Altered Layer Research Articles

Degradation mechanism of lead-vanado-iodoapatite in NaCl solution

Degradation mechanism of lead-vanado-iodoapatite (IAPT) in NaCl solution at 90 °C was investigated through systematic characterization of surface morphology, microstructure and microchemistry evolution of the alteration layer. Nano-scale characterization indicated that IAPT crystals degraded from grain boundaries toward inner grains, forming ultrafine Cl-bearing crystallites. A coupled interface dissolution-reprecipitation replacement mechanism and the ion exchange between Cl− and I− across the degradation zone may together play roles in determining the nano-scale degradation behavior of IAPT. This work highlights the degradation through multiple reactions and elemental transport across the liquid-solid and solid-solid interfaces at length scales from sub-millimeter to nanoscale.

Three-dimensional electrical resistivity structure of the Kuju volcanic group, Central Kyushu, Japan revealed by magnetotelluric survey data

The Kuju volcano group in the southwestern part of the Oita prefecture, Japan, consists of more than 20 volcanoes distributed over an area of approximately 400 sq. km. On October 11, 1995, a phreatic eruption occurred on the eastern flank of Mt. Hossho, in the central part of the Kuju volcano. A second eruption occurred in mid-December 1995, producing an ash volume of approximately 5000 m3. Therefore, clarifying the thermal and hydrologic processes occurring beneath the entire Kuju volcano area is important because the geothermal areas beneath active volcanoes may be important to the development of geothermal energy. The largest geothermal power plant in Japan (Otake–Hatchobaru power plant; 120 MW) is located 5 km west of Mt. Hossho and Mt. Iwoyama. To identify the structures associated with the hydrothermal systems beneath the Kuju volcano area, we performed a wide-band magnetotelluric survey. The survey data were collected at 63 observation points around the Kuju volcano from September–October 2015. The resulting primary three-dimensional resistivity structure model clarified that a resistivity structure (~50 Ω·m) predominates the Kuju volcano area. A highly conductive structure (~10 Ω·m) exists around the Iwoyama–Kuju area, extending in depth towards the northeastern part of Iwoyama. Based on the results of previous studies, the former can be interpreted as a lava and pyroclastic deposit whereas the latter corresponds to a magmatic fluid pathway and hydrothermal altered layers.

Corrosion mechanism analysis of the front-side metallization of a crystalline silicon PV module by a high-temperature and high-humidity test

The failure of photovoltaic modules in high-temperature and high-humidity tests is generally caused by the loss of electrical contact between the cell emitter and finger metallization, due to the corrosion of the glass layer by the acetic acid which is generated by the hydrolysis of the ethylene vinyl-acetate copolymer that is used as the encapsulant. In this study, a high-temperature high-humidity test was conducted on a photovoltaic module—namely, a solar cell with reduced output—to examine the metallization–Si wafer interface. The results confirmed that Pb was eluted from the glass layer to form an alteration layer that inhibits electron movement from the emitter layer to the metallization, thereby decreasing the output of the module. The corrosion products generated from the eluted Pb indicated that the front-side metallization was statically exposed to the same environment inside the PV module as that in an aqueous solution of acetic acid.

Near-field corrosion interactions between glass and corrosion resistant alloys

This study explores the corrosion interactions between model nuclear waste glass materials and corrosion resistant alloys, under accelerated conditions that simulate the near field of a nuclear waste repository. The interactions between the corrosion of stainless steel (SS) 316, alloy G30, or alloy 625, and international simple glass or soda-lime silica glass are systematically studied. The dissimilar materials were exposed in close proximity to each other in different electrolytes at 90 °C. After exposure, the glass surface exposed near metals showed different regimes of corrosion, with distinct surface morphologies and chemistries that were likely affected by the local environment created by the localized corrosion of metals. Surface and solution analyses showed that the corrosion rate of glass was enhanced by the presence of metals. Infrared spectroscopy data suggested the local build-up of stresses in the contact area of glass, which may lead to the mechanical instability of the glass alteration layer. On the other hand, the effect of glass on metal corrosion is strongly dependent on the leaching solution. In electrolytes containing abundant aggressive anions such as Cl−, glass seems to suppress the localized corrosion of SS by the precipitation of a Si-rich surface film that protects the SS substrate from solutions. However, in less aggressive electrolytes, the corrosion rate of SS was increased by the presence of glass corrosion products. Overall, our study showed that the hidden and localized damage on glass in contact with metals may enhance the release rate of glass components compared to typical uniform glass corrosion.

