Corrosion of simulated nuclear waste glass

Glasses are suitable host matrices for the immobilization of high-level radioactive wastes. The corrosion behaviour of nuclear waste glass in water is of considerable importance, since a potential route for returning of radionuclides to the biosphere is their leaching from the waste form into groundwater and subsequent transport by the groundwater to the surface. In this study, the preparation and characterization of borosilicate glasses of different chemical composition were investigated. Borosilicate glasses were doped with simulated nuclear waste oxides. The chemical corrosion in water of these glasses was followed by measuring the leach rates (g·cm^–2·day^–1), as a function of time. It was found that a simulated nuclear waste glass with the chemical composition (weight %), 15.61% Na₂O, 10.39% B₂O₃, 45.31% SiO₂, 13.42% ZnO, 6.61% TiO₂ and 8.66% waste oxides, is characterized by low melting temperature and with good corrosion resistance in water. Influence of passive layers on the leaching behaviour of nuclear waste glasses is discussed.     

Thermodynamic Approach for Predicting Actinide and Rare Earth Concentrations in Leachates from Radioactive Waste Glasses

Studies aimed primarily at determining leach rates of different elements from doped glasses have resulted in computerized models for predicting leachate concentrations. However, leach rate related data should be limited to predicting the stability behavior of the glass matrix; the radionuclide release data based on these studies are empirical and are highly dependant on many variables and processes which have not been systematically evaluated and thus do not provide a reliable method of predicting leachate concentrations. A better approach is available for those elements that can readily form relatively insoluble solids during preparation of glass or glass/water interactions. This alternate approach relies on the experimental solubilities of pulverized doped glasses, in a wide range of well-controlled important variables such as pH and pe, and their comparisons at the given aqueous composition to predicted solubilities of known solid phases from the thermodynamic data. These comparisons are used to indirectly identify specific solubility-controlling solids in doped glass/water systems to determine scientifically defensible aqueous concentrations of different elements for any given groundwater composition, independent of glass dissolution kinetics and independent of time. This paper summarizes data available for the application of this alternate approach to reliably predict concentrations of thorium, uranium, neptunium, plutonium, and trivalent actinides and rare earth elements leachable from the doped glasses. The thermodynamic data, in addition to that reported in recent critical reviews, includes new data that were developed for the solubility products of Th₃(PO₄)₄(s) and the solid solutions of trivalent actinides and rare earth hydroxides. Thermodynamic interpretations of the doped glass solubility data show specifically that tetravalent actinide hydrous/crystalline oxides and solid solutions of trivalent actinides and rare earths hydroxides in non-phosphate glasses and Th₃(PO₄)₄(s) and MPO₄(s), where M denotes trivalent actinides or rare earths, in phosphate-containing glasses are the dominant solubility-controlling solids. Needed future research in this area is briefly outlined.     

General, spontaneous ion replacement reaction for the synthesis of micro- and nanostructured metal oxides

A novel spontaneous ion replacement route based on the solubility difference as the driving force to synthesize a number of metal oxides has been established. We present a comprehensive study on the ion replacement reaction for chemical synthesis of micro- and nanostructured Mn2O3, ZnO, CuO, CdO, Al2O3, and CaO samples. This novel approach described herein is derived from the solubility difference between two carbonate salts, in which a metal cation can be driven from one liquid phase into another solid phase in the solution system. The resulting metal carbonate salts are initially formed and subsequently calcined to form highly crystallined metal oxides. The variation of pH values, reaction temperature, and reagent shapes can vary the solubility of these two carbonate salts, which thus changes the final morphology of metal oxides. The present work makes a progress to simply and mildly synthesize metal oxides with various morphologies, due to the fact that materials with a desired morphology are a key engineering step toward their shape-dependent chemical and physical properties.     

Magnetic susceptibility studies of lead oxyhalide glasses containing transition metal oxides

Magnetic susceptibility studies of lead oxyhalide glasses containing high concentrations of transition metal oxides such as MnO and Fe₂O₃ have been performed. While they exhibit predominantly antiferromagnetic interactions, the low temperature (<100K) region is dominated by paramagnetic contributions. The behaviour in these glasses is found to be similar to that of covalent oxide glasses and is different from that of purely ionic sulphate glasses. Communication No. 324 from the Solid State and Structural Chemistry Unit.     

