Study of the chromatographic properties of uranium and plutonium complexes using supercritical carbon dioxide chromatography

We have developed the nucleation of a supercritical phase chromatograph for the determination of radioactive metal cations (uranium (VI) and plutonium (IV)). This technique uses carbon dioxide as the mobile phase, a non-toxic, non-flammable gaseous fluid at atmospheric pressure. It therefore makes an excellent contribution to solving the problem of liquid waste generated by the nuclear industry. Because it is only slightly polar, supercritical carbon dioxide cannot be used to solubilise metal cations. The formation of low-polarity molecular complexes is therefore necessary in order to mask the polarity of the cations. A literature review led to the selection of two complexing agents: 1-phenyl-3-methyl-4-benzoyl-5-pyrazolinone for plutonium (IV) and 2,6-diacetyl-pyridine bis(benzoyl-hydrazone) for uranium (VI). The latter is selective for uranyl and cupric cations. To increase its solubility in supercritical carbon dioxide, we synthesised several derivatives bearing alkyl chains. We studied the chromatographic properties of the different complexing agents and complexes as a function of temperature, pressure and the nature of the mobile and stationary phases. This study showed a reversal in the retention order of the complexes as a function of the modifier content in the carbon dioxide, which was only observed with stationary phases grafted with polymers and polar protic modifiers. The modifier content causing co-elution of the complexes decreases as a function of their lipophilicity and increases as a function of its affinity for the residual silanol sites of the above-mentioned stationary phases. For plutonium (IV), we therefore chose beta-diketones as complexing agents. The complexes formed were eluted with pure carbon dioxide in the form of perfectly symmetrical peaks. The quantitative aspect of the separations was addressed for uranium (VI) and plutonium (IV). Their determinations can be obtained in less than three minutes. (author)