Interaction of Np( V ) with borate in alkaline, dilute-to-concentrated, NaCl and MgCl 2 solutions

The interaction of Np( V ) with borate was investigated in 0.1 – 5.0 M NaCl and 0.25 – 4.5 M MgCl 2 solutions with 7.2 ≤ pH m ≤ 10.0 (pH m = – log[H + ]) and 0.004 M ≤ [B] tot ≤ 0.16 M. Experiments were performed under an Ar-atmosphere at T = (22 ± 2) °C using a combination of under- and oversaturation solubility experiments, NIR spectroscopy, and extensive solid phase characterization. A bathochromic shift ( ≈ 5 nm) in the Np( V ) band at λ = 980 nm indicates the formation of weak Np( V ) – borate complexes under mildly alkaline pH m -conditions. The identi ﬁ cation of an isosbestic point supports the formation of a single Np( V ) – borate species in dilute MgCl 2 systems, whereas a more complex aqueous speciation (eventually involving the formation of several Np( V ) – borate species) is observed in concentrated MgCl 2 solutions. The solubility of freshly prepared NpO 2 OH(am) remained largely unaltered in NaCl and MgCl 2 solutions with [B] tot = 0.04 M within the timeframe of this study ( t ≤ 300 days). At [B] tot = 0.16 M, a kinetically hindered but very signi ﬁ cant drop in the solubility of Np( V ) (3 – 4 log 10 -units, compared to borate-free systems) was observed in NaCl and dilute MgCl 2 solutions with pH m ≤ 9. The drop in the solubility was accompanied by a clear change in the colour of the solid phase (from green to white-greyish). XRD and TEM analyses showed that the amorphous NpO 2 OH(am) “ starting material ” transformed into crystalline solid phases with similar XRD patterns in NaCl and MgCl 2 systems. XPS, SEM – EDS and EXAFS further indicated that borate and Na/Mg participate stoichiometrically in the formation of such solid phases. Additional undersaturation solubility experiments using the newly formed Na – Np( V ) – borate(cr) and Mg – Np( V ) – borate(cr) compounds further con ﬁ rmed the low solubility ([Np( V )] aq ≈ 10 − 6 – 10 − 7 M) of such solid phases in mildly alkaline pH m -conditions. The formation of these solid phases represents a previously unreported retention mechanism for the highly mobile Np( V ) under boundary conditions (pH m , [B] tot , ionic strength) of relevance to certain repository concepts for nuclear waste disposal.


Introduction
Neptunium-237 is a relevant radionuclide in the context of nuclear waste disposal because of its inventory in spent nuclear fuel and long half-life (t 1/2 = 2.14 × 10 6 a). Neptunium can be found in different redox states in the environment (+IV to +VI), which accordingly exhibit very different chemical properties. Np(IV) is expected to predominate under the reducing conditions found in anoxic near-surface groundwaters or those foreseen in deep underground repositories for the disposal of nuclear waste. Np(V), which is the predominant oxidation state under oxic conditions, can also play a role, e.g. in the early stages of repository closure, in the presence of oxidizing waste forms (i.e. high content of NO 3 − ) or in the direct vicinity of spent fuel as a result of radiolysis effects. Np(V) forms the very soluble NpO 2 OH(am) solid phase and is characterized by weak sorption. The +V oxidation state holds the lowest effective charge (Z eff ≈ +2.3) of all the oxidation states of Np that form in aqueous systems. 1 This accordingly results in weaker interactions with strong ligands (hard Lewis bases) such as hydroxide or carbonate. 2 Boron is a relatively scarce element in the Earth's crust (≈0.001% of the crust mass), where it is mostly found in evaporates such as borax (Na 2 B 4 O 7 ·10H 2 O) and kernite (Na 2 B 4 O 6 (OH) 2 ·3H 2 O). 3 Indeed, borax is a natural inclusion in the Salado Formation and has been found at the Waste Isolation Pilot Plant (WIPP), an underground repository for transuranic (TRU) waste in New Mexico, USA. Borate concentrations in "weep" brines found in the WIPP are as high as 0.18 M (or 0.044 M if expressed as B 4 O 7 2− ). 4 Boron can be also present in repositories for radioactive waste as a component of the emplaced waste, mostly (but not exclusively) coming from vitrified high level waste (HLW). 5 In the context of the accident at the Fukushima nuclear power plant, seawater supplemented with boron (as a neutron absorber) was pumped into the reactors during the first weeks after the emergency. 6 Accordingly, some of the wastes resulting from the decommissioning of the control the speciation of boron at higher [B] tot in near-neutral to weakly alkaline pH conditions. 