Uncertainty propagation in pore water chemical composition calculation using surrogate models.

Performance assessment in deep geological nuclear waste repository systems necessitates an extended knowledge of the pore water chemical conditions prevailing in host-rock formations. In the last two decades, important progress has been made in the experimental characterization and thermodynamic modeling of pore water speciation, but the influence of experimental artifacts and uncertainties of thermodynamic input parameters are seldom evaluated. In this respect, we conducted an uncertainty propagation study in a reference geochemical model describing the pore water chemistry of the Callovian-Oxfordian clay formation. Nineteen model input parameters were perturbed, including those associated to experimental characterization (leached anions, exchanged cations, cation exchange selectivity coefficients) and those associated to generic thermodynamic databases (solubilities). A set of 13 quantities of interest were studied by the use of polynomial chaos expansions built non-intrusively with a least-squares forward stepwise regression approach. Training and validation sets of simulations were carried out using the geochemical speciation code PHREEQC. The statistical results explored the marginal distribution of each quantity of interest, their bivariate correlations as well as their global sensitivity indices. The influence of the assumed distributions for input parameters uncertainties was evaluated by considering two parametric domain sizes.

Hydration Properties and Interlayer Organization in Synthetic C-S-H.

Water in calcium silicate hydrate (C-S-H) is one of the key parameters driving the macroscopic behavior of cement materials for which water vapor partial pressure has an impact on Young's modulus and the volumic properties. Several samples of C-S-H with a bulk Ca/Si ratio ranging between 0.6 and 1.6 were characterized to study their dehydration/hydration behavior under water-controlled conditions using29Si NMR, water adsorption volumetry, X-ray diffraction, and Fourier-transform near-infrared diffuse reflectance under various water pressures. Coherent with several previous studies, it was observed that an increase in the Ca/Si ratio is due to the progressive omission of Si bridging tetrahedra, with the resulting charge being compensated for by interlayer Ca, and that water conditioning influences the layer-to-layer distance and the achieved NMR spectral resolution. Water desorption experiments exhibit one step toward low relative pressure, accompanied by a decrease in the layer-to-layer distance. When sufficient energy is provided to the system (T ≥ 40 °C under vacuum) to remove the interlayer water, the shrinkage/swelling is partially reversible in our experimental conditions. A change in layer-to-layer distance of less than 3 Å is measured in the C-S-H between the wet and dried states. When the bridging SiO2 tetrahedra are omitted, interlayer Ca interacts with layer O and water interacts with the cations and potentially with the surfaces. This structural organization is interpreted as a mid-plane monolayer of water in the interlayer space, this latter accounting for about 30% of the volume of C-S-H particles.

Mechanistic and Thermodynamic Insights into Anion Exchange by Green Rust.

Fougerite is a naturally occurring green rust, that is, a layered double hydroxide (LDH) containing iron (Fe). Fougerite was identified in natural settings such as hydromorphic soils. It is one of the few inorganic materials with large anion adsorption capacity that stems from the presence of isomorphic substitutions of Fe2+ by Fe3+ in its layers. The importance of anion adsorption in the interlayer of LDH has often been highlighted, but we are still missing a mechanistic understanding and a thermodynamic framework to predict the anion uptake by green rust. We combined laboratory and in operando synchrotron X-ray diffraction and scattering experiments with geochemical modeling to contribute to filling this gap. We showed that the overall exchange process in green rusts having nanometer and micrometer sizes can be seen as a simple anion exchange mechanism without dissolution-recrystallization or interstratification processes. A thermodynamic model of ion exchange, based on the Rothmund and Kornfeld convention, made it possible to predict the interlayer composition in a large range of conditions. This multiscale characterization can serve as a starting point for the building of robust and mechanistic geochemical models that will allow predicting the role of green rust on the geochemical cycle of ions, including nutrients, in soils.

Thermodynamic evidence of giant salt deposit formation by serpentinization: an alternative mechanism to solar evaporation.

