Investigation of the Electrophoretic Mobility of the Actinides Th

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Investigation of the Electrophoretic Mobility of the Actinides Th, U, Np, Pu, and Am in Different Oxidation States Christian Willberger, Samer Amayri, Verena Häußler, Raphael Scholze, and Tobias Reich Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00997 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 11, 2019

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Analytical Chemistry

Investigation of the Electrophoretic Mobility of the Actinides Th, U, Np, Pu, and Am in Different Oxidation States Christian Willberger*, Samer Amayri, Verena Häußler, Raphael Scholze, Tobias Reich* Institute of Nuclear Chemistry, Johannes Gutenberg University Mainz, 55099 Mainz, Germany ABSTRACT: The electrophoretic mobilities (µe) of the actinides Th and U–Am in different oxidation states (prepared in 1 M HCl and 1 M HClO4) have been determined by CE-ICP-MS using 1 M acetic acid as the background electrolyte, which has proven to provide an excellent setup for trace analysis at environmentally relevant concentrations (1 · 10−9 M). The values are independent of the respective acid solution. The µe of the Pu oxidation states +III to +VI have been measured. They agree with both the available literature data and with the redox-stable analogues (Eu(III), Th(IV), Np(V), U(VI)) that have also been investigated. The trend in the µe for the actinides U–Pu was found to be An(III) > An(VI) > An(V) > An(IV). The µe of Am(III) cm2

cm2

cm2

(µe(Am(III)) = 3.86 · 10−4 ), U(IV) (µe(U(IV)) = 0.34 · 10−4 ) and U(VI) (µe(U(VI)) = 1.51 · 10−4 ) have been measured for Vs Vs Vs the first time under these experimental conditions. Furthermore, the measured µe show systematic trends that can be rationalized based on the calculated species distribution of the actinides in 1 M acetic acid and the corresponding average effective charges qeff.

INTRODUCTION Nuclear power has been used as a source of energy for many years and is still in use in many countries. During the operation of a nuclear power plant, a large amount of highlevel radioactive waste is generated, for which safe entombment has to be ensured for periods up to one million years. Most notably, the spreading of radionuclides into the environment has to be prevented at all times. Nuclear waste will in future be stored in a deep geological repository with argillaceous and crystalline rocks or salt as potential host rocks.1, 2 It is very important to gain a deep understanding of the chemical behavior of the relevant radionuclides in the respective geological formation. Long-lived actinides such as 6

(t1 = 2.14 · 10 a), 2

239

Pu

U (t1 = 4.47 · 109 a),

238

2

4

(t1 = 2.41 · 10 a) 2

and

237

Np

241

Am

(t1 = 432.2 a) are part of the high-level radioactive waste in 2

spent nuclear fuels. After a long period of enclosure these nuclides account for a significant percentage of the total radiotoxicity of the nuclear waste.3 Under environmental conditions actinides exhibit a wide distribution of possible oxidation states, with different speciations in aquifers. Plutonium, for example, displays a wide range of oxidation states from Pu(III) up to Pu(VI) in aqueous solution, whereby the lower ones exist as the free cations Pu3+ and Pu4+ and the higher ones as the plutonyl cations PuO2+ and PuO22+, not to mention the versatile possibilities of different complexation partners and coordination numbers.4 For this reason it is essential for the safety assessment of a possible repository to have an extensive knowledge of the exact speciation of the considered actinides under the expected conditions. The determination of the exact speciation of the actinides at environmentally relevant concentrations in the worst case of a possible leakage from the repository requires a reliable and

effective analytical method. For analyzing the different oxidation states, the separation capability of capillary electrophoresis (CE) was coupled with the high sensitivity of inductively coupled plasma mass spectrometry (ICP-MS). With this setup the separation and quantification of different actinide oxidation states was achieved within less than 20 minutes and limits of detection of approximately 1 · 10-9 M, which is expected for actinides in the far field of a repository.5, 6 In the present work the electrophoretic mobilities of different oxidation states of the actinides Th, U, Np, Pu, and Am was determined in 1 M acetic acid medium. The migration of actinides in a capillary is strongly affected by their speciation in solution, which is dependent on several parameters such as pH, redox potential and ligand complexation. Hence the results of such measurements under model conditions are very helpful for the realization and interpretation of investigations of authentic environmental samples in complex media concerning the long-term safety analysis of possible nuclear repositories. The CE-ICP-MS method was first described by Olesik et al in 1995.7 Since then, many applications in the field of elemental speciation have been summarized in review articles.8-11 Various domains, where CE separation schemes were applied to actinide speciation, have been reviewed by Timerbaev.12 The present study is focused on the actinides Pu and Np. The results for these elements will be discussed in comparison with the values of redox-stable homologues and compared with the available data in the literature.6, 13, 14

