The structure and composition of solid complexes comprising of Nd(III), Ca(II) and D-gluconate isolated from solutions relevant to radioactive waste disposal

Introduction

Low and intermediate level radioactive waste (L/ILW) is often stabilized by using cement or cementitious materials, and cement based materials are very popular as construction materials. In cement pore water (CPW), the major metal ion component is Ca²⁺ with a total concentration of 10⁻⁴ M to 2×10⁻² M [1]; however, Ca²⁺ concentrations could be several orders of magnitude higher in some CaCl₂ brines [2]. Under alkaline (10≤pH≤13) conditions, the degradation of various organic macromolecules present (e.g. cellulose) results in a range of low molecular weight organic-(hydroxycarboxylate) as well as sugar type (sugar carboxylate) compounds [3], the most important of them is α-D-isosaccharinate (Isa⁻) [4]. Another relevant organic compound in cementitious environments is Gluc⁻, being common cement additive (plasticizer) [5]. The mass-produced industrial starting material or additive, Gluc⁻, is often regarded as functional model of Isa⁻. It may be present in the CPW in 10⁻⁷–10⁻² M concentration [5].

Complexation of radionuclides with organic ligands like Gluc⁻ (or Isa⁻) [6], [7], [8] may increase their solubility and/or decrease their sorption on cement, and can be considered as a relevant factor contributing to the mobilization of radionuclides from an underground L/ILW repository to the environment. Trivalent radioactive nuclei prevail under the reductive conditions of radioactive waste repositories. Nd(III) can be considered as reasonably good model for trivalent lanthanides or actinides (e.g. some representative atomic radii are as follows: Nd(III): 111 pm; Pu(III): 112 pm; Am(III): 111 pm; Cm(III): 109 pm) [9], [10], [11], [12].

In an alkaline aqueous solution containing polyhydroxycarboxylate, Ca(II) and a further trivalent metal ion (e.g. Fe(III), B(III), Al(III), etc.), the formation of heteropolynuclear complexes, such as Ca(II)/Fe(III)/Gluc⁻ or Ca(II)/Al(III) or B(III)/aldarate was observed [13], [14], [15], [16]. The formation of such ternary species, which include a radionuclide, Ca(II) and Gluc⁻ (or Isa⁻) are also well known, where An(III or IV)=Nd(III), Cm(III), and Th(IV) [6], [7], [17], [18], [19]. It is remarkable that the stability of these heteropolynuclear complexes is always high, and one of the two metal ions seems to facilitate the binding of the other in the complex pointing towards a synergistic effect.

Our research group has recently reported a detailed study of the aqueous chemistry of the Nd(III)/Gluc⁻ system in the acidic to neutral pH region [20], and the results were compared with different previous studies reported [21], [22], [23]. It was shown, that Nd(III) is capable of forming both mono- and binuclear complexes in this pH region with the NdGlucH_-2 ⁰(aq) determined as the predominant major solution species at close to neutral pH. When the pH was raised over 8, the formation of a purple precipitate was observed, independent of the metal-to-ligand ratio.

It was also observed that the above precipitate got redissolved when the pH of the solution was raised above 11 indicating the stepwise formation of some charged solution species (due to ligand deprotonation and/or deprotonation of hydrate water around Nd(III)). The exact composition(s) and formation constant(s) of the complex species forming in pH≥11 solutions are not known as yet.

Some exploratory experiments were performed with regard to the effect of adding Ca(II) ions to solutions containing Nd(III) and Gluc⁻. In pH<8 solutions, at any metal-to-ligand ratio, addition of Ca(II) ions exerted no effect on the Nd(III)/Gluc⁻ system (that is, no variation in the Vis spectra, or no formation of any precipitate, etc.) However, upon addition of Ca(II) to the pH≥11 solution containing the redissolved precipitate, another pink solid complex was obtained. The formation of this solid is clearly related to the presence of the Ca(II) ions in solution.

In the current paper, we report on the characterization and comparison of the solid complex compounds that were isolated from binary (Nd(III)/Gluc⁻) and ternary (Nd(III)/Ca(II)/Gluc⁻) systems using XRD, IR, Raman, diffuse reflectance, SEM, SEM-EDX and TGA techniques.

Experimental part

Neodymium(III) chloride hexahydrate (Aldrich, 99.9%), and sodium-D-gluconate (Sigma, ≥99%) were used without any further purification. The exact concentrations of the NdCl₃ stock solutions were determined via EDTA titrations. The buffer solution was hexamethylene teramine, and methylthymol blue was used as indicator. CaCl₂ stock solution was prepared from CaCl₂·2H₂O (Sigma-Aldrich, >99%), the exact concentration was also determined via EDTA titrations. Here, the buffer solution was ammonia, and eriochrome-black-T was used as indicator.

