Cloud Point Extraction of Plutonium in Environmental Matrixes

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Cloud Point Extraction of Plutonium in Environmental Matrices Coupled to ICP-MS and Alpha Spectrometry in Highly Acidic Conditions Charles Labrecque, Laurence Whitty-Léveillé, and Dominic Larivière Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac402649v • Publication Date (Web): 30 Sep 2013 Downloaded from http://pubs.acs.org on October 6, 2013

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Analytical Chemistry

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Cloud Point Extraction of Plutonium in Environmental Matrices Coupled to ICP-

2

MS and Alpha Spectrometry in Highly Acidic Conditions

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Charles Labrecque, Laurence Whitty-Léveillé and Dominic Larivière†

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1. Laboratoire de radioécologie, Département de chimie, Université Laval, 1045

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Avenue de la Médecine, Québec, QC, Canada, G1V 0A6

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†

To whom correspondence should be addressed:

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Dominic Larivière

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Département de chimie, Faculté des sciences et de génie

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Université Laval

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1045, avenue de la Médecine, Bureau 1250D

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Pavillon Alexandre-Vachon

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Québec, Qc, Canada

16

G1V 0A6

17 18

Téléphone: 418-656-7250

19

Télécopieur : 418-656-7916

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E-mail: [email protected]

21 22

ACS Paragon Plus Environment


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Abstract

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A new cloud point extraction procedure has been developed for the quantification) of

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plutonium (IV) in environmental samples. The separation procedure can be either

26

coupled to inductively coupled plasma mass spectrometry (ICP-MS) or alpha

27

spectrometry for plutonium quantification. The method uses a combination of

28

selective ligand (P,P di-(2-ethylhexyl) methanediphosphonic acid (H2DEH[MDP]))

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and micelle shielding by bromine formation to enable quantitative extraction of Pu in

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highly acidic solutions. Cross-optimization of all parameters (non-ionic and ionic

31

surfactant, chelating agent, bromate, bromide and pH) led to optimal of the extraction

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conditions. Figures of merit of the method for the detection using alpha spectrometry

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and ICP-MS are reported (limit of detection, limit of quantification, minimal detectable

34

activity and recovery). Quantitative extractions (> 95 %) were obtained for a wide

35

variety of aqueous and digested samples (synthetic urine, waste water, drinking

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water, sea water and soil samples). The method features the first successful coupling

37

between alpha spectrometry and cloud point extraction and was the first

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demonstration of CPE suitability with metaborate fusion as a sample preparation

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approach, all techniques used extensively in nuclear industries.

40 41

Keywords

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Cloud Point Extraction, Plutonium, Soil, ICP-MS, Alpha Spectrometry, Lithium

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Metaborate Fusion


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1.

Introduction

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The presence of trans-uranian elements in the environment is a clear evidence of

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nuclear or radiological activities. These elements are radiotoxic, so reliable and fast

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environmental detection methods are important. However, as these elements are

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found in the environment at trace levels, it is important to develop strategies that lead

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to high preconcentration factors and isolation from matrix constituents1-2 even for

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refractory species, such as Pu. The importance of transuranian methodological

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development has also been highlighted by the recent events of the Fukushima-

52

Daiichi accident where plutonium traces were found in the environment. 3

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Many strategies have already been tried for the preparation of environmental

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samples and determination of plutonium. Though liquid-liquid extraction (LLE) is a

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well-established technique that gives reproducible and high extraction recoveries4 it

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produces non-negligible amount of wastes. This approach also presents some

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drawbacks and limitations associated with chemical nature of the processes. First,

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sample preparation is needed to separate actinides from matrix constituents(e.g. Al,

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Ca and Fe which interfere in environmental matrices) that interfere with the

61

measurement and the separation process. A conventional strategy to overcome this

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issue is to precipitate transuranian elements as fluoride or hydroxide compounds. 5

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Second, the preconcentration factors (PF) achievable are limited by the breakthrough

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volume and the physical volume of the column in EXC and the volume of the

65

extracting phase in LLE.