EXPERIMENTAL Capillary electrophoresis The principle of separation in electrophoresis techniques is the varying velocity of migration of different ions in solution

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in an applied electric field.15 The electrophoretic mobility µe , which is measured during capillary electrophoresis experiments, is a characteristic quantity of each ion in a particular medium. For performing capillary electrophoresis, ca. 50 µm thin fused silica capillaries are used, which are filled with a background electrolyte (in our case 1 M acetic acid). The sample is dissolved in the same medium and transferred to the capillary via hydrodynamic injection. A high voltage is applied between two platinum electrodes and the electric circuit is closed by the background electrolyte solution. One can calculate the electrophoretic mobility of an ion µi by knowing the migration times of the electroosmotic flow (EOF) tEOF and of the examined species ti at a given capillary length lk and applied voltage U. µi =

lk 2

1

U

ti

-

1 tEOF

(1)

The EOF describes the movement of the whole electrolyte solution inside the electric field. Neutral species migrate together with the EOF, which can be used for measuring the migration time of the EOF. In the present case 2-bromopropane was used as the neutral EOF marker. Its behavior during CE experiments was tested in comparison with the behavior of 2-bromoethanol, another EOF marker used in the literature14. Both 79Br signals had exact the same migration time, indicating the suitability of 2bromopropane as EOF marker.. CE-ICP-MS system ICP-MS is an appropriate detection method for analyzing eluates previously separated by CE. The system combines the advantages of mass spectrometry, which is a very sensitive and specific method for simultaneous multi-element detection at very low concentrations, with the benefits of inductively coupled plasma, namely a high efficiency of atomization and ionization of the sample. A detailed summary of possible applications of ICP-MS as a detection method for CE can be found in the literature.16 For an effective coupling of the CE (Agilent 7100, Agilent Technologies, Santa Clara, California, USA) with the ICP-MS (Agilent 7500 ce, Agilent Technologies, Santa Clara, California, USA), a functional connection between both systems is of utmost importance. A schematic drawing and a picture of the coupling system that was used in this work are shown in Supporting Information (SI), Figures S1 and S2. The centerpiece of the device is a MiraMist CE nebulizer (Burgener Research, Mississauga, Canada) leading to a Scott-type spray chamber (AHS Analysentechnik, Tübingen, Germany). In this parallelpath nebulizer, the carrier gas flow and the capillary are separated in two parallel tubes and the make-up solution is applied rectangularly to those. Accordingly, the mixture of the three components takes place just at the very tip of the nebulizer in a very small volume. Reviews of the importance of nebulizers for performing CE-ICP-MS measurements can be found in the literature.17-19 The make-up solution is introduced via a syringe pump (PicoPlus, Harvard Apparatus, Holliston, Massachusetts, USA). Its flow rate has to be adjusted experimentally since too small flow rates would lead to a laminar flow profile and too

