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Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Waste and Waste Management

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BORATE-TELLURATE GLASSES: AN ALTERNATIVE OF IMMOBILIZATION OF THE HAZARDOUS WASTES

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Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

WASTE AND WASTE MANAGEMENT Sewage Treatment: Uses, Processes and Impact Anna Stephens and Mark Fuller (Editors) 2009. 978-1-60692-959-9 Composting: Processing, Materials and Approaches Joseph C. Pereira and John L. Bolin (Editors) 2009. 978-1-60741-438-4 Sewage Treatment: Uses, Processes and Impact Anna Stephens and Mark Fuller (Editors) 2009. 978-1-60876-875-2

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Waste Management: Research Advances to Convert Waste to Wealth A. K. Haghi (Editor) 2010. 978-1-61668-415-0 Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes Simona Rada, Eugen Culea and Monica Culea (Authors) 2010. 978-1-61668-263-7 Management of Hazardous Residues Containing Cr(VI) María José Balart Murria (Editor) 2010. 978-1-61668-267-5 Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes Simona Rada and Eugen Culea and Monica Culea (Authors) 2010. 978-1-61668-723-6 Management of Hazardous Residues Containing Cr(VI) María José Balart Murria (Editor) 2010. 978-1-61668-267-5

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes Simona Rada and Eugen Culea and Monica Culea (Authors) 2010. 978-1-61668-723-6 Management of Hazardous Residues Containing Cr(VI) María José Balart Murria (Editor) 2010. 978-1-61668-901-8

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Waste Management: Research Advances to Convert Waste to Wealth A. K. Haghi (Editor) 2010. 978-1-61668-903-2

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved. Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Waste and Waste Management

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

BORATE-TELLURATE GLASSES: AN ALTERNATIVE OF IMMOBILIZATION OF THE HAZARDOUS WASTES

SIMONA RADA, EUGEN CULEA AND MONICA CULEA

Nova Science Publishers, Inc. New York

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

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Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

CONTENTS Preface Chapter 1

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Chapter 2

ix Immobilisation of Lead Ions in Borate-Molibdate Glass Ceramics

1

Immobilisation of Lead Ions in Borate-Tellurate Glasses and Glass Ceramics

29

Chapter 3

Plumbate Glasses

49

Chapter 4

Borate-Tellurate Glasses an Alternative of Immobilization of the Hazardous Wastes

57

Conclusions

71

Appendix

75

Index

81

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PREFACE Immobilization of high level toxic wastes by vitrification is a well established process that has been studied extensively over last 40 years. A suitable glass host is used to dissolve the high level nuclear waste to form a glassy (vitreous) homogeneous product that can be cast into suitable forms, including large glass blocks. Under suitable sonditions, it is possible to incorporate up to 25-30wt% high level waste into a glass. Vitrification has been selected for immobilization of high-level toxic waste because glass is higher stable, very durable and has the ability to incorporate a wide variety of chemical contaminants. The main advantages of the vitrification route include the fact glass is a good solvent for waste, glasses can be processed at reasonably low temperatures, glass is very tolerant of variations in waste composition, glass exhibits reasonable chemical durability, glass is radiation resistant and can accommodate changes occurring during decay of high level nuclear waste constituents. Various glass systems have been shown to be suitable for producing waste glass forms that are thermally and mechanically stable and exhibit good chemical durability. Most current research on vitrification of mixed waste has focused on borosilicate glass formulations originally developed for high-level radioctive waste. Although borosilicate glass has demonstrated good long-term chemical durability and is both thermally and physically stable, it requires relatively high process temperatures (1200-15000C) for effective encapsulation of waste [1]. These high temperature are a major drawback because volatilization of certain isotopes (99Tc and 137Cs) and heavy metals (Pb and Cd) can occur, requiring the use of secondary treatment systems to capture and stabilize these off-gas contaminants. Further, borosilicate glass processing is

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Simona Rada, Eugen Culea and Monica Culea

incompatible with even minor amounts (>1.5mole%) of P2O5 in a waste stream [1]. Insoluble phosphate phases form depending of amount of rare earth oxides present. Similarly, wastes containing >0.3mole% Cr2O3, >16mole% Al2O3, or >19mole% Fe2O3 has been flagged as outside the borosilicate glass concentration envelope [1]. Alternative vitrification processes are desirable for improved treatment of these wastes. Promising new low-temperature glasses and glass ceramics have been proposed based on advanced phosphate formulations [2]. Alternative glass compositions were proposed based on reducing melt temperatures, improving durability, or providing compatibility with specific waste stream components. Network modifiers have been adjusted to increase waste form durability while maintaining the advantage of low melting temperature. Unlike silicate glasses, whose durability is compromised by addition of modifying cations, corrosion resistance of some phosphate glasses can be enhanced with the addition of selected cations. High concentrations of P2O5 in certain wastes make phosphate glasses natural candidates for waste immobilization. The chronology of phosphate glass development should not proceed without a basic understanding of the chemistry and structure of these materials. Pure P2O5 is characterized by a random three-dimensional network of PO4 tetrahedra: one oxygen atom is doubly bonded while the other three oxygen atoms bond to other tetrahedra. Pure P2O5 is extremely unstable and reacts violently with water to form H3PO4. Metals added to the glass break up the [PO4] network as oxygen atoms preferentially bond to these metals. With increasing metal concentrations, the chains depolimerize, becoming shorter and shorter until finally the vitreous nature of the material gives way to formation of more stable crystalline phases. While metal additions initially improve stability of the structure, their increased concentration, decreases melt viscosity, increasing potential for crystal formation (devitrification). Indeed, the requirement for rapid cooling was also noted as a drawback to early efforts pursuing phosphate glasses as final waste forms. The binary phosphate glass systems, while of scientific curiosity, have found little commercial applicability due to their rapid degradation in water. However, an understanding of physical and chemical behavior of these systems is useful in evaluating performance of more complicated glasses. The addition of iron to lead phosphate glasses was found to dramatically increase chemical durability of the glass, by a factor of about 104 for a 9wt% iron oxide addition [3-5]. Also, the tendency for the glass to crystallize on cooling was greatly suppresed. Without the iron modifier, lead metaphosphate glasses completely crystallized in air at 3000C within a few hours. In contrast,

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Preface

xi

iron-lead-phosphate glasses were heated for several hundred hours at 5000C without any signs of devitrification. However, these glass systems were found to be hightly durable but the glass melts were considered hightly corrosive with existing glass melting equipment. Thus, borosilicate glasses were found to be, overall, more favorable for toxic waste treatment. Various combinations of phosphate glass ceramic matrices have been manufactured at the research level. Certain glass ceramics or glass composites may possess higher chemical durability than single phase glasses with respect to dissolution in water or corrosion resistance in harsh environments. Glass ceramics may be formulated on devitrification, consisting of a major, thermodynamically stable crystalline phase and a relatively durable, vitreous matrix. They are important candidates for nuclear waste immobilization [6]. Glass ceramics based on partial crystallization of precursor glasses from the silicate or phosphate systems were developed for the possible solidification of fuel recycle waste or the imobilization of high level waste [7]. These glass ceramics exhibit good chemical durability, high melting temperatures and associated losses of volatile fission products from the melts restrict potential use of these materials. Although many different types of glass and ceramic materials have been investigated as possible candidates for the immobilization of high level waste, at the present time borosilicate glass is the generally accepted first generation waste form. As a result of this decision, many commercial vitrifications plants are now in operation throughout the world using borosilicate glass as the first generation host for the immobilization of high level toxic waste. Many ceramic phases are known to posses superior chemical durabilities to borosilicate glasses under typical repository conditions. Ceramics, however, are generally multyphase systems containing many minor phases in addition to the major crystalline phases, and it can be difficult to predict long term behaviour in repository environments. In addition, the technology associated with the manufacture of crystalline ceramics is, in general, more complex than that associated with the production of glasses. Ceramics and glass appear to be basically equivalent in terms of maturity of production technology, timing of waste form production, and in cost of production. On the other hand, glass has advantages over ceramic in its theoretical ability to accept a wide range of impurities into its structure and its potential ability to withstand radiation damage. Overall, ceramics are proven to have corrosion rates at least one or more orders of magnitude lower than glass and should better contain the waste over the geological time frames necessary for repositories. Therefore, glass ceramics may offer a useful

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compromise between glasses and ceramics, being easier and less expensive to prepare than conventional ceramics, but offering higher durability than glasses. Glass ceramics are defined as polycrystalline ceramic materials prepared by the controlled bulk crystallization of suitable glasses [8]. Crystallization of conventional glasses normally occurs by the nucleation of crystals at external surfaces. This crystallization behavior generally gives rise to a coarse microstructure with large anisotropic crystals that grow inwards from the surface of the glass. Such materials are usually weak mechanically. The success of the glass ceramic process in producing mechanically strong, fine-grained polycrystalline ceramic materials depends on inducing a high crystal nucleation density within the bulk of the glass by providing a very large number of internal heterogeneities from which the major crystalline phases can form and grow. This can be achieved in practice by the use of specific nucleating agents that are added to the glass batch. Nucleating agents act either by inducing the glass to phase separate on a very fine scale, or by forming small crystallites (of the nucleating phase itself or of some compound formed by reaction with the constituents in the glass). In either case, much small heterogeneity is produced, onto which the major crystalline phases can nucleate and grow [8, 9]. A number of different glass ceramic familities have been proposed for the immobilization of the high level toxic waste as: silicate [10] and phosphate [11, 12] glass ceramics. Glass ceramic offers an easily manufacture relative to conventional ceramics, coupled with higher durability than glasses, a number of further potential advantages have been associated with the use of glass ceramics as waste forms [13]. These include higher thermal stabilities than borosilicate glass, superior mechanic properties and a ability to tailor many of their properties to meet the challenges of specific applications. In addition, glass ceramics are more tolerant of variations in waste composition than are corresponding crystalline ceramics prepared by conventional routes. Glass ceramics require an additional heat treatment relative to conventional glasses, thus leading to greater processing complexity. The future is directed towards the identification of similar glasses and glass ceramics with improved durability and processing characteristics, which will mainly be achieved through compositional modifications of existing waste forms. Then, other types of glasses and glass ceramics waste forms with lower manufacturing temperature and easier processing route also offer potential for further study.

