Detecting impurity-specific effects on structure and radiolytic hydrogen production in aluminum hydroxide

Abstract

While radiolytic hydrogen (H₂) generation is an intrinsic property of aqueous and mineral radiolysis in nuclear waste systems, detection of the sub-ns events leading to H₂ generation is challenging. Interfacial processes involving key mineral phases in the sludge, e.g., gibbsite (α-Al(OH)₃), have been implicated, with impurities affecting the amount of H₂ generated. To understand why gibbsite synthesized from nitrate precursors produces less H₂ than gibbsite from chloride precursors, we paired ²⁷Al multiple quantum magic angle spinning (MQMAS) NMR spectroscopy to determine structural heterogeneity with transverse-field muon spin rotation (TF-μSR) to probe electron availability. MQMAS revealed greater structural disorder in the gibbsite synthesized with nitrate (NO₃-gibbsite). Correspondingly, TF-μSR showed a larger diamagnetic fraction for NO₃-gibbsite, indicating reduced persistence of μ⁺-electron bound states (muonium or other radicals) and thus fewer electrons available for reaction on the sub-ns timescale. This establishes a correlation between impurity-induced disorder and electron loss. The diamagnetic fraction serves as a signature for these sub-ns events, as it provides a key constraint for predictive models without currently resolving whether the electron is lost to direct chemical scavenging or trapping at lattice defects.