B18-Core EXAFS
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Diamond Proposal Number(s):
[31395, 37736]
Abstract: Selenium-79, a radionuclide present in higher-activity radioactive wastes destined for deep geological disposal, is mobile under oxic conditions, where Se(IV) and Se(VI) dominate. Anoxic batch microcosm incubations were constructed containing Wyoming MX80 bentonite (a candidate buffer material in geological disposal) and artificial groundwater with or without steel coupons to represent canister materials. Se(VI)(aq) bioreduced and was removed by 7 days when lactate was added as an electron donor, after which sulfate reduction occurred. With H2 gas as the electron donor, Se(VI) bioreduction slowed, with complete removal at 14 days and minimal sulfate reduction thereafter. 16S rRNA gene sequencing highlighted the dominance of Anaerobacillus spp. (44% at 28 days) during Se(VI)-reduction, and in the lactate-amended systems, there was a subsequent enrichment in sulfate-reducing bacteria affiliated with Desulfosporosinus spp. (60% relative abundance at 84 days). Extended X-ray absorption fine structure (EXAFS) analyses identified monoclinic Se(0) as the bioreduction product after 28 days, but by 84 days this evolved to trigonal Se(0) in the absence of steel coupons or was further reduced to FeSe2 with steel present. The reduction of Se(VI)(aq) to poorly soluble Se(0)/FeSe2 mediated by indigenous bentonite microbial communities highlights their potential importance in promoting Se-79 retention during deep geological disposal.
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Jan 2026
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[21441, 18594]
Abstract: Vanadium (V) is a widespread trace element in the environment. At high concentration, due to anthropogenic emission such as mining activities and fossil fuels, it can be toxic to marine systems and terrestrial plants. However, the geochemical behaviour of V remains poorly understood. Here, this study aimed to understand the molecular scale V speciation, adsorption behaviour and bonding mechanisms of V(V) onto hematite as a function of geochemical factors (pH, ionic strength, and V(V) concentration), applying a multi-technique approach comprising aqueous chemical analysis and XAS supported by ATR-FTIR and PHREEQC geochemical speciation calculations. From these data, tetrahedral monovanadate formed a corner-sharing bidentate surface complex at 1.20 mM V(V) and pH 9 and 0.12 mM V(V) at all pH, while octahedral decavanadate formed at a V(V) concentration of 1.20 mM and pH < 7. The dominant coordination environment changed gradually from a mixture of octahedrally and tetrahedrally coordinated V at pH 3 to tetrahedral monovanadate at pH 9. These results demonstrate the marked effects of pH and initial V concentration on V(V) speciation at hematite surfaces, in turn affording predictions of the environmental behaviour of heavy metals released during a variety of anthropogenic activities (e.g. mining) across a range of geochemical conditions. It is envisioned these results will contribute to strategies for the treatment of lands contaminated with heavy metals predominantly through adsorption processes (e.g. mine sites).
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Dec 2025
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B18-Core EXAFS
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Diamond Proposal Number(s):
[37736]
Open Access
Abstract: 99Tc is a long-lived radioactive fission product whose subsurface mobility is governed by redox conditions. Under oxic conditions, soluble Tc(VII)O4– is mobile, whereas under reducing conditions, poorly soluble Tc(IV) phases limit transport. Microcosm studies have frequently reported TcO2-like solids and, less consistently, Tc(IV)-sulfides. The stability of Tc(IV)-sulfides under environmentally relevant conditions remains unclear. Here, we used flowing sediment columns representative of the Sellafield subsurface to examine Tc speciation and stability over ∼1 year. Under reducing conditions, >90% of added TcO4– (400 μg) was retained under both Fe(III)- and sulfate-reducing conditions. X-ray absorption spectroscopy showed TcO2-like phases dominated in Fe(III)-reducing columns, while Tc(IV)-sulfides dominated after sustained sulfate reduction. Sequential extractions indicated that Tc in sulfidic sediments was more recalcitrant (≤23% released by weak acids) than in Fe(III)-reducing systems (∼60% released). With oxic groundwater pumping, effluent Tc sourced from the sediments rose rapidly. Over 160 days, the sulfidic columns remobilized ∼25% of their Tc inventory compared to ∼50% in Fe(III)-reducing columns. The Tc(IV)-sulfides also gradually oxidized to form TcO2 phases. While Tc(IV)-sulfides may enhance Tc retention under reducing conditions, TcO2 phases more likely govern 99Tc mobility during long-term redox cycling. Our findings provide new constraints for modeling Tc fate at contaminated sites and in radioactive waste disposal.
