Charged Clay Surfaces Research Articles

Silica Nanorings on the Surfaces of Layered Silicate

A simple approach to the synthesis of clay-silica nanocomposites is presented. Silica nanorings on the edges of clay sheets were synthesized by using a modified Stöber method. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and fluorescence spectroscopy were employed to characterize the prepared nanocomposites. TEM results show that the average size of the nanorings increases with the growth of silica. XRD results indicate that the layered structures of clay can be found in the nanocomposite and the growth of silica nanorings expands the d spacing of clay platelets. The mechanism of the formation of the nanorings is discussed. The preparation of polystyrene (PS) brushes on the surfaces of silica nanorings by atom-transfer radical polymerization is also reported. The polymer nanocomposite with negatively charged clay surfaces and hydrophobic polymer brushes on the silica nanorings can be used in Pickering emulsions, and PS colloidal particles with clay-silica on the surfaces were prepared.

Diffusion and sorption of Cs +, I − and HTO in samples of the argillaceous Wakkanai Formation from the Horonobe URL, Japan: Clay-based modeling approach

Diffusion and sorption behaviors of cationic Cs +, anionic I − and neutral HTO in samples of the Wakkanai Formation from the Horonobe underground research laboratory (URL), Japan, were investigated as a function of ionic strength ( I) of synthetic groundwater by through-diffusion and batch sorption experiments and mechanistic modeling. The effective diffusivities ( D e) measured by through-diffusion experiments showed cation excess and anion exclusion effects, which were strongly dependent on I; D e for Cs + decreased as I increased, D e for I − showed the opposite dependency and D e for HTO showed no dependence. The sorption of Cs + measured by through-diffusion and batch sorption experiments were described by Freundlich isotherms with consistent parameters and decreased with I as a result of competitive ion exchange. Diffusion and sorption behaviors were interpreted by assuming the clay components of illite and smectite control diffusion and sorption mechanisms. The component additive (CA) sorption model, which includes illite and smectite contents and their ion exchange constants, provided a reasonable account of the Cs + sorption trends measured as functions of I and Cs concentration. The diffusion model was developed by coupling the electrical double layer (EDL) model, describing the change of ionic concentrations (cation excess and anion deficit) and viscoelectric effects caused by electrostatic interaction at negatively charged clay surfaces, and a simplified pore model assuming one type of pore shape and includes their size distribution. When averaging the electrostatic effects by using the pore surface area distribution, the model could predict the cation excess and anion exclusion effects, and its dependence on I reasonably well. This result implies the nanoscale pores dominating the pore surface area can strongly impact on ionic diffusion in argillaceous rocks. The clay-based modeling approach described here provides a useful tool to predict ionic diffusion and sorption in argillaceous rocks.

The molecular aggregation of pyronin Y in natural bentonite clay suspension

The molecular aggregation and spectroscopic properties of Pyronin Y (PyY) in the suspension containing natural bentonite clay were studied using molecular absorption, steady-state and time-resolved fluorescence spectroscopy techniques. Interaction between the clay particles and the cationic dye compounds in aqueous solution resulted in significant changes in spectral properties of PyY compared to its molecular behavior in deionized water at the same concentration. These changes were due to the formation of dimer and aggregate of PyY in the clay suspension as well as the presence of the dye monomer. The H-type aggregates of PyY in the clay suspension were identified by the observation of a blue-shifted absorption band of the dye compared to that of its monomer. In spite of diluted dye concentrations, the H-aggregate of PyY in the clay suspension was formed. The intensive aggregation in the clay suspension attributed to the localized high dye concentration on the negatively charged clay surfaces. Adsorption sites of PyY on the clay particles were discussed by deconvulated absorption and excitation spectra. Fluorescence spectroscopy studies revealed that the fluorescence intensity of PyY in the clay suspension is decreased by H-aggregates drastically. Moreover, the presence of H-aggregates in the clay suspension resulted in the decrease of fluorescence lifetime and quantum yield of PyY compared to those in deionized water.

