A rapid sequential chromatographic separation of U- and Th-decay series radionuclides in water samples

Talanta 207 (2020) 120282

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A rapid sequential chromatographic separation of U- and Th-decay series radionuclides in water samples

T

Claire Dalencourt, Mohamed Nait Chabane, Jean-Christophe Tremblay-Cantin, Dominic Larivière∗ Laboratoire de Radioécologie, Département de Chimie, Université Laval, 1045 Avenue de La Médecine, Québec, QC, G1V 0A6, Canada

ARTICLE INFO

ABSTRACT

Keywords: Sequential extraction Radioactivity Extraction chromatography Analig Ra-01 TRU resin Sr resin

A new sequential protocol for the separation and preconcentration of U, Th, Ra, Po and Pb for the same sample aliquot has been designed. The optimized stacking of chromatographic resins [TRU, Sr and a new hybrid Ra resin (composed of Analig Ra-01 and cation exchange AG50Wx8)] enables a rapid loading of the sample (less than 75 min for 300 mL of samples) while ensuring a high retention of the analytes of interest. The use of a hybrid Ra resin allows the complete and selective extraction of Ra on a solid support, a feature lacking in other sequential separation procedures. A loading medium composed of 1 M HNO3 and chloride ions (as NH4Cl) was found suitable for the retention of all analytes of interest onto the stacked chromatographic resins. The proposed elution conditions facilitate the complete segregation of individual elements in 5 distinctive fractions, reducing the risk of spectral and non-spectral interferences. This feature enables the proper quantification of the radioisotopes either by ICP-MS or alpha spectrometry below national and international regulatory guidelines. The mean chemical recoveries for the separation are 93, 98, 105, 88, and 98% for U, Th, Ra, Po and Pb, respectively. Reproducible yields were obtained independently of the water type tested, demonstrating the versatility and the robustness of the proposed methodology.

1. Introduction Naturally Occurring Radioactive Materials (NORM) are a source of concern for governmental health regulators as they are ubiquitous in every environmental media, such as water, soil and food [1,2]. Water is a medium of particular concern as it is a major constituent of many natural ecosystems [3] and is a significant vector for the assimilation of radioactivity in both flora and fauna [4]. Natural radioactivity found in the water originates essentially from 40 K, a primordial radionuclide, and decay products from U- and Th-series [5,6]. Decay products of these series, found in rock, soils, and sediments are leached when contacted with water, based on hydrogeological processes that are affected by physical and chemical parameters [7]. They could also accumulate in water via anthropogenic activities such as mining, hydrometallurgical processes, and fertilizer production. In this context, these radionuclides are identified as technologically enhanced NORM, abbreviated as TENORM [8]. Screening and monitoring of radionuclides in water bodies are regulated acts in most countries [9–12]. Amongst the naturally-occurring ones, 238U, 234U, 232Th, 230Th, 228Th, 228Ra, 226Ra, 210Pb and 210Po

∗

are recognized by the World Health Organization (WHO) [8] as contributors in internal dose assessments. Indeed, after ingestion, these radionuclides are absorbed and distributed to various organs via the bloodstream [13] before being excreted through urine or feces, or accumulated in tissues [14,15]. In most environmental media, affiliated radionuclides are assumed to be in secular equilibrium with each other's, however, such a scenario is seldom encountered in groundwater due to the difference in chemical mobility of each element [16]. This alteration in the secular equilibrium scenario could result in an under- or overestimation of the actual dose received through water consumption if solely the activity of 235,238U or 232 Th would be monitored [17]. Thus, the individual monitoring of the members of the U- and Th-decay series in water is essential, as highlighted in a study of the Maryland river water, where 1700-fold discrepancies between 210Po and 238U activities were reported in some collected samples [18]. The U.S. Environmental Protection Agency recommends methodologies for the quantification of radionuclides in drinking water [19] that are based, as a screening step, on gross alpha and beta counting; and when gross counting guidelines are exceeded, on radiochemical

Corresponding author. E-mail address: [email protected] (D. Larivière).

https://doi.org/10.1016/j.talanta.2019.120282 Received 14 May 2019; Received in revised form 19 August 2019; Accepted 19 August 2019 Available online 21 August 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved.