Temperature‐Induced Intersite Charge Transfer Involving Cr ions in A‐Site‐Ordered Perovskites ACu3Cr4O12 (A=La and Y)

Changes in the valence state of transition‐metal ions in oxides drastically modify the chemical and physical properties of the compounds. Intersite charge transfer (ISCT), which involves simultaneous changes in the valence states of two valence‐variable transition‐metal cations at different crystallographic sites, further expands opportunities to show multifunctional properties. To explore new ISCT materials, we focus on A‐site‐ordered perovskite‐structure oxides with the chemical formula AA′₃B₄O₁₂, which contain different transition‐metal cations at the square‐planar A′ and octahedral B sites. We have obtained new A‐site‐ordered perovskites LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ by synthesis under high‐pressure and high‐temperature conditions and found that they showed temperature‐induced ISCT between A′‐site Cu and B‐site Cr ions. The compounds are the first examples of those, in which Cr ions are involved in temperature‐induced ISCT. In contrast to the previously reported ISCT compounds, LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ showed positive‐thermal‐expansion‐like volume changes at the ISCT transition.     

Thermal Behaviour of Silicate-Phosphate Glasses in Relation to Their Biochemical Activity

Glasses of theK₂O–MgO–CaO–SiO₂ and K₂O–MgO–CaO–P₂O₅–SiO₂ systems with microelements (Fe, Mo, Zn, B, Mn, Cu) were obtained by melting of apatite and serpentinite rocks, K₂CO³ and oxides of appropriately metals mixtures. The amorphous state of glasses and the structural changes during their heating were determined by RDF and DTA/DSC methods. The solubility of the glasses in 2 mass% citric acid solution was studied by chemical analysis (ICP-AES, EDS) and SEM observations.It has been found that the biochemical activity of silicate-phosphate glasses depends on the content of P₂O₅ and K₂O in their framework and is related to crystallisation ability of these glasses.This revised version was published online in November 2005 with corrections to the Cover Date.     

Classification of oxide glasses: A polarizability approach

A classification of binary oxide glasses has been proposed taking into account the values obtained on their refractive index-based oxide ion polarizability α_O2−(n₀), optical basicity Λ(n₀), metallization criterion M(n₀), interaction parameter A(n₀), and ion's effective charges as well as O1s and metal binding energies determined by XPS. Four groups of oxide glasses have been established: glasses formed by two glass-forming acidic oxides; glasses formed by glass-forming acidic oxide and modifier's basic oxide; glasses formed by glass-forming acidic and conditional glass-forming basic oxide; glasses formed by two basic oxides. The role of electronic ion polarizability in chemical bonding of oxide glasses has been also estimated. Good agreement has been found with the previous results concerning classification of simple oxides. The results obtained probably provide good basis for prediction of type of bonding in oxide glasses on the basis of refractive index as well as for prediction of new nonlinear optical materials.     

Calculation of viscosity–temperature curves for glass obtained from four wastewater treatment plants in Egypt

This article establishes the relationship between the chemical composition, temperature and viscosity of glasses obtained from the four sludge treatment plants of urban and industrial wastewater from the Nile Delta in Egypt. In order to determine the working conditions of these glasses and their growth temperature, different techniques have been used: differential thermal analysis, hot stage microscopy and dilatometry. We used a prototype of hot stage microscopy, with the help of an image analysis programme developed in the present study. The chemical composition of major oxides sludge ranging from: SiO₂ (36–48 wt%), Al₂O₃ (9–16 wt%), CaO (5–25 wt%), P₂O₅ (1.5–11 wt%), and Fe₂O₃ (~9 wt%), this composition is close to a basalt rock, being necessary to incorporate some raw materials to adjust it to the basalt rock that has a good viscosity-temperature curve. The glass transition temperatures of the four glasses obtained vary between 650 and 725 °C and the growth occurs between 938 and 1,033 °C. We also obtained the viscosity–temperature curves with the aid of the hot stage microscopy that has allowed us to determine the working temperatures of the four glasses, ranging from 926 to 1,419 °C, depending on the type of forming process used.     

