the middle of the solid sample in a vessel of 120 cm3 . Great care was taken to prevent mixing. In the second method, homogeneous samples were prepared. Masses of samples were 10, 40, 80, and 150 g. Efficiency was determined as described above.

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The isolated 210 Pb was determined by counting of liquid sample (100,000–200,000 s). Efficiency curves were used to calculate activities, i.e. to correct according to the sample quantity. Activity was calculated from the relation A = cps/(R×E×P), where cps is net count per second, E the detection efficiency for a given (thickness) quantity and geometry, P the absolute transition probability for gamma decay, and R is the chemical recovery. Minimum detectable activity MDA was calculated from the relation MDA = 4.66Sb /(E×P×V) [12] where Sb is estimated standard error of the background net count rate and V is sample volume. The isolated 210 Pb was also determined by liquid scintillation counting as described in reference [12]. 2.4. Instruments The Canberra gamma detection system was used for the 210 Pb determination. This system consists of the high purity broad energy Ge detector (Canberra BE3830), the original shield, and Genie PC software. Detector has the following physical characteristic: active diameter of 70 mm, active area 3800 mm2 , thickness 30 mm, distance from window 5 mm, window thickness 0.5 mm, window material—carbon epoxy. The instruments used for the detection and quantitative determination of cations were the atomic absorption spectrometer Perkin–Elmer 3110 and Spectrophotometer Varian Cary 3. Detection of radioactive Am, Sr and Y was carried out on the liquid scintillation spectrometer Packard Tri-Carb 2770 Tr/Sl. pH meter Methrom 781. 2.5. Chemicals The EiChrom extraction chromatographic Sr resin (100– 150 ␮m) and the Fluka Amberlite CG-400 strong anion exchange resin (100–200 mesh) were used. The isotope standard solution was received from Analytics Atlanta US (210 Pb, 241 Am, 242 Pu, 89,90 Sr). All other chemicals used in this paper were of analytical grade. Ultima Gold LLT, liquid scintillation cocktail (Packard). 3. Results and discussions 3.1. Binding of Pb on the Sr resin As we mentioned before, Sr resin is very selective and efficient for the separation of strontium and lead from a wide variety of metal ions. Its selectivity depends mainly on the kind of acids and acid concentration. Therefore, by changing the kind of acid and their concentrations Sr and Pb can be easily separated from other ions. Additionally, Sr resin shows extremely high selectivity for lead ions in low concentration of nitric and hydrochloric acid. Table 1 shows the results of Pb distribution coefficient determination on the Sr resin for the solutions of HCl and HNO3 . It can be seen that Kd of Pb increases from 0.1 to 0.5 M HCl and 0.1–1 M HNO3 and then decreases for both solutions. The obtained results correspond well with the results of Horwitz et al. [19] and Vajda et al. [18] for acid solutions without other ions.

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Table 1 Distribution coefficients of lead for Sr resin and solution of nitric and hydrochloric acid Kd Pb (ml g−1 )

Kd Pb (ml g−1 ) seawater solution

C (HCl) 0.10 0.25 0.50 1 3 5 8

367 1871 1700 1220 375 73 nb

591 566 550 386 50 12 nb

C (HNO3 ), mol dm−3 0.10 0.25 0.50 1 3 5 8

970 1641 1800 2034 710 418 110

396 491 576 633 419 292 88

Pb(NO3 )2

122

nb: not bonded.

From their results and our results it can be seen that Sr resin becomes very specific for Pb ions if the diluted acid is used for the bonding. Namely, in concentration range of 0.1–0.5 M HNO3 or HCl only Po and Sr are weakly bound to the resin. More than 30 elements do not bind on resin. Among them are Bi and Y. It is particularly interesting to note that the Sr resin strongly binds Pb, even from redistilled water (lead nitrate dissolved in de-ionised water)—Table 1. Therefore, a column experiment was made, lead was bound to the Sr column filled with the Sr resin (1.5 g in column of I.D. = 1 cm) and then eluted with water. Hundred milliliters of water were passed through column and lead did not appear in the eluate. Finally, it was eluted with 8 M HCl. In addition, Pb can be easily separated from Sr and many other elements on the Sr column—Fig. 1. It should be mentioned that Pb can be separated from other elements by elution with HCl, HNO3 or some other eluents as described in Horwitz et al. [19]. It is specially favorable that lead is strongly bound from diluted acid solutions, which means that binding from large volumes