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procedures for the quantification of individual radionuclides [8]. The latter requirement could be a burden for radioanalytical laboratories as each of these methods has unique chemical specifications. In order to provide a fast and reliable alternative to individual radionuclide assessment, the development of multi-radioanalyte quantification strategies applicable to a single aliquot of a sample would be a welcome feature [20]. In that perspective, the use of sequential radiochemical separations seems a well-suited strategy [21,22]. A number of these separation schemes have been published to determine transition metals, lanthanides, transuranic elements or fission products [23–25]. Yet very few have been published regarding radionuclides originating from Uand Th- decay series. The first sequential separation for the quantification of U, Th, Ra, Po and Pb for a single sample aliquot was published in 1994 by Godoy et al. [26] and used handcrafted extraction chromatography (EXC) resin with trioctylphosphine oxide (TOPO) as an extractant impregnated onto a silica gel support in order to preconcentrate U, Th, and Po while Ra and Pb remained in the eluate. Then each element was eluted, co-precipitated and counted by radiometric approaches. A similar strategy proposed by Lozano et al. [27] is based on multiple co-precipitation steps. This approach had a faster turnaround time compared to commonly used radiochemical methods for the individual preconcentration of U, Th, 210 Pb and 226Ra [28]. The same research group also demonstrated that sequential extraction of U, Th and Ra was possible by employing three distinctive sequential liquid-liquid extractions after pH adjustment of the aqueous phase between each extraction. This strategy greatly improved analytical throughput, however, it was labor-intensive [29]. With the development of new extraction chromatographic resins, many of the traditional methodologies based on sequential precipitations or solvent extractions have been replaced by EXC [30]. For instance, Oliveira and Carvalho [31] have proposed a sequential strategy to monitor naturallyoccurring radionuclides (U, Th, Ra, Po, Pb) through sequential chromatographic extractions (cationic exchange and EXC resins). Through the sequential use of UTEVA, AG50W and AG1 resins, the authors reported chemical recoveries up to 86% in water matrices. However, the specific nature of the loading conditions required for each resin used resulted in additional modification steps before each extraction. Thus, the development of an analytical procedure based on a common loading matrix for all the resins used could significantly improve the analytical throughput of the methodology as well as its ease of use. This article proposes a new sequential extraction methodology for the separation of individual radioelements (U, Th, Ra, Po, Pb) employing a common loading matrix for all the resins used. The methodology uses stacked EXC (impregnated and molecular recognition) and cation exchange resins. The methodology is robust and has been applied successfully to several common types of water samples. The detailed investigation carried out to establish this technique is described in the following sections.

and hydrochloric acids, as well as ammonium chloride salt, used for the preparation of the various loading and elution phases were obtained from BDH (Radnor, USA). Nitrilotriacetic acid (NTA) and ammonium citrate were purchased from Alfa Aesar (Tewksbury, USA) and FisherScientific (Ottawa, Canada), respectively, whereas ammonium oxalate and oxalic acid were obtained from SigmaAldrich (Oakville, Canada). All chemicals used for the preparation of solutions were reagent-grade purities or better. Calibration curves and elemental standards were prepared from 1000 mg L−1 standard solutions of Ba, Sr, Ca, Pb, Th, U and Rh purchased from SCP Sciences (Baie d’Urfé, Canada) while 226Ra solutions were prepared from a standard solution purchased from NIST (Gaithersburg, USA). 209Po and 210Pb solutions, used as internal standards or to spike samples, were purchased from Eckert & Ziegler (Valencia, USA). Whatman grade 1 cellulose filters (GE, Chicago, USA) were used to filter water samples prior to acidification and matrix adjustment. 2.2. Samples The performances of the proposed methodology were evaluated using synthetic groundwater and river water surrogate samples, prepared following the procedure proposed by Li et al. [32] and Palomar et al. [33], respectively. Samples of demineralized, tap, bottled drinking, sea and mining waste waters were used to assess analytical performances. Mining wastewater was collected in Saint-Honoré, QC, Canada. Sr, Ba and Ca concentrations in these samples are presented in Table S1. All water samples were analyzed, either with or without spiking radionuclides of interest. In order to prepare for the loading of the samples onto the resins with the proposed procedure, 20 mL of concentrated HNO3 (15.7 M) and 28 mmol NH4Cl were added to each 300 mL fraction of samples, previously filtered on Whatman grade 1 filters. The addition of these chemicals is required to ensure the retention of the analytes of interest onto the various resins used (vide infra). No additional sample pretreatment was performed onto the samples prior to the loading step. 2.3. Method The proposed sequential extraction system is composed of a 2 mL cartridge of TRU resin directly followed by a 2 mL cartridge of Sr resin and finally a column composed of 0.2 g of Analig Ra-01 and 2.4 g of AG50Wx8 mixed thoroughly together (HRa resin). Loading flow rate was set at 4 mL min−1 (6.3 mL min−1 cm−2 and 3.2 mL min−1 cm−2 for TRU and Sr resins and HRa resin, respectively) with a peristaltic pump. This value was selected as it was below the maximum recommended flow rate for the retention of Ra on the Analig Ra-01 resin (i.e. 10 mL min−1) [34]. Resins were washed according to the elution protocol (steps IV to IX in Table 1) and conditioned (30 mL of 1 M HNO3/0.09 M NH4Cl; step I) prior to sample loading (300 mL, step II). A rinsing step (30 mL; step III) was performed after sample loading to minimize cross-contamination using a solution of 1 M HNO3 containing 0.09 M NH4Cl (Fig. 1 and Table 1). After the sample loading and resin rinsing steps, the columns were separated and elution steps were performed on individual resins to recover five distinctive fractions, each containing a separated analyte. As polonium is retained on two distinct resins, it was eluted from TRU resin with 15 mL 8 M HNO3 (Step IV) and from Sr resin (Step V) with 15 mL 0.1 M HNO3. Both fractions were combined to obtain a single final Po fraction. Optimal and selective elution conditions used in this procedure were obtained from publication dealing with specific resins [35–37] and are discussed in the next few lines. Thorium and uranium were eluted from TRU resin with 30 mL of 1 M HCl (Step VI) and 0.1 M (NH4)HC2O4/0.025 M HCl (Step VII), respectively, according to Horwitz et al. [38] and Moody et al. [39]. Note that the (NH4)HC2O4 solution was prepared by adding the appropriate equimolar amounts of (NH4)2C2O4 and H2C2O4. Lead was stripped from Sr resin with 10 mL of 0.05 M (NH4)2C6H6O7 (Step VIII), as proposed by the supplier of the

