Sorption of uranium onto nanscale zero-valent iron particles

The sorption of aqueous uranium onto nanoscale zero-valent iron particles has been, studied in the present work. The research has determined both the mechanisms and rates of reaction under a range of geochemical and redox conditions applicable to natural waters: Sorption experiments using uranium contaminated groundwater taken from the Lisava Uranium Mine, Banat, SW Romania and synthetic surrogate solutions were tested using a variety of analytical techniques including: inductively coupled plasma mass spectrometry, inductively coupled atomic emission spectrometry, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The results provide clear evidence that nanoscale zero-valent iron particles are highly effective for the rapid removal of aqueous uranium. Two mechanisms have been determined: (i) Adsorption followed by reductive precipitation. Aqueous hexavalent uranium IS adsorbed and then chemically reduced by metallic and/or ferrous iron, resulting in the surface mediated precipitation of tetravalent uranium oxide. (ii) Adsorption, complexation and/or incorporation. Aqueous hexavalent uraruum IS removed from solution without chemical reduction via surface mediated adsorption, complexation and/or structural incorporation/entrapment with ferric iron oxides and/or hydroxides. The long-term removal and retention of uranium on different iron-based nanopowders was also studied. For waters containing appreciable concentrations of complexing agents, namely dissolved carbonate, significant uranium re-release was recorded. The mechanism is attributed to the ingress of atmospheric oxygen and other associated gases (including C02) back into the experimental solutions, facilitating the reformation of thermodynamically stable uranyl carbonate complexes. To improve uranium retention the effect of vacuum annealing on the structure and surface chemistry of nanoscale magnetite, zero-valent iron and zero-valent iron-nickel particles was tested. Results highlight the key role that changes in nanoparticle crystallinity, surface oxide stoichiometry and impurity phases (H20, hydroxide, carbon, etc.) have on the material's aqueous corrosion and associated uranium removal efficacy. Results provide clear evidence that vacuum annealing can be applied to improve the aqueous reactivity of both nanoscale zero- valent iron and nanoscale zero-valent iron-nickel particles.