Selective separation of strontium by multitopic ion‐pair receptor: A DFT exploration

Selective extraction of a radionuclide in the presence of other interfering ions is one of the vital steps in the back‐end‐of‐the‐nuclear fuel cycle. The presence of interfering cations (such as Ca 2+ ) in the radioactive waste and involvement of multiple separation steps are known to be bottlenecks in the efficient Sr 2+ extraction. Here, using free energy corrected density functional theory, we have proposed a two‐step Sr 2+ extraction methodology in nitrate media in the presence of interfering Ca 2+ ion using a multitopic ion‐pair receptor, which was earlier reported to be strongly selective for K + (Kim et al. J. Am. Chem. Soc . 2012, 134 , 1782–1792). To depict the correct free energy trend in the proposed extraction processes, the most probable binding mode of the metal (Sr 2+ , Ca 2+ , and K + ) nitrates in the host are identified. In excellent agreement with the previously reported experiment, Crown/Pyrrole (C/P) binding is noted to be the most preferable mode for KNO 3 , where K + andNO 3 −occupied the Crown (C) and Pyrrole (P) site, respectively. However, the divalent metal ions (Ca 2+ and Sr 2+ ) are noted to marginally prefer Crown/Crown‐Pyrrole (C/CP) mode, in which metal reside at the C site while two nitrates occupy the P site and also simultaneously bind at the outer sphere of C site to coordinate with the metal via monodentate motif. Based on the free energy of extraction, we predict that the selective separation of chemically alike Ca 2+ /Sr 2+ pair is indeed achievable using this receptor. We propose that once [Sr(NO 3 ) 2 ] is extracted in organic media, the receptor's high affinity toward K + in nitrate media can be used to back strip Sr 2+ to the aqueous phase.