78 79 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Spectroscopy Group Beamline I20 (Scanning Branchline) The discovery of a new type of uraniumwill inform radioactivewaste disposal Related publication: Townsend L. T., Shaw S., Ofili N. E. R., Kaltsoyannis N.,Walton A. S., Mosselmans J. F.W., Neil T. S., Lloyd J. R., Heath S., Hibberd R. &Morris K. Formation of a U(VI)–Persulfide Complex during Environmentally Relevant Sulfidation of Iron (Oxyhydr)oxides. Environ. Sci. Technol. 54, 129-136 (2020). DOI: 10.1021/acs.est.9b03180 Publication keywords: Iron; Redox reactions; Sulfides; Uranium; Extended X-ray absorption fine structure W hen packaged, therewill be 750,000m 3 of higher activity radioactivewaste in the UK that needs to be disposed of safely - enough to fill 2/3 ofWembley Stadium. The plan is to dispose of this waste deep in the subsurface of the Earth, in an engineered facility known as a Geological Disposal Facility (GDF). To safely dispose of waste in a GDF, we need to understand the processes that could affect the mobility of uranium in the environment. One key process that affects uranium mobility in the environment is the reaction with sulfide (sulfidation) produced by microbes naturally present in the subsurface. This means that the effects of sulfidation on uranium in a GDF scenario need to be further understood. Researchers used X-ray Absorption Spectroscopy (XAS) on the I20-Scanning branchline to investigate what happened to the chemistry of uranium. The high sensitivity of this powerful beamline allowed them to look at the low, environmental concentrations of uranium in their samples. In an unexpected result, the scientists saw the reactionwith sulfide caused uranium release into solution for a very brief time, beforemoving into a highly immobile state. They attributed this unusual chemistry to the formation of a new form of uranium known as a U(VI)-persulfide, which had not been previously identified in an environmental system. The presence of this new formof uraniumgives significant insight into uranium environmental chemistry and highlights the unique insight Diamond Light Source can provide in underpinning radioactive waste disposal. In the UK, radioactive waste is planned to be disposed of deep in the subsurface in a Geological Disposal Facility (GDF). In order to ensure this disposal is effective, safe and implemented correctly, a safety case underpinned by fundamental and applied scientific research is needed to support the development of this nationally significant infrastructure project. This research involves understanding how the radioactive elements within the waste are affected by, and interact with, the environment once disposal has taken place. As uranium is the largest radionuclide by mass in radioactive waste, it is important to thoroughly understand how the naturally occurring processes in the deep subsurface will affect its chemistry and therefore its mobility in the environment. One of the biogeochemical processes that takes place in subsurface environments is the production of sulfide by microbes that are naturally present. This sulfide can then react withminerals that are ubiquitously present in this GDF subsurface environment and other elements, like uranium, in a process known as sulfidation.These reactions need to be understood if a GDF is to be commissioned as previous field and laboratory studies have suggested that sulfidation may affect environmental mobility, although a molecular scale understanding was not achieved 1-3 . In order to fully understand the effects of sulfidation on uranium environmental behaviour, experiments were performed at the University of Manchester, using a highly controlled abiotic methodology under conditions mimicking natural subsurface conditions (i.e. pH 7 and pH 9.5), to produce samples that could be analysed at Diamond Light Source using X-ray Absorption Spectroscopy (XAS). XAS is a particularly useful method of analysis due to its ability to provide information about the local atomic environment and chemistry on the element of interest (in this case uranium) within a highly complex sample. Through the power of the I20-Scanning branchline, samples with very low concentrations of analyte could be analysed and the chemistry of uranium elucidated throughout the process of sulfidation. The results of the study have produced a molecular-scale understanding of the mechanisms that control uranium mobility under sulfidic conditions relevant to geological disposal of radioactive waste. Upon initiation of sulfidation, uranium