3.1.1. NTf2 participation in the uranium complex
To obtain information about the chemical environment of NTf
2− in the binary mixtures before and after uranium extraction,
19F-NMR and FT-IR measurements were carried out. Note that NTf
2− gives only one resonance band in the
19F-NMR spectra. Its evolution provides information on the environmental change of NTf
2−.
19F-NMR spectra of [TOAH]
2[SO
4] + [TOAH][NTf
2] mixtures are plotted in
Figure 4a for x[TOAH]
2[SO
4] = 50%. It shows that the chemical shift for NTf
2− is not modified before and after uranium extraction, which indicates that NTf
2− does not directly coordinate uranyl.
This conclusion is reinforced by a Fourier transform infra-red (FT-IR) analysis shown in
Figure 4b. It can be observed that the characteristic bands of NTf2 [
34] are not affected by the presence of uranium. NTf
2 therefore does not participate directly in the complex with uranium, and the extraction non-linearities of the mixture cannot be explained by the formation of a synergistic or antagonistic mixed complex of uranium with sulphates and NTf
2.
3.1.2. UV-vis spectroscopy and EXAFS, study of first neighbors
To determine whether the origin of the nonlinear extraction is due to uranium speciation, UV-vis uranium spectroscopy and X-ray absorption spectroscopy (EXAFS) measurements were performed for various x[TOAH]2[SO4] ratios.
UV-vis spectra recorded after contact with a uranium solution (
Figure 5a), show the same type of fingerprint signal for 25 to 100 % ratios as for the conventional medium (TOA + dodecane + octanol) shown in dotted orange line. The different trend observed for the ratio 0% is due to the absence of uranium in this sample. As demonstrated by Servaes et al. who studied the speciation of uranyl nitrate complexes in acetonitrile and ionic liquid [
35], this fingerprint is characteristic of a trigonal symmetric uranium complex. In their study, spectra of UO
2(NO
3)
3− complexes in acetonitrile or in [C
4mim][NTf
2] show signals similar to those observed for UO
2(CO
3)
34− and UO
2(CH
3COO
3)
3 complexes [
36]. By comparing the experimental spectra with those reported in the literature, it can therefore be concluded that for all x[TOAH]
2[SO
4] ratios, the uranyl sulphate complex exhibits D
3h trigonal symmetry, suggesting that uranyl is complexed with 3 sulphates in an ionic liquid medium. EXAFS spectroscopy measurements were carried out to confirm this hypothesis.
Figure 5b displays EXAFS spectra measured at the L
3 threshold of uranium and the corresponding Fourier transforms. No EXAFS spectra could be measured for x[TOAH]
2[SO
4] ratios lower than 25 % as the uranium concentrations was too small in these samples. For the higher x[TOAH]
2[SO
4] ratios, it shows that experimental spectra (solid lines) obtained with the [TOAH]
2[SO
4] + [TOAH][NTf
2] mixtures are very similar to the one measured in conventional media (orange spectrum, TOA + dodecane). It can also be noted that in addition to the first U-O
yl contribution of linear transdioxo bonds (R + φ = 1.2 Å) and that of the equatorial U-O
eq oxygen coordination layer (R + φ = 1.8 to 2.4 Å), the spectra show intense contributions in the second coordination sphere (R + φ > 2.5 Å) due to sulfur atoms of sulphate anions coordinated with UO
22+ [
33,
37]
In a previous study carried out in a conventional medium (TOA + dodecane), coupling a classical fit with molecular dynamics simulations enabled to unravel the speciation of uranium in the organic phase: the complex consists of three sulphate anions coordinated to uranyl via U-O bonds in the first coordination sphere [
33]. The study also showed that uranyl trisulphate complexes shift from a 3 bidentate sulphate configuration to a 2 bidentate and 1 monodentate configuration, with the most favorable configuration remaining the 3 bidentate sulphate configuration.
In order to probe more precisely the local structure of uranyl in [TOAH]
2[SO
4] + [TOAH][NTf
2] mixtures, a fit of the EXAFS spectra was performed by applying a structural model of three bidentate sulphates in the first uranium sphere. The fitted parameters are presented in
Table 2 and the corresponding fitted spectra are plotted as dotted lines in
Figure 5b.
Good fits are obtained for all the x[TOAH]
2[SO
4] ratios, confirming the structure with three bidentate sulphates. As expected from the qualitative study of the data, U-O
yl, U-O
eq and U-S bond lengths do not vary. However, although this does not lead to outliers, the quality of fit deteriorates for the higher x[TOAH]
2[SO
4] ratios: the R factor increases from 3.9 % to 6.0 % (see
Figure 6a). Furthermore, as shown in
Figure 6b, the Debye-Waller factor increases suggesting more disorder in the complexes. The evolution of these two parameters indicates that the U-3 sulphate complex is destabilized, which could explain a loss of uranium extraction for the high x[TOAH]
2[SO
4] ratios.