Pentachlorophenol radical cations generated on Fe(III)-montmorillonite initiate octachlorodibenzo-p-dioxin formation in clays: density functional theory and fourier transform infrared studies.

Octachlorodibenzodioxin (OCDD) forms spontaneously from pentachlorophenol (PCP) on the surfaces of Fe(III)-saturated smectite clay. (1) Here, we used in situ Fourier transform infrared (FTIR) methods and quantum mechanical calculations to determine the mechanism by which this reaction is initiated. As the clay was dehydrated, vibrational spectra showed new peaks that grew and then reversibly disappeared as the clay rehydrated. First-principle density functional theory calculations of hydrated Fe/PCP clusters reproduced these transient FTIR peaks when inner-sphere complexation and concomitant electron transfer produced Fe(II) and PCP radical cations. Thus, our experimental (FTIR) and theoretical (quantum mechanical) results mutually support the hypothesis that OCDD formation on Fe-smectite surfaces is initiated by the reversible formation of metastable PCP radical cations via single-electron transfer from PCP to Fe(III). The negatively charged clay surface apparently selects for this reaction mechanism by stabilizing PCP radical cations.

Cation exchange interaction between antibiotic ciprofloxacin and montmorillonite

Exploring the interactions between antibiotics and soils/minerals is of great importance in resolving their fate, transport, and elimination in the environment due to their frequent detection in wastewater, river water, sewage sludge and soils. This study focused on determining the adsorption properties and mechanisms of interaction between antibiotic ciprofloxacin and montmorillonite (SAz-1), a swelling dioctahedral mineral with Ca 2+ as the main interlayer cation. In acidic and neutral aqueous solutions, a stoichiometric exchange between ciprofloxacin and interlayer cations yielded an adsorption capacity as high as 330 mg/g, corresponding to 1.0 mmol/g. When solution pH was above its p K a2 (8.7), adsorption of ciprofloxacin was greatly reduced due to the net repulsion between the negatively charged clay surfaces and the ciprofloxacin anion. The uptake of ciprofloxacin expanded the basal spacing ( d 0 0 1 ) of montmorillonite from 15.04 to 17.23 Å near its adsorption capacity, confirming cation exchange within the interlayers in addition to surface adsorption. Fourier transform infrared results further suggested that the protonated amine group of ciprofloxacin in its cationic form was electrostatically attracted to negatively charged sites of clay surfaces, and that the carboxylic acid group was hydrogen bonded to the basal oxygen atoms of the silicate layers. The results indicate that montmorillonite is an effective sorbent to remove ciprofloxacin from water.

Linking the Diffusion of Water in Compacted Clays at Two Different Time Scales: Tracer Through-Diffusion and Quasielastic Neutron Scattering

Diffusion of water and solutes through compacted clays or claystones is important when assessing the barrier function of engineered or geological barriers in waste disposal. The shape and the connectivity of the pore network as well as electrostatic interactions between the diffusant and the charged clay surfaces or cations compensating negative surface charges affect the resistance of the porous medium to diffusion. Comparing diffusion measurements performed at different spatial or time scales allows identification and extraction of the different factors. We quantified the electrostatic constraint q for five different highly compacted clays (rhob = 1.85 +/- 0.05 g/cm3) using quasielastic neutron scattering (QENS) data. We then compared the QENS data with macroscopic diffusion data for the same clays and could derive the true geometric tortuosities G of the samples. Knowing the geometric and electrostatic factors for the different clays is essential when trying to predict diffusion coefficients for other conditions. We furthermore compared the activation energies Ea for diffusion at the two measurement scales. Because Ea is mostly influenced by the local, pore scale surroundings of the water, we expected the results to be similar at both scales. This was indeed the case for the nonswelling clays kaolinite and illite, which had Ea values lower than that of bulk water, but not for montmorillonite, which had values lower than that in bulk water at the microscopic scale, but larger at the macroscopic scale. The differences could be connected to the strongly temperature dependent mobility of the cations in the clays, which may act as local barriers in the narrow pores at low temperatures.

