using CE-ICP-MS

Radiochim. Acta 98, 71–75 (2010) / DOI 10.1524/ract.2010.1687 © by Oldenbourg Wissenschaftsverlag, München

Determination of the stability constants of nitrate complexes of Np(V) and Pu(V) using CE-ICP-MS By S. Topin∗, J. Aupiais∗ and N. Baglan CEA, DAM, DIF, 91297 Arpajon, France (Received May 28, 2009; accepted in revised form August 24, 2009)

Pentavalent actinide / Hyphenated technique / Nitrate / Plutonium / Neptunium / Capillary electrophoresis

Summary. The interactions of pentavalent Np and Pu with nitrate ligands has been investigated in perchlorate/nitrate media by capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS). The binding constant for ◦ the complex PuO2 NO3 , log10 βPuO = 0.14 ± 0.14, is estab2 NO3 lished for the first time while a new value for NpO2 NO3 , ◦ log10 βNpO = 0.13 ± 0.14, is proposed. The usual analogy 2 NO3 applied between Np(V) and Pu(V) interactions seems to be valid for nitrates.

1. Introduction The analogy between Pu(V) and Np(V) is commonly used to predict the Pu(V) behaviour in the presence of inorganic ligands. Indeed, only few reliable experimental data are reported in the literature concerning the Pu(V) behaviour in carbonate media [1, 2] and, more recently, in sulphate and chloride media [3]. This last study shows the analogy seems to be valid for the outer sphere interaction with chloride ions whereas a slight shift can exist for the inner sphere interaction with the sulphate [3]. The absence of data for most of the Pu(V) interactions with inorganic ligands is strongly regretted in the thermodynamical data base (TDB) [1, 4] since Pu(V) plays a key role on the overall plutonium behaviour in oxic conditions (surface water) [5, 6]. Indeed, this mono-charged species (Pu(V) = PuO2 + ) is the less reactive form of Pu, characterised by a high mobility in the environment and stressing the importance to improve the knowledge of its chemical behaviour. Therefore, it appears essential to complete the thermodynamical database by investigating the interaction with common inorganic ligands. The present study focuses on the An(V)-nitrate interaction which is mainly described by the formation of only a 1 : 1 complex, AnO2 + + NO3 − ⇔ AnO2 NO3 [AnO2 NO3 ] , βAnO2 NO3 = [AnO2 + ][NO3 − ]

with (1)

*Authors for correspondence (E-mail: [email protected], [email protected]).

and for which the only available data are for Np(V): log10 βNpO2 NO3 = −(0.25 ± 0.05) at 2 M HClO4 (20 ◦ C) [7], log10 βNpO2 NO3 = −0.28 at 8 M HClO4 (20 ◦ C) [8], log10 βNpO2 NO3 = −(0.55 ± 0.09) at 2 M NaClO4 (20 ◦ C) [9] and log10 βNpO2 NO3 = −0.52 at 8.5 M NaClO4 (20 ◦ C) [10]. These binding constants are extremely weak illustrating the absence of inner sphere interaction for [NO3 − ] < 6 M [9] and indicating that the interaction is exclusively ionic. It must be noted that the strength of the interaction is of the same order than that of chloride ligands [1, 3] which also involves outer sphere interaction in dilute media [11]. In this work, the capillary electrophoresis (CE) has been used. This technique has been commonly employed to extrapolate binding constant in the biological field [12]. It allows to check, in-situ, the oxidation state of the species by their mobilities. When coupling with ICP-MS (CE-ICPMS), it allows to perform measurement at trace level, close to the environmental level [13], and below the solubility limits. Trace concentrations and absence of exchange with the other phase allows to preserve the +5 oxidation state of Pu too. Moreover, CE-ICP-MS has shown recently its high capacity to perform metal speciation measurements [2, 3, 14]. A methodology to investigate the labile complexes by CE-ICP-MS has been especially used for the Pu(V) and Np(V) interactions with carbonates [3], sulfates [1] and chlorides [1]. This methodology is based on the kinetic properties of the labile complexes at the CE separation timescale [15], for which there is a fast and permanent ligand exchange resulting in a single average peak. Its position, providing the overall electrophoretic mobility µov , characterises the equilibrium between the metal and the complexes. The electrophoretic mobility is determined experimentally by Ll µ= V

