Long-term Dissolution Research Articles

Nanoporous silica gel structures and evolution from reactive force field-based molecular dynamics simulations

Nanoporous silica-rich gel formed on silicate glass surfaces during dissolution in aqueous environment is critical in elucidating the corrosion mechanisms and the long-term residual dissolution behaviors. Silica gel models were created using two types of methods with reactive force field-based molecular dynamics simulations. The results show that the remnant silica gels created from the ISG bulk structure have a more isolated and closed pore morphology and slightly higher glass network connectivity. This contrasts with the gel structures created by hydrogarnet defect formation that exhibit more connected pore morphologies. The remnant gel structures show lower water diffusivity which was explained by the nano-confinement effect of water molecules due to frequent interactions of water molecules with adjacent silica walls and the more isolated pore morphology in the remnant gel structures. These results reveal the complexity in terms of micro and atomic structures of these silica gels, and both structure features have impact on water transport in the gel layer hence the passivating effect that controls the long-term dissolution behavior of these glasses.

Structural role of ZrO2 and its impact on properties of boroaluminosilicate nuclear waste glasses

Addition of zirconia (ZrO2) to nuclear waste glasses, even in small amount, significantly affects physical properties such as chemical durability, density, viscosity, and glass transition temperature. Hence ZrO2 plays an important role in the development of nuclear waste glass compositions. It was found recently that addition of zirconia decreases the initial dissolution rate but increases long-term dissolution by changing the protective properties such as porosity of alteration layers. In this study, the International Simple Glass (ISG) with different amounts of ZrO2/SiO2 substitution was simulated using classical molecular dynamics (MD) simulations and recently developed composition-dependent potential sets. Local structural descriptors such as bond distances, bond angle distributions, and coordination numbers were systematically studied and compared with experimental values. Zr K-edge extended X-ray absorption fine structure spectra were also calculated and compared with experiment to resolve the local coordination environment around Zr. Medium-range structural information (e.g., Qn distribution, network connectivity, and ring-size distribution) showed that ZrO2/SiO2 substitution increases the overall network connectivity and the amount of smaller sized rings, which will decrease the initial dissolution rate by strengthening the glass network and limiting water diffusivity. Finally, diffusion and dynamic properties of the ions were studied and discussed to develop a better understanding of the chemical durability of these glasses.

A 3D Numerical Approach to Assess the Temporal Evolution of Settlement Damage to Buildings on Cavities Subject to Weathering

The goal of this paper is to show how a recently developed advanced chemo-hydro-mechanical (CHM)-coupled constitutive and numerical model for soft rocks can be applied to predict the temporal evolution of settlement damage to buildings on cavities subject to weathering. In particular, a Building Damage Index (BDI) and its evolution with time is proposed. The definition of the BDI is inspired by the work of Boscardin and Cording (J Geotech Eng 115:1–21, 1989) and uses the surface differential settlements obtained by finite element (FE) analyses to assess how far a building is from a non-acceptable service condition. By modelling the reactive transport of chemical species in 3D and using a coupled CHM constitutive and numerical model, it is possible to simulate weathering scenarios and monitor the temporal evolution of surface settlements making the BDI time dependent. This approach is applied to evaluate the damage evolution of two buildings lying on two anthropic caves in a calcarenite deposit belonging to the Calcarenite di Gravina Formation. Standard and advanced experimental tests are performed on the in situ material, and the results are used to calibrate the constitutive model. The soundness of both constitutive relationship and reactive transport solver is subsequently tested by simulating two laboratory-scale boundary value experiments. The first is a model footing test on dry and wet calcarenite, while the second is a small-scale pillar that, after the saturation-induced short-term water weakening, fails due to a long-term dissolution weathering process. Finally, both 2D and 3D coupled FE analyses simulating different weathering scenarios and corresponding settlements affecting the buildings above the considered cavities are presented. Particular attention is placed on assessing the BDI and its temporal evolution.

