title: NeptuniumVRetention by Siderite under Anoxic Conditions: Precipitation of NpO2-Like Nanoparticles and of NpIVPentacarbonate.
abstract: The NpVretention by siderite, an FeIIcarbonate mineral with relevance for the near-field of high-level radioactive waste repositories, was investigated under anoxic conditions. Batch sorption experiments show that siderite has a high affinity for aqueous NpVO2+across pH 7 to 13 as expressed by solid-water distribution coefficients, log Rd,> 5, similar to the log Rddetermined for the (solely) tetravalent actinide Th on calcite, suggesting reduction of NpVto NpIVby siderite. Np L3-edge X-ray absorption near edge (XANES) spectroscopy conducted in a pH range typical for siderite-containing host rocks (7-8), confirmed the tetravalent Np oxidation state. Extended X-ray absorption fine-structure (EXAFS) spectroscopy revealed a local structure in line with NpO2-like nanoparticles with diameter < 1 nm, a result further corroborated by high-resolution transmission electron microscopy (HRTEM). The low solubility of these NpO2-like nanoparticles (∼10-9M), along with their negligible surface charge at neutral pH conditions which favors particle aggregation, suggest an efficient retention of Np in the near-field of radioactive waste repositories. When NpVwas added to ferrous carbonate solution, the subsequent precipitation of siderite did not lead to a structural incorporation of NpIVby siderite, but caused precipitation of a NpIVpentacarbonate phase.
authors: Institute of Resource Ecology, Helmholtz-Zentrum Dresden - Rossendorf, D-01314, Germany.|The Rossendorf Beamline at ESRF, F-38043 Grenoble, France.
authors: Andreas C
authors: AC
authors: Scheinost
authors: Institute of Resource Ecology, Helmholtz-Zentrum Dresden - Rossendorf, D-01314, Germany.
authors: Robin
authors: R
authors: Steudtner
authors: Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden - Rossendorf, D-01314, Germany.
authors: René
authors: R
authors: Hübner
authors: Institute of Resource Ecology, Helmholtz-Zentrum Dresden - Rossendorf, D-01314, Germany.
authors: Stephan
authors: S
authors: Weiss
authors: Institute of Resource Ecology, Helmholtz-Zentrum Dresden - Rossendorf, D-01314, Germany.
authors: Frank
authors: F
authors: Bok
mesh_terms: D002119:Calcium Carbonate / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry; D053758:Nanoparticles; D009405:Neptunium / Q000737:chemistry*; D010084:Oxidation-Reduction; D056928:X-Ray Absorption Spectroscopy
chemical_list: D008903:Minerals; D009405:Neptunium; D002119:Calcium Carbonate