Solubility phenomena in industrial and natural system

The importance of solubility phenomena in industrial and natural systems is illustrated mainly by examples related to nuclear energy development. Illustrative examples are given of the consequences of changes in solubility due to gradients of temperature (hydrothermal mass transport), chemical composition (hydrothermal alteration of minerals) electrochemical potential (corrosion) and mechanical stress (fracture mechanics). The chemical systems of interest are primarily insoluble oxides in high temperature water, for example: transition metal oxides on surfaces of structural metal alloys of power plant steam generators; actinide and other heavy metal oxides of nuclear fuels; silicate glasses for immobilizing fission product wastes, and minerals in the rocks of an underground waste vault in contact with heated ground water. Requirements for improved solubility measurement equipment and techniques, experimental data and theoretical methods are noted. Some recent solubility data and their shortcomings are outlined for specific examples, including UO₂, basic lead carbonates, potassium aluminosilicate (feldspars), three to five component borosilicate glasses and zirconium hydride (in Zr metal).Presentation to First International Symposium on Solubility Phenomena, University of Western Ontario, London, Ontario, August 21–23, 1984.     

IC-ICP-MS applied to the separation and determination of traces of plutonium and uranium in aqueous leachates and acid rinse solutions of UO2 doped with Pu

It is expected that spent nuclear fuel, today mainly UO₂, may become exposed to groundwater after extended storage in a deep geologic repository. After 1000 years, the radioactivity of the fuel will be constituted essentially by α-emissions by long-lived actinides. The α-emissions play a significant role in determining the dissolution behavior of uranium, because the radiolysis of water results in the formation of oxidizing chemical species near the fuel surface. In order to study this effect, UO₂ doped with 0.1 and 10 wt.% of a strong α-emitter (namely ²³⁸Pu) was subject to leaching at room temperature (RT) in deionized water. In order to study the mechanisms of leaching in simulated conditions, very precise and accurate techniques need to be employed. In this paper, the results obtained by inductively coupled plasma mass spectrometry coupled with ion chromatography for the determination of traces of ²³⁸Pu and uranium in aqueous leachates’ solutions are illustrated.     

Influence of composition on corroding process of Na<Subscript>2</Subscript>O-K<Subscript>2</Subscript>O-CaO-ZrO<Subscript>2</Subscript>-SiO<Subscript>2</Subscript> glasses

The corroding process of six glasses of the Na₂O-K₂O-CaO-ZrO₂-SiO₂ system with ZrO₂content 0–2.13 mass % by water was observed during static tests at 121°C and pressure of 0.25 MPa in steam sterilizer. Significant increase of Na⁺ and K⁺ content in leachates was observed after the addition of ZrO₂ into glass. Further increase of the content of ZrO₂ in glasses slowed down the rate of Na⁺ and K⁺ leaching. The leaching process of SiO₂ as well as Na⁺, K⁺, and Ca²⁺ ions was evaluated on the basis of comparison with model leaching processes. Variation of the concentrations of Na⁺, K⁺, Ca²⁺, and SiO₂ in leachates with time was described by empirical equation. Observed changes in the initial leaching rates of Na⁺, K⁺, Ca²⁺, and SiO₂ can be ascribed to the content of ZrO₂ in glasses. The presence of ZrO₂ in glasses reduced the overall rate of glass dissolution.     

Third- and second-order optical nonlinearity of Ge-Ga-S-PbI2 chalcohalide glasses

Two series of metal iodide doped chalcohalide glasses (100−2x)GeS₂·xGa₂S₃·xPbI₂ (0?x?20) and (100−x)(0.8GeS₂·0.2Ga₂S₃)·xPbI₂ (0?x?15) were prepared and characterized. The microstructure of these glasses has been studied by Raman scattering spectra. Utilizing femtosecond time-resolved optical Kerr effect (OKE) technique at the wavelength of 820 nm, a largest third-order nonlinearity χ⁽³⁾ of 2.07×10⁻¹³ esu was obtained for the 90GeS₂·5Ga₂S₃·5PbI₂ glass, and it decreases with the addition of PbI₂ in both two series. After thermally poled, second-harmonic generation (SHG) has been observed in these glasses according to Maker fringe method and a large second-order nonlinearity χ⁽²⁾ as well as 4 pm/V was obtained for the 70GeS₂·15Ga₂S₃·15PbI₂ glass. The variations of χ⁽²⁾ and χ⁽³⁾ on glass composition are ascribed to the evolution of micro-structural units in glass. These novel chalcohalide glasses would be expected to be the promising candidate materials for nonlinear optical devices.     