Fig. 1. Separation of Sr from Pb on Sr resin column (I.D. = 1 cm, 8 cm bed height, 2 g Sr resin 100–150 ␮m, flow rate 1 ml min−1 ). Elution with 0.5 M HNO3 and 8 M HCl.

will need a small amount of concentrated acid for preparing the binding solution (as described in Section 2). Seawater and drinking water samples contain significant amounts of Ca, Mg, K, Cl and other ions. As Horwitz showed, high concentration of matrix constituents can cause significant decreasing of strontium retention on Sr resin. In practical application this means that during the separation on Sr column the presence of high concentration of matrix constituents can cause a significant loss of strontium. The results of lead Kd determination obtained for the solution of HCl and HNO3 in seawater – Table 1 – clearly showed that matrix constituents caused same effect (as in strontium case). The presence of higher concentration of matrix constituents reduces binding strength of lead, but because binding strength it so high, it can be expected that this reduction cannot cause significant loss of lead during the isolation. Although Sr resin has some advantages over other methods for the simultaneous separation of Pb and Sr from the environmental samples its application in the creation of rapid method for the 89,90 Sr determination also has some limitation. One limit is mentioned in the introductory part. Another one is in connection with the previous discussion about decreasing of strontium retention on Sr resin. Namely, in practice small amount of strontium should be separated from large amount of mayor constituents. For the optimum recovery of Sr several conditions should be fulfilled. Firstly, concentration of nitric acid in the sample should be minimally 3 M. Secondly, the volume of the sample must be so large that concentration of mayor constituent cannot cause significant loss of strontium. According to this volume column dimension and mass of resin should be chosen. Those parameters determine the time and quantity of acid needed for the analysis and in some cases procedure lose its advantages. However, these limitations can be avoided in combination with the separation on anion exchange column because this separation method enables isolation of yttrium and efficient separation of Ca, Mg and other mayor constituents from strontium, lead and yttrium. 3.2. Binding and separation of Y, Sr and Pb on the anion exchange column In our previous papers we showed how strontium and yttrium can be isolated from a complex mixture by using an anion exchanger and alcohol solution of nitric acid [20–22]. Strontium and yttrium can be bonded on a strong base anion exchanger in NO3 − form from the alcohol solution of nitric acid, and can be separated from alkaline elements, Ca, Mg, etc. on the chromatographic column. Binding strength of Sr and Y and other cations depends on applied alcohol and nitric acid concentration. However, some other cations from the alcoholic solution can be bound on the anion exchanger [23,25]. Table 2 shows exchanger binding ability (and distribution coefficients) for several cations. It is obvious from these results that Pb, Th, Am, Bi and Pu are strongly bound on the anion exchanger, while U is weakly bound. The composition of alcoholic mixture, types of cations and type of exchanger determine the binding strength, i.e., the method and possibility of separation depend on them. Fig. 2 shows that

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Table 2 Distribution coefficients of cations for Amberlite CG-400 in nitrate form and alcoholic solution of nitric acid Element

Kd of element (ml g−1 ) 0.25 M HNO3 in methanol

Kd of element (ml g−1 ) 0.25 M HNO3 in ethanol

Pb U Th Am Y Sr Na, K, Cs Mg, Ca Ba, Ra Fe, Co, Ni, Al, Cu, Cr, Mn La, Ce, Bi

700 19 4100 1770 40 49 nb nb, wb sb nb sb

2000 26 514 1047 138 136 wb nb, b nb nb sb

Fig. 3. Separation of Sr an Pb on anion exchange column (I.D. = 1 cm, 15 cm bed height, 5 g of Amberlite CG-400 in nitrate form, 100–200 mesh, flow rate 1 ml min−1 ). Elution with 0.25 M HNO3 in methanol.

nb: not bonded; sb: strongly bonded; wb: weakly bonded.