2. Material and method 2.1. Material The separation procedure developed in this study employs a molecular recognition technology resin (Analig Ra-01, 60–100 μm mesh, bed density: 0.4 g mL−1) from IBC (American Fork, USA), a cation exchange resin (AG50Wx8, hydrogen form, 100–200 μm mesh, bed density: 0.8 g mL−1), a Sr resin (4,4′(5′)-di-t-butylcyclohexano-18-crown-6, 50–100 μm mesh, bed density: 0.35 g mL−1) and a TRU resin (octylphenyl-N,N-di-isobutyl carbamoylphosphine oxide, 50–100 μm mesh, bed density: 0.37 g mL−1) available from Eichrom Technologies (Lisle, USA) and purchased either as pre-packed 2 mL cartridges (7 mm internal diameter) or as bulk materials. Polypropylene 5 mL columns (12.6 mm internal diameter) used to cast the hybrid Ra resin (HRa resin) were purchased from BioRad (part #7324661, Mississauga, Canada). Ultrapure water used for the preparation of standards and solutions was obtained using a Milli-Q system from EMD Millipore (Billerica, USA). Nitric 2

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Table 1 Chromatographic conditions for the sequential procedure for the separation of Th, U, Pb, Po and Ra. Step

Description

Resina

Matrix

Concentration (M)

Volume (mL)

Targeted element

I II III IV V VI VII VIII IX

Conditioning Loading Rinsing Po Elutionb Po Elutionb Th Elution U Elution Pb Elution Ra Elution

TRU; Sr resin; HRa TRU; Sr resin; HRa TRU; Sr resin; HRa TRU Sr resin TRU TRU Sr resin HRa

HNO3/NH4Cl HNO3/NH4Cl HNO3/NH4Cl HNO3 HNO3 HCl (NH4)HC2O4/HCl (NH4)2C6H6O7 NTA pH 10

1/0.09 1/0.09 1/0.09 8 0.1 1 0.1/0.025 0.05 0.12

30 300 30 15 15 30 30 10 30

none none none Po Po Th U Pb Ra

a b

The resins are presented in the consecutive order in which they are stacked. These fractions are combined to form the final Po fraction.

resin [40], whereas radium was released from HRa resin with 30 mL 0.12 M NTA at pH 10 (Step IX), as previously demonstrated [41]. In order to assess the absence of traces of analytes in the chemicals used, blanks were performed systematically during this investigation and, when relevant, their contribution in terms of radioactivity was

subtracted. 2.4. Distribution coefficients for HRa resin Distribution coefficients (Kd) were determined in HNO3 and HCl for

