From phosphate rocks to uranium raw materials: hybrid materials designed for selective separation of uranium from phosphoric acid

Abstract

Innovative hybrid materials with high capacities to selectively extract uranium ions from phosphoric acid media were developed by grafting phosphorous-based ligands within the pores of mesoporous silica (SBA15) or mesoporous carbon (CMK3).

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Phosphate rocks represent an unconventional source of uranium.^1–4 Several processes based on solvent extraction have already been developed to extract uranium from phosphoric acid.^5–11 Despite efforts at recycling, these processes suffer from considerable drawbacks such as the use of significant volumes of organic solvent or possible loss of extracting complexes,^5,6,12 and the use of large amounts of water, which is a challenge in the arid locations typical of the phosphate industries. To avoid these difficulties, solid-phase extraction may be a smart alternative. Since the 1980s, some researchers have proposed the use of ion-exchange resins as efficient solid supports for extraction of uranium ions from phosphoric acid.^13–15 The principal benefit of solid-phase extraction is the absence of any organic solutions. While these resins display high extraction capacities¹⁴ (between 100 and 200 g kg⁻¹), they are limited to the extraction of tetravalent uranium, i.e., U(IV), and require the addition of Fe(0) to reduce U(VI) (predominant form of uranium in rocks containing phosphoric acid) to U(IV). Also their selectivity is generally low with regards to the other common impurities present in the phosphate rocks (mainly iron); some adsorption is irreversible, which leads to a small possibility of regeneration,¹² and only a limited number of extraction cycles can be performed due to fouling issues.^13,16 Moreover, these resins tend to swell,¹⁶ which makes it difficult to implement the process. To get around most of these drawbacks, use of an inorganic framework with a selective extraction component is a promising alternative option. Mesoporous inorganic supports are distinguished by high available surface areas, but also by high chemical, thermal and mechanical stability, hence their high potential in adsorption applications. Recently obtained results¹⁷ demonstrate that effects of phosphoric acid damage depend strongly on the wall thickness and the pore diameter of the silica used; SBA15 appears to be stable even when exposed to 5 M phosphoric acid for 96 h.

Functionalization of mesoporous silica^18–22 or carbon supports^23,24 by specific organic complexes has been proposed in numerous papers for decontamination of uranium ions from aqueous effluents^25,26 using dilute nitric acid.^27,28 Nevertheless, to the best of our knowledge, this approach has not yet been used for selective extraction of uranium from phosphoric acid. This study is an attempt to apply the general concept of functionalization of mesoporous silica (SBA15) or mesoporous carbon (CMK3) by an extracting agent designed for the selective extraction of uranium ions from phosphoric acid media.

Uranium(VI) in phosphoric acid has been reported to exist in many forms (charged and uncharged) depending on phosphoric acid concentration.^29–31 Recently it has been reported that uranium(VI) in aqueous solutions of 0.73–7.08 mol L⁻¹ phosphoric acid likely exists as UO₂(H₂PO₄)_n(H₃PO₄)_m^(2−m) (where n + m = 3).³² To extract uranium from phosphoric acid, it is necessary to choose organic ligands showing a higher affinity for uranyl ions than for phosphate ions. In the case of phosphorous-based ligands, this affinity is linked to the functional group bonded to the phosphoryl group (P=O). To enhance the selective affinity of the functionalized solid we have chosen to combine an amido function to the phosphoryl group as selective molecule. Such multifunctional combinations of amido and phosphonate groups, for instance as bidentate³³ or tridentate³⁴ carbamoylphosphonate ligands, have shown their potential to extract actinides. Only few examples of mesoporous supports designed to sequester actinides have been developed based on carbamoylphosphonates.³⁵ Prompted by these previous investigations, and recent results obtained for the selective extraction and quantitative recovery of uranium(VI) from phosphoric acid with carbamoylphosphonate ligands,^9,10 we designed and synthesized two ligands: a bidentate ligand (named BP) and a tridentate ligand (named TR), which are able to be introduced on a solid-state matrix.