Dissolution of plasma treated non-radioactive surrogate cemented concentrates and ion exchange resins in KOH solution at 40 °C

In Belgium, plasma incineration is a possible option to (re-)condition low and intermediate level short-lived waste streams in order to obtain end products which can fulfill the criteria developed for a near surface disposal facility. In this study, the chemical durability of plasma treated non-radioactive surrogate cemented concentrates and ion exchange resins (or plasma slags) was investigated by performing MCC-1 leach tests in a KOH solution at pH 13.5 and 40 °C. Results showed that initial dissolution rates of plasma slags were similar to that of SON68-I glass, and final dissolution rates around one order of magnitude lower. SEM analysis revealed the presence of alteration layers rich in K, Ca and Fe, which probably act as diffusion barriers and, associated with the increase of Si and Al concentration in solution, lead to the decrease of the plasma slag dissolution rates.

Hydration and reaction mechanisms on sodium silicate glass surfaces from molecular dynamics simulations with reactive force fields

AbstractRecent development of reactive force fields have enabled molecular dynamics simulations of interactions between silicate glasses and water at the atomistic scale. While multicomponent silicate glasses encompass a wide variety of compositions and properties, one common structural feature in these glasses is the combination of the network structure that is made up of silica tetrahedra linked through corner sharing interspersed with network modifiers like alkali and alkaline‐earth ions that break up the Si–O–Si linkages by forming nonbridging oxygen. In reactions with water, ion exchange between alkali ions in the glass and proton or hydronium in the solution, as well as hydrolysis reaction of the Si–O–Si linkages and subsequent silanol formation, is observed and well documented. We have used a set of recently developed reactive force field to investigate the reactions between water and the surfaces of silica and sodium silicate glasses of different compositions for reactions up to 8 nanoseconds. Our results indicate sodium leaching into water and diffusion of water molecules up to 25 Å into the glass surface. We examined the structural and compositional changes inside the glass and around the diffused ions and use these to explain the rates of silanol formation at the surface. We also observed proton transport in the glass which has an indirect influence on the silanol formation rates. While the surface of the glass was rough to start with, it undergoes further modification into a hydrated gel‐like structure in the glass for up to 5 Å in the higher alkali containing glasses. It was found that the leached sodium ions remain close to the interface and that fragments of silicate network from the surface is capable of dislodging from the bulk glass and enter the aqueous solution. These simulations thus provide insights into the formation and structure of an alteration layers commonly observed in multicomponent silicate glasses corroded in aqueous solutions.

Comparative structural investigations of nuclear waste glass alteration layers and sol-gel synthesized aerogels

While various glass alteration layer formation mechanisms have been debated in recent years, the glass alteration community generally agrees that more information on physical properties of the alteration layers is needed to further the understanding of their impacts on overall glass alteration. In this work, pore volumes and solid structures of glass (International Simple Glass, ISG) alteration layers formed in solutions of various pH conditions in initially dilute conditions at 90 °C are evaluated with positron annihilation spectroscopy, small-angle X-ray scattering, and scanning transmission electron microscopy. Pore volumes of alteration layers formed at pH 9 were found to be at their lowest near the surfaces of the alteration layers. Solid structures of alteration layers are compared with those of synthetic aerogels of comparable compositions produced under various pH conditions. Alteration layers formed at pH 11 on ISG were shown to contain large structures (>10 nm) similar to synthetic aerogels created under neutral and basic conditions whereas alteration layers formed at pH 9 did not. Available dissolved silica species defined by silica solubility were proposed to have the greatest impact on alteration layer structure.

Surface analysis of corroded XV–XVI century copper coins by μ-XRF and μ-PIXE/μ-EBS self-consistent analysis

Particle Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF) are nowadays routinely used for analysing historical coins. However, its characterization, especially in the presence of a superficial chemically altered layer (corrosion), is difficult due to its layered structure which cannot be probed using these two techniques in the general case. Elastic backscattering (both Rutherford and non-Rutherford) spectrometry is commonly used to determine depth profiles but by its own it is insufficient to unveil these complex structures. Using a PIXE detector and a particle detector recording simultaneously it is possible to perform a self-consistent analysis to obtain results about concentration variation of different elements along depth. In the present work this method was tested for two Portuguese centenary copper-based coins, and XRF was used to complement PIXE information. It is shown that elemental depth profiles with the evaluation of the superficial altered layer can be clearly solved providing relevant results about corrosion extension and composition.