1 1 − tM teof

,

(2)

where L (cm) is the total length of the capillary, l (cm) the length between the capillary inlet and the detection window, tM (s) the migration time of the species M between the injection and the channeltron detection of the ICP-MS and teof (s) the migration time of the N,N-dimethylformamide (DMF) (neutral species used to determine the electro-osmotic flow) between the injection and the UV detection at l.

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S. Topin, J. Aupiais and N. Baglan

The overall electrophoretic mobility, in the case of the formation of the 1 : 1 AnO2 NO3 complexes, writes µov =

µAnO2 + , 1 + βAnO2NO3 [NO3 − ]

(3)

where µAnO2 + is the electrophoretic mobility of the free metal, AnO2 + . It allows determining βAnO2 NO3 by fitting the µov data vs. [NO3 − ] in mol L−1 . The βAnO2 NO3 value is then extrapolated to zero ionic strength using the SIT formula which writes in this case: ◦ log βAnO2 NO3 + 2D = log βAnO − ∆(εij m j ) . 2 NO3i

(4)

Here D = 0.509Im1/2 /(1 + 1.5Im1/2 ) is the Debye–Hückel term (at 1 bar and 25 ◦ C), Im is the ionic strength in molal unit. The SIT parameter ∆(εij m j ) writes, ∆(εij m j ) = εAnO2 + ,ClO4 − m ClO4 − + εAnO2 + ,NO3 − m NO3 − + εNO3 − ,Na+ m Na+ ,

(5)

where εAnO2 + ,ClO4 − , εAnO2 + ,NO3 − , εNO3 − ,Na+ are respectively the pair interaction coefficients between AnO2 + , ClO4 − and NO3 − and between NO3 − and Na+ and where m ClO4 − , m NO3 − , m Na+ are respectively the concentration of ClO4 − , NO3 − and Na+ in molal unit. A previous investigation [16] has established that almost all the binding constants cited by the TDB [1] can be solely explained in term of activity factor variation. Therefore, it seems primordial to use the capability of CE-ICP-MS to investigate the An(V)-interaction with nitrate ions with several purposes: (1) to establish, for the first time, the binding constant of Pu(V) with the nitrate ligand, (2) to re-investigate the binding constant of Np(V) with the nitrate ligand, and (3) to assess that the Pu(V)/Np(V) analogy can be used confidently in nitrate medium.

2. Experimental 2.1 Apparatus and interface A commercial Beckman Coulter P/ACE MDQ capillary electrophoresis system (Fullerton, USA) equipped with a UV detection mode was used for all the separations. It was provided with a tailor-made capillary cartridge support designed for the adaptation of an external detector. Conventional fused–silica 50 µm internal diameter capillaries (Beckman Coulter, Fullerton, USA) of 60 cm lengths were used for the separations. An optical window is located at 10.2 cm of the capillary inlet. The capillary was maintained at a constant temperature, close to 25 ◦ C, thanks to a liquid coolant wrapping the capillary. The capillary outlet is located at 1 mm back to the nose of the nebulizer in order to ensure the electrical continuity of the circuit. Only a few portion of the capillary, outside the CE apparatus, is not temperature-controlled. New fused silica capillaries were preconditioned prior to use by rinsing with 1 M HCl (Prolabo, Titrinorm), 1 M NaOH (Prolabo, Titrinorm) and deionised water. The capillary was daily rinsed with water and 1 M NaOH prior the experiments. An Axiom (VG Elemental, Winsford, Cheshire, UK) inductively coupled plasma sector field mass spectrometer