8,9 Only a very limited number of experimental studies available in the literature deal with the impact of borate on the solution chemistry of actinides. [10][11][12][13][14][15] Most of these studies focus on Nd(III), Eu(III) and Cm(III) as analogues of the trivalent actinides Am(III) and Pu(III). [10][11][12][13] Spectroscopic and solubility data indicated the formation of relatively weak Cm(III)borate(aq) and Ln(III)-borate(aq) complexes in mildly alkaline aqueous solutions. [10][11][12][13] On the other hand, the most distinct feature observed in  was the formation of a previously unreported Nd(III)-borate(s) amorphous solid phase that defined solubility limits well below the solubility of Nd(OH) 3 (s) in weakly alkaline solutions. 13 In her PhD thesis, Hinz revealed also no (or very minor) impact of borate on the solubility of Th(IV), very likely as a result of the strong An(IV) hydrolysis that cannot be outcompeted by borate complexation. 14 The formation of An(VI)-borate(aq) aqueous complexes was confirmed in solubility (for U(VI) 14 ) and spectroscopic (for Pu(VI) 15 ) studies. So far, no experimental studies dealing with the interaction of Np(V) with borates in aqueous solutions are available in the literature. Based on the systematics along the different oxidation states of the actinide series, it can be assumed that NpO 2 + may also form weak complexes with borate. The scarcity of investigations dedicated to actinideborate interactions in aqueous systems is also reflected in the absence of any thermodynamic data selection for actinideborate aqueous complexes or solid compounds in the current NEA-TDB reviews. 16 There are no actinide-borate minerals reported to naturally occur in the environment. The first crystalline structure of a uranium-borate compound was only reported in the 80's by Behm,17 [18][19][20] molten B(OH) 3 at T > 170°C (ref. [21][22][23][24] or molten CH 3 B(OH) 2 at T > 90°C. 25 Several neptunium-borate compounds were prepared using molten B(OH) 3 , 26,27 but this synthetic route resulted in a number of cases in borate compounds containing various oxidation states of Np. 28,29 Using instead molten CH 3 B(OH) 2 at T = 120°C,    3 at T = 220°C in chloride media. 30 From a solution chemistry perspective, it is, however, unclear if such crystalline compounds synthesized at elevated temperatures will be the solubilitycontrolling phase for Np(V) at ambient temperature.
The goal of the present study is to comprehensively investigate the interaction of borate with Np(V) in dilute-to-concentrated salt systems. The primary focus is on the aqueous speciation and the associated formation and in situ transformation of new solubility-controlling solid compounds. For this purpose, a combination of over-and undersaturation experiments, spectroscopic measurements and a systematic, multimethod solid phase characterization approach was used to investigate the solution chemistry of Np in NaCl and MgCl 2 solutions over a broad range of pH m and [B] tot . The boundary conditions investigated are of special relevance in the framework of underground repositories for the disposal of nuclear waste, but could be also of interest for more specific cases such as wastes arising from the decommissioning of the Fukushima nuclear power plant.

Chemicals
All solutions were prepared with purified water (Milli-Q academic, Millipore) and purged for 2-3 hours with Ar before use.  The Np concentration and pH m of the solubility experiments were monitored at regular time intervals for up to 300 days until no further changes in Np concentration and pH m were observed. The concentration of Np in the aqueous solution was quantified by liquid scintillation counting (LSC, PerkinElmer 1220 Quantulus) after ultrafiltration with 10 kDa filters (∼1.5 nm, Pall Life Sciences). An aliquot of the resulting filtrate was mixed with 10 mL of LSC-cocktail (PerkinElmer Ultima Gold XR), and the α activity was measured for 30 minutes using α/β-discrimination to eliminate the contribution from the 233 Pa daughter nuclide. Approximately 1 mg of the corresponding solid was separated from the solution by centrifugation (4000g) in the glovebox and washed 3 times with ethanol (2 mL) under an Aratmosphere. The washed solid was dried in the glovebox and characterized by XRD using a Bruker D8 Advance diffractometer (Cu Kα radiation) equipped with a Sol-X detector. XRD data were collected within 5°≤ 2Θ ≤ 60°, with a step size of 0.04°and 6 seconds of accumulation time per step. An airtight sample holder with a dome cover (Bruker) was used for the measurements.

