Hot New Papers from LES
Water retention and diffusion in unsaturated clays: Connecting atomistic and pore scale simulations
Molecular diffusion is the dominant solute transport process in clays and claystones that are considered as sealing materials in the deep geological disposal of radioactive waste. These materials are typically water saturated, but during construction and later, at elevated temperatures or when gas may be produced, unsaturated conditions prevail. Investigating the clay's water retention properties as well as solute transport under unsaturated conditions is therefore mandatory. Here, functional dependencies of these properties were derived from atomistic and pore-scale simulations. In the absence of tomographic maps that resolve all pores in clays, model clay structure maps with different pore size distributions were generated using a previously developed algorithm. Upscaled water retention functions and upscaled diffusion coefficients of unsaturated samples were derived from these maps based on the shifted Young-Laplace equation that considers film adsorption and capillary condensation. Pore-scale parameters (water film thickness, diffusion coefficients) used for the upscaling were taken from Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, thus connecting molecular and pore-scale simulations. We focused on effects of the pore size distribution and of the adsorbed water film on upscaled parameters. Sample-scale diffusion coefficients were clearly reduced in unsaturated samples compared to the saturated state, with less reduction when including adsorbed water films. The reduction was stronger in samples with a narrow size distribution of the interparticle pores as compared to those with a wide distribution (but equal mean size). The results follow the trends of the experimental data, even though the scale of the simulations is still clearly smaller than that of typical experiments.
Modelling Ra-bearing baryte nucleation/precipitation kinetics at the pore scale: application to radioactive waste disposal
Baryte is of interest to nuclear waste disposal as the main scavenger of 226Ra, a long-lived nuclide playing a major role in the safety assessment of planned disposal sites. In specific repository setups, Ba and Ra released from the nuclear waste will react with sulphate-rich pore water, potentially leading to formation of Ra-bearing baryte. Baryte has a complex kinetic behaviour and its precipitation may strongly be inhibited. Because highly supersaturated solutions may persist metastably, it can be anticipated that the migration of Ra through the repository near-field will strongly depend on parameters related to nucleation and precipitation kinetics, so that thermodynamic equilibrium calculations will not be sufficient for a reliable prediction of 226Ra mobility. In this study, we implement Classical Nucleation Theory (CNT) and a saturation-state dependent precipitation rate equation into a Lattice-Boltzmann (LB) reactive transport code to model Ra-bearing baryte precipitation within a porous medium analogous to fragmented nuclear waste glass. In the simulations, baryte precipitation is induced by counter-diffusion of BaCl2 and Na2SO4 solutions. Radium co-precipitation is taken into account by assuming a fixed partition coefficient and constant Ra concentration at the BaCl2 injection boundary. Both homogeneous and heterogeneous growth were considered. Critical CNT parameters, particularly supersaturation-dependent induction times, were calibrated against independent turbidity and X-ray absorption experiments. The model allows exploring the influence of baryte nucleation/precipitation kinetics on the partitioning of Ra between aqueous phase and solid at the pore (micrometre) scale. Our results indicate that quantitative knowledge of kinetic and nucleation parameters is essential to predict radionuclide transport towards the geosphere in nuclear waste repository systems.
Formation of low-molecular-weight organic compounds during anoxic corrosion of zero-valent iron
Speciation of carbon during the anoxic corrosion of steel is poorly known, whereas its knowledge would be of great importance in connection with assessments of the safe disposal of 14C-containing irradiated steel in repositories for radioactive waste. The chemical form of the 14C-bearing organic compounds determines routes of migration from engineered barrier systems and their reactivity at disposal sites. Batch-type corrosion experiments with unirradiated iron powders reported in this study show for the first time that both reduced and oxidized carbon species are present in corroding iron-water systems in anoxic conditions. Methane and volatile C2–C4 alkanes and alkenes were produced during the course of corrosion whereas formate, acetate, and oxalate were released to solution in the early stage of the corrosion process. Evidence is provided that reduced and oxidized hydrocarbons were produced by two different processes. Formation of reduced hydrocarbons occurred at the surface of iron particles by a Fischer-Tropsch-type mechanism, whereas oxidized hydrocarbons were produced in the course of oxidative pretreatment of iron particles and released instantaneously from the surface in contact with alkaline solution. Results from this study have implications for safety assessments of radioactive waste disposal sites as they suggest predominant formation of alkanes and alkenes during anoxic steel corrosion and instantaneous release of only a small fraction of carbide carbon as oxidized hydrocarbons.
Quantification of dissolved organic 14C-containing compounds by accelerator mass spectrometry in a corrosion experiment with irradiated steel
Low- and intermediate-level (L/ILW) radioactive waste produced in Switzerland consists of large amounts of 14C-containing irradiated steel. 14C will be released during the anoxic corrosion of the steel in the cementitious near field of an L/ILW repository. In this study, a corrosion experiment with irradiated steel was carried out to determine the speciation of 14C released during the corrosion process in conditions similar to those anticipated in the near field of a cement-based repository. The development of the experimental setup, including installation of the reactor and development of suitable analytical methods based on compound-specific 14C analysis with accelerator mass spectrometry (CSRA AMS) is reported. Time-dependent increase in the total content of 14C-bearing organic compounds in solution (TO14C) was determined by AMS and the main organic corrosion products that are 14C bearing formate, acetate and lactate were identified by CSRA AMS after a pre-concentration step. The concentration of the 14C-bearing organic compounds was found to be very low (fmol to pmol 14C/L). Stable carbon compounds were identified and quantified while the source of stable carbon in the system has not yet been identified and the temporal evolution of the concentration of these carbon species is presently not understood.
