Showing papers in "Clay Minerals in 2013"

A review of microbial redox interactions with structural Fe in clay minerals

Abstract: Virtually all 2:1 clay minerals contain some Fe in their crystal structure, which may undergo redox reaction with surrounding redox-active species causing potentially significant changes in the chemical and physical properties of the clay mineral and its surrounding matrix. This phenomenon was originally of interest mostly as a laboratory experiment using strong inorganic reduction agents, but the discovery that the structural Fe could be reduced by microorganisms in natural soils and sediments opened the way for this to become a practical method for altering the chemical and physical properties of soils and sediments in situ . The purpose of this report was to review the body of literature that has been published since the inception of this field of inquiry and to complement, update, and complete three other reviews that have been published during the intervening years. Studies of microbial reduction of structural Fe in smectites have revealed the extent of reduction, effects on chemical and physical properties, reversibility (or lack thereof) of microbial reduction, stoichiometry, possible reaction mechanism, and types of organisms involved. Some organisms are also capable of oxidizing structural Fe, such as in biotite or reduced smectite, while one appears to be able to do both. Illitic layers resist reduction by microorganisms, but this can be partially overcome by the presence of an electron shuttle compound such as anthraquinone-2,6-disulfonate, which also enhances the extent of reduction in smectites. Microorganisms may be employed as an in situ reducing agent to drive redox cycles for structural Fe in constituent clay minerals of soils and sediments, which in turn can serve as an abiotic source for redox-mediated remediation of environmental contaminants.

On the mechanism of exfoliation of ‘Vermiculite’

Abstract: Six samples of 'Vermiculite' have been studied to investigate the mechanism of its well known but poorly understood property to exfoliate. The samples were analysed quantitatively by XRD to determine their precise mineralogical composition. Electron microprobe methods, including elemental mapping of native potassium and of caesium (introduced by cation exchange) were used to examine variation in the chemical composition of the particles. Most of the samples examined show heterogeneous mineralogical compositions which occur as distinct zones within the volume of individual particles, presenting a mosaic texture. Exfoliation is related to this mosaic distribution of the different mineral phases within the particles. Lateral phase boundaries between vermiculite and mica layers, or vermiculite and chlorite layers are postulated to prevent or impede the escape of gas from a particle, resulting in exfoliation when the pressure exceeds the interlayer bonding forces that hold the layers together. This mechanism provides a common explanation for the exfoliation of 'Vermiculite' by thermal methods or by treatment with H2O2. Paradoxically, one sample which consists of pure vermiculite, in the mineralogical sense of the term, demonstrates that pure vermiculite does not and should not exhibit the property of exfoliation. Our explanation of the mechanism of exfoliation explains the commonly observed particle size dependence of exfoliation and the tendency for obviously poly-phase 'Vermiculite' samples to show the largest coefficients of expansion.

Micro-scale experimental investigation of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock

Abstract: An experimental study of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock under hydration is presented. The investigation, which combines environmental scanning electron microscopy (ESEM) and digital image correlation techniques, has been carried out at the micrometric scale of the composite microstructure of the rock. Specimens were hydrated in the ESEM over a wide range of relative humidity and observations conducted on two planes: plane 1 parallel to the bedding plane, and plane 2 perpendicular to it. The observations reveal that the local swelling (which can be quantified at a local gauge length of about 5 µm) is strongly anisotropic in both planes. The global swelling, measured over areas of about 500 µm in width, is also clearly anisotropic in plane 2 (with major swelling direction normal to the bedding plane), but not in plane 1. The global isotropy in plane 1 arises from the uniform distribution of the orientation of anisotropic local strains, while the anisotropic swelling in plane 2 is due to a preferred local orientation. © 2013 Mineralogical Society.

Mineralogical investigations of the first package of the alternative buffer material test – I. Alteration of bentonites

Abstract: Bentonite, which is envisaged as a promising engineered barrier material for the safe disposal of highly radioactive waste, was and is investigated in different large scale tests. The main focus was and is on the stability (or durability) of the bentonite. However, most countries concentrated on one or a few different bentonites only, regardless of the fact that bentonite performance in different applications is highly variable. Therefore, SKB (Svensk Karnbranslehantering) set up the first large scale test which aimed at a direct comparison of different bentonites. This test was termed the ‘alternative buffer material test’ and considers eleven different clays which were either compacted (blocks) or put into cages to keep the material together. One so-called package consisted of thirty different blocks placed on top of each other. These blocks surrounded a heated iron tube 10 cm in diameter. Altogether three packages were installed in the underground test laboratory Aspo, Sweden. The first package was terminated 28 months after installation and the bentonite had been exposed for the maximum temperature (130°C) for about one year. Almost all geochemical and mineralogical alterations of the different bentonites (apart from exchangeable cations) were restricted to the contact between iron and bentonite. The increase of the Fe 2 O 3 content was attributed to corrosion of the tube. However, the typical 7 or 14 A smectite alteration product was not found. At the contact of one sample, siderite was precipitated. Some samples showed anhydrite and organic carbon accumulation and some showed dissolution of clinoptilolite and cristobalite. IR spectroscopy, XRD, and XRF data indicated the formation of trioctahedral minerals/domains in the case of some bentonites. Even more data has to be collected before unambiguous conclusions concerning both alteration mechanisms and bentonite differences can be drawn.