(ICP-SF-MS) was coupled with the capillary electrophoresis. The intrinsic detection limit is about 10−16 M [13]. In hyphenated mode, the detection limit is close to 10−12 M. A commercial parallel path micro-nebulizer (Mira Mist CE, Burgener, Mississauga, Canada) which operates with a make-up liquid (2% HNO3 , 10% EtOH, 1 ppb Bi as internal standard) interfaces both apparatus. Ethanol improves the signal stability by decreasing the superficial tension of water droplets. The make-up liquid is introduced by a syringe pump (11 Pico Plus, Harvard Apparatus, Holliston, Massachusetts, USA) at a nominal flow rate of 9 µL min−1 . The nebulizer is connected to a borosilicate spray chamber (Mini glass chamber, Burgener, Mississauga, Canada). The ICP-MS operates in the low resolution mode (R = 362). Bi is used to check the variation of sensitivity during the experiments.

2.2 Electrolyte and sample preparation Stock solutions of 237 Np in 0.1 M perchloric acid were used. Neptunium was purified on an anionic resin and then precipitated at oxidation state +V as NpO2 OH·xH2 O (x ∼ 2.5). A 10−5 M Np(V) stock solution was prepared in HClO4 by dissolving an appropriate mass of NpO2 OH·xH2 O. A Pu(V) stock solution was prepared according to the following procedure: 0.5 mL of 3 × 10−7 M Pu(IV) (95.5% 239 Pu and 4.5% 240 Pu) in 4 M HNO3 was evaporated to dryness in a Teflon® beaker. Then 2 mL of 70% HClO4 was added and the solution was fumed at 400 ◦ C. Note that evaporating a concentrated HClO4 solution may cause explosion, especially in presence of organic materials (even at trace level). Do not perform this procedure with a larger volume of concentrated HClO4 solution and make sure that no organic materials are present [17]. This step was repeated twice in order to fully oxidize Pu(IV). This procedure allows to quickly obtain Pu(VI) [18]. Finally the residue was dissolved in 1 mL of 1 M NaClO4 solution prepared from weighted amount of NaClO4 (Merck, sodium perchlorate monohydrate). Pu(VI) is spontaneously reduced to the pentavalent state and is stable over several days [19]. The final concentration of Pu(V) varied from 10−7 M to 3 × 10−7 M depending on the quality of the preparation. Due to the high sensitivity of ICP-MS, it was not necessary to optimize the procedure. The absence of other oxidation states was assessed by the presence of only one representative peak on the electropherograms (Fig. 1). Background electrolytes (BGE) are prepared from weighted amounts of NaNO3 (Prolabo, Normapur ≥ 99.5%) and NaClO4 dissolved in Millipore deionised water (AlphaQ, 18.2 mΩ cm). Various concentrations of nitrate (from 0 to 0.9 M) are obtained. The ionic strength (I) is adjusted by NaClO4 . The pH is close to 6 in all experiments. Samples are prepared by mixing 10 µL Pu(V) solution, 90 µL BGE, 0.2 µL DMF (BDH, AnalaR ≥ 99.5%) and 0.2 µL Np stock solution. Before each separation, the capillary is washed with BGE during 5 min at 20 psi. Separations are achieved within 15 min. Sample injections are hydrodynamically carried out at 1 psi during 4 s. Separations are performed at +4 kV and at constant pressure of 0.8 psi (to avoid clogging). The voltage value is chosen with respect to the Ohm law. The buffer


73

Determination of the stability constants of nitrate complexes of Np(V) and Pu(V) using CE-ICP-MS

Fig. 1. Electrophoretic mobilities of 2 × 10−8 M Pu and 2 × 10−8 M Np in (NaNO3 /NaClO4 ) media at I = 1 M.

vial is changed between runs to minimize the effects of the electrolysis. The determination of the temperature excess due to the Joule heating is important in CE. Under our experimental conditions and in the worst case, the temperature excess has been determined as ∆T = 1 ◦ C between the centre of the capillary and the liquid coolant using a procedure developed by Grossman [20]. The gradient of temperature inside the capillary does not exceed 0.3 ◦ C. The temperature is therefore considered as homogeneous across the capillary. In conclusion, the temperature is 25 ± 1 ◦ C in all experiments.

