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Controls on the fate and speciation of neptunium-(V) during iron (oxyhydr)oxide crystallization.

DOI: 10.1021/acs.est.5b05571 DOI Help

Authors: Pieter Bots (University of Manchester) , Sam Shaw (University of Manchester) , Gareth Law (Centre for Radiochemistry Research and Research Centre for Radwaste and Decommissioning, University of Manchester, U.K.) , Tim Marshall (University of Manchester) , Fred Mosselmans (Diamond Light Source) , Katherine Morris (University of Manchester)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Environmental Science & Technology

State: Published (Approved)
Published: February 2016
Diamond Proposal Number(s): 9621

Abstract: The speciation and fate of neptunium as Np(V)O2+ during the crystallization of ferrihydrite to hematite and goethite was explored in a range of systems. Adsorption of NpO2+ to iron(III) (oxyhydr)oxide phases was reversible and for ferrihydrite, occurred through the formation of mononuclear bidentate surface complexes. By contrast, chemical extractions and X-ray absorption spectroscopy (XAS) analyses showed the incorporation of Np(V) into the structure of hematite during its crystallization from ferrihydrite (pH 10.5). This occurred through direct replacement of octahedrally coordinated Fe(III) by Np(V) in neptunate-like coordination. Subsequent XAS analyses on mixed goethite and hematite crystallization products (pH 9.5 and 11) showed that Np(V) was incorporated during crystallization. Conversely, there was limited evidence for Np(V) incorporation during goethite crystallization at the extreme pH of 13.3. This is likely due to the formation of a Np(V) hydroxide precipitate preventing incorporation into the goethite particles. Overall these data highlight the complex behaviour of Np(V) during the crystallization of iron(III) (oxyhydr)oxides, and demonstrate clear evidence for neptunium incorporation into environmentally important mineral phases. This extends our knowledge of the range of geochemical conditions under which there is potential for long term immobilisation of radiotoxic Np in natural and engineered environments.

Journal Keywords: neptunium, iron(III) (oxyhydr)oxides, immobilisation, XAS

Subject Areas: Earth Science, Chemistry, Energy

Instruments: B18-Core EXAFS

Other Facilities: ANKA