Structures of multinuclear U(VI) species on the hydroxylated α-SiO2(001) surface: insights from DFT calculations.

Multinuclear U(VI) species may be dominant in aqueous solutions under environmental conditions, while the structures of the multinuclear U(VI) species on mineral surfaces remain unclear. This work reports the structural and bonding properties of the possible surface complexes of three aqueous multinuclear U(VI) species, i.e., (UO2)2(OH)3+, (UO2)2(OH)22+ and (UO2)3(O)(OH)3+, on the hydroxylated α-SiO2(001) surface based on density functional theory (DFT) calculations. The results show that (UO2)2(OH)22+ and (UO2)3(O)(OH)3+ tend to form end-on structures at SiO(H)SiO(H) sites, whereas (UO2)2(OH)3+ prefers a side-on structure at SiO(H)O(H)-SiO(H)O(H) sites. The main driving forces for the formation of the multinuclear U(VI) surface complexes are electrostatic interactions and partially covalent chemical bonds. The Os-2p orbital hybridizes strongly with U-5f and U-6d orbitals, with a decreasing binding strength in the sequence of (UO2)2(OH)3+ > (UO2)2(OH)22+ > (UO2)3(O)(OH)3+ for the adsorption at the same type of surface sites. For the adsorption of the same multinuclear U(VI) species, the binding energy increases with the deprotonation extent of the identical sites. In addition, hydrogen bonds between surface hydroxyls and coordination waters as well as the acyl oxygen of uranyl moieties contribute to the formation of the multinuclear U(VI) surface complexes. The U-5f electron delocalization of far-side U atoms in the end-on structures of (UO2)2(OH)22+ and (UO2)3(O)(OH)3+ surface complexes also contributes slightly to the overall binding energy. Overall, this study provides insights into the adsorption behavior of multinuclear U(VI) on silica.

Acid-Base Properties of Cis-Vacant Montmorillonite Edge Surfaces: A Combined First-Principles Molecular Dynamics and Surface Complexation Modeling Approach.

Montmorillonite layer edge surfaces have pH-dependent properties, which arises from the acid-base reactivity of their surface functional groups. Edge surface acidity (with intrinsic reaction equilibrium constant, pKa) is a chemical property that is affected by crystal structure. While a cis-vacant structure predominates in natural montmorillonites, prior molecular-level studies assume a centrosymmetric trans-vacant configuration, which potentially leads to an incorrect prediction of montmorillonite acid-base surface properties. We computed intrinsic acidity constants of the surface sites of a montmorillonite layer with a cis-vacant structure using the first-principles molecular dynamics-based vertical energy gap method. We evaluated pKa values for both non-substituted and Mg-substituted layers on common edge surfaces (i.e., surfaces perpendicular to [010], [01̅0], [110], and [1̅1̅0] crystallographic directions). The functional groups ≡Si(OH), ≡Al(OH2)2/≡Al(OH)(OH2), and ≡SiO(OH)Al sites on surfaces perpendicular to [010] and [01̅0] and ≡Si(OH)U, ≡Si(OH)L, ≡Al(OH2), and ≡Al(OH2)2 on surfaces perpendicular to [110] and [1̅1̅0] determine the proton reactivity of non-substituted cis-vacant edge surfaces. Moreover, the structural OH sites on edge surfaces had extremely high pKa values, which do not show reactivity at a common pH. Meanwhile, Mg2+ substitution results in an increase in pKa values at local or adjacent sites, in which the effect is limited by the distance between the sites. A surface complexation model was built with predicted pKa values, which enabled us to predict surface properties as a function of pH and ionic strength. Edge surface charge of both trans- and cis-vacant models has little dependence on Mg2+ substitutions, but the dependence on the crystal plane orientation is strong. In particular, at pH below 7, edge surfaces are positively or negatively charged depending on their orientation. Implications of these findings on contaminant adsorption by smectites are discussed.

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.

Mechanistic Understanding of Uranyl Ion Complexation on Montmorillonite Edges: A Combined First-Principles Molecular Dynamics-Surface Complexation Modeling Approach.