3. Results and discussion 3.1 Results For each experiment, a single peak is observed (Fig. 1) evidencing that the kinetics of ligand exchange is too fast to separate the complex and the free species even for higher voltage and longer capillary. It also shows that the relation Eq. (3) is valid to treat the data in the case of nitrate interaction. The overall mobilities, µov , for Np and Pu, decrease by about a factor 1.5 with increasing the nitrate concentration from 0 M to 0.9 M (Fig. 2 for Pu and Table 1), evidencing the decrease of the overall charge of the main migrating species between the mono-charged free species, AnO2 + , and the neutral complex AnO2 NO3 . The overall mobility is always widely positive even in the higher complexing media indicating that the strength of the interaction is small (Table 1). This trend is similar to that observed for the interaction with the chloride ligands [1], indicating that the binding constant of AnO2 NO3 might be close to that of AnO2 Cl. The µov values are similar for Np and Pu (Table 1, Fig. 1) indicating that these two elements are at the same oxidation state in the present conditions. Indeed, although Np and Pu can exist in solution in the oxidation

Fig. 2. Comparison of µov experimentally determined ( ) and µov calculated from βPuO2 NO3 (Table 2) using Eq. (3) (curve).

Table 1. Overall electrophoretic mobilities for Np and Pu in nitrate media. I = 1 mol L−1 and T = 25 ± 1 ◦ C. Uncertainties at 95% confidence level. Mobilities are expressed with three digits after the decimal point, regardless of the significance of the last digit. [NO3 − ] (M) 0.00 0.05 0.10 0.20 0.25 0.35 0.50 0.60 0.80 0.90

µov × 104 (cm2 V−1 s−1 ) Np

Pu

2.372 ± 0.053 2.268 ± 0.062 2.185 ± 0.070 2.054 ± 0.062 1.879 ± 0.066 1.831 ± 0.068 1.761 ± 0.080 1.741 ± 0.065 1.650 ± 0.073 1.618 ± 0.075

2.405 ± 0.058 2.295 ± 0.058 2.223 ± 0.066 2.076 ± 0.070 1.903 ± 0.065 1.842 ± 0.065 1.767 ± 0.086 1.762 ± 0.065 1.662 ± 0.065 1.630 ± 0.070


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Table 2. ∆(εij m j ) and log10 βAnO2 NO3 calculated for Np and Pu (Eqs. (3) and (4)) (I = 1 M). [NO3 − ] (M)

∆(εij m j ) (Np)

log10 βNpO2 NO3

∆(εij m j ) (Pu)

log10 βPuO2 NO3

0.00 0.05 0.10 0.20 0.25 0.35 0.50 0.60 0.80 0.90

−0.21 ± 0.08 −0.20 ± 0.09 −0.19 ± 0.09 −0.18 ± 0.09 −0.17 ± 0.10 −0.15 ± 0.10 −0.13 ± 0.11 −0.11 ± 0.11 −0.08 ± 0.12 −0.06 ± 0.13

−0.07 ± 0.10 −0.07 ± 0.10 −0.08 ± 0.10 −0.10 ± 0.10 −0.11 ± 0.10 −0.13 ± 0.10 −0.15 ± 0.10 −0.17 ± 0.10 −0.21 ± 0.10 −0.22 ± 0.10

−0.20 ± 0.02 −0.19 ± 0.02 −0.18 ± 0.03 −0.17 ± 0.03 −0.16 ± 0.03 −0.14 ± 0.04 −0.12 ± 0.05 −0.10 ± 0.05 −0.07 ± 0.06 −0.06 ± 0.07

−0.06 ± 0.10 −0.07 ± 0.10 −0.08 ± 0.10 −0.09 ± 0.10 −0.10 ± 0.10 −0.12 ± 0.10 −0.14 ± 0.10 −0.16 ± 0.10 −0.19 ± 0.10 −0.21 ± 0.10

Table 3. Stability constants log10 βAnO2 NO3 at 25 ◦ C. Method

I

T (◦ C)

