Hot New Papers from LES
Generating isocoulombic reactions as a tool for systematic evaluation of temperature trends of thermodynamic properties: Application to aquocomplexes of lanthanides and actinides
Most of the available thermodynamic data concerning radioactive waste disposal are restricted to values of reaction equilibrium constants (logKo298) at 25 oC and 1 bar. Simple estimation methods such as isocoulombic reactions can be used for extrapolating the properties of reactions involving aqueous species and minerals to elevated temperatures. The aim of this study was to validate the applicability of various alternative isocoulombic reactions to estimate logKoT values of aqueouscomplexation reactions for lanthanides and actinides to elevated temperatures while taking advantage of new additional literature data, and to identify criteria for choosing the ‘‘best” reactions. For each chemical species of interest, a systematic approach using dedicated software and database allowed us to identify the isocoulombic reactions and types of extrapolation that yield the best estimates of standard thermodynamic properties at elevated temperatures, when very limited or no experimental data are available. We have tested aqueous complexation reactions for selected lanthanides and actinides of different valences with chloride, fluoride, sulfate, carbonate, nitrate, phosphate and silicate ligands. ‘‘Model” complexation reactions, having known temperature trends, were systematically combined with complex formation reactions of interest whose temperature trends are unknown, into many alternative isocoulombic reactions. For each ion, we investigated which of the generated isocoulombic reactions provide the best estimates for logeKoT of the reaction of interest at elevated temperatures in order to compile the guidelines for choosing the optimal ones, then applying these guidelines to ‘‘prediction” subsets. In most cases, knowing only logeKoT at 25 oC (for the reaction of interest), it was possible to obtain rather accurate estimates of logeKoT values at elevated temperatures using isocoulombic reactions that exchange ions with similar charge and hydration properties (hydrated ionic radius and structure of the hydration shell) and known logmKoT of model reactions. These ions and their complexes interact with the solvent in comparable ways, so that their similar heat capacity and entropy effects largely cancel out on both sides of an ‘‘optimal” isocoulombic reaction.
Fe(III) uptake by calcium silicate hydrates
Wet chemistry and spectroscopic investigations were conducted to study Fe(III) uptake by calcium silicate hydrate (C–S–H) at different Ca/Si ratios. Wet chemistry experiments were carried out by using a 55Fe radiotracer while 29Si NMR and XAS spectroscopy were performed on C–S–H phases loaded with different Fe(III) concentrations. Sorption kinetics experiments indicate that equilibrium was attained within 30 days. Over the studied concentration range, Fe(III) sorption was linear, irrespective of the difference in pH of the suspension and the Ca/Si of C–S–H. In addition, Fe(III) sorption on C–S–H phases was significantly stronger than Al(III) sorption. The total Fe(III) uptake by C–S–H phases, however, was limited by the lower solubility of Fe(OH)3 compared to Al(OH)3; up to 1 mol Fe/kg C–S–H (molar Fe(III)/Si ≈ 0.001) could be taken up. 29Si NMR and EXAFS data suggest Fe(III) uptake in octahedral coordination into the interlayer of the C–S–H phases with Ca/Si ratios 1.2 and 1.5. However, such an uptake mechanism appears unlikely in the case of the C–S–H phase with Ca/Si 0.8 because structural parameters of Fe(III) deduced from EXAFS are different.
Processes leading to reduced and oxidised carbon compounds during corrosion of zero-valent iron in alkaline anoxic conditions
The Swiss disposal concept foresees that carbon-14 (14C) is predominantly released from irradiated steel disposed of in a cement-based repository for low- and intermediate-level radioactive waste. To predict how 14C migrates in the cementitious environment of the repository near field and subsequently in the host rock, knowledge about the carbon speciation during anoxic steel corrosion in alkaline conditions is therefore essential. To this end, batch-type corrosion experiments with carbon-containing zero-valent iron (ZVI) powders subject to oxidative pre-treatments were carried out in NaOH solution at pH 11 and 12.5. Alkanes and alkenes (C1–C7) were identified in the gas phase and produced on the iron surface by a Fischer-Tropsch type mechanism. The kind of oxidative pre-treatment has an effect on the production rate of hydrocarbons (HCs). In the liquid phase, carboxylic acids were identified and produced during the oxidative pre-treatment of the ZVI powders. They are released instantaneously from the oxide layer upon contact with the alkaline solution. The kind of oxidative treatment and the exposure time to oxic conditions directly influence the amount of carboxylic acids accommodated in the oxide layer.
