Novel diamide covalent organic polymers (COPs) for arresting actinides U(VI), Pu(IV) and Am(III) from highly acidic nuclear stream

Abstract

Targeted towards the hitherto unaddressed challenge of actinide capture under harsh radiochemical conditions (nitric acid molarity ≥2 M, gamma radiation field 500 kGy), novel diamide covalent organic polymers (COPs) were successfully synthesized and characterized. The completely incinerable 1,3 and 1,4-Diamide COPs were endowed with exceptional radiochemical stability, high surface area (>1400 m² g⁻¹) and remarkable sorption capacity for the actinides viz. U(VI), Pu(IV) and Am(III). Batch sorption studies with Am(III), Pu(IV) and U(VI) from nitric acid medium revealed higher uptake with 1,3 over 1,4-Diamide COP, in general. Distribution co-efficient, k_D was found to enhance with nitric acid molarity, attaining saturation after 4 M. Sorption was found to follow pseudo second order kinetics, Langmuir adsorption isotherm and was enthalpy driven. 1,3-Diamide COPs yielded high sorption capacity for U(VI), Pu(IV) and Am(III) (368.8, 452.3 and 189.3 mg g⁻¹ respectively), from 4 M nitric acid medium. Actinide removal was >90 % from simulated nuclear streams. Mode of complexation of COPs with uranyl was probed experimentally by X-ray Photoelectron Spectroscopy and Fourier transform Infrared Spectroscopy. Additional insights for optimized ligand and metal–ligand binding geometries were obtained by theoretical modelling.