Assessing the propensity of landfill soils to undergo reductive iron dissolution

Naturally occurring iron from soil and aquifer sediments at waste disposal sites often becomes liberated into groundwater as a result of reductive dissolution. Research was conducted to evaluate an appropriate procedure for assessing a soil’s propensity to undergo iron reductive dissolution. Soil samples collected from waste disposal sites in Florida were characterized by pH, organic carbon content, total iron content, amorphous iron content, citrate-dithionite-bicarbonate extractable iron, and qualitative X-ray diffraction analysis, followed by a series of extraction tests designed to simulate the reductive dissolution process. Over a 30-day period, biological reducing tests released 13–260 mg/kg Fe(II) from soils, and a chemical reducing test released 2.2–178 mg/kg Fe(II) from soils. Soil amorphous iron content was shown to be the most effective parameter for assessment of iron reductive dissolution potential through standard soil characterization. These results suggest that biological reducing tests may be helpful for assessing long-term soil iron reductive dissolution potential, and that soil amorphous iron content provides a good indication of the potential for a soil to undergo reductive dissolution at a landfill site.

Chemical durability of UK vitrified high level waste in Si-saturated solutions

In the UK, the Waste Vitrification Plant (WVP) at Sellafield converts the highly active liquid (HAL) waste from the reprocessing of spent nuclear fuel into a glass wasteform suitable for long term storage and disposal. After the completion of reprocessing operations, the highly active liquor (HAL) storage tanks will be emptied and washed out to remove any accumulated solids. These solids are expected to contain high concentrations of molybdenum. Hence, a new glass formulation has been developed specifically for immobilisation of these waste streams. This ‘Ca/Zn’ glass allows for significantly higher waste loadings by the formation of CaMoO4 crystals when the Mo content exceeds its solubility limit in the glass.The product quality of the new Ca/Zn glass is currently undergoing validation, which includes measuring the chemical durability to demonstrate its suitability for long-term storage and disposal. Previous powder-based leach tests have indicated a very slow approach to the low residual dissolution rate regime in contrast to other HLW glasses. Hence, this study investigated the leaching of Ca/Zn glass powders in Si-saturated solutions at high surface area to volume ratios in order to probe the long-term dissolution rate.

Discussion on Damages of Karst to Roadbed and Its Countermeasures

Because of the long-term dissolution of water, the formation of karst caves and fissures in rock formations are called karst. In Karst area, deformation and water damage are the main forms of karst damage to subgrade. Uneven settlement of embankment and instability of subgrade are the concrete manifestations of karst roadbed diseases. Therefore, considering the scale effect of karst, the concrete treatment measures of karst roadbed should be put forward and the principle of prevention and treatment of “dredging, crossing, reinforcing and blocking” should be followed, based on the characteristics of various karst forms, such as section funnel, depression, karst cave, solution crack and karst spring.

Long-Term Dissolution Behavior of Steelmaking Slag and Its Composite Materials in Seawater

Steelmaking slag is considered to be a suitable candidate material for rehabilitating marine environments damaged by sea desertification because it can supply nutrient elements, and its positive effects have been phenomenologically proven. To fully understand the role of steelmaking slag in the rehabilitation process and maximize its efficiency, elucidation of the dissolution mechanism of nutrient elements from slag-based materials is essential. The present study focused on the long-term dissolution behaviors of steelmaking slag, a slag–soil composite material, and a slag–humic substance composite material. Column experiments with a duration of 1152 h clarified that a marked increase in the pH value as well as the significant dissolution of elements occurred in the initial stage, while the subsequent increase in the concentrations of elements was small, and the Fe contained in the original seawater precipitated owing to the locally high pH in the column.

Stability of Clay Particle-Coated Microbubbles in Alkanes against Dissolution Induced by Heating.

We investigated the dissolution and morphological dynamics of air bubbles in alkanes stabilized by fluorinated colloidal clay particles when subjected to temperature changes. A model for bubble dissolution with time-dependent temperature reveals that increasing the temperature enhances the bubble dissolution rate in alkanes, opposite to the behavior in water, because of the differing trends in gas solubility. Experimental results for uncoated air bubbles in decane and hexadecane confirm this prediction. Clay-coated bubbles in decane and hexadecane are shown to be stable in air-saturated oil at constant temperature, where dissolution is driven mainly by the Laplace pressure. When the temperature increases from ambient, the particle-coated bubbles are prone to dissolution as the oil phase becomes undersaturated. The interfacial layer of particles is observed to undergo buckling and crumpling, without shedding of clay particles. Increasing the concentration of particles is shown to enhance the bubble stability by providing a higher resistance to dissolution. When subjected to complex temperature cycles, for which the effect of time-dependent temperature is dominant, the clay-coated bubbles can resist long-term dissolution in conditions under which uncoated bubbles dissolve completely. These results underpin the design of ultrastable oil foams stabilized by solid particles with improved shelf life under changing environmental conditions.

