Mechanism unravelling for highly efficient and selective (TcO4-)-Tc-99 sequestration utilising crown ether based solvent system from nuclear liquid waste: experimental and computational investigations

Selective and efficient separation of pertechnetate (TcO4-) 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 Tc-99 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 TcO4- moiety. Optimum ligand concentration for Tc-99 extraction was evaluated. Relevant interference of the anions was studied systematically. The effect of solution pH was analysed on the extraction efficiency of Tc-99. 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 TcO4- ions. Density functional theory (DFT) calculations were carried out to understand the nature of the complexation of TcO4- ion with DB18C6 in different solvents systems and to elucidate the key aspect behind ionic selectivity and enhanced the Tc-99 extraction efficiency of DB18C6 in the studied diluent systems. The Delta E and Delta G values for different modeled complexation reactions were evaluated systematically. From the calculated free energy of complexation of TcO4- with DB18C6, it was observed that the consideration of explicit solvent plays a vital role in predicting the experimental selectivity.