Cix Dis, TOA Dis, DNNS Dis, DNNS Pot CE-ICP-MS

2 M HClO4 8 M HClO4 2 M NaClO4 8.5 M NaClO4 2 M HClO4 0.9 M NaNO3 / 0.1 M NaClO4 0M

25 25 25 25 25 25 ± 1

CE-ICP-MS

25 ± 1

log10 βNpO2 NO3

log10 βNpO2 (NO3 )2 −

log10 βPuO2 NO3

−(0.25 ± 0.05) − − −0.28 − − −(0.55 ± 0.09) − − −0.52 − − −(1.60 ± 0.02) −(1.37 ± 0.01) − −(0.22 ± 0.10) − −(0.21 ± 0.10) 0.13 ± 0.14

state from 3+ to 6+, at pH 6, the trivalent and tetravalent form would be completely hydrolized. Moreover, Np(V) can only be oxidized into Np(VI) in acidic solution and with a oxidizing agent which would be detected as anionic species. In addition, the µAnO2 + ≈ 2.4 × 10−4 cm2 V−1 s−1 value, measured in the non-complexing medium, is equal to those previously determined, in the same condition (1 M NaClO4 ), for which the 5+ oxidation state has been clearly established [2, 3]. The combination of Eqs. (3), (4), (5) allows to treat the µov data and to simultaneously obtain the log10 βAnO2 NO3 and ◦ values by an iterative procedure which minilog10 βAnO 2 NO3 mize the square sum of the difference between the µov experimental values and the theoretical one (Fig. 2). The εAnO2 + ,NO3 − = 0.08 ± 0.10 kg mol−1 value was roughly fitted according to an empirical relation, εNpO2 + ,X− = εNa+ ,X− + 0.12 kg mol−1 [21], whereas the εNpO2 + ,ClO4 − = 0.25 ± 0.05, the εPuO2 + ,ClO4 − = 0.24 ± 0.05 and the εNO3 − ,Na+ = −(0.04 ± 0.03) kg mol−1 values are those reported in the TDB [1]. The log10 βAnO2 NO3 values when varying from about −0.10 to −0.25 (Table 2) enable to obtain the best fit of the µov data, between 0 M NaNO3 and 0.9 M NaNO3 (Fig. 2 for Pu), and to establish standard binding constant ◦ ◦ = 0.13 ± 0.14 and log10 βPuO = values: log10 βNpO 2 NO3 2 NO3 0.14 ± 0.14 (Table 3). The uncertainties associated with the ◦ log10 βAnO2 NO3 values (±0.10) and with the log10 βAnO 2 NO3 have been determined by varying µov , µAnO2 + and the SIT parameters, εAnO2 + ,NO3 − , εAnO2 + ,ClO4 − and εNO3 − ,Na+ , within their uncertainties. The good consistency between the µov experimental values and the re-calculated ones (Fig. 2 for Pu) confirms the accuracy of all the used parameters. The log10 βAnO2 NO3 value, multiplied by about 2.5 between 0 M NaNO3 and 0.9 M NaNO3 at I = 1 M, illustrates the neces-

−

0.14 ± 0.14

Ref. [15] [16] [8] [9] [17] This work This work

sity to include the medium effect into the fitting process by adding the εAnO2 + ,NO3 − m NO3 − parameters. The Np and Pu binding constants are found to be similar in each experiment (Tables 2 and 3) in agreement with the analogy between Np(V) and Pu(V).

