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.
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