Uranium Chemistry & Geological Disposal of Radioactive Waste: New Insights using the Diamond Light Source

A new paper to be published on 16 December provides a significant new insight into our understanding of uranium biogeochemistry and could help with the UK’s nuclear legacy. 

Conducted by a team of researchers from the University of Manchester, Diamond Light Source and Radioactive Waste Management, their work shows for the first time how uranium forms a uranium-sulfur complex under conditions generally found in the environment and how this compound can be an important intermediary in uranium immobilisation. Published in Environmental Science & Technology, the paper is called "Formation of a U(VI)-persulfide complex during environmentally relevant sulfidation of iron (oxyhydr)oxides" 1

Professor Katherine Morris, Associate Dean for Research Facilities in the Faculty of Science and Engineering, University of Manchester and the Research Director for the BNFL Research Centre in Radwaste Disposal explains why recreating and studying these chemical complexes is highly relevant for understanding and dealing with radioactive waste. She explains: “To be able to predict the behaviour of the uranium during geological disposal, we need to take into account that it may have interacted with other processes taking place in the ground. These so-called biogeochemical reactions are often a complex set of interactions between dissolved chemical species, mineral surfaces, and microorganisms.”

The recent study is the first time that researchers have shown that a uranium-sulfide complex can form under conditions representative of a deep underground environment.  This complex then transforms further into highly immobile uranium oxide nanoparticles.

In the experiment, the researchers studied uranium when it sits at the surface of the mineral ferrihydrite, which is a widespread mineral in the environment. The researchers used an X-ray based method called X-ray Absorption Spectroscopy (XAS) to study the samples at Diamond Light Source, the UK’s national Synchrotron. The XAS data, in combination with computational modelling, showed that during the sulfidation reaction, a short-lived and novel U(VI)-persulfide complex formed during this biogeochemical process.

Professor Sam Shaw, Co-Investigator and Professor of Environmental Mineralogy at the University of Manchester; “Shining the synchrotron beam onto the sample causes the uranium within to emit X-rays. By analysing the X-ray signal from the samples our team were able to determine the chemical form of uranium, and to which other elements it is bound. To further validate the theory on the formation pathway of the uranium-sulfur complexes, our team also made computer simulations to conclude which type of complex is more likely to form. This is the first observation of this form of uranium under aqueous conditions, and provides new insight into how uranium behaves in environments where sulfide is present. This work demonstrates the deep understanding we can develop of these complex systems and this knowledge will help underpin efforts to manage radioactive wastes in a geological disposal facility.”

Dr Luke Townsend, Postdoctoral Fellow in Environmental Radiochemistry at The University of Manchester, who undertook this research as part of his PhD further adds:

“When trying to mimic environmental processes in the laboratory, it’s a challenge to produce accurate, high quality, reproducible science with such complex experiments, whilst also maintaining relevance to the geodisposal environment. However, obtaining exciting results such as these makes all the hard work and commitment to the project from myself and the group, both in our labs in Manchester and on the beamlines at Diamond, completely worthwhile.”

The XAS measurements were performed at Diamond on beamlines I20 and B18 by the researchers who used highly controlled sulfidation experiments that mimic biogeochemical processes in the deep underground environment. This was combined with geochemical analyses and computational modelling to track and understand uranium behaviour.

Physical Science Director at Diamond, Laurent Chapon concludes; “This is another example of how Diamond’s state of the art analytical tools are enabling scientists to follow complex processes and help them to tackle 21st century challenges. In this instance, our beamlines allowed the users to gain real insight into the environmental relevance of this new uranium-sulfur complex, which feeds into our understanding of geological disposal.”


More about the paper and methods used:

The paper is called "Formation of a U(VI)-persulfide complex during environmentally relevant sulfidation of iron (oxyhydr)oxides" (http://dx.doi.org/10.1021/acs.est.9b03180) and the authors, from the University of Manchester, Diamond Light Source and Radioactive Waste Management, are: Luke Townsend, Samuel Shaw; Naomi Ofili,  Nikolas Kaltsoyannis ; Alex Walton,  Frederick J. Mosselmans; Thomas Neill, Jonathan Lloyd; Sarah Heath; Rosemary Hibberd; Katherine Morris

The work is funded by EPSRC and Radioactive Waste Management and was performed by Luke Townsend and the team using I20 and B18 beamlines at Diamond.

Furthering our understanding of uranium in the environment

When we want to store or dispose of radioactive waste, or clean-up nuclear sites and mines, it's essential to understand how uranium interacts with, and moves through, the environment. This interaction is primarily controlled by the oxidation state of the uranium, with U(VI) relatively mobile in the environment, and U(IV) mainly immobile. However, biogeochemical reactions (a complex set of interactions between dissolved species, mineral surfaces and biological activity) can alter the oxidation state of uranium, and hence its mobility in the environment.

This picture is complicated further in contaminated land and geodisposal systems, where interactions with iron and sulfide minerals are important factors in controlling uranium mobility. Previous studies have shown that transformations of iron (oxyhydr)oxide minerals result in significant impacts on the oxidation state and mobility of uranium.

While uranium-sulfide compounds have not been identified under environmentally relevant conditions, confirming their presence provides a significant new insight into our understanding of uranium biogeochemistry. More generally, the UK has a substantial nuclear legacy which needs to be cleaned up and managed, and this work demonstrates the deep understanding we can develop of these complex systems. In turn, this knowledge underpins efforts to clean-up and manage radioactive wastes and develops highly skilled people to work on these nationally important challenges.

About Diamond Light Source: www.diamond.ac.uk 

Diamond Light Source is the UK’s national synchrotron, providing industrial and academic user communities with access to state-of-the-art analytical tools to enable world-changing science. Shaped like a huge ring, it works like a giant microscope, accelerating electrons to near light speeds, to produce a light 10 billion times brighter than the Sun, which is then directed off into 33 laboratories known as ‘beamlines’. In addition to these, Diamond offer access to several integrated laboratories including the Electron Bio-imaging Centre (eBIC) and the Electron Physical Science Imaging Centre (ePSIC).

Diamond serves as an agent of change, addressing 21st century challenges such as disease, clean energy, food security and more. Since operations started, more than 14,000 researchers from both academia and industry have used Diamond to conduct experiments, with the support of approximately 700 world-class staff. More than 8,000 scientific articles have been published by our users and scientists.

Funded by the UK Government through the Science and Technology Facilities Council (STFC), and by the Wellcome Trust, Diamond is one of the most advanced scientific facilities in the world, and its pioneering capabilities are helping to keep the UK at the forefront of scientific research.

About The University of Manchester

The University of Manchester, a member of the prestigious Russell Group, is one of the UK’s largest single-site universities with more than 40,000 students – including more than 10,000 from overseas. It is consistently ranked among the world’s elite for graduate employability.

The University is also one of the country’s major research institutions, rated fifth in the UK in terms of ‘research power’ (REF 2014). World-class research is carried out across a diverse range of fields including cancer, advanced materials, global inequalities, energy and industrial biotechnology.

No fewer than 25 Nobel laureates have either worked or studied here.

It is the only UK university to have social responsibility among its core strategic objectives, with staff and students alike dedicated to making a positive difference in communities around the world.

Manchester is ranked 27th in the world in the QS World University Rankings (2020) and 6th in the UK.  It is also ranked 8th in Reuters Top 100: Europe's most innovative universities (2019).

Visit www.manchester.ac.uk for further information.

This press news story has been adapted from an original press release from The University of Manchester.