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Preface

xiii

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We propose than needful for alternative vitrification processes borate glasses and glass ceramics formulations. The borate-tellurate glasses appear to offer advantages of reduced process temperature and improved chemical durability compared with phosphate glasses. Although there are many reports in the literature about glass ceramics, these were undesirable due to high processing temperature (>14000C) and poor chemical durability. The boratemolibdate glass ceramics was synthesised at 12250C. This new formulation differs in the type and quantity of network modifiers added, resulting in a glass ceramic matrix with improved chemical durability and reduced melting temperature. The reduced processing temperature makes these borate-tellurate glasses and borate-molibdate glass ceramics suitable for the use of immobilization of high level and mixed waste, as well as for many alternative commercial glass and glass ceramic products. The present contribution compares the borate-tellurate glasses and boratemolybdate glass ceramics, processes considered for the immobilization of hazardous wastes, and discusses the merits and limitations of each.

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Chapter 1

IMMOBILISATION OF LEAD IONS IN BORATEMOLIBDATE GLASS CERAMICS

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1.1. INTRODUCTION Vitrification of hazardous industrial wastes exploiting low-cost vitrifying agents for production of vitreous and glass-ceramic materials has drawn considerable scientific and technological attention [14]. The vitrification method has been accepted as one of the most appropriate techniques for the stabilization of various waste forms and solid residues from waste incineration, since the rising environmental and economic costs. Vitrification concentrates a number of important merits such as, a large waste volume reduction, low cost application, negligible mass of by products and the ability to produce marketable materials. It is also a well tested method that has been successfully applied for the stabilization of urban, municipal, industrial, natural and radioactive waste forms [15-18]. Vitrification products can be either vitreous or glass-ceramic, depending on the batch composition, temperature and dutration of co-melting. Devitrification is an important part of the stabilization process, only if can be controlled to produce glass-ceramic materials with superior mechanical properties. It occurs spontaneously in the canister after melt casting or deliberately during thermal annealing. In the latter case, it is necessary that the devitrification process must not impair the chemical resistance achieved via vitrification. Post-annealing chemical stability depends on the distribution of the toxic element or compound in the microstructure of the glass ceramics product. It has been shown that leach resistance is increased in the following

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cases: i) when the polluting agent is entrapped in glassy islands shielded from a crystalline matrix [21] and ii) when it participates in the formation of crystal phases that have grown in a stable vitreous matrix [20]. The toxic elements and/or chemical substances can either participate on the formation of the glass network or be captivated in the form of precipitates or crystal phases in the glass network. In all cases, they are entrapped on the volume of the vitreous product and become chemically inactive. The large scale implementation of this method requires low cost vitrifying and melting agents, a sufficiently high waste content and pours temperatures easily attainable from a commercial furnace. The vitrification process is widely applied for radioactive waste storage and recently has been studied for many waste class inertization, as urban and industrial residues [22-24]. Recently, galvanic solid residue (that contains heavy metals ions) with silica and feldspat filter wastes from ceramic industries was utilized [24]. Vitreous materials for these applications are used because of the structural arrangement that support easily the incorporation of many kinds of chemical elements, such as the heavy and transition metal ions. The glass usually also shows a good environment chemical stability, that is a very interesting characteristic to residues inertization [25]. The waste vitrification process is more complex than the simple dilution of the waste components in the glassy matrix. The melting high temperature effect can reduce some melt components to their oxides forms. In this sense it is inevitable that the oxides from harmful wastes, interact with oxides from glass raw materials resulting a modified glass. Residual materials usually show very complex chemical compositions, they are composed of many chemical species and some of them show one or more coordination numbers. The contamination of soils by toxic metals (lead ions) is a widespread, serious problem that demands immediate action either by removal or immobilization, which is defined as a process which puts the metal into a chemical form, probably as a mineral, which will be inert and highly insoluble under conditions that will exist in the soil. If metals are to be immobilized, this might be achieved by the addition of sufficient amounts of the anions as oxides, phosphates, molibdates, which can form the inert mineral [26]. Solidification or stabilization of heavy metal-bearing sludge, industrial residues and contaminated soil is an attractive technology to reduce their toxicity and facilitate handling prior to landfill. In terminology, stabilization is a process of converting a toxic waste to a physically and chemically to

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Immobilisation of Lead Ions in Borate-Molibdate Glass Ceramics

3

produce a less toxic or less mobile form. It involves chemical interactions between waste and the binding agent. By comparison, solidification converts liquid waste, semisolid sludge or a powder into a monolithic form or granular material that will allow relatively easy handling and transportation to landfill sites. It does not necessarily imply any form of chemical reaction has occurred. Lead is ranked as the number one priority hazardous substance. The selection of glasses and operating parameters depends upon an understanding of the chemistry of the solidification/stabilization process. The overall process of immobilization in the glasses includes a combination of solution processes, interfacial phenomena and solid-state reactions. It is complex, especially in the presence of heavy metals. This paper is a review for the possible immobilization of the lead oxides into borate-molibdate and borate-tellurate glasses and glass ceramics. Upon devitrification of the glass products, the effect of composition on the ability to produce glass-ceramics materials was investigated by X-ray diffraction, FT-IR spectroscopy, EPR spectroscopy and DFT calculations. We attempt to illuminate structural aspects of the lead-borate-molibdate and lead-borate-tellurate glass systems by investigating the influence of the addition of lead oxide on the host glass matrices. The structural changes have been analyzed with increasing lead ions concentration. Boron can form a large variety of compounds because of the complexity of the structures involved. Boron atoms coordinate with oxygen not only in four-fold coordination ([BO4] tetrahedral units) but also in three-fold coordination ([BO3] triangular units from various borate groups and the boroxol rings). These [BO3] and [BO4] structural units may further link together via common oxygen atoms to form isolated rings and cages or polymerize into infinite chains, sheets and networks. The lead oxide can either participate on the formation of the glass network or be captured in the form of crystal phases in the glass matrix. Thus the lead ions become chemically inactive. On the other hand, lead oxide is unique in its influence on the structure of glasses [27]. It was observed in earlier studies that when PbO is added to vitreous network forming oxides, it acts both as a network modifier or/and as a network former, depending upon its concentration in the host glass [28]. This role depends on the type of ionic or covalent bond between lead and oxygen atoms [29]. Lead oxide may act as a network modifier in the same way as alkali oxide or rare earth oxide [30] disrupting the bonds connecting neighboring [BO3] and [BO4] groups. PbO can be incorporated into the glass as network-forming Pb-O groups ([PbO4] and/or [PbO3]). Additional oxygen

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for the coordination requirement of lead oxide to form network-formers are naturally provided by metal oxide present in the glass or even by a further molecule of lead oxide itself. Incorporation of lead oxide in the borate matrix results in the creation of trigonal [BO3] and tetrahedral [BO4] units. Equilibrium of the structural conversion between [BO3] and [BO4] units in the borate network depends on the chemical composition and the kind of modifiers. The infrared data revealed that increase of the amount of PbO influences the rearrangement of borate framework. In addition, the gradual increase of lead oxide in the glass results to transformation of some tetrahedral [BO4] units into trigonal [BO3] units, a reduction of the proportion of the tetrahedral [BO4] units and the forming of crystal phases in the glass network. Tellurium dioxide is a conditional glass former. It is very difficult to form pure vitreous TeO2 and it has been suggested that this is due to the lone pair of electrons in one of the equatorial positions of the [TeO4] polyhedron. Tellurate glasses are high index optical glasses possessing high levels of infrared transmission and have potential applications as acoustic and optical materials used in laser or as photochromic glasses. The doping of tellurate glasses with a higher content of PbO causes the modification of the basic structural units such as the [TeO4] trigonal bipyramid and the [TeO3] trigonal pyramid with one of the equatorial position occupied by a lone pair of electrons [26-32]. Accordingly, the increase in the number of non-bridging oxygen atoms would decrease the connectivity of the glass network and would necessite quite a radical rearrangement of the network formed by the [TeOn] polyedra. MoO3 – containing glasses have been the subject of many investigations due to their catalytic properties. The ions of molybdenum inculcate high activity and selectivity in a series of oxidation reactions of practical importance in the glass matrices [33, 34]. Molybdenum oxide as such belongs to the intermediate class of glass-forming oxides, it is an incipient glass network former and does not readily form the glass but does so in the presence of the modifier oxides like PbO with [MoO4] structural units and it may also act as modifier with [MoO6] and [Mo+5O3] structural units [35]. The molybdenum ions when dissolved in the borate glass systems, exist at least in two stable valence states, Mo(V) and Mo(VI), these ions acting both as network formers (with [MoO4] structural units, alternate with [BO4] structural units) and as network modifiers depending upon their concentration and chemical composition of the host glass network. Our EPR studies in the glass systems containing molybdenum ions have identified the presence of

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Immobilisation of Lead Ions in Borate-Molibdate Glass Ceramics

