Paul Wersin

A surface complexation model of the carbonate mineral-aqueous solution interface

A surface complexation model for the chemical structure and reactivity of the carbonatewater interface is presented. The model postulates the formation of the hydration species >CO3H0 and >MeOH0 at the surface of a divalent metal carbonate MeCO3 (Me = Ca, Mn, Fe, etc.). The existence of these primary hydration species is supported by spectroscopic data. The following reactions are proposed to govern surface speciation in the MeCO3(s)-H2O-CO2 system: CO3H0a2>CO3− + H+(1) CO3H0 + Me2+a2>CO3Me+ + H+(2) MeOH2+a2 >MeOH0 + H+(3) MeOH0a2 >MeO− + H+(4) MeOH0 + CO2a2 >MeHCO3o (5) MeOH0 + CO2a2 >MeCO3−+ H+(6) It is shown in this paper that the surface complexation model provides a systematic explanation of surface charge development and dissolution kinetics of carbonate minerals. The intrinsic stability constants of the surface complexation reactions agree well with the equilibrium constants of the corresponding complexation reactions in homogeneous solution.

Early diagenetic influences on iron transformations in a freshwater lake sediment

Abstract Iron transformations in a calcium carbonate rich fresh-water sediment were studied by analyzing the relevant constituents of both interstitial water and solid matter. Analysis of interstitial water shows that the observed redox sequence NO − 3 NH + 4 , MnO 2 Mn(II) , FeOOH/Fe(II), SO 2− 4 S(−II) is roughly in agreement with that predicted by the Gibbs Free Energy for the corresponding reactions. In contrast to marine sediments, these redox transitions occur in the uppermost sediments, i.e., at depths of 0–4 cm. Deeper in the sedimentary sequence, the depth profile for dissolved iron exhibits a steady non-linear increase up to 400 μmol dm−3. In this anoxic zone, according to thermodynamic predictions, iron (II)-minerals such as iron sulfide, siderite, and vivianite should precipitate while Fe(III) oxides should be completely dissolved. However, microscopic analysis showed that Fe(III) oxides were present throughout the studied sediment. Furthermore, scanning electron microscope/energy dispersive spectroscopy analysis suggests the presence of iron sulfide could be verified but not that of siderite or vivianite. These observations indicate kinetic control of iron transformations. We have investigated the importance of kinetic control of iron distribution in anoxic sediments using a diagenetic model for dissolved iron(II). A rough estimate of time scales for dissolution and precipitation rates was made by imposing limiting boundary conditions. Using the calculated rate constant, we established that more than 1000 years would be required for the complete dissolution of Fe(III) oxides, which is agreement with our observations and experimental data from the literature. Calculated precipitation rates of Fe(II) for a given mineral phase such as siderite yield a maximum value of 3 μg(FeCO3) g−1(dry sediment) yr−1. Such low rates would explain the absence of siderite and vivianite. Finally, it can be inferred from the Mn T Fe T ratio in the sediments that this ratio depends on the redox conditions of the sediment-water interface at the time of deposition. Thus, this ratio can be used as “paleo-redox indicator” in lacustrine sediments.

On the influence of carbonate in mineral dissolution: II. The solubility of FeCO3 (s) at 25°C and 1 atm total pressure

The solubility of siderite in acid, neutral, and alkaline medium was investigated. The experiments were performed as potentiometric titrations under two constant partial pressures of CO2 (g). This yielded the following equilibrium constants at 25°C in a 1.0 M NaClO4 medium (quoted errors through this work are given at 2σ confidence level). 1. 1) The solubility constant of FeCO3(s) FeCO3 (s) + 2H+ = Fe2 + CO2 (g) + H2O, with log K∗s0 = 7.59 ± 0.08. 2. 2) The formation constants for the two determined Fe(II) carbonate complexes: Fe2+CO2(g) + H2O = FeCO3(aq) + 2H+, with log β11 = −12.9 ± 0.12Fe2+ + 2CO2 (g) + 2H2O = Fe(CO3)22− + 4H+, with log β11 = − 28.4 ± 0.10. By extrapolation to infinite dilution, the following reaction equilibria are estimated for the infinite dilution standard state: FeCO3 (s) = Fe2+ + CO32−, with log Ksp = − 10.8 ± 0.2Fe2+ + CO32− = FeCO3 (aq), with log β11 = 5.5 ± 0.2Fe2+ + 2CO32− = Fe(CO3)22−, with log β2 = 7.1 ± 0.2. The determined solubility constant is in fair agreement with previously derived solubility data. The identified complexes and their formation constants emphasize their importance in various anoxic waters. However, carbonate complex formation does not account for the supersaturation conditions with regard to siderite equilibrium observed in many other present-day environments.