Using these molecules, three kinds of materials were then synthesized (Fig. 1): BP@SBA from the bidentate ligand (BP), which was post-grafted onto mesoporous silica (SBA15) by silanisation; and TR@SBA and TR@CMK obtained after an amidation reaction between the tridentate ligand (TR) and an intermediate functionalized support containing an amino group (NH₂@SBA or NH₂@CMK^36–38) (see ESI†). The grafting efficiency of the silica support was followed and assessed by CP-MAS NMR (²⁹Si, ¹³C, ³¹P), whereas in the case of the CMK support it was followed using Raman spectroscopy, using the ratio between the D-band intensity and the G-band intensity (see ESI†).^36,37 Chemical stability of the intermediate material was followed and assessed by measuring total organic carbon. Indeed, less than 10% of the organic part was observed to leach after 96 hours in a phosphoric acid solution (5 M).

Fig. 1 Hybrid materials used: BP@SBA (left) and TR@SBA or TR@CMK (right).

After silanisation of APTES and amidation reaction with TR, the pore size diameter was observed to clearly decrease, which indicates that the pores filled with the organic species (Table S1, ESI†). This hybrid material was previously observed as a functionalized monolayer on a mesoporous support (FMMS).³⁹ Combining SAXS (Fig. S1†) and adsorption experiments yields an easy method for the determination of the “wall thickness”⁴⁰ (Fig. S2†) as well as the size of the grafted molecule compared to that of the initial material (SBA15). These calculated sizes were compared to those obtained according to Tanford's empirical formula assuming only hydrocarbon chains.⁴¹ In regards to the value obtained, these two types of evaluations are consistent. Based on these measurements, the TR-grafted molecule appears to have a length of around 1 nm.

TGA experiments, as well as elementary analysis, allow the determination of the grafting ratio at each step (Table 1). For the intermediate materials NH₂@SBA and NH₂@CMK, the amount of amino group grafted within the pores is higher for the silica support than for the carbon support, even with a lower available surface area. For the amidation step, around 30% of the amino groups present in the pores of amorphous carbon (CMK3) react with TR ligand, compared to only 16% when using NH₂@SBA. This low reaction yield may be best explained by the high density of amino groups that cover the surface of the solid and the large size of the TR molecules that react with these free amino groups. Thus, during the amidation reaction, the TR molecule should cover several amino groups, hence avoiding further reaction, as schematically shown on Fig. 1. This phenomenon is less significant in the case of carbon-based materials due to the low density of amino groups in NH₂@CMK3, but also because the mesostructure of CMK3 is more open than that of SBA15, resulting in the absence of micropores. Using the same Tanford assumption, the width of this molecule could be estimated to be equal to 1.7 nm. Consequently one TR-grafted molecule should cover about 2.3 nm² of the available surface. According to the graft ratio evaluated by elemental analysis or TGA, these grafted molecules should envelop 290 m² g⁻¹ of the SBA, which matches its specific area (286 m² g⁻¹). We thus argue that the rate of TR grafting onto SBA15 is limited by the steric hindrance of this organic compound. The graft ratio of BP@SBA is lower, and this molecule does not cover the entire available surface. This behavior is likely caused by its high volume, which could block the entrances to the pores during the grafting step.

Table 1 Graft ratio (GR) evaluated by TGA and elemental analysis

	Sample	Elemental analysis			TGA
		%C	%N	GR mmol g^−1	Δm (%)	GR mmol g^−1
SBA15	SBA15	—	—	—	—	—
	NH2@SBA	5.1	1.7	1.4	9.4	1.6
	TR@SBA	12.2	1.9	0.21	20.5	0.23
	BP@SBA	4.9		0.17	6	0.13
CMK3	NH2@CMK		1.1	0.79
	TR@CMK		1.3	0.26

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The extraction capacity (Q_Uext = ([U]_i − [U]_f)V/m) of the hybrid material TR@SBA was investigated by mixing 250 mg of the solid with 10 mL of three different synthetic solutions containing different concentrations of U(VI) and 1 mol L⁻¹ of H₃PO₄ for 24 hours at room temperature and under vigorous stirring. Experimental kinetics studies using SBA15 hybrids have shown that equilibrium is reached after a few hours for uranium extraction from acidic media.^42,43

