As the dominant mineral phases of the high-level radioactive waste (HLW) sludge at the Hanford Site in southwest Washington State, gibbsite and boehmite play a role in radiolytic hydrogen generation. This is an important issue for worker safety, and adds complexity to the processing of HLW sludges for vitrification. Limited evidence suggests that dehydrogenation of surface hydroxyls in boehmite and gibbsite occurs in radiation environments. In this work, platelet nanoparticles of gibbsite and boehmite with high aspect ratios were synthesized and subjected to 60Co irradiation with dose levels of approximately 5 and 20 Mrad, respectively, the latter would correspond to about 1 year of irradiation dose in a typical Hanford tank. The resulting nanoparticles were analyzed by sum frequency generation – vibrational spectroscopy (SFG-VS), a surface-specific technique, and other methods including Raman spectroscopy, electron paramagnetic resonance (EPR), high resolution atomic force microscopy (HRAFM) and X-ray diffraction analysis (XRD) before and after irradiation. While both HRAFM and XRD showed that 60Co irradiation caused no change in crystallinity and surface atomic structures for either gibbsite or boehmite, SFG-VS results clearly indicated over 80% removal of the surface hydroxyls after irradiation in both. Raman spectra of the irradiated samples revealed a decrease in the bulk hydroxyl content, suggesting dehydrogenation of the internal hydroxyls as well. EPR data confirmed formation of significant levels of hydrogen radical and radiation induced oxygen defects, such as O- and O3-, and most of the radiolytic products formed within the gibbsite and boehmite structures remain long after the irradiation.

Role of surface hydroxyls on the radiolysis of gibbsite and boehmite nano-platelets

2019 Spring National Meeting

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