Fabrication of hollow spheres of metal oxide using fructose-derived carbonaceous spheres as sacrificial templates

In this report, fructose-derived carbonaceous spheres were utilized as sacrificial templates for the fabrication of metal oxide hollow spheres (MOHSs) by a facile hydrothermal approach. Hollow spheres of a series of crystalline metal oxides (α-Fe₂O₃, Cr₂O₃, Co₃O₄, NiO, and ZnO) have been fabricated, utilizing the metal chloride as the oxide precursors. Heating of an aqueous solution of the metal chloride and fructose to moderate temperature in an autoclave affords a spherical composite consisting of a metal precursor shell sheathing a carbonaceous core. Subsequent removal of the interior carbonaceous cores by thermal treatment through oxidation in air produces free-standing crystalline oxides hollow spheres. The MOHSs were characterized by means of SEM, TEM, XRD, IR spectroscopy, energy dispersive X-ray (EDX) and sorption measurements. The results show convincingly that using fructose as a sacrificial template after application of a hydrothermal synthesis route could be a favourable sacrificial template for the fabrication of various MOHSs.     

The progress of microwave-assisted hydrothermal method in the synthesis of functional nanomaterials

Developing simple, rapid, and environmentally friendly synthetic methodologies for the preparation of functional nanomaterials is of great importance for broadening and improving their potential applications. In comparison with other methods, the microwave-assisted hydrothermal method possesses and combines the merits of microwave and hydrothermal methods, which can achieve the high temperature and high pressure for a short time from several minutes to several hours in a closed reaction system. In this review, the synthesis of various types of functional nanomaterials such as metals oxides, metal composite oxides, inorganic biomaterials (hydroxyapatite and calcium carbonate), and metal sulfides via the microwave-assisted hydrothermal method is summarized. The special properties and applications of functional nanomaterials by the microwave-assisted hydrothermal method are compared with others methods. The future developments of this promising method are put forward.     

Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance studies of octadecyl modified metal oxides obtained from different silane precursors

Octadecyl (C₁₈) modified metal oxide substrates, including titania, zirconia, hafnia, and alumina, are prepared using two types of silylating reagents, n-octadecyltrihydridosilane and n-octadecyltrichlorosilane. Fourier transform infrared (FTIR) and solid-state ²⁹Si nuclear magnetic resonance (NMR) measurements are performed to examine the cross-linking of the silanes. Solid-state ¹³C NMR spectroscopy provides information about the conformation and mobility of surface-immobilized alkyl chains. Variable temperature FTIR investigations are carried out to study the influence of the organosilane precursors and metal oxides on the conformational order of the alkyl modified systems. It is found that grafting by means of n-octadecyltrichlorosilane yields higher grafting densities than surface modification with n-octadecyltrihydridosilane. Combined pyridine adsorption and diffuse reflectance infrared Fourier transform (DRIFT) measurements are performed on the titania and hafnia substrates to evaluate potential surface heterogeneities, i.e. Lewis and Brønsted sites. Differences in the alkyl chain conformational order within the series of C₁₈ modified metal oxides are explained by the presence of island structures. The reduced C₁₈ conformational order for the samples grafted with n-octadecyltrihydridosilane is traced back to the lower grafting density which in turn points to a lower reactivity of this silylating reagent. The most striking result is the higher conformational order of the C₁₈ chains grafted in the present surface modified metal oxides when compared with silica-based systems. This finding is attributed to the lower porosity of the metal oxide supports along with more closely packed chains on the surface.     

