Mechanism unravelling for highly efficient and selective (99)TcO(4)(−) sequestration utilising crown ether based solvent system from nuclear liquid waste: experimental and computational investigations

Selective and efficient separation of pertechnetate (TcO(4)(−)) from nuclear waste is desirable for the safe and secure management of radioactive waste. Here, we have projected dibenzo-18-crown-6 ether (DB18C6) in a highly polar nitrobenzene medium for enhancing the removal efficiency of (99)Tc from reprocessing plant low level waste (LLW). An effort was made to determine the stoichiometry of metal–ligand complex by slope ratio method, revealing that one ligand (DB18C6) binds with one TcO(4)(−) moiety. Optimum ligand concentration for (99)Tc extraction was evaluated. Relevant interference of the anions was studied systematically. The effect of solution pH was analysed on the extraction efficiency of (99)Tc. A kinetic study was carried out for maximum extraction of metal ions. A quantitative stripping study was also achieved for metal ions with a suitable stripping solution. After evaluation of all essential parameters, selectivity and feasibility studies were finally carried out with actual low level reprocessing plant waste to demonstrate a laboratory scale process for effective separation of TcO(4)(−) ions. Density functional theory (DFT) calculations were carried out to understand the nature of the complexation of TcO(4)(−) ion with DB18C6 in different solvents systems and to elucidate the key aspect behind ionic selectivity and enhanced the (99)Tc extraction efficiency of DB18C6 in the studied diluent systems. The ΔE and ΔG values for different modeled complexation reactions were evaluated systematically. From the calculated free energy of complexation of TcO(4)(−) with DB18C6, it was observed that the consideration of explicit solvent plays a vital role in predicting the experimental selectivity.