binding strength of lead and strontium decreases with increasing of HNO3 concentration in the mixture. Results in the Table 2 also show that binding strength of Y, Pb and Sr increases with decreasing of alcohol polarity while binding strength of Th and Am decreases with decreasing of alcohol polarity. It should be mentioned that we cannot speak about an ion exchange process which determines binding ability. It is some kind of (ad)sorption, which probably depends on the electrostatic attraction between cations and nitrate ions in the solution and the functional group of exchangers. The actual mechanism is unknown and beyond the scope of this paper, but it was previously considered, when the method of separation of strontium from calcium was developed [20]. This separation method can also be used for the simultaneous isolation of yttrium, strontium and lead from the sample. Lead can be separated (like Y and Sr) from alkaline elements, Mg and Ca, on the anion exchanger by means of the solutions of nitric acid in methanol and/or ethanol—Figs. 3 and 4. The results in Fig. 3 show that Sr can be separated from Pb and other elements by elution with 0.25 M in methanol, but elution with methanol caused overlapping with uranium. The results in Fig. 4 show that Sr and Y can be completely separated from other elements by elution with 0.25 M in ethanol and methanol. It should be mentioned that Y and Sr cannot be mutually separated in this

Fig. 2. (a) Distribution coefficients of Pb. (b) Distribution coefficients of Sr. Amberlite CG-400 in nitrate form and mixture of nitric acid and alcohol. Dependence of Kd on the concentration of acid in alcohol.

Fig. 4. Separation of Sr and Pb on anion exchange column (I.D. = 1 cm, 11 cm bed height, 5 g of Amberlite CG-400 in nitrate form, 100–200 mesh, flow rate 1 ml min−1 ). Elution with 0.25 M HNO3 in ethanol and methanol.

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Table 3 Distribution coefficients of cations for Amberlite CG-400 in nitrate form and alcoholic-water solution of nitric acid 0.25 M HNO3 in 100% methanol 90% methanol + 10% H2 O 80–20% 70–30% 60–40% 50–50% 100% ethanol 90% ethanol + 10% H2 O 80–20% 70–30% 60–40% 50–50%

Kd Pb (ml g−1 )

Kd Sr (ml g−1 )

Kd Th (ml g−1 )

Kd Am (ml g−1 )

Kd Y (ml g−1 )

Kd Bi (ml g−1 )

700 165 81 42 26 3

49 8 nb nb nb nb

4100 1026 74 4.3 nb nb

1771 31 nb nb nb nb

44 3 nb nb nb nb

1396 615 368 234 68 20

2000 283 99 55 35 5.0

136 31 nb nb nb nb

514 138 47 19 15 6.9

1047 24 nb nb nb nb

138 15 nb nb nb nb

466 384 179 59 32 19

nb: not bonded.

manner. In both cases Pb, Am, Bi and Th were strongly retained on the column but can easily be stripped with water. Since Pb binds very strongly to the exchanger from alcohol solutions, in practice it means that separation procedure will be time consuming (a large quantity of eluent is needed to isolate it). For the acceleration of the separation of Y and Sr from Pb, it is necessary to change the composition of the eluent. The best way to do that is to elute Sr and Y after calcium separation with 0.25 M HNO3 in an ethanol and water mixture (80:20)—Fig. 5. The results of determination of the distribution coefficient of Pb, Th, Am, Y, Bi and Sr, shown in Table 3, indicate that the increasing of water content in alcoholic mixture significantly weakens the bond between the cation and the exchanger, i.e., binding strength of strontium strongly decreases. It can be expected if we know that presence of high portion of alcohol makes possible of ions binding [20]. The binding strength of Am also strongly decreases with an increase in the water content, so it can be expected that it will be eluted together with Sr and Y. Yttrium behavior is similar to strontium. The difference in distribution coefficients of Pb and Th for an ethanol solution shows that it can be assumed that it will be possible to separate Pb from Th too, i.e. thorium will be eluted from the column before lead. Bismuth also strongly binds to the exchanger from both alcoholic mixtures but in difference with Pb more strongly from methanol. In alcohol–water mixture Bi behaves similarly to lead (see Kd in Table 3). Additionally, as Kd of Bi larger then Kd of Pb it can be concluded that their mutual separation is also possible. The fact that Bi and Pb can be strongly bounded on exchanger has practical value because it makes possible simultaneous binding of Pb and Bi, and their isolation from the sample. Since the primary goal of the paper is the isolation and gamma spectrometry determination of 210 Pb, the separation of bismuth (and polonium) from on the anion exchanger was not of a primary interest. Behavior of Bi is important when 210 Pb is determined through 210 Bi-210 Po. It should be pointed out that (as the obtained results show) alcoholic solutions and a strong base anion exchanger can be used for the isolation and separation of bismuth and some long-lived radionuclides (Am, Th, Pu). Also presence of water in alcoholic mixture is useful because solubility of nitrate salts (important for the sample preparation for