A coherent approach for cation surface diffusion in clay minerals and cation sorption models: Diffusion of Cs+ and Eu3+ in compacted illite as case examples
The conceptual setup and the parametrisation of surface diffusion models for different types of cations in negatively charged clay minerals has still not been fully developed. In particular, the contribution of different types of surface-associated cationic species to the overall mass transfer rates is an open question. Further, the transferability of sorption data gained on dispersed clay suspensions to the compacted clay minerals or consolidated clay rocks remains also unanswered. This contribution presents experimental results for the diffusion of 134Cs+ and 152Eu3+ in and sorption on compacted illite (bulk-dry densities of 1900 and 1700 kg m−3, respectively) and their interpretation using thermodynamic and transport modelling. Different solution parameters, such as pH, ionic strength and the concentration of stable isotope background were varied. The results give information on both diffusional processes of the two radionuclides and their sorption behaviour at relevant solid-to-liquid ratios. The existing two site protolysis non electrostatic surface complexation and cation exchange (2SPNE-SC/CE) model turned out to be fully valid for the description of the equilibrium distribution of the test cations between the solution and the clay phase in the compacted state. The model had to be extended by an electrical double layer description of cationic species bound to the planar surfaces, involving mobile species in the diffuse layer, in order to successfully model the observed diffusion profiles. Despite the notable differences in sorption behaviour of Cs+ and Eu3+ on illite, their diffusional behaviour could thus be satisfyingly described using the same model approach.
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
Understanding the mechanism of ion diffusion in hardened cement paste is of great importance for predicting long-term durability of concrete structures. Gel pores in calcium silicate hydrate (CeSeH) phase forms dominant pathway for transport in cement paste with low w/c ratios where the electrical double layer effects play an important role. Experimental results suggest that the effective diffusivity of chloride ions is similar as that of tritiated water (HTO) and higher than the sodium ions. This difference can be attributed to the electrical double layer near the charged CeSeH surfaces. In order to understand species transport processes in CeSeH and to quantify its effective diffusivity, a multiscale modeling technique has been proposed to combine atomic-scale and pore-scale modeling. At the pore scale, the lattice Boltzmann method is used to solve a modified Nernst Planck equation to model transport of ions in gel pores. The modified Nernst Planck equation accounts for steric and ion-ion correlation effects by using correction term for excess chemical potential computed through the results from the grand canonical Monte Carlo scheme at atomic scale and in turn bridges atomic scale model with pore scale model. Quantitative analysis of pore size influence on effective diffusivity carried out by this multiscale model shows that the contribution of the Stern layer to ion transport is not negligible for pores with diameter less than 10 nm. The developed model is able to reproduce qualitatively the trends of the diffusivity of different ions reported in literature.
3 Quantum-chemical modelling of clay mineral surfaces and clay mineral–surface–adsorbate interactions (book chapter)
Atomistic simulations provide insight into the crystal structure of minerals, surfaces, and mineral-fluid interaction mechanisms. Such modelling has been successfully applied to better understand structural, thermodynamic, and transport properties of clay minerals, the thermodynamics of ions adsorption, and clay mineral surface reactivity at an atomic scale. In principle, quantum mechanics- based modelling allows system description without use of any empirical system-dependent parameters. In practice, however, the complete quantum mechanical description of the condensed matter is only feasible for small systems containing few atoms, due to the limitations of currently available computational resources. Therefore, the simulations of complex reactive processes rely on a number of approximations at different levels of theory. These approximations are chosen as a compromise between the computational accuracy and the ability to include the relevant chemical processes.
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)
Clay minerals are important adsorbents in soils and sediments for hazardous contaminants in the environment. A whole variety of adsorption models for clay minerals has been developed over the past few decades. In the first part of this chapter, a brief overview of existing models describing cation exchange and surface complexation is presented. The second part presents for a large number of heavy metals and radionuclides exhibiting oxidation states from + I to + VI experimental adsorption data onto montmorillonite (Mt) and illite. A quasimechanistic nonelectrostatic adsorption model is applied to describe quantitatively the uptake of these elements over a broad pH, background electrolyte, and adsorbate concentration range. In the third part, special focus is put on the adsorption behaviour of iron on Mt. Wherever possible, a multidisciplinary approach is followed, whereby the wet chemistry studies are complemented by modelling and spectroscopic investigations with the aim of validating the underlying assumptions in the adsorption model.