Preparation and characterization of purified Na-activated bentonite from Karak (Pakistan) for pharmaceutical use

Abstract: The purpose of this paper was to prepare purified Na- bentonite with improved properties for use in the pharmaceutical industry. Calcium bentonite from the Shagia region of Karak district, Pakistan, was activated with various proportions of sodium carbonate (2, 3, 5 and 8 wt.%) and purified by sedimentation to remove impurities, especially quartz. X-ray diffraction (XRD), and swelling volume confirmed the conversion of raw bentonite to sodium bentonite by using 5% Na 2 CO 3 . The sodium bentonite (K5) obtained by activation met the chemical and microbiological requirements set by the pharmacopeias regarding the toxic trace elemental content (Pb and As), absence of E. coli , total aerobic microbial contents and physicochemical properties such as swelling volume, pH and sedimentation volume. Therefore K5 bentonite could be designated as potentially suitable for pharmaceutical applications. The CEC, surface area, porosity, pH, gel formation and swelling volume indicated that K5 bentonite could be used in the formulation of oral suspension and in topical application.

Mineralogical investigations of the first package of the alternative buffer material test - II. Exchangeable cation population rearrangement

Abstract: Bentonites are candidate materials for the encapsulation of radioactive waste. In the ‘Alternative Buffer Material test’ (ABM), compacted ring-shaped blocks of eleven different buffer materials (mainly bentonites) were packed vertically on top of each other with an iron tube as heater in the centre. These buffer materials started with various exchangeable cation populations (EC population ). The first ‘ABM package’ was terminated 28 months after installation and the bentonites had been exposed to the maximum temperature (130°C) for about one year. The aim of the present study is first: to describe modification of the cation exchange population, and second to understand the influence of the groundwater on cation exchange at different scales. No significant horizontal variation of any exchangeable cation (EC) was detected between 1 and 7 cm distance from contact with the iron tube. Large total differences of the EC populations , however, were observed for the individual blocks after the field experiment ( n = 21 blocks) with respect to the composition of the reference materials. The average cation exchange capacity (CEC) values of the analysed bentonites ( n = 9 blocks) decreased by 5.5 meq/100 (1.1–8.8 meq/100 g) after the experiment. Exchangeable Na + and Mg 2+ decreased on average, whereas Ca 2+ increased. This trend was pronounced in the top region of the parcel (upper seven blocks). Although most changes occurred on the large scale of the whole test parcel, small but important changes were also recorded in the vertical direction on the centimetre scale. The observed differences cannot be explained assuming simply that a bentonite reacts only with neighbouring blocks, which would mean that the system was more or less closed. The differences are much larger and the only conclusion from this observation is that the whole package seems to be influenced by the groundwater which was added from a water tank at the experiment site, enabling at least partial equilibration between the different blocks.

Montmorillonite mitigates the toxic effect of heavy oil on hydrocarbon-degrading bacterial growth: implications for marine oil spill bioremediation

Abstract: The ability of montmorillonite to mitigate the toxic effect of heavy oil from the Nakhodka oil spill, by growth of hydrocarbon-degrading bacteria and enable bioremediation was studied. Montmorillonite enhanced the bacterial growth significantly (P 0.05). Transmission electron microscopic observation showed that the hydrocarbon-degrading bacterial cells were covered and encrusted with montmorillonite particles. Scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (STEM-EDS) also showed that the surrounding of the bacterial cells was frequently rich in Si but not in Al. Fourier transform infrared (FTIR) spectroscopy indicated that the heavy oil-bacterial cell-montmorillonite particle complex retained the composition of both water and heavy oil. X-ray powder diffractrometery (XRD) analysis revealed that heavy oil and heavy oil-bacteria did not change the basal spacing of montmorillonite over a period of 24 days. The enhancement of hydrocarbon-degrading bacterial growth is attributed to montmorillonite likely serving as both bacterial growth-supporting carrier and protective outer layer against high concentrations of heavy oil that inhibit growth. These results shed light on the interactions in oil-bacteria-clay complexes and could potentially be used in marine oil spill bioremediation.