column separation [20,23]) is better in water–alcohol mixture than in pure alcohol. However, as we seen a higher quantity of water and a lower concentration of nitrates [20,23] can cause earlier cation elution (and their loss during isolation). This fact should be useful when the solution of the real sample will be prepared for the binding and separation on the column. All mentioned facts are used in order to develop novel methods for the isolation and rapid determination of lead and strontium isotopes in complex samples (Fig. 5). 3.3. Detection efficiency determination The simplest way for the 210 Pb determination is direct measurement on high-resolution gamma spectrometry system, but it has already been mentioned, that determination strongly depends on parameters which influences on counting efficiency. For an accurate determination, it is necessary to know how counting efficiency depends on those parameters. Fig. 6 shows the results of determination of counting efficiency depending on the kind and quantity of liquid and solid samples. The methodology is described in the experimental part.

Fig. 5. Separation of Sr, Y from Pb on anion exchange column (I.D. = 1 cm, 8 cm bed height, 3 g of Amberlite CG-400 in nitrate form 100–200 mesh, flow rate 1 ml min−1 ). Elution with 0.25 M HNO3 in ethanol–water mixture.

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theless, counting efficiency on gamma detectors is smaller in relation to liquid scintillation counting efficiency so it is better sometimes to choose this type of detection. In that case, chemical isolation is the key step for the reliable determination. Finally, the choice of the determination method depends on the kind of the sample, disposable instruments and time, etc. 3.4. Isolation of Pb from water samples on the Sr column and rapid determination of 210 Pb

Fig. 6. Results of counting efficiency determination of 210 Pb in liquid and solid samples.

Results showed that efficiency (expectedly) decreases with the increase in mass (thickness) of the sample. Alcohols, water and HCl were taken in consideration because isolation can be performed with those solutions. For a solid sample, detection efficiency is higher than the efficiency in the liquid sample with the same mass (but lower thickness), but it strongly decreases with the increase in mass (thickness) of the sample for a point source. It means that the detector “does not see” the 210 Pb above a certain thickness (mass) of the sample in the given geometry. The curve in Fig. 6 shows how efficiency decreases with the mass increasing in the case of solid homogeneous sample. This is important for the direct determination because it indicates that an increase in mass (thickness) of a solid sample in the given geometry will not significantly improve the determination limit. Previous results also show that chemical isolation in the case when large amount of sample should be taken for analysis (in case of very low activities) is reasonable. For example, in seawater, the reduction of volume by evaporating results in a large quantity of residue (around 100 g/30 l) so increasing of the sample volume does not result in significantly lower determination limit. On the other hand, if 210 Pb can be efficiently isolated from a large volume of sample the determination limit will be proportionally lower. From this point of view for the determination in a soil and sediment samples, chemical isolation followed by gamma detection makes no sense, because more than 10 g of the sample cannot be completely dissolve. Never-

Previous results of the determination of distribution coefficients clearly show that Pb can be easily bound directly to Sr resin from the diluted solution of HCl and HNO3 . Through the procedure described in the experimental part, 210 Pb was isolated from seawater and drinking water samples, and determined by gamma spectrometry and liquid scintillation counting (the procedure was tested by determination of 210 Pb in real sample). The results of determination of 210 Pb are given in Table 4. From these results it can be concluded that the proposed procedure can be successfully used for the isolation and determination of 210 Pb. It should be mentioned that this determination procedure is simple, fast and gives high recovery of Pb. It avoids precipitation and a large consumption of chemicals. By increasing the column radius and the resin mass, while keeping a constant height of the resin column, it is possible to additionally increase the flow rate and accelerate the isolation procedure. The flow rate must not be too high, in order to avoid any loss of Pb. The applied flow rate of 10 ml min−1 , for a column with the radius of 1.1 cm enables isolation in approximately 14 h. The Sr resin column can also be used for direct and simultaneous isolation of Sr and Pb from a liquid sample because both elements can be bound on resin from a HNO3 solution. Such a method is appropriate when working with volumes up to 1 l. It should be remarked that direct isolation can be very easily realized continuously, by connecting the column with a pump that can produce enough over-pressure to overcome the resistance of the resin column with the control of flow rate. 3.5. Isolation of Sr, Y and Pb on the anion exchange column and rapid 89,90 Sr, 210 Pb determination For a simultaneous isolation of Pb, Y and Sr from water samples, the method with anion exchanger was used. Pb, Y and Sr