In-situ diffusion of HTO, 22Na+, Cs+ and I- in Opalinus Clay at the Mont Terri underground rock laboratory

Summary The diffusion properties of the Opalinus Clay were studied in the underground research laboratory at Mont Terri (Canton Jura, Switzerland) and the results were compared with diffusion data measured in the laboratory on small-scale samples. The diffusion of HTO, 22Na+, Cs+ and I- were investigated for a period of 10 months. The diffusion equipment used in the field experiment was designed in such a way that a solution of tracers was circulated through a sintered metal screen placed at the end of a borehole drilled in the formation. The concentration decrease caused by the diffusion of tracers into the rock could be followed with time and allowed first estimations of the effective diffusion coefficient. After 10 months, the diffusion zone was overcored and the tracer profiles measured. From these profiles, effective diffusion coefficients and rock capacity factors could be extracted by applying a two-dimensional transport model including diffusion and sorption. The simulations were done with the reactive transport code CRUNCH. In addition, results obtained from through-diffusion experiments on small-sized samples with HTO, 36Cl- and 22Na+ are presented and compared with the in situ data. In all cases, excellent agreement between the two data sets exists. Results for Cs+ indicated five times higher diffusion rates relative to HTO. Corresponding laboratory diffusion measurements are still lacking. However, our Cs+ data are in qualitative agreement with through-diffusion data for Callovo–Oxfordian argillite rock samples, which also indicate significantly higher effective diffusivities for Cs+ relative to HTO.

From adsorption to precipitation: Sorption of Mn2+ on FeCO3(s)

Sorption of Mn2+ on siderite was investigated as a function of Mn concentration and time at constant pH, pco2, pϵ, and ionic strength. The log [Mn2+]versus log ΓMn plot revealed an S-shaped isotherm which was modeled on an extended version of the adsorption/precipitation model previously formulated for metal oxides. At low concentrations, Mn was adsorbed up to the inflection point of the isotherm, where nearly all surface carbonate sites are covered by Mn2+. At higher concentrations, ESR spectroscopy gave evidence for a co-precipitation phenomenon by the change in the line width of the ESR signal, ruling out the other suggested mechanisms (e.g., adsorption on sites with lower reactivity). In the MnCO3—FeCO3 solid solution that was formed, and within the range of isomorphous substitution considered, the activity coefficient of MnCO3(s) remained remarkably constant. The total solubility product was about 500 times larger than that of the end members. The present version of the model explicitly includes the solid phase activity coefficients, thus allowing the use of “thermodynamic” and not “adjustable” solubility constants. The kinetics of Mn2+ sorption at high Mn concentration exhibited a three step mechanism, identified by ESR. A rapid initial adsorption was followed by an intermediate surface complexation process before the co-precipitation reaction itself occurred, giving rise to a solid solution. The observed increased solubility of such a solid solution relative to that of the end members is a common feature in reducing tropical environments.

Geochemical modelling of bentonite porewater in high-level waste repositories

The description of the geochemical properties of the bentonite backfill that serves as engineered barrier for nuclear repositories is a central issue for performance assessment since these play a large role in determining the fate of contaminants released from the waste. In this study the porewater chemistry of bentonite was assessed with a thermodynamic modelling approach that includes ion exchange, surface complexation and mineral equilibrium reactions. The focus was to identify the geochemical reactions controlling the major ion chemistry and acid-base properties and to explore parameter uncertainties specifically at high compaction degrees. First, the adequacy of the approach was tested with two distinct surface complexation models by describing recent experimental data performed at highly varying solid/liquid ratios and ionic strengths. The results indicate adequate prediction of the entire experimental data set. Second, the modelling was extended to repository conditions, taking as an example the current Swiss concept for high-level waste where the compacted bentonite backfill is surrounded by argillaceous rock. The main reactions controlling major ion chemistry were found to be calcite equilibrium and concurrent Na-Ca exchange reactions and de-protonation of functional surface groups. Third, a sensitivity analysis of the main model parameters was performed. The results thereof indicate a remarkable robustness of the model with regard to parameter uncertainties. The bentonite system is characterised by a large acid-base buffering capacity which leads to stable pH-conditions. The uncertainty in pH was found to be mainly induced by the pCO(2) of the surrounding host rock. The results of a simple diffusion-reaction model indicate only minor changes of porewater composition with time, which is primarily due to the geochemical similarities of the bentonite and the argillaceous host rock. Overall, the results show the usefulness of simple thermodynamic models to describe porewater chemistry of expandable clays although significant uncertainties with regard to the effects of swelling and physico-chemical properties of the interstitial water remain.