Thermochemistry of phosphate glasses for immobilization of dangerous waste

Vitrification is currently considered to be an effective method for immobilization of radioactive waste. It is based on the enclosing of harmful elements in the structure of the glass. This work presents the results of studies on the thermal properties of glasses from P₂O₅–Al₂O₃–Na₂O and P₂O₅–Al₂O₃–Fe₂O₃–Na₂O systems for rendering nuclear waste in the form of salts such as sulfates, halides, and phosphates with high sodium content. These substances are not accepted by borosilicate glass, commonly used up to now for nuclear waste immobilization. Formation of sinters of glass-waste mixtures was selected as the method for immobilization, and the thermal chemistry of this process was studied. CaCl₂ was used as the model chloride waste substance. The process of immobilization consists of its sintering with Na, Al, Fe-phosphate glasses containing more than 50 wt% P₂O₅ as the amorphous matrix. Thermal analysis showed that all glasses exhibit an ability for crystallization, with that the intensiveness of this process is determined by the chemical composition of these glasses. The addition of Fe₂O₃ to the glass intensified crystallization process. Leaching of components of sinters tests established that glass containing Fe₂O₃ in its composition most effectively binds waste in comparison to Al₂O₃ containing phosphate glass. The test results allow for the statement that the waste substance in the form of chloride salts such as CaCl₂ is stable bound in the glass–crystalline sinters, which ensures its effective immobilization.     

Crystallization and second harmonic generation in potassium-sodium niobiosilicate glasses

Transparent glasses having molar composition (23−x)K₂O·xNa₂O·27Nb₂O₅·50SiO₂ (x=0, 5, 10, 15 and 23) have been synthesized by the melt-quenching technique and their devitrification behaviour has been investigated by DTA and XRD. Depending on the composition, the glasses showed a glass transition temperature in the range 660-680 °C and devitrified in several steps. XRD measurements showed that the replacement of K₂O by Na₂O strongly affects the crystallization behaviour. Particularly, in the glasses with only potassium or low sodium content the first devitrification step is related to the crystallization of an unidentified phase, while in the glass containing only sodium, NaNbO₃ crystallizes. For an intermediate sodium content (x=10 and 15) a potassium sodium niobate crystalline phase, belonging to the tungsten-bronze family, is formed by bulk nucleation. This system looks promising to produce active nanostructured glasses as the tungsten-bronze type crystals have ferroelectric, electro-optical and non-linear optical properties. Preliminary measurements evidenced SHG activity in the crystallized glasses containing this phase.     

Diffusion of actinides in glasses containing simulated radioactive wastes

Diffusion coefficients of radionuclides²³⁷Np,²³⁹Pu and²⁴¹Am in simulated alumina phosphate and alumina borosilicate glasses at temperatures lower than their transformation temperature were determined. Actinides are known to be the least mobile elements. In particular, the diffusion coefficients of actinides in alumina phosphate glasses at 673 K are about 10^–18 m²·s^–1, and in alumina borosilicate at 773 K about 10^–19 m²·s^–1. It is shown that crystallization of glasses leads to increasing²³⁷Np diffusion mobility. It is also shown that a rather small quantity of water absorbed by a crystallized alumina phosphate glass in tensifies low-temperature migration of²³⁷Np.     

