As-formed and vacuum annealed zero-valent iron nanoparticles (nano-Fe0) and magnetite nanoparticles (nano-Fe3O4) 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-Fe3O4. 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？μg？L？1 until 4 weeks reaction. XPS analysis of reacted nanoparticulate solids confirmed the partial chemical reduction of to in both nano-Fe0 water treatment systems, but with a greater amount of 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. 1. Introduction Iron nanoparticles (hereafter nano-Fe0) in recent years have received much attention as a potential alternative to conventional remediation technologies. By virtue of their size (0–100？nm) engineered nanoparticles offer a significantly greater surface area to volume ratio and higher surface energy [1] and resultantly offer similar or slightly enhanced reactivity to conventional materials but at a fraction of the mass. By using a smaller mass of reactive material to achieve the same objective (i.e., site remediation), both raw materials and energy are conserved [2], with significant potential savings in cost. The key driver behind the emergence of nano-Fe0 for water treatment, however, is the advantage of subsurface deployment via injection as a liquid suspension, with the potential for aqueous contaminant treatment at almost any location and depth in terrestrial groundwater systems. Although nano-Fe0 have proven highly effective for the removal of a wide range of aqueous contaminants from simple synthetic solutions, in recent years, the performance of nano-Fe0 for the remediation of chemically complex and/or “real” solutions in dissolved oxygen containing waters has yielded a contrasting result [3–7]. It has been outlined that the efficacy of nano-Fe0 can be
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