66 67

Among already-established analytical strategies that lead to high preconcentration

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factors, cloud point extraction has shown great promise since being introduced by

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Watanabe et al.6 for the extraction of Zn (II) using 1-(2-pyridylazo)-2-naphtol. Since

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then, many strategies have been proffered for the extraction of numerous elements. 7

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However, cloud point extraction has mostly been limited to metals in food and water

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samples, or to organic compounds such as polycyclic aromatic hydrocarbons(PAH)

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and pesticides in environmental samples. Most CPE (cloud point extraction)

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alternatives developed for solid environmental samples rely on leaching instead of

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total sample dissolution8-15, which limits the recoveries of refractory elements in

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solution such as transuranian elements. Most of these publications rely on filtration to

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remove the undissolved matrix, including silica, from the digestate. This strategy



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does not guarantee that transuranian elements could not be sorbed or trapped in

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these silicate structures. For most actinide species, complete sample digestion is

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required as they are present in the environment as refractory entities, which will be

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found prominently in the undigested portion.16 As an example, Roy et al.

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demonstrated that for uranium and thorium, incomplete dissolution led to improper

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quantification(up to 15%).17 Thus, the developed preconcentration method must be

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compatible with a digestion strategy yielding a complete and stable dissolution of

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samples. Based on these premises, transuranian elements would greatly benefit from

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the preconcentration methods achieved by CPE. The concept of CPE for

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transuranian elements has already been proposed, but experimental results have

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been modest.18 Only the CPE methods for uranium and thorium in aqueous samples

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have led to acceptable figures of merit. 19-20 We recently reported a CPE method for

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uranium in slightly acidic solutions (pH=3) using H2DEH[MDP],21 while this pH was

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significantly than other publications, this pH range was still incompatible with typical

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digestion. Adding bromine was found to provide a higher resistance to the acidity of

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the phase separation process. Meeravali et al. used bromine, mainly as an oxidant,

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for the extraction of thallium in highly acidic media (3 M HCl)22 and enable extraction

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in conditions compatible with sample preparation.

96 97

Since, Pu isotopic composition is frequently used as a tool to assess the origin of

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exposure, analytical instrumentation enabling the determination of isotopic signature

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at ultra-trace concentrations such as accelerator mass spectrometry (AMS) and

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thermal ionization mass spectrometry (TIMS) are regarded as prime technique for

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such purposes. However, these instruments are not widely accessible and have

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limited sample throughput, which limits their suitability, for example during a nuclear

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emergency. Inductively coupled plasma mass spectrometry (ICP-MS), while less

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sensitive, is common in most analytical laboratories and can provide detection

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sensitive to the pg L-1 for most elements.23 Since the atomic masses of most

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actinides are located in a region (m/z =230 – 256) where low background signals are

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detected, interesting detection limits could be achieved through the use of sample

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preparation techniques such as CPE that generate high preconcentration factors.

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Yet, even with a high preconcentration factor, it would still be more challenging to

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measure short-lived isotopes (t½ < 1 600 y) at environmental levels using mass

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spectrometric instrumentation than with radiometric methods such as alpha


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spectrometry or liquid scintillation counting. Therefore, a multi-technique approach

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with both MS and radiometric instrumentation yielding a complete isotopic signature

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of transuranian elements such as Pu would be highly beneficial in nuclear and

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environmental sciences.

116 117

In this report, we have developed an approach for the analysis of plutonium in acidic

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media that is compatible with typical sample preparation methods. CPE was

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developed around H2DEH[MDP] as a ligand. H2DEH[MDP] has a known high

120

extraction potential for all actinides, especially plutonium, as its K d (distribution

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constant) is among the highest of any actinides.24 While H2DEH[MDP] has never

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been used in cloud point extraction system to extract plutonium, the ligand is used in

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a commercially available solid phase extraction (SPE) cartridge. The cartridge has

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already been applied to sample preparation, mainly

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measurements.25 Experiment on LLE extraction has shown high affinity to plutonium

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whereas little affinity was observed for calcium.4 Recent work has also been done on

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the usage of the specific ligand on a polymer ligand film (PLF) for the extraction of

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plutonium.26 Following the method development, analytical performances were

129

evaluated in both digested and aqueous environmental matrices.