New vitreous matrix for chromium waste immobilization

Common waste glasses (window, bottle glass or tableware) with fly ash form a glass matrix for chromium waste immobilization. Soluble chromium from residual waters was adsorbed on fly ash; the resulting solid contained 23.7% Cr⁶⁺. The three glass wastes, chromium-containing fly ash, and borax were used to make glasses in weight ratios waste glass: borax: fly ash of 1: 1: 1 and 1.5: 0.5: 1. The hydrolytic stability ranged from 18.46 to 28.13 µg g^?1 soluble Na₂O, qualifying them in the HGB1 class. The chemical stability, characterized by the dissolution rate, was 0.011–0.077 µg cm^?2 h^?1, depending on the glass composition and the aggressive medium pH. The chromium leachability is influnced by the glass composition and the pH of the leaching solution, ranging between 0–0.015% of the total chromium. Chromium waste vitrification is a viabile solution with multiple economic advantages.

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Effect of γ-dose on the crystal structure and leaching behavior of TiO<Subscript>2</Subscript> matrix labeled with <Superscript>181</Superscript>Hf/<Superscript>181</Superscript>Ta tracer

A new method for the possible incorporation of nuclear wastes has been attempted here by using ceramic matrix of TiO₂ as a host precursor for confinement. Hafnium is used as a simulant for actinide high-level waste. After incorporating ¹⁸¹Hf tracer into TiO₂ matrix, the leaching property of the resulting matrix was studied in water, sodium chloride and humic acid solutions. The leaching was measured in each of the case by following the radioactivity of ¹⁸¹Hf. TiO₂ matrix has also been exposed to γ-radiation in order to simulate the radiation field for nuclear waste. It has been investigated with a nuclear technique called time differential perturbed Angular Correlation (TDPAC) that the lattice structure of titania remains undisturbed even under a strong radiation field. The leaching of ¹⁸¹Hf has also been studied after irradiating the TiO₂ matrix with γ-radiation and the leaching behavior was observed not to change from that before irradiation.     

Application of instrumental radioanalytical techniques to nuclear waste testing and characterization

This paper describes the validation of a multi-technique analytical methodology that uses inductively coupled plasma-mass spectrometry, α-spectrometry, and γ-spectrometry for the routine analysis of samples containing transuranic radionuclides. This methodology is capable of the determination of concentrations of both²³⁸Pu and²⁴¹Pu in the presence of²³⁸U and²⁴¹Am without the need for chemical separations. The relative merits of these three techniques were evaluated as they are applied in a nuclear waste material and spent nuclear fuel testing program by analytical (1) standards and (2) solutions prepared from the dissolution of glasses doped with²³⁷Np,²³⁹Pu, and²⁴¹Am. The uncertainty associated with technique was within ±4% for standards and ±10% for doped nuclear waste glasses. The methodology was then used to analyze three fully radioactive waste glasses.     

A device for uranium series leaching from glass fiber in HEPA filter

For the disposal of a high efficiency particulate air (HEPA) glass filter into the environment, the glass fiber should be leached to lower its radioactive concentration to the clearance level. To derive an optimum method for the removal of uranium series from a HEPA glass fiber, five methods were applied in this study. That is, chemical leaching by a 4.0?M HNO₃?C0.1?M Ce(IV) solution, chemical leaching by a 5 wt% NaOH solution, chemical leaching by a 0.5?M H₂O₂?C1.0?M Na₂CO₃ solution, chemical consecutive chemical leaching by a 4.0?M HNO₃ solution, and repeated chemical leaching by a 4.0?M HNO₃ solution were used to remove the uranium series. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after leaching for 5?h by the 4.0?M HNO₃?C0.1?M Ce(IV) solution were 2.1, 0.3, 1.1, and 1.2?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after leaching for 36?h by 4.0?M HNO₃?C0.1?M Ce(IV) solution were 76.9, 3.4, 63.7, and 71.9?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after leaching for 8?h by a 0.5?M H₂O₂?C1.0?M Na₂CO₃ solution were 8.9, 0.0, 1.91, and 6.4?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after consecutive leaching for 8?h by the 4.0?M HNO₃ solution were 2.08, 0.12, 1.55, and 2.0?Bq/g. The residual radioactivity concentrations of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th in glass after three repetitions of leaching for 3?h by the 4.0?M HNO₃ solution were 0.02, 0.02, 0.29, and 0.26?Bq/g. Meanwhile, the removal efficiencies of ²³⁸U, ²³⁵U, ²²⁶Ra, and ²³⁴Th from the waste solution after its precipitation?Cfiltration treatment with NaOH and alum for reuse of the 4.0?M HNO₃ waste solution were 100, 100, 93.3, and 100%.     