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Dec 2025
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Open Access
Abstract: The release of geogenic arsenic into groundwater, driven by reductive dissolution of Fe(III)/As(V) oxide phases, poses a severe health risk to millions in South and Southeast Asia. However, the microbes and electron donors responsible for the reductive dissolution remain unclear, due to complex a(biotic) interactions in sediments (traditionally used in microcosm incubation studies). In this study, indigenous microbial communities were sampled from arsenic-prone aquifers in Kandal Province, Cambodia, by filtering groundwater through sands coated with Fe(III)/As(V) minerals. This provided a streamlined inocula to study fundamental Fe(III)/As(V) reduction processes in controlled laboratory experiments. Anoxic incubations with contrasting electron donors suggested that biolabile organics are the main drivers of Fe(III) and As(V) reduction in the sampled aquifers, but methane can also contribute to Fe(III) reduction (at a slower rate) in the absence of labile organics. Known Fe(III)-reducing bacteria (e.g. Geobacter and Geothrix) were implicated in Fe(III)/As(V) reduction. Methane-driven Fe(III) reduction appeared to be mediated by proteobacterial methanotrophs (e.g., Methylomonas and Methylosinus), either directly or via symbiotic interactions with Geobacter through labile organic intermediates (suggested by acetate generation) highlighting the flexibility of proteobacterial methanotrophs under anoxic conditions. No methane-driven As(V) reduction was implicated in this study, while nominal As(V) reduction driven by aquatic organics (sorbed from the groundwater during filtration) was evident in control incubations suggesting some decoupling between Fe(III) and As(V) reduction. Furthermore, the sand filtration approach offers a promising method for producing simplified inocula for further studies of microbe-organic-mineral interactions in arsenic-prone aquifers and other complex biogeochemical systems.
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Oct 2025
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B18-Core EXAFS
I14-Hard X-ray Nanoprobe
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[25930, 24074, 21441]
Open Access
Abstract: Uranium (U) is a natural radioactive metal and a persistent environmental pollutant. Characterising the influence of arbuscular mycorrhizal fungi (AMF) on U bioaccumulation and partitioning in plants is crucial to understand U soil-to-plant transfer mechanisms. High resolution elemental mapping, spectroscopy and microscopy techniques were conducted on uranyl nitrate dosed Plantago lanceolata roots colonised with Rhizophagus irregularis. U-rich particles accumulated within the root cells, with higher abundance in epidermal and outer cortex cells of mycorrhizal root samples than in non-mycorrhizal roots. Electron microscopy determined two different crystalline U phases, an acicular crystal and a novel rounded aggregate formation, the latter of which was only found within the mycorrhizal root cells. Multiple imaging and spectroscopic techniques enabled the dominant elements with these U biominerals to be determined. Co-localisation between U, phosphorus and oxygen indicated the dominance of U-phosphate biominerals, but metals including calcium and zinc were also found to co-localise. The most dominant U compound was uranyl orthophosphate, likely accompanied by autunite. This study demonstrates alteration in U localisation and U particle morphology within Plantago roots as a direct consequence of AMF colonisation. This knowledge will allow more accurate U food-chain transfer modelling and better assessment of AMF-assisted phytoremediation feasibility.
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Jun 2025
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Thomas S.
Neill
,
Katherine
Morris
,
Scott
Harrison
,
Pete
Apps
,
Nick
Bryan
,
Stephen
Parry
,
J. Frederick W.
Mosselmans
,
Giannantonio
Cibin
,
Bruce
Rigby
,
Francis R.
