Formation of stable uranium(VI) colloidal nanoparticles in conditions relevant to radioactive waste disposal

DOI: 10.1021/la502832j

Authors: Pieter Bots (University of Manchester) , Katherine Morris (University of Manchester) , Rosemary Hibberd (University of Manchester) , Gareth Law (University of Manchester) , 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)
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

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 behavior of radionuclides needs to be constrained under these extreme conditions to minimize the environmental hazard from the wastes. For uranium, a key component of many radioactive wastes, thermodynamic modeling 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 characterizing the potential for formation and medium-term stability of U(VI) colloids is important in underpinning our understanding of U behavior in waste disposal. Reflecting this, we applied conventional geochemical and microscopy techniques combined with synchrotron based in situ and ex situ X-ray techniques (small-angle X-ray scattering and X-ray adsorption spectroscopy (XAS)) to characterize 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 (sodium–uranate)-type crystallographic structure. The presented results have clear and hitherto unrecognized implications for the mobility of U(VI) in cementitious environments, in particular those associated with the geological disposal of nuclear waste.

Journal Keywords: Extended X-ray absorption fine structure; Nanoparticles; Transmission electron microscopy; Uranium; X-ray scattering

Subject Areas: Environment, Earth Science, Materials


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

Added On: 27/10/2014 10:59

Documents:
la502832j.pdf

Discipline Tags:

Earth Sciences & Environment Radioactive Materials Materials Science Nuclear Waste

Technical Tags:

Scattering Spectroscopy Small Angle X-ray Scattering (SAXS) X-ray Absorption Spectroscopy (XAS) Extended X-ray Absorption Fine Structure (EXAFS)