B18-Core EXAFS
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Diamond Proposal Number(s):
[12767, 13599]
Abstract: Geological disposal is the globally preferred long-term solution for higher activity radioactive wastes (HAW) including intermediate level waste (ILW). In a cementitious disposal system, cellulosic waste items present in ILW may undergo alkaline hydrolysis, producing significant quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides. Although microbial degradation of ISA has been demonstrated, its impact upon the fate of radionuclides in a geological disposal facility (GDF) is a topic of ongoing research. This study investigates the fate of U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching conditions similar to the far field of a GDF, containing ISA as the sole carbon source, and elevated phosphate concentrations, incubated both (i) under fermentation and (ii) Fe(III)-reducing conditions. In the ISA-fermentation experiment, U(VI) was precipitated as insoluble U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority of the uranium was precipitated as reduced U(IV)-phosphates, presumably formed via enzymatic reduction mediated by metal-reducing bacteria, including Geobacter species. Overall, this suggests the establishment of a microbially mediated “bio-barrier” extending into the far field geosphere surrounding a GDF is possible and this biobarrier has the potential to evolve in response to GDF evolution and can have a controlling impact on the fate of radionuclides.
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Mar 2021
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B18-Core EXAFS
I18-Microfocus Spectroscopy
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Adam J.
Fuller
,
Peter
Leary
,
Neil D.
Gray
,
Helena S.
Davies
,
J. Frederick W.
Mosselmans
,
Filipa
Cox
,
Clare H.
Robinson
,
Jon K.
Pittman
,
Clare M.
Mccann
,
Michael
Muir
,
Margaret C.
Graham
,
Satoshi
Utsunomiya
,
William R.
Bower
,
Katherine
Morris
,
Samuel
Shaw
,
Pieter
Bots
,
Francis R.
Livens
,
Gareth T. W.
Law
Diamond Proposal Number(s):
[10163, 12767, 12477]
Open Access
Abstract: Understanding the long-term fate, stability, and bioavailability of uranium (U) in the environment is important for the management of nuclear legacy sites and radioactive wastes. Analysis of U behavior at natural analogue sites permits evaluation of U biogeochemistry under conditions more representative of long-term equilibrium. Here, we have used bulk geochemical and microbial community analysis of soils, coupled with X-ray absorption spectroscopy and μ-focus X-ray fluorescence mapping, to gain a mechanistic understanding of the fate of U transported into an organic-rich soil from a pitchblende vein at the UK Needle's Eye Natural Analogue site. U is highly enriched in the Needle's Eye soils (∼1600 mg kg−1). We show that this enrichment is largely controlled by U(VI) complexation with soil organic matter and not U(VI) bioreduction. Instead, organic-associated U(VI) seems to remain stable under microbially-mediated Fe(III)-reducing conditions. U(IV) (as non-crystalline U(IV)) was only observed at greater depths at the site (>25 cm); the soil here was comparatively mineral-rich, organic-poor, and sulfate-reducing/methanogenic. Furthermore, nanocrystalline UO2, an alternative product of U(VI) reduction in soils, was not observed at the site, and U did not appear to be associated with Fe-bearing minerals. Organic-rich soils appear to have the potential to impede U groundwater transport, irrespective of ambient redox conditions.
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Apr 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[17114]
Abstract: Colloidal silica is a nanoparticulate material that could have a transformative effect on environmental risk management at nuclear legacy sites through their use in in-situ installation of injectable hydraulic barriers. In order to utilize such nanoparticulate material as a barrier, we require detailed understanding of its impact on the geochemistry of radionuclides in the environment (e.g. fission products such as Sr and Cs). Here we show, through combining leaching experiments with XAS analyses, that colloidal silica induces several competing effects on the mobility of Sr and Cs. First, cations within the colloidal silica gel compete with Sr and Cs for surface complexation sites. Second, an increased number of surface complexation sites is provided by the silica nanoparticles and finally, the elevated pH within the colloidal silica increases the surface complexation to clay minerals and the silica nanoparticles. XAS analyses show that Sr and Cs complex predominantly with the clay mineral phases in the soil through inner-sphere surface complexes (Sr) and through complexation on the clay basal surfaces at Si vacancy sites (Cs). For binary soil – colloidal silica gel systems, a fraction of the Sr and Cs complexes with the amorphous silica-like surfaces through the formation of outer-sphere surface complexes. Importantly, the net effect of nanoparticulate colloidal silica gel is to increase the retention of Sr and Cs, when compared to untreated soil and waste materials. Our research opens the door to applications of colloidal silica gel to form barriers within risk management strategies at legacy nuclear sites.