In conclusion, 19F-NMR, UV-vis and EXAFS data indicate that there is no mixed complexes formed with sulphates and NTf2 in the first coordination sphere. However, destabilization of the bidentate sulphate complex is observed when the x[TOAH]2[SO4] ratio reaches 100 %, which may be related to the decrease of uranium extraction observed for these high ratios.
As extracted water is very important at these ratios, it would have been interesting to evaluate its contribution to the complexes. However, it was not possible to include it distinctly in the fits, as the signatures of the monodentate U-SO4 and U-H2O bonds are too similar (same distance, same properties). EXAFS therefore does not allow us to conclude if water competes or not with uranium sulphate.
3.1.3. FT-IR spectroscopy, looking at the second coordination sphere and beyond
Fourier transform infrared (FT-IR) spectra of protonated tertiary amines mixtures are presented in
Figure 7 and ESI 2 and 3. Three main bands can be distinguished in the region 800-1000 cm
−1. Identification of resonance bands has been performed according to results proposed in the literature based on a IR study dedicated to the structure of tridecylammonium sulphate in presence of uranium [
38]. The three bands were therefore assigned to monodentate water-bound sulphates ν (S-OH) at 855 cm
−1, to the uranyl motif ν (UO
22+) at 914 cm
−1 and to bidentate uranium-bound sulphates ν (S-(O...U)
2) at 967 cm
−1.
A linear shift of the ν(S-OH) absorption band towards the lower frequencies can be observed with the molar ratio of sulphate. It indicates that sulphate becomes increasingly free (see ESI 4). The area under these ν(S-OH) band curve was plotted in
Figure 8a. It shows an increase after the ratio x[TOAH]
2[SO
4] = 30% that follows the trend of the water extraction profile (see
Figure 3b). This first observation suggests that the sulphate are not only involved in the uranyl complexes but also more and more connected with water which concentration is significantly increasing for the high x[TOAH]
2[SO
4] ratios.
To highlight the bands related to uranium, it was necessary to subtract the signal of the pre-contacted samples. After subtraction, each band of interest was fitted with a Voigt function. The area under the curve (AUC) is plotted in
Figure 8b,c as a function of x[TOAH]
2[SO
4] for the ν (UO
22+) and ν (S-(O...U)
2) bands.
It is interesting to note that the area under the bands attributed to ν (UO
22+) and ν (S-(O...U)
2) bonds follows a similar trend to that of the uranium extraction profile (plotted in
Figure 2a): it is initially constant up to x[TOAH]
2[SO
4] = 12.5%, then intensifies up to x[TOAH]
2[SO
4] = 75 %, and decreases thereafter.
This infrared study indicates therefore that for x[TOAH]2[SO4] ratios above 75 %, the uranium-sulphate bond is less present, to the benefit of the sulphate-water bond, suggesting a competition between the sulphate-uranium and sulphate-water complexes.
To interpret the overall uranium extraction profile of the [TOAH]
2[SO
4] + [TOAH][NTf
2] mixture (
Figure 2a), it is necessary to take into account all the data obtained by the spectroscopic methods, as well as the water extraction data and the acid release from the organic to the aqueous phase (
Figure 3b,c). Spectroscopic data indicates that the NTf
2 anion is not involved in the extraction of uranium. As in the conventional medium (TOA + dodecane), uranium is extracted by TOAH
+ protonated amines and three bidendate sulphates in the first coordination sphere.
For low x[TOAH]2[SO4] ratios of 0-25 %, uranium extraction does not increase, while the amount of sulphates introduced into the organic phase does. This effect can be explained by the increased release of H+ protons from the organic phase into the aqueous phase. This release reduces the number of protonated amines available, inhibiting uranium extraction.
From x[TOAH]2[SO4] = 25 %, proton release stabilizes and then decreases, leading to an increase in the number of TOAH+ protonated amines available. This increase in TOAH+ accompanied by an increase in sulphates with x[TOAH]2[SO4] is therefore consistent with the very sharp increase in uranium extraction between x[TOAH]2[SO4] = 25 and 75 %.
Beyond this ratio, the very sharp increase in extracted water leads, as previously suggested, to a competition between the sulphate-uranium and sulphate-water complexes, which destabilizes the uranyl tri-sulphate complexes and leads to saturation, then to a decrease in uranium extraction.
This study shows that the main elements responsible for uranium extraction are sulphates and protonated amines, and that the availability of the latter is regulated by the concentration of extracted water, as well as by the release of acidic protons from the organic phase to the aqueous phase.
A deeper understanding of these phenomena, would require to interprete the nonlinear behavior of water extraction and the origin of H+ proton release. The increased extraction of water can be partially explained by the difference in hydrophilicity between NTf2− and SO42− anions, but its nonlinear extraction might be related to the activity of the aqueous phase and to the chemical potential equilibria between the species present in the aqueous and organic phases. The chemical potential equilibrium may also be responsible for the release of protons from the organic phase to the aqueous phase. These equilibria are however very difficult to write down and predict.