5

octahedral coordinated Mo(V) ions along with distorted octahedrons approaching tetragons. It is known that several ceramics such as zirconates, titanates and phosphates may incorporate high amounts of lanthanide and actinide in their structure and exhibit very good chemical resistance against water [36]. Radiaoactive single-phase ceramic waste forms remain difficult to prepare (grinding+sintering) than radioactive glasses (melting+casting) in nuclear facilities. To benefit at the same time from the ease of glass preparation and from the very good long-term behavior of ceramics, the preparation of glassceramic waste forms consisting of small crystals (which would preferentially incorporate lead ions) homogeneously dispersed in a durable glassy matrix – acting as a second barrier of containment – appears as an interesting alternative (double containment principle). However, for such an application, wastes need to be incorporated with a strong partitioning ratio in the crystalline phase. Morever, due to the existence of a residual glassy phase embedding the crystals, glass-ceramics could accommodate more easily waste composition fluctuations and impurities than ceramics. Indeed, because of the crystalline structure of ceramics, impurities and waste composition fluctuations could generate low durability parasitic phases containing radioactive elements in ceramic waste forms. A widespread set of very different borate glasses with optical, magnetic, super-ionic conductivity and other technologically interesting properties are currently produced. The optimization of such properties as a function of composition and other preparation parameters requires a good knowledge of the microscopic glassy structure. For many years, glasses containing transition metal ions have attracted attention because of their potential applications in electrochemical, electronic and electro-optic devices [37]. MoO3 containing glasses have been the subject of many investigations due to their catalytic properties. The ions of molybdenum inculcate high activity and selectivity in a series of oxidation reactions of practical importance in the glass matrices [38, 39]. Molybdenum oxide as such belongs to the intermediate class of glass-forming oxides; it is an incipient glass network former and does not readily form the glass but does so in the presence of the modifier oxides like PbO with [MoO4] structural units and it may also act modifier with [MoO6] and [Mo+5O3] structural units [40, 41]. The presence of high content of lead oxide in lead borate glasses can be considered as a former/modifier in the glassy matrix having basically B2O3 as a glassy former. The role of low and high concentrations of MoO3 in glass has not been received much attention. Therefore, it was thought of interest to study

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the correlation between the properties of lead borate glasses containing MoO3 and their network structures and also to gain more information about the state of Mo in such glass system. This paper examines interactions between glasses/glass-ceramics phase and heavy metal, mechanisms of heavy metal immobilization and factors controlling the effectiveness of glasses/glass-ceramics based solidification/stabilization.

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1.2. EXPERIMENTAL Glasses were prepared by mixing and melting of appropriate amounts of lead oxide, boric acid and molybdenum oxide (VI) of high purity (99,99%, Aldrich Chemical Co.). Reagents were melted at 12250C at 20 minutes and quenched. The glassy sample was subject to heat treatment applied at 5000C for 5h. The samples were analyzed by means of X-ray diffraction using a XRD6000 Shimadzu Diffractometer, with a monochromator of graphite for Cu-Kα radiation (λ=1.54Å) at room temperature. The FT-IR absorption spectra of the glasses in the 350-1700cm-1 spectral range were obtained with a JASCO FTIR 6200 spectrometer using the standard KBr pellet disc technique. The spectra were carried out with a standard resolution of 2cm-1. EPR measurements were performed at room temperature using ADANI Portable EPR PS 8400-type spectrometer, in X frequency band (9.4GHz) and a field modulation of 100kHz. The microwave power used was 5mW.

1.3. RESULTS AND DISCUSSION 1.3.1. X-Ray Diffraction and FTIR Spectroscopy The X-ray diffraction patterns reveal any crystalline phase in the samples with PbO content up to 40mol%. The glass-ceramic matrix contains B2O3 crystalline phase (figure 1.1, Appendix-Table 4.1). After doped with lead oxide some structural changes were observed and a new PbMoO4 crystalline phase appeared in the structure of the samples with up to 10mol%PbO (figure 1.1, Appendix-Table 4.2). In the glass ceramics samples with up to

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Immobilisation of Lead Ions in Borate-Molibdate Glass Ceramics

7

20mol%PbO disappear PbMoO4 crystalline phase and appears very small PbO crystalline phase (figure 1.2, Appendix-Table 4.3). While in the other glass samples with high PbO contents (50≤x ≤100%) found only vitreous phase. 0

1225 C

B2O3 * PbMoO4

intensity [a.u.]

0%

1%

*

5%

* 10%

*

*

*

10

20

*

*

30

40

50

** 60

2theta [degree]

0

1225 C 100% intensity [a.u.]

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Figure 1.1. X-ray diffraction patterns for xPbO(100-x)[2.7B2O3·0.1MoO3] samples with x=0-10mol%.

80% 50% 40%

30% PbO 20% 10

20

30

40

50

60

2theta [degree]

Figure 1.2. X-ray diffraction patterns for xPbO(100-x)[2.7B2O3·0.1MoO3] samples with x=20-100mol%.

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Simona Rada, Eugen Culea and Monica Culea

8

These results are very important from three motives: 1

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2 3

A high PbO content (x≥50mol%) shows a relaxation of the glass ceramics network, an accommodation of the network with the excess of oxygen and increasing of the content of vitreous phase. This could be explained by considering that the excess of oxygen may be accommodated by the formation of covalent Pb-O bonds lead to the open structure of the glass and the obtained of new molybdenum-leadborate glasses. We note that the PbO glass matrix was not reported till now. A low PbO content (5≤x≤10mol%) produces the apparition of lead molybdate, PbMoO4, crystalline phase.

Lead molybdate crystals have recently gained increasingly attracting interest as possible acousto-optical materials, modulators, ion conductors, scintillators in nuclear instruments [42, 43]. The lead molybdate has a tetragonal structure with a=5.433Å, c=12.110Å, point group symmetry of 4/m and space group symmetry of I41/a [44]. This structure has eight symmetry elements in an elementary cell that includes two formula units of PbMoO4. The [MoO4] tetrahedral units do not share common oxygens atoms but are linked through Pb-O bonds rather than by O-O interactions. There are four short Mo-O bonds of 1.77Å each, directed towards the corners of a slightly compressed regular tetrahedron [45]. Each lead is surrounded by eight oxygen atoms, belonging to one of eight different [MoO4] tetrahedral units. Four of them lie at a distance of 2.61Å and four others at slightly larger distance of 2.63Å. These eight oxygen atoms are placed at the corners of two interpenetrating tetrahedral forming an eightfold coordination group around each lead atom [46]. Each Mo sites is surrounded by four equivalent O sites at a bond length of 1.75Å in approximately tetrahedral S4 symmetry around that site. Each Pb site is surrounded by eight O sites at bond lengths of 2.40Å and 2.44Å. In this system PbO have an essential role in the transformation of glass ceramics in glasses. Probably, one of the advantages of PbO-containing glass systems is that they do not easily crystallize. This is because the PbO glass systems form [PbO4] structures easily since Pb plays the role of an intermediate due to its ionic field strength [47]. In the spectrum of the 2.7B2O3·0.1MoO3 glass ceramic matrix (figure 1.3, Table 1.1), the strong absorption bands are located at ~546, 640, 800, 1200 and 1500cm-1. These infrared absorption bands can be assigned as follows:

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The peak located at about ~550cm-1 is attributed to the bending vibrations of various borate and isolated [BO3] [48-51]. ii The band centered at about 640cm-1 can be attributed the condensed molybdate units vibrations such as [MoO4] and [Mo2O7] [52-56]. iii The strong IR absorption band at about ~800cm-1 was assigned to the B-O stretching vibrations of the [BO3] triangular units and the Mo-O stretching vibrations of [MoO4] structural units [52-55]. iv The absorption band at ∼1200cm-1 was assigned to the B-O bonds from isolated pyroborate groups [56]. v The region ranges between 1400 and 1600cm-1 and its spectral features are due to the to B-O- stretching vibrations in [BO3] units from various types of borate groups [56]. i

Accordingly, the [BO3], [MoO4] and [Mo2O7] structural units build the studied glass ceramic network.

0

absorbance [a.u.]

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1225 C

0% 1% 5% 10% 20% 30% 40% 400

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wavenumber [cm ] Figure 1.3. FT-IR spectra of the xPbO(100-x)[2.7B2O3·0.1MoO3]glasses and glass ceramics with x=0-40mol%.

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Table 1.1. Infrared absorption bands and their assignment of the [BO4], [BO3], [PbO4], [PbOn], [MoO4] [MoO6] structural units Peak position (cm-1) [BO4] groups

Assignment 550-630 690-720 720-780

900-1100 [BO3] groups

1190-1240 1240-1350

Lead ions

1350-1400 1420-1550 400-500 460-475 720, 980, 1100cm-1

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Molybdate groups

579 630-640

760-880 920-980

Bending vibrations of various borate and isolated BO33Oxygen bridges between two trigonal atoms Oxygen bridges between one tetrahedral and one trigonal boron atom Di-, Tri-, tetra- and penta-borate groups Boroxol rings and ortho-borate groups Boroxol rings B-O vibration of various borate rings Penta-, meta- and pyro-borate units Covalent Pb-O Pb–O bonds in [PbO4] units Pb-O bonds in [PbO3] and/or [PbO4] units Mo-O bonds in [MoO6] and [MoO3] units Mo-O-Mo Condensed molybdate such as [MoO4] and [Mo2O7] Stretching vibrations of [MoO4] anions Mo-O stretching vibrations of [MoO6]

The experimental FTIR spectra of xPbO·(100-x)[2.7B2O3·0.1MoO3] glass ceramics system with various contents of lead oxide (0 ≤ x ≤ 100 mol%) consisting of broad peaks and shoulders are presented in figures 1.3 and 1.4. The examination of these spectra shows that the increase of PbO content up to 40mol% (figure 1.3) modifies systematically the characteristic bands as follows: i

The bands located at about ~550cm-1 decreases with the increasing the concentration of the lead ions up to 30mol%, after that it is disappears. As was previously mentioned, the presence of this band is attributed to the bending vibrations of various borate and isolated [BO3].