Carbonate dissolution mechanisms in the presence of electrolytes revealed by grand canonical and kinetic Monte Carlo modeling
Dissolution of carbonate minerals is a complex multistep process, characterized by the particular sequence of steps dependent on pH and background electrolyte concentration. Currently, available dissolution models for carbonates do not consider dependence of the surface speciation on the local surface topography. We have developed a new approach combining grand canonical Monte Carlo (GCMC) and kinetic Monte Carlo (KMC) methods to investigate the influence of water pH and electrolyte concentration onto processes of surface charging and dissolution of carbonates. GCMC simulations of the calcite−electrolyte system are used to calculate populations of protonated sites. We consider two basic speciation models characterized by different spatial charge distributions at the surface: “ionic”, where surface >CO32− sites are represented by “−2” charges at the corresponding lattice positions; and “oxygen”, where surface >CO32− sites are represented by triplets of “−2/3” charges at the positions of oxygen atoms. The speciation of carbonate ion protonation probabilities is found to be controlled by local charge densities and the presence of electrolyte species. In all simulation results, protonation affinity of the surface >CO32− sites followed the trend kink (most acidic) > step > terrace (least acidic), with the same trend observed with respect to adsorption probabilities of Cl− ions. The influence of protonated site concentrations obtained in GCMC simulations was investigated in KMC simulations. The direct comparison of simulated and experimental data showed that the oxygen model, with an assumption of congruent dissolution, reproduces both the pH dependence of the calcite dissolution rate and the morphology of the calcite surface. On the basis of the considered model, we could identify four key factors that define pH-dependent dissolution mechanisms of calcite: (1) increase of the kink site propagation rate at pH < 10; (2) increase of kink site generation frequency at pH 4−7; (3) increase of monolayer pit generation frequency at pH = 2−4; and (4) acceleration of kink site propagation and generation at pH 2−4 due to the second protonation step. The combined GCMC + KMC approach shows great potential in resolving surface speciation of carbonates as functions of solvent composition and surface geometry and their influence on the dissolution mechanisms and rates. Generally, this approach could potentially be applied to any other mineral−fluid system.
Review of the current status and challenges for a holistic process-based description of mass transport and mineral reactivity in porous media
Reactivity of minerals is controlled by chemical processes at mineral- fluid interfaces acting at different time- and length scales. Various modeling approaches are available to characterize scale-specific aspects of mineral-fluid interface chemistry. Most fundamental aspects of mineral reactivity are provided by atomic scale simulations. Several attempts have been made to interpret macroscopic observation based on atomic scale simulations alone. Many of them have failed however, because of neglecting the pore scale transport phenomena. Pore scale simulation, provide an elegant way to link idealized nanometer scale atomistic description of mineral reactivity with structural and compositional heterogeneities of natural systems. The main challenges are the spatial and temporal coupling of physical models and the upscaling of transport parameters for the macroscopic interpretation of the system behavior. This paper summarizes the current molecular-scale knowledge on mineral-fluid inter- face chemistry, obtained from complementary coarse-grain simulation approaches. Using the most recent developments in this field, we highlight the complexity and challenges of the pore-scale modeling and suggest a roadmap for the process-based description of mineral dissolution/precipitation across different scales.
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
Through-diffusion experiments with 36Cl-, 35SO42− and HTO in Opalinus Clay (OPA) samples from a deep borehole in North-East Switzerland (Benken; BE) have been performed. The effect of burial depth on the experimental results has been investigated. It could be shown that the effective diffusion coefficients decrease with sample depth for all three tracers. Moreover, there was a good correlation with the texture of the samples. The diffusion coefficients for HTO are the largest (De = 5.4–8.8 × 10−12 m2 s−1), followed by those for 36Cl- (De = 0.7–1.9 × 10−12 m2 s−1), and finally 35SO42− (De = 0.2–0.6 × 10−12 m2 s−1). 36Cl- was partially excluded from the total porosity resulting in an accessible porosity smaller than the total porosity (εCl = 0.041–0.064). 35SO42−, on the other hand, showed interaction with OPA resulting in a capacity factor (α) larger than the total porosity (εtot = 0.13–0.16). Using extended Archie's law the accessible porosity for 35SO42− was estimated between 0.013 and 0.030. This enabled to evaluate the sorption coefficient of 35SO42− from the measured capacity factor, resulting in values of Kd between 6 × 10−5 and 9 × 10−5 m3 kg−1.