Continuous hydrothermal synthesis of CoFe2O4 nanoparticles

We have investigated the continuous hydrothermal synthesis and crystallization of spinel CoFe₂O₄ via the reaction of ferric nitrate and cobalt nitrate with sodium hydroxide. The reaction was carried out in water at temperatures ranging from 475 to 675 K and pressures of 25 MPa. The relative solubility of the precipitating cations was found to play a critical role in attaining the correct product. It was found necessary to control pH and temperature in order to prevent premature precipitation of iron in the reactor. Two variations of the continuous hydrothermal technique were examined—cold mixing and hot mixing. The cold mixing experiments produced a product with less impurity than the hot mixing experiments. Furthermore, the cold mixing configuration was successful in producing uniform nanoparticles of CoFe₂O₄. A mechanism of particle formation was postulated involving the precipitation of metal hydroxides at ambient conditions, dissolution of the hydroxides as temperature is increased followed by rapid precipitation of metal oxides at elevated temperatures. The hot mixing experiments, on the other hand, simply involve the precipitation of metal oxides due to the addition of the hot hydroxide solution. In both cases, very fine particles of CoFe₂O₄ are produced in the range of the processing conditions investigated.     

Temperature effect on hydrothermal synthesis of wairakite and hsianghualite

The zeolite minerals wairakite (Ca₈(Al₁₆Si₃₂O₉₆)x16H₂O) and hsianghualite (Li₁₆Ca₂₄(Be₂₄Si₂₄O₉₆)xF₁₆) were synthesized in a temperature range between 150°C and 500°C by hydrothermal treatment of artificial glasses of the respective same composition at one kbar H₂O pressure. The crystal symmetry of wairakite varied systematically with temperature under the given experimental conditions starting from orthorhombic symmetry at low synthesis temperatures, tetragonal at medium, leading to cubic symmetry at highest zeolite formation temperatures. In contrast, the crystal symmetry of hsianghualite remained cubic in the whole temperature range of synthesis. The crystal sizes varied between 500 nm and 100 μm. The investigations showed the direct correlation between chemical composition of the starting materials and the formed zeolite phase under the given pressure-temperature conditions.     

Interaction of rare-earth oxides with binary molten mixtures of zirconium and alkali metal fluorides

Isothermal saturation, inductively coupled plasma mass spectrometry (ICP-MS), and X-ray powder diffraction were used to study solubilities of La₂O₃, Sm₂O₃, and Ho₂O₃ in the eutectic mixtures of LiF-ZrF₄, NaF-ZrF₄, and KF-ZrF₄ systems at temperatures from 873 to 1073 K. The solubilities of rare earth oxides in the molten fluorozirconate systems studied decrease in the increasing order of Ln³⁺ ion radii and in the increasing order of alkali metal cation radii in the binary mixture. The thermodynamic parameters of dissolution calculated for the oxides imply the dominance of a chemical dissolution mechanism, and this implication was supported by X-ray powder diffraction data.     

Effect of the oxidation state of copper on the solution-solid phase equilibria in CuClx-MCl-H2O systems

The effect of the oxidation state of the metal on the solubility was analyzed using the example of CuCl_x-MCl-H₂O (x = 1, 2; M = Li-Cs, NH₄) ternary systems. The prevalence of acido complex formation is responsible for an essential similarity of the solubility isotherms: the presence of crystallization branches of complex salts and salting-in (or weak salting-out) of copper(I) and (II) chlorides, that increases in the order LiCl < NaCl < KCl < CsCl. The different stability of copper(I) and (II) chloride complexes results in that the chemical individuality of alkali metal cations is differently reflected in the shape of the solubility isotherms: The stronger complex formation in copper(I) systems results in leveling of this effect and the weaker complex formation with copper(II), in its differentiation.     

Nuclear Waste Storage in Gel-Derived Materials

For long life nuclear wastes (essentially actinides) research is in progress to propose new matrices with an improvement of the chemical durability. High silica content glasses present high chemical durability, and we describe a process to prepare silica glass embedding the nuclear waste. Porous silica (gel) is used as a host matrix for nuclear waste. Neodymium oxide and cerium oxide are used to simulate the actinide oxides. The gel is soaked in a solution containing the simulant in nitrate salt form.We investigate the effect of the porous network features (pore size distribution) on the ability of the material to soak up the simulating salts. A new kind of gel (composite aerogel) is proposed owing to its large permeability. After drying and nitrate decomposition, the composite material is fully sintered trapping the nuclear waste.The glass-ceramic materials (silica glass + simulating oxide) have been synthesized with simulant content close to 20 weight percent. We show that the 2 simulant oxides behave differently because of their ability to form silicate phases. The chemical durability of the glass-ceramics is investigated. The large improvement of the chemical durability (60 times) compared to the classical borosilicate glasses shows that this containment process can be a suitable way to confine actinides and long life nuclear wastes.     