Table 4 Results of determination of 210 Pb in seawater and drinking water samples after direct isolation on Sr column Sample volume (l)

RPb (%)

210 Pb

(dpm l−1 )

Knownb 1 dw 1 sw 10 dw 10 sw

98 97 92 90

240 300 24 30

R: recovery; dw: drinking water; sw: seawater. a Determination on LSC. b Deviation is expressed from total uncertainity of standards. c Deviation is expressed as average value of three determination.

± ± ± ±

10 15 2 2

210 Pb

Determinedc 250 330 27 33

± ± ± ±

35 40 5 5

(dpm l−1 )a

Knownb

Determinedc

24 ± 2 30 ± 2

22 ± 5 28 ± 4

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tillation counter (after separation from 210 Pb on Sr resin) in the same way as 210 Pb.

were precipitated from the samples of seawater and drinking water with ammonium carbonate. It should be noted that the precipitation from drinking water needs the addition of calcium for the improvement of precipitation. Sr, Y and Pb were separated from alkaline elements Mg and a part of Ca on the anion exchange column and alcohol solution of nitric acid. After that, Pb, Y and Sr were stripped with water (together with other elements). Two procedures followed. In the first, 210 Pb was determined by gamma spectrometry in the presence of all elements Th, Bi, Sr, Am that were eluted from the column with water. After that, the same sample was used for 89,90 Sr determination. Pb and Sr were separated on Sr column as described in the experimental part. It should be mentioned that highly energetic electrons, can produce X-rays of different energies around the detector (“bremsthralung”), which fall in the low energetic range and increase background, so the presence of higher activity of ␤-emitters can cause determination error. Before the 210 Pb determination, the background radiation level was determined by counting of “blank” water sample and water sample in the presence of 89,90 Sr (activity was 1 Bq l−1 ) and only normal background fluctuation was registered. 89,90 Sr was determined on LSC by Cherenkov counting. In the second, after the stripping with water, the water solution was evaporated almost to dryness, dissolved in alcohol solution and put on a smaller-size column (see Section 2), where Y and Sr were separated from lead with an 0.25 M HNO3 in ethanol–water solution (10 ml 5 M HNO3 + 40 ml H2 O + 150 ml ethanol). After the separation, strontium and yttrium were separated in the classical way [23], while 90 Sr were determined through 90 Y by Cherenkov counting on LSC. 89 Sr was determined by Cherenkov counting on LSC immediately after yttrium separation. Table 5 shows results for both determination procedures. It can be seen that isolation of Pb is very efficient from the results of recovery determination. The results also indicate that the developed procedures can be used for the rapid 210 Pb and 89,90 Sr determination in drinking water and seawater samples. It should be mentioned that Bi can be isolated with Sr and Pb on anion exchange column. 210 Bi produces Cherenkov radiation and 210 Pb can be determined by Cherenkov counting of 210 Bi. In order to estimate the possibility of 210 Bi determination by Cherenkov counting the counting efficiency of 210 Bi was determined. It is amount 7.7% for water and 8.4% for 5 M HNO3 for our instruments. Because of low efficiency (low average energy of its beta particles) determination of 210 Pb through 210 Bi is inconvenient. 210 Bi can be determined on liquid scinTable 5 Results of determination of Amberlite CG-400 Sample volume (l)

10 dw 10 sw

89,90 Sr

RSr (%)

