Ferrihydrite Formation: The Role of Fe₁₃ Keggin Clusters

In Abstract

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.

Click here to read the full article - DOI: 10.1021/acs.est.6b02481