Fig. 1. Sequential extraction process. 3

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3. Results and discussion 3.1. Optimization of HRa resin Radium is a troublesome analyte in this sequential extraction procedure since there is almost no known ligand that is sufficiently selective to perform a simple extraction, contrary to the other radionuclides investigated in this study. Analig Ra-01 resin has demonstrated a significant degree of selectivity [44]. However, its hefty price tag is a setback to its acceptance as an analytical tool in routine radiochemistry, resulting in very few published applications. In that respect, combining Analig Ra-01 resin with a generic and cheaper cation exchange resin could provide a balance between cost and selectivity, and potentially a synergistic extraction effect. A preliminary hybrid resin (HRa) composed of Analig Ra-01 and AG50Wx8 resins in various massic proportions was tested in order to evaluate the minimal quantities of Ra-01 required to quantitatively and selectively retain radium from a specific loading phase. To investigate the impact of the proportion of Ra-01 resin used on radium retention, the content of AG50Wx8 and Ra-01 were changed while maintaining the total volume of the HRa resin constant at 1.7 mL. The loading phase (300 mL) used for this experiment contains 250 mBq 226 Ra, in 1 M HNO3 with 0.09 M NH4Cl (vide infra). Results in radium recovery are presented in Fig. 2. As expected, due to the competitive effect of the H+ at such acidic pH [45,46], a 1.7 mL cartridge containing only AG50Wx8 resin is not able to reach the degree of extraction observed with a cartridge containing the same volume of Ra-01 resin only (20 vs 100%, respectively). This behavior was anticipated as other studies have shown that Ra can be extracted in mild to relatively acidic conditions without significant changes in the distribution coefficients on the Ra-01 due to its high degree of selectivity of Ra ions [44]. On the contrary, as AG50W resin is an ion exchange resin, a strong site competition is expected between H+ ions, present in higher proportion in the sample, and Ra2+ ions [37,45,46]. When both resins are mixed, a quantitative recovery is observed when the massic content in Ra-01 exceeds 37%, which corresponds to ca. 0.36 g of Ra-01 for 0.6 g of AG50Wx8. As the previous results were obtained for a pure Ra matrix and that other studies have shown the co-extraction of other alkaline earth metals (AEMs) for both Analig Ra-01 [44] and AG50Wx8 [45,46] can be substantial and reduce Ra retention, a composite solution containing Ra (0.8 Bq L−1), Ca (166 μM), Sr (166 μM), and Ba (166 μM) was prepared. In addition to potentially reduce Ra retention on the abovementioned resins, several studies have shown that various polyatomic interference based on AEM could hamper the accurate quantification of Ra using ICP-MS [41]. When the same total bed volume of HRa as in the previous experiment was used (1.7 mL), Sr and Ba(Ca), were extracted up to approximately 35% and 25%, respectively, while a significant reduction in Ra extraction efficiency was observed (Table 2). These results highlighted the need to have a sufficient number of sites on the resin to ensure a quantitative Ra retention despite the presence of other AEMs. Since performances in chromatographic retention are related to resin total bed volume, an additional test was performed to determine the impact of maintaining the amount of Ra-01 constant in the resin while increasing the bed volume to 4.0 mL with AG50Wx8. This strategy was successful in increasing the Ra extraction yield while reducing those for the other AEMs. Further optimization was done to decrease Ra-01:AG50Wx8 massic ratio by varying the amount of Ra-01 and the total bed volume in order to further reduce the retention of Sr and Ba while maintaining a Ra yield above 90%. A resin with a ratio of 1:12 provided sufficient Ra retention while a column with a larger volume (4.5 mL) but a similar amount of Ra-01 (1:16) resulted in a lower Ra recovery and higher amounts of Ca, Sr, and Ba retained. In addition to ensuring a quantitative extraction of Ra in water matrices, this composition (1:12, 3.5 mL total bed volume) ensure a limited used of Ra-01 resin during the preparation of the HRa resin, thus providing a

Fig. 2. Radium recovery as a function of the relative content of Ra-01 in HRa resin. Uncertainties presented in this figure indicate instrumental variation measured on a single replicate.

the hybrid Ra resin. The following procedure was used for this purpose. Triplicates of solutions (10 mL) containing 50 μg L−1 of either Ba, Ca or Sr or 50 Bq L−1 of 226Ra at the appropriate acid molarity (0.02, 0.1, 0.5, 1 and 5 M) were shaken 2 h with 0.052 g of HRa resin (0.004 g of Ra-01 and 0.048 g of AG50Wx8). For the determination of the Kd values of Ca at molarities of 0.02 M and 0.1 M, a solution containing 1000 μg L−1 of this element was used in both acids to ensure the adequate detection of unretained ions. A blank (in triplicate) was also measured for each acid molarity tested. Subsequently, the samples were centrifuged at 3600 g for 10 min and an aliquot was pipetted and spiked with Rh for analysis. Distribution coefficients (Kd; mL g−1) were calculated using the following equation:

Kd =

C0 Cf

Cf V m

(1) −1

where C0 and Cf are the initial and final concentrations (μg L or Bq L−1), respectively, V the volume of solution (mL) and m the mass of the HRa resin (g). 2.5. Analyses As the method design and optimization were performed using pure elemental solutions of thorium, uranium and lead, their elemental and isotopic quantification were carried out by inductively coupled plasma atomic emission spectrometer (ICP-AES, model Optima 3000, PerkinElmer, Woodbridge, Canada) and tandem mass spectrometer (ICP-MS/MS, model 8900, Agilent Technologies, Santa Clara, USA), respectively. Sr, Ba and Ca were also measured by mass spectrometry to determine their impact on 226Ra retention. 226Ra and 210Pb were quantified by ICP-MS/MS according to instrumental configurations reported in literature [41,42] with Rh as an internal standard. Samples were enriched with 50 Bq L−1 of 210Pb prior to analysis to ensure adequate quantification. 210Po was spontaneously deposited on a 2 cm diameter silver or copper disk (Alfa Aesar, Haverhill, USA) based on a modified version of the procedure published by Arunachalam et al. [43] for food samples. Deposition was performed for 4 h at 85 °C in 0.2 M HCl solution (110 mL) under vigorous stirring. The planchets were counted for up to 24 h on an alpha spectrometer (Canberra Alpha Analyst, Canberra, Canada) in order to achieve a statistically significant number of counts. Method detection limits (MDL) for the various instruments used were calculated after blank subtraction using the following equation:

