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

Uranium(VI) Chemistry in Strong Alkaline Solution: Speciation and Oxygen Exchange Mechanism

The mechanism by which oxygen bound in UO₂ ²⁺ exchanges with that from water under strong alkaline conditions remains a subject of controversy. Two recent NMR studies independently revealed that the key intermediate species is a binuclear uranyl­(VI) hydroxide, presumably of the stoichiometry [(UO₂(OH)₄ ^2–)­(UO₂(OH)₅ ^3–)]. The presence of UO₂(OH)₅ ^3– in highly alkaline solution was postulated in earlier experimental studies, yet the species has been little characterized. Quantum-chemical calculations (DFT and MP2) show that hydrolysis of UO₂(OH)₄ ^2– yields UO₃(OH)₃ ^3– preferentially over UO₂(OH)₅ ^3–. X-ray absorption spectroscopy was used to study the uranium­(VI) speciation in a highly alkaline solution supporting the existence of a species with three U–O bonds, as expected for UO₃(OH)₃ ^3–. Therefore, we explored the oxygen exchange pathway through the binuclear adduct [(UO₂(OH)₄ ^2–)­(UO₃(OH)₃ ^3–)] by quantum-chemical calculations. Assuming that the rate-dominating step is proton transfer between the oxygen atoms, the activation Gibbs energy for the intramolecular proton transfer within [(UO₂(OH)₄ ^2–)­(UO₃(OH)₃ ^3–)] at the B3LYP level was estimated to be 64.7 kJ mol^–1. This value is in good agreement with the activation energy for “yl”–oxygen exchange in [(UO₂(OH)₄ ^2–)­(UO₂(OH)₅ ^3–)] obtained from experiment by Szabó and Grenthe (<i>Inorg. Chem.</i> <b>2010</b>, <i>49</i>, 4928–4933), which is 60.8 ± 2.4 kJ mol^–1. Both the presence of UO₃(OH)₃ ^3– and the scenario of an “yl”–oxygen exchange through a binuclear species in strong alkaline solution are supported by the present study

Speciation Studies of Metals in Trace Concentrations: The Mononuclear Uranyl(VI) Hydroxo Complexes

A direct luminescence spectroscopic experimental setup for the determination of complex stability constants of mononuclear uranyl­(VI) hydrolysis species is presented. The occurrence of polynuclear species is prevented by using a low uranyl­(VI) concentration of 10^–8 M (2.4 ppb). Time-resolved laser-induced fluorescence spectra were recorded in the pH range from 3 to 10.5. Deconvolution with parallel factor analysis (PARAFAC) resulted in three hydrolysis complexes. A tentative assignment was based on thermodynamic calculations: UO₂²⁺, UO₂(OH)⁺, UO₂(OH)₂, UO₂(OH)₃^–. An implementation of a Newton–Raphson algorithm into PARAFAC allowed a direct extraction of complex stability constants during deconvolution yielding log­(β_1M,1°C)_1:1 = −4.6, log­(β_1M,1°C)_1:2 = −12.2, log­(β_1M,1°C)_1:3 = −22.3. Extrapolation to standard conditions gave log­(β⁰)_1:1 = −3.9, log­(β⁰)_1:2 = −10.9, and log­(β⁰)_1:3 = −20.7. Luminescence characteristics (band position, lifetime) of the individual mononuclear hydroxo species were derived to serve as a reference data set for further investigations. A correlation of luminescence spectroscopic features with Raman frequencies was demonstrated for the mononuclear uranyl­(VI) hydroxo complexes for the first time. Thereby a signal-to-structure correlation was achieved and the complex assignment validated

The Sorption Processes of U(VI) onto SiO₂ in the Presence of Phosphate: from Binary Surface Species to Precipitation

The ternary system containing aqueous U­(VI), aqueous phosphate and solid SiO₂ was comprehensively investigated using a batch sorption technique, in situ attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy, time-resolved luminescence spectroscopy (TRLS), and surface complexation modeling (SCM). The batch sorption studies on silica gel (10 g/L) in the pH range 2.5 to 5 showed no significant increase in U­(VI) uptake in the presence of phosphate at equimolar concentration of 20 μM, but significant increase in U­(VI) uptake was observed for higher phosphate concentrations. In situ infrared and luminescence spectroscopic studies evidence the formation of two binary U­(VI) surface species in the absence of phosphate, whereas after prolonged sorption in the presence of phosphate, the formation of a surface precipitate, most likely an autunite-like phase, is strongly suggested. From SCM, excellent fitting results were obtained exclusively considering two binary uranyl surface species and the formation of a solid uranyl phosphate phase. Ternary surface complexes were not needed to explain the data. The results of this study indicate that the sorption of U­(VI) on SiO₂ in the presence of inorganic phosphate initially involves binary surface-sorption species and evolves toward surface precipitation