The solid complex precipitating from the Nd(III)/Gluc⁻ system was prepared and isolated as follows. In a typical experiment, 25 mL 0.50 M NdCl₃ and 25 mL 0.50 M of sodium-D-gluconate was mixed and under vigorous stirring NaOH solution with ca. 1 M concentration was slowly added to the mixture until a precipitation formed at pH∼8. The pH of the solution was monitored with a calibrated glass electrode. The solid was collected on a filter paper, washed three times with distilled water, and dried in a in a desiccator for 1 week. The dry, pink precipitate was pulverized in an agate mortar, and stored in an airtight glass container.

For the ternary Nd(III)/Ca(II)/Gluc⁻ system, in a typical experiment, 25 mL of 0.50 M NdCl₃ and 25 mL of 0.50 M sodium D-gluconate was mixed, and under vigorous stirring, NaOH solution with ca. 1 M concentration was slowly added to the mixture until a precipitation formed at pH∼8 which was found to redissolve at ca. pH∼12–12.5 (to reach this pH, the addition of ca. 20 mL of 1 M NaOH was necessary). To the mixture, 0.5 M CaCl₂ solution was added until a precipitation formed (the pH of the solution dropped to 12–12.2). The precipitation was filtered and washed with cool (approximately 5°C) deionized water and dried in a drying cupboard. After drying, the pink precipitate was pulverized in an agate mortar, and stored in an airtight glass container.

For the spectroscopic measurements, as references, Nd₂O₃ (Merck, 99.9%) Nd(OH)₃ (Aldrich, 99.9%) and Ca-D-gluconate (both Sigma, ≥99%) were used.

X-ray diffractograms were obtained using a Rigaku XRD-6000 diffractometer instrument. Traces were registered in the range of 2Θ=5−80° with 4°/min scan speed using CuKα (λ=1.5418 Å) radiation at 40 kV and at 30 mA.

IR spectra with 4 cm⁻¹ resolution were recorded with a BIO-RAD Digilab Division FTS-65A/896 FT-IR (Fourier-transform infrared) spectrophotometer. 256 scans were collected for each spectrum in the 4000–600 cm⁻¹ wavenumber range. The IR instrument was set to ATR (attenuated total reflectance) mode.

To register Raman spectra, a Thermo Scientific™ DXR™ Raman microscope was used at an excitation wavelength of 535 nm for “green”, and 720 nm for “red” laser measurements. The applied laser power was 10 mW. Each recorded spectrum is an average of 20 spectra with an exposition time of 6 s.

The diffuse reflectance (DR) spectra of the solid samples were recorded on an Ocean Optics UV-Vis USB4000 diode array spectrophotometer in the 230–890 nm wavelength range (resolution 0.2 nm, integration time 0.5 s). The temperature was 22±2°C. The incident angle was 45°. MgO was employed as reference. For the measurement, an Ocean Optics DH-2000-BAL light source was used, consisting of a deuterium and a halogen lamp.

The morphologies of the materials were visualized by scanning electron microscopy (SEM-Hitachi S-4700 instrument). Before the measurements, the samples were placed into conductive carbon adhesive tapes, and a few nm of gold-palladium alloy films were condensed onto the surface of the samples to avoid charging. The elemental analysis was conducted by energy dispersive X-ray (EDAX) measurements (Röntec QX2 spectrometer equipped with Be window and coupled to the SEM).

The thermal properties of the solids were studied by a Setaram Labsys derivatograph applying constant flow of air at 3°C/min heating rate. For the analysis between 25 and 1000°C, 45–55 mg of the samples were placed into high-purity alpha alumina crucibles.

ICP-OES measurements were carried out with using a Perkin Elmer Optima 7000 DV ICP-OES spectrometer. Sample solutions were made by accurate weighing of the specimens, and three parallel measurements were made for each sample, and an yttrium containing internal standard (1 mg/L) was used throughout.

Results and discussion

Characterization of the solid complexes

The scanning electron microscope (SEM) images taken illustrate that the morphology of the solid complexes obtained from the binary and ternary systems. The simultaneously obtained EDAX images show an even distribution of the two metals in the solid samples (Fig. 1).

Fig. 1:

SEM images obtained for NdGlucH₋₁(OH)·2H₂O (top, left) and for CaNdGlucH₋₁(OH)₃·4H₂O (top, right). At the bottom, the EDX distribution of Ca (left) and Nd (right) of the ternary solid complex is shown.

FT-IR spectra of the two complexes were also recorded and compared with that Nd(OH)₃, Ca-D-gluconate and Na-D-gluconate as references (Fig. 2). The data obtained showed that gluconate is present in both complexes, since the asymmetric (denoted by * in Fig. 2) and symmetric (denoted by ⁺ in Fig. 2) carboxylate peaks of the sodium gluconate (1633 and 1398 cm⁻¹, respectively) can be detected either in the NdGlucH₋₁(OH)·2H₂O (with the asymmetric peak at 1664 cm⁻¹ and the symmetric at 1498 cm⁻¹) and in the CaNdGlucH₋₁·(OH)₃·4H₂O (peaks at 1659 cm⁻¹ and 1500 cm⁻¹). The band positions in the two complexes are almost identical, which suggests similar binding mode of the carboxylate moiety. The difference Δ [Δ=ν_asym(COO−)−ν_sym(COO−)] between the asymmetric and symmetric carboxylate vibrations gives information about the coordination mode of the carboxylate group. The coordination can be either bidentate chelating (Δ_complex<Δ_{sodium salt}) or bidentate bridging (Δ_complex~Δ_{sodium salt}) or monodentate (Δ_complex>Δ_{sodium salt}) [24]. As Δ_complex=160–170 cm⁻¹ and Δ_{sodium salt}=235 cm⁻¹ in our case, the carboxylate group is supposed to be in a chelating bidentate coordination mode in both solid complexes [25], [26], [27].