Livens
,
Samuel
Shaw
Diamond Proposal Number(s):
[17243, 21441]
Open Access
Abstract: Colloids present a challenge for nuclear decommissioning and disposal due to their potential to mobilise radionuclides. Waste retrieval and decommissioning of storage ponds for spent nuclear fuel and silos for radioactive waste at the Sellafield nuclear facility, UK, are high priorities. The particulates characterised here originate from facilities >60 years old and provide a unique opportunity to investigate the long-term fate of radionuclides in an aquatic, engineered storage environment. Radioactive effluents were obtained from a legacy pond and characterised using ultrafiltration, transmission electron microscopy (TEM) and actinide L3 edge X-ray absorption spectroscopy (XAS). TEM analysis showed discrete UO2-like nanoparticles, 5-10 nm in size, often co-associated with Mg-Al- and Fe-(oxyhydr)oxide colloidal phases. Uranium XAS indicated a mix of uranium oxidation states with EXAFS suggesting U(IV)-oxide nanoparticles and sorbed U(VI). Pu XANES identified Pu(IV) as the dominant oxidation state. Both U and Pu associates with large, Mg/Al- and Fe-(oxyhydr)oxide agglomerates highlights the potential for pseudo-colloid formation, explaining the basis of current particle filtration / abatement of technology. This study, which examines novel samples from a complex, highly radioactive facility using advanced techniques, provides a new understanding of radionuclide speciation and mobility in these environments and informs radioactive effluent treatment and disposal.
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Feb 2025
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[24074, 21441]
Open Access
Abstract: Operations at uranium (U)-mining and nuclear facilities have left a global legacy of significant radionuclide contamination in groundwaters which must be managed to minimize environmental harm. Uranium groundwater contamination is present at several sites globally, including Oak Ridge National Laboratory and Hanford, USA and Sellafield nuclear site, UK. In situ phosphate biomineralisation offers a promising method for radionuclide (including 90Sr and U) remediation at these sites. Typically, phosphate-generating amendments are injected into the subsurface to sequester select radionuclides in groundwaters by precipitation of poorly soluble Ca-phosphate phases and subsequent adsorption and/or incorporation of radionuclides to these poorly soluble phases, a remediation route being explored for both U and 90Sr. In this study, we investigate the mechanisms of U-phosphate precipitation in two phosphate-generating amendments (Ca-citrate/Na-phosphate and glycerol phosphate) under conditions relevant to Sellafield, UK. Using aerobic batch sediment experiments, we show both Ca-citrate/Na-phosphate and glycerol phosphate amendments are effective at enhancing removal of U(VI) from representative groundwaters (from 94% to >97%). Aqueous geochemical data coupled to speciation modelling highlighted that precipitation of U(VI) phosphate phases was the likely mechanism of U(VI) removal from groundwaters. Further X-ray absorption spectroscopy (XAS) analysis of solids confirmed U was present as a highly insoluble uranyl orthophosphate-like phase after treatment with both Ca-citrate/Na-phosphate and glycerol phosphate amendments. These data provide underpinning information on U-phosphate remediation in Sellafield relevant conditions thus expanding the range of treatment options for radionuclide contaminated groundwaters and defining the transport and fate of U during phosphate biomineralisation.
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Feb 2025
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B18-Core EXAFS
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Diamond Proposal Number(s):
[31395]
Open Access
Abstract: The significant abundance of uranium in radioactive waste inventories worldwide necessitates a thorough understanding of its behavior. In this work, the speciation of uranyl(VI), (UO22+) in a gibbsite system under ambient conditions has been determined as a function of pH by deconvolution and analysis of luminescence spectroscopic data. Uniquely, a combined experimental and statistical approach utilizing time-resolved luminescence spectroscopy and parallel factor analysis (PARAFAC) of excitation emission matrices has been successfully utilized to identify four separate luminescent U(VI) species in the uranyl-gibbsite system for the first time. The speciation of all luminescent U(VI) species in an environmentally relevant system over a pH range of 6–11 is discerned through the analysis of emission fingerprints at low temperature (20 K). Comparison of the deconvoluted luminescence spectra with mineral standards and geochemical models of the system allows the assignment of the luminescent chemical species as metaschoepite, Na-compreignacite, surface adsorbed ≡AlO2–UO2(OH) and ≡AlO2–UO2(CO3)24– complexes, with assignments supported by fitting of extended X-ray absorption fine structure data. The combined spectroscopic techniques in this study show that assignment and quantification of uranyl(VI) species in a sorption system over a large pH range can be accurately achieved using PARAFAC to deconvolute a three way emission spectroscopic data set.