Solid phase preparation and solubility experiments
Solid samples for XPS, SEM and TEM analysis were prepared using the same approach as described for XRD, but with a significantly reduced amount of sample (10-50 µg). After drying, the washed solid was pressed on an indium foil and analysed with an XP spectrometer (ULVAC-PHI, Inc., model PHI 5000 VersaProbe II) equipped with a scanning microprobe X-ray source (monochromatic Al Kα (1486.7 eV)). Survey scans were recorded with a source power of 31 W of the scanning microprobe X-ray source and a pass energy of 187.85 eV of the analyzer, step size 0.8 eV, to identify the elements and to determine their atomic concentrations at the sample surface. A FEI Quanta 650 FEG environmental scanning electron microscope (now Thermo Fisher Scientific Inc.) was applied to analyse the sample surfaces.
High-angle annular dark-field scanning TEM (HAADF-STEM), electron energy-loss spectroscopy (EELS), energy-dispersive X-ray spectroscopy (EDS), and selected-area electron diffraction (SAED) were performed using a FEI Tecnai G2 F20 X-TWIN equipment operated at 200 kV. EELS and EDS were performed with a STEM mode (STEM-EELS and STEM-EDS). SAED patterns were taken from a sample area of about 200 nm in diameter. Rotational profiles of the SAED patterns were obtained by using ImageJ software.
2.4.2 XAFS techniques. Neptunium L III -edge X-ray absorption spectra were recorded at the INE-Beamline at the KARA synchrotron source (formerly ANKA), KIT Campus North, in Karlsruhe (Germany). 33 The beamline is equipped with a Ge(422) double crystal monochromator (DCM) coupled with a collimating and a focusing Rh-coated mirror before and after the DCM, respectively. Approximately 1 mg of each investigated solid phase was transferred together with ≈300 μL of the supernatant solution to a polyethylene vial under an Ar atmosphere. The vials were centrifuged at 4000g for 5 minutes to compact the solid at the bottom of the vial. These were then mounted in a gas-tight cell with Kapton® film ( polyimide) windows inside the Ar-glovebox and transported to the INE-beamline. XAFS measurements were performed under continuous Ar-flow within 1 day after sample preparation.
Bulk X-ray absorption spectroscopic (XAS) measurements at the Np L III -edge at 17 610 eV were performed in fluorescence mode at room temperature using a Ge solid-state detector. The monochromator was calibrated for the Np-L III edge by assigning the energy of 17 038 eV to the first inflection point of the K-edge absorption spectrum of the Y metal foil. Multiple scans were run on each sample.
Extended X-ray absorption (EXAFS) spectra were extracted from raw data with the ATHENA interface of the IFFEFIT software. 34 The Fourier transforms (FTs) were obtained from the k 3 -weighted χ(k) functions using a Kaiser-Bessel window function with an apodization parameter of 1. Multishell fits were performed in real space (FT −1 ) across the range of the first two to three shells. Amplitude and phase shifts functions were calculated using the FEFF 8.4 code 35 and the self-consistency loop. 36 The amplitude reduction factor S 0 2 was set to the value of 0.8. 37 Structural information was obtained by following a multi-shell approach for EXAFS data fitting. The fit was limited to parameters describing the Np coordination to surrounding oxygen and boron atoms (neighbouring atomic distances (R), EXAFS Debye-Waller factors (σ 2 ), coordination numbers (N) and relative shift in ionization energy E 0 (ΔE 0 )).  The figure includes solubility data obtained from over-and undersaturation conditions. In the latter case either NpO 2 OH(am) or ternary Na/Mg-Np(V)-borate(cr) solid phases were used as the "starting material". Np(V) solubility data reported in the literature for borate-free systems under analogous pH m and ionic strength conditions are appended to the figure for comparison purposes, [38][39][40] as well as the solubility curves of NpO 2 OH(am,fresh) calculated with the thermodynamic data selected in the NEA-TDB. 16 As shown in Fig. 1 (Fig. 1a).