The evaporation of seawater in arid climates is currently the main accepted driving mechanism for the formation of ancient and recent salt deposits in shallow basins. However, the deposition of huge amounts of marine salts, including the formation of tens of metres of highly soluble types (tachyhydrite and bischofite) during the Aptian in the South Atlantic and during the Messinian Salinity Crisis, are inconsistent with the wet and warm palaeoclimate conditions reconstructed for these periods. Recently, a debate has been developed that opposes the classic model of evaporite deposition and argues for the generation of salt by serpentinization. The products of the latter process can be called "dehydratites". The associated geochemical processes involve the consumption of massive amounts of pure water, leading to the production of concentrated brines. Here, we investigate thermodynamic calculations that account for high salinities and the production of soluble salts and MgCl2-rich brines through sub-seafloor serpentinization processes. Our results indicate that salt and brine formation occurs during serpentinization and that the brine composition and salt assemblages are dependent on the temperature and CO2 partial pressure. Our findings help explain the presence and sustainability of highly soluble salts that appear inconsistent with reconstructed climatic conditions and demonstrate that the presence of highly soluble salts probably has implications for global tectonics and palaeoclimate reconstructions.

Thermodynamic and crystallographic model for anion uptake by hydrated calcium aluminate (AFm): an example of molybdenum.

Amongst all cement phases, hydrated calcium aluminate (AFm) plays a major role in the retention of anionic species. Molybdenum (Mo), whose 93Mo isotope is considered a major steel activation product, will be released mainly under the form of MoO42- in a radioactive waste repository. Understanding its fate is of primary importance in a safety analysis of such disposal. This necessitates models that can both predict quantitatively the sorption of Mo by AFm and determine the nature of the sorption process (i.e., reversible adsorption or incorporation). This study investigated the Cl-/MoO42- exchange processes occurring in an AFm initially containing interlayer Cl in alkaline conditions using flow-through experiments. The evolution of the solid phase was characterized using an electron probe microanalyzer and synchrotron high-energy X-ray scattering. All data, together with their quantitative modeling, coherently indicated that Mo replaced Cl in the AFm interlayer. The structure of the interlayer is described with unprecedented atomic-scale detail based on a combination of real- and reciprocal-space analyses of total X-ray scattering data. In addition, modeling of several independent chemical experiments elucidated that Cl-/OH- exchange processes occur together with Cl-/MoO42- exchange. This competitive effect must be considered when determining the Cl-/MoO42- selectivity constant.

Deciphering mineralogical changes and carbonation development during hydration and ageing of a consolidated ternary blended cement paste.

To understand the main properties of cement, a ubiquitous material, a sound description of its chemistry and mineralogy, including its reactivity in aggressive environments and its mechanical properties, is vital. In particular, the porosity distribution and associated sample carbonation, both of which affect cement's properties and durability, should be quantified accurately, and their kinetics and mechanisms of formation known both in detail and in situ. However, traditional methods of cement mineralogy analysis (e.g. chemical mapping) involve sample preparation (e.g. slicing) that can be destructive and/or expose cement to the atmosphere, leading to preparation artefacts (e.g. dehydration). In addition, the kinetics of mineralogical development during hydration, and associated porosity development, cannot be examined. To circumvent these issues, X-ray diffraction computed tomography (XRD-CT) has been used. This allowed the mineralogy of ternary blended cement composed of clinker, fly ash and blast furnace slag to be deciphered. Consistent with previous results obtained for both powdered samples and dilute systems, it was possible, using a consolidated cement paste (with a water-to-solid ratio akin to that used in civil engineering), to determine that the mineralogy consists of alite (only detected in the in situ hydration experiment), calcite, calcium silicate hydrates (C-S-H), ettringite, mullite, portlandite, and an amorphous fraction of unreacted slag and fly ash. Mineralogical evolution during the first hydration steps indicated fast ferrite reactivity. Insights were also gained into how the cement porosity evolves over time and into associated spatially and time-resolved carbonation mechanisms. It was observed that macroporosity developed in less than 30 h of hydration, with pore sizes reaching about 100-150 µm in width. Carbonation was not observed for this time scale, but was found to affect the first 100 µm of cement located around macropores in a sample cured for six months. Regarding this carbonation, the only mineral detected was calcite.

