Radiation-Induced Surface Modification of Gibbsite in NaNO3 Environments

Abstract

Gibbsite (α-Al(OH)3) is a dominant mineral phase in nuclear waste sludges, where it is continuously exposed to ionizing radiation in nitrate-rich alkaline environments. In this work, we investigate the effect of gamma (γ)-irradiation up to 750 kGy on the surface chemistry of gibbsite particles dispersed in anoxic aqueous sodium nitrate (NaNO3) solutions, and dried gibbsite particles with adsorbed sodium nitrate. Prior to irradiation, DRIFTS and Raman analyses reveal adsorption of NO3– ions onto the gibbsite surface. Exposure to γ-radiation results in progressive radiolytic dissociation of surface-associated NO3– ions along with concurrent modification of the gibbsite surface. The relative surface concentration of nitrogen-containing species measured with XPS changes with increasing dose, indicating reduced presence of intact NO3– ions at higher irradiation levels. This trend is concurrent with radiation-induced depletion of surface hydroxyl groups confirmed by vibrational sum frequency generation spectroscopy, and the simultaneous formation of nitrate-derived surface functionalities that alter the interfacial binding environment. γ-irradiation of gibbsite initiates cleavage of surface hydroxyl groups, generating primary radicals such as H• and surface-bound oxygen species (Al–O•). These species subsequently participate in secondary reactions. Electron paramagnetic resonance spectroscopy detects the formation of a small amount of •NO2 radicals from nitrate radiolysis adsorbed on the surface of gibbsite. However, these species do not significantly affect the intensity or stability of H• or oxygen-centered radicals that occur as trapped sites in the bulk, indicating that nitrate-derived radicals are spatially separated from the primary bulk radical generation sites. These results demonstrate that γ-irradiation drives concurrent NO3– radiolysis and modification of hydroxyl groups on the gibbsite surface, producing reactive oxygen and nitrogen species that alter the surface structure and electrostatic properties.