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high flow rates would result in a tailback of the sample in the capillary. In the present experiments a 1.25% HNO3 solution with 10% ethanol was used. Additionally, the make-up solution contained 5 ppb 89Y, 107Rh, 140Ce, and 209Bi as internal standards. Reagents Nitric acid, 2-bromopropane, hydroxylamine hydrochloride (NH3OH+Cl−), 238U wires, and Rh ICP standard solution were supplied by Merck (Darmstadt, Germany). Acetic acid and perchloric acid were received from Riedel-de Haën (Seelze, Germany), and hydrochloric acid from Fisher Scientific (Loughborough, UK). The ICP-MS elemental standards were supplied by High-Purity Standards (Charleston, South Carolina, USA) (Bi, Ce and Y), Peak Performance (CPI International, Santa Rosa, California, USA) (Eu and U) and Accu TraceTM (Accu Standard, New Haven, Connecticut, USA) (Th). All reagents were of pro-analysis quality or better. MilliQ water (18.2 MΩ, SynergyTM Millipore water system, Millipore GmbH, Schwalbach, Germany) was used for dilutions. All solutions were filtered through 0.2 µm syringe filters (Nalgene, Rochester, New York, USA) prior to their application in CE to prevent clogging. All actinide stock solutions were prepared both in 1 M HCl and in 1 M HClO4 to see if there is an effect on the electrophoretic mobility of the ions after diluting in 1 M acetic acid depending on the medium of the stock solution. 239 Pu(VI) and 237Np(VI) stock solutions were prepared by repeated evaporation with 1 M HClO4 or 1 M HCl respectively under the addition of very small amounts of NaF to prevent colloid formation. Starting from this hexavalent state, the tri-, tetra-, and pentavalent oxidation states were obtained by electrolytic reduction and oxidation reactions. Besides this, a 237Np(V) stock solution was prepared as described in the literature.20 Proceeding from this, 237Np(IV) was yielded by chemical reduction with hydroxyl ammonium hydrochloride. 238 U(VI) was obtained by dilution of the ICP-MS standard solution to the required concentration. Dissolving 238U wires in 1 M HCl produced 238U(IV) solutions. As Am(III) is stable in the trivalent oxidation state, there was only the need to adjust the medium of the stock solution by evaporation until near dryness and dissolving the residue in 1 mL of the respective 1 M acid. The Th(IV) and Eu(III) solutions were diluted from the ICPMS standard solutions with 1 M HCl or 1 M HClO4, respectively. The purity of the different oxidation states of the investigated actinides was controlled by UV-Vis measurements (Tidas 100, J & M Analytik AG, Essingen, Germany). The concentrations of the respective solutions were measured via liquidscintillation counting for Pu or γ-ray spectrometry measurements for U, Np, and Am. Some oxidation states turned out to be much more instable than expected under the used conditions, which required certain additions. These were chosen to ensure a reducing (NH3OH+Cl−) or oxidizing (NaClO) environment to stabilize the very reactive oxidation states without further complexation properties.21 Therefore, NaClO was added to the Np(VI) samples and hydroxyl ammonium hydrochloride to the Pu(III) samples.


2

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Analytical Chemistry If the concentration of the stock solutions exceeded 5 · 10-5 M, the respective solutions were diluted to this specific concentration. Solutions with concentrations less than -5

5 · 10 M were used without further dilution. Sample preparation and parameters for CE-ICP-MS Unless stated otherwise, samples for CE-ICP-MS measurements were prepared by diluting 2 µL of the analyte solution with 198 µL 1 M acetic acid in conical micro-inserts of borosilicate glass (Carl Roth AG, Arlesheim, Switzerland). To each sample, 1 µL 2-bromopropane was added as the EOF marker. The final concentration of the analyte was (5.00 ± 0.45) · 10-7 M. The micro-inserts were transferred into the CE system by placing them in polyethylene vials sealed with polyethylene olefin snap caps (both Agilent Technologies, Santa Clara, California, USA). The fused silica capillaries (Polymicro Technologies, Phoenix, Arizona, USA) were preconditioned before every use by flashing several times with MilliQ-Water, 0.1 M sodium hydroxide solution, 0.1 M hydrochloric acid, and 1 M acetic acid. In between the measurements the capillary was flushed with 1 M freshly prepared acetic acid to remove the remaining analytes and to prevent effects caused by altered acetic acid due to the applied high voltage. Temperature stability is a crucial point in performing CE experiments since a variation of 1 °C can lead to a change in the electrophoretic mobility of about 2%. We controlled the temperature at 25 °C by the air cooling device of the Agilent apparatus. Only a short part of the capillary leading out of the CE apparatus inside the nebulizer could not be temperature controlled. The temperature rise inside the capillary due to Joule heating was calculated following a procedure given in the literature22 to be on average about 0.5 °C and to not exceed 1 °C under the used experimental conditions. All evaluations were undertaken with the MassHunter Workstation software (G7200B, Agilent Technologies, Santa Clara, California, USA). All parameters during the measurements are summarized in Table S1.