Bentonite pore distribution based on SAXS, chloride exclusion and NMR studies

Abstract: Water-saturated bentonite is planned to be used in many countries as an important barrier component in high-level nuclear waste (HLW) repositories. Knowledge about the microstructure of the bentonite and the distribution of water between interlayer (IL) and non-interlayer (non-IL) pores is important for modelling of long-term processes. In this work the microstructure of water-saturated samples prepared from MX-80 bentonite was studied with nuclear magnetic resonance (NMR) and small-angle X-ray scattering spectroscopy (SAXS) coupled with chloride exclusion modelling. The sample dry densities ranged between 0.7 and 1.6 g/cm3. The NMR technique was used to get information about the relative amounts of different water types. Water in smaller volume domains has a shorter relaxation time than that in larger domains due to the average closer proximity of the water to the paramagnetic Fe at the layer surfaces. The results were obtained using 1H NMR T 1ρ relaxation time measurements with the short inter-pulse CPMG method. The interpretation of the NMR results was made by fitting a sum of discrete exponentials to the observed decay curves. The SAXS measurement on bentonite samples was used to get information about the size distribution of the IL distance of montmorillonite. The chloride porosity measurements and Donnan exclusion calculations were used together with the SAXS results to evaluate the bentonite microstructure. In the model, the montmorillonite layers were organized in stacks having IL water between the layers and non-IL water between the stacks. In the modelling, the number of layers in the stacks was used as fitting parameters which determined the IL and non-IL surface areas. The fitting parameters were adjusted so that the modelled chloride concentration was equal to the measured one. The NMR studies and SAXS studies coupled with the Cl porosity measurements provided very similar pictures of how the porewater is divided in two phases in bentonite.

Incorporation of Al in iron oxyhydroxides: implications for the structure of ferrihydrite

Abstract: The incomplete miscibility of Al into the ferrihydrite structure has been a recurring issue in understanding the environmental geochemistry of this important oxyhydroxide. During co-precipitation from acidic aqueous solution, ferrihydrite has been observed to accept only up to ∼25 at.% Al without the formation of multi-phasic Al and Fe oxyhydroxides. Using basic chemistry and crystal-chemical relationships we propose here that the saturation limit of Al substitution in the structure of Fe oxyhydroxides is controlled by Al–Al avoidance in a manner that conforms to Pauling’s distortion rule. Employing this hypothesis, we show that the predicted miscibility limit for Al incorporation is 25 at.% in ferrihydrite and 33 at.% in goethite, in agreement with previous observations. These results indicate that the classical f-phase model for ferrihydrite best represents observations. Incorrect assignment of Fe site occupancy and other shortcomings of the akdalaite/tohdite model for ferrihydrite are also discussed.

Microstructural investigation of calcium montmorillonite

Abstract: Bentonite clay is planned to form a part of deep-geological repositories of spent nuclear fuel in several countries. The extremely long operation time of the repository requires an in-depth understanding of the structure and properties of used materials. In this work the microstructure of a simplified system of Ca-montmorillonite is investigated using a set of complementary methods: X-ray diffraction, small angle X-ray scattering, nuclear magnetic resonance, transmission electron microscopy and ion exclusion. The paper presents experimental results obtained from compacted, water saturated samples in the dry density range 0.6–1.5 g/cm 3 . It can be observed that different methods yield similar quantification of water present in the interlamellar space. Combined results support the multiple porosity concept of the bentonite structure.

Lead removal from aqueous solutions by natural Greek bentonites

Abstract: Three bentonite samples (B1, B2, B3) from Milos Island, Greece, were investigated by XRD, AAS, DTA-TG, FTIR and specific surface area measurement techniques. A laboratory batch study has been performed to investigate the adsorption characteristics of lead ions (Pb2+) onto natural bentonite samples. The effect of various physicochemical factors that influence adsorption, such as solution pH (2–6), adsorbent dosage (1–10 g L–1), contact time (20–360 min), and initial metal ion concentration (5–150 mg L–1) was studied. A number of available models like the Lagergren pseudo first-order kinetic model, the pseudo second-order kinetic model and intra-particle diffusion were utilized to evaluate the adsorption kinetics. The adsorption of Pb2+ was modelled with the Langmuir, Freundlich and D-R isotherms. The maximum Pb2+ adsorption capacities for B1, B2 and B3 were 85.47 mg g–1, 73.42 mg g–1 and 48.66 mg g–1, respectively.