MDL = 3

B

CF

(2)

where σB represent the standard deviation of the signal of the method blank for 10 replicates and CF is the concentration factor. CF is obtained by dividing the initial volume of the sample (i.e. 300 mL) by the volume of the collected fraction for the selected analyte (e.g. 30 mL). 4

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Table 2 Radium and AEM extraction yield based on changes in the composition and total bed volume of the HRa resin. HRa massic composition (Ra-01: AG50Wx8)

1 1 1 1

: : : :

1.7 6.9 12 16

Mass of Ra-01 (g)

0.36 0.36 0.20 0.20

Mass of AG50Wx8 (g)

0.60 2.48 2.40 3.20

Total bed volume (mL)

1.7 4.0 3.5 4.5

Recovery (%) Sr

Ba

36 ± 1 2.0 ± 0.2 1.9 ± 0.1 3.0 ± 0.2

27 19 12 16

± ± ± ±

1 2 1 1

Ca

Ra

26 ± 1 8.1 ± 0.8 8.2 ± 0.4 11.0 ± 0.7

65 ± 15 100 ± 20 90 ± 17 65 ± 11

100 mL g−1, except for HCl where a steep decrease was measured at 5 M. At lower molarities, the extractive behavior of the HRa resin tends to mimic the one of the AG50Wx8 resin. Indeed, for molarities below 0.1 M, Kd above 2000 mL g−1 were measured, which are higher than of Ra-01 resin, suggesting that the cation exchange site of the AG50Wx8 plays a role in the extraction of Ra. This behavior is coherent with the Kd values reported for the AG50Wx8 resin which presents distribution coefficient above 10 000 mL g−1 in those conditions. Finally, at the extraction conditions proposed on the procedure (1 M HNO3), the trend observed in Table 2 (Ra > Ba > Ca > Sr) is consistent with Kd values expressed in Fig. 3. 3.2. Sequential extraction optimization 3.2.1. Choice of the order for the resin stacking The resins for the sequential extraction of U, Th, Ra, Po and Pb were first selected and subsequently, the order in which they would be stacked was determined as a means to ideally ensure a complete retention on a single resin. To achieve a high degree of separation between the selected analytes while minimizing the number of distinct resins used, three were selected: TRU, Sr resin and HRa (presented in the previous section). Note that other resins or sequences could be used to achieve the extraction of the selected radioelements, but they would result in sample phase loading incompatibility, the need of additional resins or retention on more than one resin. In addition, the selected resins allow the use of acidic conditions commonly used for water sample preservation. TRU resin is a judicious choice as it allows the selective separation of Th from U in 1 M HCl, as their distribution coefficients are 2 and 400, respectively, as opposed to 0.04 and 2 with the UTEVA resin [38] (Fig. S3). Sr resin was chosen due to its higher separation factor between Pb and Po than the Pb resin [47,48] (Fig. S4). Note that distribution coefficients presented in Fig. S4 were converted from retention factors (k’) for TRU and Sr resins by applying a conversion factor as described by Horwitz [49]. As described previously, HRa resin was chosen over its own constituents (AG50Wx8 or Ra-01 resins) based on its extractive performances in acidic media as well as its more affordable price tag. The chemical nature of loading medium in the sequential procedure (1 M HNO3) was selected in order to ensure the complete retention of the analytes while providing a condition suitable for all resins (vide infra). As Sr resin [49,50] retains Po, Pb and some Th at 1 M HNO3, it was placed after the TRU resin in order to ensure that Th is extracted on a single resin. In the proposed medium, TRU resin extracts partially Po and completely the actinides (U and Th), while Pb and Ra are not retained by this resin [36]. Therefore, it was chosen as the first to be in contact with the sample in the sequential procedure. Finally, as the HRa resin retains Pb, Ra and has some affinity with charged ions such as uranyl [44–46] (Figs. S1 and S2), it was placed at the end of the stacked cartridges sequence. Based on these constraints, the stacking of the resin should be in this order: TRU, Sr resin and then HRa. Initially, resins were individually tested to determine their respective Th, U, Pb, Po and Ra retention patterns. As hypothesized, all radioelements were correctly retained on the correct resin, supporting the proposed stacking order (data not shown). This order also limits the retention of Sr and Ba on HRa resin as these elements have a known affinity with the Sr resin [50].