Mapping pigments and binders in 15th century Gothic works of art using a combination of visible and near infrared hyperspectral imaging

Visible and near infrared hyperspectral imaging have allowed unprecedented analyses of two major works of art of the National Museum of Catalan Art (MNAC), 15th century Gothic altarpieces: (1) Madonna of the ‘Consellers’ (Lluís Dalmau, 1443–1445) [1], an impressive piece of Catalan painting, inspired by J. van Eyck and (2) Princess Eudoxia exorcised in front of St. Stephen's tomb (Vergós Group, 1495–1500) part of the altarpiece of Sant Esteve de Granollers.Visible and near infrared (VNIR) hyperspectral imaging was used to highlight the artist's handling of materials in achieving light and shadow effects in the red draperies. The refinement in the modelling could be observed thanks to the distribution images of the red pigments. Short-wavelength infrared (SWIR) hyperspectral imaging revealed blackened azurite on visually altered areas of the two paintings. It is possible to map the presence of azurite below the altered layer and obtain a better understanding of the earlier appearance of the paintings. It was moreover possible to identify and map the binders used in both works of art: the use of oil in Dalmau's painting, as the earliest Catalan oil painting, is confirmed here whereas in the Vergós's painting both egg and oil may have been used in separate areas.

Hydrogen bonding interactions of H2O and SiOH on a boroaluminosilicate glass corroded in aqueous solution

Hydrogen bonding interactions play an important role in many chemical and physical processes occurring in bulk liquids and at interfaces. In this study, hydrous species (H2O and Si-OH) on nano-porous alteration layers (gels) formed on a boroaluminosilicate glass called International Simple Glass corroded in aqueous solutions at pH 7 and pH 9, and initially saturated with soluble silicon-containing species were analyzed using linear and non-linear vibrational spectroscopy in combination with molecular dynamics simulations. The simulation results revealed various possible types of hydrogen bonds among these hydrous species in nanoconfinement environments with their populations depending on pore-size distribution. The nano-porous gels formed on corroded glass surfaces enhance hydrogen bond strength between hydrous species as revealed by attenuated total reflectance infrared spectroscopy. Sum frequency generation spectroscopy showed some significant differences in hydrogen bonding interactions on alteration layers formed at pH 7 and pH 9. The glass dissolution under the leaching conditions used in this study has been known to be ten times faster at pH 7 in comparison to that at pH 9 due to unknown reasons. The simulation and experimental results obtained in this study indicate that the water mobility in the gel formed at pH 9 could be slower than that in the gel formed at pH 7, and as a result, the leaching rate at pH 9 is slower than that at pH 7.

Evolution of cations speciation during the initial leaching stage of alkali-borosilicate-glasses

AbstractAlkali-borosilicate glasses (ABS) are used as host immobilization matrices for different radioactive waste streams and are characterized by their ability to incorporate a wide variety of metal oxides with respectively high waste loadings. The vitreous wasteform is also characterized by very good physical and chemical durability. The durability of three ABS compositions were analyzed by investigating their leaching behavior using the MCC1 test protocol and these data were used to investigate the waste components retention in the altered layer and the evolution of the interfacial water composition during the test. The results indicated that the Mg species evolution is exceptional with respect to other alkaline elements and dependent on glass matrix composition and leaching progress, while transition elements speciation is fairly constant throughout leaching process and independent on glass compositions. Si and B species are changing during leaching process and are affected by waste composition. For modified wasteform sample, evolution of Mg, Si and B species is respectively constant, whereas at highest waste loading, these elements have fairly constant speciation evolution within the first 2 weeks of leaching.

Complications of small incision lenticule extraction.

The procedure of small incision lenticule extraction (SMILE) was introduced in 2011, and since then there has been an increase in the number of cases undergoing this procedure worldwide. The surgery has a learning curve and may be associated with problems in the intraoperative and postoperative periods. The intraoperative problems during SMILE surgery include the loss of suction, the occurrence of altered or irregular opaque bubble layer and black spots, difficulty in lenticular dissection and extraction, cap perforation, incision-related problems, and decentered ablation. Most of the postoperative problems are similar as in other laser refractive procedures, but with decreased incidence. The identification of risk factors, clinical features, and management of complications of SMILE help to obtain optimum refractive outcomes.

The dissolution of simulant vitrified intermediate level nuclear waste in young cement water

AbstractIt is pertinent to the safety case for geological disposal in the UK that the behaviour of vitrified wastes in proximity to cementitious materials is understood. In this study, vitrified simulant intermediate level nuclear waste (ILW) was subject to dissolution in a synthetic cement water solution to simulate disposal conditions. Results show that the presence of alkali / alkaline earth elements in the cementitious solution can be favourable, at least in the short-term, leading to lower dissolution rates associated with incorporation of these elements into the altered layer of the glass.

