Neptunium^V Retention by Siderite under Anoxic Conditions: Precipitation of NpO₂–Like Nanoparticles and of Np^IV Pentacarbonate

The Np^V retention by siderite, an Fe^II carbonate 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 Np^VO₂⁺ across pH 7 to 13 as expressed by solid-water distribution coefficients, log <i>R</i>_d, > 5, similar to the log <i>R</i>_d determined for the (solely) tetravalent actinide Th on calcite, suggesting reduction of Np^V to Np^IV by 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^–9 M), 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 Np^V was added to ferrous carbonate solution, the subsequent precipitation of siderite did not lead to a structural incorporation of Np^IV by siderite, but caused precipitation of a Np^IV pentacarbonate phase