Restoration of in situ leached uranium mines with iron nanoparticles

In situ leaching (ISL) provides a method of extracting uranium from the subsurface without direct excavation or perturbation. Following ore removal at an ISL site, environmental responsibility lies with subsequent restoration of the groundwater system. The experiments outlined in the current study were driven by the possibility of injecting metallic iron nanoparticles into the ore zone with the purpose of (i) immobilising residual soluble uranium (VI) as insoluble uranium (IV) oxide and (ii) restoration of anoxic conditions within the subsurface. The study also explores the possibility of persistent inflow of oxygen into the ore zone or change in the redox conditions of the geological zone, Solutions highly concentrated in uranium (1000 ppm) with an initial pH ranging from 3 to 7 were studied in presence of zero valent iron nanoparticles under mildly oxic conditions (1.2 % O2 and 0.0017 % CO2) to simulate the oxidizing conditions of a exhausted uranium mine. Characterisation of both solid and solution phases indicated that at 4 hours period of reaction the Eh stabilized at values ranging from -0.1 to -0.4 V. The addition of iron nanoparticles triggered the reductive precipitation of UO2, which was demonstrated to be the main process responsible for the removal of uranium from solution at reaction times between 1 to 4 hours [1,2] . The reoxidation of uranite precipitated on the nanoparticle surface was studied at mildly acidic and at neutral-basic conditions to account for the possible disruption of the reducing conditions in the geological zone. Despite thermodynamic modelling calculations of the studied system using NEA-TDB [3] indicate UO3.2H2O as the only uranium solid phase for 4 < pH < 9, the experimental results indicated that a Fe-uranyl phase with becquerelite-like crystalline structure stabilized at neutral basic pH [4]. [1] Chadwick (1973) Chem. Phys. Lett. 21 (2), 291-294. [2] Scott et al. (2005) Geochim. Cosmochi. Acta 69 (24). [3] Guillaumont et al. (2003) Chemical Thermodynamics 5. NEA OECD, Elsevier. [4] Burns et al. (1996) The Canadian Mineralogist 34.