Evidence of Multiple Sorption Modes in Layered Double Hydroxides Using Mo As Structural Probe.

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO42-) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modeling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO4 precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanisms-edge sorption, interfacial dissolution-reprecipitation-are at play and control anion uptake under environmentally relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.

Quantitative X-ray pair distribution function analysis of nanocrystalline calcium silicate hydrates: a contribution to the understanding of cement chemistry.

The structural evolution of nanocrystalline calcium silicate hydrate (C-S-H) as a function of its calcium to silicon (Ca/Si) ratio has been probed using qualitative and quantitative X-ray atomic pair distribution function analysis of synchrotron X-ray scattering data. Whatever the Ca/Si ratio, the C-S-H structure is similar to that of tobermorite. When the Ca/Si ratio increases from ∼0.6 to ∼1.2, Si wollastonite-like chains progressively depolymerize through preferential omission of Si bridging tetrahedra. When the Ca/Si ratio approaches ∼1.5, nanosheets of portlandite are detected in samples aged for 1 d, while microcrystalline portlandite is detected in samples aged for 1 year. High-resolution transmission electron microscopy imaging shows that the tobermorite-like structure is maintained to Ca/Si > 3.

Structure of nanocrystalline calcium silicate hydrates: insights from X-ray diffraction, synchrotron X-ray absorption and nuclear magnetic resonance.

The structure of nanocrystalline calcium silicate hydrates (C-S-H) having Ca/Si ratios ranging between 0.57 ± 0.05 and 1.47 ± 0.04 was studied using an electron probe micro-analyser, powder X-ray diffraction, 29Si magic angle spinning NMR, and Fourier-transform infrared and synchrotron X-ray absorption spectroscopies. All samples can be described as nanocrystalline and defective tobermorite. At low Ca/Si ratio, the Si chains are defect free and the Si Q3 and Q2 environments account, respectively, for up to 40.2 ± 1.5% and 55.6 ± 3.0% of the total Si, with part of the Q3 Si being attributable to remnants of the synthesis reactant. As the Ca/Si ratio increases up to 0.87 ± 0.02, the Si Q3 environment decreases down to 0 and is preferentially replaced by the Q2 environment, which reaches 87.9 ± 2.0%. At higher ratios, Q2 decreases down to 32.0 ± 7.6% for Ca/Si = 1.38 ± 0.03 and is replaced by the Q1 environment, which peaks at 68.1 ± 3.8%. The combination of X-ray diffraction and NMR allowed capturing the depolymerization of Si chains as well as a two-step variation in the layer-to-layer distance. This latter first increases from ∼11.3 Š(for samples having a Ca/Si ratio <∼0.6) up to 12.25 Šat Ca/Si = 0.87 ± 0.02, probably as a result of a weaker layer-to-layer connectivity, and then decreases down to 11 Šwhen the Ca/Si ratio reaches 1.38 ± 0.03. The decrease in layer-to-layer distance results from the incorporation of interlayer Ca that may form a Ca(OH)2-like structure, nanocrystalline and intermixed with C-S-H layers, at high Ca/Si ratios.

Mineral precipitation-induced porosity reduction and its effect on transport parameters in diffusion-controlled porous media.

