| Quantum-mechanical calculations were performed to model the interactions of U(VI), Np(V), Pu(VI), and Eu(III) with carboxyl and hydroxyl functional groups of graphene oxide (GO). Models were created for sorption to a single sheet of GO. For U(VI) and Eu(III), models were also created for sorption to two sheets to evaluate the possibility of cation-induced GO aggregation via cation cross-linking of GO sheets. The quantum-mechanical calculations were used to characterize the sorption reactions in terms of energetics and vibrational peak shifts. The calculations performed used Gaussian09 with the B3LYP hybrid functional. Sorption was seen as favorable for all cations considered; the order of energetic favorability for sorption to one sheet was Pu(VI) > Eu(III) > U(VI) > Np(V). In general, differences in favorability of sorption to carboxyl versus hydroxyl groups were relatively small; sorption to deprotonated groups was more favorable (i.e., lower sorption energy) than sorption to protonated groups. For U(VI) and Eu(III), sorption to two GO sheets was more favorable than sorption to a single sheet, indicating that cation-induced GO aggregation via cation cross-linking of GO sheets is likely an important mechanism for U(VI) and Eu(III) interaction with GO. For U(VI), Np(V), and Pu(VI), the vibrational peak shifts associated with the sorption reactions varied depending on the cation, functional group, and number of GO sheets involved in the sorption. The magnitudes of these shifts may provide insight for the development of analytical methods for evaluation of the interactions between the cations studied and GO. |
