Localizing tetrahedral aluminum in nitrate-bearing gibbsite to constrain defect-impurity coupling

Abstract

The enhanced radiolytic stability of gibbsite (α-Al(OH)₃) containing trace nitrate (NO₃⁻) is a phenomenon in nuclear waste management, but its structural origins remain unresolved. Motivated by the detection of minority tetrahedral aluminum (T_d) defects in synthetic gibbsite, we hypothesized that these sites may participate in NO₃⁻ retention or mediate H₂ suppression. To evaluate this, we combined orthogonal techniques comprised of spatially selective solid-state ²⁷Al MAS NMR, comparative spectroscopy, and density functional theory (DFT) modeling. Paramagnetic editing and dynamic nuclear polarization (DNP) MAS NMR confirm that T_d defects are confined to the particle interior. DFT calculations reveal no energetic stabilization of NO₃⁻ near T_d sites. Comparative NMR analysis shows that T_d is also present in chloride-bearing gibbsite, which exhibits high radiolytic hydrogen yields. These three independent disqualifications rule out T_d as a structural contributor to nitrate-mediated suppression and narrow the scope of defect-driven explanations. The findings redirect mechanistic attention away from coordination defects and toward redox-active impurity pathways, providing a refined foundation for understanding radiation tolerance in Al(OH)₃.