Effective separation of Am(III) and Eu(III) from HNO 3 solutions using CyMe 4 -BTPhen-functionalized silica-coated magnetic nanoparticles†

It has been shown that CyMe 4 -BTPhen-functionalized silica-coated maghemite (γ-Fe 2 O 3 ) magnetic nanoparticles (MNPs) are capable of quantitative separation of Am(III) from Eu(III) from HNO 3 solutions. These MNPs also show a small but significant selectivity for Am(III) over Cm(III) with a 10 separation factor of around 2 in 4 M HNO 3 . The water molecule in the cavity of the BTPhen may also play an important part in the selectivity. A key step in closing the nuclear fuel cycle may involve the partitioning and transmutation of irradiated nuclear fuel. In the 15 case of the minor actinides (Am and Cm) this requires their selective separation from the chemically similar trivalent lanthanides. 1 One approach to this has resulted in the development of a combination of two partitioning processes to be applied to post PUREX raffinate. This protocol is based on the 20 co-separation of trivalent actinides and lanthanides by a diamide-based ligand (DIAMEX) process, followed by selective separation of trivalent actinides in a SANEX (Selective ActiNide EXtraction) process. 2 One of the SANEX processes considered utilizes liquid-liquid extraction using nitrogen bearing ligands 25 such as the 2,6-bis(1,2,4-triazine-3-yl)pyridines

