Synthesis of new carbazole derivative extractants and study on extraction of perrhenate/pertechnetate

It is very necessary to remove radioactive TcO4− from nuclear waste because TcO4− is harmful to the environment and public health due to the extremely high mobility and long-term radioactivity. In this paper, four carbazole derivative extractants L1-L4 were synthesized, and their extraction behavior for ReO4− (as an analog of radioactive TcO4−) was preliminarily studied in the chloroform-HNO3 system. Compared with L1-L3, L4 showed excellent extractability toward ReO4−, and DRe is more than 100 at pH = 2 when the initial concentration of L4 is 50 mmol/L, with the high selectivity for ReO4− over other competition anions due to the pre-organized cavity of L4 matching the size of ReO4−. L4 also showed the high SFRe(VII)/U(VI)(5000). And more, ReO4− could be easily and efficiently stripped from the loaded organic phase by Na2CO3 solution with about 100% of the stripping yield. The mechanism was further studied by NMR, HRMS, and theoretical calculation, and the results found that the process of ReO4− extracted by L4 conformed to the anion-exchange extraction model, and the extraction compound is [H2L2+·ReO4−][NO3−]. Finally, the distribution ratio (4–33) of TcO4− by L4 is better than ReO4− (1–5). This research provides a new idea for the design of extractants for efficient separation of TcO4−.