A method for simultaneous determination of 210Pb and 212Pb in drinking water samples

A sensitive and accurate method for determination of Pb and Pb in drinking water samples was developed. In the method Pb was pre-concentrated as hydroxides, separated from alkaline earth elements as PbS precipitate, purified by an anion exchange resin chromatography column, precipitated as PbSO4 for source preparation and counted by a low background β-counter. The procedure was checked with a reference material supplied by the IAEA, and the obtained data were in good agreement with the recommended values, showing the recommended procedure can provide reliable results. The minimum detectable activity of the method was 0.039 mBq L−1 for Pb and 0.033 mBq L−1 for Pb if a 48 liter of water sample was analyzed. Seventeen drinking water samples were analyzed with a Pb recovery of 88.8± 5.5%, and the typical activity concentration was in the range of 0.191-15.1 mBq L−1 for Pb and of 1.12-5.77 mBq L−1 for Pb.


Introduction
There is an elevated concern about the radiological characteristics and impact of drinking water.In fact, the naturally occurring radionuclides, such as those in the uranium series and the thorium series ( 238 U, 234 U, 226 Ra, 228 Ra, 222 Rn, 210 Pb, 210 Po), exist ubiquitously in drinking water, and often contribute significantly to internal dose to the population.Although gross α and β activity measurements can serve as a screening tool for authority to control drinking water quality, for dose estimation it is necessary that the specific radionuclide in drinking water should be identified and their individual activity concentration measured, due to the fact that the dose coefficients are only related to the specific radionuclides.
Among the concerned radionuclides, lead isotopes are not only biologically but also radiologically harmful.There are 4 radioactive lead isotopes in nature, i.e. β-and γ-emitting 210 Pb (T 1/2 : 22.23 yr) and 214 Pb (T 1/2 : 26.8 min) in the uranium series, β-and γ-emitting 212 Pb (T 1/2 : 10.64 h) in the thorium series, and β-and γ-emitting 211 Pb (T 1/2 : 36.1 min) in the actinium series.Due to its chemical behaviors, lead is rather insoluble in natural water and is usually readily adsorbed onto solid particles, and the contents of lead isotopes in water are relatively low.Among the four lead isotopes, 214 Pb and 211 Pb, possessing very short half-lives, low contents and/or poor abundance, are not measurable in the effective time of sample preparation and separation, but 210 Pb and 212 Pb are.Especially, 210 Pb together with its granddaughter 210 Po (T 1/2 : 138.4 d) can be of great concern from the standpoints of radiation protection due to their radiotoxicity, as they can accumulate in sources of food, for instance, in particular forms of marine life, reindeer and caribou, and contribute to about nearly half of the dose from total internal irradiation by ingested natural radionuclides [1][2][3][4][5].Therefore, the routine monitoring of the 210 Pb concentration is very important.
For the purpose of studies on the accumulation and migration rate, cycling process in environmental media, bioavailability of the specific contaminant, survey of contamination level in environment and impact assessment, the monitoring of 210 Pb in biological and environmental materials often requires methods that should be sensitive, reliable and applicable to samples of considerable chemical complexity.There are five main kinds of method for the measurement of 210 Pb: (1) direct counting of the low-energy (46.5 keV) γ-ray of 210 Pb using γ-spectrometry equipped with Ge(Li) detector [6,7]; (2) separation of 210 Po, being an indirect decay product of 210 Pb, and counting of its α activity by α-spectrometry [8,9]; (3) co-precipitation of 210 Pb with Ba as a sulphate, dissolving the sulphate in EDTA, mixing the obtained solution with the scintillation cocktail and measuring by liquid scintillation counting [10,11]; (4) separation of 210 Bi, the direct progeny of 210 Pb, and counting of its β activity [12], and (5) separation of Pb and counting of the β activity of the in-growing 210 Bi [13][14][15][16].In the literatures [14,15], the advantages and disadvantages of these methods have been discussed in detail.
It is concluded that the Pb separation method is one of the most practical, sensitive and less time-consuming methods.
In contrast with 210 Pb, 212 Pb, being a decay progeny of thoron ( 220 Rn), owing to its short half-life the methodology studies on 212 Pb determination in water samples are scarce [17].
Based on the Pb separation procedures [14,15], more experiments were made on herein, seeking for further developing and improving the 210 Pb and 212 Pb separation conditions in water sample.The quality control tests and real sample analyses showed that the developed method for 210 Pb and 212 Pb determination in big quantity of water samples is a very sensitive and accurate technique, and can serve as a very useful tool for 210 Pb and 212 Pb studies in the fields of health physics, geochronology and environmental science.