BACKGROUND: In geochemically perturbed systems where porewater and mineral assemblages are unequilibrated the processes of mineral precipitation and dissolution may change important transport properties such as porosity and pore diffusion coefficients. These reactions might alter the sealing capabilities of the rock by complete pore-scale precipitation (cementation) of the system or by opening new migration pathways through mineral dissolution. In actual 1D continuum reactive transport codes the coupling of transport and porosity is generally accomplished through the empirical Archie's law. There is very little reported data on systems with changing porosity under well controlled conditions to constrain model input parameters. In this study celestite (SrSO4) was precipitated in the pore space of a compacted sand column under diffusion controlled conditions and the effect on the fluid migration properties was investigated by means of three complementary experimental approaches: (1) tritiated water (HTO) tracer through diffusion, (2) computed micro-tomography (µ-CT) imaging and (3) post-mortem analysis of the precipitate (selective dissolution, SEM/EDX). RESULTS: The through-diffusion experiments reached steady state after 15 days, at which point celestite precipitation ceased and the non-reactive HTO flux became constant. The pore space in the precipitation zone remained fully connected using a 6 µm µ-CT spatial resolution with 25 % porosity reduction in the approx. 0.35 mm thick dense precipitation zone. The porosity and transport parameters prior to pore-scale precipitation were in good agreement with a porosity of 0.42 ± 0.09 (HTO) and 0.40 ± 0.03 (µ-CT), as was the mass of SrSO4 precipitate estimated by µ-CT at 25 ± 5 mg and selective dissolution 21.7 ± 0.4 mg, respectively. However, using this data as input parameters the 1D single continuum reactive transport model was not able to accurately reproduce both the celestite precipitation front and the remaining connected porosity. The model assumed there was a direct linkage of porosity to the effective diffusivity using only one cementation value over the whole porosity range of the system investigated. CONCLUSIONS: The 1D single continuous model either underestimated the remaining connected porosity in the precipitation zone, or overestimated the amount of precipitate. These findings support the need to implement a modified, extended Archie's law to the reactive transport model and show that pore-scale precipitation transforms a system (following Archie's simple power law with only micropores present) towards a system similar to clays with micro- and nanoporosity. Graphical abstract.

Effects of charging on the chromophores of dissolved organic matter from the Rio Negro basin.

This study demonstrates that the deprotonation of dissolved organic matter (DOM) originating from a small creek characteristic for DOM-rich waters located in the Rio Negro basin can be quantified based on measurements of pH effects on its absorbance spectra. The method was ascertained by the data of Near-Edge X-Ray Absorbance Spectroscopy (NEXAFS), potentiometric titration to quantify the structural and compositional differences between the colloidal and hydrophobic fractions that contribute 91% of black-water creek DOM. Changes in the absorbance spectra of the DOM fractions caused by deprotonation quantified via numeric deconvolution which indicated the presence of six well-resolved Gaussian bands in the differential spectra. The emergence of these bands was determined to be associated with the engagement of carboxylic and phenolic functionalities and changes of inter-chromophore interactions in DOM molecules. Interpretation of the data based on the NICA-Donnan approach showed that behavior of DOM chromophores was consistent with results of potentiometric titrations. Similar trends were observed for changes of the spectral slope of the DOM absorbance spectra in the range of wavelengths 325-375 nm (DSlope325-375). The behavior of DSlope325-375 values was modeled based on the NICA-Donnan approach and correlated with potentiometrically-estimated charges attributed to the carboxylic and phenolic groups. The correlations between DSlope325-375 and charges of low- and high-affinity protonation-active groups in DOM were monotonic but not linear and had important differences between the colloidal and hydrophobic fractions.

X-ray diffraction: a powerful tool to probe and understand the structure of nanocrystalline calcium silicate hydrates.

X-ray diffraction (XRD) patterns were calculated and compared to literature data with the aim of investigating the crystal structure of nanocrystalline calcium silicate hydrates (C-S-H), the main binding phase in hydrated Portland cement pastes. Published XRD patterns from C-S-H of Ca/Si ratios ranging from ~ 0.6 to ~ 1.7 are fully compatible with nanocrystalline and turbostratic tobermorite. Even at a ratio close or slightly higher than that of jennite (Ca/Si = 1.5) this latter mineral, which is required in some models to describe the structure of C-S-H, is not detected in the experimental XRD patterns. The 001 basal reflection from C-S-H, positioned at ~ 13.5 Å when the C-S-H structural Ca/Si ratio is low (< 0.9), shifts towards smaller d values and sharpens with increasing Ca/Si ratio, to reach ~ 11.2 Å when the Ca/Si ratio is higher than 1.5. Calculations indicate that the sharpening of the 001 reflection may be related to a crystallite size along c* (i.e. a mean number of stacked layers) increasing with the C-S-H Ca/Si ratio. Such an increase would contribute to the observed shift of the 001 reflection, but fails to quantitatively explain it. It is proposed that the observed shift could result from interstratification of at least two tobermorite-like layers, one having a high and the other a low Ca/Si ratio with a basal spacing of 11.3 and 14 Å, respectively.