Computer simulation study of the materials properties of intercalated and exfoliated poly(ethylene)glycol clay nanocomposites

Very large-scale molecular dynamics simulations are performed to investigate the effects of montmorillonite clay filler on poly(ethylene) glycol in the formation of clay-polymer nanocomposites. We present the results of MD simulations of intercalated and exfoliated nanocomposites at sizes which approach those of a realistic clay platelet. The simulations allow us to determine the difference between polymer adsorbed on the surface of the clay and that more remote from it. All polymers arrange themselves in layers parallel to the surface, each layer being approximately 4 A thick. We find the polymer conformation of the inner-most layer is distinct, due to complexation with the counterions found near the charged clay surface. The diffusion of polymer within this layer is much lower than that of others layers, which effectively increases the size of the nanofiller and makes gas permeation more tortuous. We perform non-equilibrium molecular dynamics simulations by imposing a strain on the model and analysing the stress response. By partitioning the stress response into clay and different polymer layers, we find that the inner-most polymer layer has a much higher Young's modulus than the remaining layers in the direction of the polymer chains.

Hydrophobation of Clay Particles by Asphaltenes Adsorption

In order to throw light on the mechanism of organic compounds–clay interactions and to control the preparation of hydrophobic fillers, the effect of asphaltene adsorption from water-saturated toluene onto various clay minerals (kaolinite, illite) was studied. The modification of the clay surface charge and wettability were characterized, respectively, by microelectrophoresis and Washburn contact angle measurements. Further, the alteration of the surface area and chemical composition of the clay particles due to asphaltene adsorption were assessed using various methods such as N2-adsorption/desorption, X-ray diffraction (XRD) measurements, FT-IR and scanning electronic microscopy. The data indicate that upon asphaltene adsorption, the clay particles become hydrophobic, their contact angle with water increases, while their surface charge and surface energy are reduced. Furthermore, according to XRD data, no swelling of the clay by asphaltenes occurs in the water-saturated toluene, indicating that asphaltenes cover the clay's external surface and lead to their aggregation, either in water or in the vacuum dried state. Finally, we elucidate the mechanisms by which asphaltenes adsorb and modify the clay surface characteristics. Such mechanisms involve mainly interactions that occur between the asphaltenes' surface groups and silanol/aluminol groups of the mineral.

Translational diffusion of water and its dependence on temperature in charged and uncharged clays: A neutron scattering study

The water diffusion in four different, highly compacted clays [montmorillonite in the Na- and Ca-forms, illite in the Na- and Ca-forms, kaolinite, and pyrophyllite (bulk dry density rho(b)=1.85+/-0.05 gcm(3))] was studied at the atomic level by means of quasielastic neutron scattering. The experiments were performed on two time-of-flight spectrometers and at three different energy resolutions [FOCUS at SINQ, PSI (3.65 and 5.75 A), and TOFTOF at FRM II (10 A)] for reliable data analysis and at temperatures between 27 and 95 degrees C. Two different jump diffusion models were used to describe the translational motion. Both models describe the data equally well and give the following ranking of diffusion coefficients: Na-montmorillonite<or=Ca-montmorillonite<Ca-illite<Na-illite<water<or=pyrophyllite<or=kaolinite. Uncharged clays had slightly larger diffusion coefficients than that of bulk water due to their hydrophobic surfaces. The time between jumps, tau(t), follows the sequence: Ca-montmorillonite>or=Na-montmorillonite>Ca-illite>Na-illite>or=kaolinite>pyrophyllite>or=water, in both jump diffusion models. For clays with a permanent layer charge (montmorillonite and illite) a reduction in the water content by a factor of 2 resulted in a decrease in the self-diffusion coefficients and an increase in the time between jumps as compared to the full saturation. The uncharged clay kaolinite exhibited no change in the water mobility between the two hydration states. The rotational relaxation time of water was affected by the charged clay surfaces, especially in the case of montmorillonite; the uncharged clays presented a waterlike behavior. The activation energies for translational diffusion were calculated from the Arrhenius law, which adequately describes the systems in the studied temperature range. Na- and Ca-montmorillonite (approximately 11-12 kJmol), Na-illite (approximately 13 kJmol), kaolinite and pyrophyllite (approximately 14 kJmol), and Ca-illite (approximately 15 kJmol) all had lower activation energies than bulk water (approximately 17 kJmol in this study). This may originate from the reduced number and strength of the H-bonds between water and the clay surfaces, or ions, as compared to those in bulk water. Our comparative study suggests that the compensating cations in swelling clays have only a minor effect on the water diffusion rates at these high densities, whereas these cations influence the water motion in non-swelling clays.