Systematic first-principles molecular dynamics (FPMD) simulations were carried out to study the structures, free energies, and acidity constants of UO22+ surface complexes on montmorillonite in order to elucidate the surface complexation mechanisms of the uranyl ion (UO22+) on clay mineral edges at the atomic scale. Four representative complexing sites were investigated, that is, ≡Al(OH)2 and ≡AlOHSiO on the (010) surface and ≡AlOHOa and ≡SiOOa on the (110) surface. The results show that uranyl ions form bidentate complexes on these sites. All calculated binding free energies for these complexes are very similar. These bidentate complexes can be hydrolyzed, and their corresponding derived p Ka values (around 5.0 and 9.0 for p Ka1 and p Ka2, respectively) indicate that UO2(OH)+ and UO2(OH)2 surface groups are the dominant surface species in the environmental pH range. The OH groups of UO2(OH)2 surface complexes can act as complexing sites for subsequent metals. Additional simulations showed that such multinuclear adsorption is feasible and can be important at high pH. Furthermore, FPMD simulation results served as input parameters for an electrostatic thermodynamic surface complexation model (SCM) that adequately reproduced adsorption data from the literature. Overall, this study provides an improved understanding of UO22+ complexation on clay mineral edge surfaces.

Selenite Uptake by Ca-Al LDH: A Description of Intercalated Anion Coordination Geometries.

Layered double hydroxides (LDHs) are anion exchangers with a strong potential to scavenge anionic contaminants in aquatic environments. Here, the uptake of selenite (SeO32-) by Ca-Al LDHs was investigated as a function of Se concentration. Thermodynamic modeling of batch sorption isotherms shows that the formation of SeO32--intercalated AFm (hydrated calcium aluminate monosubstituent) phase, AFm-SeO3, is the dominant mechanism controlling the retention of Se at medium loadings. AFm-Cl2 shows much stronger affinity and larger distribution ratio (Rd ∼ 17800 L kg-1) toward SeO32- than AFm-SO4 (Rd ∼ 705 L kg-1). At stoichiometric SeO32- loading for anion exchange, the newly formed AFm-SeO3 phase results in two basal spacing, i.e., 9.93 ± 0.06 Å and ∼11.03 ± 0.03 Å. Extended X-ray absorption fine structure (EXAFS) spectra indicate that the intercalated SeO32- forms inner-sphere complexes with the Ca-Al-O layers. In situ X-ray diffraction (XRD) shows that basal spacing of Ca-Al LDHs have a remarkable linear relationship with the size of hydrated intercalated anions (i.e., Cl-, SO42-, MoO42-, and SeO32-). Contrary to AFm-SeO3 with inner-sphere SeO32- complexes in the interlayer, the phase with hydrogen-bonded inner-sphere complexed SeO32- is kinetically favored but thermodynamically unstable. This work offers new insights about the determination of intercalated anion coordination geometries via XRD analyses.

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.

Modeling the Acid-Base Properties of Montmorillonite Edge Surfaces.

The surface reactivity of clay minerals remains challenging to characterize because of a duality of adsorption surfaces and mechanisms that does not exist in the case of simple oxide surfaces: edge surfaces of clay minerals have a variable proton surface charge arising from hydroxyl functional groups, whereas basal surfaces have a permanent negative charge arising from isomorphic substitutions. Hence, the relationship between surface charge and surface potential on edge surfaces cannot be described using the Gouy-Chapman relation, because of a spillover of negative electrostatic potential from the basal surface onto the edge surface. While surface complexation models can be modified to account for these features, a predictive fit of experimental data was not possible until recently, because of uncertainty regarding the densities and intrinsic pKa values of edge functional groups. Here, we reexamine this problem in light of new knowledge on intrinsic pKa values obtained over the past decade using ab initio molecular dynamics simulations, and we propose a new formalism to describe edge functional groups. Our simulation results yield reasonable predictions of the best available experimental acid-base titration data.

Influence of salinity gradients on the diffusion of water and ionic species in dual porosity clay samples.

Most of the available data on diffusion in natural clayey rocks consider tracer diffusion in the absence of a salinity gradient despite the fact that such gradients are frequently found in natural and engineered subsurface environments. To assess the role of such gradients on the diffusion properties of clayey materials, through-diffusion experiments were carried out in the presence and absence of a salinity gradient using salt-diffusion and radioisotope tracer techniques. The experiments were carried out with vermiculite samples that contained equal proportions of interparticle and interlayer porosities so as to assess also the role played by the two types of porosities on the diffusion of water and ions. Data were interpreted using both a classical Fickian diffusion model and with a reactive transport code, CrunchClay that can handle multi-porosity diffusion processes in the presence of charged surfaces. By combining experimental and simulated data, we demonstrated that (i) the flux of water diffusing through vermiculite interlayer porosity was minor compared to that diffusing through the interparticle porosity, and (ii) a model considering at least three types of porous volumes (interlayer, interparticle diffuse layer, and bulk interparticle) was necessary to reproduce consistently the variations of neutral and charged species diffusion as a function of salinity gradient conditions.

