High selective extraction for thorium(IV) with NTAamide in nitric acid solution: Synthesis, solvent extraction and structure studies

• The selective solvent extraction of Th(IV) with NTAamide was investigated for the first time.
• NTAamide exhibits high separation factors of Th(IV) over U(VI) and lanthanides(III) in the range between 2.5 × 102 and 10 × 103.
• The detailed extraction mechanism for Th(IV) was discussed and the thermodynamic parameters were presented.
• The crystal structure of Th(IV) complex with NTAamide was obtained.

N,N,N′,N′,N″,N″-hexaoctylnitrilotriacetamide (NTAamide(C8)) and N,N,N′,N′,N″,N″-hexabutylnitrilotriacetamide (NTAamide(C4)) were synthesized for selective solvent extraction of Th(IV) from U(VI) and lanthanides(III) (Lns(III)) and for coordination between Th(IV) and NTAamide, respectively. NTAamide(C8) exhibits excellent extraction power and selectivity toward Th(IV) over U(VI) and Lns(III) in a wide range of acidity from 0.1 to 10.0 M HNO3. Very high SFTh/M in the range between 2.5 × 102 and 10 × 103 was observed. Slope analysis shows that Th(IV), U(VI) and Lns(III) were all extracted as mono-solvated species at 3.0 M HNO3. The thermodynamic parameters describing the extraction of Th(IV) indicate that the process is endothermic and spontaneous at 298 K. The single-crystal X-ray diffraction observations also establish that 1:1 complex is formed between NTAamide and thorium nitrate, which are in good accordance with the solvent extraction results, and NTAamide is coordinated with Th(IV) in a tetradentate coordination mode.