Separation of An(III) from Ln(III) elements

Separation of Am and Cm, known as the minor actinides (MA), from the fission Ln elements is a key goal of several countries seeking to substantially reduce the mass of radionuclides requiring long-term disposition in a geo­logical repository. The implementation of a partitioning and transmutation technology necessitates the removal of the Ln elements because of rela­tively large neutron capture cross sections and incompatibility with pro­posed transmutation targets. The separation of the MA from Ln is, however, a difficult chemical separation to achieve as there are minor differences in the chemical properties of these elements. Studies in recent years have shown that ligands containing soft donor atoms such as N and S offer a higher probability of being more selective for An(III) elements (Kikuchi et al., 2004, Zhu, 1995, Modolo et al., 1998b, Wei et al., 2000a, Iki et al., 1998, Morohashi et al., 2001, Mathur et al., 2001). Although the exact mechanism for this preferential complexing behavior has not been entirely elucidated, many researchers are investigating the merits of using soft donor ligands in solid-phase extraction to perform An(III) separations from Ln(III) ele­ments in high level liquid waste (HLLW) solutions subsequent to upstream removal of U and Pu.

The N-donor ligand, 2,6,-bistriazinylpyridine (BTP) has shown promise for separating An(III) from Ln(III) via differing nitrate dependencies in

solutions of moderate acidity (pH ~2). The solubility of the ligand can also be modified via the addition of alky chains. The functionalized forms 2,6-bis — (5,6-dibutyl-1,2,4-triazine-3-yl)-pyridine (n-Bu-BTP or tro-Bu-BTP) have been synthesized (Fig. 13.7) and a solid-phase extraction material devel­oped by impregnating n-Bu-BTP into a silica-co-polymer support (Wei et al., 2004a, Wei et al., 2000b, Hoshi et al., 2006).

The silica co-polymer (SiO2-P) support is prepared by polymerizing a mixture of formylstyrene and divinylbenzene monomers saturated within the pores of spherical silica particles resulting in 17.6% (w/w) co-polymer embedded in SiO2. The ко-Bu-BTP ligand (2.5g) was incorporated into the SiO2-P support (5g) via the dry impregnation method and the material tested in batch and small (20 cm3 bed) column tests (Hoshi et al., 2006). Excellent decontamination of Am, Cm, and heavy Ln(III) elements from fission products and light Ln(III) elements was achieved using a simulated HLLW solution (w/o U, Pu) containing 1 M NaNO3-0.01 M HNO3. Heavy Ln(III) elements were removed from the column with 0.3 M NaNO3-0.01 M HNO3. The strongly sorbed An(III) elements were then eluted from the column by reducing the nitrate concentration with pure water. The resulting separation factors between Am and Ln(III) are listed in Table 13.2.

The observed nitrate dependency of An(III) and Ln(III) extraction by BTP may thus be exploited by the addition of nitrate salts and is quite promising. Batch tests were also performed to examine the stability of BTP against nitric acid. The authors (Hoshi et al., 2006) determined that

13.8 Chemical structure of Cyanex-301 ligand.

extractant losses increase with increasing nitric acid concentration due to protonation and that branched Ao-Bu-BTP shows more acid stability than n-Bu-BTP. The acid effects may not be an insurmountable issue since the material is used at very low acid concentrations.

A biopolymer microcapsule containing Cyanex 301 (bis(2,4,4-trimethyl — pentyl)dithiophosphinic acid) extractant has been reported by Mimura et al., (2001). The solid phase extraction resin was prepared by mixing the extractant with 1-2.5% (w/w) sodium alginate and then adding drop-wise to 0.5 M nitrate salt solution or 1 M HCl. The beads were allowed to harden in the bath overnight and then filtered and dried. The resin was tested in a small (4.9 cm3) chromatographic column and the researchers were able to achieve a separation factor of 20 between Am and Eu in a pH range of 1-2 using freshly made resin (<1 day old).

A satisfactory separation of Am from Ln(III) elements with newly pre­pared Cyanex 301-SiO2-P resin has also been reported (Wei et al., 2000b). However, similar testing with 4-week old resin did not produce an accept­able separation between Am and Ln(III). These authors concluded that oxidation impurities formed in the Cyanex 301 are responsible for the poor separation and that methods for stabilizing the extractant are needed. The structure of the Cyanex 301 compound is shown in Fig. 13.8.

A solid-phase extraction resin for separating MA from Ln(III) elements has been prepared by impregnating p-tert-butylthiacalix[4]arene com­pounds in a SiO2-P support (Kikuchi et al., 2004) The structure of the ligands (CAPS, CAPS-SO2) is shown in Fig. 13.9.

Batch contacts performed with the CAPS, CAPS-SO2-SiO2-P resin did not show any measureable extraction of Am or Ln elements at pH = 2. However, raising the solution pH to 4 resulted in a separation factor of 10 for Am over Nd and Eu with the CAPS-SiO2-P resin. A separation factor of 500 for Am over Nd and Eu was achieved with CAPS-SO2-SiO2-P resin in tests at the higher pH. A proposed flow sheet for separating the MA is shown in Fig. 13.10.

The effects of gamma radiation on SiO2-P particles loaded with Cyanex 301, CAPS and CAPS-SO2 were evaluated by irradiating the resin particles to a dose of 1 MGy in a weak acid (pH = 4) at ambient temperature (Kikuchi et al., 2004). The irradiation caused a 30% degradation of the Cyanex 301 exchanger and an increased retention of Eu, thus substantially

13.9 Chemical structure of CAPS and CAPS-SO2 ligands.

Minor Ac & Ln feed solution

reducing the separation ability between Am and Eu. The irradiation resulted in only 1% degradation of CAPS-SO2-SiO2-P exchanger and the high sepa­ration factor of Am relative to Nd and Eu remained constant during and following irradiation. Irradiation of the CAPS-SiO2-P exchanger also pro­duced an increased distribution coefficient for Am. The authors suggest that the sulfur in the CAPS ligand was oxidized during irradiation to become sulfonyl groups, i. e. fully or partially transforming it to CAPS-SO2, which they postulate is the factor contributing to a higher Am selectivity. Although confirmatory analytical data are not given, the hypotheses are certainly credible and merit more detailed investigation.