Hydrogeological conditions for the occurrence of two magnesium-rich natural mineral waters in Serbia and their physiological significance

Numerous medical studies have shown that magnesium has a remarkable significance for the human physiology. Thus, the possibility of using two magnesium-rich natural waters from Serbia as magnesium supplements and the hydrogeological features of these occurrences are presented through the results of SEM–EDS and XPRD analyses of water dry residues and comparisons with crystalline phases of two types of synthetic magnesium supplements available in the Serbian market. The results indicate that major phases in the dry residues of the water samples are Mg phases, whereas the synthetic supplements (besides Mg phases) contain significant amounts of fillers, such as citric acid and sodium bicarbonate. The mineral phases of dry residues of these two natural mineral waters also indirectly indicate the local hydrogeological conditions during the movement of groundwater. Hydromagnesite, as the main and only crystalline phase in the dry residue of the first Mg-rich (330 mg/l) natural mineral water, indicates a deep groundwater circulation and long-term dissolution of olivine or serpentine minerals, considering that it is tapped from the well in which only gneiss was identified. Smaller quantities of hydromagnesite and higher content of amorphous phase in the dry residue of the second Mg-rich mineral water could probably indicate a rapid circulation and shorter dissolution of serpentine minerals through which the water passes directly near the surface. The high content of Mg-rich crystal phases (up to 80%) in the dry residue of tested waters indicates their high physiological value as (natural) dietary supplements.

Lithological influences on contemporary and long-term regolith weathering at the Luquillo Critical Zone Observatory

Lithologic differences give rise to the differential weatherability of the Earth’s surface and globally variable silicate weathering fluxes, which provide an important negative feedback on climate over geologic timescales. To isolate the influence of lithology on weathering rates and mechanisms, we compare two nearby catchments in the Luquillo Critical Zone Observatory in Puerto Rico, which have similar climate history, relief and vegetation, but differ in bedrock lithology. Regolith and pore water samples with depth were collected from two ridgetops and at three sites along a slope transect in the volcaniclastic Bisley catchment and compared to existing data from the granitic Río Icacos catchment. The depth variations of solid-state and pore water chemistry and quantitative mineralogy were used to calculate mass transfer (tau) and weathering solute profiles, which in turn were used to determine weathering mechanisms and to estimate weathering rates.Regolith formed on both lithologies is highly leached of most labile elements, although Mg and K are less depleted in the granitic than in the volcaniclastic profiles, reflecting residual biotite in the granitic regolith not present in the volcaniclastics. Profiles of both lithologies that terminate at bedrock corestones are less weathered at depth, near the rock-regolith interfaces. Mg fluxes in the volcaniclastics derive primarily from dissolution of chlorite near the rock-regolith interface and from dissolution of illite and secondary phases in the upper regolith, whereas in the granitic profile, Mg and K fluxes derive from biotite dissolution. Long-term mineral dissolution rates and weathering fluxes were determined by integrating mass losses over the thickness of solid-state weathering fronts, and are therefore averages over the timescale of regolith development. Resulting long-term dissolution rates for minerals in the volcaniclastic regolith include chlorite: 8.9×10−14molm−2s−1, illite: 2.1×10−14molm−2s−1 and kaolinite: 4.0×10−14molm−2s−1. Long-term weathering fluxes are several orders of magnitude lower in the granitic regolith than in the volcaniclastic, despite higher abundances of several elements in the granitic regolith. Contemporary weathering fluxes were determined from net (rain-corrected) solute profiles and thus represent rates over the residence time of water in the regolith. Contemporary weathering fluxes within the granitic regolith are similar to the long-term fluxes. In contrast, the long-term fluxes are faster than the contemporary fluxes in the volcaniclastic regolith. Contemporary fluxes in the granitic regolith are generally also slightly faster than in the volcaniclastic. The differences in weathering fluxes over space and time between these two watersheds indicate significant lithologic control of chemical weathering mechanisms and rates.