was observed to be liberated into solution transiently, in spite of the highly reduced conditions that would usually partition uranium to the solid phase as a reduced species known as uraninite or U(IV)O 2 . This unusual uranium chemistry was studied further using XAS on I20- Scanning and revealed that a new environmental form of uranium was present during sulfidation. Through detailed analysis of the X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) (Fig. 1), this new form of uranium, known as a U(VI)-persulfide, was shown to be key in understanding why uraniumwas present in solution during this reaction. This new, transient form of uranium was shown to consist of a persulfide ligand (S 2 2- ), in addition to water molecules, coordinated to the equator of a uranyl moiety (UO 2 2+ ) (Fig. 2). To reinforce the findings from the XAS data, computational modelling of the U(VI)-persulfide and its interactions with mineral surfaces present in the system was performed. The modelled structure (Fig. 2) matched well with the values produced through analysis of the XAS data and highlighted how this form of uranium was weakly associated with the surface of minerals, providing a possible explanation for why uranium is released into solution during sulfidation. Not only does this result provide significant insight into science underpinning a GDF, it also sheds light on uranium environmental chemistry that had not previously been identified.Whilst U-S complexes have been previously identified and synthesised in laboratories 4,5 , the conditions used to produce these species were not representative of natural, environmentally relevant systems. However, this form of uranium, and the release of uranium to solution, was transient, lasting only a matter of hours. The ultimate fate of uranium was the reduced form of uranium, U(IV), in a nanocrystalline solid mineral phase known as uraninite (U(IV)O 2 ) (Fig. 2). This means that uranium will likely be immobile over long timescales and under conditions relevant to geological disposal of radioactive waste. The results of this study have highlighted the complexity surrounding uranium chemistry in environmental systems, particularly under sulfidic conditions. The new, transient form of uranium, U(VI)-persulfide, has been shown to play a key role in uraniummobility, however, ultimately uranium is immobileasasolidU(IV)mineralphase.Thesefindingsarekeytounderpinning the safety case for the implementation of geological disposal of radioactivewaste in the UK, in addition to broadening and deepening the knowledge of uranium chemistry in environmentally relevant systems. References: 1. Anderson R.T. et al. Stimulating the In Situ Activity of Geobacter Species To Remove Uranium from the Groundwater of a Uranium-Contaminated Aquifer. Appl. Environ. Microbiol. 69 (10), 5884-5891 (2003). DOI: 10.1128/AEM.69.10.5884-5891.2003 2. AlexandratosV. G. et al. Sulfidization of lepidocrocite and its effect on uraniumphase distribution and reduction. Geochim. Cosmochim. Acta 142 , 570-586 (2014). DOI: 10.1016/j.gca.2014.08.009 3. Alexandratos,V. G. et al. Fate of adsorbed U(VI) during sulfidization of lepidocrocite and hematite. Environ. Sci. Technol. 51 (11), 2140-2150 (2017). DOI: 10.1021/acs.est.6b05453 4. Grant, D. J. et al. Synthesis of a Uranyl Persulfide Complex and Quantum Chemical Studies of Formation andTopologies of Hypothetical Uranyl Persulfide Cage Clusters. Inorg. Chem. 51 (14), 7801-7809 (2012). DOI: 10.1021/ic3008574 5. Ward, M. D. et al. The [U 2 (μ-S 2 ) 2 Cl 8 ] 4– anion: synthesis and characterization of the uraniumdouble salt Cs 5 [U 2 (μ-S 2 ) 2 Cl 8 ]I. Inorg. Chem. 54 (6), 3055-3060 (2015). DOI: 10.1021/acs.inorgchem.5b00234 Funding acknowledgement: EPSRC and RadioactiveWaste Management Ltd. co-funded the PhD studentship to L.T.T. via the Next Generation Nuclear CDT (EP/L015390/1).We are grateful to the Science andTechnology Facilities Council for a PhD studentship to N.E.R.O. (ST/R504580/1).We would further like to acknowledge the support of Env Rad Net (ST/K001787/1 and ST/N002474/1) and the HEC Materials Chemistry Consortium, which is funded by the EPSRC (EP/L000202, EP/R029431) for access to ARCHER, the UK National Supercomputing Service (http://www.archer.ac.uk ). Corresponding author: Dr LukeTownsend, University of Manchester, firstname.lastname@example.org Figure 1: XAS data obtained from I20-Scanning branchline taken throughout the experiments run at pH 7 and pH 9.5. Figure 2: Overall reaction scheme for the sulfidation experiment performed in this study.