Sorption behaviour of Pu4+ and PuO22+ on amido amine-functionalized carbon nanotubes: experimental and computational study

Abstract

Amido amine-functionalized multi-walled carbon nanotubes (MWCNT-AA) were used for efficient and selective solid phase separation of plutonium(IV) and plutonium(VI). Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Tempkin isotherms were employed for understanding the sorption mechanism and Lagergren first order kinetics, an intra-particle diffusion model, and pseudo second order kinetics were applied to understand the sorption kinetics. The sorption proceeded through monolayer coverage of MWCNT-AA with capacities of 91.2 mg g⁻¹ and 89.4 mg g⁻¹ for Pu⁴⁺ and PuO₂²⁺, respectively following a Langmuir isotherm while the sorption kinetics followed a pseudo second order reaction with rate constants of 3.86 × 10⁻⁵ and 3.19 × 10⁻⁵ g mg⁻¹ min⁻¹ for Pu⁴⁺ and PuO₂²⁺ respectively. This MWCNT-AA showed high radiolytic stability and a method was developed for almost quantitative back extraction of plutonium in both the oxidation states from MWCNT-AA. Finally, the sorbent MWCNT-AA was employed for processing synthetic high level waste solution obtained from a research reactor origin. Moreover, density functional theory calculation was performed to examine the coordination and interaction behaviour of Pu⁴⁺ and PuO₂²⁺ ions towards MWCNT-AA. The present DFT study reveals that Pu is deca-coordinated (two from each of four nitrates and one AA) in the case of Pu⁴⁺ and octa-coordinated (two from each of two nitrates and one AA, and one from each of two oxo groups) in PuO₂²⁺. The calculated free energy of complexation was found to be almost three times higher for Pu⁴⁺ than PuO₂²⁺ both in the gas and aqueous phase, which thus confirms the experimentally observed higher sorption of Pu⁴⁺ compared to PuO₂²⁺ by MWCNT-AA.