A spatially resolved surface kinetic model for forsterite dissolution

The development of complex alteration layers on silicate mineral surfaces undergoing dissolution is a widely observed phenomenon. Given the complexity of these layers, most kinetic models used to predict rates of mineral–fluid interactions do not explicitly consider their formation. As a result, the relationship between the development of the altered layers and the final dissolution rate is poorly understood. To improve our understanding of the relationship between the alteration layer and the dissolution rate, we developed a spatially resolved surface kinetic model for olivine dissolution and applied it to a series of closed-system experiments consisting of three-phases (water (±NaCl), olivine, and supercritical CO2) at conditions relevant to in situ mineral carbonation (i.e. 60°C, 100bar CO2). We also measured the corresponding δ26/24Mg of the dissolved Mg during early stages of dissolution. Analysis of the solid reaction products indicates the formation of Mg-depleted layers on the olivine surface as quickly as 2days after the experiment was started and before the bulk solution reached saturation with respect to amorphous silica. The δ26/24Mg of the dissolved Mg decreased by approximately 0.4‰ in the first stages of the experiment and then approached the value of the initial olivine (−0.35‰) as the steady-state dissolution rate was approached. We attribute the preferential release of 24Mg to a kinetic effect associated with the formation of a Mg-depleted layer that develops as protons exchange for Mg2+.We used experimental data to calibrate a surface kinetic model for olivine dissolution that includes crystalline olivine, a distinct “active layer” from which Mg can be preferentially removed, and secondary amorphous silica precipitation. By coupling the spatial arrangement of ions with the kinetics, this model is able to reproduce both the early and steady-state long-term dissolution rates, and the kinetic isotope fractionation. In the early stages of olivine dissolution the overall dissolution rate is controlled by exchange of protons for Mg, while the steady-state dissolution rate is controlled by the net removal of both Mg and Si from the active layer. Modeling results further indicate the importance of the spatial coupling of individual reactions that occur during olivine dissolution. The inclusion of Mg isotopes in this study demonstrates the utility of using isotopic variations to constrain interfacial mass transfer processes. Alternative kinetic frameworks, such as the one presented here, may provide new approaches for modeling fluid–rock interactions.

Management options for Fukushima corium

The loss of core cooling for units 1–3 during the accident at Fukushima Dai-ichi caused major fuel damage. Although full details are not yet available, fuel melting produced corium within the reactor pressure vessels that has, to an unknown degree, melted through into the primary containment. The present priority is cooling the damaged reactors and managing contaminated water, but planning of longer term decommissioning has already begun. Management of highly damaged fuel and corium will be of primary concern, with the main options being recovery for reprocessing or packaging for direct disposal. Although the latter option may have significant cost advantages, it presents some novel safety challenges for both operational and post-closure phases. Concerns include criticality management and modelling of long-term dissolution of materials having highly variable composition. Further R&D is required to fill knowledge gaps – of which the most sensitive may involve determination of the extent to which small “hot particles” of corium have been produced.

除塩された水田環境における各種製鋼スラグ系肥料の長期溶出挙動(津波被災水田の製鋼スラグ系肥料による復興−2)

除塩された水田環境における各種製鋼スラグ系肥料の長期溶出挙動(津波被災水田の製鋼スラグ系肥料による復興−2)

Experimental Study on the Water-Induced Weakening of Calcarenites

Carbonatic rocks, such as calcarenites, are very often subject to damage processes, causing a progressive degradation of their mechanical properties. In nature, in some cases, this phenomenon can cause the collapse of cliffs and underground cavities, with dangerous consequences for the anthropic environment. In this paper, the results of an experimental campaign, intended to both clarify and quantify the mechanical consequences of this process, are illustrated. To achieve such a goal, suitable physical and geotechnical indices are introduced and different time scales to describe the physical/chemical reactions induced by the water saturation of the material are taken into consideration. In particular, the authors have observed: (1) a short-term marked and instantaneous reduction in strength when water fills the pores of the rock; (2) a long-term dissolution; and (3) a progressive chemically induced reduction in the grain size. To describe the degradation processes induced by the material water saturation, owing to the complexity of the hydro-chemo-mechanical phenomena taking place within the material, suitably designed tests under controlled “weathering” conditions were also performed and discussed.

