title: Controls on the Fate and Speciation of Np(V) During Iron (Oxyhydr)oxide Crystallization.
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 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 behavior 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 immobilization of radiotoxic Np in natural and engineered environments.
authors.affiliation: Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester , Manchester, M13 9PL, United Kingdom.
authors.forename: Pieter
authors.initials: P
authors.lastname: Bots
authors.affiliation: Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester , Manchester, M13 9PL, United Kingdom.
authors.forename: Samuel
authors.initials: S
authors.lastname: Shaw
authors.affiliation: Centre for Radiochemistry Research and Research Centre for Radwaste Disposal, School of Chemistry, The University of Manchester , Manchester, M13 9PL, United Kingdom.
authors.forename: Gareth T W
authors.initials: GT
authors.lastname: Law
authors.affiliation: Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester , Manchester, M13 9PL, United Kingdom.
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Marshall
authors.affiliation: Diamond Light Source, Ltd. , Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom.
authors.forename: J Frederick W
authors.initials: JF
authors.lastname: Mosselmans
authors.affiliation: Research Centre for Radwaste Disposal and Williamson Research Centre for Molecular Environmental Science, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester , Manchester, M13 9PL, United Kingdom.
authors.forename: Katherine
authors.initials: K
authors.lastname: Morris
mesh_terms: D000327:Adsorption; D003460:Crystallization; D005290:Ferric Compounds / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D058085:Iron Compounds / Q000737:chemistry; D046529:Microscopy, Electron, Transmission; D008903:Minerals / Q000737:chemistry; D009405:Neptunium / Q000737:chemistry*; D011850:Radioactive Waste; D013696:Temperature; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
chemical_list: D005290:Ferric Compounds; D058085:Iron Compounds; D008903:Minerals; D011850:Radioactive Waste; C094886:goethite; C000499:ferric oxide; D009405:Neptunium; C092844:ferric oxyhydroxide