Resolving experimental biases in the interpretation of diffusion experiments with a user-friendly numerical reactive transport approach.

The reactive transport code CrunchClay was used to derive effective diffusion coefficients (De), clay porosities (ε), and adsorption distribution coefficients (KD) from through-diffusion data while considering accurately the influence of unavoidable experimental biases on the estimation of these diffusion parameters. These effects include the presence of filters holding the solid sample in place, the variations in concentration gradients across the diffusion cell due to sampling events, the impact of tubing/dead volumes on the estimation of diffusive fluxes and sample porosity, and the effects of O-ring-filter setups on the delivery of solutions to the clay packing. Doing so, the direct modeling of the measurements of (radio)tracer concentrations in reservoirs is more accurate than that of data converted directly into diffusive fluxes. While the above-mentioned effects have already been described individually in the literature, a consistent modeling approach addressing all these issues at the same time has never been described nor made easily available to the community. A graphical user interface, CrunchEase, was created, which supports the user by automating the creation of input files, the running of simulations, and the extraction and comparison of data and simulation results. While a classical model considering an effective diffusion coefficient, a porosity and a solid/solution distribution coefficient (De-ε-KD) may be implemented in any reactive transport code, the development of CrunchEase makes it easy to apply by experimentalists without a background in reactive transport modeling. CrunchEase makes it also possible to transition more easily from a De-ε-KD modeling approach to a state-of-the-art process-based understanding modeling approach using the full capabilities of CrunchClay, which include surface complexation modeling and a multi-porosity description of the clay packing with charged diffuse layers.

Smectite phase separation is driven by hydration-mediated interfacial charge.

Smectite clay minerals have an outsize impact on the response of clay-rich media to common stimuli, such as hydration and ion exchange, motivating extensive effort to understand behaviors resulting from these processes such as swelling and exfoliation. Smectites are common and historic systems for investigating colloidal and interfacial phenomena, with two swelling regimes commonly identified across myriad clays: osmotic swelling at high water activity and crystalline swelling at low water activity. However, no current swelling model seamlessly spans the full ranges of water, salt and clay content encountered in natural or engineered settings. Here, we show that structures previously rationalized as either osmotic or crystalline coexist as a rich array of distinct colloidal phases that differ by water content, layer stacking thickness, and curvature. We present an analytical model for intermolecular potentials among water, salt and clay in both mono- and divalent electrolytes that predicts swelling pressures across high and low water activities. Our results indicate that all clay swelling is osmotic swelling, but that the osmotic pressure of charged mineral interfaces becomes attractive and dominates that of the electrolyte at high clay activities. Global energy minima are often not reached on experimental timescales due to many local energy minima that promote long-lived intermediate states with vast differences in clay, ion, and water mobilities, leading to hyperdiffusive layer dynamics driven by variable hydration-mediated interfacial charge. Teaser Distinct colloidal phases of swelling clays emerge via ion (de)hydration at mineral interfaces that drives hyperdiffusive layer dynamics as metastable smectites approach equilibrium.

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.

Retention of arsenic, chromium and boron on an outcropping clay-rich rock formation (the Tégulines Clay, eastern France).

The retention behavior of three toxic chemicals, As, Cr and B, was investigated for an outcropping rock formation, the Albian Tégulines Clay (France, Aube). At a shallow depth, Tégulines Clay is affected by weathering processes leading to contrasted geochemical conditions with depth. One of the main features of the weathering is the occurrence of a redox transition zone near the surface. Batch sorption experiments of As(V), As(III), Cr(VI) and B were performed on samples collected at two depths representative either of oxidized or reduced mineral assemblages. Batch sorption experiments highlighted a distinct behavior between As, Cr and B oxyanions. Cr(VI) retention behavior was dominated by redox phenomena, notably its reduction to Cr(III). The in-situ redox state of the Tégulines Clay samples has a significant effect on Cr retention. On the contrary, As(V) reduction into As(III) is moderate and its retention slightly affected by the in-situ redox state of the Tégulines Clay. As(V) retention is higher than As(III) retention in agreement with literature data. B retention is strongly related to its natural abundance in the Tégulines clay samples. Distribution coefficient of B corrected from its natural content is expected to be very low for in-situ conditions. Finally, the retention and mobility of these oxyanions were affected by clay mineralogy, natural abundance, and reducing capacity of the Tegulines Clay.

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.

Spectral induced polarization of Na-montmorillonite dispersions.

