The effect of vacuum annealing of magnetite and zero-valent iron nanoparticles on the removal of aqueous uranium

As-formed and vacuum annealed zero-valent iron nanoparticles (nano-Fe ⁰) and magnetite nanoparticles (nano-Fe were tested for the removal of uranium from carbonate-rich mine water. Nanoparticles were introduced to batch systems containing the mine water under oxygen conditions representative of near-surface waters, with a uranyl solution studied as a simple comparator system. Despite the vacuum annealed nano-Fe⁰ having a 64.6% lower surface area than the standard nano-Fe⁰, similar U removal (>98%) was recorded during the initial stages of reaction with the mine water. In contrast, ≤15% U removal was recorded for the mine water treated with both as-formed and vacuum annealed nano-Fe Over extended reaction periods (>1 week), appreciable U rerelease was recorded for the mine water solutions treated using nano-Fe⁰, whilst the vacuum annealed material maintained U at <50 g L^-1 until 4 weeks reaction. XPS analysis of reacted nanoparticulate solids confirmed the partial chemical reduction of U VI to U IV in both nano-Fe⁰ water treatment systems, but with a greater amount of U IV detected on the vacuum annealed particles. Results suggest that vacuum annealing can enhance the aqueous reactivity of nano-Fe⁰ and, for waters of complex chemistry, can improve the longevity of aqueous U removal.