A simple approach to the identification of trioctahedral smectites by X-ray diffraction

Abstract: A new method for identifying the trioctahedral smectites saponite, stevensite and hectorite is proposed in this study. The method is based on differences in the X-ray diffraction (XRD) patterns of the three smectites after (a) heating at 500°C for 90 min and (b) glycerol solvation of the Cs-forms of the smectites for 20 h. After heating at 500°C, well below the dehydroxylation temperature of the three smectites, saponite and hectorite re-expand upon ethylene glycol (EG) solvation, whereas stevensite layers remain collapsed. Saponite forms one-layer and hectorite two-layer complexes after Cs-saturation and glycerol solvation. Cs-stevensite displays a gradual increase in d 001 with increasing solvation time in glycerol vapours and forms two-layer glycerol complexes with prolonged solvation. Except for the individual Mg-smectites, the proposed method may be used to identify compositional heterogeneity that may exist in the smectites. Furthermore, it should be useful in identifying the individual trioctahedral Mg-smectites when present in mixtures, and in detecting interstratified layers of different Mg-trioctahedral smectites. Application of the method revealed that the SYnL-1 laponite (CMS Source Clay Project) is not homogeneous but consists of hectorite, stevensite and possibly mixed-layer hectorite/stevensite layers.

Iron-bearing smectites: a revised relationship between structural Fe, b cell edge lengths and refractive indices

Abstract: Structural iron in smectites correlates with the cell edge length b and increases the refractive index. The cell edge length b is usually obtained from the position of the (060) reflection, but in this work we show that such b values differ from the values obtained from Rietveld fits because contributions from (hkl) reflections shift the position of the (060) reflection. The correlation between Fe and cell edge length b was significant (r2 > 0.99); the relationship is b [A] = 8.9977(0.0035) + 0.1117(0.0032) × Fetot. Furthermore, we present for the first time measurements of the refractive index n of Fe-bearing smectites, applying a recently published turbidity method (Weidler & Friedrich, 2007). The refractive index correlates both with structural iron (r2 = 0.64) and with b (r2 = 0.94).

Synthesis, characterization and electrorheological properties of biodegradable chitosan/bentonite composites

Abstract: Biodegradable chitosan/bentonite composites with three different compositions were synthesized by the intercalation method using cetyltrimethylammonium bromide as the cationic surfactant. The composites were characterized using conductivity, density, particle size measurements, FTIR, TGA, XRD and SEM methods. Colloidal stabilities of the suspensions prepared in silicone oil (SO) were observed to increase with decreasing density. The effects of dispersed particle concentration, shear rate, electric field strength, electric field frequency and temperature on the electrorheological (ER) activities of the suspensions were investigated. The electric field viscosities of the suspensions showed typical shear thinning non-Newtonian viscoelastic behaviour. Yield stresses of the suspensions were observed to change in proportion to the square of applied electric field (E). Further, according to creep and creep-recovery analysis, reversible viscoelastic deformations were observed in the suspensions under E ≠ 0 kV mm–1.

Mineralogy and grain-size distribution of clay-rich rock units of the Algarve Basin (South Portugal)

Abstract: A detailed survey of the most clay-rich rock units of the Meso-Cenozoic geological section exposed in the Algarve Basin (South Portugal) was performed and data were analysed for the grain-size distribution and mineralogy (whole rock and clay fraction), aimed at a compositional study of the clay-rich sediments and their depositional environment. Granulometry was obtained using wet sieving and laser diffraction by attenuation of X-rays, and the mineralogical study was carried out by X-ray diffraction. Most clay-rich rock units of the Algarve are classified as silty clays and clayey silts, and only a minority is coarser. The mineralogical study enabled us to define two main types of clays: (1) non-calcareous clays, consisting largely of quartz and clay minerals, with goethite as the typical Fe-rich phase (sediments of Carboniferous, Neogene and Quaternary age and Cretaceous siliciclastic clays); and (2) calcareous clays, which can be calcite-rich clays (Middle and Upper Jurassic) or dolomite-rich clays (Triassic and Lower Jurassic), the latter typically containing hematite as an accessory phase. Plagioclase, K-feldspar, and Ti-oxides are often accessory phases, whereas ankerite, anhydrite, gypsum and opal are rare. In the clay fraction illite generally predominates, resulting probably from weathering of preexisting rocks, as well as the less frequent Fe-chlorite, pointing to incipient chemical alteration under semi-arid climatic conditions. Kaolinite occurs in diverse proportions, being especially abundant in Cretaceous and Cenozoic units; it is mainly related to chemical weathering in continental environments under humid conditions. As the Atlantic Ocean opened during Triassic and the continental environment evolved to a shallow-marine environment with evaporitic conditions, smectite became more expressive, being sometimes accompanied by other Mg-rich phases (chlorite, sepiolite, corrensite and palygorskite). Especially during the Cenozoic the proportion of different phases in the clay mineral association of the sediments reflects the control of tectonic movements and fluctuations in sea level during their deposition.