Apparatus and reagents
210 Bi for 210 Pb determination and 212 Pb-212 Bi for 212 Pb determination were measured using a 10-channel low-level β-counter (Berthold LB770, Germany).The counting efficiency of the instrument for the 210 Pb measurement was calibrated with a PbSO 4 precipitate source obtained from a standard 210 Pb solution and that for the 212 Pb measurement was done with a PbSO 4 precipitate source separated from a standard 232 U solution that is old enough and has reached radioactive equilibrium between 232 U and its progeny 212 Pb.The obtained counting efficiencies were 48.16% for 210 Bi and 88.36% for 212 Pb-212 Bi, respectively.The reagent background was of ≤ 0.0053 cps.
The 232 U and 210 Pb standard solutions for instrument calibration, the reference material (IAEA-315) for quality control and the BIO-RAD-AG 1-X4 resin (100-200 mesh) for Pb separation were supplied by Amersham (UK), the IAEA and the Bio-Rad Laboratories (Canada), respectively.Pb(NO 3 ) 2 was used to prepare the carrier solution for Pb separation and all other reagents were analytical grade.

Column preparation
The anion-exchange resin, BIO-RAD-AG 1-X4 (100-200 mesh), was sequentially treated with 6 NaOH, 6 M HCl and distilled water to remove any fine particles as well as other unexpected components.Twelve grams of the resin were then loaded in an ion-exchange column with dimensions of 13 mm internal diameter and of 250 mm length.Before use, the column was conditioned with 20 mL of 1.5 M HCl.

Preliminary tests
Preliminary tests for determination of 210 Pb and 212 Pb in water samples were primarily based on the procedure reported in the literatures [14,18], in which Pb separation was conducted by coprecipitation with lead and/or iron hydroxide, absorption with a BIO-RAD-AG 1-X4 anion-exchange resin column, purification with Na 2 S to precipitate Pb as PbS in 6 M ammonium acetate and source preparation as PbSO 4 .

Eliminating the interference of alkaline earth elements
Lead forms two series of compound, the stable plumbous salts in which it is bivalent and forms the Pb 2+ in many ways resembling the Ba 2+ , and the less stable covalent plumbic compounds resembling the stannic compound, in witch it is quadrivalent.The plumbic compounds are either insoluble or hydrolyzed by water to lead dioxide PbO 2 .The fate and mobility of lead in environmental water are governed by its chemical and biological behaviors.Due to formation of many precipitates, such as PbO, PbO 2 , PbS, PbCO 3 , PbSO 4 , lead halides etc., the concentration of dissolved Pb in environmental waters including drinking water is generally low and variable, depending on formation of soluble complexes.Therefore, for accurate determination of 210 Pb and 212 Pb in environmental water a big sampling volume up to 20-100 L is needed.
When the procedure mentioned above was used to treat such a big volume of water sample, in many cases a big quantity of precipitate was obtained in the process of Pb pre-concentration.It was found that the major part of the precipitate is formed by carbonates, due to the fact that (1) the environmental or mineral water samples often contain a certain amount of HCO − 3 (30-1343 mg L −1 ), and (2) many transitional 3 at the basic condition.Although the carbonates can be destroyed by addition of HCl or HNO 3 during heating, a big quantity of cations, especially Ca 2+ , are left in the solution, which interfere the Pb 2+ adsorption on the resin exchange column and deteriorate the effective separation from uranium, radium etc. in the next step.
In order to solve this problem, Jia and Torri [15] have tried to separate Pb as PbSO 4 precipitate first and this way is working well for 0.5 g of soil or sediment, but it seems not very effective when a big quantity of Ca present in the sample solution, as a big quantity of PbSO 4 as well as CaSO 4 is obtained and the latter is not easily soluble in 10-20 mL of 6 M NH 4 Ac.A big volume of 6 M NH 4 Ac (100-300 mL) could dissolve CaSO 4 completely, but low Pb recovery was observed due to the increasing solubility of PbSO 4 .
The second test was to reverse the original analytical procedure of resin exchange separation and PbS purification, i.e. after Pb coprecipitation and dissolution, PbS precipitation was made first with addition of 20-30 g of NH 4 Ac and 8 mL of 0.5 M Na 2 S at pH 6-7.In this case, all the alkaline earth elements remain in solution and are eliminated by centrifugation, and the obtained black PbS and FeS etc. are dissolved by HCl for further purification by resin exchange column.This modification is very successful and with advantages of (1) high Pb recovery and short analytical time, (2) eliminating the most of silicon gel before resin separation and preventing from column blocking, and (3) improving the decontamination effects from main α-and/or β-emitters, such as uranium, thorium, radium and their other progenies.

