Investigation into the Adsorption Mechanism of Three Varieties of Carboxylated Graphene Oxide for Pentahydrate Actinide (An = Pa–Pu) Compounds

The high radioactivity and toxicity of nuclear wastewater, significant contributors to environmental pollution, have drawn considerable attention from the scientific community. This research delves into the adsorption mechanism of actinide cations, namely, Pa-(V), U-(VI), Np-(V), and Pu-(VI), on carboxylated graphene oxide (GO). Specifically, we examined three distinct carboxylated functional groups situated on the graphene edges: -COOH, -O-COOH, and -NH-COOH. Utilizing density functional theory (DFT), we conducted a systematic investigation into the structures, bonding characteristics, and adsorption energies of Pa-(V), U-(VI), Np-(V), and Pu-(VI) when interacting with GO. Rigorous geometric analysis and resonant frequency calculations demonstrate that Pu-(VI) exhibits stronger bonding with GO than Pa-(V), U-(VI), and Np-(V) cations. Furthermore, analysis of adsorption energies shows that among the tested actinide cations, Pu-(VI) displays the highest affinity for GO, whereas U-(VI) exhibits the lowest. Concurrently, Np-(V) demonstrates a marginally higher affinity than Pa-(V), consistent with previous experiments. Electrostatic potential (ESP) and atomic charge analyses suggest that the -NH group acts as an electron donor toward adjacent -COOH groups, enhancing their electrostatic attraction for hydrated actinide cations. By elucidating interaction mechanisms at molecular scales, this research may pave the way for designing GO-based materials in radioactive wastewater treatment.