Abstract
Fe(II) bearing iron (oxyhydr)oxides were directly co-precipitated with Np(V)O2+ under anaerobic conditions to form Np doped magnetite and green rust. These environmentally relevant mineral phases were then characterised using geochemical and spectroscopic analyses. The Np doped mineral phases were then oxidised in air over 224 days with solution chemistry and end-point oxidation solid samples collected for further characterisation. Analysis using chemical extractions and X-ray absorption spectroscopy (XAS) techniques confirmed that Np(V) was initially reduced to Np(IV) during co-precipitation of both magnetite and green rust. Extended X-Ray Absorption Fine Structure (EXAFS) modelling suggested the Np(IV) formed a bidentate binuclear sorption complex to both minerals. Furthermore, following oxidation in air over several months, the sorbed Np(IV) was partially oxidised to Np(V), but very little remobilisation to solution occurred during oxidation. Here, linear combination fitting of the X-Ray Absorption Near Edge Structure (XANES) for the end-point oxidation samples for both mineral phases suggested approximately 50% oxidation to Np(V) had occurred over 7 months of oxidation in air. Both the reduction of Np(V) to Np(IV) and inner sphere sorption in association with iron (oxyhydr)oxides, and the strong retention of Np(IV) and Np(V) species with these phases under robust oxidation conditions, have important implications in understanding the mobility of neptunium in a range of engineered and natural environments.
Highlights
Neptunium (237 Np) is an important transuranic radionuclide in higher activity radioactive wastes due to its long half-life (2.13 × 106 years), potentially high environmental mobility as Np(V), and significant radiotoxicity [1,2,3,4]
The aims of the current work were to determine the products formed during the direct co-precipitation of magnetite and green rust with Np(V) under alkaline and anaerobic conditions of relevance to radioactive waste disposal using the experimental approach described in Roberts et al (2017) [17]
Analysis of X-ray diffraction (XRD) patterns for the parallel, non-active samples on mineral co-precipitation confirmed magnetite (Fe(II)/Fe(III) = 0.6) and green rust (Fe(II)/Fe(III) = 2.0) to be the dominant products formed during co-precipitation (Figure 1)
Summary
Neptunium (237 Np) is an important transuranic radionuclide in higher activity radioactive wastes due to its long half-life (2.13 × 106 years), potentially high environmental mobility as Np(V), and significant radiotoxicity [1,2,3,4]. The dioxygenyl neptunyl species (Np(V)O2 + ) dominates with a high predicted mobility in the environment. At alkaline pH, the solubility of metal ions including Np(V) and Np(IV) is expected to be low. Many disposal concepts for intermediate level radioactive wastes use grouting with cement, which develops alkaline conditions intended to reduce the mobility and solubility of radionuclides in waste [9]. Intermediate level wastes are typically expected to be grouted in steel
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