RESULTS AND DISCUSSION The electrophoretic mobilities of the Pu oxidation states +III to +VI in the given medium were determined. To assign the observed peaks in the measured electropherograms to the correct oxidation states, a set of redox-stable analogues was investigated and the obtained migration times were compared with those of the Pu measurements. First the electropherograms of the redox-stable analogues and then the results of the Pu mobility measurements are presented., followed by a discussion of the electrophoretic mobilities and their systematics.. Electrophoretic mobilities of redox-stable oxidations states As already mentioned, a set of redox-stable elements as homologues for the different Pu oxidation states was investigated first. These analogues for the +III to the +VI oxidation states were 153Eu(III), 232Th(IV), 237Np(V), and 238U(VI). All elements were mixed in one and the same solution with a concentration of ci = 5 · 10-7 M each. The measurements were

undertaken with both 1 M HCl and 1 M HClO4 as sample solution to test whether there exists an influence of the acidic medium on the electrophoretic mobility measured in the 1 M acetic acid medium. If this was the case, a difference between a non-complexing agent (ClO4-) and a weakly complexing agent (Cl-) with respect to the obtained electrophoretic mobilities would occur. A typical electropherogram is shown in Figure 1. The electrophoretic mobilities calculated using Equation (1) for the different elements dissolved in both 1 M HCl and 1 M HClO4 solutions are given in Table 1. As one can see from these values, the choice of the medium in which the oxidation states were prepared has no significant effect on the electrophoretic migration of the ions. The two values for the electrophoretic mobility of each element are nearly identical within the limits of error. The 100-fold dilution during sample preparation for CE-ICP-MS measurements and the high excess of acetic acid on the capillary create a medium in which the acid of the initial solution, i.e., HCl or HClO4, becomes negligible.

Figure 1. Electropherogram of 5 • 10-7 M redox-stable analogues measured in 1 M acetic acid (pH 2.4) with U = 25 keV and υ = 25 °C.

As can be seen in Figure 1, sharp peaks were obtained both for the investigated species and of the EOF marker. The apparent differences in the peak areas result from discrepancies in the ionization efficiencies of the elements in the plasma of the ICP-MS system and, in case of Eu, from the fact that only mass 153 was detected, which has a natural abundance of just 52.2% (mass 151 with 47.8% was not detected). The detected migration times increase from 153Eu via 232Th and 237Np to 238U, which corresponds to the reverse of the trend for the electrophoretic mobilities. Eu(III) migrates fastest, which results in the highest electrophoretic mobility, followed by Th(IV) and Np(V). The slowest ion on the capillary is U(VI), so this species has the lowest electrophoretic mobility. Following this order of elution for the different oxidation states of the analogues, the order of the electrophoretic mobilities from high to low is as follows: Eu III > Th IV ≈ Np V > U(VI)

(2)

To understand the migration of these elements, one has to take into account the different complexation behavior of the individual ions with the surrounding ligands. Initially, when the metal ions are dissolved in 1 M HCl or HClO4, they are


3


present as aquo ions of Eu3+, Th4+, NpO2+ and UO22+. After dilution in 1 M acetic acid and during the CE, the main ligand is the acetate anion. The complexation with acetate leads to a shielding effect, so the applied electric field acts upon only a smaller effective charge. Furthermore, in performing CE experiments one has to take into account not only the crystallographic or ionic radius, respectively, of the investigated ion but rather the radius of the first or even the second coordination sphere. This coordination radius significantly affects the overall size of the migrating ions. As a comparable measure of the coordination radius, the mean metal-oxygen distance in the hydrated ion can be considered. In EXAFS studies the following bond distances were determined: 2.42 Å for U(VI),23 2.49 Å for Np(V),24 2.45 Å for Th(IV),25 and 2.42 Å for Eu(III).26 Since the difference in the metal oxygen bond distances given above is ≤ 0.07 Å, the order of the quotient of charge q and radius r is governed by the ionic charge of the complexed ions. To take this influence into account, a speciation calculation with the experimental conditions used in this work was performed. From the speciation of each ion a mean effective charge qeff was calculated (for details see SI). They resulted as 0.70 for U(VI), 0.88 for Np(V), 1.12 for Th(IV), and 2.15 for Eu(III) (see also Table 3). Neglecting size differences between different species, the predicted order of elecis: trophoretic mobilites based on qeff Eu III > Th IV ≈ Np V > U(VI). This agrees with the experimental results (Figure 1, Table 1). Additionally, it is known for tetravalent actinides to have a strong tendency toward sorption on surfaces, so that it cannot completely be ruled out that the migration of the +IV actinides is slowed down by interactions with the inner capillary wall.27 One can conclude that all the factors mentioned above have to be taken into account for the interpretation of a given order of electrophoretic mobilities. On the other hand it is possible to rationalize the measured mobilities of the ions based on their speciation during CE. Besides the already discussed elements, a sample of Am(III) was investigated to compare the electrophoretic mobility of a trivalent actinide with the value of the lanthanide Eu(III). The results show that Am(III) has a similar mobility as Eu(III) whereby the migration of Am(III) is slightly slower (Table 1). Nevertheless, it is also the fastest of all oxidation states. The metal-oxygen distance for Am(III) is 2.48 Å28. This is only 0.06 Å longer than the value for Eu(III) aquo ion. Therefore, the average effective charge of Eu(III) and Am(III) is the dominating factor during their migration on the capillary. The speciation calculations (for details see SI) gave qeff(Eu(III)) = 2.15 and qeff(Am(III)) = 1.92, indicating a slightly higher mobility of Eu(III) compared to Am(III) as it was observed in the experiment. It should be noted that some literature data show the opposite trend. Lundqvist et al.29 ob-