The mechanism of the 210 Pb and 212 Pb measurements
As mentioned above, due to their short half-lives and/or low abundance, unsupported 214 Pb and 211 Pb in water sample are not detectable after chemical separation.The unsupported 212 Pb in water is difficult to be directly detected by γ-spectrometry due to its low activity concentration in most of the samples and there is few report concerning the determination of 212 Pb in water samples by chemical separation methods.On the contrary, there are a number of reports about the determination of 210 Pb in water samples by physical and chemical methods [7,14].Pb-210 through its daughter 210 Bi can be determined with ease by the most routinely used instrumentlow background β-counter.As reported in the reference [14], more accurate 210 Pb concentration can be obtained when 210 Pb and 210 Bi have reached the secular equilibrium about 30 days after the Pb source preparation.
During experiments for determination of 210 Pb in water, it was observed that 212 Pb through both 212 Pb and 212 Bi can be determined simultaneously with 210 Pb using low-background β-counter after pre-concentration and separation.Figure 1 is a typical one, which shows the β counting rate as a function of time after the Pb source preparation from a mineral water sample based on the recommended procedure.After careful treatment of the data, it is found that the figure can be very well resolved into two fractions.The first fraction, located in the counting time of 0-2 days, is dominated by the 212 Pb-212 Bi decay, as shown in fig.2; while second fraction, located in the time after 2 day counting, is characterized by 210 Bi ingrowth from 210 Pb (fig.3).Therefore, after deducting the count contribution of the instrumental and reagent backgrounds and the contribution of 210 Bi ingrowth, the first fraction can be used to calculate the 212 Pb activity concentration in water through extrapolation to the Pb separation time, and from the second fraction, the 210 Pb activity concentration can be calculated.

Pre-concentration of Pb from water samples
Thirty to fifty mL of concentrated HCl, 40 mg of Fe 3+ (40 mg Fe 3+ mL −1 ) carrier, 25 mg of Pb 2+ (25 mg Pb 2+ mL −1 ) carrier are added to 20-50 L of water sample.After 30 min stirring for isotopic exchange between carriers and analytes, the solution is adjusted to pH 9-10 with concentrated ammonia solution to precipitate iron and lead as hydroxides and carbonates, and mixed well.After the precipitate settled down, the supernatant is carefully siphoned off and the precipitate slurry is centrifuged at 4000 rpm.The supernatant is discarded and the precipitate is dissolved with 30-40 mL of concentrated HCl.The solution is then transferred to a beaker and heated to boil for digestion with 2 mL of 30% H 2 O 2 .

Separation of Pb from alkaline earth elements as PbS
The obtained solution (about 150 mL) is neutralized to pH 1.0-1.5 with ammonia solution, and 20-30 g of NH 4 Ac are added and dissolved by heating.Eight mL of 0.5 M Na 2 S is added, and in this case PbS is precipitated while most of Ca 2+ and Mg 2+ will remain in the solution.After centrifugation, the supernatant is discarded and the black precipitate is collected.Dissolving the precipitate with 4 mL of concentrated HCl and 26 mL of water, digestion is made by adding 2 mL of 30% H 2 O 2 , then the solution is filtered through a Millipore filter paper (pore size: 0.1 μm).

Purification of Pb with anion-exchange resin column
The obtained solution in an acidity of 1.5 M HCl is passed through a preconditioned anion-exchange resin column at room temperature and a free flow rate.After washing with 40 mL of 1.5 M HCl, Pb is eluted with 60 mL of distilled water at free flow rate, and the separation time of the pair 210 Pb/ 210 Bi and 212 Pb is recorded.

Source preparation, measurement and activity concentration calculation
Three mL of concentrated H 2 SO 4 are added to the collected eluant, which is then evaporated until fuming to destroy the organic matters by oxidation with 1 mL of 30% H 2 O 2 .Both the precipitate and the solution are centrifuged.The supernatant is discarded and the precipitate is filtered on a weighed filter paper with a diameter of 24 mm (Whatman 42).The sample is dried at 110 • C until constant weight (about 1 h) and weighed again to calculate the Pb chemical yield.Pb-212 is measured immediately after the chemical separation. 210Pb is determined by measuring the ingrowth activity of its progeny 210 Bi (T 1/2 : 120 h) by a low background β-counter some time after the separation (about one month being suitable).The 210 Pb activity concentration in the water sample (C P b-210 , Bq L −1 ) can be calculated according to eq. ( 1): where, A Bi-210 , the net count rate of 210 Bi (cps); λ Bi , the 210 Bi decay constant (min −1 ); t, the 210 Bi ingrowth time after 210 Pb separation (min); η, the detection efficiency for 210 Bi; Y, the chemical yield and; v, the sampling volume (L).
The 212 Pb activity concentration in the water sample (C P b-212 , Bq L −1 ) can be calculated by eq. ( 2): where, A P b-Bi-212 , the net count rate subtracting the contribution of the blank and 210 Bi (cps); λ P b-212 , the 212 Pb decay constant (min −1 ); t, the time from 212 Pb separation to counting (min); η, the detection efficiency for 212 Pb( 212 Bi).

