Controls on the fate and speciation of Np(V) during iron (oxyhydr)oxide crystallization

Pieter Bots, Samuel Shaw, Gareth T.W. Law, Timothy A. Marshall, J. Frederick .W. Mosselmans, Katherine Morris

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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.
Original languageEnglish
Pages (from-to)3382-3390
Number of pages9
JournalEnvironmental Science and Technology
Volume50
Issue number7
Early online date25 Feb 2016
DOIs
Publication statusPublished - 5 Apr 2016

Keywords

  • chemical analysis
  • hematite
  • iron
  • iron compounds
  • x-ray
  • absorption spectroscopy

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    Developing Analytical Capability and Knowledge of Uranium and Neptunium Behaviour in the Environment ? Exploitation of the I20 Facility atDIAMOND

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