New insights shine light on uranium chemistry and disposal of radioactive waste
A new research paper provides a significant new insight into our understanding of uranium biogeochemistry and could help with the UK’s substantial 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 under conditions generally found in the environment, that uranium may exist as a more water-soluble form in a uranium-sulfur complex.
Professor Katherine Morris, Associate Dean for Research Facilities in the Faculty of Science and Engineering, The 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 or cleaning up of nuclear sites and mines.
“To be able to predict the behaviour of the uranium found around a former nuclear site or mine, we need to take into account that it might also 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 activity of microorganisms.” says Prof Morris.
“Properties such as, for example, how easily the uranium dissolves in water, which significantly affects its mobility through the ecosystem, may change when it gets bound to certain other elements in entities called complexes.”
It has been noted in earlier field studies that in environments that are low in oxygen and richer in sulphur, uranium tends to become more soluble and mobile. This new study is, published in, Environmental Science and Technology, is the first time that researchers confirm that the mobile uranium-sulfide complex may form under conditions similar to those in the environment.
To be able to predict the behaviour of the uranium found around a former nuclear site or mine, we need to take into account that it might also 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 activity of microorganisms.
In the experiment, the researchers studied uranium when it sits at the surface of ferrihydrite, which is a widespread mineral present 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, facilitating the release of uranium into an aqueous solution.
Professor Sam Shaw, Co-Investigator and Professor of Environmental Mineralogy at The University of Manchester said: “Shining the synchrotron beam on to the sample causes the uranium within the samples to emit X-rays. By analysing the X-ray signal from the samples the team were able to conclude 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, the team also made computer simulations to conclude which type of complex is more likely to form. This is the first observation of this uranium species under aqueous conditions and provides new insight into how uranium behaves in environments where sulfide is present. The reaction happens between sulfur and uranium on the surface of mineral as they transform.
“This work demonstrates the deep understanding we can develop of these complex systems and this knowledge could help underpin efforts to manage radioactive wastes in a geological disposal faciltiy.”
Dr Luke Townsend, Postdoctoral Fellow in Environmental Radiochemistry at The University of Manchester, who undertook this research as part of his PhD further said: “When trying to mimic environmental processes in the laboratory, it’s always a challenge to produce, accurate, high quality, reproducible science whilst also maintaining relevance to the initial question that was asked. You are both tested by the complexity in front of you, but also conscious of the overall contribution the work can make. However, through the ample hard work and commitment to the project, both in our labs in Manchester and on the beamlines here at Diamond, is all worthwhile when we get exciting results such as these.”
The XAS measurements were performed at Diamond on beamlines I20 and B18 by the researchers combining highly controlled iron (oxyhydr) oxide sulfidation experiments, using geochemical analyses, X-ray Absorption Spectroscopy (XAS), with 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 world-changing science and helping to tackle 21st century challenges. In this instance, our beamlines enabled the users to gain real insight into this newly confirmed form of uranium-sulphur complexes. These findings are like pages in the challenging book of waste remediation and Diamond’s capabilities could help with entire chapters of that book so more contributions together with our user community still remain to be made.”
Published in Environmental Science & Technology, the paper is called "Formation of a U(VI)-persulfide complex during environmentally relevant sulfidation of iron (oxyhydr)oxides"