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Experimental

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Instrumentation

to make gross alpha

132 133

An ICP-QQQ-MS (Agilent 8800, Mississauga, ON, Canada) instrument was used for

134

the mass spectrometric determination of Pu isotopes. The optimized instrument

135

conditions are presented in Table S1. ‡ A centrifuge (Thermo Fisher Scientific Sorvall

136

Legend XTR, Bremen, Germany) was used to accelerate the phase separation

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process and during the soil solution preparation with the appropriate rotor according

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to the volume. The pH values were measured with a pH meter (Sartorius PT-15,

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Mississauga, ON, Canada). An automated fusion unit (M-4 fluxer, Corporation

140

Scientifique Claisse, Québec, Canada) and a MARS 5 microwave system (CEM

141

Corporation, Matthews, NC, USA) were used for fusion and acid digestion of soil

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samples, respectively. Both approaches were used to compare the applicability of the

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proposed CPE method, parameter used for the sample preparation via the fusion and

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MW digestions are shown in ESI. ‡



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Reagents

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High purity water with a resistivity of 18.2 MΩ cm -1 provided by a Milli-Q water

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purification system (Millipore, Etobicoke, ON, Canada) was used throughout this

150

investigation. A solution of

151

and Technologies (NIST, Gaithersburg, MD) was used in the developmental stage of

152

the CPE system and as a yield tracer of Pu in ICP-MS and alpha spectrometry.

242

Pu, obtained from the National Institute of Standards

153 154

Rhodium (SCP Science, Baie D’Urfé, QC, Canada) was used as an internal standard

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for all spectrometric determinations. For alpha spectrometry,

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tracer to determine the recovery. Certified reference materials (EP-L-3, EP-H-3 and

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EU-L-3, low level waste water) were bought from SCP Science (Baie D’Urfé, QC,

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Canada). A stock solution of reagent-grade Triton X-114 (Sigma-Aldrich Canada,

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Oakville, ON, Canada) was used as a surfactant in the CPE. Other reagents used in

160

the development of the CPE system were: KOH (Fisher Scientific, Ottawa, ON,

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Canada), HNO3 and KBr (Anachemia Chemical, Montreal, QC, Canada), KBrO3

162

(J.T.Baker Chemical co., Phillipsburg, NJ), and cetyl trimethyl ammonium bromide

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(CTAB) (Acros Chemicals, Ottawa, ON, Canada). These reagents were used without

164

any purification. P,P di(2-ethylhexyl) methanediphosphonic acid (H2DEH[MDP]) was

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synthesized using the procedure proposed by Stepinski et al.27 using Sigma-Aldrich

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reagents (Sigma-Aldrich Canada, Oakville, ON, Canada). Purity was assessed via

167

NMR spectroscopy using a Bruker AC 300 MHz NMR

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dissolved in an aqueous solution of Triton X-114 to enhance its solubility. Synthetic

169

urine and artificial seawater matrices were made based on the protocol established

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by McCurdy et al.28 and Kester et al. 29, respectively.

242

Pu was used as a

31

P and 1H. H2DEH[MDP] was

171 172

CPE system

173

An aliquot samples with a volume ranging from 6.5 to 50 mL was spiked with a

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known concentration of plutonium, and the acid concentration was adjusted to

175

approximately 2-4 M HNO3. The following parameters will be given for a typical

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solution of 6.5 mL although scale-up of the reaction was performed to achieved high

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preconcentration factor. 100 µL of a solution of 0.01 M of CTAB was added to the


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aliquoted sample, followed by 400 µL of an aqueous solution containing of 0.25 mM

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of H2DEH[MDP] dissolved in 0.2 mM of TTX-114. 0.15 mL of 0.1 M KBrO3 and 0.5 ml

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of 0.2 M KBr were added to complete the CPE system.