52 58

and

210 Pb

89 Sr

3.6. Methods comparison The described method for direct isolation of lead from water samples on the column filled with Sr resin makes possible its rapid separation in one step and subsequent gamma spectrometric determination of 210 Pb. In relation to other methods, this method saves time and chemicals in the case when only 210 Pb should be determined. It should be mentioned that main limitations of 210 Pb determinations by gamma spectrometry (after chemical isolation) lie in higher detection limit in relation to detection limit by liquid scintillation counting. The reason for that is low gamma transition probability and lower detection efficiency. The minimum detectable activity of 210 Pb amounts 6 mBq l−1 (counting time 100000 s, recovery 85%, gamma probability 0.0405 and efficiency 0.21). This value is several times higher than the values obtained by liquid scintillation detection [12]. However, environmental analysis often requires determination of other isotopes of interest. Their determination usually requires pre-concentration and chemical separation from the sample (except for a limited number of ␥-emitters which can be directly determined) so the principle of economy (in sense of time and chemicals savings) imposes separation of as many isotopes as possible from the same sample. As we mentioned before radioactive isotopes of strontium are some of the most interesting and Sr resin can be successfully used for simultaneous separation (after pre-concentration) of Pb and Sr from sample. However, for the development of the rapid method of determination of 89,90 Sr its use is limited. It can be used for the isolation when higher activity of 89 Sr and 90 Sr is expected because in this case 89 Sr and 90 Sr can be simultaneously determined by Cherenkov counting on LSC [23]. In case when the low activity is expected the rapid determination of 90 Sr, 90 Y should be isolated because 90 Sr is determined through 90 Y. In the normal circumstances 90 Sr is in radioactive equilibrium with 90 Y in samples and 89 Sr is not present (89 Sr is present after nuclear accident) so 90 Sr can be determined through 90 Y immediately after isolation. Because yttrium cannot be bind to the Sr resin, Sr resin cannot be used in this type of determination. It should be mentioned that Bi, Po and Sr cannot be simultaneously isolated from the sample so if 210 Pb is determined through Bi or Po, 89,90 Sr cannot be determined and vice versa.

in seawater and drinking water samples after precipitation and isolation on an anion exchange column filled with (dpm l−1 )

90 Sr

(dpm l−1 )

RPb (%)

Knowna

Determinedb

Knowna

Determinedb

6.0 ± 0.1 3.0 ± 0.1

4.9 ± 1.1 3.5 ± 0.3

12.0 ± 0.2 2.8 ± 0.1

14 ± 1 2.9 ± 0.3

R: recovery; dw: drinking water; sw: seawater; dpm: desintegration per minute. a Deviation is expressed from total uncertainity of standards. b Deviation is expressed as average value of three determination.

80 82

210 Pb

(dpm l−1 )

Knowna

Determinedb

24 ± 2 30 ± 2

26 ± 5 35 ± 7

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As it can be seen from the results, yttrium and strontium can be separated from all other elements on the anion exchange column. Lead also can be separated from Y, Sr and many other elements (except Bi, Am, Pu, Th) so this separation can be useful in development of determination methods not only for 210 Pb and 89,90 Sr. The main advantage of the anion exchange procedure is the possibility of separation of several elements in one step; Y (and Sr) which enable rapid determination of 89,90 Sr, and Pb, Bi, Th, Am and Pu whose isotopes can be determined on appropriate manner. 210 Pb can be determined as described or through 210 Bi and ␣-emitters by alpha spectrometry after electro deposition. Generally a method of separation with Sr resin is the most appropriate when the analysis performed requires a small amount of the sample and where column loading with other elements does not cause a significant decrease of the strontium capacity factor [19]. When we perform an analysis that requires a large amount of the sample (and consequently expect large quantity of other cations) this method becomes less advantageous, because the presence of other elements can cause significant loss of strontium (due to the decrease of capacity factor). Therefore, complementary use of Sr resin and anion exchanger (in combination with alcoholic solution as eluent) is also shown in paper. Considering the fact that great amounts of the present macro constituents can cause earlier elution of Sr during isolation on the Sr column anion exchanger can be use in first step of isolation for their removal. After the separation of K, Ca, Mg and other elements from Y, Sr, Pb, Pu, Am, Th on the anion exchange column these elements can easily be stripped with water. After that, separation of Y from Sr and Sr from Pb can be performed. This separation can be achieved with small quantity of Sr resin. 4. Conclusion The results of this study show that yttrium, strontium and lead can be strongly bound to the anion exchanger in nitrate form and separated from other elements by elution with alcoholic solution of nitric acid. Lead can be easily bound and separated from other elements on the Sr resin column with dilute solution of nitric and hydrochloric acid. The Sr resin makes possible direct isolation of 210 Pb from water samples while anion exchanger can be used for the separation of 210 Pb, 89,90 Sr and 90 Y after precipitation from samples. The described procedures provide efficient isolation of 210 Pb and 89,90 Sr from a seawater and drinking water samples