Montmorillonite (Mt) clays have a high specific surface area and surface charge, which confer them remarkable adsorption properties. Nevertheless, their electrochemical and aggregation behavior are not completely elucidated because of the complexity of their microstructural and interfacial properties. In this work, the conductive and dispersive properties of Na-Mt suspensions of weight fractions 0.5-5.2% were investigated for the first time using the spectral induced polarization method. A four-electrode system was used to reduce errors introduced by electrode polarization and contact resistances. Complex conductivity spectra in the low-frequency range of 0.1Hz to 45kHz were successfully described using a triple layer model of the basal surface of Mt and a complex conductivity model that considers conduction of the diffuse layer and polarization of the Stern layer. Aggregate size distributions were inferred from inverted relaxation time distributions. We found that the negative and permanent surface charge of the basal plane of Na-Mt controls its quadrature (imaginary) conductivity, which is not very sensitive to pH and salinity (NaCl) in the 100Hz to 45kHz frequency range. For lower frequencies, the sudden increase of the quadrature conductivity at the highest salinities was explained by considering coagulation of Na-Mt particles.

The electrophoretic mobility of montmorillonite. Zeta potential and surface conductivity effects.

Clay minerals have remarkable adsorption properties because of their high specific surface area and surface charge density, which give rise to high electrochemical properties. These electrochemical properties cannot be directly measured, and models must be developed to estimate the electrostatic potential at the vicinity of clay mineral surfaces. In this context, an important model prediction is the zeta potential, which is thought to be representative of the electrostatic potential at the plane of shear. The zeta potential is usually deduced from electrophoretic measurements but for clay minerals, high surface conductivity decreases their mobility, thereby impeding straightforward interpretation of these measurements. By combining a surface complexation, conductivity and electrophoretic mobility model, we were able to reconcile zeta potential predictions with electrophoretic measurements on montmorillonite immersed in NaCl aqueous solutions. The electrochemical properties of the Stern and diffuse layers of the basal surfaces were computed by a triple-layer model. Computed zeta potentials have considerably higher amplitudes than measured zeta potentials calculated with the Smoluchowski equation. Our model successfully reproduced measured electrophoretic mobilities. This confirmed our assumptions that surface conductivity may be responsible for montmorillonite's low electrophoretic mobility and that the zeta potential may be located at the beginning of the diffuse layer.

The titration of clay minerals II. Structure-based model and implications for clay reactivity.

The potentiometric titration and CEC data presented in part I are modeled in this paper, part II. Two models are compared: the two pK, three complexation sites plus exchange sites nonelectrostatic model developed by Baeyens and Bradbury and a model based on the MUSIC approach developed by Hiemstra and Van Riemsdijk. Both morphological and structural information is used to develop this new model. Morphological information is taken from the literature, while structural information is taken from the literature and constrained by supporting FTIR experiments. The Baeyens and Bradbury model is found to reproduce the general tendency of the titration curve, whereas the model based on the Hiemstra and Van Riemsdijk MUSIC approach provides a better fit to the experimental data. The former uses only 3 edge reaction sites, whereas the latter uses at least 27 edge reaction sites. Five main reactive sites are sufficient to fit the MUSIC model curve, but the model allows us to derive the properties of 22 other reactive sites. Logically, the greater the number of sites, the better the fit. Nevertheless, fewer adjustable parameters are necessary for the Hiemstra and Van Riemsdijk MUSIC model than for the Baeyens and Bradbury model, thanks to structural and morphological constraints. The precision of the potentiometric titration curve is insufficient to verify that the properties of the 27 sites given by the MUSIC model are effective. Thus, we coupled some properties of clay minerals, such as dissolution, to the modeled acid-base properties of these sites to assess our model. We then questioned the ability of simplified models such as the Baeyens and Bradbury model to predict the interactions between clay minerals and solutions in natural environments. In addition, we derived the cation exchange selectivity coefficients for CaCl+ ionic pairs and H+ from our CEC data and gave an estimate for the CaOH+ selectivity coefficient.

The titration of clay minerals I. Discontinuous backtitration technique combined with CEC measurements.