Fig. 3. Distribution coefficients of HRa resin depending on (a) nitric acid and (b) hydrochloric acid concentration.

significant reduction in the preparation cost compared to a pure Ra-01 resin counterpart. Therefore, for the rest of this investigation, a 3.5 mL cartridge of HRa resin composed of 0.20 and 2.40 g of Ra-01 and AG50Wx8 resins, respectively, was used for the retention of Ra in the sequential procedure. While the amount of HRa resin necessary to ensure a complete retention of Ra was previously assessed, its chromatographic performances compared to its individual counterpart (Ra-01 and AG50Wx8) was not. Fig. 3 presents the distribution coefficient (Kd) of HRa resin for Ra, Ba, Ca and Sr in HNO3 and HCl media. For comparison purpose, Figs. S1 and S2 present the distribution coefficient curves for the individual component of the HRa resin, i.e. Ra-01 and AG50Wx8. As a general trend, the distribution coefficients for Ra on the HRa resin correspond to those of the Ra-01 resin in highly acidic conditions (0.5–5 M). As for Ra-01, a decrease in the Kd of Ra is observed with an increase in HNO3 molality with a minimal Kd of approximately 5

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Fig. 4. Polonium eluted from a 15 mL 0.1 M HNO3 after its retention on a Sr resin as a function of the concentration of ammonium chloride presents in 1 M HNO3 during the loading phase.

3.2.2. Chemical composition of the sample loading phase The chemical composition of the loading phase is critical for this extraction procedure since it dictates the absorption capacity and selectivity of each analyte on each resin. Highly acidic conditions will result in an incomplete retention of Ra on HRa resin due to its partial cationic exchange nature whereas diluted acidic conditions will result in limited retention of U, Th, Po, and Pb on TRU and Sr resins. Based on the distribution coefficients published for the resins used (Figs. S3 and S4), high extraction yields are achievable in 1 M HNO3, except for Po on the Sr resin as shown in Fig. 4 (absence of NH4Cl). Previous work by Nelson et al. [36] showed that Po retention on Sr resin could be enhanced by the presence of chloride ions. To investigate this effect with the proposed procedure, ammonium chloride was chosen as a chloride ion source instead of HCl in order to avoid altering the pH of the loading phase. The impact of the presence of NH4Cl in 1 M HNO3 for concentrations ranging from 0 to 0.19 M was assessed and a maximum in Po recovery was reached at 0.09 M (Fig. 4) for 88 mBq of 210Po. In these conditions, a complete extraction of Po was observed on the Sr resin, suggesting that Po would be completely extracted on both TRU and Sr resins during the sequential procedure. It was also demonstrated that the addition of NH4Cl did not affect the extraction yield of the other analytes on the other resins (results not shown). Thus, for the rest of the investigation, the loading matrix used was composed of 1 M HNO3 containing 0.09 M NH4Cl.

Fig. 5. Profiles of the 5 eluted fractions for a 300 mL of a multianalyte standard in 1 M HNO3 (0.09 M NH4Cl) containing Th (43 nM), U (42 nM), Pb (48 nM), 226 Ra (15 Bq L−1) and 210Po (13.2 Bq L−1). Table 3 Recoveries (%) of selected radionuclides in the specified fractions. Fraction

Element

Po Th U Pb Ra

Cumulative

IV and V

VI

VII

VIII

IX

103.0 0.6 < 0.1 1.7 < 0.1

N.A. 90.9 < 0.1 1.3 < 0.1

N.A. 5.7 99.1 1.9 < 0.1

N.A. 1.9 < 0.1 88.4 < 0.1

N.A. 2.5 < 0.1 5.2 99.6

103.0 101.6 99.1 98.5 99.6

N.A. Not analyzed.

3.2.3. Volume required for analyte stripping The performances of the sequential extraction were evaluated using 300 mL of 1 M HNO3 (0.09 M NH4Cl) containing Th (43 nM), U (42 nM), Pb (48 nM), 226Ra (15 Bq L−1), and 210Po (13.2 Bq L−1). A total volume of 40 mL of stripping solutions was collected in 5 mL fractions to determine the minimal required stripping volumes for each analyte. Results from Fig. 5 demonstrates that for a volume smaller than 30 mL all analytes are eluted. Pb is efficiently eluted from the Sr resin in only 10 mL of 0.05 M (NH4)2C6H6O7. As presented in Table 3, recoveries achieved with the proposed stripping conditions and volumes were greater than 99% for most analytes measured in their respective fractions. Only, Th and Pb showed partial elution in other fractions (10.7 and 10.1%, respectively). In addition, the resins and the elution conditions used resulted in a high degree of selectivity enabling each analyte to be segregated in its expected fraction while minimizing its co-elution in other fractions. This feature is interesting as it could enable the use of other radiochemical counting techniques such as gas-