Twenty-four hours of shaking time was used in the experiments to ensure that thermodynamic equilibrium was reached. The uranium concentration for each solution, before and after contact with the solid, was analyzed using X-ray fluorescence spectroscopy. An increase of the initial amount of U in solution leads to a rise of extraction capacity up to a maximum, close to 10 g kg⁻¹ (equal to 0.042 mmol g⁻¹) for both TR-containing materials (Fig. 2). Thus, the amount of TR complex grafted onto the SBA and CMK solid evaluated previously and the determination of the uranium extraction capacity lead to the ratio [U]_extracted : [TR], which, at roughly 1 : 5, is similar for TR@SBA and TR@CMK. The extraction capacity of BP@SBA in the same experimental conditions was evaluated to be much lower, close to 0.2 g kg⁻¹. It was demonstrated that the remaining free amino groups⁴³ or silanol groups⁴² do not interact with uranyl cation at pH values less than 3. Therefore, we conclude that the efficiency of this kind of hybrid solid comes essentially from the TR or BP molecule grafted onto the available surface rather than from the nature of the support or from the free surface amino or silanol groups.

Fig. 2 Extraction isotherm of TR@SBA material (black circle) and TR@CMK material (blue square).

Comparable performances were reported for different ionic exchange resins under reasonably similar conditions with extraction capacities between 5 and 20 g kg⁻¹ for aminophosphonic resin,¹³ which are much higher than those obtained with biopolymeric microcapsules containing the molecule DEHPA–TOPO (2 g kg⁻¹).¹⁵ The extraction capacities obtained for our materials are 10 times lower than those of Kabay et al.,¹⁴ but in this case such resins containing phosphonic acid are non-selective, especially against iron.

Uranium elution from TR@CMK was eluted from TR@SBA or TR@CMK using respectively nitric acid (1 M) or KOH solution (0.5 M). Up to 40% of the uranium previously extracted with a single contact was recovered.

To complete this study, both TR@SBA and BP@SBA were evaluated to selectively extract uranium(VI) from the competitive iron(III) ion, in phosphoric acid. Experimental results were evaluated in terms of the selectivity coefficient S_U/Fe = Kd_U/Kd_Fe (with the dissociation coefficient Kd_i = Q_iext/[i]_f).

The silica support loaded with TR was able to extract uranium selectively from iron (S_U/Fe = 100), in a range of H₃PO₄ concentrations up to 1 mol L⁻¹. In the case of BP, the value of the selectivity coefficient is only 3. Beyond the amount of grafted ligand, this difference can be explained through the extraction mechanism involved for each ligand, and can be related to the differences of the structures of BP and TRF or both functionalized materials with TR ligand (TR@SBA and TR@CMK); five groups may be involved in the extraction of U: two amido groups (N–C=O), a phosphoryl group (P=O) for the complexation mechanism, and a phosphonate group (P–OH) and an amino group (NH) for ion exchange (Fig. 1). In the case of the hybrid solid loaded with an organosilane (BP), three groups may be involved in the extraction of U: an amido group, a phosphoryl group and a phosphonate group.

To decipher the extraction mechanisms, the dependence of U(VI) extraction on the concentration of H₃PO₄ was investigated for the three different materials: BP@SBA, TR@CMK, TR@SBA (Fig. 3). In each case, the extraction of uranium decreases gradually as the H₃PO₄ concentration increases, as already observed with impregnated resin.¹³ The effect of acid concentration on the extraction of uranium was evaluated in terms of the solid extraction coefficient (E_s(U) = [U]_extracted/[U]_{remaining in solution}) and slope analysis (Fig. 3). For BP@SBA materials, a relationship between log(E_s(U)) and log(H₃PO₄) with a slope of 1.3 was observed, which supports a cation-exchange mechanism with the release of about mainly 1 mole of H₃PO₄; here, only the phosphonate group could be involved in this cation-exchange mechanism. Thus, the following reaction may be proposed:

UO₂(H₂PO₄)₂·H₃PO₄ + HBP@SBA ⇔ UO₂(H₂PO₄)·H₃PO₄ − BP@SBA + H₃PO₄

Fig. 3 Logarithm of solid extraction coefficient E_s(U) versus log([H₃PO₄]) (feed solution 500 mg L⁻¹ of U(VI)); volume to solid-mass ratio: 40 mL g⁻¹ (10 mL of the feed solution and 250 mg of solid). Black circle TR@SBA; blue square TR@CMK; and red open circle BP@SBA.