Effective separation of Am(III) and Eu(III) from HNO 3 solutions using CyMe 4 -BTPhen-functionalized silica-coated magnetic nanoparticles † Ashfaq Afsar, a Laurence M. Harwood,* a Michael J. Hudson, a Petr Distler b and Jan John b It has been shown that CyMe 4 -BTPhen-functionalized silica-coated maghemite (c-Fe 2 O 3 ) magnetic nanoparticles (MNPs) are capable of quantitative separation of Am(III) from Eu(III) from HNO 3 solutions.
These MNPs also show a small but significant selectivity for Am(III) over Cm(III) with a separation factor of around 2 in 4 M HNO 3 .The water molecule in the cavity of the BTPhen may also play an important part in the selectivity.
A key step in closing the nuclear fuel cycle may involve the partitioning and transmutation of irradiated nuclear fuel.In the case of the minor actinides (Am and Cm) this requires their selective separation from the chemically similar trivalent lanthanides. 1 One approach to this has resulted in the development of a combination of two partitioning processes to be applied to post PUREX raffinate.This protocol is based on the co-separation of trivalent actinides and lanthanides by a diamide-based ligand (DIAMEX) process, followed by selective separation of trivalent actinides in a SANEX (Selective ActiNide EXtraction) process. 2 One of the SANEX processes considered utilizes liquid-liquid extraction using nitrogen bearing ligands such as the 2,6-bis(1,2,4-triazine-3yl)pyridines (BTPs), [3][4][5] or 6,6 0 -bis(1,2,4-triazin-3-yl)-2,2'-bipyridines (BTBPs) [4][5][6][7][8] (see Fig. 1) dissolved in an organic diluent.
0][11][12][13] This ligand has specific differences from the BTBPs.CyMe 4 -BTPhen is more preorganized for complex formation; it has a dipole moment and so is more surface active at the interface. 12Consequently CyMe 4 -BTPhen shows faster rates of metal ion extraction and stripping together with distribution ratios that are two orders of magnitude higher for An(III) extraction in liquid-liquid extraction experiments than its non-preorganized BTBP counterpart. 12owever, selective extraction of minor actinides by a liquidliquid extraction process comes with certain disadvantages, such as the requirement for substantial liquid storage and containment and generation of significant amounts of secondary waste. 14,15Thus there is a requirement for new systems that are capable of polishing the raffinates from the SANEX and related processes as well as for dealing with low level activity of liquid wastes.
Recently magnetic separation technology has attracted attention in the area of spent nuclear fuel separation. 14,16It is proposed that when magnetic nanoparticles (MNPs) are combined with ligands such as CyMe 4 -BTPhen, these functionalized MNPs could be used to extract the minor actinides and the radioactive material could then be collected magnetically in preference to centrifugation.Finally, the MNPs could be recycled by stripping the radioactive elements from the conjugates, generating a very small amount of secondary waste.
Owing to the acidic nature of the post-PUREX aqueous raffinate (typically 4 M HNO 3 ), unmodified iron or iron oxide based MNPs cannot be used in this medium.It was proposed to solve this problem by using a silica coating in order to provide a chemically unreactive surface to the MNPs whilst not affecting the core. 17,18Furthermore, the free Si-OH surface groups can allow effective covalent binding of organic functional groups. 17,18n the work reported herein, we have investigated the separation of minor actinides from lanthanides using CyMe 4 -BTPhenfunctionalized SiO 2 -coated MNPs.
The 5-BrCyMe 4 -BTPhen ligand 4 was synthesized by a protocol previously described 11 with the modification that the final step was performed using tetramethylcyclohexane-1,2-dione.Replacement of the bromine with a 4-hydroxyphenol linking group was successfully achieved by Suzuki coupling 22  The organic content on the MNPs was further investigated using thermal gravimetric analysis (TGA) under nitrogen (Fig. 2).Below 150 1C, the mass loss is quite small, probably corresponding to removal of absorbed water.After that, there is a more-or-less linear mass loss between ca.250-700 1C corresponding to decomposition of the organic components.From this, it can be estimated that the amount of CyMe 4 -BTPhen bound onto the MNP is about ca.20% w/w (ESI †).Further mass loss above ca.800 1C can be attributed to the loss of carbon -perhaps during the formation of iron carbide. 23,24he aqueous solutions for the solvent extraction experiments were prepared by spiking nitric acid solutions (0.001-4 M) with stock solutions of 241 Am, 152 Eu and 244 Cm and then adding 600 mL of spiked aqueous solution to 18 mg of CyMe 4 -BTPhenfunctionalized MNP 6.The suspension was sonicated for 10 min and shaken at 1800 rpm for 90 min.After centrifuging for 10 min, aliquots of the aqueous solutions (supernatant) were separated and taken for measurements.The distribution ratios, D, were calculated as the ratio between the radioactivity (a-and g-emissions) of each isotope in the standard solution and the supernatants after removal of MNP 6.The separation factor is SF Am/Eu = D Am /D Eu or SF Am/Cm = D Am /D Cm (Table 2).
Extractions were studied at nitric acid concentrations of 0.001 M, 0.1 M, 1 M and particularly 4 M.The distribution ratios and separation factors for the extraction of Am(III) and Eu(III) from nitric acid solutions at these concentrations are shown in Fig. 3. High distribution ratios (D 4 700) were obtained for both Am(III) and Eu(III) at 0.001 M HNO 3 solution with no significant selectivity (SF Am/Eu = 1.7 AE 0.1) for Am(III) over Eu(III).At 0.1 M HNO 3 , the D value for Am(III) remained    was retained on the MNP 6.The resulting separation factor (SF Am/Eu = estimated to be 41300) is far superior to that observed for CyMe 4 -BTPhen (SF Am/Eu = 400) 9,10 in solvent extraction experiments under similar conditions and means that quantitative separation of Am(III) from Eu(III) is possible at this concentration of HNO 3 (Table 3).Distribution ratios for Am(III) and Cm(III), and the separation factors at different nitric acid concentrations were also examined (Fig. 4).The D values for both Am(III) and Cm(III) decreased with increasing nitric acid concentration, in agreement with the earlier results, resulting in a small but significant SF Am/Cm = 2.2 AE 0.4 at 4 M HNO 3 .
We propose that the shortness of the linking-chain on the MNP constrains the CyMe 4 -BTPhen ligand to form 1 : 1 complexes with M(III) cations. 25For the quadridentate CyMe 4 -BTPhen ligand, the dominant metal-ligand complex stoichiometry in solution is 1 : 2 however, species proposed to be 1 : 1 complexes can be observed by 1 H-NMR titrations at high Ln(III) loadings and a crystal of a 1 : 1 complex [Y(CyMe 4 -BTPhen)(NO 3 ) 3 ].MeCN has been isolated and structurally characterized in the solid state. 12,26In the crystal, the yttrium(III) cation is 10-coordinate being bonded to the tetradentate CyMe 4 -BTPhen and to three bidentate nitrate ions.
In the BTPhen moiety, the strongly bound water molecule in the central cavity may also play an important role in the separation of Am(III) from Eu(III).The initial attack of the cation is probably on the N(2) of the triazine ring in the trans-rotamer (Fig. 5).This bound metal then seeks to bind with other nitrogens and as the cis-rotamer is forming, the strongly bound water molecule is displaced.The Am(III) subsequently binds to all four nitrogen atoms in the BTPhen, while the Eu(III) cations are unable to bind, particularly at higher nitric acid concentrations, thus providing for the quantitative separation of Am(III) from Eu(III).Further studies into this proposed mechanism are continuing.
In summary, the CyMe 4 -BTPhen ligand has been covalently bound to SiO 2 -coated MNPs by a phenyl ether linkage after functionalization at C-5 of the phenanthroline.The MNP 4 exhibits very high selectivity for Am(III) over Eu(III) at 4 M HNO 3 (with a separation factor in excess of 1300).This MNP also shows a small but significant selectivity for Am(III) over Cm(III) with a nominal separation factor of around 2 in 4 M HNO 3 .We propose that this technology may well prove effective for polishing the raffinate from SANEX-type processes and for remediation of contaminated water or soils.
The authors acknowledge the EPSRC for financial support (A.A.).Use of the Chemical Analysis Facility (CAF) and Centre for Advanced Microscopy (CfAM) at the University of Reading is also gratefully acknowledged.We also would like to thank Dr Peter Harris for his assistance with Transmission Electron Microscopy (TEM).

Fig. 3
Fig. 3 Extraction of Am(III) and Eu(III) by MNP 6 as a function of nitric acid concentration.

Fig. 4
Fig. 4 Extraction of Am(III) and Cm(III) by MNP 6 as a function of nitric acid concentration.

Table 3
Extraction of Am(III) and Cm(III) by MNP 6 as a function of nitric acid concentration