Accumulation of Tobacco mosaic virus (TMV) at different depths clay and loamy sand textural soils due to tobacco waste application

The effects of tobacco waste (TW) application to the soil surface on the accumulation of Tobacco mosaic virus (TMV) in clay and loamy sand textural soils at various depths were investigated in two different fields. The tobacco waste had been found to be infected with TMV. Eighteen months after TW application to the soil surface, soils were sampled at 20 cm intervals through to 80 cm depth. The DAS-ELISA method was performed to determine infection of soil with TMV. The viruses persisted in clay soil for a long period compared with loamy sand soil. There was no accumulation of TMV at any depth of loamy sand soil in Experimental Field 2. TMV adsorption to soil particles in 0-60 cm depth of clay soil was determined in all TW treatments in Experimental Field 1. The highest ELISA Absorbance (A405) values in all treatments were determined in the 20-40 cm soil depth that had the highest clay content. ELISA A405 values of TMV at different depths of clay soil gave significant correlations with clay content (r = 0.793**), EC values (r = 0.421**) and soil pH (r = -0.405**). Adsorption of TMV to net negatively charged clay particle surfaces increased with increasing EC values of soil solution. Decreasing soil pH and infiltration rate increased adsorption of TMV to clay particles. Higher infiltration rate and lower clay content in loamy sand soil caused leaching of TMV from the soil profile.

Equilibrium self-polarization of clay

A condition of equilibrium between charged clay particle surfaces and the solution contained in the pore space of the rock is derived using electrical double layer theory. This condition is the relation linking the cation concentration in the middle surface of the pores with the exchange capacity of the clay, the total ion charges, and the specific surface area of the particles.

An Ion Diffusion Model in Semi-Permeable Clay Materials

Clay materials typically contain negative surface charges that induce electrostatic fields (or diffuse double layers) in electrolytes. During ion diffusion in a porous medium of clay materials, ions dynamically interact with the electrostatic fields associated with individual clay grains by depressing or expanding the electrostatic double layers, which subsequently affects ionic fluxes. Current theory of ion transport in porous media, however, cannot explicitly account for the dynamic interactions. Here we proposed a model by coupling electrodynamics and nonequilibrium thermodynamics (EDNT) to describe ion diffusion in clay materials as a complex function of factors including clay surface charge density, tortuosity, porosity, chemicoosmotic coefficient, and ion self-diffusivity. The model was validated by comparing the calculated and measured apparent ion diffusion coefficients in clay materials as a function of ionic strength. At transitional states, ion diffusive fluxes are dynamically related to the electrostatic fields, which shrink or expand as ion diffusion occurs. At steady states, the electrostatic fields are time-invariant and ion diffusive fluxes conform to flux and concentration gradient relationships; and apparent diffusivity can be approximated by the ion diffusivity in bulk electrolytes corrected by a tortuosity factor and macroscopic concentration discontinuities at the interfaces between clay materials and bulk solutions.