For TR@SBA and TR@CMK materials, the dependence of the solid extraction coefficient as a function of H₃PO₄ concentration is far different, where the extraction is not affected by the acidity of the solution. A slope of −0.1 is obtained here; we suggest a uranium extraction through a solvation mechanism without release of any H₃PO₄. A chelation would occur through coordination with the oxygens of the phosphoryl group and the amido group.

These results demonstrate the potential of using phosphorous-based grafted materials to extract uranium ions in phosphoric media, even in the presence of iron ions. The grafting efficiency is limited by the steric hindrance of the organic compound used. Extraction mechanisms depend on the H₃PO₄ concentration, with complexation for the tridentate ligand or ion-exchange mechanisms for the bidentate ligand.

Notes and references

1. A. Boitsov, G. Capus, F. J. Dahlkamp, S. Kidd, G. Klassen, J. M. McMurray, H. Miyada, R. Shani, W. N. Szymanski, D. H. Underhill and I. Vera, Analysis of Uranium Supply to 2050, International Atomic Energy Agency, 2001.
2. C. K. Gupta and H. Singh, Uranium Resource Processing: Secondary Resource, Springer, Heidelberg, Germany, 2003.
3. A. Davister, M. G. Lyaudet, B. Schneider, Y. Volkman, I. Ezahr, T. Botella, A. Tolic, F. Hurst and S. Ajuria, The recovery of uranium from phosphoric acid, International Atomic Energy Agency, 1987.
4. S. Gabriel, A. Baschwitz, G. Mathonniere, T. Eleouet and F. Fizaine, Ann. Nucl. Energy, 2013, 58, 213–220.
5. H. Singh, R. Vijayalakshmi, S. L. Mishra and C. K. Gupta, Hydrometallurgy, 2001, 59, 69–76.
6. H. Singh, S. L. Mishra and R. Vijayalakshmi, Hydrometallurgy, 2004, 73, 63–70.
7. H. Singh, S. L. Mishra, M. Anitha, A. B. Giriyalker, R. Vijayalakshmi and M. K. Kotekar, Hydrometallurgy, 2003, 70, 197–203.
8. J. Stas, A. Dahdouh, H. Shlewit and S. Khorfan, Hydrometallurgy, 2002, 65, 23–30.
9. R. Turgis, A. Leydier, G. Arrachart, F. Burdet, S. Dourdain, G. Bernier, M. Miguirditchian and S. Pellet-Rostaing, Solvent Extr. Ion Exch., 2014, 32, 478–491.
10. R. Turgis, A. Leydier, G. Arrachart, F. Burdet, S. Dourdain, G. Bernier, M. Miguirditchian and S. Pellet-Rostaing, Solvent Extr. Ion Exch., 2014, 32, 685.
11. M. Krea and H. Khalaf, Hydrometallurgy, 2000, 58, 215–225.
12. F. J. Hurst and D. J. Crouse, Ind. Eng. Chem. Process Des. Dev., 1974, 13, 286–291.
13. S. Gonzalezluque and M. Streat, Hydrometallurgy, 1983, 11, 207–225.
14. N. Kabay, M. Demircioglu, S. Yayli, E. Gunay, M. Yuksel, M. Saglam and M. Streat, Ind. Eng. Chem. Res., 1998, 37, 1983–1990.
15. M. Outokesh, A. Tayyebi, A. Khanchi, F. Grayeli and G. Bagheri, J. Microencapsulation, 2011, 28, 248–257.
16. R. J. Ring, G. M. Ritcey, M. Roche, S. Ajuria, D. C. Seidel, K. D. Hester, G. Lyaudet, E. Mueller-Kahle, G. Xifeng, Z. You-Ru, N. Kuo Lin, X. De Chang and F. Jia Jun, Uranium Extraction Technology, International Atomic Energy Agency, Vienne, 1993.
17. S. El. Mourabit, M. Guillot, G. Toquer, J. Cambedouzou, F. Goettmann and A. Grandjean, RSC Adv., 2012, 2, 10916–10924.
