The valence state of neptunium, one of the most important radionuclides of concern for long-term emplacement of nuclear waste, primarily defines its geochemical reactions and migration behavior. We evaluate how redox potential and solid-phase stability interact and influence neptunium solubility and aqueous speciation in natural systems. Neptunium thermodynamic data for the most important valence states for natural waters, +IV and +V, are updated to correct database inconsistencies. The most significant changes are as follows:  (1) Np2O5(cr) is 2 orders of magnitude more stable than reported previously, (2) the stability of NpO2OH(aq) is reduced, (3) NpO2(OH)2 - and mixed Np(V) hydroxo-carbonato species become important at high pH, and (4) Np(OH)5 - is disregarded as a valid species. As a result, Np2O5 and Np(OH)4(am) are the stable solids in aquifers of low ionic strength, neptunium solubility decreases in the pH range 10−12 and increases at pH above 12, and both redox potential and Np(OH)4(am) solubility product control soluble neptunium concentrations at neutral pH and Eh between −0.2 and 0.3. These relationships are important for effective nuclear waste package design, such as including cement as an engineered barrier and evaluating impacts of discharged solutions on natural waters in release scenarios at nuclear waste storage facilities.

Pu L(3) X-ray near edge absorption spectra for Pu(0-VII) are reported for more than 60 chalcogenides, chlorides, hydrates, hydroxides, nitrates, carbonates, oxy-hydroxides, and other compounds both as solids and in solution, and substituted in zirconolite, perovskite, and borosilicate glass. This large database extends the known correlations between the energy and shape of these spectra from the usual association of the XANES with valence and site symmetry to higher order chemical effects. Because of the large number of compounds of these different types, a number of novel and unexpected behaviors are observed, such as effects resulting from the medium and disorder that can be as large as those from valence.

5729 4. Redox Potentials and Thermodynamic Properties 5730 4.1. Am(IV)/(III) 5730 4.2. Am(V)/Am(III) 5731 4.3. Am(VI)/Am(III) 5731 4.4. Am(VI)/(V) 5731 4.5. Am(VII)/(VI) 5731 4.6. Thermodynamic data for Am(IV), Am(V), and

Solubilities of neptunium and plutonium were studied in J-13 groundwater (ionic strength of about 3.7 mmol; total dissolved carbonate of 2.8 mmol) from the proposed Yucca Mountain Nuclear Waste Repository site, Nevada, at three different temperatures (25, 60, and 90 °C) and pH values (6.0, 7.0, and 8.5). Experiments were performed from both over-and undersaturation at defined CO 2 partial pressures. The solubility of 237 Np from oversaturation ranged from a high of (9.40 ( 1.22) × 10 -4 M at pH 6.0 and 60 °C to a low of (5.50 ( 1.97) × 10 -6 M at pH 8.5 and 90 °C. The analytical results of solubility experiments from undersaturation (temperatures of 25 and 90 °C and pH values 6, 7, and 8.5) converged on these values. The 239/240 Pu solubilities ranged from (4.70 ( 1.13) × 10 -8 M at pH 6.0 and 25 °C to (3.62 ( 1.14) × 10 -9 M at pH 8.5 and 90 °C. In general, both neptunium and plutonium solubilities decreased with increasing pH and temperature. Greenishbrown crystalline Np 2 O 5 ‚xH 2 O was identified as the solubility-limiting solid using X-ray diffraction. A mean thermodynamic solubility product for Np 2 O 5 ‚xH 2 O of log K°s p ) 5.2 ( 0.8 for the reaction Np 2 O 5 ‚xH 2 O + 2 H + h 2NpO 2 + + (x+1)H 2 O at 25 °C was calculated. Sparingly soluble Pu(IV) solids, PuO 2 ‚xH 2 O and/or amorphous plutonium(IV) hydroxide/colloids, control the solubility of plutonium in J-13 water.

Room-temperature ionic liquids (RTILs) are regarded as green solvents due to their low volatility, low flammability, and thermal stability. RTILs exhibit wide electrochemical windows, making them prime candidates as media for electrochemically driven reactions such as electro-catalysis and electro-plating for separations applications. Therefore, understanding the factors determining edges of the electrochemical window, the electrochemical stability of the RTILs, and the degradation products is crucial to improve the efficiency and applicability of these systems. We present here computational investigations of the electrochemical properties of a variety of RTILs covering a wide range of electrochemical windows. We proposed four different approaches with different degrees of approximation and computational cost from gas-phase calculations to full explicit solvation models. It was found that, whereas the simplest model has significant flaws in accuracy, implicit and explicit solvent models can be used to reliably predict experimental data. The general trend of electrochemical windows of the RTILs studied is well reproduced, showing that it increases in the order of imidazolium < ammonium < pyrrolidinium < phosphonium giving confidence to the methodology presented to use it in screening studies of ionic liquids.

Pu L(3) X-ray absorption fine structure spectra from 24 samples of PuO(2+x) (and two related Pu-substituted oxides), prepared by a variety of methods, demonstrate that (1) although the Pu sublattice remains the ordered part of the Pu distribution, the nearest-neighbor O atoms even at x = 0 are found in a multisite distribution with Pu-O distances consistent with the stable incorporation of OH(-) (and possibly H(2)O and H(+)) into the PuO(2) lattice; (2) the excess O from oxidation is found at Pu-O distances <1.9 A, consistent with the multiply bound "oxo"-type ligands found in molecular complexes of Pu(V) and Pu(VI); (3) the Pu associated with these oxo groups is most likely Pu(V), so that the excess O probably occurs as PuO(2)(+) moieties that are aperiodically distributed through the lattice; and (4) the collective interactions between these defect sites most likely cause them to cluster so as give nanoscale heterogeneity in the form of domains that may have unusual reactivity, observed as sequential oxidation by H(2)O at ambient conditions. The most accurate description of PuO(2) is therefore actually PuO(2+x-y)(OH)(2)(y).zH(2)O, with pure, ordered, homogeneous PuO(2) attained only when H(2)O is rigorously excluded and the O activity is relatively low.

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Intra- and intermolecular force field parameters for the interaction of actinyl ions (AnO2(n+), where, An = U, Np, Pu, Am and n = 1, 2) with water have been developed using quantum mechanical calculations. Water was modeled with the extended simple point charge potential (SPC/E). The resulting force field consists of a simple form in which intermolecular interactions are modeled with pairwise Lennard-Jones functions plus partial charge terms. Intramolecular bond stretching and angle bending are treated with harmonic functions. The new potentials were used to carry out extensive molecular dynamics simulations for each hydrated ion. Computed bond lengths, bond angles and coordination numbers agree well with known values and previous simulations. Hydration free energies, computed from molecular dynamics simulations as well as from quantum simulations with a solvation model, were in reasonable agreement with estimated experimental values.