Bioactive Sol-Gel Coatings for Orthopaedic Prosthesis

Sol-gel materials have been proposed in last years for clinical applications. In this work, bioactive sol-gel coatings were prepared from suspensions of up to 25% wt. of bioactive glass (CaO·SiO₂·P₂O₅) particles in a hybrid sol obtained from TEOS and MTES. Thick dip-coatings showed in vitro bioactivity after a few days, but glass-particles dissolution promotes a slight reduction in the corrosion resistance. In order to overpass this problem, a two-stage sol-gel coating system that includes a first SiO₂ hybrid film, acting as barrier against corrosion, and an external bioactive layer from glass particle suspension is proposed. The obtained samples revealed significant improvement in their electrochemical behaviour, and showed in vitro bioactivity.     

Local structure and chemical durability of FeOOH-fixed sodium silicate glass prepared from water glass

A 12M HCl solution of iron oxyhydroxide (a-FeOOH: goethite) was mixed with water glass (18Na₂O^.36SiO₂ ^.46H₂O) at room temperature. The mixture (sol) changed into a dry gel when dried at 25 °C for 120 hours in air. Glass-ceramic and glass samples were prepared when the dry gel was heated for 1-3 hours in an electric furnace at 800 and 900 °C, respectively. The ⁵⁷Fe Mössbauer spectrum of the dry gel is composed of a magnetic hyperfine structure owing to the formation of g-FeOOH (lepidocrocite). By contrast, the ⁵⁷Fe Mössbauer spectrum of glass-ceramic and glasses is composed of paramagnetic Fe(III) with distorted tetrahedral symmetry. This proves that Fe(III) atoms occupy network-forming Si(IV) sites in the FeOOH-fixed sodium silicate glass. A leaching test of the silicate glass in the acid rain simulant composed of HNO₃ (pH 3.5) and H₂SO₄ (pH 3.5) revealed high chemical durability, indicating that Fe(III) is firmly fixed in the glass matrix.     

Leaching of uranium,radium and thorium from vertisol soil by ground water

Shallow land burial is routinely used for the disposal of low-level radioactive waste. Natural processes causing leaching of radionuclides can lead to contamination of surrounding ground water and soil by the radionuclides. The comparative leachability of radionuclides U_(nat), ²²⁶Ra, ²²⁸Ra and Th_(nat) from the soil of a radioactive waste disposal site, by ground water was evaluated. The probability of leaching was obtained in the following order Ra (≈77%) > U (≈40%) > Th (≈20%). Observed ratios (OR) were calculated to correlate leachability of radionuclides to that of major cations Ca²⁺ and Mg²⁺. The leaching of the radionuclides was seen to be dependent on Ca²⁺ and SO₄²⁻ leached from the soil. This study provides sitespecific leachability of radionuclides, that can be used as indicator of the tendency for migration or retention in soil. It can play an important role during an unforeseen accident like breach of containment at the waste disposal site leading to contamination of soil and ground water and causing hazard to public via drinking water route.     

Thermodynamic analysis of radioactive graphite reprocessing by incineration in air and oxidation in molten salt

Reactor graphite waste of nuclear industry contains various radionuclides. This study deals with the behaviour of radioactive elements during thermal treatment of waste. Two thermal processes were simulated: incineration of graphite in air and oxidation of graphite in a molten salt, containing Na₂CO₃ + K₂CO₃ + 20 mass% PbO. Carbon oxides CO and CO₂, containing radioactive ¹⁴C, are the main components of the gaseous phase with partial pressures approximately 10⁴ Pa in both cases. More gaseous radionuclides are observed during incineration of graphite in air than during oxidation of graphite in the molten salt. In the latter case radionuclides are in the condensed state in the melt of sodium and potassium carbonates.     

