Unanticipated favoured adsorption affinity of Th(IV) ions towards bidentate carboxylate functionalized carbon nanotubes (CNT-COOH) over tridentate diglycolamic acid functionalized CNT: density functional theoretical investigation

The hybrid B3LYP density functional and TZVP basis set in conjunction with COSMO solvation approach have been used successfully to predict the free energy of adsorption for Th4+ ions with pristine CNTs, oxidized CNTs (CNT-COOH) and diglycolamic acid functionalized CNTs (CNT-DGA). Experimentally reported values of adsorption capacities of Th4+ by the above three types of CNT indicate that CNT-COOH has the strongest binding with Th4+, whereas p-CNT has the lowest; CNT-DGA shows less adsorption than CNT-COOH, although the former has tridentate ligands on CNTs' surfaces. This experimental observation has been demonstrated by DFT theoretical studies: gas phase calculation does not match with the above experimental fact, whereas, solvent phase calculation in the presence of nitrate ions, a more realistic approach, is able to explain. Free energy of complexation between Th4+ and CNT-COOH is higher than that of CNT-DGA in all available models of complexation reaction viz. bare Th4+ in aqueous nitrate medium, octahydrated Th4+ in nitrate and thorium nitrate ion pair in aqueous solution. The experimental fact that the presence of oxidized fullerene C-60 enhances the Th4+ adsorption by oxidized CNT has also been theoretically corroborated as oxidized C-60 has a quite high binding energy for Th4+. Present computational calculation in the search for the best nano carbon based Th4+ adsorbent parallel with experimental verification might be helpful for the design of effective nanomaterials to be used in the treatment of radioactive liquid waste.