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Following the addition of the reagents, the solution was left stirring in a covered

182

container for 30 minutes in an ice bath. The use of a lid limits emanations of bromine

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and potential exposure to vapors. The solution was then left to settle for 30 minutes

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at room temperature. Finally, the solution was centrifuged at 4700g for 10 minutes at

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a temperature of 20°C. The surfactant-rich phase (SRP) was isolated by removing

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the supernatant, then the SRP was redispersed in dilute acid solution (diluted HCl for

187

alpha spectrometry or diluted HNO3 for ICP-MS) through sonication of the sample. A

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summary of the optimal CPE system conditions is presented in Table S4.‡ Using

189

these conditions, analytical figures of merit were calculated using equations

190

presented in ESI.‡ In order to provide a deeper understanding of the impact of each

191

CPE parameter on the Pu extraction yield, ranges of concentrations near the optimal

192

value by cross optimisation were assessed and are presented in the results and

193

discussion section. Plutonium uptake were measured by alpha spectrometry on dilute

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soil solutions throughout the developmental stages of the method. Samples were

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counted using fluoride micro-precipitation proposed by Guerin et al.30

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Results and discussion

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Effect of the complexing agent

198

Although no complex aggregation values exist for plutonium with H 2DEH[MDP], it is

199

expected to be similar to thorium since their valence states are the same. A ratio of

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2.5 extractant molecules per thorium ion is found when the typical complex is

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formed.31 These values were determined in toluene but are expected to remain much

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the same for our system as the hydrophobic core of TTX-114 micelles are composed

203

by aromatic substituted molecules. The concentration of H2DEH[MDP] was always in

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excess by several orders of magnitude compared to plutonium. Thus, it is of no

205

interest to test the ligand concentrations at which the breakthrough of plutonium

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occurs, as environmental concentrations of Pu would be in the range of ppt or Bq∙kg -1

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of soil, which is well-below the capacity of the method. The main advantage of CPE

208

resides in the preconcentration factors achievable,32 which are not needed for

209

samples with high Pu concentration.



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In the proposed conditions, H2DEH[MDP] proved to be fairly selective for Pu

212

compared to the rest of the matrix. The optimization was done for a soil matrix from

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the Gentilly-2 nuclear power plant, Bécancour, QC, composed of elements that could

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easily interfere with complexation of Pu. Hence, variation from ligand concentration

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changes are higher than in the method whose development was done using ultrapure

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water, but this is to be expected as the optimization was done using high solid

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sample such as soil digested by fusion.(Figure 1a) Nonetheless, complexation of Pu

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by H2DEH[MDP] was always found to be more favorable than the complexation of Ca

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and Fe, which are found in excess in the clay based-materials on which the method

220

was tested, as evidenced by the high decontamination factors.(Table 2)

221 222

The method showed great resiliency (Figure 1a). Changes in the ligand

223

concentrations did not significantly decrease the extraction efficiencies, which

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suggests that scale-up should be possible. The method also shows that, even though

225

the extractant is amphiphilic, it does not affect the micelles’ chemical and physical

226

properties as changes in parameters such as cloud point temperature or critical

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micelle concentration would greatly affect the chemical recoveries. This would rather

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confirm that the extractant and the formed complexes are inside the hydrophobic

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core than at the interface water-micelles. The impact of the ligand concentration on

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uptake and thus CPT is minimal as they are found at lower concentration than TTX-

231

114.

232 233

Effect of the bromate

234 235

As sample preparation in radiochemistry takes place generally in highly concentrated

236

acid, there is a great need for methods that work in the same media to limit the

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number of sample preparation steps and the possible contamination from the

238

addition of reagent. Typically, salts of iodide and thiocyanide are used to influence

239

the environment of the micelles to modify the cloud point temperature. The positive

240

influence of these salts on the cloud point temperature is dependent of their

241

polarizability. However, this strategy must be used with caution; some salts have a

242

significant complexing activity (e.g. thiocyanide and citrate) and could impact the


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selectivity of the method. Some salts will be unstable in acidic media (e.g. iodide

244

which form iodine in acidic solution). Among the halogens that are kinetically stable in

245

water (iodine, bromine and chlorine), bromine was preferred as it is more stable than

246

chlorine, and easier to prepare in solution. Bromine is liquid at room temperature and

247

does not clog the nebuliser, unlike iodine, which is solid at room temperature and not

248

easily dissolved in water. The use of bromate is important as in acidic solution,

249

bromate and bromide in solution will comproportionate to form bromine following Eq.