3.2 Discussion Concerning the An(V) binding constants with nitrate ligands, the thermodynamical database provides only few data for Np(V) [1] which cannot be directly compared to those established in this work since the experimental conditions are different. However, the log10 βNpO2 NO3 = −(0.22 ± 0.10) value obtained in 0.9 M NaNO3 in this work is similar to the log10 βNpO2 NO3 = −(0.25 ± 0.05) value obtained in 2 M HClO4 by Gainar et al. [7]. Conversely, the log10 βNpO2 NO3 = −(0.55 ± 0.09)value obtained in 2 M NaClO4 by Rao et al. [9] is quite different. The other values, log10 βNpO2 NO3 = −0.52 in 8.5 M NaClO4 and log10 βNpO2 NO3 = −0.28 in 8 M HClO4 , respectively from the work of Patil et al. [10] and Lahr et al. [8] have been determined at far higher ionic strength without uncertainties. The log10 βNpO2 NO3 = −(1.60 ± 0.02) and log10 βNpO2 (NO3 )2 − = −(1.37 ± 0.05) [22] values, determined by potentiometry, have not been considered here since strong uncertainties exist for the liquid junction potential correction which can impact considerably the determination of the weak binding constant [23]. The consistencies of all these binding constants can be tested by comparison with the values obtain for the interaction with chloride ligands. Indeed, the values for chloride ligands have also been investigated in the works of Gainar et al. [7], Rao et al. [9] and Patil et al. [10] All the authors found a higher binding constants for the Np(V) interaction


Determination of the stability constants of nitrate complexes of Np(V) and Pu(V) using CE-ICP-MS

with chloride ions than with nitrate ions (∆ log10 βNpO2 NO3 ≈ 0.15 [7, 9], ≈ 1 [10]). That is surprising when considering the usual reactivity of the mono-anionic species according to the Hard Soft Acid Base theory [24]. Conversely, in our work, the usual trend is observed: βNpO2 NO3 > βNpO2 Cl (determined previously in the same condition [3]). The same trend is observed for Pu(V): log10 βPuO2 NO3 = −(0.21 ± 0.10) and log10 βNpO2 Cl = −(0.40 ± 0.07). Spahiu et al. [16] have also emitted several doubts when reviewing the data. Indeed, the authors consider first that the 20% relative uncertainty associated with the βNpO2 NO3 values quoted in the literature is quite optimistic. In addition, almost all the data have been acquired by considering the medium unchanged with the substitution of the perchlorate ions by the nitrate ones. This assumption seems to be inaccurate since the activity factors change a lot with the medium variation. Spahiu et al. [16] have reinterpreted the binding constants established by Rao et al. [9] as only an activity factor shift and consider that no complexation occurs. The εM+ ,ClO4 − = (0.058 ± 0.005) + (1.187 ± 0.057)εM+ ,NO3 − formula used by the authors leads to εNpO2 + ,NO3 − = 0.16 ± 0.05 kg mol−1 . However this formula seems not to be valid for each interaction: εNH4 + ,ClO4 − = −(0.08 ± 0.04) kg mol−1 and εNH4 + ,NO3 − = −(0.06 ± 0.03) kg mol−1 for example. It seems better to use the specific relation of Vitorge et al. [21], εNpO2 + ,X− = εNa+ ,X− + 0.12, and to extend it to Pu(V): εAnO2 + ,NO3 − = 0.08 ± 0.10 kg mol−1 . In addition, even for εNpO2 + ,NO3 − = 0.16 ± 0.05 kg mol−1 , the binding constant ◦ = 0.07 ± 0.05, would be weaker but not zero, log10 βNpO 2 NO3 indicating that in this experiment, the An(V) behaviour can not be explain solely by an activity factor shift. The log10 βPuO2 NO3 = −(0.21 ± 0.10) (in 0.9 M NO3 − ) values proposed here, for the first time, for the Pu(V) interaction with nitrate ligands is similar to those obtain for Np(V): log10 βNpO2 NO3 = −(0.22 ± 0.10). That indicates, as for chloride media [3], the suitability of the Np(V)/Pu(V) analogy in nitrate media. Moreover, regarding the data for the An(V) interaction with carbonate, sulfate, chloride and nitrate ligands, it seems that the Np(V)/Pu(V) analogy is better suited when the interaction occurs in the outer sphere (Cl− , NO3 − ) than in the inner sphere (CO3 2− , SO4 2− ) in which the Np(V) and Pu(V) difference in size and in 5 f electron position impact stronger the interaction.

75

Acknowledgment. The financial support provided by CEA/DAM/DME allowing us to couple CE with ICPMS and to perform measurements in the clean room is greatly appreciated.

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