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Mar 2020
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[11412]
Open Access
Abstract: Neptunium and uranium are important radionuclides in many aspects of the nuclear fuel cycle and are often present in radioactive wastes which require long term management. Understanding the environmental behaviour and mobility of these actinides is essential in underpinning remediation strategies and safety assessments for wastes containing these radionuclides. By combining state-of-the-art X-ray techniques (synchrotron-based Grazing Incidence XAS, and XPS) with wet chemistry techniques (ICP-MS, liquid scintillation counting and UV-Vis spectroscopy), we determined that contrary to uranium(VI), neptunium(V) interaction with magnetite is not significantly affected by the presence of bicarbonate. Uranium interactions with a magnetite surface resulted in XAS and XPS signals dominated by surface complexes of U(VI), while neptunium on the surface of magnetite was dominated by Np(IV) species. UV-Vis spectroscopy on the aqueous Np(V) species before and after interaction with magnetite showed different speciation due to the presence of carbonate. Interestingly, in the presence of bicarbonate after equilibration with magnetite, an unknown aqueous NpO2+ species was detected using UV-Vis spectroscopy, which we postulate is a ternary complex of Np(V) with carbonate and (likely) an iron species. Regardless, the Np speciation in the aqueous phase (Np(V)) and on the magnetite (111) surfaces (Np(IV)) indicate that with and without bicarbonate the interaction of Np(V) with magnetite proceeds via a surface mediated reduction mechanism. Overall, the results presented highlight the differences between uranium and neptunium interaction with magnetite, and reaffirm the potential importance of bicarbonate present in the aqueous phase.
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Feb 2019
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B18-Core EXAFS
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Diamond Proposal Number(s):
[7593, 10163, 12767]
Abstract: Technetium is a problematic contaminant at nuclear sites and little is known about how repeated microbiologically-mediated redox cycling impacts its fate in the environment. We explore this question in sediments representative of the Sellafield Ltd. site, UK, over multiple reduction and oxidation cycles spanning ~ 1.5 years. We found the amount of Tc remobilised from the sediment into solution significantly decreased after repeated redox cycles. X-ray Absorption Spectroscopy (XAS) confirmed that sediment bound Tc was present as hydrous TcO2-like chains throughout experimentation and that Tc’s increased resistance to remobilisation (via reoxidation to soluble TcO4-) resulted from both shortening of TcO2 chains during redox cycling and association of Tc(IV) with Fe phases in the sediment. We also observed that Tc(IV) remaining in solution during bioreduction was likely associated with colloidal magnetite nanoparticles. These findings highlight crucial links between Tc and Fe biogeochemical cycles that have significant implications for Tc’s long-term environmental mobility, especially under ephemeral redox conditions.
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Nov 2017
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B18-Core EXAFS
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Diamond Proposal Number(s):
[9621, 10163, 12767]
Open Access
Abstract: Understanding interactions between radionuclides and mineral phases underpins site environmental clean-up and waste management in the nuclear industry. Transport and fate of radionuclides in many subsurface environments are controlled by adsorption, redox and mineral incorporation processes. Interactions of iron (oxyhydr)oxides with uranium have been extensively studied due to the abundance of uranium as an environmental contaminant and ubiquity of iron (oxyhydr)oxides in engineered and natural environments. Despite this, detailed mechanistic information regarding the incorporation of uranium into Fe(II) bearing magnetite and green rust is sparse. Here, we present a co-precipitation study where U(VI) was reacted with environmentally relevant iron(II/III) (oxyhydr)oxide mineral phases. Based on diffraction, microscopic, dissolution and spectroscopic evidence, we show the reduction of U(VI) to U(V) and stabilisation of the U(V) by incorporation within the near-surface and bulk of the particles during co-precipitation with iron (oxyhydr)oxides. U(V) was stable in both magnetite and green rust structures and incorporated via substitution for octahedrally coordinated Fe in a uranate-like coordination environment. As the Fe(II)/Fe(III) ratio increased, a proportion of U(IV) was also precipitated as surface associated UO2. These novel observations have significant implications for the behaviour of uranium within engineered and natural environments.