250

1:

251

Bromate concentration was assessed instead of bromide for multiple reasons: it is an

252

efficient reagent for the valence adjustment of plutonium and bromide was found in

253

excess. Bromate is known as an efficient reagent that maintains plutonium in its

254

tetravalent oxidation state (by its oxidation potential). The reaction with bromine is

255

reversible according to Larsen et al., and this could results in the formation of a

256

mixture of Pu(III) and Pu(IV).33 However, the predominance of the Pu(IV) in the

257

conditions presented in the table S4 was confirmed by EXC (extraction

258

chromatography).

259 260

The use of bromine significantly improved the extraction efficiencies with acidic pH,

261

from less than 10% efficiency to quantitative recoveries of plutonium.(Figure 1b)

262

These results proved that the effect of high acidity was minimized by the presence of

263

bromine shielding around the micelles (it diminishes the stripping potential of acid in

264

solution and its disrupting impact on micelles). At the highest concentration of

265

bromate, extraction efficiencies decrease; this can be attributed to a competitive

266

complexation of plutonium with bromine.



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Fig. 1 Impact of various factors on plutonium uptake: a) H2DEH[MDP] b) Bromate c)

269

TTX-114 (vertical reference line stads at the cmc) d) CTAB e) HNO3 molarity

270

n.b.: Other parameters were kept constant at the concentrations presented in Table

271

S4.

272 273

The effects of the non-ionic surfactant (NIS)

274

Triton X-114 was deemed a logical choice as a non-ionic surfactant for this

275

investigation for multiple reasons. First, its cloud point temperature is estimated at

276

28°C, which makes its phase separation possible at room temperature. Secondly,

277

one of the main appeals of cloud point extraction as a sample preparation technique

278

over liquid-liquid extraction is that the concentration needed to obtain phase

279

separation is minimal compared to the large volumes of solvents used for liquid-liquid

280

extraction (as in the TALSPEAK process which use quantities of kerosene). 35 The

281

reduced volume limits waste production, which are costly to manage and not

282

environmental friendly. Thirdly, there is an abundance of knowledge on systems that

283

use TTX-114 and the impact of various parameters on the cloud point temperature

284

and critical micelle concentration.36

285 286

The best extraction performances are expected to be found at concentrations near

287

the critical micelle concentration (noted in Figure 1c by the vertical reference line), as

288

the phase separation process is thought to be more efficient than at higher


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289

concentration. The preconcentration factor achievable also increases when the

290

surfactant concentration is low, as it minimises the volume of the surfactant-rich

291

phase. The use of minimal quantities of non-ionic surfactant reduce the amount of ion

292

extracted in the absence of ligand and thus enhances the selectivity.37 Finally, the

293

minimization of the organic content is desirable as it prevents carbon deposits on the

294

ICP-MS lens and cones.

295 296

The optimized concentration of 0.8 mmol/L provided quantitative and reproducible

297

extraction results. (Figure 1c) Small variations

298

surfactant had little impact on the method performance; extraction efficiencies were

299

stable over an order of magnitude of non-ionic surfactant concentration.

in the concentration of non-ionic

300 301

The impact of using low concentration of non-ionic surfactant on analytical

302

performances is direct on preconcentration values for example. A concentration of

303

0.8 mmol/L for a solution of 10 mL leads to a SRP volume of less than 100 µL. Using

304

this quantity of non-ionic surfactant, preconcentration factor values of up to 140 were

305

reached. This volume is so low that it must be redispersed in a small volume to be

306

injected in the ICP-MS or it can be treated via the typical micro-precipitation used to

307

couple separation methods to alpha spectrometry.