Intraparticle surface diffusion of metal contaminants and their attenuation in microporous amorphous Al, Fe, and Mn oxides

Intraparticle surface diffusion is an important and rate-limiting process in the sorption of metal ions to microporous sorbents such as those of hydrous amorphous Al (HAO), Fe (HFO), and Mn (HMO) oxides; these minerals are abundant in the environment, exhibiting a high affinity for metal contaminants. In aquatic systems representative of natural environments, internal micropore surfaces of HAO, HFO, and HMO can account for 40 to 90% of the sorption sites. Surface diffusivities have been observed to range between 10(-16) and 10(-10) cm2 s(-1) for metals including Sr, Cd, Zn, and Ni. The combination of significant microporosity and small diffusivities results in the amorphous oxides acting as natural attenuating sinks.     

Excess entropy and thermal behavior of Cu- and Ti-doped bioactive glasses

Bioactive glasses belong to the ceramic family. They are good materials for implantation due to their excellent capacities to create an intimate bond with bones. Copper is known for its anti-inflammatory, antibacterial, and antifungal properties. Titanium is biocompatible and resistant to corrosion. These chemical elements can be introduced in bioactive glasses to provide a wide variety of uses and to enhance the physiological properties of implanted biomaterials. In this work, bioactive glasses doped with different contents of copper and titanium were synthesized by the melting method. The purpose is to study the effect of doping metal element on the thermal characteristics (T _g, T _c, and T _f). The results revealed that the increase of the content of copper and titanium in the glass matrix decreases the melting temperature and induces an increase of the thermal stability. The excess entropies of pure and doped glasses were calculated. Obtained results highlighted the decrease of the excess entropy with the increase of metal elements contents.     

Compact plasma source for removal of hydrocarbons for surface analysis

A low–energy, constricted‐anode Anders‐type plasma source was built and tested for the chemical removal of adventitious carbon on surfaces. Oxygen plasma, generated in the source, extends to the sample surface through an aperture in the anode. This plasma reacts with surface hydrocarbons and removes them in less than a minute without influencing the intrinsic surface stoichiometry of nonoxidizing samples such as minerals, glasses, and metal oxides. Adventitious carbon removal is critical for accurate binding energy determination and quantitative measurements in XPS and AES, particularly in multicomponent materials. We measure the plasma to be composed primarily of O⁺ and O₂⁺, with minor H⁺, H₂⁺, and O⁺⁺ components. Ion energy distributions were measured for O⁺ and O⁺⁺ and show all emitted ions have energies less than 50 eV, confirming chemical desorption as the primary removal mechanism. The plasma source, easily built ‘in house’, is compact and can be mounted on a 2.75‐in flange for in situ specimen cleaning prior to surface analysis. Copyright © 2009 John Wiley & Sons, Ltd.     

Ion-Interaction Approach: Pressure Effect on the Solubility of Some Minerals in Submarine Brines and Seawater

The pressure dependence of salt solubility in multiple electrolyte solutions was estimated to 1000 atm. The activity coefficients, thermodynamic solubility products, degrees of saturation, and mineral solubility were calculated with high precision only up to 300 atm because of the absence of compressibility data for mixed electrolyte solutions. The ion-interaction approach developed during the last 2 decades allows the prediction of various thermodynamic properties, including volumetric ones for multiple-solute natural solutions. This approach was applied to the estimation of the depth dependence of solubility for certain evaporite minerals in natural brines. The influence of pressure on solubility products, mean activity coefficients, and degrees of saturation of minerals, such as, halite (NaCl), anhydrite (CaSO₄), gypsum (CaSO₄·2 H₂O), celestite (SrSO₄), and barite (BaSO₄) were calculated for in situ depths in the Orca Basin (Gulf of Mexico), the Tyro and Bannock II depressions (the Mediterranean Sea), and for average seawater.