Synthesis of a new carbazole-based extractant and application on the extraction of ReO₄⁻ in H₂SO₄ system

Selective separation and recovery of TcO₄⁻/ReO₄⁻ is of great significance in spent fuel reprocessing, nuclear medicine, and rare earth metal smelting. In this paper, based on carbazole NH which is a strong hydrogen bond donor and electrostatic interaction, carbazole-based extractant L with a pre-organized intramolecular cavity matched with the size of TcO₄⁻/ReO₄⁻ was designed and synthesized for selectively extracting TcO₄⁻/ReO₄⁻. The crystal structure showed that there was a matched pre-organized intramolecular cavity of L for TcO₄⁻/ReO₄⁻. Extraction performance for ReO₄⁻ by L in various extraction conditions in H₂SO₄-CHCl₃ system was investigated. The extraction distribution ratio of ReO₄⁻ by L is more than 300 under high acidity (3 mol·L⁻¹ H₂SO₄, D_Re > 300), and is easy to back extraction by Na₂CO₃ aqueous solution(Stripping percentage > 99%). And L showed excellent selectivity for ReO₄⁻/M(Pd²⁺, Eu³⁺, Th⁴⁺, UO₂²⁺, MoO₄²⁻) at 1.5 mol·L⁻¹ H₂SO₄, SF_Re(VII)/M could reach more than 8 × 10³. The extraction slope method and HRMS showed that the extraction mechanism was an anion exchange mechanism, and the extract complex was [H₂L²⁺·2ReO₄⁻]. This study offers a new idea for the selective separation of TcO₄⁻/ReO₄⁻ by extraction in high-acidity systems.