The Tournemire industrial analogue: reactive-transport modelling of a cement–clay interface

Abstract: In the post-closure period of a geological disposal facility for radioactive waste, leaching of cement components is likely to give rise to an alkaline plume which will be in chemical disequilibrium with the host rock (which is clay in some concepts) and other engineered barrier system materials used in the facility, such as bentonite. An industrial analogue for cement-clay interaction can be found at Tournemire, southern France, where boreholes filled with concrete and cement remained in contact with the natural mudstone for 15–20 years. The boreholes have been overcored, extracted and mineralogical characterization has been performed. In this study, a reactive-transport model of the Tournemire system has been set up using the general-purpose modelling tool QPAC. Previous modelling work has been built upon by using the most up-to-date data and modelling techniques, and by adding both ion exchange and surface complexation processes in the mudstone. The main features observed at Tournemire were replicated by the model, including porosity variations and precipitation of carbonates, K-feldspar, ettringite and calcite. It was found that ion exchange needed to be included in order for C-S-H minerals to precipitate in the mudstone, providing a better match with the mineralogical characterization. The additional inclusion of surface complexation, however, led to limited calcite growth at the concrete-mudstone interface unlike samples taken from the Tournemire site that have a visible line of crusty carbonates along the interface.

Influence of alkaline (pH 8.3 - 12.0) and saline solutions on chemical, mineralogical and physical properties of two different bentonites

Abstract: The interaction of two different bulk bentonites (Na- and Ca-types) with three types of simulated cement waters (pH 9.7, 11.3 and 12.0) and one saline groundwater simulate (pH 8.3) as a reference, was studied in batch reactors at 25°C. The solution pH was monitored in order to keep the pH as steady as possible by replacing the leaching solution with fresh one when needed. After 554 days, one set of parallel samples was removed from the experiment in order to investigate the possible changes in the bentonite materials. The buffering capacity of bentonite was clearly observed, especially at the beginning of the high-pH experiments, as the pH of the leaching solutions decreased quite dramatically due to interaction with bentonite. The solution chemistry results showed a decrease of Ca content in all leachate samples, but especially in pH 12.0 experiments. Small amounts of silica were released throughout the experiment. Both bentonites in pH 12.0 experiments also released detectable amounts of Al, while in the lower pH experiments the levels were below detection limit. These observations were also supported by chemical analyses of the bentonite materials. Only minor changes were detected in the mineralogy, and they were mainly concentrated on experiments at pH 11.3 and pH 12.0. The measured swelling pressure showed an increase in pH 12.0 experiments. The results obtained in this research may facilitate modelling of bentonite interaction with high-pH solutions.

Dissolution behaviour of soil kaolinites in acidic solutions

Abstract: Highly weathered soils of the tropics and subtropics commonly have kaolinite-dominated clay fractions. Under acidic conditions prevailing in these soils kaolinite dissolution occurs, contributing to the high levels of soluble Al in these soils. This study evaluates the dissolution behaviour of kaolinites from subsurface horizons of highly weathered soils from Thailand, along with a soil kaolinite from Western Australia (WA kaolinite) and Georgia kaolinite (KGa-2). Kaolinite-dominated clay fractions were isolated from soils by sedimentation and chemically treated to remove iron oxides. The dissolution rate of kaolinites was measured in 0.01 m NaCl as a function of pH (1–4; HCl) at 25±1°C using non-stirred flow-through reactors. Kaolinite dissolution rates were calculated from the release of Al and Si at the steady state. In most of the experiments and at all pH values, the release of both Si and Al underwent a distinct transition from an initial period of rapid release to significantly slower release at the steady state. Aluminium and Si concentrations at the steady state were higher for soil kaolinites than the reference sample (KGa-2). At the steady state the dissolution of all kaolinites was stoichiometric except for the soil kaolinites from Thailand at pH 4, where the Al/Si ratio was well below the stoichiometric ratio. Log dissolution rate ( R Si) of soil kaolinites ranged from –13.75 to –12.51, with the dissolution rate increasing significantly with decreasing solution pH. However, the dissolution rate was similar or pH independent between pH 2 and 3, which is the pH range of the point of zero net charge (PZNC) for both soil and reference kaolinites. The dissolution rate of soil kaolinite was significantly higher than the KGa-2 sample at pH < 3. The results obtained on kaolinite samples from highly weathered soils show that in highly acidic systems kaolinite is a source of soluble Al. Soil kaolinites with poorly ordered small crystals dissolve faster than better crystalline reference kaolinite (KGa-2) with larger crystals.