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ii

With increasing of the PbO content can see a gradual disappearance of the bands located at about ∼640, 800 and 880cm-1. The gradual increase of lead oxide in the glass ceramic up to 5mol% and 10mol%, respectively, results to transformation of some [Mo2O7] units into [MoO4] units and the formation of the PbMoO4 crystalline phase. Also, the content of the B2O3 crystalline phase disappears gradual. By increasing of the lead ions content between 20 and 40mole% PbO, some of the [BO3] units were transformed into tetrahedral [BO4] units, some of the lead atoms now participate in the network as [PbO4] structural units and the excess of lead oxide generates the formation of PbO crystalline phase. iii The spectra of glass ceramics have exhibited a well-resolved band at ~890cm-1 and with increase in concentration of PbO up to 40mol%, the intensity of this band is observed to decrease. The gradual increasing of lead oxide in the glass ceramic up to 10mol% produces the appearance the new peak located at about ~920cm-1. In addition, when PbO concentration is rained up to 40mol%, the band at 890cm-1 is found to be shifted to 920cm-1. This peak was attributed the Mo-O stretching vibrations of [MoO6]. The gradual shifting of the band at 890 to 920cm-1 suggests the formation of partially isolated Mo-O bonds of the strongly deformed [MoO6] units [56]. iv The intensity of the band from ∼1200cm-1 decreases gradual with the increasing the content of PbO. This absorption band was assigned to B-O- asymmetric stretching vibrations in BO3 units from pyro- and ortho-borate groups. There takes place a reduction of the proportion of the pyro- and otho-borate units, in place of which [BO4] units and [BO3] units from boroxol rings are formed. v The IR absorption band centered at about ∼1465 cm-1 was attributed to the B-O- stretching vibrations in [BO3] units from various types of borate groups [31]. The position of this band is found to be shifted towards lower wavenumber (~1440cm-1) and its intensity decreases with the increasing of PbO concentration. In brief, the drastic reduction of intensity of the bands attributes to the [BO3] structural units from pyroborate, orthoborate units and various types of borate groups, it is the main factor in the apparition of the glass network and the substituted of the B-O-B and B-O-Mo continuous bonds with B-O-Pb bonds.

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The examination of the FTIR spectra of the xPbO·(100x)[2.7B2O3·0.1MoO3] glasses with x=40-100mol% (figure 1.4) show that the increase of PbO content strongly modify the characteristic IR bands as follows:

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i

The significantly different shape of the IR spectrum for x=40%PbO compared to that for x=50% reveals a structural change occurring between these compositions due to the apparition of the glasses network, in agreement to the X ray data. The adding of lead oxide now participate in the network as [PbO4] structural units yielding a change from the continuous borate-molibdate network to the continuous lead-borate network with interconnected through Pb-O-Pb and Pb-O-B bridges. ii The peak located at ~785cm-1 disappears at concentrations above 50mol%PbO and the region ranges between 900 and 1100cm-1 become more pronounced with increased lead concentration. This region is attributed to the B-O bonds in [BO4] structural units [47-51, 56]. iii Another broad absorptions from 1200 to 1560cm-1 are attributed to BO stretching vibration in [BO3] units in different borate groups. On the addition of the lead ion to the glass the bands become a larger band and its intensity decreases. iv The bands located around ~460, 700, 860, 980 and 1100cm-1 are associated with the presence of [PbO3] and/or [PbO4] units in the glass matrix. The role of each constituent in the glass network structure can be explained as follows. Molybdenum formed [MoO4] tetrahedral and worked as a network-former as evidenced from the glass-forming tendency in xPbO·(100-x)[2.7B2O3·0.1MoO3] system. By increasing of PbO content up to 50mol%, lead also took part in the glass network by forming the [PbO3] and [PbO4] polyhedra. B2O3 seemed to work through the continuous transformation of the [BO3] units into [BO4] structural units and boroxol rings. In brief, it is interest to mention that an opposite behavior of PbO has been observed in lead silicate glasses [57], respectively, where in for low concentrations (up to 40mol%) PbO acts as a network modifier and above 40mol% it acts as a network former. Then, the possibility to incorporate high amounts (100mol%) of lead oxide in these glass-ceramics was demonstrated.

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0

1225 C 100%

absorbance [a.u.]

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Figure 1.4. FT-IR spectra of the xPbO(100-x)[2.7B2O3·0.1MoO3]glasses and glass ceramics with x=40-100mol%.

1.3.2. EPR Investigation and Local Environment of Mo+5 Ions Our FTIR spectroscopic studies on lead-borate-molybdate glasses and glass ceramics suggest that [MoO6] octahedral and [MoO4] tetrahedral units exist in these materials. Electron Paramagnetic Resonance (EPR) spectroscopy, with its ability to explore the atomic-scale structure of solids, has been used to determine the local structure and bonding in these glasses and glass ceramics. Molybdenum has the electronic configuration [Kr] 4d5 5s1. Apparently, +6 Mo , Mo+5 and Mo+4 ions are the major components of molybdenum in the borate glasses under study. Mo+4 ions have two 4d electrons, to our knowledge no EPR spectrum seems so far to have been published. Mo+5 ions give a signal due to its unpaired electron, where as, Mo+6 ions (4d0 configuration) do not give any EPR signal. The environment of molybdenum ions in glasses is expected to exist mainly in the six fold coordination. However, there is a possibility of reduction of a part of molybdenum ions from Mo+6 state to Mo+5 state as was reported in a number of other glass matrices [58-64]. The Mo+5 ions are quite stable and occupy octahedral positions with distortions due to

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the Jahn-Teller effect [65-674]. The Mo+6 ions participate in the glass network with tetrahedral [MoO4] structural units. Most of these structures are characterized by the presence of tunnels, which originate from the particular behavior of Mo+5. Its electronic configuration favors the formation of an abnormally short molybdenyl Mo-O bond, so that each Mo+5O6 octahedron exhibits a free corner. The question of the oxidation state of the reduced molybdenum centers cannot be addressed by the vibrational spectroscopy, so we turn to EPR measurements. In this paper we discuss EPR to obtain more detailed information about the local structure and the extent of interaction between molybdenum sites in respective glasses and glass ceramics. When transition metal oxides such as MoO3 are melted, ions tend to lose oxygen and are present in more than one valence state. The study of electron paramagnetic resonance of these glasses and glass ceramics is therefore of interest for ascertaining the valence states of the transition ions. The molybdenum ions when dissolved in the borate glass systems, exist at least in two stable valence states, Mo(V) and Mo(VI), these ions acting both as network formers (with [MoO4] structural units, alternate with [BO4] structural units) and as network modifiers depending upon their concentration and chemical composition of the host glass network. Generally, Mo+5 formations in the molybdenum borate glasses can be related to the loss of oxygen from initial mixture during the preparation of glasses at relatively high temperatures [58, 59]. EPR spectra have been obtained and assigned to Mo+5 ions surrounded by oxygen atoms. The presence of a lead oxide, acting as a network modifier, brings about transformations of the environment of the paramagnetic ions. We relate the compositional dependence of Mo+5 EPR parameters to structural modifications in the glass network. The EPR spectra of these samples showed a very similar pattern to those found for glasses containing molybdenum [60-64]. It is interesting to remark that in the case of studied samples, the EPR spectra (figures 1.5, 1.6) have following structures: i) For PbO concentration of x≤40 and x=90mol%, respectively, the spectra exhibit parallel (g║=gz) and (g┴) perpendicular features along with hyperfine lines typical in an axial ligand field. The strong central line is due to 96Mo isotope which has zero nuclear spin (I=0) and a natural abundance of 75%. The smaller lines correspond to the hyperfine structure from odd 95Mo and 97 Mo isotopes which have spin nuclear value equal to 5/2. However, the natural abundances of these isotopes are 15.8 and 9.6%, respectively. As the

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First derivative of EPR absorbance [a.u.]

magnetic moments of these isotopes (-0.9270 and -0.9485 nuclear magnetons for 95Mo and 97Mo isotopes, respectively) are very nearly equal, the hyperfine structures resulting from them are superimposed.

40%

30% 20% 10% 5% 1%

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5000

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Magnetic Field [Gauss]

First derivative of EPR absorbance [a.u.]

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Figure 1.5. EPR spectra of xPbO(100-x)[2.7B2O3·0.1MoO3]glasses and glass ceramics with x=1-40mol%.

2000

90% 80% 70% 60% 50%

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Figure 1.6. EPR spectra of xPbO(100-x)[2.7B2O3·0.1MoO3]glasses and glass ceramics with x=40-90mol%.