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tained a higher relative migration velocity for Am(III) compared to Eu(III) in a (H,Na)ClO4 medium (varying pH and ionic strength I) at 25 °C, but the values seem to be very similar to each other. The same was found by Fourest et al.30 in a (H,Li)Cl medium (pH = 4 and varying I) at 25 °C, while in a later work31 they state, that the actinides are less mobile than the respective lanthanides. Marin et al.32 published cm2

cm2

and µe (Eu III ) = 3.75 · 10-4 µe (Am III ) = 4.25 · 10-4 Vs Vs in (H,K)Cl medium determined by paper electrophoresis (15 °C, pH < 2.5 and ! = 5 ∙ 10!! M). Furthermore in their work it is shown that the order of the electrophoretic mobilities is turned around for pH > 3.5 for the hydroxides µe (Am(OH)!! ) = 0.90 · 10-4

cm2

and

Vs 2 -4 cm 10 ). Vs

33

Rösch et al. determined ion µe (Eu OH !! ) = 2.38 · mobilities by means of a glass capillary apparatus. Their values for the electrophoretic mobilities are µe (Am(III)) = 5.48(15) · 10-4

cm2

and

Vs cm2 10-4 Vs

for pH > 2.5 (25 °C, µe (Eu(III)) = 5.12(29) · ! = 0.1 M) in a (Na,H)ClO4 medium. In a work using CE-ICPMS by Topin34 the electrophoretic mobilities are given as µe (Am(III)) = 4.62(15) · 10-4 cm2 10-4 Vs

cm2

and

Vs

in a (H,Na)Cl medium (25 °C, µe (Eu(III)) = 4.4(2) · pH = 2.0 and ! = 0.1 M). This overview shows that the absolute values for the electrophoretic mobilities of Eu(III) and Am(III) are difficult to compare among each other since they have been measured under different experimental conditions (temperature, pH, ionic strength, and medium) that affect the migration velocity of a given ion. As to our knowledge the mobilities of Am(III) and Eu(III) have never been investigated before under the experimental conditions used in this work. The same is true for the electrophoretic mobilities of Th(IV) and U(VI), where no data are given in the literature for measurements in 1 M acetic acid. The redox speciation of neptunium under comparable experimental conditions was investigated by Stöbener et al.5 However, no EOF marker was used in that work. Therefore, no values for the electrophoretic mobility were available. In his later work,35 the electrophoretic mobility is given as cm2

µe (Np V ) = 2.0 · 10-4 , which is in good agreement with Vs the present study. Graser et al.14 published a value of cm2

µe Np V = 2.4 (± 0.03) · 10-4 . This value is significantly Vs higher than the values of this study and by Stöbener et al., and cannot be reconciled with them without further analysis. A thorough discussion of this deviation, by interpreting additional Np oxidation states, can be found in Section 3.3. Other

Table 1. Electrophoretic mobilities of the redox analogues (stock solutions: 1 M HCl and 1 M HClO4). medium solution

stock

mobilities/10-4

cm2 Vs

Eu(III)

Am(III)

Th(IV)

Np(V)

U(VI)

1 M HClO4

4.19 (± 0.11)

3.86 (± 0.10)

2.18 (± 0.04)

2.09 (± 0.14)

1.51 (± 0.09)

1 M HCl

4.10 (± 0.09)

3.92 (± 0.03)

2.24 (± 0.11)

2.03 (± 0.08)

