NeptuniumV Retention by Siderite under Anoxic Conditions: Precipitation of NpO2-Like Nanoparticles and of NpIV 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 (NpO2+)-O-V 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 NPIV by siderite. Np L-3-edge X-ray absorption near edge ()CANES) 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 (similar to 10(-9) M), along with their negligible surface charge at neutral pH conditions which favors particle aggregation, suggest an efficient retention of Np 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.