[en] Understanding the geochemical mobility of U(VI) and modeling its transport is important in several contexts including ore genesis, uranium exploration, nuclear and mill-tailings waste management, and solution mining of uranium ores. Adsorption is a major control on partitioning of solutes at the mineral/solution interface. The effect of carbonate, fluoride, and phosphate complexing on adsorption of uranium was investigated. A critical compilation of stability constants of inorganic complexes and solid compounds of U(VI) necessary for proper design of experiment and for modeling transport of uranium was prepared. The general features of U(VI) adsorption in ligand-free systems are similar to those characteristic of other hydrolyzable metal ions. The adsorption processes studied were found to be reversible. The adsorption model developed in ligand-free systems, when solution complexing is taken into account, proved remarkably successful in describing adsorption of uranium in the presence of carbonate and fluoride. The presence of phosphate caused a much smaller decrease in the extent of adsorption than expected; however, a critical reassessment of the stability of UO₂²⁺.HPO₄^2- complexes, showed that phosphato complexes, if any, are extremely weak under experimental conditions. Removal of uranium may have occurred due to precipitation of sodium uranyl phosphates in addition to adsorption

[en] This report presents the development of a hydrogeochemical transport model for multicomponent systems. The model is designed for applications to proper hydrological setting, accommodation of complete suite of geochemical equilibrium processes, easy extension to deal with chemical kinetics, and least constraints of computer resources. The hydrological environment to which the model can be applied is the heterogeneous, anisotropic, saturated-unsaturated subsurface media under either transient or steady state flow conditions. The geochemical equilibrium processes included in the model are aqueous complexation, adsorption-desorption, ion exchange, precipitation-dissolution, redox, and acid-base reactions. To achieve the inclusion of the full complement of these geochemical processes, total analytical concentrations of all chemical components are chosen as the primary dependent variables in the hydrological transport equations. Attendant benefits of this choice are to make the extension of the model to deal with kinetics of adsorption-desorption, ion exchange, precipitation-dissolution, and redox relatively easy. To make the negative concentrations during the iteration between the hydrological transport and geochemical equilibrium least likely, an implicit form of transport equations are proposed. To alleviate severe constraints of computer resources in terms of central processing unit (CPU) time and CPU memory, various optional numerical schemes are incorporated in the model. The model consists of a hydrological transport module and geochemical equilibrium module. Both modules were thoroughly tested in code consistency and were found to yield plausible results. The model is verified with ten examples. 79 refs., 21 figs., 17 tabs

[en] Reference is made to a paper by D. Langmuir (Geochim. Cosmochim. Acta; 42:547 (1978)). Comments thereon are summarized as follow: 'The necessity of considering protonation and complexation of weak acid ligands, for proper interpretation of their effect on solution equilibria, has been demonstrated. The solubility of uraninite in presence of ΣF = 2 ppm has been recalculated to be 0.88 ppb at pH2.' These comments are accepted by the previous author, with some further qualification. (U.K.)

No abstract available

[en] Requirements for applying the surface-complexation modeling approach to simulating radionuclide adsorption onto geologic materials are discussed. Accurate description of adsorption behavior requires that chemical properties of both adsorbent and adsorbate be characterized in conjunction with determinations of extent of adsorption. Critical chemical properties of adsorbents include dissolution and oxidation/reduction behavior, types and densities of adsorption sites, and interaction of sites with solution components. Important adsorbate properties include hydrolysis, complexation, oxidation/reduction, and oligomerization. Adsorption behavior is described by a set of chemical reactions and binding constants between: adsorption sites and solution components, adsorbate and solution components, and adsorbate and adsorption sites. Methods for implementing such an approach are discussed;examples based on solute adsorption onto oxides are presented. The approach currently used to simulate sorption onto geologic materials, i.e., the determination of distribution coefficients, yields estimates that are disparate and subject to large errors. Implementation of the surface-complexation modeling approach would greatly improve the predictability of the role of adsorption in regulating radionuclide transport in subsurface environments. Research efforts should be directed towards understanding radionuclide adsorption onto fixed-charge minerals (e.g., clays), carbonate minerals, and poly-mineralic assemblages representative of those present at potential repositories. 123 refs., 19 figs., 18 tabs

[en] Dongarra and Langmuir (1980) recently published stability constants for the UO₂(HPO₄)₂^2- complex based on potentiometric titrations (pH measurements). Two aspects of their experimental systems, namely super-saturation with respect to uranyl tri-phosphate, and the degree of formation of UO₂(HPO₄)₂^2- and the magnitude of pH change during titration are discussed. (author)

[en] Numerous studies suggest, explicitly or implicitly that K_d's can be successfully used to model the transport of radionuclides away from proposed nuclear waste repositories as part of performance assessment calculations. Indeed, most ongoing experimental investigations of radionuclide retardation rely exclusively on the K_d, or at best the sorption isotherm, approach in systems thought to be representative of potential sites. The assumption that low (but nonzero) K_d values for all radionuclides may provide conservative estimates by underestimating the extent of contaminant retardation may not be correct for all geochemical settings. While acquiring perfect geochemical understanding of relevant systems may be neither necessary nor practical for designing repositories, it is certainly advantageous to investigate the extent to which K_d-based calculations may be good enough for performance assessment. The results of geochemical and hydrogeochemical simulations employing the K_d approach and a comprehensive approach explicitly accounting for geochemical processes for several selected scenarios will be presented. The intent of the simulations is to present enough quantitative information to allow context-specific decisions by the audience as to whether K_d would be acceptable. The results in general are applicable to a variety of candidate sites; however, specific examples are limited to tuff systems

[en] Gamma irradiation route has been explored for reduction of aqueous vanadium solutions. In situ generated 2-hydroxy-2-propyl radical has been evaluated for reduction of aqueous V(V). The process was studied for a monoprotic acid media, mixture of mono and polyprotic acid media and only polyprotic acid media. Precipitation of V(IV) occurred in the monoprotic acid media while the reduction to soluble lower oxidation state vanadium species could be obtained in both polyprotic media. (author)

[en] The present study has been undertaken with the purpose of optimizing the process of synthesis of vanadous based stainless steel compatible reagent. We have electro-synthesized and characterized a precursor which can be used for a single step electrolysis to the final product

[en] Painted structural surfaces inside a containment building are the major source of volatile organic compounds. Under severe nuclear accidental conditions, these would be exposed to a high radiation dose which would eventually degrade their molecular structural integrity and finally resulting in a source of organic iodides. In this work, the structural changes occurring in paint exposed to different total gamma doses upto 1000 kGy have been explored using ATR-IR spectroscopic analysis. With different absorbed doses, different scenario due to competitive chain-scission and chain-crosslinking reactions has been observed. (author)