Spectroscopic (NIR) measurements
The most dramatic feature observed in NaCl systems is the significant decrease in the apparent solubility of NpO 2 OH(am) occurring in solutions with [B] tot = 0.16 M and pH m ≤ 9 ( Fig. 1a and b). The drop in the solubility is accompanied by a change in the colour of the Np(V) solid phase: from the initial greenish colour which corresponded to NpO 2 OH(am) to a white-greyish coloured phase. These observations suggest the transformation of the initial amorphous Np(V) hydroxide phase into a previously unreported borate-containing Np(V) compound. This solid phase transformation was "fast" in 5.0 M NaCl solutions (significant drop in solubility observed at ≈2 weeks), but slower in 0.1 M NaCl solutions (≈270 days). In the latter case, the drop in solubility was observed only at pH m ≤ 8.5. Fig. 1a also shows that samples prepared from oversaturation conditions in 0.1 M NaCl and [B] tot = 0.16 M (red blue circles) resulted in very similar observations: a slight increase of Np(V) concentration at pH m ∼ 8.8 and a drop in the Np(V) solubility at pH m ≤ 8.5 accompanied by a change in the colour of the Np(V) solid phase. In both cases (under-and oversaturation conditions), the concentration of Np(V) in equilibrium with the newly formed solid phase is 2-4 log 10 -units (depending upon pH m and NaCl concentration) lower than the solubility of NpO 2 OH(am) in borate-free systems. We note that similar observations were reported for Nd(III) in the presence of comparable borate concentrations and pH m . 13 A comparable decrease in the solubility of NpO 2 OH(am) accompanied by a change in the colour of the solid phase from greenish to white-greyish occurs in dilute MgCl 2 systems at pH m < 9 and [B] tot = 0.16 M (dark blue symbols in Fig. 1c).  centration on the solubility of the ternary Na/Mg-Np(V)-borate (cr) phases remains unclear in Fig. 1 (Fig. 2a and b). Fig. 2a shows that the solubility of Na-Np(V)-borate(cr) in 0.5 and 5.0 M NaCl decreases with increasing borate concentration from 0.01 M to 0.03 M, but increases slightly above [B] tot ≈ 0.10 M. The decrease in solubility with increasing borate concentration can be rationalized using a generic expression for the apparent solubility constant of Na-Np(V)borate(cr) (1): Na-NpðVÞ-borateðcrÞ , "Na" þ "NpðVÞ" þ "borate" ð1Þ log K app s;0 ðNa-NpðVÞ-borateðcrÞÞ ¼ log½"Na" þ log½"NpðVÞ" þ log "borate" ½ From the expression of log K app s;0 (Na-Np(V)-borate(cr)), it follows that at constant [NaCl] and pH m , an increase in concentration of "borate" will result in a decrease of log["Np(V)"]. On the other hand, at high borate concentrations, the formation of Np(V)-borate aqueous complexes must be taken into account and can be used to reasonably justify the observed increase in solubility.
The solubility of Mg-Np(V)-borate(cr) in 0.25 and 4.5 M MgCl 2 solutions at pH m ≈ 8.5-8.7 decreases monotonically with increasing borate concentration following a slope ≈−1 (Fig. 2b). As in the case of Na-Np(V)-borate(cr), the decrease in solubility with increasing borate concentration can be properly rationalized through the use of log K app s;0 (Mg-Np(V)-borate(cr)). In contrast to the NaCl systems, the solubility of Mg-Np(V)borate(cr) does not increase above [B] tot ≈ 0.10 M. Although Np(V)-borate aqueous complexes may form also in MgCl 2 solutions (see for instance section 3.4), they have a minor impact in the solubility possibly due to the decreased ["borate"] free as a result of the binary Mg(II)-borate aqueous complexes formed. 41,42 Interestingly, the solubility of Mg-Np(V)-borate(cr) in 4.5 M MgCl 2 systems remains low within the investigated range of borate concentrations different to the data shown in Fig. 1d. This observation confirms that the lack of solid phase transformations in the solubility experiments using NpO 2 OH(am) as the "starting material" in 3.5 M MgCl 2 solutions (Fig. 1d) was due to insufficient equilibration time: a full transformation to a Mg-Np(V)-borate(cr) solid should be expected in the long-term.  reported by Wang and co-workers for Np(V)-borate compounds 25,30 was found for the collected diffractograms.