Analysis of 14C-bearing compounds released by the corrosion of irradiated steel using accelerator mass spectrometry
The combination of ion chromatography (IC) with accelerator mass spectrometry (AMS) was developed to determine the speciation of 14C-(radiocarbon) bearing organic compounds in the femto to pico molar concentration range. The development of this compound-specific radiocarbon analysis (CSRA) of carboxylic acids is reported and the application of the method on a leaching solution from neutron-irradiated steel is demonstrated. The background and the dynamic range of the AMS-based method were quantified. On using 14C-labelled standards, the measurements demonstrate the repeatability of the analytical method and the reproducible recovery of the main target carboxylic acids (i.e., acetate, formate, malonate, and oxalate). The detection limit was determined to be in the mid fmol 14C per L level while the dynamic range of the analytical method covers three orders of magnitude from the low fmol to the mid pmol 14C per L level. Cross contamination was found to be negligible during IC fractionation and was accounted for during eluate processing and 14C detection by AMS. The 14C-bearing carboxylates released from an irradiated steel nut into an alkaline leaching solution were analysed using the CSRA-based analytical method with the aim to check the applicability of the approach and develop appropriate sample preparation. The concentrations of 14C-bearing formate and acetate, the main organic corrosion products, were at a low pmol 14C per L level for convenient dimensions of the alkaline leaching experiment which demonstrates that compound-specific 14C AMS is an extremely sensitive analytical method for analyzing 14C-bearing compounds. The content of total organic 14C in solution (TO14C) determined by the direct measurement of an aliquot of the leaching solution agrees well with the sum of the 14C concentrations of the individual carboxylates within the uncertainty of the data. Furthermore, the TO14C content is in good agreement with the calculated value using the corrosion rate determined from the 60Co release and the 14C inventory of the irradiated steel specimen.
Multiscale modeling of ion diffusion in cement paste: electrical double layer effects
3 Quantum-chemical modelling of clay mineral surfaces and clay mineral–surface–adsorbate interactions (book chapter)
This chapter starts with a short overview of the methods of quantum chemistry currently applied to the simulations of clay minerals. Theoretical equations are intentionally excluded. The focus is on the physical rationale behind the methods, assumptions applied, and their consequences for the interpretation of the results. For the theoretical details of the methods, the reader is directed to the specialized text books and review articles provided as references. The second part of the chapter deals with the applications. The chapter starts with the description of the bulk crystal structure, continues with the structural properties of the surfaces and surface-fluid interfaces, and concludes with thermodynamic and structural aspects of adsorption.
5 Adsorption of heavy metals including radionuclides (book chapter)
Carbonate dissolution mechanisms in the presence of electrolytes revealed by grand canonical and kinetic Monte Carlo modeling
Review of the current status and challenges for a holistic process-based description of mass transport and mineral reactivity in porous media
The diffusion of SO42- in Opalinus Clay: Measurements of effective diffusion coefficients and evaluation of their importance in view of microbial mediated reactions in the near field of radioactive waste repositories
A comparative anion diffusion study on different argillaceous, low permeability sedimentary rocks with various pore waters
The influence of small pores on the anion transport properties of natural argillaceous rocks – A pore size distribution investigation of Opalinus Clay and Helvetic Marl
Effect of the pore water composition on the diffusive anion transport in argillaceous, low permeability sedimentary rocks
Simulating Donnan equilibria based on the Nernst-Planck equation
Retention of selenium by calcium aluminate hydrate (AFm) phases under strongly-reducing radioactive waste repository conditions
Transport of low molecular weight organic compounds in compacted illite and kaolinite
An internally consistent thermodynamic dataset for aqueous species in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl to 800 ˚C and 5 kbar
The stability of major aqueous complexes at elevated temperatures and pressures was constrained using selected reaction constant data (for example plot A and B). The Gibbs energy of formation of aqueous ions and complexes was simultaneously optimized with GEMSFITS against a large selection of solubility experiments over a wide range of conditions, taking the standard properties of minerals (unmodified) from the Holland and Powell internally consistent database (for example, plot C). The resulting thermodynamic dataset is consistent with the complex formation data, with the mineral solubility experiments, and with the standard properties of minerals from Holland and Powell database. The internally consistent dataset can be used to model natural fluid-rock interaction (for example, plot D).
Comparison of calculated and measured experimental data for: (A) the stability constant of HCO3- as function of pressure at temperatures of 55, 150 and 250 °C; (B) the association constant of CaCl+ as function of temperature at saturated water vapor pressure; (C) calcite solubility in NaCl solutions at 400 °C; (D) log(Ca/Mg) molar ratios from sedimentary fluids in equilibrium with calcite and disordered dolomite, at temperatures of 50 to 150 °C and at saturated water vapor pressure.