Previous experimental studies on clay potentiometric titration have been unable to distinguish inorganic cation exchange in the interlayer and on basal plane surfaces from specific pH-dependent sorption of cations and anions on the edges. In this study, we refined a titration technique, combining discontinuous backtitration and cation exchange capacity (CEC) measurements, and applied it to the potentiometric titration of Na- and Ca-conditioned montmorillonites. This technique can be used to accurately measure cation exchange, edge surface proton charge, dissolution of clay, and precipitation of new phases. Thus, a precise measurement of the variations of net proton surface charge is possible. This has important implications for clay surface modeling (see part II of this article) and for processes that depend on the clay surface charge, e.g., alteration, rheological processes, and contamination retention applications. In addition, this study confirms the adsorption of ionic pairs such as CaCl+ in exchange site positions and shows that CaOH+ could behave like CaCl+. This result, together with the evidence of precipitation of a Ca?Si phase over a short time-scale (1 week) at high pH and low temperature, can be used to model clay-concrete interactions more accurately. We confirmed and quantified the H+/Na+ exchange reaction at low pH. Finally, we demonstrate that both the edge surface charge and the permanent structural charge are compensated for by the nonspecific sorption of cations from solution across the entire pH range from 4 to 11. Under these conditions, the surface potential is fully screened and does not need to be invoked in modeling sorption processes on clay particles in dilute suspensions.

Modelling CEC variations versus structural iron reduction levels in dioctahedral smectites. Existing approaches, new data and model refinements.

A model was developed to describe how the 2:1 layer excess negative charge induced by the reduction of Fe(III) to Fe(II) by sodium dithionite buffered with citrate-bicarbonate is balanced and applied to nontronites. This model is based on new experimental data and extends structural interpretation introduced by a former model [36-38]. The 2:1 layer negative charge increase due to Fe(III) to Fe(II) reduction is balanced by an excess adsorption of cations in the clay interlayers and a specific sorption of H(+) from solution. Prevalence of one compensating mechanism over the other is related to the growing lattice distortion induced by structural Fe(III) reduction. At low reduction levels, cation adsorption dominates and some of the incorporated protons react with structural OH groups, leading to a dehydroxylation of the structure. Starting from a moderate reduction level, other structural changes occur, leading to a reorganisation of the octahedral and tetrahedral lattice: migration or release of cations, intense dehydroxylation and bonding of protons to undersaturated oxygen atoms. Experimental data highlight some particular properties of ferruginous smectites regarding chemical reduction. Contrary to previous assumptions, the negative layer charge of nontronites does not only increase towards a plateau value upon reduction. A peak is observed in the reduction domain. After this peak, the negative layer charge decreases upon extended reduction (>30%). The decrease is so dramatic that the layer charge of highly reduced nontronites can fall below that of its fully oxidised counterpart. Furthermore, the presence of a large amount of tetrahedral Fe seems to promote intense clay structural changes and Fe reducibility. Our newly acquired data clearly show that models currently available in the literature cannot be applied to the whole reduction range of clay structural Fe. Moreover, changes in the model normalising procedure clearly demonstrate that the investigated low tetrahedral bearing nontronites (SWa-1, GAN and NAu-1) all exhibit the same behaviour at low reduction levels. Consequently, we restricted our model to the case of moderate reduction (<30%) in low tetrahedral Fe-bearing nontronites. Our adapted model provides the relative amounts of Na(+) (p) and H(+) (ni) cations incorporated in the structure as a function of the amount of Fe reduction. Two equations enable the investigated systems to be described: p=m/(1+Kr·ω·mrel) and ni=Kr·ω·m·mrel/(1+Kr·ω·mrel); where m is the Fe(II) content, mrel, the reduction level (m/mtot), ω, the cation exchange capacity (CEC, and Kr, an empirical constant specific to the system.

Influence of surface conductivity on the apparent zeta potential of TiO2 nanoparticles.

Zeta potential is a physico-chemical parameter of particular importance in describing ion adsorption and electrostatic interactions between charged particles. Nevertheless, this fundamental parameter is ill-constrained, because its experimental interpretation is complex, particularly for very small and charged TiO(2) nanoparticles. The excess of electrical charge at the interface is responsible for surface conductance, which can significantly lower the electrophoretic measurements, and hence the apparent zeta potential. Consequently, the intrinsic zeta potential can have a larger amplitude, even in the case of simple 1:1 electrolytes like NaCl and KCl. Surface conductance of TiO(2) nanoparticles immersed in a NaCl solution is estimated using a surface complexation model, and this parameter and particle size are incorporated into Henry's model in order to determine a constrained value of the zeta potential from electrophoresis. Interior conductivity of the agglomerates is calculated using a differential self-consistent model. The amplitude of estimated zeta potential is greater than that derived from the von Smoluchowski equation and corresponds to the electric potential at the outer Helmholtz plane calculated by our surface complexation model. Consequently, the shear plane may be located close to the OHP, contradicting the assumption of the presence of a stagnant diffuse layer at the TiO(2)/water interface.