proportional or liquid scintillation counters if only the radionuclides evaluated in this article are present in the samples. 3.3. Analysis of water samples Surrogate water matrices (300 mL) were spiked with Th (26 nmol), U (25 nmol), Pb (11 nmol), 226Ra (150 mBq) and 210Po (132 mBq) in order to determine recoveries of these elements in more complex samples (demineralized water, river water, ground water). Spike recoveries for surrogate samples are presented in Table 4 and range between 83 and 110% for Th, U, Pb and Ra whereas they are slightly lower (76–86%) for Po probably due to its incomplete deposition prior to alpha counting. Analyses performed on environmental water samples (Table 4) also demonstrated high spike recoveries (72–112%), highlighting both the robustness and the versatility of the proposed method for individual radionuclides in various water matrices. Po recoveries ranging from 70 6

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Table 4 Spike recoveries in surrogate and environmental samples ± σ (%) (n = 3).

Surrogate

Demineralized water River water Ground water Tap water Bottled drinking water Sea water Mineral wastewater

Environmental

Po (%)

Th (%)

U (%)

Pb (%)

Ra (%)

82 76 86 88 98 77 72

107 ± 2 98 ± 2 102 ± 1 89 ± 3 88 ± 3 101 ± 2 103 ± 9

97 ± 2 83.3 ± 0.5 91 ± 1 100 ± 1 104 ± 4 96 ± 3 81 ± 1

103 ± 3 110 ± 14 103 ± 2 100 ± 1 84 ± 8 104 ± 3 82 ± 5

107 ± 7 97 ± 1 102 ± 2 110 ± 4 104 ± 7 108 ± 9 112 ± 6

± ± ± ± ± ± ±

7 7 7 7 5 11 2

3.4. Figures of merit of the sequential procedure

Table 5 Radiochemical analyses for various water samples (mBq L−1 ± σ) (n = 3 per matrix). 210

WHO MDL Tap water Bottled drinking water Sea water Mineral wastewater

232

238

210

226

100 3 11 ± 2 22 ± 2

1000 0.0007 0.10 ± 0.02 0.23 ± 0.03

10000 0.0004 11.7 ± 0.4 8±2

100 66 68 ± 2 67 ± 2

1000 2 53 ± 7 123 ± 13

7±1 27 ± 10

0.03 ± 0.02 2.0 ± 0.2

29.3 ± 0.4 12.3 ± 0.07

< MDL < MDL

40 ± 17 60 ± 7

Po

Th

U

Pb

As stated previously, other sequential procedures have been previously published for radionuclides separation and preconcentration in water samples. Thus, in order to assess the performances of the proposed methodology, several figures of merits were compared (Table 6). In order to facilitate the comparison, chemical yields obtained for the various types of water samples were pooled and only the mean and the standard deviation were presented. Chemical yields obtained using our methodology are comparable or better to those reported elsewhere. In addition, as for Oliveira and Carvalho approach [31], the proposed separation was able to isolate all five elements of interest using only chromatographic resins without any co-precipitation or liquid-liquid extraction. While the methodology presented by Oliveira and Carvalho was able to concentrate the amount of radioactivity in the water sample, this was done through evaporation, which is time-consuming for large volumes of water (> 100 mL). As the complete sample volume (300 mL) is loaded onto the resin at a relatively high flow rate (4 mL min−1) in the proposed strategy, approximately 75 min were required for this step, whereas the elution of each radionuclide can be performed in less than 8 min. Thus, the complete separation scheme could be performed with a turnaround time of less than 3 h. As most analytes were completely eluted in less than 30 mL, a preconcentration of the radionuclides of interest by a factor of 10 (30 for Pb) is possible using the proposed methodology. This enrichment is a welcome feature if low levels of radioactivity are expected in the samples.

Ra

MDL: method detection limit.

to 98% were achieved for the environmental samples tested. These variable recoveries are the result of incomplete spontaneous deposition, as previous hypothesized. As an example, the mineral wastewater sample tested showed a Po recovery of 109 ± 12% when a 209Po tracer was added just before the spontaneous deposition step, a significant improvement over the deposition performed without a tracer (72 ± 2%). Based on these findings, available water samples were analyzed to determine their radioactive content. The measured activities are presented in Table 5. The drinking water WHO regulatory levels [8] in mBq L−1 are also presented in this table to highlight the sensitivity of the methodology using the proposed extraction procedure. Table 6 Figures of merit of the proposed methodology. Matrix type

This work Water (n = 21)

Oliveira technique # 2 [31] Water (n = 16)

Lozano [27] Water (n = 2)

Chemical yields for water analysis ± σ (%) U Th Ra Pb Po Separation technique

Blanco Rodriguez [29] Demineralized water (n = 4)

93 ± 7 98 ± 6 105 ± 5 98 ± 9 88 ± 9 Chromatography

105 ± 5 108 ± 10 103 ± 3 N.A. N.A. Liquid-liquid extraction

71 ± 10 37 ± 10 16 ± 9 86 ± 9 47 ± 20 Chromatography

3 300 4 yes

N.A. 200–1000 N.A. yes

3 6a N.R. no

70 ± 6 63 ± 12 70 ± 9 69 ± 6 N.A. Liquid-liquid extraction; coprecipitation N.A. N.R. N.A. N.A.