A comparative anion diffusion study on different argillaceous, low permeability sedimentary rocks with various pore waters
Through-diffusion experiments with tritiated water (HTO) and 36Cl- as a function of pore water concentration (0.01–5 M) were performed on two Ordovician-age argillaceous rock samples from the Blue Mountain Fm and Queenston Fm shales of the Paleozoic intracratonic Michigan Basin in Canada. This study reveals that the effect of ionic strength on the anion-transport porosity is similar, and only the minimal anion excluded porosity is higher in the Blue Mountain Fm shale. The differences in rock sample mineralogy cannot explain this effect. It is hypothesized that the structure of the Blue Mountain Fm shale samples has led to pore space openings suffi- ciently small that they behave as interlayers. Such pores are defined as interlayer equivalent (ILE) pores. These ILE pores, as in the case of interlayer pores, can act to permanently limit the anion-accessible porosity. Pore-size distribution measurements provide further evidence of increased potential for ILE pores within the Blue Mountain Fm samples. A Donnan model, which includes consideration of both ILE and uncharged pores, is shown to describe the effect of molar concentration on the anion-accessible porosity in the argillaceous rocks investigated.
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
The pore size distribution of two natural argillaceous rock samples, Opalinus Clay (OPA) and Helvetic Marl (HM) was investigated with five different methods: NMR, NMR cryoporometry, mercury intrusion porosimetry and CO2 adsorption, as well as N2 adsorption. Due to different physical principles of these methods different ranges of pore width could be detected, from micropores (< 2 nm) to mesopores (2–50 nm) and macropores (> 50 nm). The aim was to shed light on the role of small pores on the transport properties of natural ar- gillaceous rocks, in particular to explain the differences of anion diffusion in the two argillaceous rock sam- ples. Knowing that Helvetic Marl exhibits a stronger anion exclusion than Opalinus Clay it was hypothesized that HM (with its smaller phyllosilicate and smectite content compared to OPA) has more interlayer equivalent (ILE) pores than OPA. ILE pores are defined as pores so narrow (< 0.5 nm) that diffuse double layers, formed at negatively charged surfaces, are overlapping. Accordingly, ILE pores behave similarly as interlayer pores and may block the anion diffusion. This study could not confirm the hypothesis that HM has more ILE pores. Similar pores size distributions were determined for both materials, even with a tendency of a larger fraction of small pores in OPA as compared to HM. However, all methods have limitations in the range of very small (nm) pores.
Effect of the pore water composition on the diffusive anion transport in argillaceous, low permeability sedimentary rocks
The effect of the pore water composition on the diffusive anion transport was studied for two different argillaceous, low permeability sedimentary rocks, Opalinus Clay (OPA) and Helvetic Marl (HM). The samples were saturated with different solutions with varying molar concentration and different main cations in the solution: NaCl based pore solutions and CaCl2 based pore solutions. The total porosity was measured by through-diffusion experiments with the neutral tracer HTO. Experiments performed in NaCl solutions resulted in a porosity of 0.12 for OPA and 0.03 for HM, and are consistent with results of the experiments in CaCl2 solutions. The total porosity was independent of the molar concentration, in contrast to the measured anion porosity, which increased with increasing molar concentration. It could further be observed that the pore solution based on the bivalent cation calcium shielded the negative surface charge stronger than the monovalent cation sodium, resulting in a larger measureable anion-accessible porosity in the case of CaCl2 solutions. The data was modelled based on an adapted Donnan approach of Birgersson and Karnland (2009). The model had to be adjusted with a permanent free, uncharged porosity, as well as with structural information on the permanent anion exclusion because of so-called bottleneck pores. Both parameters can only be evaluated from experiments. Nevertheless, taking these two adaptions into account, the effect of varying pore water compositions on the anion-accessible porosity of the investigated argillaceous rocks could be satisfactorily described.
Simulating Donnan equilibria based on the Nernst-Planck equation
Understanding ion transport through clays and clay membranes is important for many geochemical and environmental applications. Ion transport is affected by electrostatic forces exerted by charged clay surfaces. Anions are partly excluded from pore water near these surfaces, whereas cations are enriched. Such effects can be modeled by the Donnan approach. Here we introduce a new, comparatively simple way to represent Donnan equilibria in transport simulations. We include charged surfaces as immobile ions in the balance equation and calculate coupled transport of all components, including the immobile charges, with the Nernst-Planck equation. This results in an additional diffusion potential that influences ion transport, leading to Donnan ion distributions while maintaining local charge balance. The validity of our new approach was demonstrated by comparing Nernst-Planck simulations using the reactive transport code Flotran with analytical solutions available for simple Donnan systems. Attention has to be paid to the numerical evaluation of the electrochemical migration term in the Nernst-Planck equation to obtain correct results for asymmetric electrolytes. Sensitivity simulations demonstrate the influence of various Donnan model parameters on simulated anion accessible porosities. It is furthermore shown that the salt diffusion coefficient in a Donnan pore depends on local concentrations, in contrast to the aqueous salt diffusion coefficient. Our approach can be easily implemented into other transport codes. It is versatile and facilitates, for instance, assessing the implications of different activity models for the Donnan porosity.