Smectite clay from the Sabga deposit (Cameroon): mineralogical and physicochemical properties

Abstract: The physicochemical and mineralogical characterization of the <250 mu m particlesize fraction from six clay-rich samples from the Sabga deposit (north-west, Cameroon) were carried out to evaluate their potential applications. The major clay mineral is dioctahedral smectite and minor kaolinite is present in three of the clay samples. Cristobalite, feldspars, ilmenite and heulandite are accessory minerals. Application of the Greene-Kelly test revealed that the smectite present is montmorillonite. The chemical composition (wt.%) of the bulk clays consists of (66-70%) SiO2, (13-16%) Al2O3 and (2-7%) Fe2O3. Physico-chemical characterization of the clays showed that the cation exchange capacity (CEC) and the specific surface area range from 38 to 46 meq/100 g and from 33 to 90 m(2)/g respectively. The physical and chemical properties are fully compatible with potential uses in environmental applications. After purification and chemical modification, these materials could also be used in refining edible oil as adsorbent, waste water treatment and wine technology.

Characterization of clay deposits of Nanga-Eboko (Central Cameroon): Suitability in the production of building materials

Abstract: Abstract Clayey deposits of Nanga-Eboko (central Cameroon) were studied to assess their potential as building materials. Characterization was performed using XRD, FTIR, XRF, DTA/DTG and firing testing. Clays appear as discontinuous pockets with the same textural characteristics in three villages located on both sides of the Sanaga River. The average thickness of the exploitable layer is about 3m. The estimated tonnage ~7−17 x 105 m3 can supply a brick industry of great importance. SiO2 (~70%), Al2O3 (~15%) and Fe2O3 (~4%) are the predominant oxides of the natural clays. Quartz (~55%), kaolinite (~33%), illite (~5%) and K-feldspar (~4%) are major minerals. Clays are not suitable for building construction due to their fine-grained size and high plasticity properties. Firing properties of bricks (950 and 1050ºC) are good despite the high shrinkage values. Therefore the addition of ‘‘degreasers’’ is recommended to control shaping and drying.

Structural interpretation of anion exchange in divalent copper hydroxysalt minerals

Abstract: Minerals with the general formula Cu 4 (OH) 6 A 2 ∕ n n ± p H 2 O ( A = Cl − , NO 3 − , SO 4 2 − ) were synthesized and their behaviour following treatment with chloride, nitrate and sulfate solutions was studied by powder XRD and SEM. Two types of transformation were found: (1) an ion-exchange reaction manifested by preservation of both the precursor’s morphology and structural type; (2) a dissolution-crystallization mechanism characterized by changes in the structural type and the morphology. The results were considered by simultaneous application of the binary presentation of the structures, a bond valence approach and the ion-exchange ability. It was found that the structures of minerals with ion-exchange properties are built from similar layered structural unit of edge-shared and corner-shared Jahn-Teller square frameworks and different interstitial complexes of exchangeable ions, water molecules and cation-water groups. On the basis of their structural features the position of the investigated minerals in mineralogical classifications is also discussed.

Overview of radionuclide migration experiments in the HADES Underground Research Facility at Mol (Belgium)

Abstract: In situ migration experiments using different radiotracers have been performed in the HADES Underground Research Facility (URF), built at a depth of 225 m in the Boom Clay formation below the SCK–CEN nuclear site at Mol (Belgium). Small-scale experiments, mimicking laboratory experiments, were carried out with strongly retarded tracers (strontium, caesium, europium, americium and technetium). Contrary to europium, americium and technetium which are subjected to colloid mediated transport, the transport of strontium and caesium can be described by the classic diffusion retardation formalism. For these last two tracers, the transport parameters derived from the in situ experiments can be compared with the laboratory-derived values. For both tracers, the apparent diffusion coefficients measured in the in situ experiments agree well with the laboratory-derived values. In the large-scale experiments (of the order of metres) performed in the URF, non-retarded or slightly retarded tracers (HTO, iodide and H C 3 − 14 ) were used. The migration behaviour of these tracers was predicted based on models applied in performance assessment calculations (classic diffusion retardation) using migration parameter values measured in laboratory experiments. These blind predictions of large-scale experiments agree well in general with the experimental measurements. Fitting the experimental in situ data leads to apparent diffusion coefficients close to those determined by the laboratory experiments. The iodide and H C 3 − 14 data were fitted with a simple analytical expression, and the HTO data were additionally fitted numerically with COMSOL multiphysics, leading to about the same optimal values.