Research of surface microstructure of the steel 40CrNiMo after EDM

The aim of the work is to study the influence of modes of electrical discharge machining, on the microstructure and microhardness of the treated surface of thermally hardened steel 40CrNiMo. To fulfill the task, a sample was made of hardened steel 40CrNiMo. The sample was processed in various modes of electrical discharge machining. Microsections were made from the obtained sample. Metallographic and microdurometric analysis was carried out. The study of the microstructure showed that no clearly defined altered layer was detected in all processing modes. The study of microhardness showed that near the surface of the studied samples there is a drop in the level of microhardness. This phenomenon is caused by tempering during the surface heating during processing. The heat-affected zone does not exceed 1.2 mm, and is minimal under high-performance modes.

Acceleration of glass alteration rates induced by zeolite seeds at controlled pH

As the glass-water reaction proceeds, certain glass compositions undergo a delayed acceleration in the alteration rate – a phenomenon that is commonly referred to as Stage III behavior – which generally coincides with zeolite formation. However, studying Stage III in the laboratory is difficult because the time scale required to observe Stage III behavior can vary from months to several years, and the solution conditions that initiate this acceleration remain poorly understood. Consequently, the development of a corrosion test method capable of quantifying Stage III rates in a timely manner would be of great benefit to both the scientific community and policy makers concerned with nuclear waste disposal. In this work, we induced and measured the acceleration in the corrosion rate of the AFCI glass by seeding static corrosion tests with zeolite Na–P2 at imposed pH90°C values ranging from 9.5 to 11.5. The log10 induced Stage III rates increased linearly with increasing pH and were measured to be between the forward and residual rates of AFCI. Tests designed to compare induced Stage III rates obtained for different seeding times showed that the induced Stage III rates were faster in the absence of an alteration layer. In summary, the use of the seeded test method allows for a quantification of induced Stage III rates in a relatively short, one-month time period.

Microfluidic Study of Fracture Dissolution in Carbonate-Rich Caprocks Subjected to CO2-Charged Brine

Investigating fracture evolutions triggered by chemical interactions in caprocks of CO2 storage sites is of great importance when caprock integrity is concerned. Mineral heterogeneity is one of the factors affecting fracture evolution. We present results from flow-through experiments deploying a unique high pressure geo-material microfluidic cell to monitor the fracture evolution of four carbonate-rich caprocks: (1) a homogeneous carbonate-rich sample, (2) a heterogeneous carbonate-rich sample, (3) a heterogeneous carbonate-rich shale sample, and (4) a heterogeneous carbonate-rich organic shale sample, representing different levels of mineral heterogeneity. The results show rather smooth fracture wall dissolution for the homogeneous rock sample. For the heterogeneous sample without shale, however, an altered layer is formed around the fracture that leads to an increase in the fracture roughness. Chemical analyses of effluent solutions demonstrate a decrease in the bulk dissolution rate of calcite over tim...

Collapse of Reacted Fracture Surface Decreases Permeability and Frictional Strength

AbstractGeochemical and geomechanical perturbations of the subsurface caused by the injection of fluids present risks of leakage and seismicity. This study investigated how acidic fluid flow affects hydraulic and frictional properties of fractures using experiments with 3.8‐cm‐long specimens of Eagle Ford shale, a laminated shale with carbonate‐rich strata. In low‐pressure flow cells, one set of samples was exposed to acidic brine and another set was exposed to neutral brine. X‐ray computed tomography and energy‐dispersive X‐ray spectroscopy revealed that samples exposed to acidic brine were calcite‐depleted and had developed a porous altered layer, while the other set showed no evidence of alteration. After reaction, samples were compressed and sheared in a triaxial cell that supplied normal stress and differential pore pressure at prescribed sliding velocities, independently measuring friction and permeability. During the initial compression, the porous altered layer collapsed into fine particles that filled the fracture. This effectively impeded flow and sealed the fracture, resulting in fracture permeability to decrease 1 to 2 orders of magnitude relative to the unaltered fractures. This is a favorable outcome in subsurface applications where the goal is to reduce leakage risks. However, during shear the reacted fracture had lower frictional strength because the fine‐grained particles in the collapsed layer prevented the formation of interlocking microasperities. Therefore, coupled geochemical and geomechanical processes that could favorably seal fractures could also increase the likelihood of induced seismicity. These findings have important implications for geological carbon sequestration, pressurized fluid energy storage, geothermal energy, and other subsurface technologies.

