Neptunium V Retention by Siderite under Anoxic Conditions: Precipitation of NpO 2 -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 ^V O ₂ ⁺ across pH 7 to 13 as expressed by solid-water distribution coefficients, log R _d, > 5, similar to the log R _d determined for the (solely) tetravalent actinide Th on calcite, suggesting reduction of Np ^V to Np ^IV by siderite. Np L ₃ -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 NpO ₂ -like nanoparticles with diameter < 1 nm, a result further corroborated by high-resolution transmission electron microscopy (HRTEM). The low solubility of these NpO ₂ -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.