Biopolymer coated clay particles for the adsorption of tungsten from water

Biopolymer coated clay (i.e. chitosan coated montmorillonite clay called biosorbent) particles were synthesized. The effects of tungsten concentration in feed water (20–500 ppm) and water pH (4, 5.5 and 6.4) on the zeta potential of adsorbent particles, tungsten removal, and adsorption equilibrium were studied using biosorbent and natural clay. Coating clay particles with chitosan shifts the net surface charge of clay from negative to positive and the point of zero charge (PZC) of clay from 2.8 and 5.8. The net surface charge of biosorbent particles decreases with an increase in the tungsten concentration of feed water because the positively charged sites are consumed by the adsorption of tungsten anions. Biosorbent was found to be much more effective than natural clay for the removal of tungsten. The tungsten removal efficiencies of both biosorbent and natural clay decrease with an increase in the pH level and an increase in the tungsten concentration of the feed water. Adsorption equilibrium studies show that tungsten removal is the highest at pH 4. Adsorption equilibrium data were modeled with the Langmuir, Temkin, and Freundlich isotherms and the model parameters were obtained. Adsorption oftungsten species on both biosorbent and natural clay seemed to obey Langmuir isotherm within the range of concentrations and pH investigated. The maximum tungsten adsorption capacities of chitosan, biosorbent and natural clay were found to be 632 mg tungsten/g of chitosan, 23.9 mg tungsten/g of biosorbent, and 5.45 mg tungsten/g of natural clay at pH 4, respectively. The tungsten species adsorption on biosorbent was found to be governed by the ionic attraction between the protonated surface groups on chitosan and the negatively charged tungsten species. Whereas, the tungsten species adsorption on natural clay is governed by the positively charged clay particle edges formed by broken bonds of Al-O and Si-O.

Sorption of Pesticides on Kaolinite and Montmorillonite as a Function of Hydrophilicity

Pesticides and other organic species are adsorbed by soil via different mechanisms, with bond strengths that depend on the properties of both the soil and the pesticide. Since the clay fraction in soil is a preferential sorbent for organic matter, reference kaolinite and montmorillonite are useful models for studying the mechanism and the strength of sorption. This paper presents the results of batch experiments to investigate the interactions of kaolinite KGa-1 and montmorillonite SWy-1 with the following pesticides and organic species resulting from the natural degradation of pesticides in the environment: atrazine (1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine), simazine (1-chloro-3,5-bisethylamino-2,4,6-triazine), diuron [1,1-dimethyl-3-(3,4-dichlorophenyl)urea], aniline, 4-chlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol. Each of these chemicals has different hydrophilicity. Systems containing 2.0 g of clay were put in contact with 100.0 mL of solutions of the pesticides at known concentration ranging from 1.0 to 5.0 mg/L, and the amount of solute adsorbed was evaluated through RP-HPLC analysis of the pesticide still present in the aqueous suspension. To test for electrostatic interactions between the clay surface and the pesticides, potentiometric titration was used to determine the permanent surface charge of clays. Experiments were performed at different pH values. The results indicate that, for the chemicals studied, neutral molecules are preferentially retained relative to ionized ones, and that montmorillonite is a more effective sorbent than kaolinite.

Coupled chemical processes at clay/electrolyte interface: A batch titration study of Na-montmorillonites

The present work addresses the protolytic charge of montmorillonite, which occurs on the broken-bond sites at the particle edges. The purpose is to overcome the general difficulty arising in potentiometric titration due to coupled side reactions, which severely impede the titrant budget (partial dissolution of the clay and of secondary phases, hydrolysis and readsorption of dissolved species, cation exchange). Batch potentiometric titrations were carried out on the montmorillonite fractions extracted from two bentonites (MX80 and SWy2) to quantify their protolytic charge. The effects of equilibration time (24 h and 7 days), pH from 4 to 10, and ionic strength (0.1 and 0.01 mol L −1) were extensively studied for the MX80 sample. Quantification of dissolution was achieved by analysis of the equilibrium solutions for dissolved species and by La 3+ exchange of the readsorbed species. The results clearly show that secondary phases such as iron- or silica-rich minerals contribute to the dissolved species, according to the nature of the raw bentonite. Furthermore, readsorption affects significant amounts of dissolved species. The overconsumption of proton/hydroxide due to dissolution, readsorption, and hydrolysis of dissolved species was evaluated using a self-consistent thermodynamic calculation. The ability of such calculation to correct the raw titration curves in order to extract the titrable surface charge of montmorillonite was evaluated by comparison with the continuous titration procedure. Especially in the alkaline domain, correcting the raw batch titration curves for the measured side reactions failed to reproduce the continuous titration curves. These observations demonstrate the limitations of the batch titration method and the superiority of fast, continuous methods for quantifying the dissociable surface charge of clays.