Dry deposition measurements using water as a receptor : a chemical approach

The field measurement of dry deposition still represents a difficult task. In our approach, a 1 to 2 cm thick layer of water in a petri dish with a diameter of 22 cm, serves as a surrogate surface. The atmospheric constituents taken up by the water can be analyzed chemically by the same procedure as for the wet deposition samples. In contrast to solid surrogate surfaces, water exhibits the following advantageous properties: low and constant surface resistance, high sticking coefficient for aerosols, and predictable Sorption behavior for gases. Consequently, the deposition rates measured to the wet surface are generally higher, by up to a factor of 4 for NH4+, Cl−, NO3− and SO42−, than those to a dry surface, but still smaller than the concurrent wet deposition rates. We observed the following average dry deposition rates in μmol m−2 d−1∶ NH4+ 48.3, Ca2+ 40.7, Na+ 15.8, Mg2+ 8.4, K+ 4.2, H-Aci 36.4; SO42− 57.2, Cl− 39.2, NO3− 34.5, HSO3− 5.7, formate 4.0; acid soluble metals: Fe 2.8, Zn 0.60, Cu 0.11, Pb 0.073, Cd 0.0022. The soluble fraction of Zn, Cd, Cu, Pb and Fe in the dry deposition varied with the pH of the water phase corresponding to the adsorption tendency of these metals to oxide surfaces. The sampling method also allows tracing of regionally and locally emitted atmospheric pollutants. In our study the local pollution sources included road salting, construction work and a refuse incinerator. Finally, chemical reactions occurring in the atmosphere, such as the conversion of Cl− to HCl by HNO3 or the oxidation of SO2, can be identified by evaluating the data. The method proposed is relevant to measure reproducibly the dry deposition of a variety of compounds to water bodies and moist vegetation.

Reactive transport modelling of iron–bentonite interaction within the KBS-3H disposal concept: the Olkiluoto site as a case study

Abstract The interaction of steel with bentonite used as buffer material in high-level waste repositories may result in changes to the properties of the buffer. One of the repository designs (KBS-3H) developed by Posiva and SKB foresees the horizontal emplacement of so-called supercontainers, consisting of copper canisters surrounded by compacted bentonite and an outer perforated steel shell. The corrosion of the steel shell and the interaction of iron with the clay may impair the long-term safety functions of the buffer. The corrosion and iron–clay interaction processes within the KBS-3H concept were assessed with a kinetically based reactive transport model and a comprehensive thermodynamic database. The large uncertainty related to precipitation rates of corrosion products and iron silicates was considered by defining a series of test cases. The results generally indicate a limited effect on the stability of montmorillonite, thus affecting only a few centimetres next to the iron source. Upon complete corrosion only insignificant changes are predicted. These results are explained by (i) the diffusional constraint of mass transfer, (ii) low solubility of corrosion products and (iii) slow transformation kinetics of montmorillonite. Model results further suggest that the largest impact arises from ‘indirect’ processes, such as microbial sulphate reduction, which may lead to a strong increase in pH.

INTERACTION OF CORRODING IRON WITH BENTONITE IN THE ABM1 EXPERIMENT AT ÄSPÖ, SWEDEN: A MICROSCOPIC APPROACH

Bentonite and iron metals are common materials proposed for use in deep-seated geological repositories for radioactive waste. The inevitable corrosion of iron leads to interaction processes with the clay which may affect the sealing properties of the bentonite backfill. The objective of the present study was to improve our understanding of this process by studying the interface between iron and compacted bentonite in a geological repository-type setting. Samples of MX-80 bentonite samples which had been exposed to an iron source and elevated temperatures (up to 115°C) for 2.5 y in an in situ experiment (termed ABM1) at the Äspö Hard Rock Laboratory, Sweden, were investigated by microscopic means, including scanning electron microscopy, μ-Raman spectroscopy, spatially resolved X-ray diffraction, and X-ray fluorescence.The corrosion process led to the formation of a ~100 mm thick corrosion layer containing siderite, magnetite, some goethite, and lepidocrocite mixed with the montmorillonitic clay. Most of the corroded Fe occurred within a 10 mm-thick clay layer adjacent to the corrosion layer. An average corrosion depth of the steel of 22–35 μm and an average Fe2+ diffusivity of 1–2 × 10−13 m2/s were estimated based on the properties of the Fe-enriched clay layer. In that layer, the corrosion-derived Fe occurred predominantly in the clay matrix. The nature of this Fe could not be identified. No indications of clay transformation or newly formed clay phases were found. A slight enrichment of Mg close to the Fe—clay contact was observed. The formation of anhydrite and gypsum, and the dissolution of some SiO2 resulting from the temperature gradient in the in situ test, were also identified.

Interaction of titanium with smectite within the scope of a spent fuel repository: A spectroscopic approach

Abstract The Swedish and Finnish nuclear waste repository design, KBS-3H, foresees horizontal emplacement of copper canisters-bentonite modules surrounded by a titanium shell. The interaction of titanium with bentonite was studied here using a combination of wet chemistry and a spectroscopic approach to evaluate the potential impact of Ti corrosion on the clay. For natural analogue clays with high Ti contents, spectroscopic investigations showed that titanium occurs as crystalline TiO2. In contrast, the Ti in the MX-80 bentonite occurs in the clay structure, presumably in the octahedral sheet. Hydrothermal tests conducted at 200°C using synthetic montmorillonite showed little if any change in the montmorillonite structure at near-neutral and acidic conditions. Under alkaline conditions, limited alteration was observed, including the formation of trioctahedral clay minerals and zeolite. These changes, however, occurred independently of the addition of Ti. In the batch tests conducted at 80°C, Ti did not occur as separate TiO2 particles. The comparison of experimental data with spectroscopic simulations provides sound evidence that Ti was incorporated in a neoformed phyllosilicate structure.