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Formation of Stable Uranium(VI) Colloidal Nanoparticles in Conditions Relevant to Radioactive Waste Disposal

DOI: 10.1021/la502832j DOI Help

Authors: Pieter Bots (University of Manchester, U.K.) , Katherine Morris (University of Manchester, U.K.) , Rosemary Hibberd (University of Manchester, U.K.) , Gareth Law (University of Manchester, U.K.) , Fred Mosselmans (Diamond Light Source) , Andy Brown (University of Leeds) , James Doutch (Diamond Light Source) , Andy Smith (Diamond Light Source) , Sam Shaw (University of Manchester, U.K.)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Langmuir , VOL 30 (48)

State: Published (Approved)
Published: October 2014
Diamond Proposal Number(s): 5975 , 8544

Open Access Open Access

Abstract: The favored pathway for disposal of higher activity radioactive wastes is via deep geological disposal. Many geological disposal facility designs include cement in their engineering design. Over the long term, interaction of groundwater with the cement and waste will form a plume of a hyperalkaline leachate (pH 10 – 13) and the behaviour of radionuclides needs to be constrained under these extreme conditions to minimise the environmental hazard from the wastes. For uranium, a key component of many radioactive wastes, thermodynamic modelling predicts that at high pH, U(VI) solubility will be very low (nM or lower) and controlled by equilibrium with solid phase alkali and alkaline-earth uranates. However, the formation of U(VI) colloids could potentially enhance the mobility of U(VI) under these conditions and characterising the potential for formation and medium term stability of U(VI) colloids is important in underpinning our understanding of U behaviour in waste disposal. Reflecting this, we applied conventional geochemical and microscopy techniques combined with synchrotron based in- and ex-situ X-ray techniques (SAXS and XAS) to characterise colloidal U(VI) nanoparticles in a synthetic cement leachate (pH > 13) containing 4.2 – 252 μM U(VI). The results show that in cement leachates with 42 μM U(VI), colloids formed within hours and remained stable for several years. The colloids consisted of 1.5 – 1.8 nm nanoparticles with a proportion forming 20 – 60 nm aggregates. Using XAS and electron microscopy we were able to determine that the colloidal nanoparticles had a clarkeite (Na-uranate) type crystallographic structure. The presented results have clear and hitherto unrecognised implications for the mobility of U(VI) in cementitious environments, in particular those associated with the geological disposal of nuclear waste.

Subject Areas: Environment, Energy, Materials

Instruments: B18-Core EXAFS , I22-Small angle scattering & Diffraction