Fig. 2:

FT-IR spectra of the solid complexes and those of the inorganic and the organic compounds used as references (* and ⁺ denote symmetric and asymmetric carboxylate vibration bands, respectively).

The stretching vibrations of hydroxyl groups belonging to excess water present occur in the spectrum as a broad band appearing in the range of 3700 cm⁻¹–3000 cm⁻¹ with the highest intensity at 3316 cm⁻¹, indicating the presence of crystalline water in the complexes. Similarities in the Raman spectra of the complexes (Figure S4) suggest again that the binary and ternary species have analogous structure.

The DR spectra of the NdGlucH₋₁(OH)·2H₂O and CaNdGlucH₋₁·(OH)₃·2H₂O together with some reference spectra (Fig. 3) proves that the chemical environment of the chromophore group, in this case the Nd(III), is very much different from that in Nd(OH)₃ and in Nd₂O₃. However, having a look at the two spectra at higher magnification, only some very small differences can be observed, which are just beyond experimental uncertainty. This attests that the chromophore is in very similar environment in the binary and the ternary complexes. The DR and FT-IR observations attest that the coordination sphere of the Nd(III) is very similar (if not identical) in these solid complexes.

Fig. 3:

The Vis-DR spectra of the solid complexes together with those of some inorganic compounds used as references.

From our previous studies [20], the molar absorptivity of the solution complex species NdGlucH₋₂ ⁰(aq) has been calculated. A part of the DR spectrum of NdGlucH₋₁(OH)·2H₂O and the molar absorptivity spectrum of NdGlucH₋₂ ⁰(aq) taken from Ref. [20] are compared in Fig. 4.

Fig. 4:

Top spectrum: excerption from the molar absorbance spectrum of NdGlucH₋₂ ⁰ complex in solution. Bottom: excerption from the diffuse reflectance spectrum of the solid NdGlucH₋₁(OH)·2H₂O complex.

The spectra in Fig. 4 appear to be similar to the solid state spectrum resembling absorbance of that in solution (and this statement is true for the rest of the electron spectra of these two specimen; these are not shown). This observation suggests that in the binary system, the precipitate keeps the structure of the charge-neutral solution species, and proves that the solution structure is retained in the solid state. According to the species distribution diagram shown in Figure S5 (calculated on the basis of the formation constants presented in ref. 20), the predominant solution species at pH∼8 is the NdGlucH₋₂ ⁰(aq), and the most likely scenario is that this solution species reaches the solubility at this particular pH range and results in the NdGlucH₋₁(OH)·2H₂O precipitate formation. For the first sight, this result is not surprising. However, it has to be noted that the precipitating solid is not necessarily related directly to the predominant species in solution, rather to the one which is the least soluble under the experimental conditions employed.

Discussion and conclusions

The structure and composition of the solid complex compounds isolated from binary (Nd(III)/Gluc⁻) and ternary (Nd(III)/Ca(II)/Gluc⁻) systems have been determined. From the elemental analysis, the composition of the complexes is suggested to be NdGlucH₋₁(OH)·2H₂O and CaNdGlucH₋₁(OH)₃·2H₂O. From FT-IR, Raman and diffuse reflectance measurements and in particular from comparing the molar absorptivity spectrum of the solution species NdGlucH₋₂(aq) and the DR spectrum of the solid NdGlucH₋₁(OH)·2H₂O, it can be concluded that the coordination environment of the Nd(III) ion in these two complexes are identical. From previous studies [20], in the NdGlucH₋₂(aq), the Nd(III) is bound to one alcoholate (in position 2) and to the carboxylate of the Gluc⁻ and to further 6 water molecules, one of which is deprotonated. This way, the solution species NdGlucH₋₂(aq) is a mixed-hydroxo complex of neodymium (see figure 8 in Ref. [20]). We suggest that the structure of the solid complex is identical to this, except that from the IR spectra the binding mode of the carboxylate is likely to be bidentate, while in [20], monodentate coordination is suggested for the solution species from DFT calculations.

The coordination sphere is very similar in the CaNdGlucH₋₁(OH)₃·2H₂O ternary solid complex to that found in the NdGlucH₋₁(OH)·2H₂O binary one. This suggests that the binding of Ca(II) takes place remotely from the Nd(III) center; this would explain why the attachment of the Ca(II) to the complex exerts only a slight (hardly detectable) influence on the DR spectra. Most probably, Ca(II) forms an outer-sphere complex with the NdGlucH₋₁(OH)·2H₂O or, less likely, acts as a coprecipitating agent at the conditions of the precipitation.