Isotropic/nematic and sol/gel transitions in aqueous suspensions of size selected nontronite NAu1

Abstract: The phase behaviour of aqueous suspensions of NAu1 nontronite was studied on size-selected particles by combining osmotic pressure measurements, visual observations under polarized light, rheological experiments and Small Angle X-ray Scattering (SAXS). NAu1 suspensions display a liquid crystalline behaviour as they exhibit a Isotropic/Nematic (I/N) transition that occurs before the sol/gel transition for ionic strengths below 10 –3 M/L. This I/N transition shifts towards lower volume fractions for increasing particle anisotropy and its position in the phase diagram agrees well with the theoretical predictions for platelets. SAXS measurements reveal the presence of characteristic interparticular distances in the isotropic, nematic and gel phases. In the gel phase a local lamellar order is observed which shows that the “house of cards” model is not appropriate for describing the gel structure in swelling clay materials at low ionic strength. Furthermore, by combining results from osmotic pressure measurements and X-ray scattering, it appears that the pressure of the system can be well described using a simple Poisson-Boltzmann treatment based on the repulsion between charged infinite parallel planes. In terms of rheological properties, even if the thermodynamical status of the sol/gel transition remains partially unclear, the yield stress and elasticity of the gels can be easily renormalized for all particle sizes on the basis of the volume of the particles. Furthermore, rheological modelling of the flow curves shows that for all the particles an approach based on excluded volume effects captures most features of nontronite suspensions.

Influence of the mineralogical composition on the properties of adobe blocks from Aveiro, Portugal

Abstract: Earth materials have been used in the form of adobe or rammed products for the construction of load-bearing walls in buildings. In Portugal, the adobe has been used predominantly on the central coast, particularly in the Aveiro district. Although many old adobe buildings in Aveiro have been abandoned, some of them can be refurbished with minor maintenance and repair works. Representative samples from typical adobe blocks, from various constructions in the region were collected and characterized for their mineralogical composition, particle size, aggregate content, water uptake, durability and strength. These basic properties provide a basis for the development of adequate interventions, preserving the characteristics of the buildings. This study compares the main characteristics of adobes from two different locations (Anadia and Murtosa). The most important differences between the two groups of adobes are the particle size distribution, the mineralogical composition, the water absorption and the mechanical and durability properties. The study improved the knowledge of the traditional construction methods and will enable decisions for rehabilitation of adobe buildings to be made. Knowledge of the main characteristics of adobes allows those participating in the rehabilitation process to obtain the basis for the rehabilitation and conservation of old buildings.

Synthesis and optical properties of composites based on ZnS nanoparticles embedded in layered magadiite

Abstract: Composites based on ZnS nanparticles embedded in layered magadiite were synthesized via a three step process: protonation of Na-magadiite, ion exchange in order to introduce Zn(NH3)2+4 in the interlayer space, and addition of Na2S to form ZnS particles in the interlayer space of magadiite. The composites obtained were characterized by X-ray power diffraction (XRD), Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS), Transmission Electron Microscopy (TEM), Raman spectroscopy, Photoluminscence spectra (PL), and UV-visible transmission spectra (UV-vis). Results indicated that ZnS nanoparticles embedded in magadiite presented different optical properties and photoluminescence enhancement properties compared with those of uncovered ZnS particles (without host magadiite).

Experimental investigations of piping phenomena in bentonite-based buffer materials for an HLW repository

Abstract: During the installation of the buffer in a deposition hole of an HLW repository, it is necessary to control water flow from the fractured rock into the deposition hole. Water flow with inflow rate greater than 0.001 l/min may cause piping and erosion of the buffer, and may trigger mass redistribution of the buffer, sedimentation and material separation of bentonite materials. This paper describes the condition of parameters which cause piping and erosion; revised conditions which keep advection, inflow rate, buffer component, gap between buffer materials, gap between outside wall and buffer block, and type of water. The results from the experiment show the condition of the self-sealing function of bentonite materials, formation of piping, allowable limit of inflow rate in the case of an Na type bentonite block of 70 wt.% Kunigel V1 and 30 wt.% silica sand, or a pellet of 100 wt.% Kunigel V1. Piping and erosion continue until the engineered barrier (EB) is filled with water, and then the hydraulic gradient becomes small. Piping may lead to erosion and redistribution of material which needs to be taken into account in the long-term performance assessment.

