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.

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Sequestration of CO2 in geologic reservoirs is one of the promising technologies currently being explored to mitigate anthropogenic CO2 emissions. Large-scale deployment of geologic sequestration will require seals with a cumulative area amounting to hundreds of square kilometers per year and will require a large number of sequestration sites. We are developing a system-level model, CO2-PENS, that will predict the overall performance of sequestration systems while taking into account various processes associated with different parts of a sequestration operation, from the power plant to sequestration reservoirs to the accessible environment. The adaptability of CO2-PENS promotes application to a wide variety of sites, and its level of complexity can be increased as detailed site information becomes available. The model CO2-PENS utilizes a science-based-prediction approach by integrating information from process-level laboratory experiments, field experiments/observations, and process-level numerical modeling. The use of coupled process models in the system model of CO2-PENS provides insights into the emergent behavior of aggregate processes that could not be obtained by using individual process models. We illustrate the utility of the concept by incorporating geologic and wellbore data into a synthetic, depleted oil reservoir. In this sequestration scenario, we assess the fate of CO2 via wellbore release and resulting impacts of CO2 to a shallow aquifer and release to the atmosphere.

Subsurface injection of CO2 for enhanced hydrocarbon recovery, hydraulic fracturing of unconventional reservoirs, and geologic carbon sequestration produces a complex geochemical setting in which CO2-dominated fluids containing dissolved water and organic compounds interact with rocks and minerals. The details of these reactions are relatively unknown and benefit from additional experimentally derived data. In this study, we utilized an in situ X-ray diffraction technique to examine the carbonation reactions of forsterite (Mg2SiO4) during exposure to supercritical CO2 (scCO2) that had been equilibrated with aqueous solutions of acetate, oxalate, malonate, or citrate at 50 °C and 90 bar. The organics affected the relative abundances of the crystalline reaction products, nesquehonite (MgCO3 · 3H2O) and magnesite (MgCO3), likely due to enhanced dehydration of the Mg(2+) cations by the organic ligands. These results also indicate that the scCO2 solvated and transported the organic ligands to the forsterite surface. This phenomenon has profound implications for mineral transformations and mass transfer in the upper crust.

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.