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Simona Rada, Eugen Culea and Monica Culea

The signal with g┴ = 1.92 is much more intense than the signal with g║ =1.86. The value of g┴ is always greater than g║ in all these glasses which is characteristic of Mo+5 ions in an oxygen environment [60]. The EPR data are consistent with an oxygen environment of the Mo+5 ions forming an axially compressed octahedron. The addition of lead ions has influence on the width and shape of the signals. The values of g║ and g┴ depend critically on the local symmetry of this field. A detailed analysis of the g tensor was given by Gladney and Swalen [61]. An octahedral site with tetragonal compression would give values of g║ < g┴ < g (g = free electron value 2.0023) and A║ > A┴ [62, 63]. A similar conclusion can be drawn from the g and A values obtained in the present investigation. It is therefore, concluded that the molybdenum ions exist in glasses in octahedral coordination with a tetragonal compression and they have a C4V symmetry. ii) By increasing of the PbO concentration between 50 to 80mol%, respectively, EPR spectra show that the Mo+5 signal consisting of a single isotropic line (gx = gy = gz). This consists of the partial disappearance of the hyperfine structure and the appearance of a broad line at g┴=1.92 value characteristic for dipole-dipole and superexchange coupled Mo+5 ions [64]. Then, it is observed that the intensity of the resonance line, g┴, decreases with increasing of lead ions content (50≤x≤80mol% PbO). It can be assumed that Mo+5 ions in these glasses have an octahedral coordination and the geometric shape is associated with isotropic magnetic moments. Such a result may also indicate that the environment of molybdenum Mo+5 ions could mostly be fixed by the thermodynamic parameters of the melt composition before cooling. On the contrary at high-PbO content (90mol%) in the glasses, the EPR spectra start to exhibit a noticeable hyperfine structure. We show that molybdenum oxide acting as former enters the glasses as [MoO4] and [MoO6] structural units. iii) The resonance line width increases mildly with PbO concentration in the region where PbO acts as a network former in these glasses. It may be noted that the resonance line width, g=1.92, for x=50mol% PbO is larger than the x=1 and x=80mol% PbO. This may be due to the increase in Mo+5 concentrations which causes dipolar broadening and also due to additional bond angle and bond length strains caused by the introduction of lead into the network. On the other hand, due to the number increasing of the Mo+5 ions, the superexchange interaction appears among them and consequently a narrowing of this broad line result [65].

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Our EPR studies in the glass systems containing molybdenum ions have identified the presence of octahedral coordinated Mo+5 ions along with distorted octahedrons approaching tetragons. Changing PbO content modifies the structure of the glasses and glass ceramics. The coordination polyhedra of Mo+5 become more distorted leading the introduction of lead into the glass ceramics network and after that into glasses network. The presence of hyperfine structure could be an indicative of Mo+5-O+6 Mo bonds disruption in the network. Therefore, its absence may indirectly point to enhanced electron hopping between molybdenum centers along Mo+5O-Mo+6 bonds, occurring in lead-molybdate-borate glasses with x=50-80mol% demonstrated to be electronic conductors [66]. In electronically conducting molybdate glasses, one such interactions occurs via a so-called superexchange of an electron, such as hopping of a mobile electron along a Mo+5-O-Mo+6 linkage. Such a mechanism was observed in glasses of binary MoO3-P2O5 system [67, 68]. For glasses with PbO content between 50 and 80mol%, it seems that the shorter occupancy time of electron 4d1 results in energy broadening and consequently in the disappearance of the hyperfine structure. In this region, where the glasses behave as electronic conductors [69], the conditions for electron hopping along Mo+5-O-Mo+6 linkages are more favorable than in glasses with smaller PbO content, in which the Mo-O-Mo linkages and the network are highly disrupted, as shown by our FTIR studies. In brief, FTIR and EPR investigations on this xPbO·(100x)[2.7B2O3·0.1MoO3] system shown that MoO3 plays a network modifier or former role in function of their concentrations. For x≤40mol%PbO the molybdenum ions lead to a strong depolymerization of borate units with the short chain units or ring formations appearance and act as modifiers which weaken the network. For x≥50mol% PbO, the molybdenum atoms prefer to bridge with oxygen atoms which not participate in borate structural units and thus act as network former by Mo-O-Mo bonds formation.

1.3.3. The Devitrification Behavior of the Lead-Borate-Molibdate Glasses and Glass Ceramics The production of glass ceramic materials based on inorganic industrial waste generated by different industries is a promising line and has been used very successfully to crystallize important ceramics from glass phase. Some of these glass-ceramic materials became commercial products and found their

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B2O3 * PbMoO4

*

intensity [a.u.]

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applications in the field of abrasion-resistant materials, high-temperature insulators or multilayer ceramic substrate. Multilayer ceramic substrate is widely used for commercial and military electronic devices and has advantages in high capacity and low cost. Lowtemperature co-fired ceramics technology used glass-ceramic materials for substrate and has advantages in low dielectric constant and high component density. They use screen-printing technology to embed passive components like capacitors, resistors and inductors into multilayer ceramic substrate with low resistance precious metals as electrodes and interconnections [70-73]. They form into integrated component after co-firing process at low temperature. In order to use the lead-borate-molybdate glass ceramics than multilayer ceramic substrate, the devitrification behavior of these samples is reported. The controlled crystallization of this system is commonly employed to produce glass ceramics with desirable properties. The X-ray diffraction patterns reveal any crystalline phases in the treated samples up to 5h. By increasing of the lead oxide content up to 30mol% the PbMoO4 and B2O3 crystalline phases was detected in the samples (figure 1.7, Appendix-Tables 1 and 2). The increase of PbO concentration between 40 and 80mol% causes significant changes and the appearance of the β-PbB2O4 crystalline phase (figure 1.8, Appendix-Table 4).

5%

*

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* **

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Figure 1.7. X-ray diffraction patterns for heat-treated xPbO(100-x)[2.7B2O3·0.1MoO3] samples with x=0-30mol%.

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* PbB2O4

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intensity [a.u.]

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19

* ** * * *

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*

* *

*

50% 60% 70% 80% 90% 100% 10

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Figure 1.8. X-ray diffraction patterns for heat-treated xPbO(100-x)[2.7B2O3·0.1MoO3] samples with x=40-100mol%.

The vibrational modes of the borate, molibdate and plumbate network are shifted under influence of the lead ions, the extent and direction of the shift depending on the type of the cation. Figures 1.9 and 1.10 show the FTIR spectra of lead-borate-molibdate glass ceramics as a function of PbO concentration. It is clearly seen in figures 1.9 and 1.10 that spectra exhibit a strong compositional dependent trend. The band intensity and bandwidth also vary with the concentration of PbO. The absorption bands and the corresponding band assignments based on the literature can be summarized as follow: i) The narrowing and prominent absorption bands are formed which are not present in the parent system before the addition of PbO content. The small absorption peaks at about 640 and 780cm-1 which is seem in the parent network increase drastically by increasing of PbO content between 5 and 30mol%, after that its disappearance. These bands are assigned to the condensed molybdate units such as [MoO4] and [Mo2O7] and the stretching vibrations of [MoO4] anions. When a small PbO content is introduced, a large number of non-bridging oxygen ions exist in the glass ceramic host, the [MoO4] structural units content to increase yielding the increase of the content of PbMoO4 crystalline phase.

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ii) The peak located at about ~550cm-1 is attributed to the bending vibrations of various borate and isolated [BO3]. Its intensity increases up to 30mol% PbO, after that disappears abrupt. 0

absorbance [a.u.]

500 C, 5h

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0

500 C, 5h

100% absorbance [a.u.]

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Figure 1.9. FT-IR spectra of the heat-treated xPbO(100-x)[2.7B2O3·0.1MoO3]glasses and glass ceramics with x=5-30mol%.

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Figure 1.10. FT-IR spectra of the heat-treated xPbO(100-x)[2.7B2O3·0.1MoO3]glasses and glass ceramics with x=40-100mol%.

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The strong vibrational bands between 1200 and 1500cm-1 indicate an increasing formation of metaborate, pyroborate and orthoborate structural units in the glass ceramic matrix. So that, the [BO3] structural units with nonbridging oxygens content continues to increase yielding the increase of the content de B2O3 crystalline phase up to 30mol% PbO. By increasing of PbO content up to 40mol%, the B2O3 crystalline phase disappears abrupt and the content of [BO3] structural units decreases. After that, for the samples with x≥50mol%, the number of [BO3] structural units increase slowly. iii) For x between 20 and 30mol% PbO, a new absorption peak at about 690cm-1 appears which is attributed to combined vibrations of the [BO4] and [PbO4] structural units [74]. It attains its maximum values at the sample with 40 and 50mol% PbO, respectively. iv) The small peak at about ~460cm-1 become prominent band for sample with x=40mol% PbO. After that, by increasing gradual of the PbO concentration, its intensity decrease up to x=50mol% while increase slowly. This band is attributed to the bending vibrations of the tetrahedral [PbO4] structural units and it is sensitive to changes in the amount of PbO. v) All samples show absorption bands between 900 and 1100cm-1, indicating B-O band stretching of the tetrahedral [BO4] structural units. The strong peak at ~1200cm-1 before the addition of 40mol% PbO has disappeared. This result shows an increased formation of [BO4] structural units by the disappearance of B2O3 crystalline phase. It seems that the content of [BO4] structural units can become higher and attains its maximum values of 40mol% PbO yielding the formation of the PbB2O4 crystalline phase. These results suggested that there are three different sites for Pb+2 ions corresponding to a network former and modifier positions of Pb+2 in the glass ceramic, namely: i) For PbO content up to 30mol%, the lead ions are firstly inserted in the divalent state and they can be considered as modifiers because they have a strong affinity towards these groups containing non-bridging oxygens, which are negative-charged. Presence of multiple cations, boron and molybdenum in the glass ceramic to attract oxygen ions yield a competition between these cations. Such, the [MoO4]-2 anions are readily available for charge compensation than the [BO3] structural units leading the formation of the PbMoO4 crystalline phase because the [MoO4]-2 anion (2.35) has higher electronegativity than the [BO4]-1 anion (2.10). This preference is decided by the electronegativity of the structural groups. The group electronegativities are calculated using the Sanderson’s method [74].

