Ferrihydrite Formation: The Role of Fe13 Keggin Clusters
Journal: Environmental Science and Technology
Publication Date: 02 August, 2016
Characterising the formation pathway of iron oxide nanoparticles during radioactive effluent treatment.
Nano-sized iron oxide minerals (FeOOH), such as ferrihydrite, are ubiquitous. Ferrihydrite formation is a key part of the global biogeochemical iron cycle in oceans and rivers, and it is widely used in effluent treatment to effectively remove contaminants. Despite this, the molecular pathways of ferrihydrite formation are poorly understood.
Researchers at the University of Manchester, in partnership with Sellafield Ltd, have characterised ferrihydrite formation during radioactive effluent treatment at the Enhanced Actinide Removal Plant (EARP), Sellafield, UK. The research used the state-of-the-art Diamond Light Source synchrotron to show that ferrihydrite nanoparticles are preceded by the formation of Fe13 so-called ‘Keggin’ clusters < 1 nanometre in size. This is followed by aggregation and precipitation processes to form ferrihydrite nanoparticles which preserve the Keggin cluster in the core. In this way, the Fe13 Keggin acts as a molecular building block for ferrihydrite. The research provides essential support to the growing body of evidence for cluster-mediated pathways to solid formation, which challenge the established thermodynamic view that solid formation occurs via monomeric species.
The research provides fundamental insight into the mechanism of ferrihydrite formation and delivers essential, molecular scale understanding of the EARP treatment process which is vital in enabling clean-up of the UK’s nuclear legacy.
- EARP is an integral part of Sellafield’s effluent management solution. EARP currently treats the radioactive effluent from Magnox and oxide fuel reprocessing operations at Sellafield, and will in future support post-operational clean-out and decommissioning.
- Keggin clusters consist of oxygen and transition metal atoms, and their structure comprises a single central tetrahedrally coordinated metal ion, caged by 12 octahedrally coordinated metal ions. The iron Keggin cluster (Fe13), is less than 1 nanometre in size. The aqueous cluster is highly reactive and was identified for the first time only in 2015 (Sadeghi et al. (2015), Science, 347, 1359).
- As well as being important in effluent treatment and the natural environment, ferrihydrite is present in almost all living organisms within the protein ferritin, acting as a bioavailable store of iron.
- The Diamond Light Source synchrotron accelerates electrons to near light speeds so that they give off a light 10 billion times brighter than the sun. This light was used in this research in scattering experiments to characterise ferrihydrite formation.