Simultaneous thermal analysis of different bentonite–sodium carbonate systems: an attempt to distinguish alkali-activated bentonites from raw materials

Abstract: Alkali activation with sodium carbonates is a traditional method to improve bentonite properties for a variety of applications. In some applications, natural sodium-rich bentonites are preferred, and custom regulations require proper declaration of Na-rich bentonites, with respect to activation. Consequently, there is need for a method that can unambiguously distinguish between natural and activated Na-rich bentonites. The paper deals with the preparation of several alkali-activated sets, specifically (a) anhydrous Na 2 CO 3 with trace amounts of thermonatrite (Na 2 CO 3 ·H 2 O) and trona (Na 3 (CO 3 )(HCO 3 )·2H 2 O), hereafter called ASC, (b) mixtures of ASC with CaCO 3, and (c) mixtures of ASC with CaCO 3 and a Ca 2+ -rich bentonite at different moisture contents, to distinguish natural and alkali-activated bentonites by simultaneous thermal analysis (STA) linked with a mass spectrometer for the analysis of evolved gases. STA linked with MS revealed alkali activation of bentonites, even in the presence of CaCO 3. The moisture content during activation and storage of activated samples, however, has a strong influence on the detection of activated samples by STA-MS. Uncertainties remain with respect to unknown foreign phase contents of technical ASC used for alkali activation in practice and the influence of carbonates like dolomite or siderite, which are often present in natural bentonites.

The experimental study of bentonite swelling into fissures

Abstract: The geological disposal of radioactive waste, based on a multi-barrier concept wherein the first barrier consists of the metal waste container and the final barrier the host rock, is widely considered the only viable solution to this issue. The bentonite-based seal around the canister forms one of the barriers. The unique swelling and sealing capabilities of bentonite play a major role in repository safety concepts in that they allow the bentonite barrier to withstand serious mechanical damage without its function being compromised. This paper presents experimental research focusing on the dynamics and mechanics of the sealing of cracks and joints using bentonite-based materials. Physical models were used to simulate the contact point of bentonite-based sealants with cracks in the rock mass. The models examined the ability of the tested material to fill the crack thus preventing the creation of a preferential water pathway. The results show that in most cases total bentonite advance (for the same material) into fissures is, primarily, linearly dependent on fissure width. The absolute value of advance could be related to the overall swelling ability of the material characterized by its swell index or swelling pressure.

Dissolution of compacted montmorillonite at hyperalkaline pH and 70°C: in situ VSI and ex situ AFM measurements

Abstract: In situ measurements were carried out to quantify montmorillonite dissolution rates at a compaction pressure ranging from 0.04 to 10.00 MPa and temperature of 70°C in 0.3 m NaOH solution (pH 12.1 at 70°C) using vertical scanning interferometry (VSI) and an auto-compaction cell. Ex situ measurements of the reacted samples using atomic force microscopy (AFM) were performed to quantify the ratio of edge surface area (ESA) to total surface area (TSA) ( X ESA = ESA/TSA). Accordingly, the actual ESA for the montmorillonite examined by in situ VSI could be estimated. The X ESA value increases as a function of run duration or compaction pressure. At atmospheric pressure, X ESA is approximately 0.0054 and converges to ∼0.0107 at 10 MPa, An expression that relates reactive surface area and montmorillonite compaction ( X ESA / X ESA initial = kX ESA, k : variable factor) is kX ESA = 1.0 + 0.64628 P 0.1527 where P is in MPa. Using the calculated X ESA , dissolution rates from the in situ VSI measurements are obtained. The early dissolution ( 1.0 × 10 –11 mol/m 2 /s) than that at higher compaction pressure. The rates after >1500 min are slower, with values of less than 3 × 10 –12 mol/m 2 /s, but there is no significant dependency on the density in the range from 1.0 to 1.7 Mg/m 3 . These observed rates for compacted montmorillonite are two-orders of magnitude slower (2.63 × 10 –13 mol/m 2 /s) than dissolution rates in the suspended state.

Pathways of volcanic glass alteration in laboratory experiments through inorganic and microbially-mediated processes

Abstract: Rhyolitic obsidian was reacted with natural waters to study the effect of water chemistry and biological activity on the composition and formation mechanisms of clay. Two sets of experiments (18 months, 6 years) used fresh, hypersaline water (Mg-Na-SO 4 -Cl- and NaCl-rich) and seawater. The 6-year experiments produced the transformation of obsidian into quartz, apparently by in situ re-crystallization (Cuadros et al. , 2012). The most abundant neoformed clay was dioctahedral (typically montmorillonite), indicating chemical control by the glass (where Al > Mg). Altered glass morphology and chemistry in the 18-months experiments indicated in situ transformation to clay. Magnesium-rich (saponite) clay formed under water-chemistry control in the bulk and within biofilms with elevated Mg concentration (Cuadros et al. , 2013). The contact between microbial structures and glass was very intimate. Glass transformation into quartz may be due to some characteristic of the obsidian and/or alteration conditions. Such combination needs not to be uncommon in nature and opens new possibilities of quartz origin.