The structure and dynamics of 2-dimensional fluids in swelling clays

The interlayer pores of swelling 2:1 clays provide an ideal 2-dimensional environment in which to study confined fluids. In this paper we discuss our understanding of the structure and dynamics of interlayer fluid species in expanded clays, based primarily on the outcome of recent molecular modelling and neutron scattering studies. Counterion solvation is compared with that measured in bulk solutions, and at a local level the cation-oxygen coordination is found to be remarkably similar in these two environments. However, for the monovalent ions the contribution to the first coordination shell from the clay surfaces increases with counterion radius. This gives rise to inner-sphere (surface) complexes in the case of potassium and caesium. In this context, the location of the negative clay surface charge (i.e. arising from octahedral or tetrahedral substitution) is also found to be of major importance. Divalent cations, such as calcium, eagerly solvate to form outer-sphere complexes. These complexes are able to pin adjacent clay layers together, and thereby prevent colloidal swelling. Confined water molecules form hydrogen bonds to each other and to the clays' surfaces. In this way their local environment relaxes to close to the bulk water structure within two molecular layers of the clay surface. Finally, we discuss the way in which the simple organic molecules methane, methanol and ethylene glycol behave in the interlayer region of hydrated clays. Quasi-elastic neutron scattering of isotopically labelled interlayer CH 3OD and (CH 2OD) 2 in deuterated clay allows us to measure the diffusion of the CH 3- and CH 2-groups in both clay and liquid environments. We find that in both the one-layer methanol solvates and the two-layer glycol solvates the diffusion of the most mobile organic molecules is close to that in the bulk solution.

Polymer/silicate Interaction in Nylon 6-Clay Hybrid Studied by Temperature Programmed Pyrolysis Techniques

Nylon 6-clay hybrid (NCH) was characterized by temperature-programmed pyrolysis (TPPy) techniques such as TPPy-mass spectrometry and TPPy-gas chromatography mainly focusing on the interaction between nylon 6 molecule and the surface of silicate monolayers in the NCH samples, which leads to the superior properties of the hybrid nanomateirals. Pyrolysis profile of nylon 6 matrix in the NCH samples was gradually shifted to lower temperature region with increase in the clay content. The yields of main pyrolysis products such as e-caprolactam from the NCH samples decreased with increase in the clay content, whereas those of the nitrile compounds significantly increased. These results suggested that the clay surface accelerate cis-elimination reaction of nylon 6 matrix in the NCH samples to form the nitrile compounds rather than intramolecular amide exchange to e-caprolactam. Based on the changes in pyrolysis behaviors of nylon 6 matrix in the NCH samples, the interaction between positively charged NH proton of nylon 6 molecules and negatively charged clay surface was proposed which would stabilize the six-membered transition state during pyrolysis.

Adsorption of Sulfonamide Antimicrobial Agents to Clay Minerals

Adsorption of three sulfonamide antimicrobials to clay minerals was investigated as a function of pH, ionic strength, and type of exchangeable cation. Sulfonamide antimicrobial adsorption exhibited pronounced pH dependence consistent with sorbate speciation and clay properties. Sulfonamide antimicrobials did not intercalate into montmorillonite, and surface charge density influenced sorption by determining adsorption domain size. Adsorption edge data were best fit to a model including terms for the cationic and uncharged species. Adsorption of uncharged sulfamethazine to montmorillonite was relatively insensitive to pH, ionic strength, and type of exchangeable cation, while that to kaolinite was highly sensitive to ionic strength. Adsorption of cationic sulfamethazine to montmorillonite exceeded that of the neutral species by 1-2 orders of magnitude, but was unimportant for kaolinite atthe pH values examined. Cation exchange appeared to contribute to sorption of cationic sulfonamide species to montmorillonite. Anionic sulfamethazine adsorption was negligible. The nature of the sulfonamide R group influenced the degree of adsorption of cationic and neutral species. Our results highlight the importance of considering sulfonamide speciation and clay surface charge density in predicting the transport of these antimicrobials.