Soil flushing pilot test in a landfill polluted with liquid organic wastes from lindane production

Sites contaminated by Dense Non-Aqueous Liquid Phases (DNAPLs) containing chlorinated compounds are a ubiquitous problem caused by spills or the dumping of wastes with no concern for the environment. Their migration by gravity through the subsurface and their accumulation far below ground level make in-situ treatments the most appropriate remediation technologies. In this work, an aqueous solution containing a non-ionic and biodegradable surfactant was injected in the Sardas alluvial layer contaminated at some points with DNAPL (formed by a mixture of more than 28 chlorinated compounds) from lindane production. A volume of 5.28 m3 of an aqueous surfactant emulsion (13 g L−1) was injected at 14.5 m b g.l in the permeable layer (gravel-sand), at a flow rate of 0.6 m3 h−1 and the groundwater was monitored within a test cell (3.5 m radius) built ad hoc. The flow of the injected fluids in the subsurface was also evaluated using a conservative tracer, bromide (130 mg L−1), added to the surfactant solution. Concentration of contaminants, chloride, bromide and surfactant, surface tension and conductivity were measured at the injection point and at three monitoring points over time. High radial dispersion was noticed resulting in high dilution of the injected fluids. The surfactant was not adsorbed in the soil during the injection time, the adsorption of the surfactant took place in the meantime (15 h) between its injection and the groundwater (GW) extraction. The concentration of chlorinated compounds dissolved from the soil in the surfactant aqueous phase when equilibrium was reached (about 850 mg L−1) is related to the moderate average contamination of the soil in the test cell (about 1230 mg kg−1). In contrast, the extraction of the free DNAPL in the altered marls layer was highly enhanced due to the addition of the surfactant. Finally, it was found that the surfactant and the contamination did not migrate from the capture zone.

Geochemical and mineralogical assessment of reactivity in a full-scale heterogeneous waste-rock pile

Mine waste rock is typically stored on-site in large piles that are heterogeneous in terms of material composition and grain size. This leads to variable reactivity within the pile and makes prediction of drainage quality difficult. Here, we present data from two >100 m continuous boreholes through an operational waste-rock pile at the Antamina mine (Peru). Reactive zones with elevated sulfide and metal content exhibited increased temperatures, reduced oxygen content, as well as pervasive secondary mineral precipitation. Sharp contact between altered and unaltered layers at the meter to centimeter scale indicated strong variability in mineral reactivity. Unaltered waste rock persisted within high-sulfide zones due to low oxygen levels, and surface passivation and galvanic interactions between sulfide couples also limited alteration. Waste-rock layers with gradated alteration that resemble hardpans, as observed in tailings, were in direct contact with non-altered, less-permeable layers. Finally, leaching tests show that sulfate, Cu and Zn were efficiently leached from reactive and mineralogically altered zones, whereas As and Fe were retained through sorption, surface passivation, or secondary mineral formation. This study shows that, over and above bulk geochemical and lithological data, waste-rock reactivity may also be defined by mineralogical data and thereby contribute to improved drainage quality predictions.

Crossover from anomalous to Fickean behavior in infiltration and reaction in fractal porous media

When the solution in a porous medium is in chemical equilibrium with mineral grains and an external surface is put in contact with water, the infiltration of unsaturated solution may be accompanied by mineral reactions that change the morphology of that medium. Here we study the evolution of this infiltration process considering that the initial porous media are fractal and that the reactions form additional porous material plus soluble products. A Sierpinski carpet and a Menger sponge are the fractal models, the mineral inclusions are represented by the non-fractal blocks (squares and cubes) formed in their iterative constructions, and reaction rates are described by a thermally activated model, in conditions of slow reactions in comparison with the diffusion. The interplay between the infiltration by diffusion and the structural changes by the reactions is explained by a combination of kinetic Monte Carlo simulations of lattice models and a scaling approach. At short times, the infiltration is subdiffusive, but the dissolved mass increases anomalously fast due to delocalization of the reaction, which is distributed through a time increasing infiltrated region. At long times, normal (Fickean) infiltration and reaction are observed. The crossover between the two regimes occurs when the thickness of the altered layer formed around the mineral inclusions is of the same order of magnitude of the width a of the smallest gaps between those inclusions. The order of magnitude of the crossover time tc is estimated as a/(vk), where k is the reaction rate constant and v is the molar volume of the mineral; this time does not depend on the diffusion coefficient of reaction products or on the fractal geometry of the initial medium, which suggests the application to a variety of systems. Estimates of tc are obtained in two recently studied rocks with reported fractal properties and justify the Fickean diffusion assumption of previous models for their weathering in geological time scales. The morphological evolution of the mineral blocks that partially reacted qualitatively agrees with experiments.