Ra (30 mL) Th (30 mL) Pb (10 mL) U (30 mL) Po (30 mL) ICP-MS/MS, α-spectrometry

Ra (5 mL) Th (5 mL) U (5 mL)

Ra (50 mL) Th (40 mL) Pb (20 mL) U (15 mL) Pob (N.A.) α-spectrometry

Number of resins used Sample volume loaded (mL) Loading flowrate (mL min−1) Loading phase compatibility with all separation process Elution volumes required

Radioanalytical detection techniques used

LSC

N.A. – Not applicable, N.R. – Not reported. a Sample was evaporated and the residue was redissolved in the volume presented. b Po is isolated via spontaneous deposition from the raw water sample, thus it is not eluted from a resin. 7

Th (N.R.) U (N.R.) Ra (precipitate) Pb (precipitate) α-spectrometry, LSC

C. Dalencourt, et al.

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4. Conclusion

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The methodology presented in this article allows the sequential extraction of a number of important naturallyoccurring radionuclides from the U- and Th-decay series in large volumes of water (300 mL) with a separation turnaround time of less than 3 h. With the optimization of the sample loading conditions and the proper stacking of three selective resins (TRU, Sr resin, HRa), sequential extraction of U, Th, Ra, Pb and Po was possible. Chemical recoveries higher than 72% were achieved for all analytes in each water type investigated, suggesting a high degree of robustness and the versatility for water regulations. Finally, as selective elution of analytes was possible in small volumes with the appropriate eluents, an enrichment factor of 10 (U, Th, Ra, Po) and 30 (Pb) is possible, facilitating the detection of these radioelements. When coupled with ICP-MS/MS (U, Th, Ra, Pb) and α-spectrometry (Po), the proposed methodology enables the detection of the selected radionuclides at levels ranging from 0.0007 to 66 mBq L−1. These levels are for most radionuclides (except 210Pb), 2 orders of magnitudes lower than the WHO guidelines for drinking water. Declarations of interest None. Acknowledgments The authors are grateful to Serge Groleau for his technical guidance and help with the instrumental analyses. The authors are also grateful to Fonds de Recherche du Québec—Nature et Technologies—Développement Durable du Secteur Minier (FRQNTDDSM, grant # FT109592) for the funding of this project. Finally, the authors would like to thank Jean-Philippe Bérubé for his optimization work on the polonium spontaneous deposition on silver and copper disks. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.talanta.2019.120282. References [1] Health Canada, Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials Canadian Guidelines for the Management of Naturally Occurring Radioactive Materials (NORM), (2011). [2] United States Environmental Protection Agency, Technologically Enhanced Naturally Occurring Radioactive Materials from Uranium Mining, (2008). [3] R.H. Friis, Essentials of Environmental Health, Jones & Bartlett Learning, 2012. [4] United Nations Scientific Committee on the Effects of Atomic Radiation, Annex B : exposures of the public and workers from various sources of radiation, Sources and Effects of Ionizing Radiation, United Nations, 2008. [5] S. Krishnaswami, J. Kirk Cochran, Chapter 1. Introduction, U-Th Series Nuclides in Aquatic Systems, Radioactivity in the Environment, Elsevier, 2008, pp. 1–10. [6] J.B. Cowart, W.C. Burnett, The distribution of uranium and thorium decay-series radionuclides in the environment—a review, J. Environ. Qual. 23 (1994) 651. [7] K.M. Hiscock, V.F. Bense, Hydrogeology: Principles and Practice, John Wiley & Sons, Inc., 2014. [8] H.G. Gorchev, G. Ozolins, WHO guidelines for drinking-water quality, WHO (World Health Organ.) Chron. 38 (2011) 104–108. [9] Gouvernement du Québec, Radionucléides recommandés pour l’analyse de la radioactivité dans les matrices environnementales, (2017). [10] Health Canada, Guidelines for Canadian Drinking Water Quality Summary Table, (2017). [11] United States Environmental Protection Agency, National Primary Drinking Water Regulations, (2009). [12] The Council of the European Union, Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption, Off. J. Eur. Communities L330 (1998) 32–54. [13] International Commission on Radiological Protection, Human alimentary tract model for radiological protection, Annal. ICRP Publ. 100 (2006) 36.

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