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and enables their rapid quantitative determination. Under the appropriate conditions these procedures can be adapted and used for the isolation and determination of 210 Pb and other isotopes in different kind of samples. References [1] B. Swift, Forensic Sci. Int. 98 (1998) 119. [2] R. Vecchi, G. Marcanzzan, G. Valli, J. Environ. Radioact. 82 (2005) 251. [3] N. Preiss, M.A. Melieres, M. Pourchet, J. Geophys. Res. 101 (1996) 28847. [4] F. Raes, R. Van Dingenen, E. Vignati, J. Wilson, J.P. Putaud, J.H. Seinfield, Atmos. Environ. 34 (2000) 4215–4240. [5] H. G¨aggeler, H.R. von Gunten, U. Nyffeler, Earth Planet Sci. Lett. 33 (1976) 228. [6] E.G. San Miguel, J.P. Perez–Moreno, J.P. Bolivar, R. Garcia-Tenorio, J.E. Martin, Nucl. Instrum. Methods A 493 (2002) 111. [7] F. Cannizaro, G. Greco, M. Raneli, M.C. Spitale, E. Tomarichio, Appl. Radiat. Isot. 55 (2001) 129. [8] R. Pilvi¨o, J. LaRosa, D. Mouchel, R. Wordel, M. Bickel, T. Altzitzoglou, J. Environ. Radioact. 37 (1997) 355. [9] D. To, Anal. Chem. 65 (1993) 2701. [10] P. Blanco, J.C. Lozano, V.G. Escobar, F.V. Tome, Appl. Radiat. Isot. 60 (2004) 83. [11] G. Wallner, Appl. Radiat. Isot. 48 (1997) 511. [12] C.D. Bigin, G.T. Cook, A.B. MacKenzie, J.M. Pates, Anal. Chem. 74 (2002) 671. [13] G.A. Peck, J.D. Smith, Anal. Chim. Acta 422 (2000) 113. [14] C. Katzlberger, G. Wallner, K. Irlweck, J. Radioanal. Nucl. Chem. 249 (2001) 191. [15] I. Garcia-Orlleana, M. Garcia-Leon, Appl. Radiat. Isot. 56 (2002) 633. [16] Q. Chen, X. Hou, H. Dahlgaard, S.P. Nielsen, A. Aarkrog, J. Radioanal. Nucl. Chem. 249 (2001) 587. [17] G. Kim, N. Hussain, T.M. Church, Han-Soeb Yang, Talanta 49 (1999) 851. [18] N. Vajda, J. LaRosa, R. Zeisler, P. Danesi, Gy. Kis-Benedek, J. Environ. Radioact. 37 (1997) 355. [19] E.P. Horwitz, R. Chiarizia, M.L. Dietz, Solvent Extr. Ion Exch. 10 (1992) 313. ˇ Grahek, I. Eˇskinja, K. Koˇsuti´c, S. Luli´c, K. Kvastek, Anal. Chim. [20] Z. Acta 379 (1999) 107. ˇ Grahek, K. Koˇsuti´c, S. Luli´c, J. Radioanal. Nucl. Chem. 242 (1999) [21] Z. 33. ˇ Cerjan-Stefanovi´c, Croat. Chem. ˇ Grahek, I. Eˇskinja, K. Koˇsuti´c, S. [22] Z. Acta 73 (2000) 795. ˇ Grahek, M. Roˇzmari´c Maˇcefat, Anal. Chim. Acta 534 (2005) 271. [23] Z. [24] IAEA Technical report series 118, Reference methods for marine radioactivity studies, Viena, 1970, pp. 93–127. [25] J. Korkisch, F. Tera, Anal. Chem. 33 (1961) 1264.