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Sep 2024
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B18-Core EXAFS
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Diamond Proposal Number(s):
[24074]
Open Access
Abstract: Microbial ureolysis offers the potential to remove metals including Sr2+ as carbonate minerals via the generation of alkalinity coupled to NH4+ and HCO3– production. Here, we investigated the potential for bacteria, indigenous to sediments representative of the U.K. Sellafield nuclear site where 90Sr is present as a groundwater contaminant, to utilize urea in order to target Sr2+-associated (Ca)CO3 formation in sediment microcosm studies. Strontium removal was enhanced in most sediments in the presence of urea only, coinciding with a significant pH increase. Adding the biostimulation agents acetate/lactate, Fe(III), and yeast extract to further enhance microbial metabolism, including ureolysis, enhanced ureolysis and increased Sr and Ca removal. Environmental scanning electron microscopy analyses suggested that coprecipitation of Ca and Sr occurred, with evidence of Sr associated with calcium carbonate polymorphs. Sr K-edge X-ray absorption spectroscopy analysis was conducted on authentic Sellafield sediments stimulated with Fe(III) and quarry outcrop sediments amended with yeast extract. Spectra from the treated Sellafield and quarry sediments showed Sr2+ local coordination environments indicative of incorporation into calcite and vaterite crystal structures, respectively. 16S rRNA gene analysis identified ureolytic bacteria of the genus Sporosarcina in these incubations, suggesting they have a key role in enhancing strontium removal. The onset of ureolysis also appeared to enhance the microbial reduction of Fe(III), potentially via a tight coupling between Fe(III) and NH4+ as an electron donor for metal reduction. This suggests ureolysis may support the immobilization of 90Sr via coprecipitation with insoluble calcium carbonate and cofacilitate reductive precipitation of certain redox active radionuclides, e.g., uranium.
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Feb 2024
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B18-Core EXAFS
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Diamond Proposal Number(s):
[24074]
Open Access
Abstract: Historical operations at nuclear mega-facilities such as Hanford, USA, and Sellafield, UK have led to a legacy of radioactivity-contaminated land. Calcium phosphate phases (e.g., hydroxyapatite) can adsorb and/or incorporate radionuclides, including 90Sr. Past work has shown that aqueous injection of Ca-phosphate-generating solutions into the contaminated ground on both laboratory and field scales can reduce the amount of aqueous 90Sr in the systems. Here, two microbially mediated phosphate amendment techniques which precipitated Ca-phosphate, (i) Ca-citrate/Na-phosphate and (ii) glycerol phosphate, were tested in batch experiments alongside an abiotic treatment ((iii) polyphosphate), using stable Sr and site relevant groundwaters and sediments. All three amendments led to enhanced Sr removal from the solution compared to the sediment-only control. The Ca-citrate/Na-phosphate treatment removed 97%, glycerol phosphate 60%, and polyphosphate 55% of the initial Sr. At experimental end points, scanning electron microscopy showed that Sr-containing, Ca-phosphate phases were deposited on sediment grains, and XAS analyses of the sediments amended with Ca-citrate/Na-phosphate and glycerol phosphate confirmed Sr incorporation into Ca-phosphates occurred. Overall, Ca-phosphate-generating treatments have the potential to be applied in a range of nuclear sites and are a key option within the toolkit for 90Sr groundwater remediation.
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Aug 2023
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