Results and discussion
The results were given in tables 1-2, and the reported uncertainty of the obtained activity concentration of radionuclides for individual analysis of a sample was 1 standard deviation (SD), which was estimated from the uncertainties associated with the instrument calibration, the addition of the carrier to the sample and the counting statistics of the sample and the blank etc.

Quality control
The step of Pb separation from alkaline earth elements as PbS precipitate in the method could also be used to simplify the procedure for 210 Pb determination in soil if the reliable results could be confirmed.For the purpose of quality control, reference material IAEA-315 Marine Sediment supplied by the IAEA was analyzed to check the recommended procedure.The reference material of about 2 g was leached based on the procedure given in the literature [15], and the obtained leachate was analyzed following the procedure for water samples.The precision was evaluated by the relative standard deviation obtained from a set of six analyses.The accuracy was assessed by the term of relative bias, which reflects the difference between the experimental mean and recommended value of the 210 Pb activity concentration.Due to the presence of unsupported 210 Pb in the IAEA-315, the fraction of unsupported 210 Pb had to be corrected to the reference date.The obtained 210 Pb and 212 Pb activity concentration in the IAEA-315 is shown in table 1.The mean 210 Pb concentration in the IAEA-315 was found to be 30.8± 1.8 Bq kg −1 (decay correction to the date of 1st Jan. 1993).It was observed that the relative standard deviation obtained from a set of six analyses of the IAEA-315 is ±5.9% for 210 Pb.Since all being less than ±10% the precision for the analyses is well acceptable as far as such a low activity is concerned.The relative bias obtained from the analyses is +2.3% for 210 Pb, showing that the mean activity concentration of 210 Pb is in good agreement with the recommended value of 30.1 Bq kg −1 (the 95% confidence interval: 26.0-33.7 Bq kg −1 ).
The mean 212 Pb concentration in the IAEA-315 was found to be 24.9 ± 3.0 Bq kg −1 .Although the IAEA did not issue any recommended value for 212 Pb, the reliability of the 212 Pb activity concentration may be judged from the recommended value for 228 Th that is in secular equilibrium with its predecessors 228 Ra and 232 Th.In fact, the 212 Pb activity in the IAEA-315 sample is nearly in equilibrium with 228 Th, as (1) the half-lives of its decay products are short, and (2) 220 Rn escaped from the sealed container is negligible (≤ 2%).In this case, the obtained relative standard deviation and the relative bias of 212 Pb are ±12% and −7.8% respectively.The obtained deviation and bias for 212 Pb are bigger than those for 210 Pb, mainly due to multi-corrections for the instrument background, reagent background, as well as interference of the 210 Bi ingrowth from 210 Pb.But the data of 212 Pb can still be considered as acceptable and in good agreement with the recommended value [27.0 (24.0-28.9)Bq kg −1 ] for 228 Th.

Concentration of 210 Pb and 212 Pb in drinking water
For the purpose of application of the established method, seventeen brands of drinking water samples were collected and analyzed.As shown in table 2, the typical activity concentration in the analyzed drinking water samples is in the range of 0.191-15.1 mBq L −1 for 210 Pb and of 1.12-5.77mBq L −1 for 212 Pb.The calculated activity ratio for 212 Pb/ 210 Pb is in the range of 0.289-16.9 and with an average value of 2.58 ± 4.08.In fact, the concentrations of uranium isotopes, radium isotopes and 210 Po in the same water samples have also been determined and the relevant data have been reported elsewhere [19].If compared with the activity concentration of uranium isotopes, radium isotopes and 210 Po [19], it is shown that: (1) the 210 Pb and 212 Pb activity concentration in nearly all water samples is much lower than that of uranium isotopes, but more or less in the same levels as 226 Ra, 228 Ra and 210 Po, and (2) the activity disequilibria are always observable for the pair of

Figure 1 :
Figure 1: The 212 Pb-212 Bi decay and 210 Bi ingrowth curve of a Pb source obtained from a potable water sample collected in Italy.

Figure 2 :
Figure 2: The 212 Pb-212 Bi decay curve of a Pb source obtained from a potable water sample collected in Italy (same water as shown in fig.1).

Figure 3 :
Figure 3: The 210 Pb decay and 210 Bi ingrowth curves (after subtracting the 212 Pb contribution) of a Pb source obtained from a potable water sample collected in Italy (same water as shown in fig.1).

Table 2 :
The 210 Pb and 212 Pb activity concentration (in mBq L −1 ) in drinking water samples collected in Italy.