Micro-channel as a New Tool to Investigate Glass Dissolution Kinetics

A reliable modeling of the long-term dissolution of HLW glass requires sufficient evaluation of the glass dissolution kinetics including a sound understandings of reaction mechanism. For the evaluation of glass dissolution kinetics, we need much more data on the glass dissolution rate measured precisely, consistently and systematically under various well-constrained test conditions. The current standard test methods, unfortunately, cannot provide enough data for the kinetic evaluation. Therefore, we should improve or develop the test methods to provide precise and consitent data for the kinetic evaluation. In the present paper, therefore, major standard test methods currently applied to the measurement of glass dissolution are summarized with their advantages and disadvantages, and some newly developed test methods are introduced. In addition, problems to be solved are discussed to advance the kinetic evaluation.

Magnetic investigation of peat sediments from the mountain in Arid Regions

Detailed and systematic magnetic analysis was conducted on continuous peat depositional sequences from the Altay Mountains in arid regions, Northwest China. Based on magnetic properties, such as composition, concentration and grain-size, the magnetic-variation mechanism of these minerals can be discussed in peat sediments that are rich in organic matter under redox conditions. The results are showed as follows: The concentration of ferrimagnetic minerals has a low enrichment degree. Magnetite, hematite, a great deal of diamagnetic organic matter and some paramagnetic components have been identified in peat sediments, and there is no magnetotactic bacteria (MTB). The grain sizes are mainly fine particles, but meanwhile there are also coarse particles-multidomain (MD). The results further reveal that the major variations of ferrimagnetic minerals in the peat sediments occur during the short period of post-depositional processes. As for the possible mechanism, we argue that the pseudo single domain (PSD) and MD magnetite on the peatland surface are dissolved and refined in the acid waterlogged sub-oxic conditions, which results in low concentration and smaller grain sizes. Furthermore, we also speculate that no long-term burial-diagenetic dissolution processes take place in the peat sediments and little magnetite dissolution happens under the condition of alkaline and reductive environments in which the peat is buried deeply.

Contribution of atom-probe tomography to a better understanding of glass alteration mechanisms: Application to a nuclear glass specimen altered 25 years in a granitic environment

We report and discuss results of atom probe tomography (APT) and energy-filtered transmission electron microscopy (EFTEM) applied to a borosilicate glass sample of nuclear interest altered for 25.75years at 90°C in a confined granitic medium in order to better understand the rate-limiting mechanisms under conditions representative of a deep geological repository for vitrified radioactive waste. The APT technique allows the 3D reconstruction of the elemental distribution at the reactive interphase with sub-nanometer precision. Profiles of the B distribution at pristine glass/hydrated glass interface obtained by different techniques are compared to show the challenge of accurate measurements of diffusion profiles at this buried interface on the nanometer length scale. Our results show that 1) Li from the glass and hydrogen from the solution exhibit anti-correlated 15nm wide gradients located between the pristine glass and the hydrated glass layer, and 2) boron exhibits an unexpectedly sharp profile (~3nm width) located just outside of the Li/H interdiffusion layer; this sharp profile is more consistent with a dissolution front than a diffusion-controlled release of boron. The resulting apparent diffusion coefficients derived from the Li and H profiles are DLi=1.5×10−22m2·s−1 and DH=6.8×10−23m2·s−1. These values are around two orders of magnitude lower than those observed at the very beginning of the alteration process, which suggests that interdiffusion is slowed at high reaction progress by local conditions that could be related to the porous structure of the interphase. As a result, the accessibility of water to the pristine glass could be the rate-limiting step in these conditions. More generally, these findings strongly support the importance of interdiffusion coupled with hydrolysis reactions of the silicate network on the long-term dissolution rate, contrary to what has been suggested by recent interfacial dissolution–precipitation models for silicate minerals.

