Formation of U(vi) peroxide nanoclusters from cascade reactions with a persulfate radical initiator

Abstract

Radiolysis of water in high radiation fields generates a variety of reactive oxygen species that influence the chemical behavior and complexation of hexavalent uranium. This study investigates the behavior of interaction of a uranyl cation (UO₂²⁺(VI)) with a series of free radicals that are formed in situ via activation of the free radical initiator persulphate (S₂O₈²⁻), which releases both SO₄˙⁻ and ˙OH species in the solution. Electron Paramagnetic Resonance (EPR) and Raman spectroscopy were used to evaluate the presence of the hydroperoxyl radical (HO₂˙) and superoxide radicals (O₂˙⁻) that are formed within the solution through radical cascade reactions. In addition, a uranyl peroxide cluster solid (NaU₂₄) was crystallized and characterized using single crystal X-ray diffraction (SCXRD), vibrational spectroscopy, and EPR spectroscopy. The presence of the hydroperoxyl radical (HO₂˙) and superoxide radicals (O₂˙⁻) was also observed in the solid-state compound, but spectroscopic evidence suggests that it was associated with the Na⁺ network and not the cluster itself. Density functional theory (DFT) calculations were also utilized to further confirm the radical species produced and determine the potential stabilization of radicals detected within the cluster and lattice.