Retention of selenium by calcium aluminate hydrate (AFm) phases under strongly-reducing radioactive waste repository conditions
Safety assessment studies of future nuclear waste repositories carried out in many countries predict selenium-79 to be a critical radionuclide due to its presence as anions in three relevant oxidation states (VI, IV, -II) resulting in weak retardation by most common rock minerals. This assumption, however, ignores its potential uptake by AFm phases, positively charged anion exchangers, which are present in significant quantities in the cementitious materials used in artificial barriers. Here we report for the first time wet chemistry and spectroscopic data on the interaction of the most relevant selenium anion species under the expected strongly reducing conditions, i.e. HSe-, with two AFm phases commonly found in cement, monocarbonate (AFm-MC) and hemicarbonate (AFm-HC). Batch sorption experiments showed that HSe- is retained much more strongly by AFm-HC (solid-liquid distribution ratio, Rd, of 100±50 L kg-1) than by AFm-MC (Rd = 4±2 L kg-1) at the equilibrium pH (~12). X-ray absorption fine-structure (XAFS) spectroscopy revealed that the larger d-spacing in AFm-HC (d-spacing = 8.2 Å) provides easy access for HSe- to the AFm interlayer space for sorption, whereas the smaller d-spacing of AFm-MC (d-spacing = 7.55 Å) hinders interlayer access and limits HSe- sorption mostly to the outer planar surfaces and edges of the latter AFm phase. XAFS spectra further demonstrated that Se(-II) prevalently sorbed in the interlayers of AFm-HC, is better protected from oxidation than Se(-II) prevalently sorbed onto the outer surfaces of AFm-MC. The quantitative sorption data along with the molecular-scale process understanding obtained from this study provide crucial insight into the Se retention by the cementitious near-field of a radioactive waste repository under reducing conditions.
Transport of low molecular weight organic compounds in compacted illite and kaolinite
14C-containing dissolved organic compounds may significantly contribute to the calculated annual overall dose emanated from a deep geological repository for radioactive waste. To date, there is a general lack of knowledge concerning the transport behaviour of low molecular weight organic compounds in the geosphere. The present work is aiming at a generic approach to measure weak adsorption of such compounds onto selected clay minerals. Percolation experiments were employed to sensitively measure the retardation of low molecular weight carboxylates and alcohols in compacted illite and kaolinite as a function of the ionic strength. Detection limits of ~10-5m3kg-1 for the involved sorption distribution coefficients were attained thereby. The adsorption of alcohols on clays was near the detection limit and assumed to occur predominately via H-bonding. The adsorption of organic anions was influenced by several factors such as molecular structure, type of clay surfaces and the chemical composition of the aqueous phase. It was found that the relative position of neighbouring hydroxyl groups strongly influ- enced the retardation behaviour. Alpha-hydroxylated carboxylates, such as lactate, were found to be most retarded. Ligand exchange at the edge aluminol sites is the most probable explanation for the uptake of the negatively charged organic test compounds by the clay surface. The breakthrough behaviour of organic anions was additionally impacted by anion exclusion in illite. The demonstrated weak retardation of the test compounds can be robustly introduced in transport models, leading thus to a much lower contribution of 14C to the expected long-term overall dose.
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
An internally consistent thermodynamic dataset for aqueous species in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl was generated using the thermodynamic database for minerals of Holland and Powell (1998; updated Thermocalc dataset ds55). This dataset makes it possible to perform geochemical and reactive transport modeling with high levels of accuracy and reliability.
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.
Deciphering pore-level precipitation mechanisms
Mineral precipitation and dissolution in aqueous solutions has a significant effect on solute transport and structural properties of porous media. The understanding of the involved physical mechanisms, which cover a large range of spatial and temporal scales, plays a key role in several geochemical and industrial processes. Here, by coupling pore scale reactive transport simulations with classical nucleation theory, we demonstrate how the interplay between homogeneous and heterogeneous precipitation kinetics along with the non-linear dependence on solute concentration affects the evolution of the system. Such phenomena are usually neglected in pure macroscopic modelling. Comprehensive parametric analysis and comparison with laboratory experiments confirm that incorporation of detailed microscale physical processes in the models is compulsory. This sheds light on the inherent coupling mechanisms and bridges the gap between atomistic processes and macroscopic observations.