308

The effects of the ionic surfactant

309

Ionic surfactants were added to the CPE system to enhance the solubility of polar

310

molecules and the extraction efficiencies. As they impact the cloud point temperature,

311

they can be used to tune the system to a desired temperature. The most common

312

ionic surfactants used include anionic SDS (sodium dodecyl sulfate) and cationic

313

CTAB (cethyl trimethyl ammonium bromide). CTAB was used for the system and its

314

concentration optimization was done throughout a wide range of concentration. The

315

system was found to be responsive to the presence of CTAB as chemical recoveries

316

went from 70 % to quantitative levels following the addition of CTAB. (Figure 1d)

317

The effects of acid concentration

318

In order to favor the use of CPE, the determination in environmental matrices, optimal

319

extraction conditions needs to be achieved in acidic media as discussed previously.

320

Such conditions are especially critical in the case of plutonium as its hydroxide is



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321

formed at low pH (< 2).38 However, in acidic conditions plutonium can be present

322

simultaneously as various species.39 However as demonstrated, only Pu(IV) is

323

present in the conditions tested. This shows that CPE enables actinide valence

324

adjustment as well as preconcentration, just as in solid phase extraction used for

325

actinide separation.

326

The pH range rendered possible by the formation of in situ Br2 is more compatible

327

with typical sample preparation techniques such as automated borate fusion, which

328

uses HNO3 concentrations from 1–3 M. The bromate concentration determined

329

previously is deemed to be ideal since with lower concentrations of bromate in

330

solution, strong acidic conditions are needed to efficiently form enough Br2 to have

331

reproducible extractions. The acid concentration at which Pu uptake is maximal

332

(Figure 1e) is consistent with the one reported by Horwitz et al.24 on the analogue

333

Actinide SPE cartridge.

334

Experimentation with acid concentrations also shows that an extraction mechanism

335

based on a micellar phase equilibrium is predominant, and aqueous solubility is not

336

the primary factor for the uptake efficiency determination.

337 338

Analytical Performances on Environmental Samples

339

The method presented was tested with environmental matrices to determine its

340

applicability for the analysis of real samples. A wide array of samples were selected

341

to determine the efficiencies and verify any possible interferences from the

342

matrix.(Table 1) The method proved to be reliable over a wide range of samples. To

343

determine the best-suited digestion system, automated borate fusion and microwave

344

(MW) digestion were compared. Soil digested with automated fusion led to a

345

complete dissolution with no residue. MW-digestion, using HNO3 and HF (10:1), led

346

to an incomplete dissolution that required filtration. Uptake using MW were lower than

347

for fusion which is due to the presence of F- which is a potent ligand for plutonium.

348

This affirmation is supported by the observed decrease of the uranium, neptunium

349

and americium uptake for the extractant used on a solid support. 24 Determination of

350

Pu in HQ-G2 by MW digestion was not attempted as significant dilution would have

351

been necessary to prevent damages to ICP-MS rendering Pu quantification

352

impossible. Otherwise,

significant quantities of a fluoride scavenger would be


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353


necessary minimizing the separation accomplished by CPE.

354 355

Table 1: Determination of plutonium in environmental matrices Digestion Method

Spike Recoveries (%) Alpha Spectrometry

ICP-MS (n=4)

LiBO2:LiBr fusion

95 ± 2

99 ± 9

MW-digestion (HNO3:HF)(10:1)

30 ± 20

N.A.

N.A.

97

93 ± 9

N.A.

100

95 ± 9

SCP-EU-L-3 (waste water)

N.A.

93

109 ± 4

SCP-EP-L-3 (drinking water)

N.A.

102

100 ± 7

SCP-EP-H-3 (drinking water)

N.A.

98

109 ± 4

IAEA-384 (sediment)

LiBO2:LiBr fusion

102 ± 7

95 ± 9

Soil (HQ-G2)

[28]

Urine [29]

Sea Water

356 357

N.A. = not applicable

358

Soil extraction by borate fusion led to quantitative results, even though potential

359

competitive ions such as iron and calcium are present in abundance. 24 The quantity

360

of calcium ions in the surfactant-rich phase is low as the resolution of the peaks in

361

alpha spectrometry does not decrease when soils are analyzed. The presence of

362

competing ions was validated by ICP-OES. (Table 2) Lithium metaborate used for the

363

sample preparation is reduced in the SRP (decontaminating factor of 20) minimising

364

the presence of Li+ and BO2- in the SRP, one of the main concerns of coupling borate

365

fusion to ICP-MS.