Sorption, dissolution and pH determine the long-term equilibration and toxicity of coated and uncoated ZnO nanoparticles in soil

To assess the effect of long-term dissolution on bioavailability and toxicity, triethoxyoctylsilane coated and uncoated zinc oxide nanoparticles (ZnO-NP), non-nano ZnO and ZnCl2 were equilibrated in natural soil for up to twelve months. Zn concentrations in pore water increased with time for all ZnO forms but peaked at intermediate concentrations of ZnO-NP and non-nano ZnO, while for coated ZnO-NP such a clear peak only was seen after 12 months. Dose-related increases in soil pH may explain decreased soluble Zn levels due to fixation of Zn released from ZnO at higher soil concentrations. At T = 0 uncoated ZnO-NP and non-nano ZnO were equally toxic to the springtail Folsomia candida, but not as toxic as coated ZnO-NP, and ZnCl2 being most toxic. After three months equilibration toxicity to F. candida was already reduced for all Zn forms, except for coated ZnO-NP which showed reduced toxicity only after 12 months equilibration.

Silicon budget of eutrophic Lake Kasumigaura, Japan

Determining the dynamics of silicon in lakes, one of the essential nutrients for diatoms, is valuable for understanding aquatic environmental problems. The dissolved silicon (DSi) and biogenic silicon (BSi) budgets in Lake Kasumigaura, a shallow eutrophic lake in Japan, during the last three decades were assessed based on the analysis of dated sediment cores and a water quality database. Sediment cores (100 cm long) were taken at the center of Lake Kasumigaura in 2005, 2007, and 2009 and at two other sites in 2007. BSi contents of the dated sediments were determined by wet alkaline digestion. The net sedimentation rates of BSi were defined as the difference between the DSi load from inflowing rivers and the DSi and BSi loads from the outflow of the lake, calculated using DSi concentrations and diatom abundances in the lake from 1980 to 2007 and DSi concentrations of the inflowing rivers during 1994, 2007, and 2009. The gross sedimentation rates of BSi were estimated by multiplying BSi concentrations in lake water by the diatom sinking rate reported by previous studies. Budgetary calculations based on the database showed that 60–70 % of DSi inputs from the inflowing rivers during the 27 years could ultimately be accumulated as diatom frustules in bottom sediments in the lake. The sediment analysis revealed that the amount of BSi accumulated in the lake from 1980 to 2007 was 2.0–2.6 × 1011 g, similar to the amount based on the database of 1.3–2.4 × 1011 g. Although the gross sedimentation rates of BSi likely increased, the net sedimentation rates of BSi decreased significantly from 6–10 × 109 g year−1 in the 1980s to 2–6 × 109 g year−1 in the 2000s, suggesting a fast recycling of BSi in recent years caused by an increase in sediment resuspension and regeneration. The sediment core information and the water quality database can be used for calculating the long-term silicon budgets in Lake Kasumigaura. An increase in the DSi release rates was identified, which is consistent with recent sediment resuspension. Comparing the sediment core information with the database suggests the long-term dissolution of sediment BSi; however, analysis of the BSi content in sediment cores representing a much longer time period is needed to confirm this.

Effect of composition on the short-term and long-term dissolution rates of ten borosilicate glasses of increasing complexity from 3 to 30 oxides

Ten borosilicate glass compositions consisting of a ternary sodium borosilicate containing increasing numbers of some of the key elements (Al, Ca, Zr, Ce) present in nuclear glasses were leached in pure water at 90°C and monitored for up to 14years. They were then characterized to establish correlations between the glass composition and the short- and long-term alteration rates. We first qualitatively explain the variations of the initial dissolution rate by structural considerations. Then we evidence a qualitative inverse correlation between the initial and residual rates. This counterintuitive result is in fact related to the effect of gel reorganization on the diffusive properties of the passivating layer. Since no equilibrium can be reached between glass and solution, these long-term experiments help in understanding how glasses behave once the solution is saturated with respect to the main glass formers. Very efficient synergy between Ca and hardener elements (Al or Zr) leads to the lowest residual rates, compared with glasses having only one of the two categories of elements. We also confirm the detrimental effect of precipitation of silicate minerals on the residual rate.