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Sep 2017
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B18-Core EXAFS
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Diamond Proposal Number(s):
[8269]
Abstract: Calcite formation via an amorphous calcium carbonate (ACC) precursor phase potentially offers a method for enhanced incorporation of incompatible trace metals, including Sr2+. In batch crystallisation experiments where CaCl2 was rapidly mixed with Na2CO3 solutions the Sr2+ : Me2+ ratio was varied from 0.001 to 0.1; and, the pathway of calcite precipitation was directed by either the presence or absence of high Mg2+ concentrations (i.e. using a Mg2+ : total Me2+ ratio of 0.1). In the Mg-free experiments crystallisation proceeded via ACC → vaterite → calcite and average Kd Sr values were between 0.44-0.74. At low Sr2+ concentrations (Sr2+ : Me2+ ratio ≤ 0.01) EXAFS analysis revealed that the Sr2+ was incorporated into calcite in the 6 fold coordinate Ca2+ site. However, at higher Sr2+ concentrations (Sr2+ : Me2+ ratio = 0.1), Sr2+ was incorporated into calcite in a 9-fold site with a local coordination similar to Ca2+ in aragonite, but calcite-like at longer distances (i.e. > 3.5 Å). In the high-Mg experiments the reaction proceeded via an ACC → calcite pathway with higher Kd Sr of 0.90-0.97 due to the presence of Mg2+ stabilising the ACC phase and promoting rapid calcite nucleation in conjunction with higher Sr2+ incorporation. Increased Sr2+ concentrations also coincided with higher Mg2+ uptake in these experiments. Sr2+ was incorporated into calcite in a 9-fold coordinate site in all the high-Mg experiments regardless of initial Sr2+ concentrations, likely as a result of very rapid crystallisation kinetics and the presence of smaller Mg2+ ions compensating for the addition of larger Sr2+ ions in the calcite lattice. These experiments show that the enhanced uptake of Sr2+ ions can be achieved by calcite precipitation via ACC, and may offer a rapid, low temperature, low-cost, method for removal of several incompatible Me2+ ions (e.g. Pb2+, Ba2+, Sr2+) during effluent treatment.
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Jan 2017
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I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[11075, 12704]
Abstract: Ferrihydrite is the most common iron oxyhydroxide found in soil and is a key sequester of contaminants in the environment. Ferrihydrite formation is also a common component of many treatment processes for cleanup of industrial effluents. Here we characterize ferrihydrite formation during the titration of an acidic ferric nitrate solution with NaOH. In situ SAXS measurements supported by ex situ TEM indicate that initially Fe13 Keggin clusters (radius ∼ 0.45 nm) form in solution at pH 0.12–1.5 and are persistent for at least 18 days. The Fe13 clusters begin to aggregate above ∼ pH 1, initially forming highly linear structures. Above pH ∼ 2 densification of the aggregates occurs in conjunction with precipitation of low molecular weight Fe(III) species (e.g., monomers, dimers) to form mass fractal aggregates of ferrihydrite nanoparticles (∼3 nm) in which the Fe13 Keggin motif is preserved. SAXS analysis indicates the ferrihydrite particles have a core–shell structure consisting of a Keggin center surrounded by a Fe-depleted shell, supporting the surface depleted model of ferrihydrite. Overall, we present the first direct evidence for the role of Fe13 clusters in the pathway of ferrihydrite formation during base hydrolysis, showing clear structural continuity from isolated Fe13 Keggins to the ferrihydrite particle structure. The results have direct relevance to the fundamental understanding of ferrihydrite formation in environmental, engineered, and industrial processes.
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Aug 2016
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B18-Core EXAFS
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Diamond Proposal Number(s):
[9621]
Open Access
Abstract: The speciation and fate of neptunium as Np(V)O2+ during the crystallization of ferrihydrite to hematite and goethite was explored in a range of systems. Adsorption of NpO2+ to iron(III) (oxyhydr)oxide phases was reversible and for ferrihydrite, occurred through the formation of mononuclear bidentate surface complexes. By contrast, chemical extractions and X-ray absorption spectroscopy (XAS) analyses showed the incorporation of Np(V) into the structure of hematite during its crystallization from ferrihydrite (pH 10.5). This occurred through direct replacement of octahedrally coordinated Fe(III) by Np(V) in neptunate-like coordination. Subsequent XAS analyses on mixed goethite and hematite crystallization products (pH 9.5 and 11) showed that Np(V) was incorporated during crystallization. Conversely, there was limited evidence for Np(V) incorporation during goethite crystallization at the extreme pH of 13.3. This is likely due to the formation of a Np(V) hydroxide precipitate preventing incorporation into the
goethite particles. Overall these data highlight the complex behaviour of Np(V) during the
crystallization of iron(III) (oxyhydr)oxides, and demonstrate clear evidence for neptunium
incorporation into environmentally important mineral phases. This extends our knowledge of the range of geochemical conditions under which there is potential for long term immobilisation of radiotoxic Np in natural and engineered environments.
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Feb 2016
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B18-Core EXAFS
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Diamond Proposal Number(s):
[7367, 7593, 8070]
Open Access
Abstract: Uranium incorporation into magnetite and its behaviour during subsequent oxidation has been investigated at high pH to determine the uranium retention mechanism(s) on formation and oxidative perturbation of magnetite in systems relevant to radioactive waste disposal. Ferrihydrite was exposed to U(VI)aq
containing cement leachates (pH 10.5–13.1) and crystallization of magnetite was induced via addition of Fe(II)aq. A combination of XRD, chemical extraction and XAS techniques provided direct evidence that U(VI) was reduced and incorporated into the magnetite structure, possibly as U(V), with a significant fraction recalcitrant to oxidative remobilization. Immobilization of U(VI) by reduction and incorporation into
magnetite at high pH, and with significant stability upon reoxidation, has clear and important implications for limiting uranium migration in geological disposal of radioactive wastes.
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Dec 2015
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