366 367

The method performances was also demonstrated via the analysis of a certified

368

reference material. (IAEA-384) (Table 3) The ratios obtained from the analysis of the

369

material were within acceptable error limits of the expected value and proved that the

370

total recoveries can be achieved by fusion dissolution. This also demonstrates that

371

the method is suitable for the analysis of environmental materials and the

372

determination of the isotopic composition. The method also showed great

373

performance using complementary instruments (ICP-MS and alpha spectrometry),

374

enabling the measurement of both short-lived and long lived Pu isotopes.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

375 376

Based on the results presented in Table 1 and Table 3, it is clear that the proposed

377

approach is applicable for the determination of plutonium even though H 2DEH[MDP]

378

is known to extract other actinides, such as uranium. The presence of an interfering

379

signal from 238U at m/z=239 (as abundance sensitivity or hydride) could explain some

380

of the positive bias reported in table 3. However it was demonstrated that for a 10

381

mg∙L-1 uranium standard, extraction efficiency in the conditions presented is

382

approximately 30%. This observation suggests that some level of separation between

383

U and Pu could be achieved, especially if the conditions were optimized for this

384

purpose.

385

Analytical figures of merit

386

The proposed method features high chemical recoveries and a low detection limit,

387

and is compatible with alpha and mass spectrometric instrumentation. The

388

combination of multiple instruments allows for an almost complete isotopic

389

investigation, which is potentially useful for safety and nuclear forensic applications.

390

The use of radiometric and mass spectrometry allows the determination of both long-

391

lived isotopes and short-lived isotopes with good precision. A minimal detectable

392

activity40 of 0.02 mBq/L and a detection limit varying between 15 and 200 pg∙L -1 are

393

acheivable by alpha spectrometry and ICP-MS respectively. The proposed method

394

compares well to other published liquid extraction methods with respect to LOD(limit

395

of detection), PF, and extraction efficiency. (Table 4) As the method is based on

396

CPE, high preconcentration factors >140 are achieved, which is particularly

397

attractive, especially at very acidic pH. Typically, preconcentration factors are

398

hampered because higher quantities of TTX-114 are required41 which affect poorly

399

on the overall analytical performances. As proposed, the use of Br2 results in lower

400

TTX-114 concentrations and thus higher PF. The main advantage of the proposed

401

method over other existing extraction methods is the minimal sample preparation

402

needed before the actual extraction. As proposed, the only sample preparation

403

needed before performing CPE is the elimination of silica in solution using PEG-

404

6000.42 This step, although necessary as silica is unstable in highly acidic solution, is

405

not time-consuming and does decrease the extraction yield. In this respect, the

406

proposed CPE does not use CaF2 co-precipitation involving HF or hydroxide

407

precipitation used in most methods.23 These methods are not selective as multiple


Page 15 of 21

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

408


ions and alkaline earth ions will follow actinides in the precipitation process.

409 410

Even with minimal sample preparation, the method proposed yielded an excellent

411

plutonium extraction (>98%) and selectivity towards this element compared to other

412

matrix constituents such as alkaline earth and transition metals in solution (table 2).

413

Most decontamination factors values measured were near 50 for high solid content

414

solutions. Note that lanthanum’s decontamination factor is low, as a result of its low

415

abundance in environmental matrices which complicates its detection by ICP-OES.

416 417

The proposed method also compares well against the analogue resin method. The

418

elution from the Actinides resin, which contains H2DEH[MDP], in aqueous media is

419

not quantitative, as the complex formed is too stable. Hence, the complex is usually

420

stripped with EtOH, which is incompatible at high concentrations with mass

421

spectrometry, 43 though the proposed method with redispersion of the SRP through

422

sonication is simple and quantitative. Croudace et al.44 also proposed to digest the

423

resin using borate fusion, but this strategy worked against the main advantage of

424

these methods as separation from the highly concentrated lithium borate matrix is

425

nullified.