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ii) By increasing of the lead ions content up to 40mole%, Pb+2 ions will coordinate more with non-bridging oxygens and will practice now the [BO3] structural units. Lead ions disrupt the bonds connecting neighboring [BO3] and [BO4] structural units and will make the formation of PbB2O4 crystalline phase. iii) High PbO content over 50mol% leading the decreased of the number of individual Pb+2 ions and some of the lead atoms now participate in the network as [PbO4] structural units. Then, the diffusion of different ions and ionic complexes of the lead will decrease the rate of crystal growth and the conversion of the glass into crystalline material become more difficult, in agreement to the X ray data. A possible mechanism which explain the scheme of the structural modifications it is represented on the basis of following equations: i) For x=5-30mol%PbO B2O3 and MoO3 are known to be glass formers, where as PbO can behave like a modifier. The modifier and former roles of the PbO, B2O3 and MoO3 can be represented as: PbO → Pb+2 + O-2 B2O3 ≡ 2 [BO3/2]0 MoO3 ≡ [MoO3/2O3/2]0 2 [BO3/2]0 + O-2 → 2 [BO4/2]-1 2 [MoO3/2O3/2]0 + O-2 → Mo2O7 _____________________________________________ [2BO3/2 · MoO3/2O3/2]0 + 2 O-2 → 2 [BO4/2]-1 + Mo2O7 Pb+2 + Mo2O7 → PbMoO4 + [MoO3/2O3/2]0 Pb+2 + 2 [BO4/2]-1 → PbO + 2 [BO3/2]0 Variations observed in the FTIR spectra and X-ray diffraction patterns for sample with 5≤x≤30mol% are consistent with the scheme of modification explained above. It is evident that there are the PbMoO4 and B2O3 crystalline phases and the content of [BO3] structural units attains maximum values. ii) For x=40mol% PbO [MoO4]-2 → [MoO3/2O3/2]0 + O-2 2 [MoO4]-2 → Mo2O7 + O-2 2 [BO3/2]0 + O-2 → 2 [BO4/2]-1 Pb+2 + 2 [BO4/2]-1 → PbB2O4

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These typical modifications of the network is illustrated in our FTIR and X-ray results by the formation of the PbB2O4 crystalline phase and the high intensity of characteristic bands of the [BO4] structural units (950-1050cm-1). iii) For x=50mol%PbO B2O4-2 → 2 [BO3/2]0 + O-2 B2O4-2 → 2 [BO4/2]-1 Pb+2 + 4 O-2 → 2 [PbO4]-4 Pb O + 2 [BO3/2]0 → Pb+2 + 2 [BO4/2]-1

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The remaining PbO reacts with the [BO3/2]0 structural units and the Pb+2 ions with the excess of oxygen ions and now, the lead ions can participate as former network. From the above discussion we noted that the conversion of Pb+2 ions and [BO3/2]0 structural units into [PbO4] and [BO4] structural units leads to increasing dimensionality of the network. This compositional evolution of the structure could be explained by considering that the excess of oxygen may be accommodated by the change from the continuous borate network to the continuous plumbate-borate network with interconnected through Pb-O-B and B-O-B bridges. These results can be a possible explanation for weakly crystallization of the samples. iv) For 60≤x≤90mol% PbO 2 [BO3/2]0 + O-2 → 2 [BO4/2]-1 Pb+2 + 2 [BO4/2]-1 → 2 [BO3/2]0 + PbO 2 PbO + O-2 → 2 [PbO3/2]0 It is evident from vibrations bands of the [BO3] structural units (1200-1400cm1 ) and X-ray data that there is a high possibility of reconversion of the four coordinated borons into three coordinated borons. v) For x=100mol% PbO 2 [PbO3/2]0 + O-2 → 2 [PbO4/2]-1 2 [PbO4/2]-1 + Pb+2 → 2 [PbO3/2]0 + PbO According to these results, it can be concluded that most of the lead ions incorporated into the glass ceramic is not located inside clusters and the Pb+2 content is distributed into two sites attributed to network modifier and network former. In this study, lead ions essentially act as a network former, thus increasing the glass stability towards the devitrification process. Moreover, presence of

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multiple cations in the glass always increases the tendency of network formers to attract oxygen ions due to competition between the cations themselves. The unit which has higher electronegativity value picks up oxygen ion and gets modified. This tendency increases further if the size of cations increases.

1.4. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8]

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[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

C. M. Jantzen, J. Non-Cryst. Solids. 84 (1986) 215. B. C. Sales, Materials Research Society Bulletin XII(5) (1987) 32. B. C. Sales, L. A. Boatner, Science. 226 (1984) 45. B. C. Sales, L. A. Boatner J. Non-Cryst. Solids. 79 (1986) 83. B. C. Sales, M. M. Abraham, J. B. Bates, L. A. Boatner, J. Non-Cryst. Solids. 71 (1985) 103. W. Donald, B. L. Metcalfe, R. N. J. Taylor, J. Mater. Sci. 32 (1997) 5851. G. H. Beall, H. I. Rittler, U. S. Patent. 4, 314, 909, 1982. P. W. McMillan, Glass Ceramics, 2nd Ed., Academic Press, London, 1979. I. W. Donald, Encyclopedia of science and engineering, suplimetary Vol. 3, edited by R. W. Cahn (Pergamon, Oxford) 1993, 1689. P. J. Hayward, Glass Technol. 29 (1988) 122. Y. Nan, W. E. Lee, P. F. James, J. Amer. Ceram. Soc. 75 (1992) 1641. M. Langlet, M. Saltzburg, R. D. Shannon, J. Mater. Sci. 27 (1992) 972. I. W. Donald, B. L. Metcalfe, R. N. Taylor, J. Mater. Sci. 32 (1997) 5851. Th. Kehagias, Ph. Komninou, P. Kavouras, K. Chrissafis, G. Nouet, Th. Karakostas, J. Eur. Ceram. Soc., 26 (2006) 1141. M. Romero, R. D. Rawling, J. M. Rincon, J. Eur. Ceram. Soc. 19 (1999) 2049. P. Pisciella, S. Crisucci, A. Karamanov, M. Pelino, Waste manag. 20 (2000) 561. L. Barbieri, A. Corradi, I. Lancellotti, J. Eur. Ceram. Soc. 20 (2000) 2477. G. Scarinci, G. Brusatin, L. Barbieri, A. Corradi, I. Lancellotti, J. Eur. Ceram. Soc. 20 (2000) 2485. A. Dwivedi, Y. Berta, R. F. Speyer, J. Mater. Sci. 29 (1994) 2304. P. Kavouras, G. Kaimakamis, T. A. Ioannidis, Th. Kehagias, P. Komninou, S. Kokkou, Waste Manag. 23 (2003) 361.

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[21] L. Barbieri, A. C. Bonamartini, I. Lancellotti, J. Eur. Ceram. Soc., 20 (2000) 2477. [22] P. Kavouras, P. Komninou, K. Chrissa, G. Kaimakamis, S. Kokkou, K. Paraskevopoulos, T. Karakostas, J. Eur. Ceram. Soc., 23 (2003) 1305. [23] J. Yan, I. Neretnieks, Sci. Total Environ., 172 (1995) 95. [24] J. Sterpenich, G. Libourel, Chem. Geol. 174 (2001) 181. [25] A. C. Silva, S. R. H. Mello-Castanho, J. Eur. Ceram. Soc., 27 (2007) 565. [26] S. K. Porter, K. G. Scheckel, C. A. Imperllitteri, J. A. Ryan, Critical Reviews in Environmental Science and Technology, 34(6) (2004) 495. [27] M. Ganguli, K. J. Rao, J. Solid State Chem. 145 (1999) 65. [28] S. Ram, K. Ram, J. Mater. Sci. 23 (1988) 4541. [29] T. Takaishi, J. Jin, T. Uchino, T. Yoko, J. Am. Ceram. Soc. 83(10) (2000) 2543. [30] S. H. Kim, T. Yoko, J. Am. Ceram. Soc. 78 (1995) 1061. [31] R. Iordanova, V. Dimitrov, D. Klissurski, J. Non-Cryst. Solids. 180 (1994) 58. [32] D. Klissurski, Y. Pesheva, N. Abadjeva, Appl. Catal. 77 (1991) 55. [33] M. Pal, K. Hirota, H. Sakata, J. Appl. Phys. 34 (2001) 459. [34] I. W. Donald, B. L. Metcalfe, R. N. J. Taylor, J. Mater. Sci., 32 (1997) 5851. [35] M. El-Hofy, I. Z. Hager, Phys. Stat. Sol. A 182 (2000) 697. [36] L. Bih, M. El. Omari, J. Reau, M. Haddad, D. Boudlich, A. Yacoubi, A. Nadciri, Solid State Ionics. 132 (2000) 71. [37] R. Iordanova, V. Dimitrov, D. Klissurski, J. Non-Cryst. Solids. 58 (1994) 58. [38] D. Klissurski, Y. Pesheva, N. Abadjeva, Appl. Catal. 77 (1991) 55. [39] M. Pal, K. Hirota, H. Sakata, J. Appl. Phys. 34 (2001) 459. [40] M. El-Hofy, I. Z. Hager, Phys. Stat. Sol. 182(A) (2000) 697. [41] C. Zeng, J. Cryst. Growth. 171 (1997) 136. [42] S. Takano, S. Esashi, K. Mori, T. Namikata, J. Cryst. Growth. 24&25 (1974) 437. [43] R. Sailer, G. McCarthy, North Dakota State University, Fargo, North Dakota, ICDD Grant-in-Aid (1992). [44] J. Leciejewicz, Zeitschrift fur Kristallographie. 121 (1965) 158. [45] T. M. Bochkova, M. D. Volnyanskii, D. M. Volnyanskii, V. S. Shchetinkin, Phys. Sol. St. 45 (2003) 244. [46] J. Krogh. Moe, Phys. Chem. Glasses, 10(2) (1969) 46. [47] S. Rada, M. Culea, E. Culea, J. Phys. Chem. A 112(44) (2008) 11251.

