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.
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.
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 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.
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
Iron occurs in clay minerals in both ferric and ferrous forms. Depending on its oxidation state and the environmental conditions, it can participate in redox reactions and influence the sorption processes at surfaces of clay minerals. Knowing the oxidation state and the preferential structural position of Fe2+ and Fe3+ is essential for the detailed understanding of the mechanism and kinetics of such processes. In this study, molecular dynamics (MD) calculations based on density functional theory (DFT+U) were applied to simulate the incorporated Fe in bulk montmorillonite and to explain the measured Fe K-edge X-ray absorption fine structure (XAFS) spectra. The analysis of the experimental data and simulation results suggested that iron in montmorillonite is preferentially incorporated as Fe3+ into the octahedral layer. The simulations showed that there is no preferential occupation of cis- or trans-sites by Fe2+ and Fe3+ in bulk montmorillonite. A very good agreement between the ab initio simulated and the measured XAFS spectra demonstrate the robustness of the employed simulation approach.
Quantitative description of thermodynamic and molecular mechanism of Al incorporation into calcium-silicate hydrates (C-S-H), the main binder in hydrated cement paste, is essential for development of novel cementitious materials with a lower CO2 footprint. Thermodynamics integration
based on ab initio molecular dynamic simulations was applied to estimate the Gibbs free energy of the Al exchange between different silica tetrahedral sites forming the dreierketten-chains at the C-S-H surface and aqueous Al(OH)4− anions. The calculations confirm that the Al substitute for Si into bridging tetrahedral sites with an estimated equilibrium constant KAl/Si ∼ 1. Al for Si substitution is further found to favor the crosslinking between adjacent chains of the same C-S-H layer. This result is in a good agreement with recent conclusions made from 27Al MAS NMR spectroscopy results. Mesoscale Monte Carlo simulations were performed with the calculated KAl/Si to interpret experimental observations of Al incorporation into C-S-H. The simulation results suggest that the chemical affinity of Al to C-S-H is controlled by electrostatic interactions and the Al(OH)4−/Si(OH)3O− aqueous molar ratio.
The anion exclusion behavior in two different clay stones, Opalinus Clay (OPA) and Helvetic Marl (HM), was studied using a well-established experimental through-diffusion technique. The ionic strength of the pore water was varied between 0.01 and 5 M to evaluate its effect on the diffusion of HTO and 36Cl−. The total porosity determined by HTO-diffusion was independent of the ionic strength, while the anion accessible porosity varies with the ionic strength of the pore water. In the case of Opalinus Clay, the anion accessible porosity increases from 3% at low ionic strength (0.01 M) up to 8.4% at high ionic strength (5 M), whereas the anion accessible porosity of Helvetic Marl increases from 0.6% up to only 1.1%. The anion exclusion effect in HM is thus more pronounced than that in OPA, even at high ionic strength. This observation can be correlated to differences in mineralogy and to the fact that HM has a larger fraction of interlayer equivalent pores. Interlayer equivalent pores are small pores in compressed clay stones that are small enough to have, because of overlapping electric double layers, properties similar to those of interlayers and are therefore rather inaccessible for anions.