426

Table 2 Amount of concomitant ions left after CPE on digested soils and the corresponding decontamination factor. [Elements]

Supernatant (ppm)

SRP (ppm)

Decontamination Factor

Li+

3300 ± 200

60 ± 9

55

B3+

4000 ± 200

155 ± 40

25

Ca2+

13.2 ± 0.2

1320

Fe3+

284 ± 3

7±1

40

Sr2+

2.16 ± 0.06

0.03 ± 0.01

79

La3+

0.023 ± 0.04

0.002

12

Al3+

273 ± 6

4,5 ± 0.7

61

Ba2+

0.92 ± 0.02

18

427



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

428

Page 16 of 21

Table 3: Activity Measured in IAEA-384 Reference Material in comparison to Reference values. (n=4) IAEA-384

429 430 431 432

433 434

Alpha Spectrometry (Bq∙kg-1)

ICP-MS (Bq∙kg-1)

Referred Values (Bq∙kg-1)45

Bias (%)

Pu-238

36 ± 3

-

38.9 ± 0.6

-7.4

Pu-239

-

110 ± 20

97 ± 9

13.4

Pu-[239-240]

107 ± 9

-

107 ± 2

0

Pu-241a

-

-

56 ± 5

-

Pu-242 b

-

-

1.9 ± 0.5c

-

a) β- emitter (liquid scintillation counting required); b) Used as a tracer c) Pu-242 values in italic as too few data were reported

Table 4 : Analytical performance of the proposed method with Pu

a

Extraction Technique

Extractant

Diluent

PF

M.D.A. (mBq∙L-1)

LOD 239 Pu (pg∙L-1)

LOQ 239 Pu (pg∙L-1)

LOD 242 Pu (pg∙L-1)

CPE

H2DEH[MDP]

TTX-114

>140

0.02

15

50

200

LLE

Malonamide

C4mimTf2

N.P.

N.P.

N.P.

N.P.

N.P.

LLE

TBP

Dodecane

N.P.

0,2

N.P.

N.P.

N.P.

LOQ Extraction Instruments References 242 Pu Efficiency % (pg∙L-1) (n=4) ICP-QMS ; 700 >98 Alpha This work Spectrometry; N.P. 99 LSC 46 N.P.

60 ± 7

Alpha Spectrometry

LOQ = limit of quantification; LOD = limit of detection; PF = preconcentration factor; N.P. = not provided

435 436

Conclusion

437

The proposed CPE method is well suited for the preconcentration and determination

438

of plutonium in a wide array of samples and showed an exceptional preconcentration

439

factor for highly acidic samples. We believe that the methodology could become a

440

green alternative to its liquid-liquid extraction counterpart, which uses much larger

441

quantities of solvent. The conditions found are consistent with the conditions used for

442

uranium extraction in a similar system,47 as H2DEH[MDP], TTX-114 and the CTAB

443

dependence is also similar despite the major differences in the extraction pH, high

444

solid matrices, and the use of a Br2/BrO3- system.

445 446

The proposed method also demonstrates the applicability of the CPE system to solid

447

samples dissolved by borate fusion. This coupling was possible due to the resistance

448

of the proposed method to highly saline media. Using this coupling, the determination

449

of the specific activity of many isotopes of plutonium was possible by both ICP-MS

450

and alpha spectrometry.


47

Page 17 of 21

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451 452

Acknowledgements

453

This work was financially supported by the Natural Sciences and Engineering

454

Research Council of Canada (NSERC) and the Canadian Foundation for Innovation

455

(CFI). The authors would like to thank Corporation Scientifique Claisse for providing

456

the automated fusion unit.

457

Author Informations

458

a

459

genie, Universite Laval, 1045 Avenue de la Medecine, Bureau 1250D, Pavillon

460

Alexandre-Vachon, Quebec, QC, Canada G1V 0A6. E-mail:

461

[email protected]; Fax: +418 656-7916; Tel: +418-656-7250

Laboratoire de Radioecologie, Departement de chimie, Faculte des sciences et de

462 463

Notes

464

The authors declare no competing ﬁnancial interest.

465

References

466 467

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343x185mm (72 x 72 DPI)


Cloud Point Extraction of Plutonium in Environmental Matrixes

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