Oxidation state selective sorption behavior of plutonium using N,N-dialkylamide functionalized carbon nanotubes: experimental study and DFT calculation

Abstract

Selective phase separation of Pu⁴⁺ and PuO₂²⁺ was performed using N,N-dialkylamide functionalized multi-walled carbon nanotubes (AFMWCNTs). To understand the sorption kinetics, three widely accepted kinetic models (Lagergren first order kinetics, intra particle diffusion model and pseudo second order kinetics) were investigated. The sorption kinetics followed a pseudo second order kinetics with rate constants of 2.50 × 10⁻⁵ g mg⁻¹ min⁻¹ and 4.30 × 10⁻⁵ g mg⁻¹ min⁻¹ for Pu⁴⁺ and PuO₂²⁺ respectively. The analysis of the sorption mechanism through Langmuir, Freundlich, Dubinin–Rodushkevich (D–R) and Temkin isotherms revealed that the sorption proceeds via heterogeneous, non-ideal multi-layer adsorption following the Freundlich isotherm. The radiolytic stability of the AFMWCNTs and the stripping behavior of plutonium from the loaded AFMWCNTs were also investigated and finally AFMWCNTs were employed for the processing of simulated high level waste solutions originating from Research Reactors (RRs) and Fast Breeder Reactors (FBRs). Density functional theory calculation was used to understand the higher selectivity of tetra valent plutonium over hexa valent plutonium. The structural parameters of the AFMWCNT and its complexes of Pu⁴⁺ and PuO₂²⁺ were optimized along with the evaluation of their binding energy in the gas phase as well as solution phase. Orbital bonding analysis was carried out to rationalize the selectivity of Pu⁴⁺ions over PuO₂²⁺ with AFMWCNTs.