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Stability, composition and core-shell particle structure of uranium(IV)-silicate colloids Thomas Samuel Neill, Katherine Morris, Carolyn I. Pearce, Nicholas K. Sherriff, M. Grace Burke, Philip Chater, Arne Janssen, Louise S. Natrajan, and Samuel Shaw Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01756 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018
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Stability, composition and core-shell particle
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structure of uranium(IV)-silicate colloids
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Thomas S. Neill‡, Katherine Morris‡, Carolyn I. Pearce§, Nicholas K Sherriff⊥, M. Grace
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Burke†, Philip A. Chater+, Arne Janssen†+, Louise Natrajan# and Samuel Shaw‡*
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‡
Research Centre for Radwaste and Disposal, Williamson Research Centre, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
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§
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⊥
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Pacific Northwest National Laboratory, Richland, WA 99354, USA
National Nuclear Laboratory, Chadwick House, Warrington Road, Birchwood Park, Warrington WA3 6AE, UK
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†
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Materials Performance Centre, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
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#
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School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
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+
Diamond Light Source, Harwell Campus, Didcot, Oxfordshire, OX11 0DE, UK
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KEYWORDS Colloids, uranium(IV), core-shell, uranium silicate, nuclear fuel storage,
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EXAFS, SAXS
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ABSTRACT
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Uranium is typically the most abundant radionuclide by mass in radioactive wastes and is a
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significant component of effluent streams at nuclear facilities. Actinide (IV) (An(IV))
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colloids formed via various pathways, including corrosion of spent nuclear fuel, have the
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potential to greatly enhance the mobility of poorly soluble An(IV) forms, including uranium.
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This is particularly important in conditions relevant to decommissioning of nuclear facilities
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and the geological disposal of radioactive waste. Previous studies have suggested that silicate
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could stabilise U(IV) colloids. Here the formation, composition and structure of U(IV)-
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silicate colloids under the alkaline conditions relevant to spent nuclear fuel storage and
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disposal were investigated using a range of state of the art techniques. The colloids are
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formed across a range of pH conditions (9-10.5) and silicate concentrations (2-4 mM) and
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have a primary particle size 1-10 nm, also forming suspended aggregates < 220 nm. X-ray
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absorption spectroscopy, ultrafiltration and scanning transmission electron microscopy
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confirm the particles are U(IV)-silicates. Additional evidence from X-ray diffraction and pair
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distribution function data suggests the primary particles are composed of a UO2-rich core and
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a U-silicate shell.
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U(OH)3(H3SiO4)3-2 complexes in solution indicating they are likely particle precursors.
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Finally, these colloids form under a range of condition relevant to nuclear fuel storage and
U(IV)-silicate colloids formation correlates with the formation of
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geological disposal of radioactive waste and represent a potential pathway for U mobility in
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these systems.
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TEXT
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INTRODUCTION
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Uranium (U) is the most abundant radionuclide by mass in both the nuclear fuel cycle and
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many higher activity radioactive waste inventories1. Its mobility in engineered and
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environmental systems is a major concern in a range of scenarios, including waste storage
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systems (e.g. nuclear fuel storage ponds), contaminated land and geodisposal facilities. The
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speciation of U is key to controlling its mobility and therefore informing effluent
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treatment/remediation options. Of particular importance is the oxidation state of U. U(VI),
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typically present in aqueous systems as uranyl UO22+, is relatively soluble, particularly in the
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presence of carbonate2. By contrast, U(IV) is poorly soluble under the anoxic and neutral-
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basic pH conditions expected in radioactive waste storage/disposal and contaminated land
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scenarios3,4 and is often present as UO2(s)5,6 or non-crystalline U(IV)7–9 in anoxic systems.
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This means U is considered to be immobile under reducing conditions. Therefore, reduction
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from soluble U(VI) to U(IV), either biotically10,11 or abiotically12, is seen as a possible
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immobilization mechanism for U in environmental and waste treatment systems. However,
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studies have highlighted that colloids can mobilise actinides in a range of scenarios13–16.
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These include interim storage facilities at sites such as Sellafield, UK, where legacy spent
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fuel storage ponds are maintained at a high pH and where colloids are present17–20.