Results of XPS analyses of the Np(V) secondary phases formed in 0.1 M NaCl, 5.0 M NaCl and 0.25 M MgCl 2 solutions are summarized in Table 1. These data confirm the stoichiometric contribution of boron and Na/Mg in the investigated Np(V) solid phases. EDS results of the Np(V) solid equilibrated in 5.0 M NaCl shows the presence of Cl together with an excess of Na, compared to the Np(V) compound equilibrated in 0.1 M NaCl. This observation can be explained by the presence of NaCl resulting from incomplete removal of the 5.0 M NaCl matrix solution during sample preparation, as also confirmed by XRD (see Fig. 3). XPS data in Table 1 can be used to define tentative stoichiometries for the solid phases that formed in NaCl and MgCl 2 solutions, namely NpO 2 [B 5 O 6 (OH) 4 ]·2NaOH(cr) and (NpO 2 ) 2 [B 5 O 6 (OH) 4 ] 2 ·3Mg(OH) 2 (cr). These proposed stoichiometries should be considered hypothetical until there is more definitive experimental evidence (e.g. single crystal analysis). Fig. 4 shows SEM images of Np(V)-borate solid phases formed in NaCl and MgCl 2 solutions. Solid phases collected from samples in 0.1 M NaCl (Fig. 4a) and 0.25 M MgCl 2 solutions (Fig. 4b) show a homogeneous distribution of Np(V) in the entire investigated area. The sample equilibrated in 0.1 M NaCl contains very thin (∼20 nm) hexagonal platelets with a diameter of ∼500 nm. The structure of the sample equilibrated in 0.25 M MgCl 2 looks similar in shape but appears less crystalline. The Np(V)-borate solid phase formed in 5.0 M NaCl (Fig. 4c) clearly shows the co-existence of two phases. Here, massive, crystalline hexagonal blocks appear surrounded by platelet-like particles. EDS indicated the predominance of Na and Cl in the block structures, whereas the less crystalline phase corresponds to the newly formed Np(V)-borate phase whose composition was summarized in Table 1. 3.2.2 STEM-EELS, STEM-EDS and SAED. Fig. 5a and b show STEM-EELS and STEM-EDS spectra obtained from an individual platelet particle of the Np(V)-borate solid formed in 0.1 M NaCl. The HAADF-STEM image of the platelet particle indicated by an arrow is shown in the inset of Fig. 5a.  Consistent with the XPS results summarized in Table 1, STEM-EELS and STEM-EDS spectra support the co-existence of Np, B, O and Na in the sample. On the other hand, SAED patterns taken along the normal directions of the platelet particles of the sample indicate at least four types of crystal structures as shown in Fig. 6a(a1, a2, b1 and b2). The SAED patterns show hexagonal-like grids with the clear presence of deformations, which lead to distinct diffraction peaks (Fig. 6b). This feature likely reflects the morphology of the hexagonal-like shaped particles seen in the SEM and HAADF-STEM images of the sample (Fig. 4 and 5a). Fig. 6b and c show radial intensity profiles of the SAED patterns (radial profiles) and the XRD profile, respectively. XRD patterns are shown in terms of reciprocal d-spacing (nm −1 ) instead of degrees, to allow the comparison between XRD and radial profiles. This conversion is required since the wavelength of the Cu K-α X-ray (0.15406 nm) is different from that of the electron beam at 200 kV (0.00251 nm), causing differences in the Bragg angles in each case. Diffraction peaks marked as 1 to 11 in the radial profiles (Fig. 6b) show a good match with the peak positions in the XRD profile (Fig. 6c), although some diffraction peaks in the latter profile are not observed by SAED. This comparison shows that the Np(V)-borate solid formed in 0.1 M NaCl contain several (at least four) phases or polymorphs with Both spectra taken from the same particle area at the same time. Inset shows a HAADF-STEM image of a platelet particle of the sample. The carbon signal in the EELS spectrum mainly originates from a carbon thin film below the particle, which is not visible in the HAADF-STEM image. The copper signal originates from the Cu TEM grid.  Fig. 7 together with the corresponding best fit models. The k 3 -weighted χ(k) spectra of the three investigated solids are very similar. The greatest differences are observed in the k-space ∼6-8 Å −1 , where the oscillation splits show a beat pattern between 6-7 Å −1 and a maxima at ∼7.7 Å −1 for the NaCl samples. In contrast, this oscillation is dampened in the MgCl 2 sample with a maxima at ∼7.4 Å −1 . This difference reflects the presence of different cations (Na + or Mg 2+ ) in the Np(V)-borate structure.
