Fast and selective reversed-phase high performance liquid chromatographic separation of UO22+ and Th4+ ions using surface modified C18 silica monolithic supports with target specific ionophoric ligands

Abstract

Reprocessing nuclear-spent fuels is highly demanded for enhanced resource efficacy and removal of the associated radiotoxicity. The present work elucidates the rapid separation of UO₂²⁺ and Th⁴⁺ ions using a reversed-phase high-performance liquid chromatographic (RP-HPLC) technique by dynamically modifying the surface of a C₁₈ silica monolith column with target-specific ionophoric ligands. For the dynamic modification, four analogous aromatic amide ligands, N¹,N¹,N³,N³,N⁵,N⁵-hexa(alkyl)benzene-1,3,5-tricarboxamide (alkyl = butyl, hexyl, octyl, and decyl) as column modifiers were synthesized. The complexation properties and retention profiles of the amide-based column modifiers for the selective and sequential separation of UO₂²⁺ and Th⁴⁺ ions were investigated. In addition, the selective separation of UO₂²⁺ and Th⁴⁺ ions among the competitive ions of similar chemical properties were also studied. The ionophore immobilized C₁₈ silica monolith columns demonstrated a varying degree of retention behavior for UO₂²⁺ and Th⁴⁺ ions (UO₂²⁺ is retained longer than Th⁴⁺ under all analytical conditions), eventually leading to rapid separations within a period of ≤5 min. A 0.1 M solution of 2-hydroxyisobutyric acid (HIBA, 1 mL min⁻¹) served as the mobile phase, and the qualitative and quantitative assessment of the sequentially separated 5f metal ions was achieved through post-column derivatization reaction, using arsenazo(III) as a post-column reagent (PCR; 1.5 mL min⁻¹) prior to analysis using a UV-vis detector, at 665 nm (λ_max). The developed technique was further evaluated by standardizing various analytical parameters, including modifier concentration, mobile phase pH, mobile phase flow rate, etc., to yield the best chromatographic separation. Also, the conceptual role of alkyl chain length (in the modifier) on the retention behavior of the studied metal ions was evaluated for cutting-edge future applications.