Discriminating factors affecting incorporation: comparison of the fate of Eu3+-Cm3+ in the Sr carbonate-sulfate system.

The aim of this work is to assess the effect of ligand strength, symmetry, and coordination number on solid solution formation of trivalent actinides and lanthanides in carbonate and sulfate minerals. This is of particular importance in radionuclide migration where trivalent actinides such as Pu, Am, and Cm are responsible for the majority of radiotoxicity after 1000 years. Time-resolved laser fluorescence spectroscopy was used to study trace concentrations of the dopant ion after interaction with the mineral phase. This study expands on previous work with aragonite and gypsum where it was found that aragonite incorporates Eu(3+) and Cm(3+) while only surface sorption is observed in gypsum. This study uses isostructural minerals strontianite (SrCO(3)) and celestite (SrSO(4)) to decouple the effect of structure from that due to the anion. It is demonstrated that while distribution coefficients can predict the amount of dopant ion associated with the mineral phase, they do not have any correlation with solid solution formation. This substitution mechanism is most likely dictated by the symmetry of the site being substituted and the electronic structure of the dopant atom.

Metal speciation in landfill leachates with a focus on the influence of organic matter.

This study characterises the heavy-metal content in leachates collected from eight landfills in France. In order to identify heavy metal occurrence in the different size fractions of leachates, a cascade filtration protocol was applied directly in the field, under a nitrogen gas atmosphere to avoid metal oxidation. The results of analyses performed on the leachates suggest that most of the metals are concentrated in the <30 kDa fraction, while lead, copper and cadmium show an association with larger particles. Initial speciation calculations, without considering metal association with organic matter, suggest that leachate concentrations in lead, copper, nickel and zinc are super-saturated with respect to sulphur phases. Speciation calculations that account for metal complexation with organic matter, considered as fulvic acids based on C1(s) NEXAFS spectroscopy, show that this mechanism is not sufficient to explain such deviation from equilibrium conditions. It is therefore hypothesized that the deviation results also from the influence of biological activity on the kinetics of mineral phase precipitation and dissolution, thus providing a dynamic system. The results of chemical analyses of sampled fluids are compared with speciation calculations and some implications for the assessment of metal mobility and natural attenuation in a context of landfill risk assessment are discussed.

Spectral and temporal luminescent properties of Eu(III) in humic substance solutions from different origins.

Although a high heterogeneity of composition is awaited for humic substances, their complexation properties do not seem to greatly depend on their origins. The information on the difference in the structure of these complexes is scarce. To participate in the filling of this lack, a study of the spectral and temporal evolution of the Eu(III) luminescence implied in humic substance (HS) complexes is presented. Seven different extracts, namely Suwannee River fulvic acid (SRFA) and humic acid (SRHA), and Leonardite HA (LHA) from the International Humic Substances Society (USA), humic acid from Gorleben (GohyHA), and from the Kleiner Kranichsee bog (KFA, KHA) from Germany, and purified commercial Aldrich HA (PAHA), were made to contact with Eu(III). Eu(III)-HS time-resolved luminescence properties were compared with aqueous Eu(3+) at pH 5. Using an excitation wavelength of 394 nm, the typical bi-exponential luminescence decay for Eu(III)-HS complexes is common to all the samples. The components tau(1) and tau(2) are in the same order of magnitude for all the samples, i.e., 40 (7)F(2) transition exhibits the most striking differences. A slight blue shift is observed compared to aqueous Eu(3+) (lambda(max) = 615.4 nm), and the humic samples share almost the same lambda(max) approximately 614.5 nm. The main differences between the samples reside in a shoulder around lambda approximately 612.5 nm, modelled by a mixed Gaussian-Lorentzian band around lambda approximately 612 nm. SRFA shows the most intense shoulder with an intensity ratio of I(612.5)/I(614.7) = 1.1, KFA/KHA/SRHA share almost the same ratio I(612.5)/I(614.7) = 1.2-1.3, whilst the LHA/GohyHA/PAHA group has a I(612.5)/I(614.5) = 1.5-1.6. This shows that for the two groups of complexes, despite comparable complexing properties, slightly different symmetries are awaited.