The differences of the spectral features observed in the k 3 -weighted χ(k) spectra are reflected in the k 3 -weighted EXAFS function of the Fourier back-transform spectra (Fig. 7c). The first two shells of the Radial Structure Functions (RSF) (modulus, |FT|, and imaginary parts, ImFT) described at R + Δ ∼ 1.4 and ∼1.9 Å represent the axial (O ax ) and equatorial oxygen atoms (O eq ) (Fig. 7b). The distances to the axial and equatorial oxygen atoms are very similar in all the samples analysed. A very small shift to longer distances is observed for the third shell (corresponding to B) depending upon the concentration and composition of background electrolyte. The structural parameters that result from the EXAFS fit and calculated paths of an atomic cluster based on a starting structure of Np(V)-borate (NpO 2 [B 3 O 4 (OH) 2 ]) 25 are shown in Table 2. For the fit, data were transformed in the k-space between ∼4.3-11.9 Å −1 and in the R-space between ∼1.1-3.7 Å. A stepby-step approach was followed to model the experimental spectra. Each shell (O ax , O eq and B) were fitted separately and subsequently the best fits were used for the final structural model.
As already observed in the Radial Structure Functions (RSF) (Fig. 7b)     importantly impacts the aqueous speciation of Np(V) in weakly alkaline systems. Analogous spectroscopic studies on-going at LANL using NaCl instead of MgCl 2 as background electrolyte confirm this observation. As in the case of dilute MgCl 2 solutions, the observation of an isosbestic point in dilute to concentrated NaCl solutions confirms the formation of one main Np(V)-borate aqueous species. The more complex picture observed in the present work in concentrated MgCl 2 solutions reflects also the complexity of the Mg-borate binary system. As already proposed by Felmy and Weare (1986) 41  Due to the limited dataset collected in this work and to the complex aqueous speciation of boron in MgCl 2 solutions, no thermodynamic modelling of the spectroscopic data was attempted in this study.

Conclusions
Under-and oversaturation solubility experiments with Np(V) in combination with spectroscopic investigations and a comprehensive, multimethod solid phase characterization confirm the impact of borate on the aqueous speciation and especially on the solubility of Np(V) in mildly alkaline, dilute to concentrated NaCl and MgCl 2 solutions.
Spectroscopic data confirm the formation of at least one Np(V)-borate complex in MgCl 2 solutions with [B] tot ≥ 0.04 M, although the exact stoichiometry of the complex/es formed remains so far undefined. In spite of forming the Np(V)-borate complex/es, the presence of borate does not significantly increase the solubility of Np(V) in alkaline NaCl and MgCl 2 solutions. On the contrary and similarly to Nd(III), a significant drop in the Np(V) solubility (3 to 4 log 10 -units) occurs in borate-bearing NaCl and MgCl 2 solutions with pH m ≤ 9. The drop in solubility is accompanied by a clear change in the colour of the initial solid (from green to white-greyish), supporting the formation of a new solid phase. Solid phase characterization using XRD, XPS, SEM-EDS, TEM and EXAFS confirms the formation of hitherto unknown Na-Np(V)borate(cr) and Mg-Np(V)-borate(cr) solid phases in NaCl and dilute MgCl 2 , respectively. Although the undersaturation solubility experiments with the Mg-Np(V)-borate(cr) phase exhibit a very low solubility in 4.5 M MgCl 2 solutions, the transformation of NpO 2 OH(am) was kinetically hindered and was not observed (within the timeframe of this study) in such concentrated brines. The in situ formation of Na-Np(V)-borate(cr) and Mg-Np(V)-borate(cr) solid phases in aqueous solutions at ambient temperature conditions highlights a previously unreported retention mechanism for the highly mobile Np(V) under boundary conditions ( pH m , [B] tot , ionic strength) that is potentially relevant in the context of nuclear waste disposal.

Conflicts of interest
There are no conflicts to declare.