1.48 (± 0.04)


4

Page 5 of 9

studies on the electrophoretic mobility of Np(V) were done by Aupiais et al.36 They determined a value of cm2

µe (Np V ) = 2.94(7) · 10-4 for zero ionic strength (25 °C, Vs HClO4 medium with creatinine buffer, pH = 5.00). Rösch et cm2

al.37 published a value of µe (Np V ) = 2.25(20) · 10-4 in 1 Vs M (Na,H)ClO4 solution (25 °C, 1 < pH < 2). Furthermore, it is shown in their work that the electrophoretic mobility of Np(V) in acetic acid medium decreases by about 50% with the acetate concentration increasing from !(acetate) = 10!! M to !(acetate) = 10!! M. Once again, direct comparison of these data with the present results is not possible because of the differences in the experimental conditions, but the values seem to fit well in the overall picture. Separation and identification of Pu oxidation states For the investigation of the Pu oxidation states, the samples were prepared electrochemically starting from Pu(VI) as described above. With this method all four Pu oxidation states in the range from +III to +VI could be produced in 1 M HClO4. During the CE-ICP-MS measurements it emerged that only Pu(IV) and Pu(VI) were stable under the given experimental conditions. Pu(III) was oxidized and Pu(V) disproportionated during dilution in 1 M acetic acid, so that only electropherograms of Pu(IV) and Pu(VI) were obtained. The +III oxidation state could be stabilized by adding an excess amount of hydroxyl ammonium hydrochloride. Samples with Pu(V) were prepared by adding a Pu(VI) aliquot to synthetic Opalinus Clay pore water (OPA PW) at pH 7.6 (the composition is described elsewhere).38 The electropherograms are presented in Figure 2. Here the ICP-MS signal for mass 239 is plotted as a function of the electrophoretic mobility as calculated via Equation (1). Pu(V) in Opalinus Clay pore water

1E+4

Pu(III) 1E+4

8E+5 7E+5 6E+5

8E+3

5E+5

6E+3

4E+5

Pu(VI)

3E+5

Pu(IV)

4E+3

signal Pu(V) / cps

signal Pu(III), Pu(IV), Pu(VI) / cps

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


2E+5 2E+3

1E+5

0E+0

0E+0 5.0

4.0

3.0

2.0

1.0

electrophoretic mobility/10-‐4 cm2V-‐1s-‐1

0.0

Figure 2. Electrophoretic mobilities of the different (5 • 10-7 M) Pu species measured in 1 M acetic acid (pH 2.4) with U = 25 keV and υ = 25 °C.

For the oxidation states +III, +IV, and +VI one peak was obtained. These samples contained no other oxidation states at concentrations higher than the detection limit of the used method. In the OPA PW sample, beside the dominant peak with the mobility assigned to Pu(V), a second, smaller peak with the mobility of Pu(IV) can be seen. Evidently, the medium here was also unable to completely preserve the +V oxidation state. In the UV-Vis spectrum only Pu(V) could be seen,

