Adsorption study of U and Th by N,O-hybrid donor ligand-immobilized hydrogels

ABSTRACT

The thorium (Th) fuel cycle is attracting attention again because of its potential merits over the uranium (U) fuel cycle, but the associated back-end technology remains insufficiently mature compared with that of the U fuel cycle. In the chemical reprocessing of spent Th oxide fuels, effective separation of UO₂²⁺ from Th⁴⁺ is required and a quite high dose rate is expected in reprocessing because of the strong radiation by ²⁰⁸Tl generated from ²³²U, an associated product of ²³³U. Therefore, gel/liquid extraction was applied to U/Th separation．Introducing ligands chemically into the hydrogels (via the C–C bonds) can minimize ligand leaching, and the ligands are expected to be more resistant to radiation. Some of the N,O-hybrid donor ligands that have polymerizable functional groups – Bis(di-N-allyl-N-tollyl) phenanthlorine diamide (Allyl-Tol-PTDA) and N,N,N’,N’-tetraallylpyridine-2,6-dicarboxamine (Tet-Allyl-PDA) for gel adsorbent synthesis and N,N,N’,N’-tetrahexylpyridine-2,6-dicarboxamine (Tet-Hex-PDA) – were synthesized for solvent extraction because Allyl-Tol-PTDA has low solubility in n-dodecane. Allyl-Tol-PDA and Tet-Allyl-PDA were then polymerized with N-isopropylacrylamide (NIPA) and N,N’-methylenbisacrylamide (BIS) to give thermosensitive hydrogels, namely PTDA-gel and PDA-gel. The general extraction behaviors of Allyl-Tol-PTDA and Tet-Hex-PDA instead of Allyl-Tol-PDA and the adsorption behaviors of PTDA-gel and PDA-gel were compared with those of UO₂²⁺ and Th⁴⁺ in HNO₃. Consequently, differing extraction/adsorption behavior between solvent extraction and gel adsorption was determined. The PTDA gel showed no strong dependency on acidity and temperature with UO₂²⁺, but Th⁴⁺ showed a local maximum with varying acidity. By contrast, the PDA-gel showed temperature dependency: UO₂²⁺ adsorbed more at higher temperature and Th⁴⁺ adsorbed more at lower temperature. Therefore, U/Th separation is possible with (i) PTDA-gel at higher temperature and acidity, and (ii) PDA-gel at higher temperature and lower acidity.

KEYWORDS:

• Gel/liquid extraction
• uranium
• thorium
• adsorption
• thermosensitive gels

Additional information

Funding

This research was supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Challenging Research (Exploratory, 18K19043) and the Interuniversity Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposal nos. 17K0086 and 18K0108); Tohoku University.