Extraction of Th(Ⅳ) and U(Ⅵ) from Nitric Acid Solutions by N,N,Nꞌ,Nꞌ,Nꞌꞌ,Nꞌꞌ-hexaoctylnitrilotriacetamide

High-purity thorium is essential for producing key radioactive nuclides such as ²³³U and ²²⁵Ac. However, thorium naturally co-occurs with uranium and rare earth elements (REE) in minerals like monazite. Even commercial thorium reagents typically contain trace uranium and REE impurities. Insufficient thorium purity during the conversion of ²³²Th to either ²³³U or ²²⁵Ac can impede neutron or proton absorption, reducing conversion efficiency. Consequently, the primary challenge in producing high-purity thorium lies in separating and removing these trace impurities from crude thorium products. Currently, solvent extraction serves as the principal method for Th/U separation, widely used for thorium purification due to its efficiency, operational simplicity, and cost-effectiveness. Common extractants include neutral phosphorus compounds (e.g., tributyl phosphate (TBP)) and amine extractants (e.g., tricaprylmethyl ammonium chloride (Aliquat 336)). While TBP serves as a low-cost, extensively used option, it suffers from moderate extraction ability, constrained separation efficiency, and relatively high aqueous solubility. Amine extractants such as Aliquat 336 offer excellent thorium selectivity but exhibit poor solubility in common alkane diluents (e.g., kerosene, n-dodecane), necessitating phase modifiers. Due to its low loaded capacity, even with modifiers, emulsification or third-phase formation frequently occurs in systems with high aqueous-phase metal ion concentrations, severely limiting applicability. To address this, a novel extractant of N,N,N',N',N'',N''-hexaoctyl- nitrilotriacetamide (NTAamide(n-Oct)) was developed for isolating high-purity thorium in nitric acid medium containing uranium and REE. In the present work, using n-dodecane as a diluent, the extraction of Th(Ⅳ) and U(Ⅵ) from nitric acid solutions by NTAamide(n-Oct) was investigated, focusing mainly on the effects of acidity, extractant concentration, and temperature. Within the acidity range of 0.1-6.0 mol/L, NTAamide(n-Oct) exhibits significantly higher extraction for Th(Ⅳ) than U(Ⅵ), demonstrating excellent selectivity for Th(Ⅳ). This selectivity further increases as the U(Ⅵ)/Th(Ⅳ) ratio in the aqueous phase decreases, demonstrating a promising application prospect in the preparation of high-purity thorium. Slope analysis indicates the formation of 1∶1 complexes between NTAamide(n-Oct) and both metal ions. Increasing temperature reduces the distribution ratios, confirming that the extraction of Th(Ⅳ) and U(Ⅵ) is exothermic and spontaneous. However, Th(Ⅳ) extraction is accompanied by an entropy increase, while U(Ⅵ) extraction shows an entropy decrease. Additionally, infrared spectroscopy and single-crystal X-ray diffraction reveal that coordination involves all three carbonyl oxygen atoms and the central amine nitrogen atom of the NTAamide ligand with Th(Ⅳ), forming a 1∶1 complex consistent with solvent extraction results. In nitric acid medium, the extraction of Th(Ⅳ) and U(Ⅵ) by NTAamide(n-Oct) follows a neutral complexation extraction model.