so in this case a partial reduction from Pu(V) to Pu(IV) took place during the dilution step with 1 M acetic acid. The detected migration times increase from Pu(III) via Pu(VI) and Pu(V) to Pu(IV). Accordingly the order of the electrophoretic mobilities of the different Pu oxidation states (Table 2) can be summarized as follows: Pu III > Pu VI > Pu V > Pu(IV) (3) As in the case of the redox-stable analogues, the trivalent Pu species shows the highest mobility. However, compared to Th(IV) and U(VI) the order of migration for the tetravalent and the hexavalent Pu is reversed. Pu(IV) has the lowest mobility of all Pu oxidation states. It should be noted that µe changes significantly from Th(IV) to Pu(IV), i.e., from 2.18 • 10-4 to 0.96 • 10-4 cm2/Vs, respectively. The same order of electrophoretic mobilites as for Pu was also determined for Np, i.e, Np VI > Np V > Np(IV) (see Table 3). A comparison with published µe values for the different Pu oxidation states is presented in Table 2. The first investigation on the different Pu oxidation states was carried out by Kuczewski et al.6 under the same conditions as in the present experiment. In that work no electrophoretic mobilities were given due to the fact that no EOF marker was measured. Therefore, the µe values were estimated from the electropherograms shown in that publication by calculating a theoretical migration time of the EOF marker from the electrophoretic mobility of the Np(V) species (which was also measured as a +V analogue) and the corresponding value obtained in this study. All Pu mobilities could then be determined using Equation (1) as explained above. As one can see from Table 2, this leads to results that are in good agreement with the values of the present study; deviations can be explained by the difficulty of the exact reading of the particular migration times. The migration order of the Pu oxidation states is the same, which is also confirmed in other works.13, 39 Recently Graser et al.14 published values for the electrophoretic mobilities of different plutonium species that were measured under the same conditions as used in the present work. As can be seen from Table 2, the order of migration for the different Pu oxidation states is the same in both studies. Except for a slightly larger difference in case of Pu(IV) and Pu(VI), the µe values for Pu(III) and Pu(V) agree within their limits of error. The comparison of our results with the results for Pu(III) and Pu(V) given in the works of Topin et al.40-42 shows the necessity of comparing electrophoretic mobilities under the same experimental conditions, such as pH, ionic strength and temperature. The present work was carried out at pH 2.4 and ionic strengths 1.0 mol/L, whereas Topin et al. measured at lower ionic strengths and variable pH values. The deviations of the electrophoretic mobilities can thus be seen as a consequence of the varying experimental conditions. Trends in mobility values and estimation of mobilities Table 3 summarizes all µe values that were determined in this study. For U, Np, and Pu it was possible to determine µe values for several oxidations states of each element. Based on these data, one can derive the following empirical relationship (Eq. 4) between the electrophoretic mobilities of one particular actinide (An) in different oxidations states (if they can be formed).


5


Page 6 of 9

Table 2. Electrophoretic mobilities of the Pu oxidation states and comparison with literature. mobility/10-4

this work 1 M HClO4

cm2 Vs

Pu(III)

Pu(IV)

Pu(V)

3.92 (± 0.06)

0.96 (± 0.04)

OPA PW

Pu(VI) 2.20 (± 0.03)

1.65 (± 0.05)

literature

Pu(III)

Pu(IV)

Pu(V)

Pu(VI)

Kuczewski6 *

3.85

0.82

1.69

2.10

Graser14

4.0 (± 0.07)

1.2 (± 0.08)

1.6 (± 0.06)

2.4 (± 0.08)

Topin

41 **

Topin

48 **

1.93 (± 0.12)

Topin

46, 49, **

2.360–2.405

*

4.61 (± 0.15)

calculated from the electropherograms shown in the publication

**

different pH values and ionic strengths

µe (An III ) > µe (An VI ) > µe (An V ) > µe (An IV ) (4)

-‐1.72 × 10-‐4 -‐0.55× 10-‐4 -‐0.69 × 10-‐4 cm2/Vs

This allows estimating an unknown electrophoretic mobility of an element based on its other measured values using Eq. 4. Estimates for µe of U(III), Np(III), and U(V) based on this approach can be found in SI. It should be noted that the numerical values given in Eq. 4 are only applicable for identical conditions in the CE-ICP-MS measurements, in particular the choice of background electrolyte, ionic strength, temperature etc. In case of Pu, similar differences in µe between the different oxidation states as in Eq. 4 were observed in previous studies6, 14 as discussed before. However, there is a discrepancy for Np. The electrophoretic mobilities of Np(V) and Np(VI) are given by Graser et al.14 as µe Np V

= 2.4 (± 0.06) · 10-4

cm2

Vs cm2 10-4 , Vs

and

so here the order of the µe Np VI = 2.2 (± 0.05) · An(V) and An(VI) is contrary to the trend for Pu and to the Np results of the present study. Since Np and Pu are chemically very similar, no explanation for the reversed order in the electrophoretic mobilities of Np(V) and Np(VI) in the previous work can be given here. The discussion in this Section showed that differences in the electrophoretic mobility of similar species are governed most-

ly by their average effective charge during the migration. A manifestation of this strong correlation between µe and qeff can be obtained by calculating the ratios qeff/µe. With three exceptions, the qeff/µe values in Table 3 fall in the range of 0.42 to with an average value of 0.52 • 104 Vs/cm2 0.48 (± 0.04) • 104 Vs/cm2. Strong deviations from this average are observed for tetravalent U, Np, and Pu. Since the µe values for these species agree with Eq. 4, the discrepancy in the µe/qeff ratios must be due the calculated qeff. As can be seen from Table S2, the types of acetate complexes reported in the literature for tetravalent Th, U, Np, and Pu differ significantly. For Th(IV) 1:1 to 1:5 acetate complexes are reported; for U(IV), Np(IV), and Pu(IV) only 1:1 and 1:2 acetate complexes are considered in the database, leading to more positive values of qeff. Therefore, the complexation reactions of acetate with tetravalent U, Np, and Pu should be reinvestigated.