Sensitivity of the acid–base properties of clays to the methods of preparation and measurement: 1. Literature review

Measuring and modeling the surface charge of clays, and more especially smectites, has become an important issue in the use of bentonites as a waste confinement material aimed at retarding migration of water and solutes. Therefore, many studies of the acid–base properties of montmorillonite have appeared recently in the literature, following older studies principally devoted to cation exchange. It is striking that beyond the consensus about the complex nature of the surface charge of clays, there are many discrepancies, especially concerning the dissociable charge, that prevents intercomparison among the published data. However, a general trend is observed regarding the absence of common intersection point on raw titration curves at different ionic strengths. Analysis of the literature shows that these discrepancies originate from the experimental procedures for the preparation of the clays and for the quantification of their surface charge. The present work is an attempt to understand how these procedures can impact the final results. Three critical operations can be identified as having significant effects on the surface properties of the studied clays. The first one is the preparation of purified clay from the raw material: the use of acid or chelation treatments, and the repeated washings in deionized water result in partial dissolution of the clays. Then storage of the purified clay in dry or wet conditions strongly influences the equilibria in the subsequent experiments respectively by precipitation or enhanced dissolution. The third critical operation is the quantification of the surface charge by potentiometric titration, which requires the use of strong acids and bases. As a consequence, besides dissociation of surface sites, many secondary titrant consuming reactions were described in the literature, such as cation exchange, dissolution, hydrolysis, or precipitation. The cumulated effects make it difficult to derive proper dissociation constants, and to build adequate models. The inadequation of the classical surface complexation models to describe the acid–base behavior of clays is illustrated by the electrokinetic behavior of smectites, which is independent from the pH and the ionic strength. Therefore, there is still a need on one hand for accurate data recorded in controlled conditions, and on the other hand for new models taking into account the complex nature of the charge of clays.

Long-term diffusion experiment at Mont Terri: first results from field and laboratory data

The diffusion of radionuclides is an important safety aspect for nuclear waste disposal in argillaceous host rocks. A long-term diffusion experiment, termed DI-A, is being carried out at the Mont Terri Rock Laboratory in the Opalinus Clay formation. The aim of this experiment is the understanding of the migration and sorption behaviour of cationic and anionic species in consolidated clays. This study reports on the experimental layout and the first results obtained from the DI-A experiment, which include the investigation of HTO, 22Na +, Cs +, and I − migration during a period of 1 year by analysing these tracers in the water circulating in the borehole. In addition, results obtained from through-diffusion experiments on small-sized samples with HTO, I −, and 36Cl − are presented. The decrease of tracer concentrations in the borehole is fastest for Cs +, followed by 22Na +, HTO, and finally I −. The chemical composition of the artificial pore water in the borehole shows very little variation with time, thus indicating almost no chemical disturbance around the borehole. Through-diffusion experiments in the laboratory that were performed parallel to the bedding plane with two different methods yielded effective diffusion coefficients for HTO of 4–5×10 −11 m 2 s −1 and significantly lower ones for anions Cl − and I − (0.7–1.6×10 −11 m 2 s −1). The results indicate the importance of anion exclusion effects arising from the negatively charged clay surfaces. Furthermore, they demonstrate the anisotropic diffusion properties of the clay formation with significantly increased diffusion rates parallel to bedding relative to the perpendicular direction. The tracer data of the in situ experiment were successfully described with 2D diffusion models using diffusion and sorption parameters obtained from the above mentioned and other laboratory studies. The modelling results indicate that HTO and I − diffused with no retardation. The retardation of Na + and Cs + could be described by empirical sorption expressions from previously derived batch sorption (Cs +) or diffusion (Na +) experiments. Overall, the obtained results demonstrate the feasibility of the technical concept to study the diffusion of non-sorbing and sorbing tracers in consolidated clays.