Fractionation of Suwannee River fulvic acid and aldrich humic acid on alpha-Al2O3: spectroscopic evidence.

Sorptive fractionation of Suwannee River Fulvic Acid (SRFA) and Purified Aldrich Humic Acid (PAHA) on alpha-Al2O3 at pH 6 was probed in the supernatant using different spectroscopic techniques. Comparison of dissolved organic carbon (DOC) analysis with UV/vis spectrophotometric measurements at 254 nm, including specific UV absorbance (SUVA) calculation, revealed a decrease in chromophoric compounds for the nonsorbed extracts after a 24 h contact time. This fractionation, only observable below a certain ratio between initial number of sites of humic substances and of alpha-Al2O3, seems to indicate a higher fractionation for PAHA. C(1s) near-edge X-ray absorption fine structure spectroscopy (NEXAFS) confirmed this trend and points to a decrease in phenolic moieties in the supernatant and to an eventual increase in phenolic moieties on the surface. Time-resolved luminescence spectroscopy (TRLS) of Eu(III) as luminescent probe showed a decrease in the ratio between the (5)D0-->(7)F2 and (5)D0-->(7)F1 transitions for the fractionated organic matter (OM) that is thought to be associated with a lower energy transfer from the OM to Eu(III) due to the loss of polar aromatics. These modifications in the supernatant are a hint for the modification of sorbed humic extracts on the surface.

Spectromicroscopy mapping of colloidal/particulate organic matter in Lake Brienz, Switzerland.

Transmission electron microscopy (TEM), soft X-ray scanning transmission X-ray microscopy (STXM), and mu-FTIR spectromicroscopy were used to map colloidal/ particulate material in an ultra-oligotrophic lake, Lake Brienz, Switzerland, with a special focus on organic functionality. Within the statistical margin of error and the uncertainties arising from the representativeness of the results, the research reveals that organic material was associated with potassium-rich inorganic colloids present in surface and deep water (depths of 1 and 100 m, respectively), which indicates a vertical transfer of aggregates by sedimentation. Pure organic colloids could only be detected in surface waters. In addition, correlation map analysis of synchrotron-based mu-FTIR and carbon K-edge STXM spectromicroscopic data using spectra from the Lütschine and Aare Rivers as target spectra revealed spectral similarities with organic components from both tributary rivers in deeper regions (100 m) of the lake. The results prove that STXM and mu-FTIR can characterize colloidal and particulate organic material in low organic carbon systems.

Generation of humic and fulvic acid from Callovo-Oxfordian clay under high alkaline conditions.

Low-carbon-containing clay from four different depths (447 to 516 m) of the Meuse Haute Marne (MHM) site is kept in contact with alkaline solution simulating conditions expected from cement dissolution in the near-field of a nuclear waste repository. Original organic material in the clay consists mainly of aliphatic hydrophobic compounds basically without oxygen-containing functional groups. After contact with 'solid young fluid' (mimicking cement dissolution, initial pH 13.22) for approximately one and a half years, high concentrations of hydrophilic organic matter are found (243-355 mg DOC/L). Characterization by solubility behavior, UV/Vis absorption, IR and fluorescence properties show that the dissolved hydrophilic organic matter has the characteristic features of humic and fulvic acids. Estimation of humic and fulvic acid content via UV/Vis spectroscopy results in 97.5 (+/-9.7)% of DOC being humic and fulvic acid. The results indicate that this could be an important source of complexing mobile organic matter influencing the mobility of radionuclides in a nuclear waste repository under consideration for this site. Investigations were conducted under oxic conditions representing the situation in the excavation disturbed zone (EDZ) of an underground facility. Sample amounts were very small and thus some characterization results are partly of preliminary character.