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Additionally, colloids have the potential to mobilize radionuclides in geodisposal facilities21,
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at nuclear weapons testing sites13, U mine drainage sites22, and during ore deposit
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formation23,24. It is therefore important to have a detailed understanding of the formation, 3 ACS Paragon Plus Environment
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composition, structure and surface properties of colloidal U(IV) species under relevant
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(geo)chemical conditions to assess their impact on U speciation and mobility in a range of
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natural and engineered environments. Despite significant research into colloidal transport of
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radionuclides, U colloid formation and stability mechanisms are poorly constrained. There is
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a particular scarcity of knowledge under alkaline, anoxic conditions relevant to spent nuclear
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fuel storage25 and long term geodisposal26.
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The formation of U oxide colloids at acidic pH has been well established for both U(IV)27
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and U(VI)28. Under environmentally relevant anoxic, circumneutral pH conditions, UO2
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colloids have been shown to form during anaerobic corrosion of spent nuclear fuel21. The
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resultant colloidal particles were < 250 nm and showed resistance to oxidation, persisting for
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70 days during exposure to air before oxidation to U(VI)-silicate
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K2(UO2)2Si6O15·4H2O). In addition, colloidal U(IV)-silicate nanoparticles have been shown
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to form under anoxic conditions at pH 6.9-9.529 similar to those within radioactive waste
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storage and disposal environments. The nanoparticles, 1 mM)30,31. Furthermore, recent studies suggest that the
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presence of U(IV)-silicate solution complexes and colloids are essential precursors to
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coffinite formation32,33. Understanding the formation of U(IV)-silicate species is therefore
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essential to understanding U(IV)-silicate mineral formation as well as U(IV) mobility. U
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silicate solution complexes have also been observed within groundwaters at the underground
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research laboratory in Horonobe, Japan34. Here, U was associated with low molecular weight
(weeksite,
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polysilicates in saline groundwaters from 500 m depth providing field based evidence that
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U(IV)-silicate solution species and/or nanoparticles have the potential to enhance U(IV)
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mobility in anoxic systems.
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Studies of actinide(IV) (An(IV)) silicate colloids have found that Th(IV) and Np(IV)
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silicate colloids can form under similar condition to those favoring U(IV)-silicate colloid
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formation35–37. X-ray photoelectron spectroscopy (XPS) of Th(IV)-silicate particles suggested
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that the structure of the individual nanoparticles may not be homogeneous, with evidence for
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silica enrichment at the particle surface under initially elevated Si concentrations. Enrichment
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of silicate on the surface of An(IV) particles (where An = U, Th, Np) is thought to stabilize
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the colloidal nanoparticles via increasing the charge and reducing the hydrophobicity of the
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particle surface37. This mechanism of stabilization has also been observed for other silicate
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containing colloidal systems including Fe(III) hydrolysis products38 and amorphous calcium
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carbonate where silica was observed at the particle surface39. Overall, An(IV)-silicate
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nanoparticles have been reported across a range of systems, and may have a significant
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impact on An(IV) mobility in reducing engineered and natural environments where actinides
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and elevated Si concentrations exist (e.g. groundwater). Information on the formation,
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composition, structure and stability of U(IV)-silicate phases and colloids at conditions
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relevant to interim spent nuclear fuel storage and geodisposal of radioactive waste (i.e. pH
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>10) is crucial to predicting the long term mobility and fate of An(IV) in natural and
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engineered systems.
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In this study we have characterized U(IV)-silicate colloids at a range of silicate
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concentrations and high pH conditions ([Si] 0-4 mM, pH 9-12) relevant to spent nuclear fuel
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storage and radioactive waste disposal. This was achieved using a multi-technique approach
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to determine the size, stability and structure of the colloidal nanoparticles. Here, a
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combination of ultrafiltration, small angle X-ray scattering (SAXS), dynamic light scattering 5 ACS Paragon Plus Environment
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(DLS), zeta potential analysis, scanning transmission electron microscopy (STEM), extended
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X-ray absorption fine structure (EXAFS) spectroscopy, powder X-ray diffraction (XRD) and
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X-ray Pair Distribution Function (PDF) provided unprecedented atomic to nanoscale insight
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into properties of U(IV)-silicate colloids. Results show the U(IV) colloids consisted of
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particles