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[48] E. I. Kamitsos, A. Patsis, G. D. Chryssikos, J. Non-Cryst. Solids, 152 (1993) 246. [49] C. P. Varsamis, E. I. Kamitsos, N. Machida, T. Minami, J. Phys. Chem. 101(B) (1997) 3734. [50] S. Rada, M. Bosca, M. Culea, R. Muntean, P. Pascuta, Vibrat. Spectrosc. 48(2) 2008 285. [51] E. I. Kamitsos, Phys. Chem. Glasses. 44(2) (2003) 79. [52] A. Mogus-Milankovic, A. Santic, A. Gajovic, D. E. Day, J. Non-Cryst. Solids. 76 (2003) 325. [53] P. Znasik, M. Jamnicky, J. Non-Cryst. Solids 74 (1992) 146. [54] U. Selveraj, K. J. Rao, J. Non-Cryst. Solids 104 (1988) 500. [55] S. Rada, M. Culea, M. Neumann, E. Culea, Chem. Phys. Letters 460 (2008) 196. [56] P. W. Wang, L. Zhang, J. Non-Cryst. Solids. 194 (1996) 129. [57] M. Elahi, M. H. Kekmat-Shoar, C. A. Hogart, K. A. K. Lott, J. Mater. Sci., 14 (1979) 1997. [58] U. Selvaraj, H. G. Kershava Sundar, K. J. Rao, J. Chem. Soc. Faraday Trans. I 85 (2) (1989) 251. [59] C. Sanchez, L. Livage, J. P. Launay, M. Fournier, Y. Jeannin, J. Am. Ceram. Soc. 104 (1982) 3194. [60] H. M. Gladney, J. D. Swalen, J. Chem. Phys. 42 (1977) 399. [61] R. Muncaster, S. Parke, J. Non-Cryst. Solids. 24 (1977) 399. [62] B. Yasoda, R. P. S. Chakradhar, J. L. Rao, N. O. Gopal, Mater. Chem. Phys. 106 (2007) 33. [63] O. Cozar, I. Ardelean, S. Simon, L. David, Solid State Commun. 85(5) (1993) 461. [64] L. Bih, M. El. Omari, J. M. Reau, M. Haddad, D. Boudlich, A. Yacoubi, A. Nadiri, Solid State Ionics. 132 (2000) 71. [65] A. Mansigh, J. K. Vaid, R. P. Tandon, J. Phys. C: Solid State Phys. 10 (1977) 4061. [66] S. Muthupari, S. Prabakar, K. J. Rao, J. Phys. Chem. Solids. 57 (1996) 553. [67] U. Selvaraj, K. J. Rao, Chem. Phys. 123 (1988) 141. [68] U. Selvaraj, K. J. Rao, J. Non-Cryst. Solids. 104 (1988) 300. [69] R. R. Tummala, J. Am. Ceram. Soc. 74(5) (1991) 895. [70] D. M. Mattox, S. R. Gurkovich, Ceram. Eng. Sci. Proc. Ceram. Bull. 9(11-12) (1988) 1567.

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[71] S. K. Muralidhar, A. S. Shaikh, G. J. Roberts, D. J. Leandri, D. L. Hankey, T. J. Vlach, Low dielectric, low temperature fired glassceramics, U.S.Patent No5, 164, 342 (1992). [72] Y. H. Jo, M. S. Kang, K. W. Chung, Y. S. Cho, Materials Research Bulletin. 43 (2008) 361. [73] G. Sharma, K. Singh, M. S. Mohan, H. Singh, S. Bindra, Radiation Physics and Chemistry. 75 (2006) 959. [74] R. T. Sanderson, Polar Covalence, Academic, New York, 1983.

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Chapter 2

IMMOBILISATION OF LEAD IONS IN BORATE-TELLURATE GLASSES AND GLASS CERAMICS

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2.1. INTRODUCTION TeO2-based glasses possess great interest for applications in lasers and non-linear optics according to their high refractive index, high third-order nonlinear susceptibility, good infrared transmittance and low melting point [1, 2]. Recent approaches revealed some contradictions about the binary tellurate glasses structure. Some authors [3] reported that these glasses are formed by three-dimensional network composed of [TeO4] trigonal bypiramids linked in infinite chains by shared vertices and in the glass formation, some Te-O bonds are broken creating [TeO3] trigonal pyramids. Other authors [4] have shown that in TeO2 based glasses containing alkaline oxide as network modifiers, the glass structure change from trigonal bipyramids to [TeO3+1] polyhedral and then to [TeO3] trigonal pyramids, as the alkaline oxide concentration increases. Pure B2O3 contains only three-coordinated boron atoms, and some of these units are transformed into four-coordinated tetrahedral as the alkali oxide content increases. The amorphous network is built by [BO3] and [BO4] units linked by bridging oxygen atoms. The fraction of boron atoms that is fourcoordinated increase with increasing alkali oxide concentration in the region over 33%, and the non-bridging oxygen atoms are formed, the [BO4] units decrease [5-7]. In the binary borate-tellurate glasses the structure-composition relationship is more complicated than that of the borate or tellurate glasses,

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and a lot of models were raised to explain the relationship in the past several years [8].

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2.2. EXPERIMENTAL Glasses were prepared by mixing and melting of appropriate amounts of tellurium dioxide, boric acid and lead oxide (II) of high purity (99,99%, Aldrich Chemical Co.). Reagents were melted at 8750C for 30 minutes and quenched. The glassy sample was subject to heat treatment applied at 4500C for 5h. The samples were analyzed by means of X-ray diffraction using a XRD6000 Shimadzu Diffractometer, with a monochromator of graphite for Cu-Kα radiation (λ=1.54Å) at room temperature. The FTIR absorption spectra of the studied glasses were measured for each glass sample over the range 380-1600cm-1 of wavenumber. A JASCO FTIR 6200 spectrometer was used in conjunction with the potassium bromide, disc technique. Samples of glass weighing 0.002g were mixed and ground with 0.300g KBr. After which the mixture was pressed at 10 tons for 3 min under vacuum, to yield transparent discs suitable for mounting in the spectrometer. The precision of the absorption band maxima is ± 2 cm-1. Computer simulations are a valuable means for investigating atomic details of materials enabling us to retrieve information inaccessible by many experimental techniques. The geometry optimization of the proposed model structure was carried out using density function theory (DFT). The DFT computations were performed with B3PW91/CEP-4G/ECP method using Gaussian’03 program package [9]. It should be noticed that only the broken bonds at the model boundary were terminated by hydrogen atoms. The molecular graphic of model was generated with MOLEKEL 4.1 program [10]. Frequency analysis followed all optimizations in order to establish the nature of the stationary points found, so that all the structures reported in this study are genuine minima on the potential energy surface at this level of theory, without any imaginary frequencies. Accordingly, frequency calculations were performed to ensure that the stationary points were minima and to calculate infrared (IR) spectra.

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2.3. RESULTS AND DISCUSSION 2.3.1. Immobilisation of Lead Ions in Borate-Tellurate Glasses FTIR spectrum of the 7TeO2·3B2O3 glass for the 400 and 1400cm-1 wavenumber range is shown in figure 2.2. Obtained bands and their assignments can be summarized as follow: The large band centered at ~625cm-1 is assigned to the stretching mode of [TeO4] trigonal bypiramidal with bridging oxygens [10-12]. The shoulder located at about ~780cm-1 indicates the presence of [TeO3] units [13, 14]. ii The broad bands in the region 800-1200cm-1 are due to the stretching of B-O bonds of [BO4] units [15, 16]. The feature of band centered in the region ~1030cm-1 is due to the pentaborate, tetraborate, triborate and diborate vibrations. The IR absorption band at about 1263cm-1 was attributed to the B-O stretching vibrations of the [BO3] units from the boroxol rings [17, 18]. iii The bands centered in the region between ~1200 and 1450cm-1 can be attributed to the [BO3] units stretching vibrations, which are associated with the vibrational mode inside the various borate rings and the non-bridging B-O- bonds, respectively [19, 20]. The absorption band located at about ~1200cm-1 was assigned to B-Oasymmetric stretching vibrations in BO3 units from pyro- and orthoborate groups. This band responsible for non-bridging oxygen atoms increase in the borate groups with increase the lead ions, thus indicating more non-bridging oxygen ions exist in borate glass.

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i

The X-ray diffraction patterns did not reveal any crystalline phase in the prepared samples up to 100mol% PbO (figure 2.1). The addition of PbO in the 7TeO2·3B2O3 glass matrix with x=0-100mol% (figures 2.2, 2.3), gives some drastically change in the FTIR spectra of these glasses as follows: i) By increasing the PbO content between 5 and 30mol%, the characteristic features of the five absorption bands were included. The increasing trends in wavenumber of the bands centered in the regions ~620, 915, 1086, 1242 and 1384cm-1 can be explained considering a conversion process of the [TeO4] structural units into [TeO3] structural units, an

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interconversion process of the [BO3] ↔ [BO4] structural units and the transformation of the large number of boroxol rings.

0

875 C

600

intensity [a.u.]

500

100%

400

60% 300

30% 200

10% 100

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10

20

30

40

50

60

-1

wavenumber [cm ] Figure 2.1. X-ray diffraction patterns for xPbO(100-x)[7TeO2· 3B2O3] samples with x=0-10mol%.

These stronger vibration bands reflect that more [BO3] units coupling with lead ions still exist in the glass host. The lead ions have a strong affinity towards the groups containing non-bridging oxygens, which is negativecharged, because they are readily available for charge compensation [21]. The B-O bonds are formed by an introduction of lead ions itself instead of the degradation of boroxol rings. ii) The accumulation of the excess of oxygen in the network generates a drastically change by increasing of PbO content between 40 and 50mol%. The centered band at about 630cm-1 shifts to 608cm-1. Then, the bands located at about 1088 and 1400cm-1 shift to the 1110 and 1433cm-1, respectively. A small band appears at about ~470cm-1. This band can be assigned the bending mode of Te-O-Te or O-Te-O linkages and also the Pb-O-Pb bending vibrations of the [PbO4] tetrahedral units.