Adsorption characteristics of traces of radium on membrane filters,glass and polyethylene

Adsorption of radium was studied on glass and polyethylene from aqueous solutions containing 8–40 pg·dm^{–3 224}Ra and on membrane filters, glass and polyethylene bottles from waste and river waters containing 2–170 pg·dm^{–3 226}Ra. The adsorption from aqueous solutions was determined as a function of pH and composition of the solutions and interpreted as due to ion exchange of Ra²⁺ ions for counter ions in the electric double layer on glass and polyethylene or due to chemisorption of RaSO₄ (RaCO₃) ion pairs on glass. Borosilicate glass adsorbed radium substantially more than polyethylene. The adsorption of dissolved forms of radium from the waste and river waters during storage and membrane filtration of the waters was negligible, but a significant loss of particulate forms of radium was sometimes observed during the storage. It has been recommended to separate dissolved and particulate forms of radium soon after the sampling and to prefer polyethylene to glass as container material for storage of dissolved forms of radium.     

Bioactive and Protective Sol-Gel Coatings on Metals for Orthopaedic Prostheses

The aim of this work has been the preparation and evaluation of sol-gel coatings for clinical applications. Research was focussed in the development of highly corrosion resistant and/or bioactive sol-gel coatings onto AISI 316L stainless steel. Hybrid SiO₂ sol-gel coatings inhibited corrosion and Fe diffusion, although no signal of bioactivity was detected. The inclusion of Ca- and P-alcoxides in the sol composition did not promote bioactivity. Bioactive coatings were obtained from suspensions prepared by adding glass (CaO·SiO₂·P₂O₅) particles to an hybrid organic-inorganic SiO₂ sol. The dissolution of glass particles promoted in vitro induction of apatite along with a slight reduction in the corrosion resistance of coated pieces. By combining an inner SiO₂ hybrid film acting as barrier against corrosion with an outer coating containing bioactive glass particles, a significant improvement in the electrochemical behaviour was observed. This double-layered coating showed in vitro signals of bioactivity, and preliminary in vivo tests gave promising results.     

Separation of cerium from other lanthanides by leaching with nitric acid rare earth(III) hydroxide-cerium(IV) oxide mixtures

A simple operational method for separating cerium from other lanthanides in natural mixtures of cerium dioxide with lanthanide(III) hydroxides, obtained from Vietnamese parisite and Mongolian bastnasite, has been developed. The method is based on drying crude Ln(OH)₃×H₂O material in air at about 200°C within 6h, followed by leaching Ln(III) with concentrated nitric acid added carefully with stirring to the dried material/water 11.5 slurry. Under optimum conditions cerium (and Th, if present) virtually does not pass into solution while the yield of leaching and the sum of REE oxides (REO) concentration in the after-leach solution reach the maximum values of 97% (mass) and 0.18 kg·dm^–3, respectively. Besides an expected decrease of the leaching yield, roasting the starting material at 600°C results in over 7% Ce content in the Ln leached.     

Influence of MgO (CaO) on the structure of silicate-phosphate glasses

Thermal and structural properties of model silicate-phosphate glasses containing the different amounts of the glass network modifiers, i.e. Mg²⁺ and Ca²⁺ were studied. To explain the changes of the parameters characterizing the glass transition effect (T_g, Δc_p) and the crystallization process (T_c, ΔH) depending on the cations modifiers additions, analysis of the bonds and chemical interactions of atoms in the structure of glasses was used. ³¹P MAS-NMR spectra of SiO₂–P₂O₅–MgO(CaO)–K₂O glasses show that the phosphate complexes are mono- and diphosphate. It has been found that increasing amounts of Mg²⁺ or Ca²⁺ cations in the structure of glasses causes the reduction of the degree of polymerization of the phosphate framework (Q¹→Q⁰). The influence of increasing of modifiers in the structure of silicate- phosphate glasses on the number of non-bridging oxygens per SiO₄ tetrahedron and density of glasses was presented.