18. L. Y. Yuan, Y. L. Liu, W. Q. Shi, Y. L. Lv, J. H. Lan, Y. L. Zhao and Z. F. Chai, Dalton Trans., 2011, 7446–7453.
19. L. Y. Yuan, Y. L. Liu, W. Q. Shi, Z. J. Li, J. H. Lan, Y. X. Feng, Y. L. Zhao, Y. L. Yuan and Z. F. Chai, J. Mater. Chem., 2012, 22, 17019–17026.
20. P. J. Lebed, K. de Souza, F. Bilodeau, D. Lariviere and F. Kleitz, Chem. Commun., 2011, 47, 11525–11527.
21. P. J. Lebed, J. D. Savoie, J. Florek, F. Bilodeau, D. Lariviere and F. Kleitz, Chem. Mater., 2012, 24, 4166–4176.
22. X. L. Wang, L. Y. Yuan, Y. F. Wang, Z. J. Li, J. H. Lan, Y. L. Liu, Y. X. Feng, Y. L. Zhao, Z. F. Chai and W. Q. Shi, Sci. China: Chem., 2012, 55, 1705–1711.
23. A. K. S. Deb, P. Ilaiyaraja, D. Ponraju and B. Venkatraman, J. Radioanal. Nucl. Chem., 2012, 291, 877–883.
24. J. H. Kim, H. I. Lee, J. W. Yeon, Y. Jung and J. M. Kim, J. Radioanal. Nucl. Chem., 2010, 286, 129–133.
25. M. Carboni, C. W. Abney, K. M. L. Taylor-Pashow, J. L. Vivero-Escoto and W. B. Lin, Ind. Eng. Chem. Res., 2013, 52, 15187–15197.
26. J. L. Vivero-Escoto, M. Carboni, C. W. Abney, K. E. deKrafft and W. B. Lin, Microporous Mesoporous Mater., 2013, 180, 22–31.
27. K. A. Venkatesan, V. Sukumaran, M. P. Antony and P. R. V. Rao, J. Radioanal. Nucl. Chem., 2004, 260, 443–450.
28. S. Sadeghi and E. Sheikhzadeh, Microchim. Acta, 2008, 163, 313–320.
29. C. F. Baes, J. Phys. Chem., 1956, 60, 878–883.
30. A. Elyahyaoui, S. Bouhlassa, M. Hussonnois, L. Brillard and R. Guillaumont, J. Less-Common Met., 1987, 135, 147–160.
31. A. Elyahyaoui, S. Bouhlassa, M. Hussonnois, L. Brillard and R. Guillaumont, J. Less-Common Met., 1988, 143, 195–206.
32. D. Beltrami, F. Mercier-Bion, G. Cote, H. Mokhtari, B. Courtaud, E. Simoni and A. Chagnes, J. Mol. Liq., 2014, 190, 42–49.
33. T. H. Siddall, J. Inorg. Nucl. Chem., 1963, 25, 883–892.
34. G. S. Conary, R. L. Meline, R. Schaeffer, E. N. Duesler and R. T. Paine, Inorg. Chim. Acta, 1992, 201, 165–176.
35. J. C. Birnbaum, B. Busche, Y. H. Lin, W. J. Shaw and G. E. Fryxell, Chem. Commun., 2002, 1374–1375.
36. H. Kaper, A. Grandjean, C. Weidenthaler, F. Schuth and F. Goettmann, Chem.–Eur. J., 2012, 18, 4099–4106.
37. H. Kaper, J. Nicolle, J. Cambedouzou and A. Grandjean, Mater. Chem. Phys., 2014, 147, 147–154.
38. C. M. Chang and Y. L. Liu, Carbon, 2009, 47, 3041–3049.
39. X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu and K. M. Kemner, Science, 1997, 276, 923–926.
40. A. Grandjean, G. Toquer and T. Zemb, J. Phys. Chem. C, 2011, 115, 11525–11532.
41. C. Tanford, The hydrophobic effect, Wiley Interscience, New York, 1973.
42. X. Wang, G. Zhu and F. Guo, Ann. Nucl. Energy, 2013, 56, 151–157.
43. Y. Liu, L. Yuan, Y. Yuan, J. Lan, Z. Li, Y. Feng, Y. Zhao, Z. Chai and W. Shi, J. Radioanal. Nucl. Chem., 2012, 292, 803–810.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis details, SAXS data, selectivity data, pores properties. See DOI: 10.1039/c4ra08703h

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