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875 C 0%

absorbance [a.u.]

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5% 10% 1,0

20%

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30% 40%

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800

1000

1200

1400

1600

-1

wavenumber [cm ]

1,8 1,6

0

875 C

100%

1,4

90% absorbance [a.u.]

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Figure 2.2. FT-IR spectra of the xPbO(100-x)[7TeO2· 3B2O3] glasses with x=040mol%.

1,2 1,0

80%

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60% 0,4

50%

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40% 0,0 200

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wavenumber [cm ]

Figure 2.3. FT-IR spectra of the xPbO(100-x)[7TeO2· 3B2O3] glasses with x=40100mol%.

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Simona Rada, Eugen Culea and Monica Culea

This compositional evolution of the structure could be explained by considering that the excess of oxygen may be accommodated by the following changes: the transformation from [TeO3] structural units into [TeO4] structural units, a conversion process from [BO4] to [BO3] structural units and the formation of the [PbO4] structural units. The formation of the structural units such as [PbO4] of PbO requires either an additional oxygen atom or two single bonded oxygen ions which can be provided by the lead oxide or from this destruction of the [TeO4] structural units. It seems that the content of [BO4] structural units cannot become higher, because the modified [BO3] units containing one or more B-O-Pb bonds are unable to accept a fourth oxygen atom. Then, the PbO behaves as a glassformer by means of the intercalation of [PbO4] entities in the [TeO4] and [BO4] chain network. iii) For x≥50mol%, the band centered at about 460cm-1 is invisible on the spectra of all series of doped glasses. It is more pronounced only for sample with content of 100%PbO. The intensity of the absorption band located at about 620cm-1 increase gradually with the increase of PbO content. The evolution of this absorption band suggests an increase of the degree of polymerization of the glass network and the doping with lead ions deforms the Te-O-Te linkages changing the distribution of [TeO4] and [TeO4] structural units in the glass matrix. The features of the band presented in the region between ~1235 and 1430cm-1, responsible for non-bridging oxygen atoms decrease with increase the lead ions. The position of these bands was found to be shifted towards lower wavenumbers with increasing the lead concentration (~1212 and 1324cm-1, respectively) indicating the conversion from [BO3] units into [BO4] structural units. The intensity of the bands located in the region between 800 and 1100cm-1 becomes more shaper and pronounced showing the increase of [BO4] structural units. In brief, the lead ions have a strong affinity towards [BO3] groups containing non-bridging oxygens. Then, the PbO behaves as a glass-former by means of the intercalation of [PbO4] entities in the [TeO4] and [BO4] chain network. The excess of oxygen can be supported into the glass network by the formation of [TeO4] and [BO4] structural units. Accordingly, the borate-tellurate glasses may be good hosts for the incorporation of high lead ions contents at low temperature and their structural properties are significantly modified. Structural changes, as recognized by analyzing band shapes of IR spectra, revealed that PbO causes a change from

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the continuous borate-tellurate network to the continuous lead-borate-tellurate network with interconnected through Pb-O-B and Te-O-Pb bridges. Then, this low temperature it is favorable for the obtained of plumbate glasses.

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2.3.2. Immobilisation of Lead Ions in Borate-Tellurate Glass Ceramics Glass-ceramics are polycrystalline solids produced by controlled crystallization of glasses. Controlled crystallization usually involves a two stage heat treatment, namely a nucleation stage and a crystallization stage. In the nucleation stage, small nuclei crystallization proceeds by growth of a new crystalline phase. It is important to determine nucleation and crystallization parameters in the preparation of glass ceramics to obtain desired microstructure and properties. The criteria that show the selection of the glasses as possible agent of immobilization of the lead waste are: i) mature production technology, low costs and timing, ii) high solubility in the lead, iii) good durability over geologic time. In general, this final point implies a more detailed characterization of waste forms, in particular chemical durability of the glass necessitate a process of devitrification. This process of devitrification is high possible due to the elevated temperatures expected (100-3000C) to be encountered in a geologic repository. In order to understand the chemical durability over time of the leadborate-tellurate glasses, the glassy samples were subject to heat treatment applied at 4500C because as far as this temperature glasses were thermodynamic stable. The X-ray diffraction patterns reveal any crystalline phase in the treated samples up to 5h, exception the sample with 80mol% PbO. By increasing of the lead oxide content up to 100mol% X-ray diffraction analysis shows following: i

ii

The composition of the glass ceramics with the PbO concentration up to 30mol% was found to consist mainly of the TeO2 crystalline phase and little B2O3 crystalline phase (figure 2.4, Appendix -Table 4.1). By the increasing of the PbO concentration up to 50mol%, can see a gradual disappearance of B2O3 crystalline phase. Then, for x between 60 and 70mole%PbO showed a drastic structural modification which causes gradual disappearance of TeO2 crystalline phase, too (figure

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2.5, Appendix -Table 4.5). Surprising for sample with 80mol% PbO can be observed two halos characteristic of the amorphous compounds. iii In the structure of glass ceramic sample with 90mol%PbO it was found the Pb2Te3O8 crystalline phase (figure 5, Appendix -Table 4.6) while for the sample with 100mol%PbO can see the PbO crystalline phase (figure 2.5, Appendix-Table 4.3). The examination of the FTIR spectra of the xPbO·(100-x)[7TeO2·3B2O3] glass ceramics with x=0-40mol% (figure 2.6) shows that the increase of PbO content strongly modify the characteristic IR bands as follows: The intensity of the peak located at about 546cm-1 gradual disappears with the increasing of PbO composition up 30mol%. This band is attributed to the bending vibrations of various borate and isolated [BO3] structural units. ii The band located in the range of 600 to 700cm-1 splits into three components centered at about ~648, 675 and 700cm-1 and its intensity decreases with increase of PbO content up to 30 mol%. For the glass ceramics with 40mol% PbO, these components shift to higher wavenumbers located at ∼662, 678 and 705cm-1, indicating the conversion of some [TeO4] into [TeO3] structural units. This result is also related by the increase of intensity of X-ray diffraction peaks corresponding to the TeO2 crystalline phase comparative with their analogues with 30mol%PbO. iii The band centered at about ~782cm-1 corresponds to the stretching vibrations in [TeO3] structural units and shifts to ∼773cm-1. iv The large absorption bands in the 880-1150cm-1 spectral region is splitting into five components located at ~884, 925, 946, 1025 and 11007cm-1. All this bands are attributed to the stretching vibration in [BO4] units [19, 22]. v The IR features located in the region ranges between 1200 and 1600cm-1 are due to the asymmetric stretching relaxation of B-O bonds of from the trigonal [BO3] units. The band centered at ~1200cm-1 was assigned to B-O- asymmetric stretching vibrations in [BO3] units from pyro- and ortho-borate groups [23]. Its intensity decreases drastic with increase of PbO content up to 40mol% and shifts to 1250cm-1.

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i

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Immobilisation of Lead Ions in Borate-Tellurate Glasses…

37

TeO2

intensity [a.u.]

* B2O3 40%

*

*

30%

*

*

20%

*

*

10% 5% 0% 10

*

* *

*

*

** 20

30

40

50

60

2 theta [degree]

Figure 2.4. X-ray diffraction patterns for xPbO(100-x)[7TeO2· 3B2O3] glass ceramics with x=0-40mol%.

*

50% 60% intensity [a.u.]

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* B2O3 TeO2

70% 80% Pb2Te3O8 90% PbO 100% 10

20

30

40

50

60

2 theta [degree]

Figure 2.5. X-ray diffraction patterns for xPbO(100-x)[7TeO2· 3B2O3] glass ceramics with x=50-100mol%.

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Simona Rada, Eugen Culea and Monica Culea

38 3

0

450 C, 5h

absorbance [a.u.]

40% 2

30% 20% 10%

1

5% 0% 0 400

600

800

1000

1200

1400

1600

-1

wavenumber [cm ]

Copyright © 2010. Nova Science Publishers, Incorporated. All rights reserved.

Figure 2.6. FT-IR spectra of the xPbO(100-x)[7TeO2· 3B2O3] glass ceramics with x=0-40mol%.

The band located at about 1442cm-1 responsible for non-bridging oxygen atoms increase in the borate groups with increase the lead ions concentration up to 5mol% indicating more non-bridging oxygen ions exist in borate glass ceramic. This stronger vibration band reflects that more [BO3] units coupling with lead ions still exist in the glass ceramic host. Then, the intensity of the infrared bands related to the [BO3] groups from varied types of borate groups (the bands located at ∼1362 to ∼1442 cm-1) seems to decrease with increasing lead oxide content in the studied glass ceramics and shift to ~1360 and 1434cm-1. These results could be explained by considering the conversion of some ortho-, pyro- and varied types of borate structural units into boroxol rings. Our studies on lead-borate-tellurate glass ceramics indicate that, if the PbO content increase up to 40mol%, the lead ions must be considered to behave as modifier. It seems that the formation of [BO4] tetrahedral structural units is reduced because the modified [BO3] units containing one or more BO-Pb bonds are unable to accept a fourth oxygen atom. In the present work, the increase of PbO content over 50mol% modify the characteristic bands of the FTIR spectra (figure 2.7) and can be characterized as follow:

Borate-Tellurate Glasses: An Alternative of Immobilization of the Hazardous Wastes : An Alternative of Immobilization of the

Immobilisation of Lead Ions in Borate-Tellurate Glasses… i

ii

39

A small band appears visible on the spectra of samples with x between 30 and 60mol% at about 460cm-1. It is attributed to the bending vibrations of the tetrahedral [PbO4] structural units and it is sensitive to changes in the amount of PbO. The content of fourcoordinated lead attains its maximum values of 100mol%PbO. For 50