SUMMARY AND CONCLUSIONS CE-ICP-MS has proven to provide an appropriate experimental setup for trace analysis of actinides in different oxidation states and speciations in aqueous solutions. With the described setup it was also possible to carry out multi-element measurements in one solution. All Pu oxidation states that are relevant under environmental conditions (III–VI) could be measured and compared to redox-stable analogues (Eu(III), Am(III), Th(IV), Np(V), U(VI)). The trend in the electrophoretic mobilities for the actinides U to Pu using 1 M acetic acid as the background electrolyte was found to be An(III) > An(VI) > An(V) > An(IV).


6

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Table 3. Measured electrophoretic mobility µe and calculated mean effective charge qeff. oxidation state

III

IV

V

VI

element

µe

qeff

10-4 cm2/Vs

ratio (qeff/µe)×104 cm2/Vs

the possibility of a better understanding of the exact speciation of an actinide ion in a given solution.

ASSOCIATED CONTENT Supporting Information

Eu

4.19 (±0.11)

2.15

0.51

Pu

3.92 (±0.06)

1.75

0.45

Am

3.86 (±0.10)

1.92

0.50

Details on the CE-ICP-MS measurements and the coupling between CE and ICP-MS (Figures S1, S2, and Table S1); calculations of the acetate speciation of metal ions and their species distribution (Table S2); estimation of electrophoretic mobilities with schema for investigated actinides (Table S3).

Th

2.18 (±0.04)

1.12

0.51

AUTHOR INFORMATION Corresponding Author

1

U

0.34 (±0.12)

2.02

5.943

Np

1.42 (±0.12)

1.96

1.383

Pu

0.96 (±0.04)

2.33

2.433

Np

2.09 (±0.14)

0.88

0.42

Pu

1.65 (±0.05)

-4

-

U

1.51 (±0.09)

0.70

0.46

2

4

* E-Mail: [email protected], [email protected]

ACKNOWLEDGMENT We thank Dr. J. Aupiais for the inspiring discussions and the very helpful advices on the topic of determining electrophoretic mobilities of actinides by CE-ICP-MS.

Np

2.60 (±0.12)

-

-

Pu

2.20 (±0.03)

1.01

0.46

We thank the anonymous reviewers of this manuscript for their qualified and elaborate remarks.

average: 0.48 ± 0.04

This work was financially supported by the Federal Ministry for Economic Affairs and Energy under contract No. 02E10981.

1) Sample was diluted in 1 M HCl instead of 1 M acetic acid. 2) Sample was diluted in 1 M HClO4 instead of 1 M acetic acid. 3) This value is not included in the calculated average ratio. 4) No thermodynamic data available.

If reliable thermodynamic data are available for speciation calculations, it is possible to estimate the electrophoretic mobility of a certain actinide based on the CE measurements of neighboring actinides in the same or other oxidations states. From experimental µe values and calculated qeff, an average value for the ratio qeff/µe can be obtained for a given experimental condition. In this study, 1 M acetic acid (pH 2.4) was used as background electrolyte in CE. The obtained qeff/µe ratio of 0.48 (± 0.04) • 104 Vs/cm2 can be used to verify the consistency of the thermodynamic data and to estimate µe based on calculated qeff. As an alternative, a rough estimate of the electrophoretic mobility could be obtained also using the model of Anderko et al.,43 as it has been done for sulphate and chloride complexes of Np(V) and Pu(V) by Topin et al..42 In future works the results from this broad overview of the separation of different actinide speciations in model samples can be used for the investigation of unknown samples, for example from experiments being carried out in connection with the long-term safety analysis for a possible nuclear repository. By comparison of the electrophoretic mobilities measured for the species in an unknown sample with the ones known from the present work, one can infer the qualitative and (by calibration) quantitative composition of the sample. Furthermore, with CE-ICP-MS measurements it is possible to gain insight into the complexation processes of actinides with several ligands in different oxidation states. This offers

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Investigation of the Electrophoretic Mobility of the Actinides Th

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